Proposal to NSF for a conceptual Design Study - MIT

Proposal to the National Science Foundation 

for a Conceptual Design Study: 

X-ray Laser User Facility 

at 

Bates Laboratory 

Massachusetts Institute of Technology 

Principal Investigator 

David E. Moncton 

Co-Principal Investigators Science Collaborators 

William S. Graves Simon Mochrie Keith A. Nelson 

Franz X. Kaertner Gregory Petsko Dagmar Ringe 

Richard Milner Henry I. Smith Andrei Tokmakoff 

Bates Senior Staff Contributors 

Manouchehr Farkhondeh William M. Fawley James Fujimoto 

Jan van der Laan Hermann Haus Erich Ippen 

Christoph Tschalaer Ian McNulty Denis B. McWhan 

Fuhua Wang Jianwei Miao Michael Pellin 

Abbi Zolfaghari Mark Schattenburg Gopal K. Shenoy 

Townsend Zwart 

April 2, 2003

SUMMARY 

Recent advances in accelerator, laser, and undulator technology have created the possibility of 

constructing a national user facility based on an intense free-electron laser at extreme ultraviolet and x-ray 

wavelengths. MIT is exploring the construction of such a facility at its Bates Laboratory site. The facility 

would produce x-ray beams with peak brilliance some ten orders of magnitude greater than is presently 

available from today’s synchrotron sources, and pulse durations from 100 femtoseconds to less than 1 

femtosecond. The wavelengths produced will range from 0.3 to 100 nm in the fundamental, with 

substantial power in the x-ray 3 rd harmonic at 0.1 nm. The possibility of future upgrades to even shorter 

wavelengths will be preserved in the design. Based on a 4 GeV superconducting linac incorporating a 

number of extraction points, the complex will include the potential for twenty or more undulators and 

x-ray beamlines. 

In order to produce beams of the highest quality, various methods of seeding the electron beam 

with high harmonics of laboratory lasers are currently under investigation, as is lasing by selfamplification 

of spontaneous emission. A number of these methods will be exploited to produce radiation 

sources matched to experimental needs. Advanced laser technology, used to seed the electron beam for 

short pulse production, will also be used to produce the electron beam and for use in pump-probe 

experiments. Thus, lasers will be an integral part of the facility. 

As described in the body of the proposal, the scope of science possible with such a facility is both 

broader and in some sense deeper than that pursued at today’s synchrotron facilities or laser sources, 

because it combines high power and coherence for the first time in the 100-0.1 nm range. The science that 

is foreseen spans many disciplines including atomic and fundamental physics, condensed matter physics 

and materials sciences, femtochemistry, structural biology, and various fields of engineering. The source 

we propose, and the experimental methods it will spawn, will generally be qualitatively new and have 

high impact in many fields of science and technology. The strength of the science and technology base in 

the northeast region, and in particular at MIT, make this a superb location for such a facility. 

But MIT is ideally suited to be the host institution for an equally important reason: the remarkable 

educational opportunities that such an endeavor would create and sustain. Beyond the enormous impact 

on graduate and post-graduate education of a facility with such a broad user science program, is the 

opportunity to provide education in the underlying accelerator sciences and technologies. Such 

technologies are becoming increasingly critical to society, but they are difficult, if not impossible to teach, 

without the existence of working accelerators. A program at MIT would have immense impact in 

developing a future generation with skills in this area. The presence of such a laboratory will also allow 

enhancements of novel teaching methods for K-12 students and teachers. Many of these educational 

opportunities will be exploited early in the proposed study by using infrastructure that currently exists at 

Bates Laboratory. 

The scale of the facility, with its technical infrastructure, is ideally matched to the 80-acre, 1.2 km 

long Bates site. The scope of such a project has been under study at Bates by a small group for nearly a 

year. The three-year study proposed here will, in the first 18 months, produce a conceptual design, an 

R&D plan, a cost estimate and schedule, and a detailed scientific case, as integral parts of a proposal for 

construction beginning in FY2007. During the balance of the proposal period, R&D will be undertaken, a 

management plan developed, and the facility design advanced to the point necessary to begin 

construction. It appears that the capital cost for the facility will be around $300M, assuming construction 

beginning in FY 2007, and depending on the number of beamlines implemented with construction funds. 

iii

TABLE OF CONTENTS 

SUMMARY........................................................................................................................................... iii 

INTRODUCTION ................................................................................................................................. 1 

1 SCIENCE AND EDUCATION ................................................................................................... 2 

1.1 Overview of Source Properties........................................................................................... 2 

1.2 Science ............................................................................................................................... 4 

1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy....................... 5 

1.2.1.1 Introduction........................................................................................... 5 

1.2.1.2 The MIT/Bates X-ray Laser.................................................................. 6 

1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy........................ 6 

1.2.1.4 Beamline Concept................................................................................. 9 

1.2.1.5 Proposed Work...................................................................................... 9 

1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy..................... 11 

1.2.2.1 Introduction........................................................................................... 11 


1.2.2.3 Beamline Concept................................................................................. 11 

1.2.2.4 Experiments with Photon Correlation Spectroscopy ............................ 12 

1.2.2.5 Proposed Work...................................................................................... 14 

1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical 

and Biophysical Systems....................................................................................... 15 

1.2.3.1 Introduction........................................................................................... 15 


1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular 

Dynamics in Solution............................................................................ 16 

1.2.3.4 Beamline Concepts and Proposed Work............................................... 17 

1.2.4 Trace Chemical Analysis in the Particle Counting Limit...................................... 20 

1.2.4.1 Introduction........................................................................................... 20 


1.2.4.3 Experimental Methods .......................................................................... 22 

1.2.4.4 Proposed Work...................................................................................... 23 

1.2.5 Structural Biology ................................................................................................. 24 

1.2.5.1 Introduction........................................................................................... 25 


1.2.5.3 Experimental Concepts ......................................................................... 27 

1.2.5.4 Proposed Work...................................................................................... 32 

1.2.6 Electron Dynamics with Attosecond Resolution................................................... 34 

1.2.6.1 Introduction........................................................................................... 34 


1.2.6.3 Experimental Concepts ......................................................................... 35 

1.2.6.4 Proposed Work...................................................................................... 36 

v

TABLE OF CONTENTS (CONT.) 

1.2.7 X-ray Microscopy With Atomic Resolution ......................................................... 37 

1.2.7.1 Introduction........................................................................................... 37 

1.2.7.2 X-ray Microscopy: Source Considerations ........................................... 38 

1.2.7.3 X-ray Microscopy: Real-Space Methods.............................................. 39 

1.2.7.4 X-ray Microscopy: Reciprocal Space Methods .................................... 42 

1.2.7.5 Conclusions........................................................................................... 45 

1.2.8 Nanometer Lithography ........................................................................................ 46 

1.2.8.1 Introduction........................................................................................... 46 

1.2.8.2 Achromatic-Interferometric Lithography.............................................. 46 

1.2.8.3 Zone-Plate Array Lithography .............................................................. 47 

1.2.9 Status of Scientific Programs at Operating UV FELs ........................................... 48 

1.2.9.1 Low Energy Undulator Test Line (LEUTL) at ANL ............................ 49 

1.2.9.2 Deep Ultraviolet FEL at BNL............................................................... 49 

1.2.9.3 Tesla Test Facility at DESY.................................................................. 51 

1.3 User Program...................................................................................................................... 52 

1.4 Education and Outreach Program....................................................................................... 53 

1.4.1 Programs During Design Study............................................................................. 54 

1.4.2 Planning for Programs During Facility Construction and Operation.................... 55 

2 TECHNICAL CONCEPT SUMMARY ...................................................................................... 56 

2.1 Facility Description ............................................................................................................ 57 

2.2 Injector ............................................................................................................................... 57 

2.3 Linac................................................................................................................................... 59 

2.4 Conventional Lasers and Seed Generation......................................................................... 59 

2.5 Undulators .......................................................................................................................... 60 

2.6 FEL Properties.................................................................................................................... 60 

2.7 Photon Beamlines............................................................................................................... 63 

2.8 Comparison to Other Sources............................................................................................. 64 

2.9 Research and Development Program ................................................................................. 67 

3 THE BATES LINEAR ACCELERATOR CENTER .................................................................. 69 

4 INTERAGENCY AND INTERNATIONAL COOPERATION ................................................. 70 

4.1 Interagency Cooperation .................................................................................................... 70 

4.2 International Cooperation................................................................................................... 71 

5 DELIVERABLES AND PROPOSED SCOPE OF WORK ........................................................ 72 

REFERENCES ...................................................................................................................................... 76 

APPENDIX A TECHNICAL CONCEPT ........................................................................................... A-1 

APPENDIX B PROJECT PLANNING.............................................................................................. B-1 

vi

INTRODUCTION 

The fields of laser and accelerator technology stand now at a point of remarkable opportunity: the 

creation of highly coherent, powerful and ultra-short pulses of x-ray radiation ranging in wavelength from 

100 to 0.1 nanometers. The x-ray laser has been the “Holy Grail” of both the laser and the x-ray research 

communities since the invention of the laser. Now, not only can such a device be built, but it can also be 

part of a very cost-effective national user facility providing many independent photon beamlines for 

different areas of research. When contemplating the impacts of an x-ray laser on human knowledge and 

technology, think of the independent impact of the x-ray and the laser as the starting point. All of that 

scientific and educational benefit has been produced without any laser reaching the x-ray regime and 

without any x-ray source producing coherent radiation. That is now possible for the first time, and it is so 

compelling that in comparing the x-ray laser with current sources, a Global Science Forum sponsored by 

the Organization for Economic Cooperation and Development (OECD) held in September 2001 [1] 

concluded that, “because of the vast increment in performance, it is very likely that entire new types of 

scientific measurement and applications will be enabled.” Now, eighteen months after that statement was 

written, most experts would call it an understatement. 

Three key elements of the facility we envision would make it unique. First, a 4-GeV linear 

accelerator with superconducting radio frequency cavities would produce such high electron pulse rates 

that twenty or more beamlines could be extracted to serve a large user community. Second, integrated 

high-harmonic generation laser technology would seed the electron beam and generate photon beams with 

high longitudinal coherence and pulse lengths significantly below 100 femtoseconds, perhaps below 1 

femtosecond. Third, taking advantage of the ability of linear accelerators to extract beams at different 

energies, we envision a facility spanning both the traditional extreme ultraviolet and x-ray wavelength 

range. This approach provides for integration and synergy between the UV and x-ray communities and 

the laser community, where scientists are anxious to move to wavelengths shorter than conventional 

table-top technology can provide with high pulse power. Here, we propose a three-year study to develop a 

construction proposal, optimizing the machine for the remarkable science and education opportunities it 

will enable, and proceeding with the necessary preparations to be ready for construction in 2007. 

1

1 SCIENCE AND EDUCATION 

The potential of x-ray laser radiation for science and education is extraordinary. One might 

determine the structure of a single molecule with one x-ray pulse without the need for crystals; probe the 

dynamics of atoms, molecules or of condensed matter on fundamental time scales and length scales 

simultaneously; study the properties of matter at very high energy densities; improve technologies for 

fabricating, inducing, and observing structure at the smallest length scales; and probe and exploit nonlinear 

phenomena in the x-ray regime. It can be safely said that the x-ray laser promises to be a 

comprehensive probe of spatial and temporal structure on all scales and at all resolutions relevant to all 

forms of condensed matter. And it will be an exquisite tool to manipulate matter as well. 

Educational opportunities are also exceptional for many reasons. The first is the scope of science 

that the facility would support, with almost every experiment involving students as is common at today’s 

synchrotron radiation facilities. The success of the User Program discussed in Section 1.3 is essential in 

this regard. Second, the technology associated with the facility spans many fields of physics and 

engineering, thus providing a context for development of a curriculum in accelerator science and 

technology, based on academic excellence at MIT and surrounding universities. Accelerators are 

becoming increasingly important to society, but few schools offer substantial programs. Third, the 

existence of a large-scale high-technology construction project would open many one-of-a-kind 

experiences for students and teachers in a rich array of subjects from environmental planning and 

architecture to project management. Finally, MIT has been a leader in educational innovation, including 

the TEAL (Technology Enabled Active Learning) classroom [2], iLAB [3], the CMSE RET (Center for 

Materials Science and Engineering Research Experience for Teachers) program [4], and the Cambridge 

University collaboration [5], to name a few. These activities have reached well beyond traditional 

graduate and undergraduate populations to include K-12 students and teachers, with a strong emphasis on 

diversity. All of these experiences will be enhanced and new ones will be generated by the proposed 

study, the eventual construction, and ultimately, the scientific operation of the x-ray laser facility. 

1.1 OVERVIEW OF SOURCE PROPERTIES 

Photon Beam Parameters. The extraordinary science potential of an x-ray laser source is a 

direct consequence of the photon beam parameters and how they compare to other sources, as discussed 

in detail in Section 2.8. The beams will have a tunable wavelength range from 100 nm to 0.1 nm 

(wavelengths shorter than 0.3 nm will be obtained from higher harmonics); they will be fully coherent in 

the transverse direction; they will be pulsed at a rate of 1 kHz, and each unseeded pulse will have a 

duration up to 100-200 femtoseconds, contain up to 1 mJ of energy, and have a bandwidth of about 0.1%. 

Of course it is the peak brilliance that is the strong suit of these machines—some ten orders of magnitude 

greater than current sources and having pulse lengths 1000 times shorter. As we will see, a great deal of 

the proposed science is driven by techniques exploiting these characteristics to access new regimes of 

temporal behavior. However, the time-average brilliance properties also exceed third-generation 

synchrotron sources by several orders of magnitude, and the flux is comparable at similar bandwidths. 

Photon Degeneracy. High peak brilliance is a deeper concept than simply having a large number 

of photons at once. For the first time, we can begin to think about an x-ray source having a large number 

of photons occupying a single quantum state at one time. This parameter, called photon degeneracy, is 

less than one for third-generation synchrotron sources at wavelengths of 0.1 nm and is only a few hundred 

at 100 nm. For the x-ray laser, it will be of order 10 9 . This increment of seven to ten orders of magnitude 

in an important source parameter will have revolutionary scientific impact. The x-ray laser is not just a 

better source than a traditional synchrotron; it is a qualitatively different kind of source. It would be more 

2

appropriate to call these sources first-generation x-ray lasers, than to call them fourth-generation 

synchrotron sources. In fact, for most of the venerable experimental methods of hard and soft x-ray 

research, it is essential that photons interact singly with the sample. Synchrotron facilities will continue to 

be the workhorses of the field for some time to come. As will become apparent, the new experimental 

methods appropriate for an x-ray laser have more in common with optical laser methods and many have 

no analog at existing synchrotron sources. 

Seeded Beams. In the preceding paragraph, we have described the unseeded beam parameters 

essentially as a starting point for discussing the improvements obtained by seeding, as we propose with 

high-harmonic generation from conventional Ti:sapphire lasers. The longitudinal structure of the 

unseeded pulses is complex; the structure is not transform-limited and it is statistically noisy. Seeding has 

the potential to reduce or eliminate these shortcomings, producing pulses that are transform-limited with 

duration perhaps below 1 femtosecond, with correspondingly lower pulse power. The science we envision 

will be greatly enhanced by, and in some specific cases entirely dependent on, these improved 

characteristics. Successfully implementing this technology is challenging, but it will pay huge scientific 

dividends. There appear to be no fundamental technical barriers. It looks straightforward at the longer 

wavelengths, with increasing difficulty as one approaches the hard x-ray regime. The synergy generated 

by the technical challenge, followed by the scientific pay-off of producing stable, reliable, transformlimited 

pulses approaching 0.1 nm wavelength, will be the intellectual driving force during the early 

operation of this facility. 

Fundamental Lengthscales and Timescales. Over the centuries, improvements in the optical 

quality of photon sources have enabled measurements with improved resolution. With the development of 

x-ray laser sources, an enormous step is now possible before physical limits are reached. Consider spatial 

resolution. A laboratory x-ray tube enables x-ray images with about 0.1 mm resolution, typical of a 

medical x-ray. The introduction of synchrotron sources moved that resolution to a few microns, and thirdgeneration 

undulator sources have the potential to resolve features below 100 nm. A concept exists for a 

30 nm “nanoprobe” facility at Argonne National Laboratory’s Advanced Photon Source. An x-ray laser 

will have full transverse coherence, which is the equivalent of achieving the diffraction limit. In that limit, 

a microscope is theoretically possible with resolution equal to the wavelength of the radiation. 

In extracting static structure information, reciprocal space methods, commonly called x-ray 

diffraction techniques, are perhaps even more important than real-space microscopy. The challenge of 

extending diffraction methods to larger length scales is the conjugate of the problem, described above, of 

extending real-space methods to shorter length scales. The full transverse coherence of an x-ray laser 

beam will permit the “perfect” diffraction experiment, that is, one in which the resolution of the 

diffraction peaks could potentially be the inverse of the size of the beam illuminating the sample. This is a 

direct consequence of the uncertainty principle, and the concept of diffraction limit. The implications of 

the situation are simple and powerful. With full transverse coherence, it will be possible to elect either 

real space methods or reciprocal space methods over the entire range of spatial length scales (say 0.1 nm 

to 0.1 mm) relevant to condensed matter. Indeed, in this limit, the most powerful approach is not to 

separate imaging and diffraction, but to combine them in one general method referred to as coherent 

imaging. Implications for the study of non-periodic structures and, particularly, individual molecules, are 

profound. 

We now turn to an analogous discussion in the time domain, where the characteristics of the 

beams we propose, particularly those seeded to approach or reach the “transform limit,” will be 

revolutionary. Consider photoemission, or inelastic x-ray or neutron scattering. In these techniques, the 

probe exchanges energy with the sample and the measurement of that energy transfer, as a function of 

3

momentum transfer, gives fundamental information on the dynamics of electrons, spins, phonons and 

other quasi-particle excitations in condensed matter systems. The accessible region of energy resolution 

for x-rays is from a few electron volts to 1 meV, with increasing difficulty at the best resolutions. For 

neutrons, the available resolution range is shifted to smaller energies by a factor of 10 to 100, but the 

limited flux of neutron beams means large samples are required. Neutron techniques cannot be widely 

applied to microscopic samples, monolayers, or small particles containing a few atoms, molecules, or 

clusters. 

The x-ray laser would open a vast new dynamical regime by permitting studies to be carried out 

in real time. There will be two distinct advantages of this capability. First, energy ranges accessible to 

conventional inelastic x-ray and neutron scattering can be probed in real time, rather than in the energy 

domain. In many complex systems, the dynamics are not well-described or understood in terms of 

“harmonic” quasi-particle excitations whose excitation energy is greater than their inverse lifetime. In 

many cases, the critical motions are large conformational changes involved, for example, in protein 

function or chemical reactions. Second, and equally important, real-time methods will naturally extend to 

times that correspond to energy resolution well outside the range of traditional energy-domain methods, 

for example to nanoseconds (i.e. microvolts), or microseconds (i.e., nanovolts). Anticipating success in 

seeding beams to get pulse lengths of 1 femtosecond, we will have a probe capable of accessing 

essentially all relevant timescales, from those of atoms and molecules to those in condensed matter in 

virtually all forms. Not only that, but such a probe, to the extent that its wavelength approaches 0.1 nm, 

will be generally sensitive to the structural character of the dynamics. 

1.2 SCIENCE 

Over the last decade, but with increasing activity in the last five years, the prospect of acceleratorbased 

x-ray laser sources has generated immense excitement in a growing scientific community. We 

estimate that over 100 workshops have been held leading to the development of four 100-plus page 

scientific cases for proposed new facilities: the Linac Coherent Light Source (LCLS) at Stanford, the 

TESLA XFEL at the Deutsches Electron Synchrotron Laboratory (DESY), the SASE-FEL project at the 

Berlin Electron Storage Ring for Synchrotron Radiation (BESSY), and the 4GLS project at the Daresbury 

Laboratory in the UK. High-level committees in the respective countries have favorably reviewed these 

cases. Virtually all the work proposed in these documents can be undertaken at the proposed MIT x-ray 

laser facility. Our website (http://mitbates.mit.edu/xfel/index.htm) contains these documents in their 

entirety. Furthermore, beyond the “paper” studies, three demonstration facilities have been built to 

produce radiation in the range of 100 nm providing the incentive for the development of real experimental 

programs, all of which could also be carried out at the proposed MIT facility. 

In consideration of all of this activity, we have concluded that the general case for the x-ray laser 

has been made clearly and convincingly. Our goal in this document is not to repeat that work, but rather 

to move toward the development of more specific experimental concepts and approaches that are driven 

by compelling scientific opportunities. During the proposed study we intend to establish and execute a 

process that will greatly expand the involvement of the external community, and result in conceptual 

designs for a specific set of about ten initial beamlines to be included in the construction proposal. This 

process is described in more detail in Section 1.3 (User Program) and Section 5 (Deliverables and 

Proposed Scope of Work). We want to emphasize here our intent that this process (1) be closely 

integrated with the design of the machine so as to maximize the science it will enable, (2) be based on a 

broad outreach activity involving many workshops and a public proposal solicitation process, and (3) 

employ peer review for all proposals using well-established criteria to determine scientific merit. 

4

To this end, we present below eight contributions by scientists and faculty at MIT, Brandeis, 

Yale, Argonne, and the Stanford Synchrotron Radiation Laboratory. It is essential that the project team 

have a scientifically strong cadre of “users” with whom to interact on a regular basis as the technical 

design of the facility is developed. It is also essential to have such a nucleus of interested and motivated 

users to organize the planned workshops and generally provide leadership to the community to develop 

and prepare the more detailed technical and scientific cases for each beamline included in the construction 

proposal. Five of these contributions are proposals in themselves (their authors are collaborators on this 

proposal) aimed at specific work that would be funded and performed under this study. The other three 

contributions (Sections 1.2.4, 1.2.7, and 1.2.8) have a somewhat different flavor. They are concepts for 

very innovative and in some cases speculative instruments in different wavelength ranges of the proposed 

facility. In two cases, concept development and R&D are underway at Argonne and Stanford with 

separate funding. For x-ray lithography, more work would be necessary before a specific proposal could 

be made to develop realistic instrument concepts. But at this stage, we do want to think broadly and 

entertain some high-risk concepts. Taken together, all of these ideas, we believe, give a flavor of some 

extremely exciting possibilities. But not all science must wait for new machines to be constructed. We 

also include a brief overview in Section 1.2.9 of science proposed or underway at existing 100 nm 

sources. 

1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy 

1.2.1.1 Introduction 

The development of time-resolved coherent laser spectroscopy has ushered in a new 

understanding of condensed matter dynamics, including the time scales for electronic and vibrational 

decoherence and relaxation, liquid-state molecular dynamics and chemical reactions, and collective 

structural rearrangements in a variety of complex media. The technology and methods used have been 

exploited extensively for practical applications as well, ranging from advanced materials characterization 

and metrology to optical coherence tomography and other biomedical assessment techniques to the 

coherent optics used routinely in photonic switching and communications. These applications, with their 

myriad benefits to society, were developed and are being extended continually by the scientists and 

engineers whose graduate and postgraduate research was devoted to inventing time-resolved coherent 

optics and spectroscopy. 

Another scientific and technological revolution will be ushered in as ultrafast coherent optics and 

spectroscopy are extended to wavelength scales 100 times shorter than those used today. Here we propose 

a study of collective structural change in condensed matter using four-wave mixing or “transient grating” 

techniques with coherent x-rays. Transient grating fringe spacings of Λ = 1-100 nm, corresponding to 

coherent scattering wavevectors q = 2π/Λ extending to nearly the edge of the Brillouin zone, will be used 

to examine acoustic modes, nonoscillatory density dynamics, polarization, and other order-parameter 

responses on the same length scales. These are the mesoscopic correlation lengths whose fluctuations 

underly the great majority of collective structural dynamics that we now measure through coherent 

spectroscopy on much coarser length scales [6–8]. Thus our current measurements tell us a great deal 

about the time scales for collective structural change, but little about the length scales or the connections 

between the two. 

When we measure the multiple time scales for density (i.e. structural) relaxation in viscoelastic 

polymer liquids or polarization relaxation in mixed ferroelectric crystals, we are observing on coarse 

length scales (far longer than the correlation lengths for these variables) the integrated outcomes of 

fluctuations whose dynamics may in fact vary sharply with (mesoscopic) length scale. Thus, we are left to 

5

speculate: are the fastest components of polymer relaxation associated with motions of polymer end 

groups, intermediate components with side chains, and slowest components with backbone and whole 

molecule motions? If so, then how do we understand the similar hierarchy of time scales observed in the 

structural relaxation dynamics of glass-forming van der Waals liquids that have no obvious corresponding 

hierarchy of structural correlation lengths [9–12]? Are the fastest dielectric relaxation components in 

mixed crystals like KnbxO3/Kta1-xO3 near ferroelectric phase transitions [13–15] associated with the 

smallest polarized nanodomain regions, those formed around isolated impurity ions? Are the intermediate 

relaxation time scales associated with those nanodomains that have encountered each other and merged 

together as the temperature has cooled toward Tc and the polarization correlation length around each 

impurity ion has grown? Are the slowest time scales associated with clusters of randomly situated 

impurity ions whose surrounding polar regions have merged? Or does collective relaxation in complex 

systems of this sort involve sequences of steps that inherently give rise to complex dynamics? These 

questions are of practical as well as fundamental interest, since the design of these materials, used widely 

in ferroelectric DRAMS, capacitors, and piezoelectric actuators, for high-bandwidth applications is 

intimately connected to the association between local structure and dynamics. 

Unambiguous association of the time and length scales of collective structural rearrangements 

requires direct and simultaneous measurement of both. This will be enabled through coherent optical 

spectroscopy using x-ray wavelengths. At the same time, the development of coherent optical methods 

that operate on nanometer length scales will open a wealth of new possibilities. Coherent x-ray machining 

and lithographic fabrication of nanometer features in advanced devices, x-ray optical trapping and 

organization of atoms, molecules, and nanomaterials into assemblies with nanometer spacings, x-ray 

coherent scattering metrology of ultrathin films, and other advanced materials—a new world of 

applications will emerge under the leadership of the scientists and engineers whose graduate and 

postgraduate research involves development of ultrafast coherent x-ray optics and spectroscopy. 

1.2.1.2 The MIT/Bates X-ray Laser 

The proposed x-ray laser will provide unprecedented capabilities for coherent ultrafast x-ray 

spectroscopy, coupled with an exceptional degree of access through construction of a beamline dedicated 

for the purpose. The anticipated output parameters are extraordinary: approximately 1 mJ energy per 

pulse at x-ray wavelengths, with pulse duration in the range of 100-200 femtoseconds. The optical fourwave 

mixing measurements that we have conducted over the years on viscoelastic materials and crystals 

that undergo structural phase transitions [9,11,12,16–24] have typically involved pulse energies of tens of 

microjoules, focused to spot sizes of tens to hundreds of microns in samples that are essentially 

transparent to the excitation light. The proposed facility will provide comparable energies at the sample, 

and far higher intensities if desired since the x-ray spot could be focused to far smaller sizes. More likely, 

comparable spot sizes will be used and the energy may need to be reduced. Transient grating experiments 

can be conducted with nearly any hard x-ray wavelength, and the availability of various wavelengths may 

be exploited to enlarge the range of coherent scattering wavevectors. The pulse duration only needs to be 

short compared to the fastest acoustic oscillation period, which will be on the order of 0.5-1 THz. If 

seeding is used to produce shorter pulse durations, they will also be suitable. 

1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy 

The time-resolved four-wave mixing setup illustrated in Figure 1 yields a transient grating fringe 

spacing Λ and corresponding coherent scattering wavevector magnitude q given by the wavelength λ of 

and the angle θ between the excitation pulses, Λ = 2π/q = λ/2sin(θ/2). As in conventional light scattering 

spectroscopy, the measurement length scale is limited to the order of the light wavelength. Time-resolved 

6

optical four-wave mixing measurements have provided an effective means for probing structural 

relaxation dynamics in both liquid and solid materials, including elucidation of dispersive responses 

involving thermal, acoustic, or optic phonon-polariton modes [9,11,12,16–24]. An example is provided in 

Figure 2, which shows data from a polymer liquid recorded as the temperature is cooled such that the 

characteristic structural relaxation time τ (an average value used for characterization of nonexponential 

relaxation dynamics) moves through the range of the acoustic oscillation period, i.e. through the range 

ωacτ ≈ 1 where ω ac is the acoustic frequency given by the acoustic (transient grating) wavevector 

magnitude qac and the acoustic velocity vac(ωac) through the dispersion relation ωac/qac = vac. The data 

show clearly the acoustic anomalies (velocity dispersion and attenuation maximum) that occur around the 

region where ωacτ ≈ 1. The data also show slower, nonoscillatory components at lower temperatures that 

reveal structural relaxation dynamics occurring on time scales slower than the acoustic oscillation period. 

This method, known as “impulsive stimulated thermal scattering” (ISTS) is based on the sudden sample 

heating resulting from the excitation pulses and the subsequent thermal expansion that launches the 

acoustic and slower responses observed through coherent scattering of the probe light. Data of this sort 

have been used extensively for study of supercooled liquid dynamics [9,11,12,19–21]. 

In other studies, excitation of acoustic waves or coherent optic phonons through impulsive 

stimulated Brillouin scattering (ISBS) or Raman scattering, respectively, has been used to generate 

material responses relevant to collective structural change including viscoelastic relaxation or structural 

phase transitions [16–19], again with the time-dependent responses probed through coherent scattering. In 

general, coherent time-domain spectroscopy of collective modes has proved particularly valuable in cases 

where the frequencies of the modes are low or the damping (or dephasing) rates are high, including the 

overdamped regime, and where the low-frequency spectrum may contain several contributions from, for 

example, structural or polarization relaxation in addition to acoustic or optic phonons. In these cases, 

conventional light-scattering spectroscopy may be unsuitable since the frequency shift may be 

prohibitively low, the Stokes and anti-Stokes lines may broaden and merge, or additional central peak 

FIGURE 1 Setup for four-wave mixing. (a) A binary phase mask pattern is used to generate two excitation 

pulses that are recombined at the sample. The variably delayed probe pulse also is split to generate a 

reference field for heterodyne detection. (b) Adaptation of the setup for x-ray wavelengths. A crystalline 

grating will be used to split the incoming excitation and probe beams, and a second crystalline grating will be 

used to recombine the pulses at the sample. 

7

FIGURE 2 Four-wave mixing data from polypropylene glycol. As T is reduced, the viscosity increases and 

the structural relaxation time scale τ lengthens, passing through the ωacτ ≈ 1 range at around 260 K where 

the acoustic damping rate is strongest. At lower T, slow components of structural relaxation are observed 

directly. At long times (0.1–1 ms), thermal diffusion between the transient grating peaks and nulls is 

observed. 

features due to relaxation processes may obscure the acoustic or optic phonon features. The advantages of 

the time-domain approach are often realized in association with collective structural rearrangements (e.g., 

structural phase transitions or structural relaxation in viscoelastic fluids), since they are characterized by 

slow and complex collective dynamics, often involving coupled low-frequency modes [6–8,20–24]. 

Recent x-ray Brillouin scattering measurements [25], while revealing interesting behavior at high 

wavevectors, also show clearly the difficulties posed by strongly damped or overdamped responses as 

well as additional central peak spectral features. 

In x-ray four-wave mixing measurements, we expect to be able to adjust the transient grating 

spatial period Λ through the region of characteristic structural correlation lengths L, i.e., we expect to gain 

full access to the wavevector regime where qL ≈ 1. Acoustic anomalies (maximum in the damping rate, 

dispersion in the velocity) similar to those that occur when ωacτ ≈ 1 should be observed, in this case 

directly revealing the correlation lengths rather than the correlation times involved in structural relaxation. 

Observation of the slower, nonacoustic dynamics observable on time scales longer than the 

acoustic period (which is very fast at high q) with variable wavevector will provide a direct window into 

the connection, if any, between the structural correlation length and time scales. If the broad distribution 

of relaxation times, with average value τ, is associated with a distribution of correlation lengths with 

average value L, then as the transient grating wavevector is varied from the qL < 1 regime through the 

qL ≈ 1 range, the dynamical response will progressively lose its slower relaxation components until, for 

qL > 1, the structural relaxation dynamics are filtered out completely because the measurement is being 

made on a length scale shorter than that of the observed structural relaxation processes. In crystalline 

solids such as the mixed ferroelectrics mentioned earlier, very similar considerations hold since strain is 

linearly (piezoelectrically) coupled to the polarization which is the order parameter [26]. Examination of 

the soft optic phonon branch [7,17] (whose displacements give rise to the polarization) in the highwavevector 

range should also be revealing. If “nanodomain” polarization correlation lengths L are 

8

associated with the correlation times measured, the results should be analogous to those described above 

for acoustic modes, namely a stiffening of the phonon response at high wavevectors as the qL ≈ 1 range is 

reached and exceeded. Thus x-ray four-wave mixing measurements of samples that under collective 

structural rearrangements will directly reveal the relevant time and length scales and the association 

between them. Other related issues such as the wavevector range at which acoustic modes in disordered 

or partially disordered materials become overdamped will also be addressed directly. 

1.2.1.4 Beamline Concept 

The details of the beamline to be constructed for x-ray coherent scattering experiments will be 

formulated during the proposed grant period. However, the beamline will certainly need apparatus for 

generation and delivery to the sample of excitation and time-delayed probe x-ray pulses. This could be 

done using crystalline grating interferometers as shown in Figure 1. Reflective optics may also be used 

for beam delivery to the sample. A separate beamsplitting step, prior to the one shown in Figure 1, is 

necessary for generation of the probe pulse, which must be variably delayed and then directed to the 

sample either through reflective or diffractive optics. The latter, as illustrated in the figure, permit 

heterodyning of the coherently scattered signal field with a reference field that originates from the probe 

beam. This methodology [27] is used extensively for optical four-wave mixing measurements. 

The beamline will also require an amplified femtosecond laser system with which time-resolved 

optical probing can be used following x-ray excitation. The optical wavelengths cannot be used for 

coherent scattering off the x-ray generated transient grating, but they can be used to assess electronic and 

other responses to x-ray excitation irrespective of length scale. An understanding of the x-ray excitation 

process and what sample dynamics are initiated by it will be a crucial element in the development of 

coherent scattering methodology at x-ray wavelengths. 

1.2.1.5 Proposed Work 

A study of the experimental feasibility and theoretical description of time-resolved four-wave 

mixing measurements conducted with coherent x-ray wavelengths on collective structural dynamics is 

proposed. This will include preliminary design of the experimental beamline at the proposed facility and 

estimates of the expected signal levels and time-dependences that might be observed. One of the science 

collaborators on this proposal, Keith Nelson, has been involved in formulation of x-ray four-wave mixing 

experiments in connection with the LCLS project. This project, if supported and brought to successful 

operation, will provide the necessary coherent x-ray pulses, but will offer far more than the energy 

required for the proposed experiments. Also its single experimental station will be used for a number of 

other experiments as well as research on condensed matter of the sort proposed here. 

Professor Nelson also is involved currently in an experimental collaboration with Professors 

Henry Kapteyn and Margaret Murnane at the University of Colorado, attempting to conduct four-wave 

mixing measurements at soft x-ray wavelengths generated through high harmonic generation of 

femtosecond optical pulses [28]. These measurements, if successful, will provide access to a range of 

wavevectors considerably wider than the range accessible through visible wavelengths, but still far 

smaller than the range reached with hard x-rays, and confined by absorption to near-surface regions. The 

measurements also will teach us much about how to conduct this class of experiment with coherent 

x-rays. For example, the experimental setup depicted in Figure 1(b) is being tried, not with crystalline 

gratings, but with 100-nm grating structures fabricated through electron-beam lithography [29]. 

Prof. Shaul Mukamel and his group at the University of Rochester has undertaken 

[30–32] theoretical treatment of x-ray four-wave mixing applied to the study of molecular response. 

