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
linac, so that the entire linac and injector configuration is technically much less challenging than
that required at 1.5 Å.
The final absolute energy spread will decrease by a factor of two with the reduced
compression (from ~2.8 MeV to 1.4 MeV), while the relative energy spread will increase with the
reduced beam energy (from 0.02% to 0.03%). The very small energy chirp along the electron
bunch can be approximately maintained, even after the L3 rf is switched off.
The beta-match at the undulator entrance, due to its permanent magnet focusing, needs to be
adjusted to produce ~6-meter average undulator beta functions, as opposed to the ~18-meter beta
functions at 1.5 Å (14.3 GeV). The re-match is accomplished using the four QM35-38
quadrupoles at undulator entrance (last four quads at right of Figure 7.36).
Table 7.9 lists the machine parameters for the 15-Å case alongside the nominal 1.5-Å case.
Parameters not listed here are the same in the two configurations. The same injector beam, shown
at top of Figure 7.6, is tracked in 2D through the re-configured LCLS accelerator with the L3-
linac rf switched off. The final longitudinal phase space at the undulator entrance, at 4.54 GeV, is
shown in Figure 7.11. The temporal distribution is very flat with a nearly constant 2-kA peak
current.
Table 7.9 Machine parameters for 15-Å SASE radiation with a saturation length of ~60 m juxtaposed
against the nominal 1.5-Å configuration, both with 1-nC of charge. Energy sensitivity is worse
(×3) at left, but also more tolerable (×3) at 15 Å.
Bunch Charge → 15 Å (long λ r) 1.5 Å (nominal) units
Final rms bunch length 45 22 µm
Norm. rms slice emittance 3.0 1.2 µm
Peak current 1.9 3.4 kA
L1 rf phase −35.1 −38.1 deg-S
L2 rf phase −40.6 −42.8 deg-S
R 56 of BC1 −40.0 −35.9 mm
R 56 of BC2 −21.7 −22.5 mm
energy spread in BC1 (rms) 1.59 1.78 %
energy spread in BC2 (rms) 0.72 0.76 %
energy spread in undulator (rms) 0.03 0.02 %
bunch length after BC1 (rms) 200 195 µm
rms gun ∆t 0 → 12% σ z 2.3 4.0 psec
rms gun ∆t 0 → 0.1% ∆E/E 0 0.4 1.4 psec
rms ∆Q/Q 0 → 12% I pk 11 6.0 %
This is just one possible long-wavelength configuration. Many long-wavelength scenarios are
possible including reduced charge, increased peak current, and reduced emittance. This specific
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