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 Matching Section ends at the main linac section L1. The photoinjector can be operated
independently of the main linac by turning off the injector bend and delivering the beam to the
injector dump. This dump will be used when commissioning the injector, and for troubleshooting
and injector studies. A spectrometer dipole on this alternative beam path allows full longitudinal
beam analysis when running in this mode.
The rf deflecter allows the longitudinal and horizontal phase space, including the slice energy
spread and slice horizontal emittance, to be measured at 150 MeV. See Section 7.8.2, Bunch
Length Diagnostics. Because of their important effect on FEL performance, emittance and energy
spread are also measured at several other locations along the path to the undulator. See
Section 7.8.1, Tranverse Emittance Diagnostics, and Section 7.8.3, Beam Energy Spread
Diagnostics, for further discussion.
Figure 6.18 also shows an electro-optic device and a streak camera in the Matching Section.
These devices measure the temporal dependence of the charge distribution within the bunch.
Slice emittance and EO diagnostics are the topics of the subsections that follow.
6.5.2 Slice Emittance
The wire-scanners described in Section 7.8.1, Transverse Emittance Diagnostics, are used to
measure the projected transverse emittance of the full electron bunch. However, it is primarily the
so-called "slice emittance," the (transverse) emittance of electrons in axial slices that are only a
fraction of the full bunch length, that determines the performance of a SASE FEL. The slice scale
of interest for a SASE FEL is the slippage length in the undulator, which for the LCLS is 0.5 µm
or 1/150 th of the FWHM length of the compressed bunch at the end of the linac. The length of an
equivalent slice at the injector is about 100 fs. Multi-particle simulations of photoinjector beams
indicate that within the bunch, the axial variation in the transverse space charge force causes a
smooth, non-filamented dilution in phase space density, with concomitant full-bunch emittance
growth relative to slice values [7]. The injector diagnostics will only be capable of measuring
emittances for slices with lengths on the order of a couple picoseconds. Strategies for minimizing
or reversing in the final beam the correlated emittance growth have received considerable
attention, as in Section 6.1.2, Emittance Compensation
Two methods to measure the slice emittance will be available in the MS: one uses the rf
deflector, the other a streak camera. As mentioned in Section 6.5.2, Standard Beamline
Diagnostic Devices, the horizontal slice emittance can be measured using the rf deflector in
combination with the well-established quadrupole scan procedure and straight-ahead wire scanner
or view screen.
As demonstrated at LANL [17], a streak camera can be used in combination with a
quadrupole scan to measure the slice emittance if the electron beam is sufficiently intense. A quad
scan relies on a set of beamwidth measurements obtained as a quadrupole lens is scanned through
a range of focal lengths, and yields the three parameters that characterize the region in phase
space occupied by the beam. In a typical application, optical radiation emitted from a screen
inserted in the beam path is imaged onto a light sensitive detector, and a beam width
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