9

However, the experiments of interest here deal explicitly with condensed matter collective responses that 

have not been considered to date. Perhaps the most important questions deal with the mechanisms 

through which such responses will be generated by the x-ray excitation pulses. What are the relevant 

mechanisms through which hard x-ray excitation pulses will interact with the samples? At optical 

wavelengths, there are essentially two excitation mechanisms that give rise to sudden, spatially periodic 

stress in the sample, thereby inducing acoustic, and in some cases, slower density responses. In ISTS, 

optical absorption and rapid thermalization produce a sudden temperature rise and a step-function applied 

stress. In ISBS, the initial, spatially periodic stress takes the form of an impulse function, which drives a 

transient acoustic response. The combination of driving forces and time-dependent responses has been 

treated in detail, and both mechanisms have been exploited for study of acoustic behavior associated with 

collective structural change [9,11,12,16,19–24]. Following excitation through either mechanism, the timedependent 

density dynamics are monitored through coherent scattering of probe light. 

At x-ray wavelengths, absorption of crossed excitation pulses leads to ionization at the 

interference maxima (i.e., the transient grating “peaks”), producing local currents and resulting in a spatial 

distribution that may be influenced by electron mobility. At the high transient grating wavevectors q that 

will be reached, thermal diffusion from grating peaks to nulls (the rate of which increases as q 2 in the 

hydrodynamic limit) may be fast compared to some or all components of the density response, in which 

case the temporal profile of the applied stress may approach that of an impulse function rather than the 

step-function profile exerted at optical wavelengths. Apart from this, ionization of molecules at the 

grating peaks will lead to a separate step-function stress since the steady-state density will be different for 

partially ionized regions of the sample than for pristine regions. These effects all arise from x-ray 

absorption and are likely to be the dominant contributions to acoustic and other density responses. 

Additional interactions that give rise to stress in a manner analogous to stimulated scattering also must be 

considered. Excitation of phonons in crystalline solids through similar mechanisms will be treated as 

well. For example, liberation of valence electrons by ultrashort pulses reduces screening within the unit 

cell, inducing coherent optic phonon responses in semiconductors even with visible excitation 

wavelengths [24,33]. X-rays should produce similar responses in insulating crystals as well. Finally, the 

detection of time-dependent responses through coherent scattering of x-ray probe pulses must also be 

treated. 

Apart from the light-matter interactions, density dynamics at high wavevectors and short times 

will be modeled approximately so that the time-dependent responses likely to be observed can be 

simulated. The purpose of the effort proposed is not to undertake a full simulation of the collective 

dynamics at high wavevectors, but to anticipate the types of time-dependent responses expected in view 

of plausible material response functions and the different excitation mechanisms at play, and on that basis 

to elaborate experimental strategies for observing the responses of interest in the most incisive manner. 

Support for a postdoctoral associate in the Nelson group will be used to undertake the theoretical 

and feasibility study described above. Professor Shaul Mukamel has indicated a strong interest in working 

collaboratively on the theoretical effort. His experience in treating x-ray four-wave mixing in molecules 

specifically [32] and nonlinear optical spectroscopy generally [34] will accelerate the effort markedly. 

Through treatment of the x-ray excitation and probing processes and consideration of the collective 

sample responses of interest, we will be able to estimate reliably the signal levels and dynamical features 

to be expected from x-ray four-wave mixing measurements conducted at the proposed accelerator, the 

information content that the experiments can be expected to provide, and the theoretical modeling that 

will be necessary in order to extract that information content. Using realistic parameters for the output 

generated by the proposed system, we will be able to assess critically the feasibility and scientific value of 

the experiments. 

10

1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy 


Complete understanding of a condensed matter system depends not only on knowledge of its 

static properties and structure, but also on how it changes in time in response to time-varying applied 

fields, or, simply, to thermally-induced spontaneous fluctuations. In addition, these sample dynamics may 

be crucial for deciding how suitable a material may be for a particular application, and surely are key for 

any materials processing required to achieve a useful final product. In many cases, we still do not 

understand how molecular interactions and motions at the Angstrom scale give rise to often-complex 

collective dynamics at mesoscopic (nanometer) and macroscopic (micrometer and longer) length scales. 

What is needed is a powerful and general means of monitoring the full dynamic response over a range of 

length scales for materials of all sorts, from proteins and long-chain polymers to ferroelectrics and 

glasses. 


In recent experiments, it has been shown that the new technique of x-ray photon correlation 

spectroscopy (XPCS) is capable of studying the slow dynamics of strongly-scattering samples at smaller 

length scales—of the order of 30 nm—than can be achieved with laser PCS [35–41]. In principle, XPCS 

yields the intermediate scattering function (IFS) for the electron density of a sample, S(Q,t), and thus 

should be a general and powerful method for characterizing the small-scale dynamics of condensed 

matter. However, the time and length scales that can be studied via XPCS even at third-generation sources 

is limited by the source brilliance, so that to extend XPCS studies to shorter times and into the nanometer 

range demands even more brilliant sources. Specifically, the coherent flux, required for XPCS, is directly 

proportional to the source brilliance. Thus, the proposed MIT/Bates x-ray laser source represents an 

extremely exciting opportunity for novel XPCS studies. 

1.2.2.3 Beamline Concept 

Generally the proposed 1 kHz time structure of the source must be carefully considered in the 

design of XPCS experiments, particularly the duty cycle, which may range from 10 to 100%. Our goal, 

under this proposal, is to develop some of the methods and instruments that will permit us to optimally 

perform XPCS experiments largely independent of the details of the duty cycle. An XPCS beamline will 

have the following key features. First, it will have an x-ray beam splitter/delay line that will separate a 

single pulse into two and introduce a variable time delay between the arrival times of the pulses at the 

sample. We envision the beam splitter using eight thin diamond crystals to give two more-or-less 

symmetric legs, each with a variable x-ray path length. The difference in x-ray path length between the 

two legs then determines the delay time between pulses via the speed of light - a maximum path length 

difference of 3 m would allow a maximum delay time of 10 ns. Two equivalent legs permit the path 

length and delay time to be nulled straightforwardly. Second, it will have a slit system that will allow the 

beam size on the sample to be accurately controlled, while introducing as little extraneous slit scattering 

as possible. Third, it will have the means to orient the sample and detector as desired. Importantly, in 

order to resolve speckle, we anticipate an unusually large sample-to-detector distance of 10 m or more. 

Finally, it will have a fast-CCD based x-ray detector, with sufficient resolution to resolve speckle and 

sufficient speed to keep up with the x-ray laser repetition rate. 

To realize XPCS at the proposed x-ray laser, there are several issues that must be addressed, 

including sample damage issues, and the construction of the x-ray beam splitter and delay line. We will 

11

work closely with the project team on these issues, which have extensions to other methods. Another 

important consideration is the x-ray detector that should be used. At Yale, we propose to focus on this 

aspect under this proposal. 

1.2.2.4 Experiments with Photon Correlation Spectroscopy 

With the time structure as proposed, two sorts of XPCS measurements can be conceived. In the 

first type of experiment, each pulse of the x-ray laser is used to illuminate a sample. The resultant 

scattered x-rays are then detected via an area detector with a read-out that is synchronized to the x-ray 

laser pulse. There is a tremendous advantage in signal-to-noise in employing an area detector. Thus, we 

envisage XPCS data from the proposed x-ray laser to consist of batches of 100 (with 10% duty cycle) 

successive CCD images each separated one from another by 1 ms. Calculation of the intensity 

autocorrelation between images separated by a delay time t, [ g 2 (t)= / 2 ] yields the ISF 

via g 2 (t) = 1 + β [ S (Q,t) / S (Q) ] 2 , permitting the sample dynamics to be studied on time scales from 

1 millisecond to tens or hundreds of seconds, although with a gap for time delays between 100 and 

900 ms. Actually, this data acquisition scheme is reminiscent of that shown to work by Lumma et al., who 

used so-called kinetics mode to rapidly acquire 20 or so successive scattering patterns before CCD 

readout [42]. Within this scheme, it is possible to examine the dynamics on time scales equal to and 

longer than the x-ray laser pulse separation. Thus, for XPCS is it desirable to maintain as much flexibility 

in the pulse structure of the source as possible. For example, a burst of 100 pulses, separated from each 

other by one tenth or one-hundredth of a millisecond once a second, would permit accesses to dynamic 

processes occurring on time scales of 10 -4 or 10 -5 s. There are nevertheless many interesting questions 

concerning the dynamics on short length scales in slow systems—in particular polymeric systems—that it 

will be possible to conclusively answer at this source, and not elsewhere, because of the very high 

brightness. 

An important example of this sort of experiment would be a definitive characterization of the 

wavevector dependent (Q-dependent) dynamics of compositional fluctuations within a block copolymer 

melt or a binary polymer blend. Many of the properties of high polymer liquids may be understood on the 

basis of the reptation model of polymer dynamics, which depicts the motion of a polymer as a random 

walk along a tube delimited by temporary entanglements with neighboring chains [43–48]. 

Microscopic evidence for the existence of the tube and its corresponding entanglement length has 

been provided by neutron spin-echo (NSE) measurements [49], which have characterized the relaxations 

within the tube (Rouse modes). However, the delay times accessible with NSE are shorter than needed to 

observe reptative relaxation directly, and important aspects of the reptation model, remain untested. After 

more than 30 years, reptation remains a topic of considerable current interest [50–52]. 

The expected intensity autocorrelation function (g2) for a polymer melt is sketched versus delay 

time in Figure 3 for several values of Q. At small times, g2 decreases as a universal function of Q t , as a 

result of Rouse modes, to a Q-dependent plateau value, given by the tube diameter. Experimentally, NSE 

measurements have been able to characterize the early time behavior (t«τd), but how this connects to the 

behavior at long times and the details of the long time relaxations themselves await XPCS measurements 

at an x-ray x-ray laser source. A remarkable prediction of the reptation model is that long-time, smallscale 

compositional fluctuations show a wavevector-independent relaxation rate (Γ), determined by the 

disentanglement time (τd) for a polymer to reptate out of its original tube [53,54]. This prediction stands 

in contrast to the to the Γ~Q 4 behavior that is expected for non-entangled copolymers on short length 

scales. Physically, the prediction arises because to to relax a small-scale compositional fluctuation, it is 

nevertheless necessary for polymers to disentangle, which requires a time τd. Thus, for QRg >1, the 

plateau is predicted to persist until the disentanglement time, irrespective of Q. 

12

FIGURE 3 Schematic intensity autocorrelation functions for a polymer melt for four wavevectors, increasing 

from A to D. 

Another as-yet untested, defining aspect of the reptation model concerns the lineshape of the ISF 

versus t for times near τd. The lineshape is predicted to consist of a sum of exponential decays [46,54,55]. 

Figure 4, for example, shows reptation-model-based predictions for the mode amplitudes and mode 

relaxation rates for a highly asymmetric diblock copolymer [55]. 

Careful XPCS measurements at the proposed x-ray laser source will allow detailed lineshape 

analyses to test these sorts of predictions. But beyond tests of existing theory, new XPCS experiments 

have the potential to yield key insight into resolving the puzzle of what determines the entanglement 

length in blends and melts. 

In addition to the polymer melts and blends discussed so far, there are many other polymeric 

systems, in which the polymer conformations can be quite different and can exhibit novel dynamics. 

Thus, we can anticipate that XPCS studies of a variety of polymer systems extending down to nanometer 

length scales will prove an exciting and fruitful research area. 

The second method for implementing XPCS at the proposed x-ray laser facility is a significant 

departure from established methods, but being able to study condensed matter dynamics on time scales 

from 1 picosecond to 10 nanosecond timescales is the potential reward. Now, we must rely on an x-ray 

FIGURE 4 Specific predictions for the relative mode amplitudes and corresponding relaxation rates for 

compositional fluctuations within an asymmetric (f=0.05) diblock copolymer melt. Note that τd=4t*. 

13

pulse splitter and delay line to create from one pulse of the x-ray laser two x-ray pulses with a variable 

separation in time (∆t). The two pulses are then permitted to scatter in succession off of the sample, and 

the scattered x-rays are detected by a CCD area detector. 

Each split pulse will generate a speckle pattern (given that the pulse width is shorter than any 

sample dynamics) with the second speckle pattern corresponding to the sample’s configuration at a time 

∆t after the first. If the sample’s configuration is essentially unchanged in ∆t, the two patterns will be the 

same. On the other hand, if the sample’s configuration has completely changed after a delay ∆t, the two 

speckle patterns will be uncorrelated. The speed of light (3×10 8 ms -1 ) together with the reasonable range 

of delay line lengths (0.3 mm to 3 m, say) implies that it may be possible to create pulses with ∆t=10 -12 s 

to 10 -8 s. 

Although this range of times is similar to the range that can be achieved with NSE, it is pertinent 

to recall that only a handful of NSE instruments exist in the world, and that with XPCS it will be possible 

to study much smaller samples of difficult-to-manufacture materials or the dynamics of thin films. One 

exciting avenue of research, for which being able to study small sample volumes will be an important 

asset, will be to study the dynamics of internal motions within protein molecules. XPCS studies 

performed on protein crystals, where crystallographic order orients the vibrational/relaxational modes, 

may prove especially informative in assessing the key motions of these fundamental molecular machines. 

It is not possible, however, to separately read out CCD images from pulses separated by 

10 -12 -10 -8 s. Nevertheless, the statistical properties of the intensity distribution in the combined image 

reveal whether the two contributing speckle patterns are correlated or uncorrelated, and indeed the extent 

of their correlation [56–59]. Although the conclusions are the same for partial coherence, for clarity we 

will consider a perfectly coherent incident x-ray beam. In the case of a sample with slow dynamics 

compared to ∆t, it is as though there is a single speckle pattern, and the variance of intensity distribution 

relative to the mean intensity squared - that is, the contrast of the speckle pattern - is 1. By contrast, the 

sum of two uncorrelated speckle patterns has a contrast of 0.5. If ∆t is varied over the range of 

characteristic times of the sample’s dynamics, one will go from a constrast of 1 for small ∆t to a contrast 

of 0.5 at large ∆t. Clearly, the variation of the contrast versus ∆t determines the characteristic relaxation 

times of the sample. Thus, the second method of carrying out XPCS involves measuring the speckle 

contrast for different settings of the delay line. 


In both of the XPCS methods that we have outlined, it is necessary to read out batches of 

100 successive CCD images each separated one from another by 1 ms. These raw data must then be 

processed to yield either correlation functions (method 1) or the speckle contrast (method 2). Because of 

the very high data rates, a development effort to realize the data acquisition and data reduction envisioned 

is necessary. Under this proposal, we will develop a prototype fast CCD detector system suitable for both 

methods. 

To this end, the first item that we need is a CCD camera capable of 1 ms readout. This is state-ofthe 

art for current technology. Specifically, we propose to purchase a CCD camera under this proposal— 

the Dalstar 1M60 from Dalsa—that can read out an array of pixels in 0.86 ms. Mochrie’s group has 

already shown that this relatively inexpensive, commercial camera may be straightforwardly modified to 

become a successful detector for XPCS [60]. 

14

In the context of XPCS experiments at the proposed x-ray laser—64 × 1024 pixels per 

1 millisecond repeated 100 times per second—the overall data rate is about 6 Megabytes per second, or 

22 Gbytes per hour, or 500 Gbytes per day. It would be theoretically possible to store these quantities of 

data for later data reduction and analysis. Practically, however, it is clear that such a data storage rate 

would be difficult to maintain for extended periods, and if it becomes necessary to interrupt data 

acquisition to do data storage, some of the gain in the signal-to-noise ratio from parallel detection in many 

channels is lost. In addition, in order to be able to make sensible decisions during the experimental run, it 

is far preferable to reduce the data in real time, so that the correlation functions are available during the 

experiment. Anticipation of CCD cameras with yet faster readouts, leading to increased data rates, also 

directs one towards real-time data reduction. Thus, we propose to develop a CCD detector system, based 

on the Dalstar 1M60, read out by 4 frame grabbers in 4 fast computers, one for each of the camera’s 

4 “taps.” The computers, in turn, will calculate intensity correlations versus time (method 1) or the 

speckle pattern contrast (method 2) in real time. The modular nature of the system means that it should be 

straightforward to upgrade to a faster detector, or faster frame grabbers, or faster computers as they 

become available and/or required. 

Next, we require computer code that carries out the necessary calculations. Writing, debugging, 

verifying, and refining this software is an essential part of the work of this proposal. A key advantage of 

our software-based data reduction scheme is that it will be possible to continuously improve, for example, 

how g2 is calculated, if improved algorithms become available. A software-based implementation also 

facilitates other methods of processing data should that be desired. Others have implemented CCD-based 

PCS (multi-speckle PCS) in real time [61,62]. The distinction here that warrants a significant 

development effort is that the raw data rates are hundreds of times higher than what was achieved 

previously. 

Finally, it is worth remarking that, although the camera system we propose is a prerequisite for 

carrying out XPCS at the proposed x-ray laser, the real-time data reduction that we will implement will in 

addition very greatly improve the user-friendliness of XPCS at third-generation rings, providing 

experimenters with the same sort of immediate review of the data as is available in light-scattering 

experiments. Thus, our proposed detector system development effort has the potential to greatly increase 

the pool of scientists who may be interested in carrying out XPCS experiments now and in the future at 

the proposed x-ray laser. 

1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical and Biophysical Systems 


In large part, the advances made in chemistry and other molecular sciences during the twentieth 

century involved molecular structure: its description, determination, creation, and manipulation. Atoms 

were described as the building blocks of molecules, and the construction of molecules from diatomics to 

DNA was essential to describing their chemical, physical, biological, and material properties. Changes to 

molecular structure through reactions (or mutations) are thus used to influence such properties or 

function. Through this development, much of molecular structure has also been largely described in timeinvariant 

or static terms. Part of the reason for this may be that many of the tools we use to characterize 

structure are time-averaged measurements. However, every chemist inherently deals with structural 

change, even at levels as simple as drawing arrows to indicate the motion of electrons in chemical 

reactions. Increasingly, researchers in the molecular sciences are emphasizing the importance of 

describing, watching, and controlling the time-evolution of molecular structure. This is of broad 

importance, whether to watch and control changes in molecular bonding during a chemical reaction, to 

15

understand the folding of a protein into its physiologically active structure, or to describe structural 

transformation or phase transitions in molecular materials. 

For studies of molecular dynamics in solution there are no broadly applicable tools to characterize 

how structure changes on appropriate timescales. For that reason, the study of solution-phase chemical 

and biophysical problems would greatly benefit from new structurally sensitive experimental methods for 

describing molecular and collective dynamics. Femtosecond x-ray spectroscopy and scattering 

experiments offer the promise of unique sensitivity to atomic, molecular, and collective structure with 

time resolution exceeding that of nuclear motions. Such techniques thereby offer tremendous 

opportunities for characterizing molecular dynamics in liquids and other amorphous systems. The 

proposed MIT/Bates x-ray laser project represents the creation of a center of intellectual activity that will 

move this under-developed area of science forward, training and developing scientific personnel in an 

emerging discipline within the chemical and biophysical sciences. 

For the exploratory work under this proposal, a variety of possible experimental applications of 

femtosecond x-ray pulses to studies in the liquid phase will be evaluated, for their use in the study of 

near-equilibrium and non-equilibrium dynamics in solution. These vary by the nature of the chemical 

system studied, the x-ray interaction employed, and the type of distance scales to be probed. The results 

of these feasibility studies will form a primary input to the design of a beamline for such experiments. In 

the following sections, a general overview of methods applicable to chemical problems in solution is 

described, followed by a more detailed description of four experiments. These four examples represent 

specific experiments whose feasibility can be evaluated quantitatively through modeling, and which 

overlap with other complementary time-resolved spectroscopies. 


Several aspects of the proposed x-ray laser make it a unique as a facility for the study of solutionphase 

molecular dynamics. The pulse length of 100 fs for unseeded beams makes it useful to all but the 

fastest studies that involve observation of the motion of nuclei, opening experimental possibilities over 

time scales from femtoseconds to milliseconds. The ability to seed beams to pulse lengths of 1 fs or less 

will then cover all the relevant timescales. The extremely high peak brilliance of this source compared 

with others is vital to time-domain spectroscopies. All such methods use multiple pulses to interrogate a 

sample, and generally involve detecting small changes in what would otherwise be already small signals. 

Perhaps the most powerful attribute of the proposed x-ray laser source is the broad tunability over the soft 

to hard x-ray energy range. For chemists this translates into the ability to spectroscopically probe distance 

scales from the very local with atom specific excitation, to the mesoscopic with scattering experiments 

that interrogate the 0.1-100 nm distance scales relevant to the study of numerous complex condensed 

phases and biological systems. 

1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular Dynamics in Solution 

Studies of molecular dynamics in solution can be broken into two broad categories. One set of 

problems involve studies of dynamics near equilibrium, such as the changing collective structure of 

liquids, conformational fluctuations and large amplitude motions in biopolymers, and dynamical 

heterogeneity in supercooled liquids and glasses. Perhaps more experimentally challenging is finding 

ways of describing the complex structural changes accompanying non-equilibrium processes, such as the 

changes in nuclear and electronic configurations during chemical reactions, or large-scale conformational 

changes in biophysical processes, such as protein folding or binding of substrates. Whether near or far 

16

from equilibrium, a broad predictive understanding of these types of processes requires experimental 

methods that probe the evolving structures in these systems. 

Generating high-brilliance coherent femtosecond x-ray pulses opens the door to numerous 

possible techniques that could be used to study solution phase problems. These techniques will require 

two or more pulsed electromagnetic fields, of which one or more act initially as a pump, to prepare a 

system by initiating a chemical process or perturbing the system from equilibrium, and one acts as a 

probe, to detect a time-dependent change. These can be broadly classified as optical-pump/x-ray probe 

and multiple-pulse x-ray experiments. Each in turn can involve time-resolved x-ray absorption 

spectroscopy (XAS) on core level electrons, scattering, or diffraction processes. A system can also be 

probed by x-ray emission, although with limited control over the time scale of the dynamics to be probed. 

The spectroscopic theory for such experiments is rapidly gaining momentum, and in the perturbative 

limit, can formally be related to third-order nonlinear spectroscopies [63,64]. 

Optical-pump/x-ray probe experiments can be used to follow photo-initiated chemical processes 

or molecular relaxation processes with structural selectivity. Resonant optical pumping can be used to 

photoinitiate chemical reactions varying from charge transfer to bond rearrangements. Near-infrared 

pumping of solvent overtones can be used to induce rapid temperature jumps for protein folding studies. 

Nonresonant optical pumping can be used as an impulse perturbation to an equilibrium system—a method 

often used in study of collective relaxation in complex liquids. Femtosecond x-ray absorption or 

scattering can then be used as a structurally sensitive probe of these initiated dynamics [65,66]. 

Multiple pulse or nonlinear x-ray absorption or scattering experiments can be used to follow the 

dynamics initiated by an x-ray absorption or scattering event. For pulses resonant with the absorption 

bands of core electrons, such experiments include techniques broadly used in the optical range such as 

pump-probe or hole-burning experiments, and transient gratings, yet with the additional control over 

molecular scale excitation wave-vectors [64]. Such experiments would be sensitive to the dynamics of 

molecular relaxation processes viewed through the core electron levels. While the lifetimes of core level 

holes are much shorter than the planned 100 fs pulse length [67], other relaxation and transport processes 

can be probed [68]. To the degree that inhomogeneous or dynamic line broadening may be present, x-ray 

transient photochemical hole burning or pump-probe could be used to probe structural dynamics or 

heterogeneity present in the broadening of absorption lineshapes. Transient grating experiments would 

follow relaxation of excitations prepared with a well-defined wave-vector on distance scales 

corresponding to molecular and collective structures. Coherent multi-pulse experiments detected with 

wave-vector selective scattering offers the ability to follow molecular dynamics through their influence 

on the interference between fields scattered from two delayed pulses. For the case of nonresonant 

excitation and probing, this is an x-ray photon correlation spectroscopy that will allow the large-scale, 

collective motions of liquids or macromolecules and the heterogeneity to be probed as a function of time 

and wavevector [69]. 

1.2.3.4 Beamline Concepts and Proposed Work 

Building on the afore-developed general concepts, four types of experiments are discussed here 

for their use in solution phase molecular dynamics, from the collective structure in liquids to chemical 

reaction dynamics in solution and rapid protein folding events. The goal of this project is to investigate 

the feasibility of these experiments theoretically and computationally, and provide input to the design of 

beamlines for such experiments. The goal is to undertake specific investigations and to communicate a 

conceptual and theoretical framework with which to describe and model such femtosecond x-ray 

experiments. The feasibility studies will also permit subsequent investigations into the experimental 

17

systems that lend themselves to femtosecond x-ray studies and complementary optical and infrared 

experiments. 

X-ray Probed Optical Kerr Effect Spectroscopy (XOKE). The femtosecond Kerr effect 

spectroscopies used in the optical regime have been widely used to study collective structural relaxation 

processes in molecular liquids, supercooled liquids, and other amorphous condensed phases [70–72]. 

OKE experiments in two-pulse or transient grating configurations have proven effective at characterizing 

the time scales for collective dynamics by measuring the decay of a transient orientational or spatial 

anisotropy induced by an optical pulse [73]. The lack of microscopic spatial selectivity makes separating 

processes such as reorientation, collective librations, density fluctuations, and heterogeneous relaxation 

difficult to distinguish. 

The selectivity of OKE experiments to collective liquid relaxation can be extended to 

characterizing the distance scales associated with relaxation processes, when combined with x-ray 

probing. Figure 5 shows a schematic of this XOKE experiment. By exerting a torque on the liquid, a 

strong femtosecond optical pulse creates an orientational anisotropy that decays with collective relaxation 

processes. A femtosecond x-ray probe pulse can be scattered off this anisotropy to follow the relaxation 

as a function of wave-vector, revealing the time scale of relaxation over different spatial dimensions. 

Probing with wavelengths in the 0.3 to 30 nm range effectively allows all relevant distance scales for 

intermolecular liquid motions to be probed. This is a fundamental step in revealing the nature of 

collective relaxation processes in liquids and other amorphous condensed phases. Particularly in 

supercooled liquids, wave-vector dependent relaxation experiments will throw insight into whether nonexponential 

relaxation behavior arises from heterogeneous dynamics in single nanometer scale domains. 

Fragile supercooled organic liquids and isotropic phase liquid crystals are both excellent 

candidates for probing structural correlation lengths and wavevector-dependent dynamics [71,74]. 

Theoretical evaluation of this experiment can work with both atomistic and hydrodynamic descriptions of 

the signals, revealing how molecular and collective reorientation as well as density fluctuations are 

observed in the experiment. Also the role of the polarization of the fields involved on the motions 

observed should be addressed [72]. 

Photoinitiated Chemical Reaction Dynamics in Solution. “Femtochemistry” refers to the study 

of optically initiated chemical reaction dynamics, which have most often been performed on simple 

chemical reactions with femtosecond electronic spectroscopy of the low-lying valence states involved 

FIGURE 5 An optically induced anisotropy in a liquid sample is probed by time-delayed small angle x-ray 

scattering using a femtosecond x-ray pulse. The change in the scattering function at different wavevectors for 

a given time delay δS(q,τ) will gradually return to an isotropic shape with collective relaxation processes. 

18

[75]. In the case of photodissociation of small molecules, the knowledge of potential energy surfaces and 

the ability to directly relate the wavelength of observation to a point on the reaction coordinate allows 

much to be said about the dynamics of the nuclei and electronic states involved [76]. For increasingly 

complex reactions, other structurally selective probing methods are required. Time-resolved x-ray and 

electron diffraction experiments are beginning to be used in this context to study the gas phase reaction 

dynamics of small molecules [77], or for probing of phase transitions in crystalline substances [78,79]. 

While scattering experiments in an isotropic solution may have some use in following the 

reactants, transition states, products, time-resolved XAFS provides an alternate approach that gives local 

information on changes in electronic and nuclear structure [79,80]. The applicability of this method has 

recently been demonstrated. XAFS probing of optically induced charge-transfer reactions have shown the 

ability of such methods to probe the structural changes accompanying excited state processes, [65,81]. 

Such techniques have broad applicability to the study of electron and proton transfer processes, 

isomerization, photodissociation and more complex reactions in solution. XAFS would also be a sensitive 

probe of the structural changes accompanying optically initiated spin state transition in iron complexes 

[82]. 

We plan to evaluate is the use of x-ray probes of femtosecond UV photodissociation of CO from 

metal carbonyls in solution. Photodissociation of metal dicarbonyls and hexacarbonyls in hydrocarbon 

solvents have been widely studied and show rich dynamical behavior and solvation effects [83]. Also, 

these lend themselves to complementary femtosecond infrared vibrational spectroscopies [84]. It is of 

interest to not only follow the time-scale for the leaving of the ligand and solvation of the fragments, but 

to determine what femtosecond x-ray spectroscopy can say about the relative geometries of fragments and 

solvent during such a process. Femtosecond XAS or XAFS probing of the metal L-edge and K-edge of 

the O will be tested as a structural probe of metal and ligand during dissociation and solvation. 

Transient Photochemical Hole Burning of X-ray Absorption Line Shapes. Little is known 

about the dynamic or static line broadening mechanisms for x-ray absorption features. Since the lifetime 

of the core hole is exceedingly fast (

larger scale, questions exist regarding the heterogeneity of denatured states, the degree of native structure 

and compactness of intermediates, and how these are reflected in the topology of protein folding free 

energy landscapes. These types of questions can be addressed by directly observing folding process and 

heterogeneity at a molecular level on all relevant time scales. Probing of protein folding or denaturing 

processes in solution with femtosecond x-ray scattering would open up the possibility of probing local 

and large-scale structures on the now largely inaccessible time-scales between 100 fs and 1 µs that govern 

the initial collapse of denatured states [88,89]. 

In crystalline systems, time-resolved x-ray diffraction has been demonstrated as effective in 

probing the localized dynamics of ligands and sidechains following picosecond excitation of 

photoinitiated protein dynamics [90]. The more conformationally flexible, large amplitude motions in 

solution have been probed through time-resolved small angle x-ray scattering (SAXS) on µs to second 

time scales, [66,91]. Such measurements have been used to follow the change in the radius of gyration for 

the protein. To access the formation of protein structure in solution on various distance scales and over 

shorter time scales, time-resolved SAXS can be used to probe folding induced by an optical pulse [89,92]. 

Perhaps the most general approach is folding initiated by a nanosecond temperature-jump experiment on a 

cold-denaturing protein. A nanosecond near-infrared pulse can be used to rapidly heat a sample through 

solvent overtone absorption, allowing the nanosecond to microsecond time scales of protein folding or 

denaturing to be studied [93]. For a single excitation pulse, a broad range of time scales can be probed 

with a sequence of micropulses from the x-ray laser pulse train. Equivalently, binding experiments can 

also be initiated with optical pulses [94]. For instance, the binding of divalent zinc or calcium ions can be 

initiated by optically releasing these ions from a photolabile ligand [95]. Binding of the ions by proteins 

or peptides could be probed either with SAXS or XAS. As with the XOKE experiments, analysis of the 

induced scattering change as a function of time and wave-vector can potentially be used to establish the 

timescales for formation of secondary and tertiary structure and the degree of compactness of the protein. 

It is possible that the analysis of transient and temperature-dependent equilibrium scattering patterns can 

be used to quantify structural heterogeneity of the folding proteins. 

1.2.4 Trace Chemical Analysis in the Particle Counting Limit 


Trace particulate analysis is at once an important and a difficult area of forefront analytical 

science research. The importance is exemplified by, but not limited to, the needs of the semiconductor 

industry to analyze devices whose features will shortly approach 100 nm and for which one impurity atom 

can dramatically affect device performance. The surface composition of submicron sized particulates is 

crucially important in assessing their environmental impact. Similarly, microbiologists attempt to measure 

the three-dimensional location and abundance of single altered molecules contained in a single cell. In the 

future, where nanotechnology promises to become an important tool, such measurements are likely to 

become even more important. The difficulty in the trace measurement of such samples arises in the need 

to be both discriminative and efficient in sample usage. In a micron-sized particle, for instance, parts per 

billion impurities have only 10 atoms present. Yet these atoms must be measured among the 10 billion 

matrix atoms present. For the analyst measurement efficiency is embodied in the concept of useful yield 

(atoms detected/atoms removed) and discrimination is embodied in the concept of minimum detectable 

limit. For most measurement techniques useful yield and minimum detectable limit are unfortunately 

mutually exclusive. 

Secondary Neutral Mass Spectrometry (SNMS) utilizing saturating Vacuum Ultra Violet (VUV) 

light pulses from the proposed x-ray laser as a photoionization source will provide a unique analytical 

20

facility with part per trillion minimum detection limits and useful yields exceeding 30%—an 

improvement of several orders of magnitude over any facility in the world for the measurement of small 

samples or for the measurement of small pixels within larger concentration maps. The SNMS technology, 

post-ionization in a high useful yield, high mass resolution time of flight mass spectrometer, has already 

been demonstrated in our laboratory [96] with dramatic effects for samples ranging from semiconductor 

wafers [97] to stardust [98,99]. 


The proposed x-ray laser will provide a unique photoionization source, capable of producing 

tunable VUV with photon numbers per pulse sufficient to ionize all of the atoms or molecules of interest 

in the several mm [98] volumes required by post-ionization techniques. The proposed x-ray laser is 

uniquely suited for a SNMS analytical facility for two reasons. First, VUV light while otherwise difficult 

to produce, is ideal for the post-ionization of atoms and molecules. Consider Figure 6. Displayed are 

histograms of the photoionization potentials for all known atoms and molecules. Note that except for the 

alkali metal atoms, all ionization potentials fall in the VUV range. Second, the proposed x-ray laser is the 

only source capable of producing sufficient light energy per pulse in this wavelength range to saturate the 

photoionization process. Remember that signal-to-noise considerations require that each and every atom 

or molecule of interest be photoionized, in order for analysis of atomic dimension samples to become 

possible. A brief overview of the methods of SNMS and its place among the myriad of analytical 

techniques will be undertaken in the next section. 

Since the advent of Antony van Leeuwenhoek’s microscope, advances in analytical 

instrumentation have led the way in our understanding of the physical world. The instrument proposed 

here allows investigation in an entirely new world. Considering the rapid advancement in nanotechnology 

and its need to advance from demonstration to device implementation, a trace microprobe with a 10 nm 

lateral and a 1 nm depth resolution would seem likely to play an important role. Yet arguments based in 

useful yield and minimum detection limits are relatively dry. In the last section, therefore, we will explore 

several near-term experiments that we propose for this instrument. While space limits the detail supplied, 

the experiments proposed are exciting and can be accomplished without fundamental extensions to 

current technology. The light pulses must be tunable in the VUV and of order mj/pulse. 

FIGURE 6 A histogram of the ionization potentials of atoms and molecules demonstrating the importance of 

the VUV wavelength region. 

21

1.2.4.3 Experimental Methods 

Commercially, Secondary Ion Mass Spectrometry (SIMS) continues to be the technique of choice 

for trace analytical measurements—particularly those requiring analyses of samples with limited 

dimensions such as surfaces and particulates [100–102]. This choice is demonstrated by the 

implementation of several multimillion dollar large-frame SIMS facilities in the U.S. and around the 

world. SIMS is often chosen because of its low detection limits (< 100 ppt in favorable cases) [102], 

superb depth resolution (< 1 nm with appropriate primary ion sources) [101], relatively high useful yields 

(approaching 10% in very favorable cases), high lateral resolution (~10 nm pixel resolution with a liquid 

metal ion source) [100], high precision (

elements including rare gases, etc. to be analyzed with 30% useful yields and ppt minimum detection 

sensitivities. 

