LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
The VISA experiment, a BNL-LLNL-SLAC-UCLA collaboration, has obtained a gain of
about 2¥10 8 and reached saturation at a radiation wavelength of 840 nm in 3.6-3.8 m using a 4-m
long undulator with distributed strong focusing quadrupoles [36]. The properties of SASE
radiation, as a function of distance through the undulator magnet have been measured and
analyzed. For the first time, observation and analysis of the statistical intensity fluctuations of
SASE radiation at saturation was studied, and compared to the data obtained during exponential
growth. These results are compared to the start-to-end numerical model of the experiment, which
follows the electron beam dynamics from photocathode emission, through acceleration, transport
and the undulator. This combination of experimental results and start-to-end simulations resulted
in a comprehensive description of the underlying beam dynamics and FEL process. Additional
results based on VISA at saturation include measurements of FEL harmonic radiation and
electron beam microbunching. The measured energy of the radiation as function of position along
the undulator and the prediction from the numerical model are shown in Figure 4.5.
SASE Intensity [µJ]
10 2
10 1
10 0
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
simulations
measurements
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
z [m]
Figure 4.5 Exponential growth and saturation in the VISA experiment at 840 nm, in a 4-m long
undulator [36]. The outer curves are the predictions for the 1-sigma limits of the
intensity fluctuations.
Similar experimental programs on SASE-FELs are being prepared at Spring8 in Japan, at
BESSY-II in Berlin, in China and at other laboratories.
4.4 LCLS: An X-Ray SASE-FEL
The first proposal for a SASE-FEL using the SLAC linac was made in 1992 [21]. The initial
design was developed by a study group until 1996 after which a design group prepared the LCLS
Design Study Report [23] that was published in 1998.
A short summary of the main LCLS parameters is given in Table 4.1. A full list can be found
in Appendix A. The components of the LCLS and its operational principals are described in full
detail in this CDR.
F E L P H Y S I C S♦ 4-15