For molecular analysis the situation is more complicated, but the opportunity for significant 

improvement over current measurement limits is extremely high. We have demonstrated previously 

[106,107] soft ionization of such molecules that is free of fragmentation with low intensity, high 

harmonic generation lasers. Unfortunately, the number of photons per pulse was low. Thus the achievable 

detection sensitivity (though among the best ever measured) was quite low. In a separate experiment we 

used fixed frequency F2 laser (157 nm) to measure samples of carefully chosen molecules. In essence, 

lacking a tunable, energetic laser, we tuned the molecular IP instead. These experiments [108], which 

demonstrated the highest useful yield for molecules ever reported, validated the soft ionization concept up 

to saturation. The x-ray laser tunability will allow the detection of any desired molecular species. The 

possibility for analysis of molecular surfaces has also been bolstered by recent rapid progress in 

sputtering studies using polyatomic projectiles. 


We propose to use this trace analysis facility to conduct two important demonstration 

experiments. All of this work is separately funded and being carried out at Argonne (see Section 1.2.9.3), 

but will contribute directly to the development of concepts for beamlines to be built at the proposed x-ray 

laser facility. The first experiment will utilize the elemental analysis capability of the instrument to make 

critical measurements fundamental to our understanding of the early evolution of the solar system. The 

second will utilize the molecular analysis capability of the instrument to make measurements aimed at 

understanding the molecular basis for chemical mutagenicity. 

There is currently a huge debate raging about the origin of our solar system and how the sun 

evolved during its formative years. The old idea that the solar system quietly condensed is being 

challenged by the X-wind model (see Figure 7), which suggests the sun underwent a short period of 

intense radiation flux [109–114]. In this model, much material of the solar system was subtended into the 

sun and strongly irradiated before being expelled into the solar system [115–117]. Cosmochemists have 

long been puzzled by the fact that meteorites in general show very different O and N isotope abundances 

than does the earth. Recent work by R. Clayton at the University of Chicago suggests that it is the earth 

rather than the meteorites that are anomalous [118]. The Genesis sample return mission is nearing 

completion, and will provide pristine collections of solar wind components including light elements 

[119,120]. In particular, we propose to investigate the N isotopic content of the solar wind. Measurements 

on dilute samples require sensitivity for N that is not possible in current instruments, yet are well within 

the projected minimum detection limits for the proposed instruments. The measurement itself requires 

only percent level isotope precision to differentiate between terrestrial and meteoritic ratios. If solar 

values are meteoritic the X-Wind model would receive significant validation. 

A molecular understanding of chemical mutagenicity requires the ability to isolate and measure 

chemically modified DNA strands from single individuals. Prof. Paul Chiarelli of Loyola University of 

Chicago has demonstrated that a small fraction of the guanine of smokers’ DNA has been chemically 

altered by reaction with polyaromatic hydrocarbons (PAHs), a common component of all types of smoke 

[121–123]. These chemical adducts are proposed to be at the heart of carcinogenesis. Current analytical 

measurements are extremely inefficient requiring digestion of the placentas of a population of women in 

order to detect a statistically significant number of DNA adducts [124–127]. The SPIRIT instrument will 

allow detection of altered DNA adducts with a useful yield of nearly 30% (one third of the molecules in 

23

FIGURE 7 Winds in the solar nebula may have been responsible for the mixing of “hot” and “cold” 

components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions 

(CAIs, formed at about 2,000 kelvins) and chondrules (formed at about 1,650 kelvins), which were created 

near the protosun and then blown (green arrows) several astronomical units away, into the region of the 

asteroids between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive, 

carbon-based “cold” components. This scenario is based on the X-wind model of the solar nebula developed 

by Frank Shu and his colleagues at the University of California, Berkeley. From J. Nuth, American Scientist, 

May-June 2001. 

the sample will generate measurable signal) [128]. This level is many orders of magnitude larger than the 

Loyola study. At this sensitivity level, altered base pairs in very small samples, even in single individuals 

may be detected. Ultimately, experiments will attempt to measure base pair sequences rather than 

individual bases to determine whether the altered base pair resides in an important gene. 

1.2.5 Structural Biology 

The Brandeis University group headed by Professors Dagmar Ringe and Gregory A. Petsko will 

participate in the MIT/Bates x-ray laser project, contributing their expertise in the area of structural 

biology. During the past 25 years this group has made seminal contributions in the application of x-ray 

diffraction techniques to the study of protein structure and function. Many of these methods have 

involved the use of synchrotron x-ray sources. The Ringe/Petsko group has developed techniques to 

determine the spatial distribution of protein motions using x-ray diffraction; established the utility of very 

high resolution x-ray structure determination to unravel enzyme catalytic mechanisms; and pioneered the 

development and use of time-resolved techniques to map the reaction pathways of crystalline enzymes at 

atomic resolution. 

All of these advances have depended on the availability of powerful, fast sources of x-rays, and 

their further development has been stymied by lack of even brighter sources with pulse rates in the 

femtosecond time regime. The proposed x-ray laser source would shatter these barriers and open up a new 

range of exciting biological experiments that would reveal the details of protein structure and function at a 

level previously impossible. The Brandeis group intends to use the proposed x-ray laser for three types of 

experiments: ultra-high resolution structure determination of crystalline proteins; time-resolved 

24

diffraction studies of proteins at work on very fast time scales; and structure determination of singleparticles 

of biological molecules and macromolecular assemblies without the need for crystals. They will 

carry out a series of feasibility studies to establish what parameters of the source and ancillary equipment 

and what methods of specimen preparation and treatment are needed to perform these experiments. 


Structural biologists wish to determine the atomic structures and to observe the dynamical 

interactions between—and within—molecules whose mass ranges from a few thousand daltons to 

millions of daltons. It is now well established that protein dynamics as well as protein structure is 

essential to protein function [129]. Individual and collective atomic motions are required to allow 

substrates and regulatory ligands to fit into binding sites on the protein surface, and to permit two or more 

proteins to dock together to make a macromolecular complex. These motions range in amplitude from a 

few hundredths of an Angstrom to several Angstroms and tend to be as short as an atomic vibration or as 

long as a side-chain libration. These same motions may guide reacting species over the activation energy 

barrier, but this is less certain. Slower motions involving whole domains of a protein are important for 

protein folding and for ligand-induced conformational changes; these may have amplitudes of many tens 

of Angstroms. The dynamical time scales of interest in proteins range from as long as many milliseconds 

to as short as a few femtoseconds. 

There are now more than 13,000 proteins whose three-dimensional structures have been 

elucidated in atomic detail. Nearly all of these are static structures that have been determined by the 

technique of x-ray crystallography, in which a single crystal of the protein is irradiated with x-rays and 

the resulting diffraction pattern is recorded. The pattern provides the amplitudes of the scattered x-rays, 

but in order to compute an image of the scattering object the phases of these waves must also be 

determined. Since they cannot be measured directly, the phases are deduced from additional experiments 

or are calculated from the structure of a homologous protein. The phased waves are then combined via 

Fourier transformation to produce an image of the electron density in the crystal. This electron density 

map is interpreted semi-automatically in terms of an atomic model of the protein; the model can be 

improved by an iterative refinement process that minimizes the sum of the squared differences between 

the measured diffraction amplitudes and those calculated from the model. The process is indicated 

schematically in Figure 8. 

X-rays 

Crystals Diffraction 

Pattern 

Phases 

Refinement 

Electron 

Density Map 

Fitting 

Atomic 

Model 

FIGURE 8 Flow chart of the steps involved in determining the three-dimensional structure of a protein by 

means of x-ray crystallography. The brightness of the x-ray source determines the minimum size crystal that 

can be used for data collection, as well as the maximum resolution that is attainable for the structure. 

25

Until about 1970 the x-rays used in protein structure determination (about 1.5 Angstroms in 

wavelength) were obtained from tubes or rotating anode generators. After the first observation of 

synchrotron radiation, it was soon realized that it represented a much more intense x-ray source. 

Synchrotrons have now evolved through three generations. First-generation sources, which include SSRL 

at Stanford, DORIS in Hamburg, and CESR at Cornell, utilized radiation from storage rings built for 

high-energy physics purposes. Second-generation machines, which initially used bending magnets again, 

were dedicated light sources; examples include SRS at Daresbury and the NSLS at Brookhaven. 

Eventually, insertion devices such as wigglers and undulators, which produce tunable radiation of 

increased brightness, were added to these facilities. It was these facilities that demonstrated to the 

structural biology community the potential of synchrotron radiation to revolutionize protein structure 

determination. Its high brilliance enabled much smaller crystals containing larger macromolecules to be 

studied, and its tunability enabled new methods for rapid phase determination [130]. This potential was 

realized with the advent of third-generation synchrotron facilities, whose construction was driven by 

increasing demand for synchrotron radiation for structural biology and materials science. Thirdgeneration 

sources are characterized by an order of magnitude lower electron beam emittance (emittance 

is the product of the beam transverse size and divergence), which leads to higher brilliance, and a larger 

number of straight sections for insertion devices. Presently operating third-generation x-ray sources 

include the ESRF in France, the APS at Argonne, and SPring-8 in Japan. 

Since third-generation sources were introduced in the early 1990s, most of the protein structures 

that have been determined have used x-rays produced by synchrotrons [131]. Yet even these sources have 

limitations. The radiation is not bright enough, or coherent enough, for direct visualization of the 

structures of large single particles. Crystals are still necessary. A limited number of time-resolved studies 

have been carried out with these sources, but the temporal regime is limited to micro-to-milliseconds in 

most cases, which is too long for observation of most short-lived intermediates in enzyme-catalyzed 

reactions. And the sources are not bright enough to enable diffraction to be recorded at true atomic 

resolution (what we call “ultra-high resolution”), i.e., resolutions beyond ~1 Angstrom, for most 

crystalline proteins. 

During the past few years, a consensus has developed among forward-thinking physicists and 

structural biologists that short-wavelength free-electron lasers driven by linear accelerators represent the 

most promising path for those cutting-edge and future applications where high brightness, coherence, and 

short bunches are important [132]. These applications, which are described in detail below, are: ultra-high 

resolution macromolecular crystallography; time-resolved crystallography; and single-particle structure 

determination. 


The proposed x-ray laser will produce a 1kHz stream of femtosecond (fs) pulses each containing 

up to 1 mJoule of x-rays with full transverse coherence in a wavelength range suitable for atomic 

resolution protein structure determination. This combination of parameters will enable us to carry out a 

number of previously impossible experiments. There is tremendous interest in structural biology in both 

time correlation studies of thermal fluctuations and pump-probe relaxation studies (using as a pump either 

an external synchronized laser or the x-ray pulse itself) [133]. Synchronization with external optical lasers 

should be possible at the picosecond level, and more precise, sub-picosecond synchronization could be 

achieved by using diffracting crystal optics to split the x-ray pulse into pump and delayed probe pulses. 

The short time structure of the x-ray pulse could be used to probe sample dynamics on a femtosecond to 

nanosecond time scale [134]. Direct observation of such fluctuations at the atomic level is impossible 

with existing technology, yet such motions are believed to be at the heart of many biological processes 

26

[135]. Coherent x-ray laser radiation would be monochromatic, but the proposed facility would also 

produce a broadband pulse of radiation, incoherent but quite intense and having a sub-picosecond pulse 

duration. This radiation could be used for ultra-fast Laue crystallography, a technique that requires a 

range of wavelengths [136]. With such radiation, structures could be determined even of very small or 

unstable samples. The high brightness of the x-ray laser would also allow protein structures to be 

determined using very small samples, perhaps down to single particles of large macromolecular 

assemblies. In addition to conventional crystallographic techniques, it should be possible to exploit the 

spatial coherence of the radiation to get structural information holographically [132]. 

A major problem that must be addressed if these concepts are to lead to practical techniques is the 

issue of radiation damage to the sample. At this point too little is known about the damage that such x-ray 

pulses will cause in biological samples (relatively little is known about radiation damage to proteins in 

general!). From the power density involved, significant damage should be expected, even at 100K [137], 

the temperature at which most samples are frozen in biological synchrotron studies. Yet the very short 

period of the pulse might allow scattering information to be collected before damage becomes apparent. 

For static structure determination, irreversible damage to the sample is not necessarily a problem, so long 

as the structure information is retrieved first, in snapshot fashion. The damage issue may be more 

problematic for dynamical measurements in which the sample must remain undamaged for a longer time. 

Experiments are needed with high-power, very fast x-ray pulses in order to understand and develop 

methods to deal with this problem. 

1.2.5.3 Experimental Concepts 

Ultra-High Resolution Crystallography. Synchrotron radiation has made it possible to collect 

diffraction data from protein crystals to resolutions approaching those previously attainable only for 

crystals of small organic and inorganic compounds [138]. The inherently weak diffraction from protein 

crystals means that only very intense x-ray sources can generate measurable diffraction intensities at 

resolutions below about 1 Angstrom. We call such resolutions “ultra-high” to emphasize that they provide 

information not obtainable from conventional “high resolution” structures at around 2 Angstroms. Still, 

even with third-generation synchrotron sources, very few protein crystals have enough diffracting power 

to yield data even to 1 Angstrom resolution. In many cases this limitation is due to imperfect packing of 

the protein in the crystal lattice and no amount of additional x-ray power will solve the problem. But in a 

significant number of instances, higher power x-ray sources will open up the possibility of ultra-high 

resolution data collection for protein crystals that cannot provide such information with any current 

sources. Further, for those proteins that already diffract to ~0.8 A resolution with third-generation x-ray 

sources, the proposed x-ray laser offers the possibility of obtaining data to 0.5 A resolution and beyond. 

At that resolution, the bonding electron density between atoms should be observable directly, providing 

information that can be compared with—and used to validate—the results of approximate computational 

methods such as density functional theory [139]. This possibility is exciting because the limited number 

of ultra-high resolution protein crystal structures that have been solved so far have revealed a wealth of 

new information about the relationship between structure and function [140]. These tantalizing glimpses 

portend a bright future for ultra-high resolution protein crystallography once more proteins can be 

visualized at that level of detail. 

Hydrogen atoms, which are present in equal number to the heavier atoms in every protein, are 

invisible at normal high resolution but become visible when data are included beyond 1 A. Hydrogen 

atom location is essential to unraveling the catalytic mechanisms of enzymes, because nearly all 

enzymatic reactions involve proton transfers between ionizable functional groups. Figure 9 (left side) 

27

FIGURE 9 Difference electron density maps at 0.8A resolution of the protein xylose isomerase, showing the 

residual electron density, in green, of hydrogen atoms that were not included in the atomic model. Nearly 

every hydrogen that must be present has electron density, but the quality differs considerably, even for 

hydrogens in the same molecule. The map on the left shows that under favorable circumstances, even the 

hydrogen atoms on a water molecule that is hydrogen bonded to two protein side chains can be observed, 

allowing the orientation (and ionization state) of the water to be determined. In our experience, the quality of 

the weak data and the maximum resolution attainable are the most important factors in whether or not 

hydrogen atoms can be located, and the quality of the x-ray source plays a dominant role in these two factors. 

shows difference electron density at 0.86 A resolution for a tryptophan residue from our structure of the 

protein xylose isomerase, one of the best-diffracting crystalline enzymes. The green positive difference 

density clearly represents the aliphatic and aromatic hydrogen atoms that were not observed (and 

therefore not included in the atomic model) at lower resolution. Figure 9 (right side) shows another region 

of the same structure, where the two hydrogen atoms on a bound water molecule can be clearly seen, 

allowing its orientation to be established unambiguously. Unfortunately, even at this resolution, only 

about 75% of the hydrogens are visible. Higher resolution data should reveal the missing ones, but thirdgeneration 

synchrotron sources are too weak to produce such data even with large crystals of xylose 

isomerase (and similar strongly-diffracting proteins). The x-ray laser source should be strong enough to 

provide data to 0.5 A resolution at least; all hydrogens should be visible then. 

Ultra-high resolution also provides directional information about atomic motions in proteins. 

Atoms in real molecules do not have point electron densities, because the thermally-driven atomic 

fluctuations result in a smearing out of the density along the principal axes of motion. At normal high 

resolution, the ratio of observable data to variable parameters is too low to allow fitting of anisotropic 

ellipsoids of motion for every atom in a protein, so isotropic parameters only can be modeled [141]. 

Isotropic atomic displacement parameters (ADPs) show the average amplitude of the motion, but not the 

most frequent directions. At ultra-high resolution, there are enough observations to permit fitting of 

anisotropic ADPs for every atom, even in a large protein [142]. Such parameters could reveal new aspects 

of the dynamics of protein function. For example, one theory for how enzymes catalyze their reactions 

hypothesizes that protein structures have evolved to favor atomic motions along the reaction pathway at 

the expense of those motions that would drive the reacting species away from one another [143]. This 

theory could be tested if anisotropic thermal parameters could be obtained for enzymes with substrates 

and substrate analogs bound. Figure 10 shows such ellipsoids of motion for part of xylose isomerase. 

Since good anisotropic ADPs can be fit even at 1 A resolution, the x-ray source will enable this valuable 

information to be obtained for hundreds more proteins than now possible. 

28

FIGURE 10 At true atomic resolution, attainable only with very bright x-ray sources, it is possible to fit 

anisotropic ellipsoids to the spread of electron denisty around each atom in a protein structure, as shown here 

in red for some of the atoms in xylose isomerase at 0.8A resolution (data collected with a third-generation 

synchrotron source). These ellipsoids provide the directionality as well as the amplitude of the motions of the 

atoms, making it possible to test theories about the roles of protein motion in protein function. 

Another exciting possibility that ultra-high resolution would open up is being able to identify 

atoms directly by electron counting. Frequently proteins bind small molecules whose chemical 

composition and structure are unknown, either because they were captured inadvertently or because they 

were covalently altered by the action of the protein. Normal high resolution does not differentiate between 

atoms of similar atomic number, but at ultra-high resolution the integrated electron density is directly 

proportional to the electron count. Figure 11 shows the electron density in our structure at 0.9 A 

resolution for a tyrosine residue in the protein aminopeptidase; note that the oxygen atoms on the side 

chain and main chain have significantly greater electron densities than do the aliphatic and aromatic 

carbon atoms; note also that the backbone nitrogen atom has density intermediate between them. 

Figure 12 shows the electron density for a sodium ion bound between two protein-side chains. Normally 

this ion might be overlooked, or mistaken for a water molecule, but at ultra-high resolution its octahedral 

coordination is unmistakable (the remaining ligands are water molecules). Not only can atoms be 

identified at ultra-high resolution, but also the distances and angles between them can be determined with 

a precision of less than 0.02 A and 0.02 o . Such precision allows many putative chemical mechanisms to 

be differentiated. 

FIGURE 11 At true atomic resolution (here 0.86A for the structure of the protein aminopeptidase) it is 

possible to identify chemical compounds by their electron density alone; this is a tyrosine. Note that the 

electron density for the phenolic oxygen and the carbonyl oxygen is about twice as large as that for the 

carbon atoms, and the electron density for the amide nitrogen is also larger. In other words, atoms can be 

distinguished directly at this resolution. 

29

FIGURE 12 Here is another region of the aminopeptidase structure at 0.86A resolution, showing a sodium 

atom coordinated to four water molecules and two protein side chains. At ordinary resolutions this bound ion 

was missed. 

Time-Resolved Crystallography. Proteins are dynamic systems and their functions require that 

they be able to change their conformations in response to ligand binding, catalysis, pH changes, etc. For 

enzymes, their substrates also change, becoming intermediates and products. These dynamic processes 

mean that proteins are machines with moving parts and understanding how they work requires stop-action 

pictures of them as they move. During the past twenty years, we and others have been developing 

techniques to visualize unstable intermediates, transient complexes, and short-lived conformations of 

crystalline proteins [144]. These techniques range from rapid data collection to freeze-quenching; which 

technique (or combination) is needed varies from protein to protein and even from species to species 

within a particular protein’s mechanism. Under favorable conditions the entire reaction pathway of, say, a 

crystalline enzyme can be mapped out in atomic detail, with structures obtainable for the enzymesubstrate 

complex, the enzyme-product complex, and all of the kinetically-significant intermediates. 

Figure 13 summarizes our work on the catalytic pathway of bacterial cytochrome P450, an 

enzyme that stereospecifically hydroxylates camphor with the aid of molecular oxygen, electrons and 

protons [145]. Among the techniques needed to produce these structures was the ability to collect data 

very rapidly. 

The method of Laue diffraction allows virtually complete data sets to be collected on a submillisecond 

time scale provided broad band-pass radiation of high intensity is available. With thirdgeneration 

sources, sometimes data sets can be obtained in microseconds. The x-ray laser would produce 

broadband spontaneous radiation in addition to monochromatic radiation; this radiation would be weaker, 

but still have much higher peak brilliance than that obtainable from synchrotron sources. We estimate that 

x-ray pulses as short as a few femtoseconds duration should be strong enough to provide essentially 

complete Laue data sets for many protein crystals, opening up the possibility of true femtosecond time 

resolution in protein crystallography. Such rapid data acquisition will make studies such as the one shown 

in Figure 13 much easier, enable previously inaccessible systems to be studied at a similar level of detail, 

and may even permit us to observe the extremely short-lived species that result from interatomic 

fluctuations as a protein moves. These fluctuations occur on a picosecond to microsecond time scale, 

except in photoactive proteins, where the functionally important motions may be of femtosecond 

duration. 

30

FIGURE 13 The complete reaction cycle of the enzyme cytochrome P450, an enzyme that in the human liver 

detoxifies foreign compounds, metabolizes drugs, and synthesizes steroid hormones. The enzyme requires a 

heme group for catalysis and uses molecular oxygen, attached to the heme iron, to insert an oxygen atom into 

the C-H bond of an unactivated hydrocarbon substrate, a reaction that normally requires a blowtorch to 

achieve without the enzyme. Arranged around each of the intermediates in this complex reaction pathway are 

the electron density maps and atomic models for the structures of each intermediate, which we were able to 

determine by time-resolved techniques requiring very powerful x-ray sources with very rapid pulse rates. 

This is the first time that an enzyme catalytic path has been observed at atomic resolution, but extension of 

this new technology to faster enzymes (P450 is rather slow) will require a new generation of x-ray sources. 

Of course, time-resolved crystallography requires that most of the protein molecules in the crystal 

lattice be in the same state at the time of data collection [146]. Homogeneity throughout the crystal can 

only be achieved in two ways: by using low temperature or other perturbation to lengthen the lifetime of 

the species of interest so that it accumulates; or by triggering the reaction in the crystal by a means faster 

than either the lifetime of the intermediate or the time required to collect the data. Light pulses provide 

the fastest and most convenient trigger, and research in our lab and elsewhere is aimed at designing and 

synthesizing chemical compounds that are inert in the dark but can be activated by a pulse of light. Such 

compounds can be “caged substrates” for crystalline enzymes. They allow reactions to be initiated 

throughout a protein crystal by a rapid light pulse; if the pulse is synchronized to an x-ray pulse, then 

stroboscopic data collection should be possible on a sub-picosecond time scale, and the x-ray laser makes 

such synchronization trivial: the source itself will produce light in the required wavelength range along 

with x-rays, at exactly the right time interval. 

Single-Particle Structure Determination. The final application we envision for the proposed 

x-ray laser is single particle structure determination. The x-rays generated by this source will have 9 to 

12 orders of magnitude higher peak brilliance than those produced the most powerful synchrotron today. 

Current x-ray sources can produce measurable diffraction intensities from crystals containing about 

10 12 particles (“particle” here means either single chain protein, oligomeric protein or complex of 

proteins). Simple mathematics suggests that a source 10 12 -fold more powerful than those might allow 

measurable scattering to be observed from a single particle if that particle were large enough. How large 

is a matter for investigation, but the structures of single-subunit or oligomeric proteins on the order of 

31

500,000 daltons in mass, or of protein complexes the size of small viruses or, say, the ribosome, might be 

directly observable from single-particle scattering without the need to form crystals [147]. 

This hypothesis is easily testable once an x-ray laser facility is operational, but three technical 

questions must be addressed before then. The first is how the data are to be used. One possibility is that 

sampling of the diffraction transform can be made sufficiently fine to allow the phases to be computed by 

an iterative algorithm [148]. Tobacco mosaic virus is one of the test specimens we propose to investigate 

in this proposal, to develop algorithms and programs suitable for carrying out the same calculations on 

non-helical particles and three-dimensional transforms. Finally, if the source is sufficiently coherent, true 

hard x-ray holography may be possible, obviating the need for phase determination [147,150]. That 

possibility can be evaluated once the source comes on line. 

The second issue that must be investigated is how to immobilize the specimen in the x-ray beam 

for the time required to collect the scattered amplitudes. A single particle cannot be mounted in a capillary 

tube or on a loop like a protein crystal. One possibility is that a beam of particles can be produced by 

electrospray or matrix-assisted laser desorption methods like those used in mass spectrometry, but we 

worry that such approaches would dissociate or otherwise damage the particle, which after all has not 

evolved to exist in a vacuum [147]. We prefer to immobilize the specimen in vitreous ice or in a 

glass-forming cryoprotective solvent mixture [150], and to “find” the particle by means of its transform. It 

is this approach that we propose to evaluate in this proposal. 

Finally, an x-ray beam powerful enough to produce measurable diffraction from a single particle 

is also powerful enough to destroy that particle almost instantly. The key word here is “almost.” It is 

possible that scattering, which occurs on a sub-femtosecond timescale, will be complete before radiation 

damage, which involves both primary and secondary events, especially if the sample is cooled to liquid 

helium temperatures. Unfortunately, no one knows if this will be the case. We propose to evaluate this 

possibility and also to search for chemical substances that can protect single particle samples from 

radiation long enough to allow diffraction data to be collected. 


We propose to perform a series of feasibility studies to establish which structural biology 

experiments can be carried out in the early stages of the development of the proposed x-ray source. These 

studies are intended to obtain information about how to stabilize single particle specimens for structure 

determination; how to process ultra-high resolution diffraction data and extract hydrogen atom positions 

and atomic displacement parameters from such data; whether it is possible to suppress radiation damage 

chemically; and how to trigger reactions in the crystalline state for time-resolved studies. 

Single particles of biological materials might be introduced into the x-ray beam by spraying 

techniques such as those used in mass spectrometry, but we suspect that many specimens will not remain 

intact under those conditions, particularly if the sample consists of multiple proteins non-covalently 

bound together. We prefer to test the hypothesis that the most versatile method will be to immobilize the 

sample by embedding it in vitreous ice. Techniques for forming glasses in aqueous solution are wellestablished 

for the cryoprotection of crystalline proteins, and we propose to test these methods, many of 

which we originated, on several different single particles: large oligomeric native proteins (we will use 

D-galactonate dehydratase, a homooctamer of ~400,000 daltons molecular mass); viruses (we will use 

tobacco mosaic virus, a non-spherical virus, and Southern bean mosaic virus, an icosahedral virus, as test 

specimens); large multiprotein complexes (we will use the eukaryotic proteasome, a complex of fourteen 

different polypeptide chains); and intact cell organelles (possibilities are the ribosome, the spliceosome, 

32

and the mitochondrion). Successful immobilization can be evaluated by electron microscopy, since 

reference structures exist for most of these test cases, and by biochemical assay for function after thawing 

to determine if the freezing process has denatured the sample. 

Using these methods, we have already been able to collect ultra-high resolution data on a small 

number of very strongly diffracting protein crystals using third-generation synchrotron sources. We can 

locate between 50–70% of the hydrogen atoms in these structures, less than expected since the resolution 

is about 0.8 A. We need to develop better techniques for processing the weak reflection data that occur at 

such resolution, and for extracting the positions of weakly scattering atoms such as hydrogen, or heavier 

atoms with multiple positions, from the computed electron density maps. We propose to carry out a 

systematic investigation of various types of maps and various methods of treating the data using a single 

model system, the protein gamma-chymotrypsin. Crystals of this protein diffract beyond 0.8 A resolution 

on third-generation sources, providing us with data that are representative of what we should be able to 

obtain for many other proteins using the much higher brilliance of the x-ray laser. 

Radiation damage is a problem that has long plagued structural biology, since most biological 

specimens sustain both primary and secondary damage from all forms of ionizing radiation. The extent of 

damage is dependent on the duration of exposure and the power of the x-ray beam, but little is known 

about the mechanisms. We have observed that protein crystals exposed to third-generation synchrotron 

radiation show decarboxylated glutamatic acid residues and oxidized tyrosine, methionine and cysteine 

side-chains. These chemical changes probably represent the products of free-radical induced processes, 

which should be, in principle, preventable by introducing radical traps into the specimen. We propose to 

investigate various traps such as t-butanol in controlled experiments on third-generation synchrotron 

sources using crystals of mutarotase, which we have already shown undergo such damage (see Figure 14). 

To carry out time-resolved studies of enzymes at work in the crystalline state, methods must be 

developed to trigger reactions in the crystal so that all the molecules are at the same stage of the reaction 

at the time of observation. Light pulses may suffice for many reactions provided chemical protecting 

groups can be developed that will allow substrate molecules to be diffused into enzyme crystals in the 

dark without their being transformed by the protein catalysts. Then, a laser light pulse—perhaps derived 

from the x-ray laser itself—can be used to “uncage” the substrate and allow it to bind to the active site 

FIGURE 14 Radiation damage in mutarotase. The electron density on the left shows that this glutamate 

residue in this crystalline protein has been decarboxylated by radiation damage from the synchrotron source 

used. On the right is electron density at the same resolution at the same temperature collected on a laboratory 

source. 

33

productively, initiating the reaction. Some such groups already exist, e.g., the nitrosyl protecting groups 

developed by Kaplan for nucleotide triphosphates. We propose to try to exploit this chemistry to cage the 

most important substrate class for biostructural studies: peptides and amino acids. 

1.2.6 Electron Dynamics with Attosecond Resolution 

The MIT Ultrafast Optics Group of Professors James Fujimoto, Hermann Haus, Erich Ippen, and 

Franz Kaertner of the Department of Electrical Engineering and Computer Science intends to participate 

in the MIT/Bates x-ray laser project. Over the past 30 years, group members have made major 

contributions to the theory and development of picosecond and femtosecond lasers and their use in 

studying ultrafast phenomena happening on those time scales. The group feels strongly that the proposed 

x-ray laser opens up exciting possibilities for the study of linear and nonlinear phenomena occurring on a 

few-femtosecond to attosecond time scale with unprecedented power levels. Members of the group have 

already demonstrated expertise and research accomplishments in the science and technologies needed to 

carry out the proposed work and to achieve success. Each of them is collaborating actively and 

successfully with one or more of the others on related research. 


The interaction of intense ultrashort pulses with matter has become increasingly important to 

future advances in the physical, chemical and biological sciences. Ultrashort pulses can be used to change 

the physical properties, the chemical composition and the biological function of matter. Controlling the 

temporal variation of the intensity and frequency of ultrashort pulses recently led to the breaking of 

selected chemical bonds in molecules thereby steering—for the first time—chemical reaction dynamics. 

The time scale relevant to the motion of atoms and molecules is the femtosecond (fs) time scale. It is the 

time scale on which their structure, and their chemical and biological properties change. Laser pulses with 

a duration of less than 10 fs are now available and permit triggering of these processes and the subsequent 

monitoring of their evolution in time. The impact of these advances on chemical and biological sciences 

was acknowledged by the 1999 Nobel Prize in chemistry. Furthermore, ultrafast light pulses have been 

used to advance the understanding of carrier transport in semiconductor materials and devices and thereby 

helped to develop faster devices for more powerful computers and communication systems. 

One may ask whether there is a need for control and measurement on an even shorter time scale, 

say below 1 femtosecond. Strongly excited atoms return into their ground state by electron relaxation 

processes within a period of typically less than a femtosecond and upon doing so they release energy by 

emitting x-rays. These extraordinarily fast inner-atomic processes are of crucial importance to the 

development of efficient, compact, laser-like sources of x-rays that would impact a wide range of fields in 

science and technology. 


The proposed MIT/proposed x-ray laser will emit VUV and soft x-ray radiation at an 

unprecedented power level and would make novel linear and nonlinear VUV and x-ray experiments 

possible. With its short wavelengths, this source will give us the possibility to push ultrafast laser physics 

far into the attosecond regime and obtain, for the first time, a tool for time-resolved nonlinear studies of 

inner processes in atoms and to access directly the motion of inner atomic electrons. The basis for these 

studies will be the generation of low power attosecond x-ray pulses through the focusing of high energy 

fs-laser pulses into gas jets and the resulting generation of high harmonics. These temporally short but 

weak pulses can then be used to seed the FEL amplifier to achieve high energy attosecond pulses. The 

34

development of this unprecedented technical capability, which is a central part of the proposed x-ray laser 

project is outlined in Appendix A and is likely to dramatically push the frontiers of several fields in 

science and technology. 

1.2.6.3 Experimental Concepts 

The opportunity to push the time resolution available in ultrafast studies into the attosecond 

domain arises from the short wavelengths provided by the x-ray laser and by its broadband amplification 

properties. The FEL is essentially a broadband amplifier in the VUV and x-ray regime that starts from 

spontaneous emission like an optically parametric amplifier. However, the optical pump beam of the 

parametric amplifier is replaced by the rf-field driving the accelerator and the medium is the electron 

beam on which radiation generated at the intended wavelength is imposed. The radiation is then further 

amplified in subsequent undulator sections. The temporal coherence of this amplified spontaneous 

radiation source is related to the length of the electron bunches used, in this case 200 fs, which does not 

represent the full bandwidth of the x-ray laser-amplifier. Therefore, this source on its own does not make 

use of its inherent capability to reach attosecond resolution. The MIT Ultrafast Optics Group proposes to 

develop, over the next three years, a low energy attosecond source in the 100–1 nm range based on 

high-harmonic generation with phase controlled femtosecond laser pulses focused into gases. These 

pulses can be used to seed the broadband FEL amplifier to generate broadband phase-coherent, and 

therefore attosecond, high-power VUV and x-ray radiation. The seed source is based on a high energy 

low rep-rate Ti:sapphire laser system, which has to be phase-stabilized for efficient and reproducible 

attosecond pulse generation. 

Already the weak attosecond VUV and x-ray seed pulses, and especially the amplified pulses, 

open up the possibility to study electron dynamics of free and inner-core electrons of atoms on these 

times scales. As visible and near infrared pico and femtosecond lasers have dramatically improved our 

understanding of basic physical, chemical, and biological processes important in many areas of science 

and technology, the high energy femto- and attosecond pulses will pave the way for an improved 

understanding and applications in the following areas. 

Gain Media and Relaxation Processes. In recent months, the arena of attosecond science was 

entered by investigators interested in the relaxation processes of inner shell electrons in Krypton using 

attosecond soft-x ray pulses [151,152]. New instruments like those proposed here will for the first time 

clearly resolve dynamic processes of such inner shell electrons. Science and technology based on such 

transitions will benefit in the same way our understanding of chemical and biological processes has 

benefited over the last 30 years from the availability of femtosecond and picosecond pulses. 

High Harmonic Generation. There is still much progress to be made in improving our 

understanding of high harmonic generation and the optimization of conditions for producing high 

harmonics. Recent studies by the groups of Katsumi Midorikawa at RIKEN, Japan and of the Margaret 

Murnane and Henry Kapteyn group at the University of Colorado, have shown that phase matching and 

adaptive feedback control of high-harmonic generation can lead to the enhancement of certain harmonics 

by one or two orders of magnitude [153–155]. This finding shows that we now have to develop the tools 

and measurement techniques that will allow us to understand the bound and free electron dynamics on 

timescale of much less than a cycle of the drive laser. We can conceive of making a movie of the electron 

trajectory of the bound and freed electrons undergoing acceleration in the drive laser field that would 

allow us to follow it as we can now follow a chemical reaction pathway. 

35

Electron Bunch Modulation. Theoretical work is essential to understand electron bunch 

modulation using intense phase-controlled light fields and the generation of shaped attosecond light 

pulses. The availability of strong-field driver pulses for attosecond seed pulse generation will also open 

up a completely new method for forming the electron bunch within the accelerator. Co-propagation of the 

optical beam, which easily reaches an electric field strength of 1 GV/m, with the approximately 

50 µm-wide electron beam, can lead to a modulation of the electron beam. Eventually, compression of the 

electron bunch below 50-200 fs becomes possible. The electron bunch may evolve into femtosecond or 

even attosecond sub-structure that can in turn be transferred onto the amplified spontaneous emission or 

the amplified seed pulses and facilitate the generation of shaped attosecond pulses. 

X-ray Material Processing. Beyond those fundamental investigations of the electron dynamics, 

completely new materials processing capabilities can be explored. The short wavelength amplified pulses 

can be used for materials processing on a nm spatial scale. No material will withstand the focused x-ray 

radiation of pulse with energies on the order of mJ focused to spot sizes on the order of a few nm. 

Thermal effects on this time scale are completely absent. 

In addition to investigating the new possibilities for science opened up by the availability of 

attosecond time resolution, a major part of this study will be to draft a layout for a possible beamline 

dedicated for attosecond science. This design has to be developed in close cooperation with the project 

staff. The most important task of this beamline is to guide the generated femto- and attosecond XUV and 

soft x-ray pulses together with the remaining electron beam in a vacuum environment into a laboratory, 

eventually even together with the optical driver pulse at 800 nm. Note, all three beams are initially lined 

up temporally with femto- to attosecond precision. The beamline must have the capability to separate 

and/or reconfigure the three beams in ways that enable a wide variety of experiments. Vacuum chambers, 

where the beams can be rearranged for possible pump probe configurations have to be laid out in such a 

way that two or more of the different kinds of radiation can be delayed with respect to each other on a 

scale of initially 100 attoseconds and later 10 attoseconds, which correspond to 30 nm or 3 nm precision, 

respectively. This is only possible if the beams stay superimposed over much of the joint pathway to the 

experiment. X-ray optics have to be developed that enable filtering of the different kinds of radiation 

between 1-1000 nm, focusing to diffraction-limited spots and spectroscopy. It is essential for this project 

that close cooperation is possible with the project staff, to take advantage of their extensive training in 

x-ray optics and vacuum technology. 


During the first 1.5 years of this project we propose to develop a Ti:sapphire-based laser system 

for VUV and soft x-ray generation. This system will be further described in Appendix A. It will be the 

basis for an exploratory study of seeding the x-ray laser with this radiation. The funding for this laser 

system shall come from other institutions. However, we would require support for one postdoctoral 

researcher from this study proposal to investigate technical aspects that are key to this research. One of 

his/her main tasks shall be to perform a theoretical study of the most promising material systems for 

scientific investigation with femtosecond to attosecond VUV sources. With those possible experiments in 

mind, plans will be developed for a dedicated beam line on the x-ray laser facility where our group and 

others may perform time-resolved studies with the generated high power XUV and soft x-ray radiation on 

a femto- to attosecond timescale. Initial experiments with the relatively low-power high harmonics 

generated with this small-scale laser system shall be performed in the second 1.5 years. In the second 

1.5 years of the study, a vacuum chamber and x-ray spectrometer capable of time-resolved studies on an 

attosecond time scale shall be developed. For that period, additional funding will be needed to support a 

36

graduate student and provide the equipment, materials and services necessary to set up the vacuum 

chamber and x-ray spectrometer. 

1.2.7 X-ray Microscopy With Atomic Resolution 


Imagine a tool with the ability to image the atoms within macromolecules, proteins, viruses, and 

nanotubes in their natural state, in three dimensions. Such a breakthrough would have enormous impact in 

structural biology, genetic engineering, materials science, nanotechnology, and many other areas of 

science and engineering. Its advent could reveal the inner machinery of cells, cure terrible diseases, and 

develop new materials with fundamentally different electrical, magnetic, and thermal properties. For 

example, despite tremendous advances in gene sequencing and proteomics, there are no direct means for 

imaging the building blocks of life with atomic resolution. 

Imaging at the atomic scale requires a spatial resolution of ~0.1 nm. Imaging systems, whether 

they utilize photons or the de Broglie waves of particles such as electrons, obey the same laws relating the 

wavelength λ, the transverse imaging resolution R, and longitudinal resolution or depth of field, DOF. 

These quantities are related through the numerical aperture (NA) of the optical system, given by NA = 

sinθ, where θ is the half-angle of the cone of rays accepted by the imaging system. We find that the 

transverse resolution is R = λ/2NA and the longitudinal resolution is DOF = λ/NA 2 . One sees 

immediately that shorter wavelengths lead to proportionately better transverse and longitudinal resolution, 

and that we require atomic-scale wavelengths to image with atomic resolution. We also see that in order 

to resolve structure in 3D, the longitudinal resolution should be comparable to the transverse resolution, 

i.e. DOF ~ R. 

Several means already exist to determine structure with atomic resolution such as atomic force 

microscopy, electron microscopy, and x-ray crystallography. Scanning-tunneling and atomic force 

microscopes are certainly capable of atomic resolution, but can only resolve structure at surfaces. On the 

other hand, for electron microscopes, a small NA is required to minimize aberrations in electron optics 

and form a sharp focus. Although the wavelengths of ~50 keV electrons in a typical electron microscope 

are much less than 1 nm, images obtained with electron microscopes typically average over tens to 

thousands of atomic layers because of the small NA and corresponding large DOF. 

Charged particle probes such as electrons and protons pose another obstacle to atomic-scale 

imaging. Due to their electric charge, these particles are subject to multiple scattering within the sample 

volume, degrading both the transverse and longitudinal resolution unless the sample is very thin. Use of 

higher energy particles (e.g., of order 1 MeV) can alleviate scattering but also causes greater material 

damage. 

By comparison, photons and neutral particles interact weakly with matter. With a brilliant enough 

source of short-wavelength neutrons, one could envision atomic-scale imaging of samples composed of 

light elements using large NA neutron optics. While such a neutron source is inconceivable at present, the 

best x-ray sources are brilliant enough for x-ray crystallography at atomic resolution and microscopy of 

non-crystalline samples with ~100 nm resolution. 

The resolution of an x-ray microscope with sufficiently large NA optics is in principle limited 

only by the wavelength used. However, the technical difficulties of fabricating focusing optics capable of 

atomic resolution are formidable due to the fundamental interaction of x-rays with matter. At x-ray 

37

wavelengths, the refractive indices of materials deviate only very slightly from unity. The complex 

refractive index can be expressed as n = 1-δ-iβ, where δ and β depend on the x-ray energy and are 

between 10 -3 to 10 -6 . Here, the real part of the index, 1-δ, is the ratio of the speed of light in the material to 

the speed of light in vacuum, and the imaginary part β describes the absorption of light in the material 

(the 1/e absorption length of x-rays is given by λ/4πβ). One consequence of such small δ and β is that 

conventional normal-incidence lenses and mirrors absorb too strongly and impart far too small of phase 

shifts to the x-ray wave to be usable at large NA. In the late 1930's Boersch, Bragg and others appreciated 

that atomic structure might be accessible by x-ray diffraction instead of via optics. 

X-ray crystallography has made a tremendous impact in structural biology, materials sciences, 

chemistry, and other areas. Due to this success it has become the method of choice for structural 

determination of biological molecules. It has the advantage of needing no optics, and the resolution is 

limited only by the x-ray wavelength and the sample quality. Crystallography, however, is only applicable 

to study of periodic structures. This condition (crystallinity) often requires the sample to be studied in a 

highly unnatural state. Many amorphous and disordered materials, including polymers, crystals with 

strains and defects, and inorganic structures such as nanotubes, are not amenable to this approach. Many 

bio-molecules stubbornly refuse to crystallize and simply cannot be accessed by these techniques. In 

biology, structures such as whole cells, sub-cellular structures, and viruses are most often non-crystalline. 

At the molecular level, 20%–40% of protein molecules, including most of the important membrane 

proteins, are difficult to crystallize and therefore are not currently accessible by x-ray crystallography. 

1.2.7.2 X-ray Microscopy: Source Considerations 

To explore aperiodic and amorphous structure at the atomic scale we are motivated to develop 

other forms of x-ray microscopy. The most challenging example of such a structure is probably a single 

macromolecule. Before discussing the merits of these approaches, we first consider the x-ray sources and 

optics needed to obtain atomic resolution. 

Source Brilliance. Of major practical importance to the operation of a microscope is the 

brilliance of its source of illumination. We can specify the spectral brilliance B or phase-space density of 

the flux emitted by a source as the number of photons emitted per unit of time, per solid angle, per area, 

and per optical bandwidth. The flux used by the microscope is that fraction of B it accepts; the accepted 

photon flux depends linearly on B. The spatially coherent flux Fc ~ λ 2 B emitted by the source is that 

portion within an acceptance ~λ 2 . Only the spatially coherent flux can profitably be focused to a 

diffraction-limited spot or be used to form interference fringes. 

High-resolution microscopes are demanding of source brilliance. The number of photons required 

to detect a feature of diameter D scales as 1/D 2 . [156] When one considers that hard x-ray microscopes 

would be flux-limited at a resolution of ~10 nm using today’s most brilliant x-ray sources, it is evident 

that more than 4 orders of magnitude improvement in average source brilliance is necessary to obtain 

images at atomic resolution by the same methods. The unprecedented increase in average brilliance of the 

MIT/Bates x-ray laser over existing sources is not only beneficial, it is essential to form images with 

atomic resolution. As we will describe in more detail, the coherence per se of the laser beam is not 

required to attain atomic resolution. Nevertheless, the source brilliance that accompanies it is necessary to 

obtain atomic-scale images, especially when they must be acquired on short time scales. 

Short Pulses and Radiation Damage. A crucial factor in microscopy is the degree to which the 

imaging process disturbs the sample under study. Artifacts arising from damage to radiation-sensitive 

biological specimens are one example. As we push the resolution in x-ray imaging applications well into 

38

the nanometer range, radiation damage to most biological and many materials science samples can be 

expected to change or destroy their internal structure irrevocably. For instance, the radiation dose required 

for imaging at a resolution below ~100 nm is near the limit where wet biological specimens show 

evidence of radiation damage [157]. Artifacts due to radiation damage may be mitigated by cryopreparation 

of the sample down to a resolution of ~10 nm, but it is difficult to avoid at doses beyond 

~10 8 Gy [158]. From 10 nm resolution down to atomic-scale x-ray imaging, it is necessary to acquire the 

entire image of the sample on time scales significantly shorter than 1 ps before blurring due to 

hydrodynamic expansion and other mechanisms occurs [159]. 

We can estimate the peak source brilliance required for “flash” imaging with a single ~100 fs 

x-ray pulse. An image consisting of 1000 2 resolution elements (e.g. pixels of size R x R), recorded with a 

Poisson-limited signal-to-noise ratio of 1% in each pixel, requires of order (10 3 ) 2 × 10 4 = 10 10 detected 

photons. Relative to the most brilliant synchrotron sources which produce ~1000 coherent photons per 

~100-ps pulse, we see that an increase in peak brilliance of at least 10 orders of magnitude is necessary 

for flash imaging at the 100 fs time scale. The unprecedented peak brilliance and pulse brevity that will be 

produced by the MIT/Bates laser presents a unique opportunity to test and apply damage-insensitive flash 

imaging in the x-ray region. By using a single ~100 fs x-ray pulse to produce an image of the specimen, 

the radiation damage problem of well blurring due to sample motion can be avoided altogether. We 

anticipate that the extremely short duration of the laser pulses will also be of enormous interest to 

“freeze” biological and chemical processes in studies of sample dynamics. The possibilities for pumpprobe 

microscopy, combined with snapshot imaging at electronic time scales, are truly exciting. 

We now consider both real-space and reciprocal-space methods for x-ray microscopy, their optics 

requirements and feasibility, and discuss the unprecedented opportunity to use a fully coherent x-ray 

source, such as the proposed MIT/Bates x-ray laser, to study matter at atomic resolution. 

1.2.7.3 X-ray Microscopy: Real-Space Methods 

With real-space or “direct” methods for imaging samples at high resolution, the object image is 

obtained “directly.” It is discernible immediately without added processing by computational or optical 

means. The magnified object image as detected is a real-space representation of the features and 

morphology of the object, that is, lengths in the object are represented directly as lengths in the image. A 

variety of real-space imaging methods exist such as scanning transmission, scanning fluorescence, 

projection imaging, full-field transmission imaging, and tomographic variants of the above. Depending on 

the method, the object image is obtained coherently, where a well-defined phase correlation exists within 

the data recorded for every point of the object, or incoherently, where the information recorded for each 

object point is uncorrelated. In both systems, a common measure of the quality of an imaging instrument 

is how smoothly it transfers all spatial frequencies (corresponding to all feature sizes) present in the 

object. Fourier optics tells us that the resolution in a coherent imaging system is half of that in an 

incoherent imaging system due to the sharp cutoff in the transfer function [160]. 

Two possibilities stand out for coherent real-space imaging with nanometer resolution: full-field 

imaging and projection imaging (Figure 15). A number of prerequisites need to be satisfied. First, these 

methods must be compatible with a flash source, in which the x-ray exposure is completed in a single 

pulse. Second, they must be compatible with fully coherent illumination on the optics and sample. Lastly, 

their imaging resolution must not be limited by the geometry, provided focusing optics of sufficient 

resolution are available. Both methods, which may be treated as a form of holography, meet these criteria 

with one important caveat. Because we have assumed use of coherent illumination, some means of 

39

(a) (b) 

FIGURE 15 Real-space x-ray microscope geometries capable of atomic resolution and compatible with 

coherent flash sources. (a) full-field and (b) projection. 

dealing with the “ringing” due to the sharp cutoff in the coherent transfer function must be applied in 

order to obtain unaberrated images of the object. This amounts to determination of the phase of the object 

wave. A variety of approaches for this have been developed as discussed below. 

Both methods have the potential for 3-D imaging, either using large NA optics or tomographic 

techniques. In tomography, a series of small-NA projections through the sample is recorded over a wide 

range of incidence angles. The projection series is assembled numerically on a computer to reconstruct a 

3-D image of the object. In doing so, a larger NA is effectively synthesized by virtue of the broad angular 

range of the projection series. Conventional tomography is an incoherent process involving only the 

intensities associated with straight or conical ray projections through the object where the wave nature 

(i.e., the phase) of the object wave does not play a part. In diffraction tomography, the complex amplitude 

of the object wave is used to form a 3-D image of the object. In order to reconstruct the object wave, 

some means of determining its phase is again necessary. In order to perform tomography with a flash 

source, it is probably necessary to record multiple simultaneous views through the sample, such as by the 

use of beam-splitting optics and faceted or spherical detectors. 

To achieve an imaging resolution well below that of visible light, all of these methods utilize 

x-ray optics that are subject to the same limitations as described above. The available options for focusing 

x-rays are refractive optics (compound lenses), diffractive optics (zone plate lenses), reflective optics 

(grazing incidence and multilayer mirrors), or some combination of these elements. 

Compound Refractive Lenses. Focusing based on refraction depends on the bending of waves 

as they pass through a change in refractive index. Refractive bending of x-rays was attempted by Röntgen 

and others shortly after the discovery of x-rays in 1896, although these early attempts failed. It was not 

realized until much later that the focus of a simple refractive lens for x-rays converges very slowly, and 

such lenses tend to be highly absorptive of the incident x-ray illumination (typically δ and β are small, 

and δ < β). Early refractive x-ray lenses had focal lengths of kilometers and very poor efficiency. 

Compound refractive lenses (CRLs), which consist of stacks of many (up to hundreds) of individual 

lenses [161], are capable of shorter focal lengths, and therefore larger NA, when used with relatively 

penetrating hard x-rays Although CRLs have steadily improved, there remain substantial challenges 

ahead to fabricate CRLs with large NA, low absorption, and minimal chromatic aberration. 

Reflective Optics. Optics based on the principle of reflection offer another approach to realizing 

an x-ray microscope. If a mirror can be designed to use very small grazing-incidence angles (~1 degree or 

less), then the phenomena of total external reflection can be exploited to realize a high efficiency optic. A 

40

significant advantage of mirrors (excluding multilayer optics, see below) is their broadband response, i.e., 

they suffer little or no chromatic aberration. Kirkpatrick and Baez (KB) in 1948 demonstrated a focusing 

grazing-incidence mirror optic for x-rays utilizing ellipses and hyperbolae of translation [162]. Wolter in 

1952 described an alternative design utilizing ellipsoids and hyperboloids of revolution [163]. Large, 

nested, grazing-incidence Wolter optics with sub arc-second resolution were employed in the Chandra 

Observatory to obtain exquisite images of astronomical x-ray sources. 

Several groups have built x-ray microscopes using KB optics with near atomic precision to 

approximate the ideal optical figure required. An imaging resolution of 80 nm has recently been quoted 

using KB optics and some improvement beyond that appears feasible. However, the refractive index for 

x-rays in matter poses a fundamental limit to this trend. The value of the real component δ of the 

refractive index determines the critical angle θC = (2δ) 1/2 for x-rays to be reflected efficiently. At x-ray 

wavelengths, δ ~ λ 2 a(Z), where a(Z) is a constant that depends only on the atomic number Z of the 

material and which increases approximately linearly with Z. For example, gold, a popular x-ray mirror 

coating, has a ~ 0.013/nm 2 . Since the graze angle is limited, we find that NAMAX = λ(2a) 1/2 , where the 

relationship sinθC ~ θC has been assumed since θC is small. From this we conclude that the microscope 

resolution, R = λ/2NAMAX = (8a) −1/2 , is a constant independent of wavelength. For example, a gold 

mirror would have R ~ 10 nm, and only slightly better could be achieved with a heavier metal such as 

uranium. 

Multilayer and Crystal Optics. Those familiar with x-ray optics might suggest using multilayer 

coated mirrors or crystal optics to achieve much larger NA. Multilayer mirrors, consisting of many 

precisely controlled nanometer-thick high-Z/low-Z pairs of materials deposited into smooth substrates, 

can have high reflection efficiencies even at large angles of incidence [164]. The number of layers and the 

degree of perfection required generally increase as the incidence angle increases, until absorption by the 

multilayer materials or scattering due to imperfections intervenes. However, the more layers, the narrower 

the angular acceptance (or equivalently, the optical bandwidth). Similarly, the Bragg reflections from the 

many participating atomic planes in perfect crystals are restricted to very narrow angular ranges. 

Consequently, while it might be possible to devise a crystal optic or a multilayer optic for hard x-rays that 

would operate efficiently at large NA, it would not operate over the angular range required to obtain a 

smooth transfer function for high fidelity imaging. 

Zone Plates. The zone plate (ZP) lens, first developed by Soret and Rayleigh in the nineteenth 

century, is a circular diffraction grating consisting of alternate absorbing or phase-shifting lines [165]. 

X-ray ZPs are made of very small and accurately placed metallic features supported by a thin x-ray 

transparent membrane [166]. It can be readily shown that the transverse resolution of a ZP is given by 

R ~ 1.22 dr where dr is the width of the smallest (outermost) zone. Like refractive lenses, ZPs are 

chromatic but the dependence of their focal length (and thus NA) depends linearly rather than 

quadratically on λ, making them easier to use over a broad energy range. 

The world’s leading high-resolution x-ray microscopes use phase ZP optics for focusing and 

undulator or bending magnets at synchrotron radiation facilities as sources. To illustrate, the soft x-ray 

XM-1 instrument at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory uses a 

zone plate as an objective lens to image samples with 30-nm resolution onto a CCD camera [167]. The 

smallest zones in ZPs fabricated to date are on the order of 20 nm, leading to a resolution of ~24 nm for 

soft x-rays [168]. A concerted effort is now being made in the U.S. to push dr down to ~10 nm. This is 

believed to be near the practical limit due to the extraordinary difficulty of patterning the required 10 nmwide 

metal lines with sufficient thickness for the ZP to have reasonable efficiency. Indeed, the challenge 

of fabricating outermost zones with the required high aspect ratio for operation at x-ray wavelengths 

41

currently limits our ability to extend the usefulness of ZPs to hard x-ray energies. ZPs are currently 

among the highest resolution x-ray optics available and offer moderate efficiency (10–30%) and NA 

(0.001–0.01). While the ultimate resolution of ZP optics will still be a factor of 10 too large to image 

atoms, they will be extremely useful as condenser optics for an atomic-resolution microscope. 

1.2.7.4 X-ray Microscopy: Reciprocal Space Methods 

Having examined the various approaches, we find that the resolution of real-space methods is 

limited by optical technology, not by optical geometry. Due to the fact that n ~ 1 for x-rays, the resultant 

NA

FIGURE 17 X-ray fluorescence holography of a cobalt oxide crystal. (a) raw hologram, (b) hologram after 

applying a low pass filter, (c) reconstructed 3-D image of the Co atoms showing nearest-neighbor atoms as 

well as unit cell. 

In both methods, the intensity pattern resulting from interference between the incident and 

scattered fluorescence waves is recorded over a spherical volume, corrected for background and other 

effects, low-pass filtered, then reconstructed by Fourier transformation to obtain an image of the atoms in 

the immediate neighborhood of the source (or detector) atom. The imaging process is coherent, but the 

incident and scattered fluorescence waves are uncorrelated with the excitation. A sign ambiguity in the 

phase results from recording the hologram intensity, producing an out-of-focus “twin image” that 

confuses the in-focus object image upon reconstruction unless the phase is determined. This problem can 

be minimized experimentally by limiting the extent of the reconstructed volume to a few unit cells or less 

through filtering, and by recording holograms at multiple energies to pin down the phase. This and several 

other factors currently limit utility of the method for atomic imaging. 

Due to the relatively weak fluorescence emission by a single source (or detector) atom, a large 

number of identical, mutually aligned copies of the structure are necessary for there to be sufficient 

fluorescence signal to record a usable hologram. This could be mitigated by using x-ray optics to focus 

the intense beam produced by the proposed MIT/Bates laser to a ~10 nm spot onto the sample, without a 

NA penalty. Although coherent illumination is not required for the holographic process, being fully 

coherent, the entire flux in the laser beam can be focused onto the sample. The improved signal-to-noise 

ratio might also help to eliminate reconstruction artifacts due to twin-image noise. 

Coherent Diffraction. The other technique combines coherent x-ray diffraction with a means of 

determining the phase. Popular phase retrieval methods include the transport of intensity method 

proposed by Nugent [174] and the oversampling method [175, 176]. When a finite object is illuminated 

by a coherent x-ray beam, the weakly scattered x-ray photons form a continuous diffraction pattern in the 

Fraunhofer region of observation. This continuous pattern can be sampled at spacing finer than the 

Nyquist frequency (i.e., the inverse of the object radius), equivalent to surrounding it with a region of zero 

amplitude. The higher the sampling frequency, the larger the known-amplitude region required. When the 

sampling frequency is high enough, the phase information can, in principle, be retrieved from the 

diffraction pattern. In practice, this can be accomplished by an iterative algorithm supplied with a random 

phase set as an initial input. By combining the measured magnitude of the Fourier transform and the 

random phase set, a new Fourier transform is assembled. A new value for the object amplitude 

corresponding to an estimate of its electron density is obtained by applying an inverse fast Fourier 

transformation on the assembled data. Based on the oversampling frequency, a “finite support” is defined 

in real space to separate the empty and the electron-dense region. Both the electron density outside the 

finite support and the negative electron density inside the support are set to zero, and a new electron 

density is obtained. The process is repeated and the phases of successive Fourier transforms are adopted 

43

for the next iteration. After a few hundreds to thousands of iterations, convergence is usually complete 

and the correct phase of the object is recovered. Because only coherent x-ray flux can be used, coherent 

diffraction naturally has a great appetite for source brilliance. 

This concept was first demonstrated experimentally in 1999 at the National Synchrotron Light 

Source [176] using coherent soft x-rays (λ = 1.7 nm) from an undulator source. The diffraction pattern of 

a test object (a matrix of 100-nm diameter, 80-nm thick gold dots on a silicon nitride membrane) was 

recorded by a CCD detector. The object image was successfully reconstructed from the diffraction pattern 

using the oversampling method after 400 iterations (~15 min on a 450 MHz Pentium II workstation). 

More recently, coherent diffraction has been successfully applied to structural determination of disordered 

materials, nanocrystals and biological samples [177,178], including with E. Coli bacteria at ~30 nm 

resolution using a wavelength of 0.2 nm [179]. Results obtained at 8 nm resolution in 2-D and 50 nm 

resolution in 3-D with a three-dimensional test object [180] are shown in Figure 18. 

(a) (b) (c) 

FIGURE 18 Coherent x-ray imaging of a nano-fabricated two-layer 3D test object. (a) Scanning electron 

microscopy image of the test object. (b) High-resolution image (~8 nm) reconstructed from a 2-D diffraction 

pattern of (a). (c) Reconstructed 3-D structure displayed with isosurface rendering. 

Single-molecule Imaging. The ultra-short, extremely intense coherent x-ray pulses expected 

from the proposed MIT/Bates x-ray laser offer a unique opportunity to image individual molecules at 

atomic resolution using coherent diffraction [181]. A proposed experimental scheme is shown in 

Figure 19. Pulses from the laser are focused to a 100 nm spot by a zone plate lens. Identical molecules are 

selected by a mass spectrometer and sprayed one by one at random orientations into the focal plane. 

Before being hit by the focused coherent x-ray laser pulse, each molecule is oriented by a polarized 

optical laser field. The diffraction patterns are recorded by an x-ray CCD camera in a high vacuum 

environment to minimize background scattering. Once recorded, the 2-D diffraction patterns from each 

molecule are assembled into a 3-D diffraction pattern then phased by the oversampling method. With 

sufficiently wide-angle (high NA) recording of the diffraction patterns, the imaging resolution is 

ultimately limited by the signal that can be obtained with a single flash, in competition with radiation 

damage to the molecule. 

FIGURE 19 Scheme for imaging a single molecule using coherent diffraction and the oversampling method. 

44

Issues for Further Study. As we have seen, continued development of x-ray optics and study of 

radiation damage are critical to the realization of atomic resolution imaging. Nano-fabrication of the nextgeneration 

of x-ray optics will be required to obtain the precision required for nano-focusing. Whether or 

not x-ray optics are ultimately capable of atomic resolution, as seems unlikely now, they will be essential 

to boost the weak signal from very dilute samples and single molecules. The phenomenal brilliance and 

full coherence of the MIT/Bates laser beam in combination with x-ray optics. are crucial to deliver an 

intense, diffraction-limited spot onto the sample 

Acquiring the sample’s image well before it is destroyed by thermal and/or other processes 

appears to be an ideal application for the ultra-short pulse length (~100 fs) radiation from the proposed 

MIT/Bates x-ray laser. The most pressing issue is therefore to determine if damage to both the optics and 

the sample would destroy either before an image could be collected, or make it impractical to develop a 

robust optical system that can withstand such extreme conditions. Because there is much we do not yet 

understand about x-ray damage mechanisms and nonlinear processes on femtosecond time scales, it is not 

yet clear how these ultra-short pulses would affect imaging. For example, will bulk refractive indices 

remain the same, allowing standard calculations for optical performance and sample image contrast to 

apply in the short-pulse regime? Perhaps nonlinearities could be exploited in more powerful methods. 

Certainly, the existence of atomic resolution images using electrons gives us confidence that similar 

results can be obtained for x-rays. 

1.2.7.5 Conclusions 

The proposed MIT/Bates x-ray laser source and others under development will deliver a peak 

brilliance exceeding 10 32 ph/s/mm 2 /mrad 2 /0.1% BW in pulses shorter than 300 fs. Based on these two 

critical specifications (brilliance and pulse width) we come to the following conclusions: 

1. Progress in x-ray optics technology will soon enable x-ray microscopes employing 

real-space imaging methods to reach ~10 nm resolution in conjunction with these 

new source properties; 

2. Reciprocal-space methods (coherent diffraction and x-ray fluorescence holography), 

combined with phase retrieval, are the most promising means of realizing 0.1 nm 

resolution in 3-D; 

3. Three-dimensional atomic-resolution imaging of individual molecules requires the 

extreme brilliance of an x-ray laser source; and 

4. The ability to record snapshot images in which atomic structure has been utterly 

frozen, potentially in combination with pump-probe or other dynamical techniques, 

requires the extremely short pulses produced by a source such as the proposed 

MIT/Bates facility. 

A number of strengths and interests can be brought to bear to realize such an instrument on a 

beamline at the MIT/Bates x-ray laser. Recent developments in high-resolution microscopy methods at 

third-generation synchrotron facilities worldwide constitute a key component of success. New phase 

retrieval algorithms are being tested experimentally at these synchrotron facilities and modeled 

theoretically at University of Melbourne, University of Illinois at Champagne-Urbana, Stanford 

University, and elsewhere. World-class expertise in x-ray optics is available at several of the nation’s 

universities including MIT and the State University of New York at Stony Brook, and at U.S. national 

laboratories including those at Argonne, Brookhaven, and Berkeley. Finally, burgeoning research in the 

biology, biotechnology, materials, and nanotechnology fields is sure to create support and future 

customers for the microscope. Clearly, the scientific payoff of an x-ray microscope with 0.1-nm 

resolution would be enormous. 

45

1.2.8 Nanometer Lithography 


There is a long and complex history of efforts to use synchrotron sources for x-ray lithography. 

Largely the technology is successful and powerful, but has not become cost competitive for commercial 

chip production. With the advent of new x-ray laser sources, it is appropriate to reconsider the issue of 

commercial lithography. Pagani et al. [182] have proposed that unseeded 50-100 nm radiation from an 

x-ray laser source would support VUV lithography based on reflective SiC mirrors, which are of high 

optical quality and extremely robust. They have argued that feature sizes down to 50 nm could be 

produced more cost-effectively than expected with the more challenging multiplayer optics required for 

14 nm EUV methods currently under development in the industry. 

Of course, it is not yet clear whether commercially viable methods will be developed using these 

new sources, but it does seem clear that new and powerful lithographic methods will be developed that 

will have very exciting consequences for a variety of research applications. The NanoStructures 

Laboratory (NSL) at MIT has been a world leader for over 20 years in the development of lithographic, 

processing and metrology techniques for the sub-100 nm domain. These developments have enabled a 

wide range of applications, ranging from x-ray astronomy to molecular manipulation. 

In lithography, photon-based techniques provide a number of fundamental advantages over 

techniques and systems that employ electrons or ions. High brightness, short-wavelength photon sources, 

such as proposed for the MIT/Bates x-ray laser facility, could have an enormous impact on the 

development of nanolithography and nanotechnology in general. The high flux, spatial coherence and 

tunability of the proposed source, are ideal for two projects we would like to pursue: achromatic 

interferometric lithography for 50 nm period gratings and grids, and zone-plate-array lithography at 

4.5 nm. 

1.2.8.2 Achromatic-Interferometric Lithography 

Figure 20 is a schematic of an achromatic-interferometric-lithography (AIL) system that we have 

designed to achieve large-area, 50 nm-period gratings and grids. This system requires a bright, highly 

collimated x-ray source with a wavelength of approximately 4.5 nm. (This wavelength is the carbon K 

edge, and enables efficient exposure of carbonaceous resists.) Techniques for making the 100 nm-period 

gratings required in the interferometer are well developed in the NSL. We have done extensive theoretical 

analysis of the exposure contrast, alignment requirements, coherence, depth of focus, etc. for the system 

depicted in Figure 20. We are confident that if the proposed x-ray laser source is available, we will 

produce high-quality, large-area gratings and grids for a wide variety of applications. 

Fine-period precision gratings and grids have an enormous range of exciting applications. For 

example, the techniques of interferometric lithography developed in the NSL were crucial in providing 

the diffraction gratings utilized in the Chandra x-ray astronomy satellite. We have also enabled 

applications in high-density magnetic-information storage, atomic spectroscopy and diffraction, 

nanometer metrology, and arrays of nanoscale field emitters for high-resolution displays. The atom 

interferometers under development at MIT, the Max Planck Institute in Goettingen, University of Vienna, 

and Arizona State University all make use of free-standing, 100 nm-period gratings fabricated in the NSL 

at MIT. 

46

FIGURE 20 Schematic of an achromatic interferometric lithography (AIL) interferometer, operating at 

4.5 nm (the Carbon K edge) suitable for exposure in carbonaceous resists of gratings and grids with 25 nm 

nominal lines and spaces. Such gratings and grids have an enormous range of applications in nanotechnology 

and science, including templated self assembly. The 100 nm-period matched transmission gratings in the 

interferometer are themselves made by AIL using 200 nm-period matched gratings and a wavelength of 

193 nm from an ArF laser. 

One particularly exciting area that would benefit from high quality 50 nm-period gratings and 

grids is what we call Templated Self-Assembly (TSA). This new initiative aims to use periodic 

lithographic patterning of a substrate to induce long-range order in the otherwise poorly ordered selfassembled 

systems. Materials such as block co-polymers, epitaxially grown quantum dots, and organic 

crystals can potentially all be controlled, manipulated, and organized via templated self-assembly. The 

ability to make 50 nm period templates would open up the exciting possibility of large-area structures of 

coherently self-assembled macromolecules. 

To date, we have not been able to produce 50 nm period gratings and grids using the achromatic 

scheme depicted in Figure 20 due to our lack of a suitable source. The proposed x-ray laser would be 

ideal for this application. The high photon energy of the proposed x-ray laser also invites the possibility 

of creating these templates in wholly new ways. By using the high energy of the photons to modify 

surface properties directly, or change a surface’s chemical affinity, we may be able to create selfassembly 

templates for a much broader range of materials than by using the simple topographic templates 

that are currently envisaged. 

1.2.8.3 Zone-Plate Array Lithography 

A second project that would make extensive use of the new x-ray laser is zone-plate-array 

lithography (ZPAL). This is depicted in Figure 21. Here again the optimal wavelength for achieving the 

highest resolution in carbonaceous resists is the 4.5 nm carbon K edge. ZPAL is a maskless lithography 

scheme that has already been demonstrated at blue and UV wavelengths. To achieve higher resolution one 

must go to shorter wavelengths. At a wavelength of 4.5 nm the anticipated resolution depends on how 

fine the zone plates can be made. With existing techniques, zone plates can be fabricated with minimum 

zone widths of about 20 nm. ZPAL has not been demonstrated at x-ray wavelengths due of the lack of a 

suitable source. The proposed facility will solve this problem. 

47

FIGURE 21 Schematic of the maskless, zone-plate-array lithography (ZPAL). Each zone plate focuses 

incident, narrow-band, collimated radiation into a focal spot. Each beamlet is multiplexed on and off by 

means of upstream micromechanics. The stage is scanned to create patterns of arbitrary geometry via a “dot 

matrix” scheme. For the ultimate resolution, the radiation should be at 4.5 nm (the carbon K edge), which 

efficiently exposes carbonaceous resists and avoids spurious back-scattering and Auger-electron effects. 

We believe that once the MIT/Bates facility is operational we will be able to demonstrate 20 nm 

resolution with ZPAL, and shortly thereafter be able to push down to even finer resolution. Compared to 

electron-beam lithography, the low energy (280 eV) of the 4.5 nm photon will not damage substrates, and 

there are no deleterious effects such as back-scattering, photoelectrons, or proximity effects to hinder 

high-resolution patterning. Finally, the massively parallel nature of the zone-plate array allows for large 

area patterning much faster than is possible with a single electron beam. Access to the type of source 

proposed in the MIT/Bates x-ray laser could be a critical stepping stone in the development of a maskless, 

high-throughput lithography system critical to the development of future nanotechnology. 

1.2.9 Status of Scientific Programs at Operating UV FELs 

In this section, we provide a brief overview of the scientific programs at operating single pass UV 

FEL facilities at Argonne National Laboratory [183], Brookhaven National Laboratory [184], and DESY 

[185], because these programs are an early indication of both the unique research opportunities and the 

strong interest by the scientific community. All of the work being undertaken could be performed on the 

proposed MIT/Bates x-ray laser source, but more importantly the lessons learned from these experiments 

will have direct bearing on the readiness of the community to utilize new sources. The existence of such 

well-defined efforts is additional confirmation of the maturity of the scientific case for x-ray laser sources, 

and the timeliness of the current proposal to proceed to the next stage, including initial experimentation 

and design of beamlines for shorter wavelength sources such as the MIT/Bates facility. For completeness 

we note that there are other UV FELs operating as low-gain oscillators [186–188], but they cannot reach 

the short pulse lengths or hard wavelengths of single-pass devices critical to the kinds of research 

discussed below. 

48 

)

1.2.9.1 Low Energy Undulator Test Line (LEUTL) at ANL 

The FEL at Argonne has demonstrated the ability to deliver sustained operation over a tunable 

wavelength range from 660 nm to 120 nm. This laser was the first SASE FEL to reach saturation in a 

non-waveguide operation mode [183], proving the feasibility of such devices and underscoring their 

promise of achieving laser-like operations well into the x-ray spectrum. 

Another phase of research is beginning, driven by the unique science that will be performed with 

this source. The recently installed end-station includes a state-of-the-art mass spectrometer, SPIRIT 

(Single-Photon Ionization Resonant Ionization to Threshold), that can address four high-impact 

experiments. The first experiment requires only high energy VUV, which would be used to softly 

photoionize DNA to very sensitively detect mutant or carcinogenic strands. The second experiment 

requires tunability and would validate a model explaining anomalous isotope ratios in the solar system. 

The third experiment takes advantage of the subpicosecond pulse length of the FEL and would selectively 

fragment molecules by pushing them through excited state pathways. The final proposed experiment 

depends on the coherent properties of the FEL light and would show speckle from mesoscopic systems. 

Coupled with a laser that can ionize any atom or molecule, SPIRIT will enable high-profile experiments 

in the fields of molecular biology, cosmochemistry, chemical physics, and materials science. 

Studies of chemical mutagencity using molecular trace analysis suitable for cancer research has 

not been possible on samples from individuals. The new end station at ANL promises to be the most 

sensitive and discriminative instrument in the world and will enable these analyses. The SPIRIT 

instrument will allow detection of altered DNA adducts with a useful yield of nearly 30% (one third of the 

molecules in the sample will generate measurable signal). This level is many orders of magnitude larger 

than previous studies. At this sensitivity level, altered base pairs in very small samples, even in single 

individuals, may be detected. 

Determination of how our sun evolved by performing isotopic analysis on extraterrestrial material 

is now possible using resonant ionization mass spectroscopy; however, accurate measurements of lighter 

elements such as oxygen is only possible with a tunable laser capable of reaching the VUV wavelengths 

and with sufficient energy per pulse. 

Chemical bonds in molecules can be selectively broken with intense short pulse light. Extending 

this practice to the VUV broadens the applicability of this technique to a large number of species and 

enables precisely studying their excited states. The broad tenability and short pulse nature of the APS 

VUV FEL would allow investigation of a significantly larger set of molecular reactions than current 

Ti:Sapphire technology. Ti:Sapphire lasers produce pulses whose wavelength bandwidth is ~100 nm. 

1.2.9.2 Deep Ultraviolet FEL at BNL 

The DUV-FEL at Brookhaven National Laboratory (BNL) operates with a fundamental at 

266 nm and a third harmonic at 89 nm. The fundamental has an output energy of 0.1 mJ in a 200 fs pulse, 

and the third harmonic has an output energy of 1µJ with a similar pulse length. The DUV-FEL is based on 

the principle of seeded, high-gain harmonic generation (HGHG) in which the FEL acts as a single-pass 

amplifier of a harmonic of the seed laser. As a result, the output radiation has the full longitudinal and 

transverse coherence and stability of the seed laser. The number of photons in the coherence volume of 

the fundamental is ~10 14 whereas for a synchrotron storage ring this degeneracy factor is 1. A group from 

the BNL Chemistry Department and the NSLS are developing the scientific research program at the 

DUV-FEL. 

49

Photodissociation Dynamics and Ion Pair Imaging. For the first experiments a fully UHV 

compatible gas-phase end-station has been constructed. It provides a single differentially pumped 

molecular beam source, which is directed to a 75 mm imaging detector. It uses second-generation velocity 

map ion optics and image acquisition based on the existing CCD-camera/centroiding approach. The first 

experiments study complex polyatomic systems undergoing reaction on multiple potential energy 

surfaces. Velocity map imaging is used to study photodissociation, as in recent experiments on CH3 + 

[189]. In this experiment, a molecular beam of CH3Cl is photodissociated by a laser into the ion pair 

CH3 + and Cl - and rotationally resolved energy release spectra are obtained for each ion. These provide 

the vibrational frequencies and rotational constants of CH3 + . The third harmonic of the DUV-FEL at 

89 nm is above the ion pair dissociation threshold for alkanes and is being used to study H - elimination. 

A second class of experiments at the DUV-FEL involves two-color, or pump probe, experiments. 

A harmonic of the seed laser is used to induce photodissociation and then the FEL is used as a universal 

soft ionization probe. By varying the delay between the two pulses, it may be possible to separate prompt 

excited state photodissociation processes which occur on time scales of femtoseconds—from those 

occurring after internal conversion to lower lying states which occur on picosecond time scales. 

Coherent Control. Molecular coherent control is based on using two indistinguishable excitation 

paths in an atom/molecule for producing quantum interference in the final state channels. For example, 

the photodissociation of a generic molecule ABC can fragment into AB, BC, and AC, or produce atomic 

fragments A, B, or C. For a single color, the branching ratio among the channels is fixed and depends on 

the couplings to the initial molecular state. However by using two colors with a well-defined phase 

relationship, one can control the relative dissociation branching ratios. The requirement for a well-defined 

phase relationship is satisfied by the fundamental and the third harmonic content of the DUV-FEL. A 

coherent control experiment cannot be performed with a SASE FEL but requires the coherent output of 

the DUV-FEL. 

Surface Chemistry and Dynamics. A second end-station will be used to probe the angular 

distribution of neutral molecules desorbed from a surface following thermal or photo-induced reaction. 

The DUV-FEL offers significant advantages in photon intensity as well as the possibility of beam 

focusing for “sheet ionization.” In order to obtain a large collection solid angle, the probe beam is focused 

along one axis to form an optical “sheet” parallel to the crystal surface. Initial experiments will focus on 

molecules which are particularly hard to detect by state-resolved spectroscopy (e.g., O2, CO2, ethylene, 

formaldehyde), but for which one-photon ion imaging is very well suited. 

Nonlinear Optics. The DUV-FEL will be used to drive nonlinear photon absorption in atoms and 

molecules. Using hydrogenic scaling laws and perturbation theory [190], it is estimated that saturation of 

the two-photon ionization of hydrogen occurs at a peak intensity of 10 14 W/cm 2 , and the peak power in 

the fundamental of the DUV-FEL will approach 2 × 10 15 W/cm 2 . Clearly the DUV-FEL exceeds the 

power needed for two-photon ionization of hydrogen by an order of magnitude. Similarly, nonlinear 

excitations in helium may be feasible using the weaker third harmonic at 86 nm. Using the calculated 

two-photon ionization cross-section [191], the estimated saturation intensity is 10 14 W/cm 2 . Assuming 

diffraction-limited performance the third harmonic can produce 2 × 10 14 W/cm 2 . Even with losses of a 

factor of 10 in the transport system, one expects an ionization probability of 0.05 and the two-photon 

ionization of helium should be observable. In each of these cases the nonlinear transition will be detected 

as an electron or ion following ionization using the chamber described above. 

In addition to the lowest nonlinear process discussed above, even higher-order ionization can be 

observed in intense laser fields (>10 12 W/cm 2 ). In this case, electron energy analyses will show a series 

50

of peaks separated by the photon energy. This process (above-threshold ionization, ATI) has been studied 

at visible wavelengths but it has not been studied below 100 nm where the low field approximations will 

break down. 

Temporal Characterization of the DUV-FEL. Ultra-fast metrology relies on a broad nonlinear 

response of some media in order to produce a correlation function. For example, a second-harmonic 

autocorrelator depends upon two-photon absorption in a nonlinear crystal with large nonlinear 

susceptibility. No suitable crystals exist below 200 nm due to strong absorption, and the two-photon 

ionization experiment using the DUV-FEL could provide a novel technique for the complete 

characterization of a short wavelength pulse. By adding a Michelson two-arm interferometer in the 

transport line, two replica pulses will be focused into the end-station and the two-photon ionization signal 

collected as a function of delay. Recording the total ion or electron yield as a function of time delay yields 

an autocorrelation of the DUV-FEL pulse duration. 

1.2.9.3 Tesla Test Facility at DESY 

A VUV FEL for wavelengths down to 6 nm will operate at the TESLA Test Facility (TTF-II) at 

DESY in 2004. The groundwork for the operation of TTF-II has been completed over the past 3 years 

with the demonstration of SASE at 100 nm using the TESLA Test Facility (TTF). Preliminary 

experiments with this FEL source have focused on understanding the SASE process and developing 

necessary diagnostics. The present goal of TTF-II is to produce coherent radiation tunable in the photon 

energy range up to 200 eV (6 nm) using a 1 GeV linac. 

The primary experimental interest in the second phase of TTF was recently reviewed (September 

2002) at DESY. The experiments proposed covered condensed matter physics, chemical physics 

(including atoms, molecules and clusters), material science, plasma, and laser science. Many of the 

proposals focused on beam diagnostics that are essential to assess the performance of the FEL at shorter 

wavelengths and to scientific research requiring synchronization of laser pump and TTF probe, or TTF 

pump and laser probe to few tens of fs. 

A condensed list of the proposed experiments for TTF-II to be performed in 

2004-2005 is provided below: 

• Adiabatic stabilization of Lithium (-1) atoms. 

• Multiphoton ionization and excitation of atoms and carbon clusters. 

• Characterization of VUV and EUV FEL pulse using rare gas photoionization. 

• Development of cross-correlator for pump-probe experiments using ponderomotive 

shift of the ionization potential to a few-fs accuracy. 

• Imaging of photoelectrons produced in a gas cell for fs synchronization. 

• Study of multiphoton ionization regime using COLTRIMS. 

• Photodissociation of molecular ions stored in a ion trap. 

• Interaction of intense VUV pulse with rare gas clusters. 

• Study of charge and energy transfer at surfaces using resonance-enhanced 

multiphoton ionization. 

• Non-linear processes as surface probe. 

• Sub-ps magnetization dynamics through sum harmonic frequency generation. 

• Use of ‘photon sieve’ to suppress higher harmonics in zone plate images and perform 

nanospectroscopy. 

• Luminescence measurements and inelastic light scattering at the FEL. 

51

• Temporal studies of biomolecules. 

• Warm dense matter. 

The TTF-II facility would begin to provide user beam time in CY 2004. The split in beam time 

will be 1,500 hours for user science and 3,000 hours for FEL studies. This split will be 2,500 hours and 

2,500 hours in year 2005, 3,000 hours to 2,500 hours in 2006, which is likely to continue in future years. 

In this proposal-round the total requested beam time exceeded 13,600 hours for 29 proposals, exceeding 

the availability by nearly a factor of four. 

First experiments have been performed recently that indicate that the clusters absorb many 

photons from the FEL simultaneously and burst by Coulomb explosion. These experiments were centered 

on the study of electron dynamics and of non-linear optical processes in atoms (He-Xe) in the VUV 

wavelength (98 nm) range at power density of up to 7 × 10 13 W/cm 2 . Some answers have been obtained 

regarding the competition between above threshold ionization and resonant multi-photon processes. 

Briefly, Xe clusters show that the Xe atoms become only singly ionized by the absorption of single 

photons and, on average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. 

The clusters are heated up and electrons are emitted after acquiring sufficient energy. Finally, the clusters 

completely disintegrate by Coulomb explosion [192,193]. 

1.3 USER PROGRAM 

All existing synchrotron, neutron, and high-magnetic-field user facilities have strong user 

programs. Indeed, users at the newest facilities have had early involvement in the design process to help 

make the best choices among various technologies and optimization strategies. They have participated 

through advisory committees and workshops well before the facilities were operational. The proposed xray 

laser user facility will require an even deeper level of user involvement because it differs from 

existing sources in important ways. In this section we outline the differences, and present an initial model 

of a user program that addresses them. 

In an x-ray laser facility, the electron beam characteristics can be separately manipulated to meet 

the source requirements for specific experiments. This stands in contrast to modern synchrotrons. For 

example, at synchrotrons the beamlines all have very similar, even identical, source characteristics. There 

are well-defined interfaces between the machine and the user. Prior to and during construction it is 

necessary to get user input generally, but the task of constructing individual beamlines is separate in 

physical location (outside the shield wall) and generally later in time than the construction of the machine 

itself. This should not be the case for the proposed x-ray laser facility. The “beamlines” here must be 

thought of as including the seed laser and the electron beam, whose parameters can be varied from 

beamline to beamline, and perhaps even varied for successive pulses in the same beamline. There is no 

clear separation between the machine and the beamline, and strong user involvement is needed at the 

earliest stages. 

This technical integration should be reflected in the user program by expanding and deepening 

the role of the early users in the design of the facility. Additionally, a strong in-house research program is 

an essential element in a successful facility. For these reasons we seek user participation from the outset 

within the project team, and will refer to users who accept involvement and responsibility at this level as 

principal users. We are encouraging strong MIT faculty participation on the design team, and the initial 

complement of principal users includes the science collaborators on this proposal. However, a rigorous 

review system will be implemented to ensure that the principal user group associated with beamline 

52

facilities included in the Conceptual Design Report is national in character, and that all successful 

principal users meet the same high standards. 

There will ultimately be many scientists who will not be principal users; in fact they will be the 

majority of the user community. Such individuals have been called general users or independent 

investigators. They are the lifeblood of a user facility and represent the future of the facility after its 

construction. New principal users, as well as facility managers, will emerge from the population of 

general users as turnover occurs during the facility lifetime. At any one time most of the science being 

generated on the floor will involve the general users, either independently or in collaboration with the 

principal users. This group needs to have a degree of access and support commensurate with their ability 

to produce the highest quality science. 

The development of a superb user program requires an overarching philosophy and a substantial 

body of implementing process and detail. It is best to develop the user policies and procedures early in the 

project planning stage. Just as we have produced a strawman design for the technical facility, the 

discussion above represents a strawman philosophy for the user program. We expect it to evolve as the 

plans for the facility evolve, as it is subject to discussion with potential users, and as it is reviewed by 

appropriately constituted advisory committees. During the study period we envision establishing three 

key standing committees to provide advice on the user program from different and complementary 

perspectives: (1) an Executive Committee elected by the User’s Organization, (2) a Research Council 

consisting of all the principal users and key facility management, and (3) a Scientific Advisory 

Committee of experienced independent scientists. It is essential to have this structure well in place by the 

beginning of construction. 

As a first step toward establishing a philosophy and developing the initial framework for its 

implementation, we will convene an ad-hoc committee chaired by Prof. Arthur Bienenstock of Stanford 

University. His committee will be charged with reviewing our user philosophy, advising on its content, 

and laying out a strategy to implement it over the study period, eventually vesting its authority in standing 

committees such as those suggested above. The work of this committee will include advice on the balance 

between in-house and external principals, the balance between beamtime for the principal users and that 

for general users, the balance of resource allocation to support general users, and advice on the balance of 

the scientific portfolio. An overriding issue will be how to extract the maximum educational benefit from 

such a unique national facility. The final details of the user program will be contained in the Project 

Management Plan completed in the second half of the study period. 

1.4 EDUCATION AND OUTREACH PROGRAM 

The mission of MIT is to advance knowledge and educate students in science and technology to 

best serve the nation and the world in the twenty-first century. Further, MIT is dedicated to providing its 

students with an education that combines rigorous academic study with the excitement of discovery. An 

x-ray laser facility, located on the Bates site in close proximity to MIT and the other excellent universities 

in the Boston area, would provide direct access to the research frontiers across a wide range of fields in 

science and engineering to a large pool of highly motivated and premier caliber students. 

Bates has a distinguished record in the education and training of more than 100 Ph.D. nuclear 

physicists over the past three decades. These students are widely sought in academia, in industry, and in 

research laboratories. In addition, undergraduate students from MIT and the Bates user institutions 

actively participate in the research activities. 

53

1.4.1 Programs During Design Study 

The first component of the education program involves the post-docs and graduate students who 

will work with the science collaborators and with the Bates staff. The budget includes support for a 

growing number of post-docs, graduate students, and undergraduates within the MIT Undergraduate 

Research Opportunities Program. 

A vibrant educational outreach portfolio must also offer exciting opportunities to K-12 students 

and teachers, as well as undergraduates from other colleges and universities. Light is fundamental to our 

everyday experience, and a facility based on extending our use of light will provide those exciting 

opportunities to teachers of young students. In the design study, we propose integrating Bates’ 

educational outreach efforts with those of MIT’s Center for Materials Science and Engineering (CMSE). 

CMSE, an NSF-funded Materials Research Science and Engineering Center, has an excellent program in 

this area. By collaborating on education programs, we can take advantage of CMSE’s experience in this 

field, while enhancing the Center’s offerings by adding an additional research area. CMSE has operated a 

very successful Research Experience for Teachers (RET) program for the past four summers. 

The objectives of the program are to provide opportunities for teachers to participate actively in 

current materials research and to develop plans to transfer materials science and engineering concepts to 

middle and high school science students. Participants are immersed in research and are highly encouraged 

to develop classroom material based on that experience. In fact, some participants return for a second 

summer specifically to create lesson plans and modules. CMSE currently places about eight teachers a 

year in its labs. We plan to invite one or two teachers to join this program to perform research at the Bates 

facility during the summer of 2003. 

RET participants will spend seven weeks working with faculty, graduate students, and post-docs 

on current research either in CMSE labs on campus or at the Bates linear accelerator. Currently, one day 

each week is devoted to learning about the research and specialized equipment in the Center’s Shared 

Experimental Facilities, followed by a group meeting where the teachers learn about each other’s work 

and discuss connections to their classroom teaching. To expand their experience, the teachers placed in 

labs at Bates will join the rest of the group on campus for these sessions. The entire group will spend one 

day together at the Bates facility learning about our research as well. In 2003, for the first time, the 

teachers will have the option of earning graduate education credits at the University of Massachusetts, 

Boston, for their work over the summer. 

In addition to the RET program, CMSE offers “content institutes” to middle and high school 

teachers. These intensive, one-week classes are designed to increase the educators’ knowledge in specific 

areas of the Massachusetts science education frameworks. Content institutes in 2002 and 2003 address the 

“engineering design process” standard. We have begun discussions with the CMSE director and education 

leader about possibilities for jointly offering a similar program using Bates staff and facilities in the 

future. 

A third area for collaboration with CMSE is in the Summer Research Internship Program. This is 

an REU program that places approximately fifteen students each summer in materials research labs on 

campus. We are exploring the possibility of placing one or more of these students in research groups at 

the Bates facility in the future. 

54

1.4.2 Planning for Programs During Facility Construction and Operation 

During the design study, we also plan to develop education and outreach programs to take 

advantage of the x-ray laser’s construction phase and, eventually, its operational phase. We would plan to 

continue, as appropriate, the programs described above and to add to them a number of new components. 

These would then be proposed as part of the facility construction proposal and included in the 

Management Plan. 

The proposed source will attract faculty and students from the many premier research institutions 

in the greater Boston area. The students who participate in the design, construction, and implementation 

of beamlines, and ultimately, in experiments, will help to meet the nation’s urgent need for well-trained xray 

scientists. Specifically, the United States has a major investment in synchrotron radiation sources. In 

order to perform the highest quality science at these facilities, each one must have qualified personnel on 

staff. As DOE-run facilities move increasingly toward a general-user mode of operation, there will be less 

opportunity for students to acquire hands-on beamline experience in those facilities. Thus, beamlines at 

the proposed x-ray laser will have the potential to play an increasingly important role in educating and 

training the next generation of x-ray scientists with a deep understanding of how a facility is assembled, 

maintained, and operated in order to create the best possible science. This role is essential for the future 

health of the x-ray science in this country. 

Another major initiative would be the development of a curriculum for accelerator science and 

technology at the graduate level. This effort would be focused around the MIT Physics Department, 

which has considerable existing strength in accelerator physics within the Laboratory for Nuclear Science 

and the Plasma Fusion Center. In addition, it would attract other accelerator physics expertise at MIT and 

in the New England region. The goal is to initiate a program of academic courses, available broadly to 

students, which would provide an education in accelerator science and technology at the doctoral level. 

Finally, we will take advantage of the construction project itself to provide educational 

opportunities in collaboration with departments at MIT that are not traditionally involved with particle 

accelerators. For example, we would envisage students from the MIT Department of Architecture and 

Planning active in the construction project, as well as students from the Sloan School of Management. 

55

2 TECHNICAL CONCEPT SUMMARY 

In recent years, a number of short wavelength FEL experiments have demonstrated key 

technologies and obtained good agreement between experiment and theory. There is now consensus in the 

FEL community that the technology demonstrations and our understanding of FEL physics have reached 

sufficient maturity to permit the construction of x-ray laser user facilities with low technical risk. The 

proposed MIT x-ray laser incorporates design features that take advantage of many recent developments. 

It blends proven technologies into a powerful new instrument that combines the high power, coherence, 

and ultrashort timescale probe of a laser with the energy reach and spatial resolution of synchrotron 

x-rays. It is a primary goal to integrate the instruments and experimental methods from both the laser and 

synchrotron radiation communities at the earliest stages of design. 

One of the important technologies for an x-ray laser user facility is a high-repetition-rate, 

high-brightness accelerator. Linacs have demonstrated the required beam quality, but most of them are 

copper structures limited by heating to low repetition rates. Low repetition rates are undesirable because 

they significantly limit the number of beamlines that can be implemented. It is only recently that the 

successes of the superconducting linacs at DESY and Jefferson Lab have demonstrated that they can also 

produce the high repetition rate required to support multiple beam lines. DESY in particular has 

demonstrated the shortest laser wavelength yet, producing saturated power output at 90 nm. Their Tesla 

Test Facility (TTF) [185] generates peak power of 1 GW and up to 100 µJ pulse energy. We propose to 

use a 4-GeV linear accelerator based on the DESY design that will produce such high electron pulse rates 

that twenty or more beamlines can be extracted to serve a large user community. 

Integrated high-harmonic generation laser technology [194,195] will seed the electron beam and 

generate photon beams with high longitudinal coherence and pulse lengths significantly below 100 

femtoseconds, perhaps below 1 femtosecond. The FEL itself will use the high gain harmonic generation 

(HGHG) method [196,197] to produce multiple harmonics of the tunable input seed. BNL has 

demonstrated saturated output in the HGHG regime at 5 µm [198] and 266 nm. The output radiation has 

the full longitudinal and transverse coherence and stability of the seed laser, providing substantial 

improvement over performance based solely on self-amplified spontaneous emission (SASE). 

The undulator length required to achieve lasing grows rapidly with decreasing wavelength, 

reaching ~100 m for 0.1 nm radiation. The construction and installation of such long undulators is greatly 

simplified if they can be made in short sections separated by stations containing focusing magnets and 

beam diagnostics. Such a segmented undulator design consisting of nine 2.4 m sections was installed and 

successfully commissioned at the LEUTL facility at ANL [199]. The diagnostic stations in this design are 

distributed over the length of the undulator, enabling important experiments on the physics of the 

amplification process. The properties of the radiation and the electron beam are sampled at each location 

yielding z-dependent measurements of SASE power and spectrum, mode size, energy fluctuations, and 

electron beam microbunching [183,200]. These experiments were the first measurements of the growth 

and saturation of the SASE output, and output of harmonic radiation [201]. Comparisons of these 

experiments with numerical simulation have confirmed our understanding of the physics of SASE. 

By taking advantage of the ability of linear accelerators to extract beams at different energies, we 

envision a facility spanning both the traditional extreme ultraviolet and x-ray wavelength range. This 

approach provides for integration and synergy between the previously separate synchrotron community 

and the new users who will come from the laser community, anxious to move to wavelengths shorter than 

conventional table-top technology can provide with high pulse power. 

56

2.1 FACILITY DESCRIPTION 

A brief summary description of the layout of the entire facility is given as an introduction, 

followed by more detailed descriptions of the important components. The description here is 

supplemented by Appendix A, which contains a fuller treatment of the initial plan for accelerator 

development, including upgrade options and cost considerations. 

A view of the proposed site is shown in Figure 22. The overall length of the proposed facility is 

less than 1 km, which fits comfortably at MIT’s Bates laboratory. A sketch of the layout of the accelerator 

and experimental halls is shown in Figure 23. The major components are the superconducting electron 

linac of length ~300 m, undulator tunnels for 4–8 undulators that are ~50 m long, and three experimental 

halls (UV, nanometer, and x-ray) following the undulators that are also ~50 m long. Each hall contains a 

number of conventional lasers used for multiple color experiments and seeding of the FEL. 

The production of x-ray laser pulses begins with generation of the electron beam in the RF 

photoinjector. The photoelectrons are produced by a conventional laser striking the photocathode 

contained in a high field RF cavity, producing ~100 pC pulses that are a few picoseconds long. The 

injector exit energy is approximately 5 MeV. After a brief drift space, the beam enters the 

superconducting linac. Space charge forces are dominant in this first section of linac, which requires that 

the beam transport be designed to minimize space charge induced emittance growth as it accelerates to 

approximately 250 MeV, beyond which the space charge forces are of less concern. At ~250 MeV the 

beam enters the first magnetic chicane, which compresses it to a length of a few hundred femtoseconds 

and increases the bunch current from a few tens of amps to a few hundred amps. Following the chicane 

the beam is accelerated to approximately 1 GeV, where it is further compressed in a second chicane, then 

enters the first beam “switchyard.” 

The switchyard selects individual pulses for delivery to the set of approximately four undulators. 

The undulators produce laser output over the wavelength range 100 nm to 10 nm. Additional pump-probe 

and seed lasers are also present in the hall. The various lasers have a design goal of 10 fs synchronization 

with each other; the means of achieving this are discussed in more detail in Section 2.4 and Appendix A. 

The beam is then further accelerated to the energy of 2 GeV where it enters the second switchyard that 

selects pulses for the nanometer hall. The range of wavelengths in this hall is from 10 nm to 1 nm, with a 

number of conventional lasers present and also synchronized to the FEL to within 10 fs. Following the 

second switchyard the beam is accelerated to its final energy of 4 GeV where it is directed by the third 

switchyard into the x-ray hall. This hall produces wavelengths from 1 nm to 0.3 nm in the fundamental, 

and in addition produces substantial third harmonic power (~1 µJ) at 0.1 nm. 

The layout of the accelerator and experimental halls will allow for a future upgrade to a higher 

energy, longer linac and the placement of additional long undulators to produce 0.1 nm radiation in the 

fundamental. 

2.2 INJECTOR 

Existing RF photoinjectors have demonstrated the performance required for the x-ray laser 

facility [202–204]. Continuing improvements in their performance will help to reduce the cost of the 

facility by reducing the undulator and linac lengths. The injector is among the most critical accelerator 

components because the electron beam that it produces cannot be improved upon in later stages of 

acceleration. 

57

FIGURE 22 Aerial view of MIT-Bates laboratory with scale. 

Injector 

laser 

1 GeV 

Seed 

laser 

2 GeV 

Laseer master oscillator 

1.2 1.2 km km 

UV Hall X-ray Hall 

Undulators 

100 nm 

30 nm 

10 nm 

SC Linac 

Seed 

laser 

Pump 

laser 

4 GeV 

10 nm 

3 nm 

1 nm 


Nanometer Hall 

Fiber link synchronization 

Seed 

laser 


1 nm 

0.3 nm 

Pump 

laser 

0.3 nm SC Linac 

0.1 nm 

FIGURE 23 Layout of laser experimental halls and accelerator (halls not to scale) 

~500 m 

Pump 

laser 

58 

Future upgrade to 0.1 

nm at 8 GeV

The injector consists of an RF cavity with photocathode and drive laser. A laser pulse lasting a 

few picoseconds produces electrons in the presence of the strong accelerating field of the RF cavity. The 

beam quality in the injector is largely determined by the laser and cathode properties, and their 

performance is critical to reaching the FEL performance goals. The temporal and transverse shapes of the 

drive laser pulse are the most critical parameters. One thrust of the study will be improvements in the 

experimental methods used to shape and measure the temporal laser and electron beam properties on a 

time scale of tens of femtoseconds. This requires pulse shaping to generate sub-ps rise and fall times, and 

sharp transverse edges. The drive laser must also meet stringent timing, stability, and reproducibility 

specifications. Ideally, it will allow flexibility in the output of different macropulse and micropulse 

patterns to suit changing user needs. 

The chosen cathode should be robust in the accelerator environment with high quantum 

efficiency and low thermal emittance, and exhibit prompt and uniform emission. The cathode properties 

have an important impact on the so-called thermal emittance, which is the minimum possible electron 

beam emittance for a given cathode material. The x-ray laser properties depend so strongly on emittance 

that an improved knowledge and reduction of thermal emittance is certain to have a large impact on 

performance. 

2.3 LINAC 

The feasibility of producing high-brightness, high-repetition-rate electron beams from a 

superconducting linac has been demonstrated at DESY’s TTF [185] and at Jlab [205]. We expect to 

choose the 1.3 GHz structures developed at DESY due to their advanced state of development and 

commercial availability. We have joined with Cornell and BESSY to study whether it is possible to costeffectively 

run the superconducting linac in CW mode. If adopted, this will provide the greatest flexibility 

for pulse sequences for the users. 

Other important accelerator components are the magnetic chicanes used for bunch compression 

and the beam switchyards. The chicanes are straightforward to produce, but must be carefully optimized 

to reduce the undesirable effects of coherent synchrotron radiation (CSR), and to minimize timing jitter. 

For the fast electron beam switches, two technologies will be examined: fast pulsed magnets and 

deflecting RF cavities. Both have demonstrated the required performance [206,207]. 

2.4 CONVENTIONAL LASERS AND SEED GENERATION 

The proposed facility takes advantage of conventional laser technology to increase the 

performance of the x-ray laser. Different lasers distributed throughout the facility will be used to generate 

the electron beam, to seed the undulators, and for use in pump-probe experiments. By seeding the various 

undulators with relatively low power at either the fundamental or a subharmonic of the desired FEL 

wavelength, the FEL can then generate amplified pulses with desirable properties of the seed such as full 

temporal coherence, extremely short pulses (

must be superimposed on the electron bunches before the FEL with a temporal uncertainty of about 10 fs. 

Breathtaking advances over the last years in frequency metrology based on ultrafast lasers and, therefore, 

also in laser stabilization and synchronization, show that such low timing jitters between different laser 

systems can be achieved and maintained over arbitrarily long times and also distances of several hundred 

meters [208–210]. The techniques and challenges of this are discussed in more detail in Appendix A. The 

seed wavelength can be made tunable when generated by an optical parametric amplifier and by selecting 

among closely spaced high harmonics. 

A central goal of the seeding program is the development of a multi-kHz source of intense 

20-30 fs pulses, as well as phase-controlled few-cycle (sub-5 fs) light pulses and their full characterization 

and control with respect to intensity, shape, pulse width, and carrier-envelope phase. These high-intensity 

pulses with precisely known phase can be used for the generation of femtosecond and attosecond seed 

pulses in the XUV and soft x-ray regime. Optimization of this process by coherent control techniques is 

likely to improve efficiencies dramatically. 

2.5 UNDULATORS 

The proposed facility will have three experimental halls, each supplied initially by perhaps four 

undulators with space for up to approximately eight undulators each, supporting a total of as many as 

30 beamlines. The total undulator lengths will reach about 10 meters for long wavelength (100 nm) and 

up to 60 meters for short wavelength (0.3 nm). Long undulators will consist of separate segments and 

short break sections where beam focusing quadrupoles, orbit correction magnets, phase adjusters and 

diagnostic devices will be located. This approach has been successfully demonstrated at ANL [199], and 

significantly eases construction of long undulators. For most of the undulators, each segment will most 

likely be a permanent-magnet planar hybrid device. The technologies to produce such undulators are well 

established in many existing devices. Electromagnetic undulators are a viable alternative at longer 

periods, and high-field superconducting undulator technology will be reviewed as part of the study. It is 

important to tune the photon energy by tuning the gap, the field strength, or the period of the undulator, 

rather than the electron beam energy, so that other experiments are not affected. Furthermore, it would be 

advantageous to adjust the effective undulator length by tuning it in sections to optimize the FEL 

properties in response to changing electron beam parameters or different seeding configurations. 

2.6 FEL PROPERTIES 

The FEL behaves as a classical analog amplifier operating at x-ray wavelengths. Its output 

properties are an amplified reproduction of the input signal, which may be generated from the emission of 

spontaneous radiation early in the undulator, or from an appropriate seed source. In addition to 

amplification of the fundamental, an FEL using a planar undulator produces substantial power in the low 

harmonics that can be used to reach successively shorter wavelengths by cascading multiple undulator 

sections tuned to successively higher harmonics. 

SASE operation is the simplest and most flexible alternative for generating the initial signal. It 

depends only on the electron beam properties, which are readily manipulated, to generate output at 

different wavelengths, and does not require subpicosecond timing synchronization to a seed optical pulse. 

Most x-ray laser proposals to date have depended on SASE generation because of a perceived lack of 

suitable seed sources at hard photon wavelengths. The drawbacks of SASE radiation are that it is not 

transform-limited longitudinally, and exhibits fluctuations in timing, frequency, and amplitude due to the 

initial noise statistics (Figure 24). The lack of temporal coherence also limits the opportunities for optical 

pulse shaping that has proven so productive for visible short-pulse, high-power lasers. 

60

The shortcomings of SASE radiation can be overcome by introduction of a seed optical pulse at 

the undulator entrance. If the seed power is substantially greater than the initial undulator radiation, then 

its phase and timing properties will dominate the spontaneous noise. The FEL output will then be a highpower 

pulse with the same time and phase characteristics as the input pulse, assuming sufficient FEL 

bandwidth. The recent successes of gas jet HHG provide the basis for a tunable seed in the UV to soft 

x-ray wavelengths. Seeding opens up many opportunities to shape the output pulse to produce short time 

durations (Figure 25) or narrow linewidth (Figure 26). Furthermore, a chirped input seed may be 

overlapped with a chirped electron beam to produce a high power chirped x-ray pulse that can then be 

optically compressed to sub-fs length with peak power approaching terawatt levels. 

SASE Operation. The initial signal amplified in the SASE process is spontaneous synchrotron 

radiation emitted by the electron bunch in the initial periods of an undulator. Each electron produces a 

wavetrain that is uncorrelated with other electrons in the bunch. This incoherent emission has intensity 

that grows linearly with the number of electrons. For the collective FEL instability to occur, the density of 

electrons in six-dimensional phase space must be high enough that they become spatially bunched 

(microbunched) by the radiation on the scale of the optical wavelength. The modulated electrons then 

emit in phase with the existing radiation, producing radiation power that scales as the square of the 

number of electrons. 

With typically a few 10 9 electrons per bunch, the enhancement of SASE power over incoherent 

undulator radiation is very large. Both the microbunching and the radiation intensity grow exponentially 

along the undulator. The time and frequency structure of the SASE optical pulse are determined by the 

statistics of spontaneous emission, the slippage length of electrons relative to the radiation, and the 

electron bunch length. Spontaneous emission results from fluctuations in the particle density and as a 

consequence it exhibits fluctuations in the power spectrum and time profile. The slippage distance is the 

difference between the optical path and the undulating electron path. Each electron slips one optical 

wavelength behind the radiation for each undulator period traveled. The cooperation length for electrons, 

or the coherence length of the radiation, is approximately the slippage that occurs in one gain length. The 

optical phase and amplitude are preserved over this length scale, but fluctuate randomly within the larger 

FEL bandwidth and bunch length. This accounts for the spikes shown in the plots of Figure 24. 

Seeded operation with high gain harmonic generation. HGHG [196] is a method of producing 

a longitudinally coherent short wavelength pulse by seeding with a longer wavelength pulse. The input 

seed is used to coherently microbunch the electron beam in a first undulator that is tuned to produce 

radiation at the seed wavelength. The non-sinusoidal trajectory of the electron beam creates harmonic 

content in the microbunching power spectrum. The beam is then introduced into a second undulator 

whose fundamental wavelength is tuned to a harmonic of the first undulator. In this second undulator the 

original harmonic (now fundamental) power undergoes exponential growth from the FEL instability. This 

process may be repeated in multiple sections to frequency-multiply the initial seed, eventually creating a 

final output that is a high harmonic of the starting wavelength. The method has been successfully 

demonstrated in the IR [197] and UV at BNL. The seed radiation for these experiments was produced by 

harmonic generation in a nonlinear crystal, which can reach wavelengths of approximately 180 nm. 

Single-stage HGHG FEL amplification can then reach to 60 nm, and cascaded HGHG can reach a few nm 

without the need to change electron beam energy. For shorter wavelengths, a new seeding source is 

required. 

61

Power (GW) 

8 

7 

6 

5 

4 

3 

2 

1 

0 

0 10 20 30 40 50 

Time (fs) 

Power (W) 

10 10 

10 10 

10 10 

10 8 

10 8 

10 8 

10 6 

10 6 

10 6 

10 4 

10 4 

10 4 

10 2 

10 2 

10 2 

10 0 

10 0 

10 0 

0 10 20 30 40 50 

Time (fs) 

Power (kW/bin) 

500 

400 

300 

200 

100 

0 

0.2995 0.3 0.3005 0.301 

Wavelength (nm) 

FIGURE 24 Plots of time structure and spectrum for SASE pulse from GINGER simulation at 0.3 nm. 

Spikes in time and frequency are due to startup from initial noise. Optical pulse is approximately the same 

length as electron pulse. Undulator length is 45 m. 

Power (GW) 

2 

1.5 

1 

0.5 

0 

0 10 20 30 40 50 

Time (fs) 

Power (W) 

10 10 

10 10 

10 10 

10 8 

10 8 

10 8 

10 6 

10 6 

10 6 

10 4 

10 4 

10 4 

10 2 

10 2 

10 2 

10 0 

10 0 

10 0 

0 10 20 30 40 50 

Time (fs) 

Power (kW/bin) 

1000 

800 

600 

400 

200 

0 

0.2995 0.3 0.3005 0.301 


FIGURE 25 Plots for ultrashort seeded pulse from GINGER simulation at 0.3 nm. Seed peak power is 

10 MW and length is 0.5 fs FWHM. Log plot of time structure shows background SASE level for unseeded 

portion of electron beam. X-ray output pulse length is 0.75 fs FWHM, much shorter than 50 fs electron pulse. 

Undulator is 20 m long. 

Power (GW) 

2 

1.5 

1 

0.5 

0 

0 10 20 30 40 50 

Time (fs) 

Power (W) 

10 10 

10 10 

10 10 

10 8 

10 8 

10 8 

10 6 

10 6 

10 6 

10 4 

10 4 

10 4 

10 2 

10 2 

10 2 

10 0 

10 0 

10 0 

0 10 20 30 40 50 

Time (fs) 

Power Power Power (MW/bin) 

500 

400 

300 

200 

100 

0 

0.2995 0.3 0.3005 0.301 


FIGURE 26 Plots for long seeded pulse from GINGER simulation at 0.3 nm. Seed peak power is 0.5 MW. 

Modest structure on time profile is due to competition from initial spontaneous noise. Spectrum is nearly 

transform limited. Undulator length is 28 m. 

62

A promising, rapidly developing candidate for seed generation of ultrashort soft x-ray pulses is 

high harmonic generation (HHG) by a conventional laser in a gas jet. In this method, few femtosecond 

Ti:Sapphire laser pulses generate ~1 fs soft x-rays in the wavelength range 10–30 nm with pulse energies 

of nanojoules. The bandwidth of these pulses is currently a few percent, which is wider than the FEL can 

support. However, near-term efforts are likely to provide pulsewidths and bandwith that are a good match 

to the FEL. Section A.4.2 describes our approach to generating the appropriate HHG seed. At shorter 

wavelengths, below 1 nm, the FEL gain bandwidth is sufficient to support sub-fs pulses. We plan to 

cascade multiple HGHG sections to achieve the shortest wavelengths. 

The peak power produced by the shortest pulses is reduced a modest amount compared to SASE 

or long pulse generation. This is due to slippage of the electrons out of the optical pulse in a single gain 

length. The effect of slippage is mitigated because the optical pulse slips into electron beam that is 

relatively higher quality due to its lack of FEL interaction. Initial simulations with GINGER indicate that 

the FEL is capable of producing sub-fs pulses at short wavelengths as shown in Figure 25. These 

simulations are fully time-dependent and use realistic electron beam assumptions. More detailed 

simulations of the entire process, from electron beam generation and seed pulse generation, through 

HGHG output, will be carried out during the course of the proposed study. 

Chirped Pulse Amplification. A further refinement to seeded operation is to use chirped pulse 

amplification (CPA), which shows promise to create the shortest, highest power pulses. In this method the 

seed radiation is chirped to produce a time-frequency correlation. The electron beam is similarly chirped 

to produce a matching time-energy correlation. This may be accomplished as part of the normal electron 

bunch compression, where the electron beam is under-compressed to produce the appropriate chirp, or it 

may be chirped by wakefield generation in dedicated small diameter beampipe sections. The chirped 

optical pulse is amplified and frequency multiplied in a HGHG FEL. The resulting x-ray pulse can be 

compressed by the ratio of chirp to transform-limited bandwidth. Seed pulses produced by HHG have a 

bandwidth of 5–10% yielding theoretical compression ratios as high as 5,000 at the shortest wavelengths. 

This could produce x-ray pulses with peak powers above 1 TW and pulse lengths much shorter than 1 fs. 

Figure 27 illustrates compressed pulse lengths and output powers for different wavelengths assuming 

linear compression of the chirped pulse. This naive estimate does not account for phase distortion due to 

the FEL process or nonlinearities in the optical compressor, effects that will reduce the peak performance 

achievable. The wide bandwidth of HHG seed pulses is well suited to CPA in the FEL. The opportunities 

for pulse shaping by CPA will be studied in detail during the three-year design study. 

2.7 PHOTON BEAMLINES 

The proposed x-ray laser will deliver higher peak power than third-generation sources, and the 

photon pulse duration will be nearly three orders of magnitude shorter. Many experiments will require a 

femtosecond pump pulse to be synchronized with a femtosecond probe pulse, or with a detector. In the 

time domain of interest here, the diffraction process from optical elements will be fundamentally different 

from conventional synchrotron sources because the pulse length and bandwidth are close to the transform 

limit. In some cases the energy bandwidth of the reflection may be narrow enough to stretch the pulse. All 

these aspects are currently being addressed in various femtosecond x-ray demonstration experiments, 

which use either HHG sources or sliced electron bunches in storage ring facilities. These sources deliver 

modest photon flux, but do provide a test bed for (1) a careful study of the beam transport using various 

optical elements, and (2) development of diagnostic, synchronization and detection methods suitable for 

the temporal domain. 

63

Chirped pulse length (s) 

10 -15 

10 -15 

10 -15 

10 -15 

10 -16 

10 -16 

10 -16 

10 -16 

10 -17 

10 -17 

10 -17 

10 -17 

10 -9 

10 -9 

10 -9 

10 -9 

10 -8 

10 -8 

10 -8 

10 -8 

Wavelength (m) 

10 -7 

10 -7 

10 -7 

10 -7 

Compressed power (W) 

10 13 

10 13 

10 13 

10 13 

10 12 

10 12 

10 12 

10 12 

10 11 

10 11 

10 11 

10 11 

10 -9 

10 -9 

10 -9 

10 -9 

10 -8 

10 -8 

10 -8 

10 -8 

Wavelength (m) 

FIGURE 27 Left plot shows theoretical minimum pulse length achievable using CPA at different 

wavelengths assuming perfect linear compression, and a 50 fs input seed with 6% chirp. Phase distortions will 

limit actual compressability. Right plot shows corresponding peak power. The three separate lines are from 

optimization of electron beam and undulator parameters at different electron beam energies. 

The beamlines at the proposed facility will include optical elements needed to tailor phase space 

parameters to meet experimental requirements. The x-ray optics system must filter the intensity, spectral, 

and spatial characteristics of the FEL beams as needed for the experiments and transport it to the sample. 

In contrast to third-generation sources, there is very little average power in the FEL beams (< 1W). 

Therefore, most of the techniques to perform these functions (slits, absorbers, mirrors, monochromators, 

etc.) are straightforward extensions of methods commonly used at synchrotron sources. The FEL beams, 

on the other hand, generate very high peak power densities (10 13 -10 16 W/cm 2 ). The main optical 

elements are crystals, mirrors, gratings, multilayers and combinations of them, e.g., multilayer gratings, 

Bragg-Fresnel optics, zone plates, etc. All these components have both fundamental and technological 

performance limits with and without the heat load. 

In defining the transport and optics requirements for various experiments, we propose to draw 

heavily on the expertise of optics specialists in both the synchrotron radiation field and laser area for 

handling ultra-bright beams and ultra-short pulses. We will assemble a detailed database of existing optics 

knowledge and develop simulation capabilities to predict the performance of the optics for soft and hard 

x-rays required for the proposed experiments. Using this process, we will then identify R&D requirements 

and collaborative strategies for the latter stages of the study or early phases of the construction phase. 

Early diagnostics will be based on extensions of proven techniques that have worked well at synchrotron 

sources and in optical laser experiments. They will require pulse-by-pulse measurements of total beam 

energy, pulse length, energy spectrum, photon beam divergence, beam centroid, spatial shape, and 

transverse coherence. 

2.8 COMPARISON TO OTHER SOURCES 

In order to appreciate the beam characteristics of the proposed MIT/Bates x-ray laser source, we 

briefly review in this section the performance of sources available today. 

Synchrotron Sources. The highest power sources of hard x-rays are modern third-generation 

synchrotron sources, while the most coherent sources are lasers. Such synchrotron sources have a degree 

of transverse coherence that approaches macroscopic length scales (i.e., microns), but have pulse lengths 

on the order of 100 ps and photon degeneracy parameters (i.e., the number of photons per quantum state) 

64 

10 -7 

10 -7 

10 -7 

10 -7

generally in the range of unity. The proposed x-ray laser can produce fully coherent pulses with 

degeneracy parameter of 10 11 , and many orders of magnitude improvement in pulse length and peak 

power over existing synchrotron sources (see Table 1). The narrow bandwidth of the x-ray laser also 

makes it competitive with third generation sources in average flux when comparing power delivered to a 

sample, where the synchrotron undulator output is typically monochromated to 0.1% bandwidth. The 

peak and average brightness of the x-ray laser surpass that of synchrotrons by several orders of 

magnitude. The pulse structure of the laser, at kilohertz repetition rates, is well-suited to studies of time 

dynamics that require relaxation time between pulses, and to synchronization with other laser sources. 

Synchrotrons produce (relatively) low intensity x-ray pulses at megahertz repetition rates that are wellsuited 

to study of average crystal structure. They will remain the workhorse instrument for experiments 

that do not require the short time duration, high brightness, and coherence of a laser source. 

Energy-recovery Linacs. There are plans to build a new generation of ring-based sources known 

as energy-recovery linacs. A low energy prototype that drives an IR free-electron laser has been 

demonstrated at Jlab [205], and Cornell has proposed a higher energy and higher current prototype [211]. 

This concept is motivated by the desire to improve electron beam emittance by providing a fresh beam 

from the linac for each turn of the ring, thereby preventing emittance degradation by synchrotron 

radiation emission. ERLs can also produce pulse lengths similar to SASE FELs (tens of femtoseconds). 

They do not have the longitudinal and full transverse coherence of a laser. The ERL represents a new 

generation of synchrotron source, with repetition rate and peak power similar to a ring, but improved 

brightness and pulse length. Significant challenges in accelerator development remain before an ERL user 

facility can be constructed. 

Short-pulse Methods. Because of the intense interest in shorter pulses of hard x-rays a number 

of other schemes have been developed such as electron-beam slicing methods at existing synchrotron 

sources [212], Thomson scattering from relativistic electron beams, and plasma sources. Although 

interesting demonstration experiments are possible with such sources, they generally have very low beam 

power and/or little coherence. 

Laser Sources. Modern laser sources have full temporal and transverse coherence and short pulse 

lengths down to the femtosecond level. They are the sources of choice for wavelengths longer than 

180 nm. Using high-harmonic generation methods, they can produce radiation with wavelengths down to 

10 nm, but with power levels of only nanojoules per pulse due to low conversion efficiency (

TABLE 1 Comparison of MIT-Bates x-ray laser to other accelerator-based sources 

APS Und. 

A SASE FEL 

MIT Bates 

Min. 

bandwidth 

seeded FEL 

Min. pulse 

length seeded 

FEL 

BESSY 

FEL 

LCLS 

FEL 

TESLA 

FEL 

Cornell 

ERL 

X-rays per pulse 

(0.1% max BW) 

1.E+08 3.E+11 3.E+11 6.E+09 1.E+13 2.E+12 2.E+12 1.E+07 

Peak power (GW) 3.E-06 4.0 4.0 4.0 7.0 8.0 20.0 7.E-05 

Peak brilliance 

(p/s/0.1%/mm-mr^2) 

3.E+22 1.E+33 3.E+35 7.E+33 5.E+32 1.E+34 3.E+34 3.E+25 

Peak flux (p/s/0.1%) 1.E+18 6.E+24 6.E+24 1.E+23 5.E+25 7.E+24 1.E+25 4.E+19 

Peak trans. coh. flux 

(p/s/0.1%) 

4.E+14 6.E+24 6.E+24 1.E+23 5.E+25 7.E+24 1.E+25 2.E+17 

Avg. flux (p/s/0.1%) 7.E+14 3.E+14 3.E+14 6.E+12 8.E+16 2.E+14 5.E+15 2.E+16 

Avg. brilliance 

(p/s/0.1%/mm-mr^2) 

4.E+19 5.E+22 1.E+25 3.E+23 1.E+24 4.E+23 1.E+25 1.E+22 

Avg. coh. flux 

(p/s/0.1%) 

2.E+11 3.E+14 3.E+14 6.E+12 8.E+16 2.E+14 5.E+15 8.E+13 

Trans. coh. fract. (%) 0.03 100 100 100 100 100 100 0.5 

Avg power (W/0.1%) 0.9 0.2 0.2 0.004 10 0.3 9 40 

Degeneracy parameter 0.03 4.E+09 3.E+11 6.E+09 8.E+11 4.E+09 1.E+08 100 

Pulse length (fs) 73000 50 50 1 200 230 200 300 

Photon beamlines 50 10-30 10-30 10-30 3 1 5 ~20 

Photon wavelength (nm) 0.015–.4 0.3–100 0.3–100 0.3–100 1.2–60 0.15–1.5 0.1–1.5 0.03–.5 

Photon energy (keV) 12.4 4.2 4.2 4.2 0.8 8.2 12.4 12.4 

Pulse frequency (Hz) 7.E+06 1000 1000 1000 8000 120 2300 1.30E+09 

TABLE 2 Comparison of MIT-Bates x-ray laser to other laser sources 

Min. 

MIT Bates 

Min. pulse 

Ti:Sa 

HHG BESSY bandwidth length SASE LCLS TESLA 

OPA Excimer gas jet FEL seeded FEL seeded FEL FEL FEL FEL 

Wavelength (nm) 800 157 5–50 1.2–60 0.3–100 0.3–100 0.3–100 0.15–1.5 0.1–1.5 

Photon Energy (eV) 1.6 7.8 30 800 4200 4200 4200 8300 12400 

Energy Pulse (mJ) 1 15 1.E-05 1 0.2 0.004 0.2 3 4 

Pulse length (fs) 30 1.E+07 1 200 50 1 50 230 200 

Relative Bandwidth 5.E-02 3.E-08 5.E-02 7.E-04 1.E-05 6.E-04 6.E-04 2.E-04 2.E-04 

Peak Power (GW) 33 0.002 0.01 7 4 4 4 8 20.0 

Photons/pulse 4.E+15 1.E+16 2.E+09 1.E+13 3.E+11 6.E+09 3.E+11 2.E+12 2.E+12 

Photons/sec 2.E+19 2.E+12 8.E+16 3.E+14 8.E+16 3.E+14 2.E+14 5.E+15 

Pulse freq. (kHz) 4 1 8 1 1 1 0.12 2 

66

2.9 RESEARCH AND DEVELOPMENT PROGRAM 

While the major technologies required for an x-ray laser user facility have demonstrated the 

required performance, significant gains in performance or reductions in cost may be achieved by 

conducting focused research and development on particular technologies. Examples are improvements in 

photoinjector performance, adaptation of TESLA cryomodules to CW operation, and development of 

HHG seed generation. In addition, we anticipate that workshops and detailed studies carried out in the 

course of producing the conceptual design report will identify other technologies that will benefit from 

further hardware development. A detailed hardware R&D plan will be developed as part of the conceptual 

design study. This plan will then be executed in the second half of the study period to assist in making 

technology choices, to offset engineering risk, and to begin prototyping of critical components that will be 

necessary to meet early construction schedule milestones. 

This section represents our initial estimates for technology development. We expect that 

development of the detailed R&D plan may reveal additional challenges requiring funding that would be 

requested in a supplemental proposal. The eventual cost and schedule of facility construction will depend 

to some extent on the maturity of the R&D program performed within this study phase. 

Linac. The ability to run the linac in CW mode rather than pulsed operation will provide 

maximum machine stability and permit flexibility in the pulse pattern. However it will use a large amount 

of AC power for RF generation and cryogenic cooling, and affects the specification of the RF system. 

MIT has joined with BESSY and Cornell to modify the TESLA cryomodules for efficient CW operation. 

We are requesting capital funds to purchase components for digital RF control hardware needed to 

operate the cavities at higher Q, and for parts to build a multicell vertical test stand. 

Seed Generation and Timing Synchronization. One of the most promising aspects of this 

facility is the generation of temporally coherent x-ray pulses through seeding of the FEL amplifier by 

harmonics of conventional lasers. Both long-pulse (50 fs) and short-pulse (1 fs) seeds will be developed. 

Capital funds are requested to purchase a commercial Ti:Sapphire laser amplifier for production of high 

harmonic radiation, and for purchase of a vacuum chamber and spectrometer for characterization of 

output radiation. 

Undulator. The ability to rapidly tune wavelength for different experiments will be 

accommodated by undulators with adjustable gaps or periods. In addition it may be necessary to vary the 

effective undulator length for optimized x-ray generation. This will be achieved by varying the gap or 

period in short sections. We are requesting capital funds to produce a prototype undulator segment in 

collaboration with the Advanced Photon Source at ANL. 

Photoinjector. The electron beam quality is largely determined by the photoemission 

characteristics and early transport of the beam in the photoinjector. Improved injector performance will 

produce lowered facility cost due to shorter undulators and improved ability to generate attosecond 

pulses. We are requesting funds to purchase a spatial light modulator to develop pulse-shaping techniques 

for the photoinjector drive laser, and a low charge emittance monitor. 

Photon Beamlines. The x-ray laser has a total time-average power similar to the levels contained 

in the beams at third generation sources after the initial monochromators. To the extent that a challenge 

exists due to power levels, it will be due to the fact that the x-ray laser beam has much higher peak power. 

Questions arise as to the fundamental nature of the interaction of these pulses with matter, and they 

include concerns about x-ray damage. Furthermore, beamline components, including mirrors and 

67

monochromators, need to maintain beam coherence while utilizing the narrow bandwidth and short pulse 

length of the photon pulses. Specialized manipulations such as pulse splitting and delay may be required 

for different experimental techniques. Methods to control polarization, to attenuate the beam, to reject 

higher harmonics, and to detect and synchronize the pulses with pump or probe lasers will be required. To 

meet these various challenges, funds are requested for an R&D program that will be developed to 

leverage work on-going at the LCLS, the TESLA XFEL project, and elsewhere, while specializing to the 

specific needs of experiments chosen for the conceptual design. 

Electron Beam Switches. Many beamlines will be run in multiplexed fashion with fast ferrite or 

RF switches steering the beam to alternate paths at kHz repetition rate. Funds are requested to purchase a 

fast high-power switched power supply that demonstrates the required performance. 

68

3 THE BATES LINEAR ACCELERATOR CENTER 

The Bates Linear Accelerator Center is located on an 80-acre site in Middleton, Massachusetts, 

about 20 miles northeast of MIT’s Cambridge campus. The Bates property is an excellent physical site for 

the x-ray laser facility in terms of geology and existing infrastructure. The x-ray laser would fit 

comfortably on the 1.2 km long site, allowing ample room for future upgrades. The use of the site would 

not change substantially from its current use, easing the process of gaining environmental and 

construction permits. In addition, the present Bates staff, consisting of 85 scientific and technical 

personnel, and the associated infrastructure can provide a nucleus around which the construction and 

operation of the x-ray laser can be realized very effectively. 

Currently, the Laboratory is funded by the U.S. Department of Energy (DOE) and operated by 

MIT as a national user facility for experimental nuclear physics used by over 200 scientists from more 

than 50 institutions worldwide. Bates delivers both polarized and unpolarized electron beams in the 

energy range 125 MeV to 1 GeV to a number of experimental areas. The present complement of research 

equipment includes a 500 MeV linear accelerator, a recirculator which doubles the energy to 1 GeV, a 

1 GeV storage ring, an energy compression system, and a polarized electron source. 

Since the initiation of experiments in 1974, the Bates laboratory has carried out frontier research 

in nuclear physics. Research highlights include the understanding of deformed nuclear structure using 

high resolution electron scattering in the 1970s; pioneering experiments on light nuclei in the 1980s; and 

the study of proton structure using parity violating electron scattering in the 1990s. 

A major impact of Bates since the start of experiments in the early 1970s has been its education 

and training of young physicists. Over 110 Ph.D.s have written their theses on research carried out at 

Bates. These students are widely sought in industry and research laboratories. Over 25 Bates-educated 

Ph.Ds are in academic positions worldwide. 

At present, a central research focus at Bates is the study of the fundamental properties of the 

nucleon, including its shape, magnetism and charge distribution. A major new detector, the Bates Large 

Acceptance Spectrometer Toroid (BLAST) has been constructed and will begin taking data in 2003 and 

continue through 2005. The present understanding between DOE and MIT calls for phase out of the 

accelerator support at Bates after the BLAST experiments are completed. This schedule is ideally 

matched to the timescale of this study and the proposed construction of the new x-ray laser facility. 

69

4 INTERAGENCY AND INTERNATIONAL COOPERATION 

There are few scientific endeavors that have as strong a multi-institutional and multi-disciplinary 

character, and as strong a network of international collaboration as the synchrotron radiation (SR) 

community. The x-ray laser facility we propose promises to further broaden and deepen these dimensions 

by developing an active partnership with the conventional laser community and including new areas of 

science and technology not present in today’s SR sources. Furthermore the science to be done and the 

technology to be utilized have developed with major international contributions and collaborations 

involving many different institutions and funding sources. We have already begun a process, which will 

grow during the proposed design study, to involve other U.S. funding agencies and their institutions, and 

to cooperate and collaborate on a fully international scale. 

4.1 INTERAGENCY COOPERATION 

In 1998 a study panel was convened by the National Research Council to look at the issue of 

interagency cooperation in the construction and operation of large-scale user facilities. Their report 

entitled “Cooperative Stewardship: Managing the Nation’s Multidisciplinary User Facilities for 

Research with Synchrotron Radiation, Neutrons and High Magnetic Fields” [220] provides excellent 

guidance for the successful interagency cooperation necessary in this project, which we intend to 

implement as appropriate during the design study. In addition to the National Science Foundation (NSF) 

as the steward agency, the other agencies with significant involvement in the research program will be the 

Department of Energy (DOE) and the National Institutes of Health (NIH). We also expect involvement, 

perhaps to a lesser degree, from NASA and the defense agencies. 

Although the major effort to establish interagency agreements will be undertaken after the 

funding is provided for the study, we have had discussions with scientists and management at DOE’s 

Brookhaven National Laboratory, Argonne National Laboratory, and Lawrence Berkeley National 

Laboratory and identified research collaborations on a broad range of topics of mutual interest. 

Brookhaven is a leader in the development of the laser seeding methods using its FEL facility known as 

the Source Development Laboratory, and this is an area of joint interest. Brookhaven also has a program 

of femtochemistry research planned for the near future using radiation from the SDL facility, as described 

in Section 1.2.9 of this proposal. Experience from this work will be available early to help guide plans for 

the use of shorter wavelength and higher power beams planned for the proposed MIT facility. 

Scientists at other DOE labs have expressed interest in research collaborations related to the 

underlying accelerator technology. We have already had considerable interaction with Argonne 

accelerator physicists who have provided codes for our use, and with experimentalists who have helped 

advise us on the development of the scientific case. An accelerator physicist from Lawrence Berkeley 

National Laboratory is also actively involved with us in accelerator simulation work, and we are in active 

discussions to broaden that collaboration. These early relationships will be extended into broader and 

more formal agreements once funding is established and resources are available to support work at MIT. 

The technology of free electron lasers, including electron guns, linacs, and undulators, has 

historically been of great interest to the defense funding agencies. We intend to develop proposals in 

appropriate areas of interest to, for example, the programs of the Joint Technology Office. In addition to 

technology development, research with high-power x-ray beams is of potential interest to the defense 

programs side of the DOE in areas related to Stockpile Stewardship. One of the major experimental 

programs identified in the scientific case for the LCLS project involved the creation of high-energydensity 

states of condensed matter. NASA would have interest in this research as well, since such matter, 

also known as warm dense matter, is a common component of the structure of the interior of planets and 

70

stars. Another example of research possible with x-ray laser sources of potential interest to NASA 

involves new methods described in Section 1.2.4 for determining chemical and molecular composition of 

microscopic samples with counting-level accuracy. 

While these research projects will be an important component of the x-ray laser’s research 

portfolio, the majority of science impact is expected in materials science, chemistry and biology. 

Therefore the funding agencies with the strongest programmatic involvement will be NSF, DOE, and 

NIH. We should also note that beyond these well-known federal agencies there are many other sources of 

funds including state agencies, industry, and private foundations. The user program (see Section 1.3) will 

be developed to allow full participation by all funding institutions in the planning and execution of 

research programs. 

Although not specifically “interagency collaboration,” we should mention the importance of 

collaboration with other NSF-funded facilities. Discussions have been undertaken with both Cornell and 

with the National High Magnetic Field Laboratory (NHFML). Cornell has been a pioneer in the 

development of superconducting cavity technology and is actively developing techniques for utilizing the 

DESY cavities on a CW basis. Collaborations with Cornell are pending funding decisions by the 

Foundation on their ERL proposal. At the NHFML, technology in superconducting magnet development 

could have important applications to undulator magnets for FEL sources. That Laboratory is currently 

undergoing a management change, following which discussions could begin to further develop this 

exciting opportunity. 

4.2 INTERNATIONAL COOPERATION 

There is now intense interest worldwide in the development of FEL-based sources. The OECD 

report characterized this interest in terms that essentially mark the transition to the next generation of 

facilities in stating: “It is likely that no new major storage-ring based facilities will be built beyond those 

currently planned at this time. The next generation of advanced photon sources will likely be free-electron 

lasers.” This consensus has further strengthened interest in collaboration among laboratories engaged in 

or considering future x-ray laser projects. 

To conduct the initial discussion of the feasibility and potential for an x-ray laser facility at 

MIT/Bates Laboratory, a small workshop of FEL experts was convened in June 2002. Leaders of projects 

in Japan and Europe attended this meeting and enthusiastically indicated their strong support for the 

project and a desire to collaborate. While projects of this relatively small scale do not lend themselves to 

joint international funding for construction, R&D collaboration as well as involvement on project 

advisory committees has proved to be invaluable. 

MIT is currently planning to become a member of the extensive collaborative network put 

together by DESY for the TESLA linear collider project. DESY initially developed the superconducting 

RF technology for achieving high-gradient acceleration for the application. As the world expert DESY 

has offered to help other labs in the collaboration seeking to use that technology in exchange for 

contributions in kind to the TESLA project. Since the TESLA project also includes plans for an x-ray 

FEL, there is a very significant mutual interest in this technology between MIT and DESY. That interest 

is strongly expressed in the letter of support from DESY. 

Other institutions whose scientists have expressed interest in collaborations include BESSY in 

Berlin, the Paul Scherrer Institute in Switzerland, and the Daresbury Laboratory in Britain. During the 

design phase, we will develop these collaborative interests much more thoroughly and put them on a 

formal basis. Extensive collaboration of this sort is actually now a necessary, essential, and expected part 

of doing business in this field. 

71

5 DELIVERABLES AND PROPOSED SCOPE OF WORK 

The schedule for the three-year study is designed to support a construction start in FY 2007. The 

principal activities, milestones, and necessary resources shown in Figure 28 reflect the experiences and 

best practices of other large construction projects such as the Advanced Photon Source, the National 

High-Field Magnet Laboratory and the Spallation Neutron Source. The activities take full account of the 

NSF process for developing projects funded by the Major Research Equipment account and will allow for 

continuing review and monitoring of the proposed work. The schedule also allows an adequate period for 

NSF review of the construction proposal, while work continues in the second half of the grant period. 

As shown on the chart, the first quarter of the study occurs in the last quarter of calendar year 

2003, and includes the preparation of a Preliminary Design Report. That work has already begun, funded 

by internal MIT resources, and will continue during the balance of 2003. It will enable the design to 

proceed to a point where a work breakdown structure (WBS) can be developed. The WBS is a 

prerequisite for the conceptual design effort, which will culminate in the preparation of a Conceptual 

Design Report (CDR) to be submitted in March 2005 as a key part of the proposal for construction. 

FIGURE 28 Schedule of activities and deliverables during three-year design study. The study is presumed to 

start on October 1, 2003, which is the first quarter of FY04. 

72

The first half of the proposed study is dedicated to two main activities—conceptual design of 

(1) beamlines and (2) accelerator systems. The initial set of beamlines will be developed through a 

proposal-based process occurring in conjunction with a number of science workshops. This effort will 

start with the nucleus of science collaborators who have contributed to this proposal. They will work as 

part of the project team to help develop the technical concept for the machine while further developing 

instrument concepts to support their scientific interests. A significant fraction of the proposed study 

resources will be allocated to the personnel support for the faculty and researchers who are generating 

these early experimental concepts. 

The science workshops are intended to significantly broaden the community of interested users, 

to identify new scientific opportunities, and to address specific technical issues associated with the unique 

experimental challenges of the beamlines. As shown in Figure 28, we envision four workshops in the first 

year followed by two each in years two and three. Following the early workshops, a general solicitation 

for letters of intent (LOI) will take place to identify the potential community of principal users who could 

lead the design and construction of the initial set of beamlines. After the LOIs have been reviewed by the 

Science Advisory Committee, proposals will be solicited and subject to thorough peer review in order to 

select the initial ten beamlines for inclusion in the CDR. Although we expect representation of the current 

group of science collaborators among the principal users included in the CDR, all will be subject to the 

same criteria and review process. Funding is requested (see Budget Justification) to permit up to ten 

groups to receive modest support for the beamline design effort. 

The second main activity during the first half of the study will be the generation of the conceptual 

design for the accelerator facility. To deliver this design it will be necessary to add personnel to the 

existing technical expertise at Bates in areas such as x-ray optics, FEL physics and superconducting RF 

engineering. It is expected that these resources will be added systematically over the course of the first 

year so that the laboratory will be able to support the conceptual design effort necessary to ensure its 

timely completion. Two workshops per year will be held to assist in the development of the technical 

design of the accelerator. To oversee progress on the conceptual design of the facility, an expert 

Accelerator Advisory Committee will be formed and reviews will be held twice per year. Once the 

conceptual design is finished in early 2005, design work will move to the next phase, known as Title I 

design, and continue through the balance of the study period. Work in this phase will be prioritized to 

facilitate the most cost-effective transition to construction in 2007. 

While the required conventional facilities are straightforward, they do represent a significant 

fraction of the anticipated project cost (see Appendix A.8) and thus cost risk. The proposed study will 

address the conceptual design of conventional facilities as an integral part of the overall design effort. 

These facilities include accelerator tunnels, experimental buildings, office buildings, staging areas, 

HVAC infrastructure and cryogenic plants. As outlined in the Budget Justification, consulting and design 

services for these systems will be required during the study. 

At the outset of the design study we will begin a strong educational program. We plan to support 

opportunities for undergraduates and graduate students not only in the traditional fields of science and 

engineering, but also in related areas such as architecture and project management. In conjunction with 

the educational opportunity program for high school teachers at the MIT campus, a summer program for 

high school teachers will be held at Bates to serve the broader community, as described in Section 1.4. 

We also plan to further develop the program plans for an accelerator science and technology curriculum. 

An R&D program is essential to the success of this endeavor. During the conceptual design 

phase, we will identify the critical technologies that need to be further developed and/or prototyped for 

73

the success of the facility. Early in the study, these activities will be focused on collaboration with other 

laboratories worldwide having research programs and equipment of direct interest to us. At the end of the 

first half of the study period we will deliver an R&D plan describing areas that will require further work 

during the balance of the study. Section 2.9 describes areas that we currently anticipate will form the basis 

for this work. The R&D funding requested is modest, but it is possible that more study and review will 

uncover the need for additional work. If so, a supplemental proposal can be submitted during a later phase 

of the study. 

Reviews by the Accelerator Advisory Committee and the Science Advisory Committee will be 

held at the beginning of the first year of the study to review the preliminary design, and at the beginning 

of the second year to review progress on the conceptual design. The reports generated by these reviews 

will help guide the development of the conceptual design and will be available to the NSF. The 

Conceptual Design Report will be completed in March, 2005, after which work will begin on a more 

detailed technical design. This effort will require the addition of significant engineering and design 

resources. The addition of personnel to accomplish these objectives is outlined in the Budget Justification 

included in this proposal. 

As discussed above, the study will greatly advance the scientific case for the eventual facility by 

virtue of the selection and planning process for specific beamlines proposed for inclusion in the facility. 

Early in the second year, the beamline designs developed by the principal users will be further integrated 

into the facility conceptual design. The integration of optical and accelerator systems will be an essential 

focus of the continuing annual reviews for both science and accelerator committees. This beamline design 

effort will require the development of an x-ray optics capability at Bates to facilitate the design process 

and to ensure integration with the accelerator design. Collaboration with experts at other laboratories will 

maximize the effectiveness of the local staff. 

We anticipate that the successful completion of the CDR and the development of a first set of 

experimental beamlines by our initial principal users will stimulate further interest in the project and new 

beamline proposals from the scientific community. To accommodate these as-yet unidentified 

experiments, the facility will be designed to allow a larger number of beamlines to be extracted than the 

ten we expect to include in the construction proposal. We believe that the mission of the proposed facility 

is best served by use of an open peer-review process to judge the scientific merits of the proposed 

experiments. Therefore we will continue to use this process to identify additional beamlines with highly 

meritorious scientific programs and work with their proposers to develop funding sources. 

An overview of our planned construction project management approach is contained in 

Appendix B. Early in the study, we will begin to expand this document to form the Preliminary 

Management Plan, to be included with the proposal for construction. In the second half of the study, an 

increased emphasis will be placed on creating a considerably more detailed Construction Management 

Plan for the proposed facility. This will include the detailed development of the organizational structure 

and human resource policies and systems, the establishment of essential financial systems including 

accounting and procurement, the definition of systems to manage change control and contingency, and 

plans for systems to track progress through earned-value analysis. A strategy for quality control and 

component acceptance will be developed. We will also finalize the required permitting and environmental 

assessment necessary to construct the facility on the Bates site. 

Implementation of the R&D plan will be an increased focus of activity in the second half of the 

proposed study, although some activity will be started earlier. The overall effort will be focused on the 

technologies required for the success of the project and will include prototyping of systems which are 

74

vital to the successful performance of the facility, e.g. the RF photoinjector, fast electron beamline 

switches, one accelerator module, laser seeding systems and short pulse instrumentation and diagnostics. 

Priorities will be based on an assessment of technical and cost/schedule risk. 

In summary, the proposed study outlines an integrated plan for development of the experimental 

program and the accelerator design, including a robust program to take advantage of many educational 

opportunities. We believe this approach, combined with conventional facility design, the development of 

a detailed project management plan, and focused R&D will support construction in 2007. 

75

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APPENDIX A: 

TECHNICAL CONCEPT 

The technical design of the facility relies on several key technologies that have been successfully 

demonstrated in recent years. Innovative experiments from the accelerator, synchrotron optics, and laser 

communities have overcome technological hurdles to enable the specification of a mature user facility 

drawing on the strengths of each of these fields. We envision a technical concept achieving complete 

integration of conventional lasers and experimental methods with those from the synchrotron and 

accelerator communities. 

This appendix is divided into sections that describe each of the technologies in more detail, from 

the photoinjector through many of the accelerator subsystems, to the FEL output. Although this proposal 

only sketches the technical detail necessary for a complete design, it does pay particular attention to the 

requirements on the conventional laser systems for several key challenges. These include producing the 

high quality electron beam, timelocking the electron beam with multiple external lasers to 

~10 femtosecond precision, and generation of ultrashort, tunable soft x-ray seed pulses. 

The electron beam provides a very flexible gain medium whose properties may be shaped for 

particular experiments. For instance, the FEL bandwidth is typically ~.001. Since the electron beam 

contains about 1 Joule of energy per pulse, it produces x-rays with up to millijoule pulse energy, many 

orders of magnitude above that available from harmonic generation by a tabletop laser. The FEL produces 

substantial power in low-order harmonics, which can be successively amplified in a cascaded scheme to 

frequency multiply a longer wavelength seed into a high-power, short-wavelength x-ray pulse. The wide 

bandwidth also supports ultrashort pulses, reaching below 1 fs for wavelengths shorter than 1 nm. Even 

shorter pulses, reaching farther into the attosecond regime, can be obtained by applying a time correlated 

energy chirp on the electron beam. This generates a chirped x-ray pulse that can then be optically 

compressed. Conversely a narrowband long pulse seed can be amplified with high fidelity for precision 

spectroscopy. 

A wide range of operating conditions are anticipated, starting with SASE output of long 

wavelength (100 nm), long pulse (~100 fs), and high pulse energy (mJ). We will extend this performance 

to reach short wavelength (0.3 nm), short pulse (1 fs), and moderate pulse energy (few µJ) from seeded 

operations. The broad spectrum of proposed user experiments requires such flexible, tunable FEL source 

characteristics. 

The demands of a multiple beamline user facility place burdens on the accelerator/laser complex 

that are routine in synchrotron facilities, but have not yet been addressed by existing laboratory lasers or 

UV FEL labs. These include very high reliability and uptime, automated operation, stability of operating 

parameters, and beam delivery to multiple beamlines. In addition, it will be necessary to synchronize the 

timing of multiple lasers at femtosecond timescales. The design of the facility will address each of these 

challenges. 

While we depend to a large extent on demonstrated technologies, they will be combined in new 

ways in the proposed facility so that the cumulative performance is strongly enhanced over SASE 

sources. The combined effects of the aggressive integration of conventional lasers, and design for seeded 

operation, result in x-ray pulses that are orders of magnitude shorter than SASE, with full coherence, and 

are timelocked to multiple lasers at femtosecond rather than picosecond timescales. 

A-1

A.1 ACCELERATOR OVERVIEW 

The FEL places stringent requirements on the quality of the electron beam. These requirements, 

along with the desired repetition rate and demonstrated performance history of different components, 

constrain the technological choices for the accelerator. The required electron beam performance is 

determined by numerical simulations. A very useful parameterization of FEL performance has been 

performed by M. Xie [1], which has been shown through both experiment and detailed time-dependent 

simulations to accurately reflect FEL performance. These fits are used here to estimate the required 

electron beam properties. The three primary measures of electron beam quality that determine 

performance are the transverse emittance, fractional energy spread, and peak current. Figure A1 illustrates 

the change in saturation length (i.e., required undulator length) at x-ray wavelength of 0.3 nm that occurs 

for varying these three parameters. This is the shortest wavelength under consideration, and so sets the 

most demanding requirements. 

It is evident that the FEL performance does not rapidly improve for peak current over 2 kA or 

energy spread below 3.0e–4, but that it always continues to improve when emittance is reduced. The most 

straightforward method of reducing the emittance is to reduce the amount of charge per pulse. It is 

important to understand that most measures of the FEL performance do not depend on the amount of 

charge produced, with the exception of total emitted energy. The gain length depends only on peak 

current, emittance, and energy spread, and the undulator parameters. We take advantage of this fact to 

improve the performance by studying reduced amounts of charge, in the range from 50 to 500 pC. This 

approach is described in Section A.2. 

Recent experience at UVFEL facilities has shown that the primary challenges for generating a 

high-quality FEL beam are to produce adequate emittance from the photoinjector at moderate beam 

charge, compress that electron beam to sub-picosecond levels without emittance growth, and suppress 

undesirable wakefield effects by proper control of the vacuum pipe impedance seen by the beam. 

Observed photoinjector performance decreases rapidly with increasing charge, and the best FEL results 

have been achieved at modest charges. The energy spread requirement is generally not challenging to 

meet, a fact that can be taken advantage of to best match the electron beam phase space to the FEL. 

Measurement and control of very high quality photon and electron beams at femtosecond timescales has 

improved markedly in recent years, so that beam properties on a 100 fs scale are routinely measured. The 

desire for ever shorter pulses and more complete control of all phase space dimensions requires that the 

fast pace of progress in controls and diagnostics continue. 

One of the key decisions to be made concerns the pulse train format required to optimize the 

science experiments. Ideally, the accelerator will run in continuous wave mode, so that there is maximum 

flexibility in the pulse train delivered, which then depends only on the laser timing. The current TESLA 

linac design runs pulsed at 1% duty factor, which reduces demand on the cryogenics and the facility 

electrical power by several MW compared to CW operation. As detailed in Section A.3, we will study the 

tradeoffs in technology and cost associated with operational mode. The injector is likely to be a room 

temperature cavity. Although it uses a small amount of power relative to the facility, heating of the cavity 

will limit the ability to run CW so that it is likely to be a pulsed device. Recently developed 

superconducting injectors [2–4] will be studied as an alternative. 

The sections below present a self-consistent proposal that describes how existing technologies 

have reached sufficient maturity to enable construction of a user facility. The design addresses each of the 

major subsystems and indicates the readiness of the systems to be assembled into a technologically 

A-2

Peak current (kA) 

Peak current (kA) 

3 

3 

2.5 2.5 

2 

2 

1.5 1.5 

40 40 40 40 40 40 

50 50 50 50 50 50 

60 60 60 60 60 60 

70 70 70 70 70 70 

80 80 80 80 80 80 

90 90 90 90 90 90 

100 100 100 100 100 100 

110 110 110 110 110 110 

120 120 120 120 120 120 

1 

1 

0.2 0.4 0.6 0.8 1 1.2 1.4 

Norm. emittance (um) 

dE/E (.01%) 

dE/E (x1.0e-4) 

5 5 

4.5 4.5 

4 4 

3.5 3.5 

3 3 

2.5 2.5 

2 2 

1.5 1.5 

1 1 

40 40 40 40 40 40 

50 50 50 50 50 50 

60 60 60 60 60 60 

70 70 70 70 70 70 

80 80 80 80 80 80 

90 90 90 90 90 90 

100 100 100 100 100 100 

110 120 

120 

110 110 

120 

120 

110 110 

120 

120 

110 

0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1 1 1.2 1.2 1.4 1.4 

Norm. emittance (µm) 

FIGURE A1 The contours show lines of constant undulator saturation length in meters for a laser operating 

at 0.3 nm. The length required becomes less sensitive to peak current above 2 kA (left plot) and relative 

energy spread below 3.0e–4 (right plot). In contrast, the FEL performance continues to benefit from 

decreasing emittance. 

feasible and cost effective facility. We rely on existing technologies to the greatest extent possible, and on 

the operating experience at existing FEL labs. This proposal does not contain the detailed specifications 

of a full proposal. We expect it to evolve into a mature and optimized design over the course of the study 

period. 

A.2 INJECTOR 

There are a number of candidates for the injector design, including RF or DC cavities with either 

thermionic or photo cathodes, high-power pulsed DC guns, and superconducting RF photoinjectors. 

Among the choices, the room temperature photoinjector stands out as the only one that has demonstrated 

the robust performance and high beam brightness required by an x-ray user facility. We expect to pursue 

the optimization of this solution while keeping abreast of developments in competing technologies. 

The injector is among the most critical accelerator components because its performance, 

including that of the drive laser and cathode material properties, largely determines the quality of the 

electron beam produced. Although the electron beam is accelerated and transported through hundreds of 

meters, its transport in the first few millimeters following the cathode presents the greatest challenge to 

generating the brightest beams. This is due to space charge effects that rapidly diminish as the beam 

reaches relativistic energy. Both simulation and analytical work [5,6] have shown that it is possible to 

generate a high charge (~1 nC) electron beam while preserving the very small phase space volume needed 

to drive an x-ray laser. These simulations run with distributions that are almost perfectly uniform. Such 

distributions have proven difficult to generate and measure experimentally. One thrust of the study will be 

improvements in the experimental methods used to shape and measure the temporal laser and electron 

beam properties on a timescale of tens of femtoseconds. A more detailed description of the drive laser is 

given in Section A.4.1. 

In practice, the beam quality requirements on the longitudinal dimensions of energy spread and 

bunch time duration have proven straightforward to meet. The principal challenge remains the production 

of transverse emittance less than 1 µm for a short bunch of high charge. Photoinjector experiments have 

A-3

succeeded in generating emittance under 2 µm at moderate charge [7], which is adequate for the proposed 

facility, but have fallen short of the very low emittance (~0.5 µm) reached in simulations. Recent 

experimental studies indicate that temporal and transverse structure in the laser profile [8], and 

non-uniform emission from the cathode, create non-uniform electron distributions that then experience 

rapid emittance growth due to non-linear space charge forces before reaching relativistic energy. 

We expect to choose the injector RF frequency to match the linac at 1.3 GHz. A good solution for 

the injector is the one in use at the TTF at DESY. This is a pillbox cavity design 1.5 cells in length with 

on-axis RF coupler and careful symmetrization to remove undesirable deflecting modes from the cavities. 

It uses a Cs2Te cathode excited by 266 nm photons. The proposed facility will likely have a different 

pulse format than TTF, which is optimized for collider operations. 

The performance of this injector design can be enhanced in several ways. One is through velocity 

bunching [9,10], where an energy chirp at low energy at the gun exit causes bunch compression by 

imparting higher velocity to the tail than the head. Velocity bunching in a straight line can be useful to 

help generate the shortest bunches while reducing the degrading effects of coherent synchrotron emission 

that occur during compression by a magnetic chicane at high energy. The addition of one or more 

independently phased RF cells for generating energy chirp will be studied for its ability to produce a 

shorter bunch while maintaining low emittance. Two standard codes developed at LANL will be used to 

design the injector cavity (code SUPERFISH [11] and model the beam physics (code PARMELA [12]). 

Members of the project team have extensive experience [13,14] with these codes, including successful 

comparisons of simulation results with sub-picosecond measurements of beam performance [15]. 

The pillbox design has high shunt impedance for efficient acceleration, but suffers from localized 

areas of high heat load that limit the ultimate gradient and/or repetition rate. An alternative is to design a 

reentrant cavity [16] similar to the high power cavities used in the SLAC B-factory. This choice allows 

high gradient and moderate power consumption for operation at the highest repetition rates. Optimization 

of such a cavity will be explored in collaboration with groups at LBNL and BNL. The duty factor and 

time structure of the bunches is influenced by the beamline user needs, and abilities of the drive laser and 

linac. It will be determined during the course of the study. 

Common photocathode materials for RF photoinjectors include Cs2Te, Cu, Mg, K2CsSb, and 

GaAs. The chosen cathode should be robust in the accelerator environment with high quantum efficiency 

and low thermal emittance, and exhibit prompt and uniform emission. The thermal emittance is 

determined by the momentum spread of the distribution of emitted electrons exclusive of the effects of 

space charge. It is important because this is the minimum possible emittance for a given material and laser 

wavelength. The x-ray laser properties depend so strongly on emittance that an improved knowledge and 

reduction of thermal emittance is certain to have a large impact on performance. It will be a focus area of 

the study and for the R&D program. We will work with condensed matter faculty at MIT to advance the 

state of the art for cathode performance. Of the materials listed, Cs2Te appears today to be the best choice. 

It has demonstrated months of operational lifetime while producing beam of sufficient quality to lase at 

90 nm at DESY. 

The RF system that drives the photoinjector must achieve a high degree of stability in amplitude 

and phase. To reach the timing goal of 10 fs for the electron beam relative to an external laser, 

PARMELA simulations indicate that the RF amplitude must be stable to 2.5e–4 at a peak gradient of 

60 MV/m, and the RF phase must be stable to .05 degrees at 1.3 GHz. These are challenging but 

attainable specifications. The design and prototyping of a low level RF system demonstrating this 

performance is included as part of the R&D program proposed in Appendix B. 

A-4

A superconducting photoinjector, if it could be shown to be robust, has the advantage of enabling 

CW operation, which gives maximum flexibility in pulse format for the beamlines. This technology has 

only recently seen its first demonstration. As part of the study we will examine its benefits and the 

prospects for producing a robust design in a timely manner. 

A.3 SUPERCONDUCTING LINAC 

The feasibility of producing high-brightness, high-repetition-rate electron beams from a 

superconducting linac has been demonstrated at DESY’s TTF [17] and at TJNAF [18]. The important 

advantage of superconducting cavities is their extremely low electrical surface resistance at a temperature 

of 2 K, reducing rf losses by 5 to 6 orders of magnitude relative to copper cavities. We expect to choose 

the 1.3 GHz structures developed at DESY due to their advanced state of development and commercial 

availability. Other important advantages of these structures are their relatively large apertures, resulting in 

relaxed alignment tolerances and insensitivity to transverse and longitudinal wake-fields, and the 

availability of existing klystrons appropriate for our application. 

The current third-generation cryomodule shown in Figure A2 contains a stack of eight nine-cell 

cavities, and is 12 m long including its associated quadrupole. The cavities are made from solid pure 

niobium and are cooled with liquid helium to 2 K. The power dissipation in the cavity wall is extremely 

small, which allows the generation of the required rf accelerating field gradient with long-duration, lowpeak-power 

rf pulses and yields a high transfer efficiency of rf power input to beam energy. In addition to 

the accelerating cavities, each cryomodule contains a quadrupole magnet, steering coil, and beam position 

monitor. The HOM-couplers and power coupler with waveguide transitions are integrated, as is the 

helium distribution system needed to operate at 2 K. The low average beam current in the proposed 

machine will yield low HOM power so that existing couplers are sufficient. 

The RF coupler used at TTF is a coaxial design that can provide a factor of 6 range of adjustment 

in external Q. At TTF it is set for a Q of 3 × 10 6 to provide the best match to the beam loaded cavity, 

where the beam current during the macropulse is ~9 mA. This is much higher than the ~10 µA of the 

proposed machine. In the latter case, the beam loading is negligible so that the Q can be raised for 

improved RF efficiency. The upper limit is set by the need to compensate frequency detuning due to 

mechanical vibrations (microphonics) and electromagnetic forces (Lorentz detuning). For the existing 

cavity, the optimum external Q is 2.6 × 10 7 in the absence of beam loading [19], yielding a RMS 

bandwidth of 25 Hz, which is adequate to control detuning. An active effort is underway among Cornell, 

BESSY, DESY, and MIT to optimize the cryomodule for CW operations. The coupler has been tested to 

1.5 MW peak power [20] at a 1% duty factor to yield 15 kW average power. This is sufficient to support 

gradients up to 25 MV/m (Table A1). 

The achievable gradient has been steadily increasing as the manufacturing process has matured so 

that 25 MV/m is routinely achieved and the best electro-polished cavities exceed 35 MV/m [20]. The 

choice of operating gradient depends on balancing initial capital costs (linac length) with operating costs 

(power consumed, klystron cost), and on whether pulsed or CW operation is selected. Table A1 lists 

parameters for the linac RF and cryogenics at different gradients. Power consumption costs can be 

reduced by an order of magnitude from the peak numbers reported in the table by running at 10% duty 

factor. It is assumed that the helium refrigerator can achieve ~30% of the Carnot efficiency, and that the 

klystrons and high power modulators can achieve 50% wall plug efficiency. CW operation offers the 

greatest flexibility for the experimental program and the best stability, but at additional cost. The optimal 

running configuration, whether pulsed or CW, will be determined over the course of the study by 

examining the needs of the science users and performing a detailed cost analysis. 

A-5

FIGURE A2 The nine-cell superconducting cavity and LHe vessel developed at DESY for the TESLA 

collider and FEL. 

TABLE A1 Facility heat load, RF, and AC power requirements 

for various operating gradients. 

Gradient (MV/m) 

R/Q (Ohm) 1020 1020 1020 

Q0 (1 × 10 10 ) 1.3 1.3 1.3 

Qext (1 × 107 ) 2.6 2.6 2.6 

RF power per cavity (kW) 4 12 23 

Dynamic heat load per 

cryomodule at 2 K (W) 

270 750 1470 

Static heat load at per 

cryomodule at 2 K (W) 

2 2 2 

RF power per klystron (kW) 5 15 29 

Number of klystrons 267 160 114 

Total cold length (m) 378 227 170 

Number of cryomodules 22 13 10 

Total heat load at 2 K (kW) 6 10 15 

Refrigerator wall power (MW) 3.6 6.0 9.0 

Total RF power (MW) 1.3 2.4 3.3 

RF wall power (MW) 2.6 4.8 6.6 

Total facility power (MW) 8 12 18 

A-6 

15 

25 

35

One of the primary technical goals is to reduce timing jitter to less than 10 fs between the FEL 

output and external lasers. This in turn places strict requirements on the electron beam energy stability. At 

relativistic energy, the timing jitter is dominated by differences in path length due to energy jitter in 

combination with longitudinal dispersion in bends. The design of the facility will minimize the number of 

bends, which should consist only of the compression chicanes and isochronous bends into the undulators. 

The total longitudinal dispersion (R56) will be kept to ~50 mm as in the LCLS design [21]. Relative 

energy jitter must then be kept below 6 × 10 -5 . This is a specification that TJNAF meets [22] in operation. 

Powering each cavity independently loosens the tolerance on the individual klystrons, so that the 

uncorrelated amplitude jitter per klystron must be below ~8 × 10 -4 , and the phase jitter less than 

2 degrees. Separate klystrons for each cavity also provide more flexible energy tuning by allowing the 

gradient to vary with each cavity, and make the operation less sensitive to individual klystron failures. 

The low level RF phase and amplitude control will be based on vector I/Q modulators and demodulators 

controlled by digital signal processors. The system used at TTF [23] serves as a baseline design. Feed 

forward will be used if necessary to reduce repetitive fluctuations, and feedback will reduce slow thermal 

or mechanical drifts. The high Q of the superconducting cavities acts as a lowpass filter to remove noise 

above ~100 Hz. 

Magnetic bunch compressors consisting of four dipole chicanes will be used to compress the 

picosecond beam produced by the injector to a pulse length of tens of femtoseconds at the undulators. The 

most likely configuration is to use two chicanes: one at ~250 MeV, which is high enough to avoid space 

charge effects, and the second at ~1 GeV near the first set of undulators. Bunch compression is 

accomplished by applying an energy-time correlation (chirp) using an RF section so that the bunch head 

is at lower energy than the tail, and then passing the bunch through the four dipole magnets where the 

shorter path taken by the tail causes it to catch up to the head. The minimum bunch length is determined 

by the shape of the energy distribution. The chirp applied is linear to high accuracy; however second- and 

third-order distortions in time caused by RF and space charge forces respectively limit the minimum pulse 

length. Space charge distortions may be reduced by lowering the charge, which in turn produces shorter 

bunches exiting the injector. These short bunches then reduce the RF distortion because the bunch sees 

less RF curvature. The lower charge bunch produces a more linear chirp allowing better magnetic 

compression. Another effective means of reducing the second-order RF distortion is to linearize the 

energy distribution with a harmonic RF cavity [24]. This option will be studied as part of the simulation 

effort. Harmonic cavities can also efficiently provide the energy chirp for bunch compression. 

Traditionally this has been done by setting the phase of the accelerator tanks off-crest, but doing so has 

drawbacks: the acceleration efficiency is reduced, RF curvature is present for phases other than the zero 

crossing, and the slope of the fundamental frequency is not as steep as a harmonic. 

The emission of coherent synchrotron radiation (CSR) by short bunches in the chicane dipole 

magnets is a significant concern. CSR causes effective emittance growth because the energy loss is not 

uniform along the bunch length, resulting in mismatched trajectories for different time slices when the 

bunch exits the dipole. The emitted CSR scales as the square of the charge, so that working with reduced 

charge is again advantageous to improving the beam brightness. In addition to emittance growth, CSR 

emission can lead to an exponentially growing instability that causes a large beam energy/time 

modulation [25]. The accelerator codes PARMELA (up to ~250 MeV) and ELEGANT (above 

~250 MeV) include CSR physics and will be used to generate detailed particle tracking for the proposed 

design. Together, these codes are capable of modeling all of the important physics effects starting with 

realistic initial distributions at the cathode and including space charge, structure misalignments, RF jitter, 

and CSR effects. 

A-7

The distribution of the electron beam to multiple undulators in a way that is most useful to the 

various experimental users will require careful study . The proposed facility includes three extraction 

points from the linac, each serving up to ~5 undulators. Extraction at different energies serves the purpose 

of a wide tuning range in wavelength for the optical beamlines. The limitation of the number of extraction 

points minimizes the required extraction hardware which would add length and expense to the linac, but 

complicates the beamline switchyards. The proposed pulse structure allows the use of conventional ferrite 

kickers followed by magnetic septa. The kickers must have deflection angles of a few mr at momenta up 

to the full linac energy, rise and fall times of

Master 

oscillator 


amplifier 

1 GeV 

Seed laser 

2 GeV 

UV Hall X-ray Hall 

Pump 

laser 

Seed laser 


100 nm 

30 nm 

10 nm 

Seed laser 

SC Linac 

4 GeV 

10 nm 

3 nm 

1 nm 


Nanometer Hall 

Fiber link synchronization 

Pump 

laser 


FIGURE A3 Possible layout for the overall laser system to be studied and developed for the proposed 

facility. All lasers, including those for the photoinjector drive, FEL seed, and pump-probe experiments, are 

clocked to a master oscillator. The photoinjector system starts with a 10 ps, 80 MHz Ti:Sapphire oscillator 

followed by a chirped pulse amplifier to reach 100 µJ, 10 ps pulses at a variable repetition rate of 1–10 kHz. 

The cathode material chosen sets requirements for the laser energy and wavelength. During the 

course of the study, the advantages of different materials will be investigated. For the purposes of the 

preliminary design, Cs2Te is chosen due to its demonstrated high quantum efficiency (>1%) and robust 

lifetime of months of operation. In contrast, metal cathodes such as Cu and Mg have at least one order of 

magnitude lower QE although they have outstanding RF and vacuum properties. Semiconductors such as 

GaAs or CsKSb have very high QE at longer wavelengths, but have not yet demonstrated sufficient 

lifetime at high voltage. The thermal emittance [27,28] of each material is ultimately the limiting factor 

for high performance at FEL wavelengths under 1 nm. R&D toward improved thermal emittance will be 

proposed for the latter half of the study period. 

Assuming CsTe as the cathode and a desired charge per bunch of 50–500 pC, the photoinjector 

drive laser is required to generate 0.1–1 µJ per pulse at 266 nm with a pulse length in the range 1–10 ps 

and a repetition rate of approximately 10–20 kHz. The precise wavelength necessary will be determined 

during the course of the study to optimize the electron beam quality. One standard system for achieving 

the necessary performance consists of a high-repetition-rate 100 MHz Ti:sapphire oscillator with center 

wavelength of 800 nm and a pulse length of about 50–100 fs. The pulses are selected with a pockels cell 

and amplified in a chirped pulse amplifier to about 10–100 µJ. These pulses are then tripled in BBO to 

achieve the desired UV-pulses at 266 nm with a pulse energy variable between 0.1–1 µJ assuming a 10% 

conversion efficiency and some power loss due to pulse shaping. The Ti:Sapphire laser technology 

proposed here is mature and well-developed and can be bought directly from laser manufacturers. 

Depending on the final pulse width that shall be achieved as well as the wavelength, alternative laser 

A-9 

1 nm 

0.3 nm 

Pump 

laser

systems that are more cost-effective and more reliable like Nd:YLF or Yb:YAG based systems shall also 

be considered in the study. These systems can be directly diode pumped and, therefore, operate much 

more reliably and at lower cost. 

A.4.2 Seeding of the FEL with High Harmonics 

All of the undulator lines at the proposed facility will have the option of generating light using the 

SASE process. However, amplifying a low power seed rather than noise has distinct advantages, 

including greater wavelength and energy stability, generation of optical pulses much shorter than the 

electron bunch, and improved time synchronization of the FEL output with other experimental lasers. The 

seed wavelength can be made tunable when generated by an optical parametric amplifier or by selecting 

among closely spaced high harmonics. 

For wavelengths longer than 180 nm, seed radiation can be generated with traditional nonlinear 

optical processes, like second or third harmonic generation and cascaded combinations thereof. In the past 

few years, it has also been shown that the generation of high harmonics in gases can provide an efficient 

pathway to coherent radiation at wavelengths shorter that 180 nm, in the VUV and soft x-ray region. This 

has been made possible by the availability of high energy laser pulses comprised of only a few cycles of 

light, a new dimension in controlling light and in controlling matter with light. Over the last two years it 

has become possible to obtain full control of this carrier-envelope phase and therefore over the electric 

field of these short pulses [29,30]. The MIT Ultrafast Optics Group has developed ultrabroadband 

femtosecond Ti:Sapphire lasers that can be phase stabilized directly from the laser [31–33]. Such a laser 

can be used as a seed laser for an amplifier delivering high energy phase-controlled femtosecond pulses. 

It has been demonstrated by the Colorado and Vienna groups that it is indeed possible to achieve soft 

x-ray radiation up into the few nm range [34,35]. Very recently, the Vienna group demonstrated the 

generation of attosecond XUV-pulses by exploiting the spectral dependence of the high-harmonic 

generation on the phase of the femtosecond pulse that produced these harmonics [36]. The energy, 

temporal shape, and timing of these XUV pulses depend sensitively on the carrier-envelope phase as 

shown in Figure A4. The first time-resolved studies using those pulses were also reported [37]. The basis 

for these achievements is the development of intense few-cycle laser pulses with pulse durations as short 

as 5 fs and peak powers exceeding 0.1 TW [38], which have now been available for several years and 

which can be used for high-field experiments [39]. A different carrier envelope phase leads to a different 

maximum field strength within one pulse. 

φ=0 

φ=π/2 

x-ray harmonic 

emission 

-4 -2 0 2 4 

Time, fs 

FIGURE A4 Phase controlled few-cycle laser pulses and XUV emission by high harmonic generation. 

In the absence of stabilization and control of the carrier-envelope phase, all these parameters are 

subject to strong fluctuations. As a consequence, reproducible production of sub-femtosecond XUV 

pulses will rely on driver pulses with reproducible carrier-envelope phase in addition to a well determined 

A-10

intensity envelope. As stated above, the reliable generation of few-cycle pulses with fixed carrierenvelope 

phase has been achieved recently by heterodyning different parts of the laser spectrum with each 

other [40–44]. These key technologies form the basis of the research project proposed here. Reliable 

control of the phase in high energy pulses is not yet achieved. In this project, we propose to accomplish 

this task and to use such a laser to generate femtosecond and attosecond x-ray pulses for use as a seed for 

the MIT/Bates laser. These objectives are equally important for the science described in Section 1.2.6, as 

well as for the FEL seeding, and are summarized in the following section. 

Laser Research Program. A central goal of this study is the development of a multi-kHz source 

of intense 20–30 fs pulses, as well as phase-controlled few-cycle (sub-5fs) light pulses and their full 

characterization and control with respect to intensity, shape, pulse width and carrier-envelope phase. 

These high-intensity pulses with precisely known phase can be used for the generation of femtosecond 

and attosecond seed pulses in the XUV and soft x-ray regime. The objectives of the above proposal will 

be pursued in the framework of the following research program. 

(a) We specifically want to develop a 20–30 fs 5–10 kHz Ti:sapphire amplifier system 

with special stretcher compressor design to achieve phase stable amplification [45]. 

For higher-order dispersion control we can use our expertise in chirped mirror design 

[46,47]. To investigate very high-harmonic seeding a phase-stabilized sub-5fs, 

mJ-scale near-infrared lasers at multi-kHz repetition rate with high energy stability 

better than 1% rms shall be developed. An alternative approach for the generation of 

few-cycle high energy phase stabilized optical pulses is by coherent addition [48] of 

already phase-controlled microjoule pulses from a long cavity laser [49,50]. Note that 

due to the coherent addition of the pulses in the last stage, no stretcher and 

compressor has to be employed. Therefore, carrier envelope phase changes can be 

monitored and actively controlled up to the last stage. This proposed overall system 

should greatly improve the phase stability of the pulse generation. 

(b) Single-shot measurement of the carrier-envelope phase of few-cycle high energy 

pulses is necessary. Two different approaches shall be implemented. The perturbative 

nonlinear optics approach—by spectral interference between the broadened laser 

pulse and its second harmonic using bulk self-phase modulation and the extreme 

nonlinear optics approach—using carrier-wave Rabi flopping in semiconductors 

[40,43,44]. 

(c) Efficient high-harmonic generation in atomic gases for the purpose of seeding the 

FEL-amplifier shall be investigated in the range of 100–1 nm. For successful seeding 

of the MIT/Bates FEL-amplifier pulse, energies of about 1–10 nJ for wavelength 

around 30 nm, i.e., around the 25 th 

harmonic are desirable. Using quantum und 

semiclassical theories for high-harmonic generation [51,52] we propose to investigate 

the relationship between pulse shape and carrier-envelope phase as well as efficiency 

and frequency cut-off of the high-harmonic spectrum. The goal is optimization for 

high efficiency and/or single attosecond XUV pulse generation. The group of 

Midorikawa et al. achieved 300 nJ soft-X ray radiation at the 27 th 

harmonic (about 

30 nm) using 16 mJ, 35 fs pump pulses [53]. For such long pulses phase control is 

not necessary. In this study, we propose to demonstrate 10 nJ seed pulses at 30 nm 

generated from 1 mJ optical pulses using 20–30 fs pulse focused in Argon [54] using 

improved phase matching conditions [55–57]. High harmonic generation with phase 

controlled sub-5 fs long high energy pulses may extend the seed radiation into the 

A-11

soft x-ray regime down to a few nanometers. Optimization of this process by 

coherent control techniques is likely to improve efficiencies dramatically. Schnürer 

et al. has studied XUV generation using few-cycle laser pulses. Even without phase 

control conversion efficiencies of about 10 -6 up to the 30 th harmonic [58] have been 

achieved in Argon. In neon, typical efficiencies of 10 -8 up to the 75 th harmonic are 

reported, resulting in 10 pJ pulses, too small yet to be useful for seeding. 

These emerging tools will open up new chapters in ultrafast science by enabling control of matter on 

a sub-light-cycle (i.e. attosecond) time scale as well as tracing, with the same time resolution, the 

processes that can be triggered and controlled. 

A.4.3 Femtosecond Phase Locking of Multiple Lasers 

A schematic of the overall laser system is shown in Figure A3. It is comprised of the master 

oscillator, photo-injector laser, seed laser system that is located about 300 m downstream of the 

photoinjector, and pump-probe lasers. For successful seeding of the FEL with femtosecond, and later 

even attosecond, VUV and soft x-ray pulses, a precise synchronization of the seed pulses has to be 

maintained to much better than the electron bunch length of ~100 fs. A timing jitter on the order of 10 fs 

corresponding to a few micron spatial offset between the seed pulse and the electron bunch must be 

maintained. At first this might seem unachievable, however, breathtaking advances over the last years in 

frequency metrology based on ultrafast lasers and, therefore, also in laser stabilization and 

synchronization, show that such low timing jitters between different laser systems can be achieved and 

maintained over arbitrarily long times and also distances of several hundred meters. The team at JILA and 

University of Colorado achieved synchronization of two independent ultrashort pulse Ti:sapphire lasers 

with pulse width around 50 fs with relative timing jitter of 1.7 fs over 2 MHz of bandwidth by locking 

their repetition rates at the fundamental and 100 th harmonic frequency via phase-locked loops [59]. Our 

group invented and demonstrated very recently an optical version of a balanced homodyne detector, i.e., a 

balanced cross correlator. This device made it possible to synchronize two independent and completely 

different laser systems (a sub-10 fs Ti:sapphire laser operating at an 800 nm center wavelength and a 40 fs 

Cr:Forsterite laser operating at 1240 nm wavelength with a relative timing jitter of only 300 attoseconds 

over a bandwidth of 2.3 MHz. Figure A5 shows a schematic of the experiment. 

The balanced current of the cross-correlators, as shown in Figure A5, is proportional to the time 

difference between the two pulses and the sign of the current carries information on the direction of the 

detuning (see Figure A6 (b)). Therefore, in the vicinity of zero de-timing this detector acts like a phase 

detector operating in the multiple THz range. At the zero-crossing of the photocurrent, this detector 

delivers a perfectly balanced signal, and therefore, amplitude noise of each of the lasers does not affect 

the detected phase. 

The signal from the balanced mixer is used to lock the repetition rates of the two lasers by 

controlling the cavity length of the Ti:sapphire laser with PZT-mounted cavity mirrors. Figure A6 shows 

the results of a timing jitter measurement with the out of loop cross-correlator. The remaining timing jitter 

at the detector’s bandwidth of 2.3 MHz revealed 299 as ± 104 as. Noise beyond this bandwidth is 

negligible. The stated error is determined from the amplitude noise measured at the peak of the crosscorrelation. 

These results show, that we are well prepared to precisely lock the photo-injection laser to the 

A-12

FIGURE A5 Experimental setup of the synchronized lasers. Cr:fo:Cr:forsterite laser, Ti:sa:Ti:sapphire 

laser; SFG: sum-frequency generation; bandpass filters transmit only the sum-frequency (496 nm = 833 nm + 

1225 nm). The third correlator is used to generate the plots shown in Figure A6 (a). 

(a) (b) 

FIGURE A6 (a) Timing jitter determined from the amplitude noise of the SFG of a third cross-correlator out 

of loop, see Figure A5. The rms-jitter measured in a 2.3 MHz BW results in 299 as ± 104 as. (b) shows the 

output of the balanced cross-correlators as a function of a temporal detuning between the two laser pulses. 

laser creating the XUV-seed pulses within the desired precision. Today, the photo-injection is typically 

carried out with a 10 Hz, 1 mJ, amplified 1 ps-Ti:Sapphire system starting from a high repetition rate, 

80 MHz seed laser. From the results described before, it is easily possible to establish synchronization 

between the 10 ps seed oscillator for the photo injector and the sub-10 fs seed laser for EUV-XUV 

A-13

generation. However this is not enough. Timing jitter can build up between the electron bunch and the 

XUV-seed due to the following processes: 

(a) The seed pulse of the XUV-system or for the photo injector has to be propagated over 

300 m in fiber or vacuum to establish the synchronization. Thermal expansion, 

acoustics and other environmental influences will introduce timing jitter on the order 

of mm, i.e., ps, which has to be compensated. However, this can be overcome. 

Delivery of optical clock signals over distances of several hundred meters in fibers 

without deterioration of the timing and phase jitter has been demonstrated recently by 

active monitoring and control of the fiber length [60]. Similar techniques can be 

explored in the first two years of the study. 

(b) Phase jitter between the accelerator RF and the photoinjector laser causes variations 

in the electron beam time to exit the photoinjector. The phase jitter is demagnified 

because the velocity variations diminish as the beam becomes relativistic. 

PARMELA simulations show that the phase jitter must be kept below 0.05 RF 

degrees at 1.3 GHz to keep the timing jitter less than 10 fs at the photoinjector exit. 

Existing photoinjector facilities achieve jitter of ~0.1 degrees. This is a challenging 

specification, but can be met with careful engineering. 

(c) Velocity variations due to energy and phase jitter are negligible downstream of the 

photoinjector. However any amplitude or phase jitter in the RF will be translated to 

time jitter by dispersive elements such as the bunch compression chicanes. The 

chicane R56 will be approximately 30 mm which implies that the energy jitter must 

be less than 0.01% to meet a timing jitter specification of 10 fs. 

All of these processes shall be analyzed theoretically to understand the scaling of the timing jitter 

introduced as function of the stability of the applied RF-field. Direct experimental studies will also be 

carried out to the extent possible in the first two years of this study. 

Development Program. The task of the first 1.5 years of this study is to develop the three 

alternative laser systems as described in Section A.4.2 for high harmonic generation and to demonstrate 

high-harmonic generation. The funding for this laser system shall come from other sponsors. We expect 

to use support from this study proposal for one postdoctoral researcher who will support the experimental 

and theoretical effort necessary for the layout of the laboratory laser system that shall later be employed at 

Bates. This initial study should show which kind of driver laser system is the most promising one for the 

XUV-seed generation for the MIT/Bates facility. The two laser systems to be developed in this project 

(first 1.5 years) can be set up in a recently renovated ultrafast laser laboratory on the MIT campus. The 

laboratory is equipped with the basic electronic and optical instrumentation as well as standard 

femtosecond laser diagnostics. The phase stabilization and locking techniques are being developed 

currently within a multi-disciplinary university research initiative (MURI) at MIT on “Enabling 

Technologies for Optical Clocks,” which involves researchers from the Physics, Electrical Engineering 

and Computer Science and the Material Science and Engineering Departments. 

In the second 1.5 years of the study a prototype of the overall laser system can be developed. It 

comprises the photo-injector laser, the drive laser for the XUV-seed and the synchronization between 

them. During that period of the study one of the two concepts of the drive laser shall be developed to an 

extent that will show the full capabilities of such a system for seeding. This will most likely involve new 

powerful pump lasers with more than 100 W of average power at 532 nm (currently, the most powerful 

A-14

laser is the Corona from Coherent delivering up to 85 W average power) and special custom designed 

laser optics, which will be available at that time. 

A.5 UNDULATORS 

We anticipate developing different types of undulators to accommodate the broad tunable range 

of photon wavelengths. Undulator length will vary from about 10 meters for long wavelength (100 nm) to 

60 meters for short wavelength (0.3 nm). Long undulators will consist of separate segments and short 

break sections where beam focusing quadrupoles, orbit correction magnets, phase adjusters and diagnostic 

devices will be located. For most of the undulators, each undulator segment will be a permanent-magnet 

planar hybrid device. The technologies to produce such undulators are well established in many existing 

devices, notably VISA [61] and LCLS [62]. A picture of the LCLS structure is shown in Figure A7. 

Table A2 lists the parameters of six possible undulators for the proposed facility. The study will have 

several objectives with respect to undulator and undulator system technology. 

Undulator parameters. The study will deliver the specifications of a set of undulators. The 

tradeoffs between period, peak field and electron energy will be evaluated to specify undulators which 

will as broadly as possible cover the wavelength and brilliance requirements of the varied users. 

Undulator segment and break section length will be optimized for all beamlines. A few standard undulator 

segments will be finalized as the building blocks for most of the undulator lines. This study will help to 

minimize the total undulator length, a major cost component of the proposed facility. 

Undulator Line Electron Beam and Optical Beam Diagnostic and Control. Optimal operation 

of FEL, electron beam, and x-ray diagnostic along the undulator lines are essential. The electron beam 

diagnostics include position and transverse profile monitors. Optical diagnostics may include spectrum, 

power, and time profile. The proposal study will deliver specifications for diagnostic and control of 

different undulator lines based on existing and developing technologies, such as the use of electro-optic 

crystals, and cross-correlators for the hard wavelength range. Standardization of these combined function 

break sections is of high priority. 

FIGURE A7 View of the LCLS undulator segment short model. 

A-15

TABLE A2 Initial estimates of undulator parameters for different photon wavelengths 

assuming a planar hybrid magnet. 

Photon energy (keV) 4.43 1.73 0.247 0.082 0.041 0.0124 

Beam energy (GeV) 4.0 4.0 2.5 2.5 1.0 1.0 

Photon wavelength (nm) 0.28 0.72 5 15 30 100 

Undulator period (mm) 18 30 30 45 30 40 

Peak magnetic field (T) 0.80 0.70 1.36 1.31 1.30 1.34 

Minimum gap (mm) 6 6 6 6 6 6 

Undulator K 1.35 1.96 3.74 5.48 3.65 5.66 

Length (m) 56.1 40.0 13.0 13.2 10.0 10.6 

Variable Gap Undulators. It is expected that users will require a tuning range in each of the 

delivered photon energies. The straightforward technique of varying the electron energy may not be 

practicable for most of the beam lines. Adjusting the undulator gap will be the main technique for finetuning 

of photon wavelength. It will be challenging to maintain precise gap control while allowing 

substantial tunability. The following parameters will be gap dependent: photon beam phase matching 

from different undulator segments, field integral errors, and required undulator length. A gap-dependent 

phase shifter and orbit correction will be implemented. On the other hand, gap variation will have great 

advantages on a number of issues for undulator line operation: accurate tuning for different segments, 

serving as a taper to increase FEL efficiency to compensate e-beam energy loss by synchrotron radiation 

and wakefield, changing effective length of a undulator line by opening the gap of a segment etc. Study is 

needed to identify all the photon wavelength tuning needs and technical challenges for variable gap 

undulators. 

Influence of Undulator Errors. Analysis will be performed to examine undulator error influence 

on FEL performance. Beam-based orbit correction procedures will be required in addition to realistic 

tolerance specifications for undulator field error and quadrupole misalignments, thermal, and long-term 

stability considerations. The requirement for an on-site undulator measurement facility will be examined. 

Undulator Vacuum Chamber Wakefield Effects. Wakefield effects will generate energy loss, 

increase energy spread, and may cause emittance growth if transverse wakes are excited. Wake 

amplitudes depend on bunch length, peak current and vacuum chamber impedance. Simulations will be 

carried out for all types of beam lines to evaluate the effects and specify the requirements for undulator 

vacuum chamber properties like surface roughness, conductivity, etc. 

Radiation Damage and Collimator. Evaluate radiation damage level to different undulators and 

assess the needs of beam collimators for individual lines. 

Superconducting Undulators. The superconducting undulators allow the production of higher 

fields at shorter undulator periods. A structure manufactured by ACCEL is now advertising a period of 

14 mm with a peak field of 1.3 T. The study will examine the feasibility of using these new devices. 

A-16

A.6 CONTROLS AND DIAGNOSTICS 

The installation of an integrated, reliable state-of-the-art controls system will be vital to the 

effective operation of the proposed facility. Effective, routine production of the x-ray laser beams requires 

stable hands-off operation of the accelerator for long time periods. The study will deliver the design of a 

control system that meets these needs. This system will take advantage of the global software effort which 

has generated the ExPerimental and Industrial Control System (EPICS) that has greatly enhanced 

machine performance and reduced development cost at many laboratories worldwide. The control system 

should allow flexible operation, automatically retuning accelerator parameters for different configurations 

and operating modes. During the study we plan to make special effort to integrate high-level accelerator 

and optical physics models into the control system to provide even greater optimization of beam delivery 

and quality to the end users. Online experimental analysis and integration of experimental results with the 

accelerator lattice models and optical beamlines will be important. Standard tools such as Matlab and 

Mathematica will be interfaced to the control system. 

The installation of accurate and reliable electron and photon beam diagnostics will be essential to 

provide monitor points for the control system above. This study will make selections of the most suitable 

diagnostics for these ultrashort femtosecond pulses both for electron and x-ray beams. Where possible, we 

will focus on the use of non-destructive measurements such as RF pick-ups and electro-optic crystals. The 

design of the diagnostics will be matched to the expected ~1 kHz repetition rate of the accelerator. 

A.7 RADIATION BEAMLINES 

The superior characteristics of the new photon beams present challenges to the beamline designer. 

For example, the proposed photon beams will deliver many orders of magnitude higher peak power than 

third-generation sources, and the photon pulse duration will be nearly three orders of magnitude shorter. 

Fortunately, some of these challenges have been addressed in laser experiments. Ultimately, the focused 

beam from the facility will deliver a peak power exceeding 10 16 W/cm 2 . While this is many orders of 

magnitude higher than the focused beams at the Advanced Photon Source, it is considerably lower than 

many terawatt lasers produce. The penetration of the high power x-rays into matter will require one to 

address new physical phenomena involved in energy-matter interactions. However, both theoretical 

calculations and early experiments at the DESY TTF facility show that the nonlinear effects in such 

interactions are small enough that they are not likely to play an important role in ultimate design of the 

beamline optical components. 

Many experiments will require a femtosecond pump pulse to be synchronized with a femtosecond 

probe pulse, or with a detector. In the time domain of interest here, the diffraction process from optical 

elements will be fundamentally different from conventional synchrotron sources because the pulse length 

and bandwidth are close to the transform limit. In some cases the energy bandwidth of the reflection may 

be narrow enough to stretch the pulse. All these aspects are currently being addressed in various 

femtosecond x-ray demonstration experiments, which use either HHG sources or sliced electron bunches 

in storage ring facilities. These sources deliver modest photon flux, but do provide a test bed for (1) a 

careful study of the beam transport using various optical elements, and (2) development of diagnostic, 

synchronization, and detection methods suitable for the temporal domain. The linac-based Stanford 

Pulsed Photon Source (SPPS) project, which will be operated during 2003–2005 as a precursor to the 

LCLS project, will develop some of these instruments and techniques for the LCLS and provide 

important data and proof-of-principle for use in the proposed MIT/Bates facility. 

A-17

A.7.1 Beam Transport and Optics 

The beamlines at the MIT/Bates facility will include optical elements needed to tailor phase space 

parameters to meet experimental requirements. The x-ray optics system must filter the intensity, spectral, 

and spatial characteristics of the FEL beams as needed for the experiments and transport it to the sample. 

In contrast to third-generation sources, there is very little average power in the FEL beams (< 1 W). 

Therefore, most of the techniques to perform these functions (slits, absorbers, mirrors, monochromators, 

etc.) are straight-forward extensions of methods commonly used at synchrotron sources. The FEL beams, 

on the other hand, generate very high peak power densities (10 13 –10 16 W/cm 2 ). The main optical 

elements are crystals, mirrors, gratings, multilayers and combinations of them, e.g., multilayer gratings, 

Bragg-Fresnel optics, zone plates, etc. All these components have both fundamental and technological 

performance limits with and without the heat load. However, excellent progress has been made in this 

area, and the applicability to MIT/Bates facility beamlines, both in the soft and in the hard x-ray, regime 

looks promising. 

The following topics are proposed for full development in the CDR: 

• 

• 

• 

• 

Beam interactions with materials constituting the beamline elements. 

Preservation of emittance/coherence, e.g., optical quality of surfaces: 

- Overview of coherence preservation of present-day optics. 

- Required generic optical performance to transport full beam coherence. 

- Special needs of specific experiments. 

Preservation of brilliance, e.g., radiation resistance, heat load: 

- Overview of brilliance preservation of present-day optics. 

- Availability of optics that can withstand FEL beams. In particular, we will 

address the instantaneous response of optical matter to high peak power over a 

few tens of femtoseconds. 

- New concepts to improve upon existing schemes and limits. 

Topics related to the time structure of FEL beams: 

- Pulse length preservation optics. 

- Optical schemes to modify/control the time structure. 

- Issues related to bandwidth of crystal reflection and pulse stretching. 

The CDR, in addition, will include a plan for the development of the specialized instrumentation 

unique to each planned experiment. 

In defining the transport and optics requirements for various experiments, we propose to draw 

heavily on the expertise of optics specialists in both synchrotron radiation field and laser area for handling 

ultra-bright beams and ultra-short pulses. It is also expected that the experience gained at the DESY TTF 

facility on the performance of soft x-ray optics during the next few years will be very useful. We will 

assemble a detailed database of existing optics knowledge and develop simulation capabilities to predict 

the performance of the optics for soft and hard x-rays required for the proposed experiments. Using this 

process, we will then identify R&D requirements and collaborative strategies for the latter stages of the 

study or early phases of the construction phase. 

A.7.2 Diagnostics, Synchronization, and Detection 

These three topics are closely related and are critical to the design of experiments that use ultra 

short x-ray pulses. In addition, the design of photon beam diagnostics will need to address the statistical 

A-18

nature of the pulse structure and the interaction of the x-ray laser pulse with matter. The early diagnostics 

will be based on extensions of proven techniques that have worked well at synchrotron sources and in 

optical laser experiments. They will require pulse-by-pulse measurements of total beam energy, pulse 

length, energy spectrum, photon beam divergence, beam centroid, spatial shape, and transverse 

coherence. These characteristics will define the basic performance parameters. 

Many schemes to detect products of interactions of VUV radiation and x-rays with samples have 

been proposed. They include conventional detectors for photons and photoelectrons, as well as mass 

spectrometers to measure atoms, ions, molecules and clusters. The techniques for jitter-free 

synchronization of a pump pulse with a probe pulse or a detector, so as to resolve sub-picosecond events, 

are currently being developed and should be available in a timely fashion to support experimental systems 

at the proposed MIT/Bates facility. Much knowledge on this subject will be gained during next couple of 

years from experiments conducted using plasma sources and the SPPS source. 

In diagnostic measurements ion chambers and x-ray streak cameras will play a prominent role. 

Micro-strip ion chambers are miniaturized multi-wire proportional chambers working in the proportional 

mode. Small distances between the readout channels improve the spatial resolution. Both the velocity of 

signal development and the detector’s rate capability are improved by short charge collection times, 

which are achieved by small distances between the electrodes. These detectors will be important 

diagnostics tools. 

Currently streak cameras are operated nearly jitter free with a resolution under 300 fs. The high 

voltage sweep ramp on the deflection plates of the streak camera is directly triggered by the laser light 

from the pump laser illuminating a photoconductive switch. Their synchronization performance in the 

environment of x-ray FELs is yet to be demonstrated. 

It is expected that many new detector concepts for various experiments will become available 

shortly. For example, a full understanding of the nonlinear interactions of photon with molecules and 

clusters measured in early experiments at the DESY TTF may be the basis for new detector concepts. 

In the CDR, a summary of the current knowledge on diagnostics, synchronization and detection 

will be presented. Their applicability will be discussed in the context of relevant demonstration 

experiments as well as the proposed scientific experiments. Special needs of specific experiments will be 

delineated. 

A.8 CONVENTIONAL FACILITIES 

Conventional facilities comprise the buildings and tunnels for the accelerator and beams, and 

accommodations for an estimated 170 employees and 100 outside users, as well as the necessary 

communications and power infrastructure. Structures that house the accelerator and beam equipment are 

based on the current concept described in previous chapters, i.e., a superconducting linac of 300 m length, 

12 undulator tunnels for 4 undulators feeding each of three experimental halls (UV, nanometer, and x-ray) 

with photon beam lines and office and lab space for on-site experimenters. Office, laboratory, and shop 

space needs were estimated based on three existing organizations: Bates, the SNS accelerator division, 

and Jefferson Lab. Unit costs for the elements of conventional construction were suggested by consultants 

[63]. A preliminary assessment of the suitability of the Bates laboratory as a site for the x-ray laser has 

been carried out by the same firm and concludes that the site appears suitable. The following tables list 

the conventional facilities envisaged for the proposed x-ray laser with some basic dimensions, occupation 

A-19

TABLE A3 Conventional construction costs and space allocation for offices and laboratories. 

Office, Labs, and Common Facilities 

net area 

(k ft 2 ) 

occupancy 

ft 2 /person 

gross area 

(k ft 2 ) 

cost 

(M$) 

Assigned offices, labs, and service areas 

(130 employees) 

21 270 35 

Special labs, shops, stores (40 technicians) 29 1000 40 

Special facilities (auditorium, high-bay area, 

water houses, control room, etc.) 

- - 13 

Total gross area 88 

Total cost @ 250$/ft2 22 

TABLE A4 Allocation of space and costs for technical buildings. 

Facility 

area 

(k ft 2 ) 

unit cost 

($/ft 2 ) 

item cost 

(M$) 

cost 

(M$) 

Injector bldg. 7 m high 2 200 0.6 

Cryo plant bldg. 7 m high 3 300 0.9 

RF Gallery, 4m wide, 400 m long 16 250 4.0 

Linac tunnel (cut & fill) 5 m wide 400 m long 20 300 6.0 

Beam switch yards: 4.9 

- UV beam tunnels 3 m wide 120 m long 3.6 300 1.08 

- NM beam tunnels 3 m wide 180 m long 5.4 300 1.62 

- XR beam tunnels 3 m wide 240 m long 7.2 300 2.16 

Undulator tunnels 2.2 

- 4 UV, 4 m wide, 40 m long 1.6 300 .48 

- 4 NM, 4 m wide, 60 m long 2.4 300 .72 

- 4 XR, 4 m wide, 80 m long 3.2 300 .96 

Experimental Halls incl. Photon beams 

(40 m wide for 4 beams each 9 m high) 

- UV hall, 40 m deep 16 400 6.4 

- NM hall, 50 m deep 20 400 8.0 

- XR hall, 60 m deep 24 400 9.6 

Labs/Offices for 100 users 50 100 5 

Total building facilities 47.6 

A-20 

24

TABLE A5 Conventional construction costs for 

miscellaneous infrastructure. 

Infrastructure 

Cost 

(M$) 

Power installation and cooling for 10 MW 1.5 

Fire, water, sewage installation 1.0 

Roads total 1 mile, 24 feet wide 0.8 

Parking for 300 cars including access 0.2 

Total infrastructure 3.0 

densities, and costs. In addition to the items in the tables, several further issues must be addressed during 

the course of the study, including: 

• Subsoil quality, load bearing, drainage 

• Required vibration limits and how to meet them 

• Expected emissions (radiation, effluents) 

• Hazard classification and permitting 

• Peak power averaging for a pulsed linac 

• Optimal facility lay-out, upgrades, and extensions 

A.9 UPGRADE OPTIONS 

The present proposal for the x-ray laser allows for three main upgrade options. First, the facility 

allows for the installation of additional undulator and x-ray lines as the scientific scope of the facility 

increases. Second, the facility can make ready use of improvements in technology as they become 

available, and third, the Bates site is well-suited for an extension of the linac that would allow the 

production of harder x-rays. 

Additional Beamlines. From the outset, the design of the site will include empty beamlines at a 

few linac energies. We anticipate that initial funding would allow for the construction of order ten 

undulator lines. We will reserve space for an additional 10–20 undulator lines and will consider the use of 

additional extraction points from the linac. Further, the study will examine the possibilities for x-ray beam 

switching. This approach is already being pursued and TTF2. 

Photoinjector and Undulator Technology. Any improvement in the performance of injector 

technology, including brightness, duty factor, peak current and reliability, could have a large impact on 

the performance of the proposed facility. The study will examine how best to incorporate the injector so 

that the facility can make optimal use of these advances as they become available. Possibilities include 

the explicit design of a rapidly swappable injector stand, similar to an architecture now in use at Bates, or 

the use of multiple injectors. 

Several laboratories and companies are now developing the technology for a superconducting RF 

photoinjector, although the technology is still at an early stage of development. Two of the chief 

advantages of such an injector will be increased duty factor and stability. The use of a superconducting 

A-21

accelerator (as compared to a copper structure) in the design will position the facility to take full 

advantage of the greater duty cycle these superconducting injectors would provide. 

As with the photoinjector, improvements in undulator technology and design have the potential to 

extend the reach of the facility without enormous capital expenditures. Superconducting undulators with 

fields exceeding 1 T and periods under 1.5 cm are now under design. The study will examine how best to 

incorporate the future use of these devices. We anticipate that one beamline may be dedicated to 

commissioning new structures so that their technological development can be accelerated. 

Linac Extension. An alternative, but straightforward, approach to the production of shorter 

wavelengths is the use of higher energy electron beams. The Bates site is 1.2 km along its long axis and 

would readily accommodate a longer linac. The preliminary design calls for an initial linac energy of 

~4 GeV to produce wavelengths as short as 0.3 nm. If the beam quality from the photoinjector can be 

improved, then wavelengths as short as 0.1 nm can be reached at energy of ~10 GeV. The major benefit 

of a later linac extension is that the choice of energy will be better matched to the injector and undulator 

technology available in the future. An attractive feature of this strategy is that the upgrade can proceed 

while the existing machine is operational. 

The detailed design to be produced in the Conceptual Design Report will include explicit 

consideration of how the proposed facility could best be constructed to accommodate future upgrades. 

Choices about what will be included in the baseline design and what will be deferred to a later upgrade 

will also depend on the continuing evaluation of the scientific merit for the first proposed experimental 

facilities. 

A.10 INITIAL COST MODEL 

This section presents preliminary cost estimates for the x-ray laser user facility. The principal 

expenditures for the facility are illustrated in Figure A8. The total estimated cost is about $300M. These 

costs can be classified into three main categories, accelerator systems, undulator and x-ray systems, and 

conventional infrastructure. 

Accelerator. (Injector, linac, RF systems, electron beamlines, controls & diagnostic systems) The 

$5M cost for the RF Photoinjector includes several systems which are relatively modest in cost, RF 

cavity, RF Klystron and Modulator, UV photoinjector laser, vacuum systems, and diagnostics. However, 

the integration of these systems is vital for the effective performance of the entire facility. This cost is 

substantially lower than the estimates for the TESLA X-FEL injector ($23M [64]) and the LCLS injector 

($20M [65]) both of which include accelerating sections following the RF photoinjector. 

The cost of the linac ($60M) assumes 200 m active length and is based on a budgetary 

memorandum from ACCEL GmbH [66] for a fully dressed linac. This linac includes cryostats with eight 

cavities each, one quadrupole per module, one beam position monitor per module, RF power couplers, 

frequency tuners and girders. ACCEL quotes 0.35M € per active meter. The preliminary cost estimate 

assumes $0.30M per active meter due to expected lower costs as manufacturing processes are improved 

and other SRF cavity producers become established. 

The RF systems total $24M for costs. This includes 200 15 kW klystrons at $60K. The costs of 

high voltage power supplies and RF circulators are well established and expected to cost $40K per RF 

station. The remaining $4M is allocated to the low-level RF control and costs and risks associated with 

system are less certain and will depend in detail on the particular design that will be chosen during the 

early stages of the proposed study. 

A-22

25 

10 

50 

Cost (M$) 

51 

5 

46 

60 

15 

FIGURE A8 Cost projections for the x-ray laser facility. 

24 

20 


SRF Linac 

RF Systems 

Electron Beamlines 

Controls & Diagnostic Systems 


Photon lines 

Conventional Laser Systems 

Cryoplant 

Buildings, Tunnels, Infrastructure 

The total cost per active meter, $0.7M per meter, including accelerator systems and linac tunnels, 

are significantly more than a projected scaled TESLA XFEL cost of $0.18M per meter, but less than the 

SNS cost of $2M per meter. Costs for the SNS are significantly higher due to the need for several 

different accelerating structures, different RF sources, and much higher average beam powers. 

Electron beamlines include two magnetic bunch compressors (

strongly on duty factor of the machine. The study will also determine what portions of the existing 6 MW 

Bates AC infrastructure could be used for the x-ray laser. 

The total estimated project cost falls between the projected LCLS cost of $270M and the TESLA 

XFEL cost of $684M. The TESLA X-FEL project has much higher cost due principally to the use of a 

20 GeV linac and longer undulator structures needed to generate a 0.1 nm x-ray. The LCLS realizes some 

significant cost savings by use of the existing linac, but undulator, x-ray beamline, and conventional 

facility costs dominate and are comparable to the proposed MIT project. The proposed study will refine 

these cost estimates and deliver a detailed cost model for the construction and operation of this facility. 

A-24

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3 E. Barthels et al., “On the way to a superconducting RF-gun: first measurements with the gun cavity,” Nucl. 

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4 H.P. Bluem, A.M.M. Todd, G.R. Neil, “Superconducting RF injector for high-power free-electron lasers,” 

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5 D.T. Palmer, “The next generation photoinjector,” Ph. D. Thesis, Stanford University (1998). 

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14 W.S. Graves et al., “Modeling and measurements of the DUVFEL photoinjector cavity RF properties,” 

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Part. Accel. Conf., Paris, 1801 (2002). 

A-25

17 V. Ayvazyan et al., “Generation of GW radiation pulses from a VUV free-electron laser operating in the 

femtosecond regime,” Phys. Rev. Lett. 88, 104802 (2002). 

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19 S.M. Gruner, M. Tigner (eds.), “Study for a proposed phase I energy recovery linac (ERL) synchrotron light 

source at Cornell University,” CHESS Technical Memo 01-003, 142 (2001). 

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APPENDIX B: 

PROJECT PLANNING 

Sophisticated project management strategies and tools are essential to efficiently execute the 

construction phase of a project of this magnitude. Increasing performance expectations regarding cost, 

schedule, and environmental stewardship require management practices that are responsive, robust, and 

fully transparent to all stakeholders. It is our intention that this project serve as an example of a new 

generation of project management within the NSF. The management plans and methods of several major 

projects with similar complexity and scope that are currently under way or recently completed in the 

United States will be reviewed, and their best practices adopted. A Project Management Plan will be 

developed detailing our approach to all key management areas including organizational development, 

project management systems, environment, safety and health, and construction cost and schedule. In this 

section, components of the project managment plan are briefly described, along with early concepts for 

implementation. 

B.1 ORGANIZATIONAL DEVELOPMENT 

Construction of the MIT x-ray laser on schedule and within budget will require development of 

an organizational structure with appropriate internal and external interfaces, as well as the Human 

Resources support to recruit and support a large team of people with a broad range of backgrounds and 

skills. 

Organizational Structure. The Project Management Plan will include a detailed description of 

the organizational structure for the project and its relationship to the organizational structure of MIT. Key 

interfaces will be defined with the academic units, in particular the Schools of Science and Engineering, 

and with the MIT business units. MIT will establish a Corporation-level committee to oversee the 

project’s overall progress and to facilitate establishment of an efficient management link between the 

project and the MIT administration. This committee will include external members as well as internal 

members from relevant departments in the Schools of Science and Engineering. At the project level, two 

advisory committees will be implemented including a Scientific Advisory Committee, with broad 

responsibility for the project scope and scientific program, and an Accelerator Advisory Committee, 

providing international-level advice and oversight to ensure that technical systems are planned and 

implemented using world-wide experience and state-of-the-art practices. Finally, we note the importance 

of the two committees of users proposed in Section 1.3 for bringing user issues and concerns in the case 

of the User’s Executive Committee, and for managing the construction and operation of beamline 

facilities in the case of the Research Council. 

On the business side, it will be our general philosophy to employ within the project only those 

functions and FTE levels that can be sustained in operation. The additional staff necessary for peak 

construction loads in areas such as Human Resources, Procurement, Finance, Legal, and Safety, for 

example, would be obtained from the relevant MIT organization on a matrix basis, retained from the 

outside on a consulting basis, or hired on a fixed-term arrangement within the project. To the maximum 

extent possible, all such workers would be located at the project site and be fully dedicated to the project. 

MIT employees working on a matrix basis would receive a position description and a performance 

appraisal from the relevant project manager similar to the direct project employees. 

B-1

Now we briefly describe our plans for the organization itself. An executive director, who would 

be a full faculty member of an appropriate MIT academic department, would lead it. A full-time project 

director with advanced management experience and skills will be appointed for the construction phase of 

the project. Other possible senior managers might include a technical director, and other associate 

directors. It is important to balance the need for a compact senior management team with the need to 

support a wide variety of management roles and responsibilities required in modern federally funded 

projects. The organization will have a set of staff functions to provide management support in the 

business areas discussed above, and a line organization with four main divisions: accelerator systems, 

laser systems, experimental systems, and conventional facilities. This organization will be optimized for 

the construction project and closely mapped onto the Work Breakdown Structure to provide clear 

accountability for delivering project components and systems. The roles and responsibilities of key 

personnel and groups within this team will be clearly specified. The Project Management Plan will also 

include a discussion of the transition to operations, including the expected changes to the organizational 

structure. 

Human Resources. Assembly of the construction team, and its later evolution to the operations 

phase, will require recruitment of staff beyond the existing personnel and expertise at MIT and Bates. The 

pace of a construction project is such that it is necessary to have a dedicated Human Resource (HR) office 

at the project site, which is as knowledgeable and efficient as possible in the hiring process. It is also 

critical to have the Bates HR office be an integral part of the MIT HR organization. Maximum use will be 

made of all MIT HR policies and procedures, and modifications will be implemented only when the 

unique demands of a construction project require doing so. The HR office for this project will be 

responsible for delivery of on-site human resources services such as workforce planning, position 

description development, recruitment, hiring, benefit counseling, performance appraisals, and dispute 

resolution. Recruiting and maintaining a diverse workforce will be a high priority. 

B.2 PROJECT MANAGEMENT SYSTEMS 

We will employ management systems and tools that are fully consistent and responsive to the 

NSF's newly implemented Management and Oversight Guide for large facility projects. 

Management Systems Platform. MIT has recently implemented the SAP (System Application 

Products) software system to provide integrated support of key management systems and databases 

including accounting, procurement, human resources, information systems and certain aspects of project 

management. The system offers all the tools necessary for integrated project management within the MIT 

environment. 

Integrated Financial System. MIT’s SAP-based finance system will be linked to the project 

through an accounting system based on the work breakdown structure. The system will incorporate 

payroll expense data, procurements, overheads, and operational and miscellaneous other costs. It will 

track cost-generating activities from initiation to payment. And it will support the cost/schedule and 

procurement systems in an integrated fashion. A project comptroller, who will be formally part of the 

MIT finance structure and report to the CFO, will manage the system. It will be necessary for this 

individual to be resident at the project site and have a small staff of perhaps two FTEs. This matrix 

position is one of the two most important in the project management. The other is the director of the 

procurement function described below. 

Procurement. The construction of the MIT x-ray laser will require a large number of 

procurements of high-technology components and systems, some of high dollar value, and virtually all 

B-2

having schedule critical and mission critical functions. The nature and volume of such procurements are 

generally not customary within the existing MIT procurement environment. It is even likely that the level 

of experience in such procurements may not be resident within the current MIT procurement staff to the 

degree necessary, and many systems and strategies such as advanced procurement planning, evaluated 

procurements, phase-funded procurements, and procurement tracking and expediting may not be 

commonly used. Often in federally funded construction projects, with uncertainties in the amount and 

timing of funding allocations, progress can be limited by traditional finance/procurement systems that 

require funding to be available at the time the procurement is initiated rather than at the time of contract 

award. These are just some examples of the many issues that will need to be studied as part of the 

management planning activity. Plans will then be developed and incorporated in the Project Management 

Plan describing the appropriate policies and the plan to implement them in a timely fashion to support 

construction in FY2007. Due to the intense pressure of a construction schedule, and the complexity of 

both the technical systems and the appropriate procurement strategies, a close working relationship will 

be required between the project’s technical management team and the procurement team. Therefore, it 

will be essential to assemble an on-site procurement office, having probably 4–6 professionals during the 

maximum construction workload. The project procurement director and staff will be employees of MIT’s 

procurement department. 

Cost and Schedule Control System. Projects of this magnitude require a robust yet workable 

cost/schedule control system, which serves the following three functions: (1) the process of developing 

the initial data, and modifying it as the project progresses, provides a degree of quantitative familiarity 

with the details of the work that is essential for the relevant managers to have in order to manage their 

resources effectively, (2) the product of the system—the so-called earned value reports produced 

monthly—are the formal way project management tracks overall progress against the original, or 

modified, project cost and schedule baselines, and (3) these reports also serve to keep external parties, 

such as the primary funding agency and review and oversight committees, informed, and they can serve 

as an effective method of supporting audit requirements. 

Change control system. Change is the basic dynamic of a construction project, and managing 

change is the most important management function. Serious problems may result from too much change 

and also from too little, but problems will always result if changes are not made in a deliberate process, 

communicated to all affected parties, and tracked within the cost/schedule control system. The currencies 

of change control are the project contingency account—an amount of funds set aside to support 

unexpected costs—and the schedule contingency—an amount of time set aside to deal with unexpected 

delays. Change control will incorporate both a SAP-based system for managing the data, and a graded 

management approval process that will delegate authority to the appropriate level in the organization 

depending on the magnitude of the proposed change. Large changes requiring funds or delays beyond the 

original baseline approved by the funding agency, or even a large fraction of remaining contingencies 

would require MIT and agency level approval. All changes will be tracked in monthly reports provided by 

the cost/schedule system. 

B.3 ENVIRONMENT, SAFETY, AND HEALTH 

The main objective of this project is to construct a scientific user facility that meets the stated 

objectives while protecting the environment and the safety of the workers, the users, and the general 

public. The Project Management Plan will address the Environment, Safety, and Health (ES&H) issues 

for the construction phase of the project, and provide ES&H guidelines for the operational phase of the 

facility. The Plan will identify an assistant project director for ES&H, who will report to the project 

director. Responsibilities will include safety analysis, establishing hazard classification, generation of 

B-3

safety assessment documents, and safety reports. The plan will also include construction and industrial 

safety and waste minimization and pollution prevention. The ES&H office will be responsible for 

obtaining all environmental licenses, authorizations, and permits required for the project. We want to 

emphasize two important activities early in the study period. 

First it will be necessary to conduct an Environmental Assessment to formally determine the 

impact that this project will have on the environment and to reach agreement on what process and steps 

will be required to mitigate that impact. At this time, the major items would appear to include the impact 

of physical construction and the impact of an increased site population. We now believe, but must 

confirm as soon as possible, that there are no impacted wetlands on site, and that there are no other site 

issues such as cultural resources that would require study and impact mitigation, and that there are no 

additional radiation-related risks associated with the facility. In fact, our estimates indicate that the 

electron beam for the x-ray laser will containing lower power than that in the current Bates facility. 

Second, it is important to begin planning for a program of construction safety that will avoid any 

significant worker injury during the course of construction. Best construction safety practices have 

regularly achieved safety records consistent with such a goal on projects of this size and larger. None of 

the expected construction work is particularly high-risk, but an aggressive and proactive program must be 

established early to avoid accidents. For example, each contractor bidding on work must be pre-qualified 

for their safety record to ensure that a competitive bidding environment does not jeopardize safety. This 

issue links to procurement policy and must be in place before construction begins. 

B.4 CONSTRUCTION COST AND SCHEDULE 

The Project Management Plan will include the detailed cost and schedule for the project 

according to the Work Breakdown Structure (WBS). We plan to estimate the capital construction cost, 

and pre-operations expenses incurred before facility completion, at a level of substantial detail suitable for 

review by external experts, and necessary to gain confidence that the project can be completed on budget. 

The estimate will include an appropriate level of contingency determined by risk analysis at an 

intermediate level of the WBS. Estimates will also be prepared for the facility operations costs, for full 

life-cycle costs and other categories required by NSF guidelines for the Large Facility Projects within the 

NSF MRE account. The costs and contingencies will be presented according to the NSF budgeting and 

cost estimating guidelines for major projects. The schedule estimate will be similarly detailed, and both 

the cost and schedule data will be integrated in a resource-loaded scheduling system capable of 

supporting management and agency planning activities with fast turn around times. Official cost and 

schedule baselines will be established at the time of construction project approval and changed 

periodically after by agreement with the funding agency as necessary to better manage the work 

remaining. 

B-4

Proposal to NSF for a conceptual Design Study - MIT

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