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
charge anywhere from 1 nC to 0.1 nC, or optimize with an intentional electron chirp at the
undulator. A qualitative summary of the effects of changes to the critical compression parameters
is listed in Table 7.2. The two R56 values are not considered free parameters here since their
values depend on rf phases and other factors.
The lower limit on the choice for BC1 energy is set by space charge forces of the shorter
bunch with 250 MeV considered a safe energy. The upper limit is set by the desire to initially
compress the bunch early in the linac to ease transverse wakefields. The chosen energy of
250 MeV also desensitizes the system to injector timing jitter and is a practical solution for L1,
which consists of one klystron powering three 3-meter S-band sections at an rf phase of −38° off
crest at an average gradient of 17.5 MV/m. The location (energy) of BC2 is set by the need to
produce a very small energy spread at 14.3 GeV. This involves a balance between the
longitudinal geometric wakefield in L3 (which scales with Linac-3 length) and the remaining δ-z
correlation just after BC2. Other factors, including synchrotron radiation, are discussed in more
detail in Section 7.4.1 (BC1) and Section 7.4.2 (BC2).
Table 7.2 Bunch compression parameter trade-offs: A qualitative summary of the effects of changes to
the bunch compression parameters. Only limitations are noted. An “increase” of rf phase, ϕ,
refers to moving farther off rf crest and σ z1 is the intermediate bunch length (after BC1, but
before BC2).
Parameter Increase Parameter Decrease Parameter
σz 1 ≈ 190 µm
|ϕ 1 | ≈ 38°
|ϕ 2 | ≈ 43°
E 1 = 250 MeV
E 2 = 4.54 GeV
• Insufficient L2 wake compensation for
L2/BC2 non-linearities.
• Requires stronger BC2 and more CSR.
• Increased L1 energy spread.
• Inefficient acceleration.
• Energy spread too large for
cancellation with L3 wake.
• Inefficient acceleration.
• Longer L1—> stronger L1 transverse
wakes and increased L1 ∆ε.
• Can increase jitter sensitivity.
• Increase BC2 ∆ε due to incoherent
synchrotron radiation, or lengthen BC2.
• Shorter L3—> insufficient L3 wake for
L2 energy spread compensation.
• CSR emittance growth increased in BC1.
• Can increase jitter sensitivity.
• Can increase jitter sensitivity.
• Increased BC1 strength and ∆ε due to CSR.
• Energy spread too small—over-compensated
with L3 wake.
• Increased BC2 strength and CSR ∆ε.
• Increase BC1 chicane strength and CSR ∆ε.
• Increased space charge forces.
• Longer L3—> L3 wake too large— over-
compensation of L2 energy spread.
• Shorter L2—> insufficient L2 wake
compensation for L2/BC2 non-linearities.
System optimization scans show that even higher BC2 energy (i.e., >4.5 GeV) can further
desensitize the final bunch length and final energy to gun timing and charge jitter. In this case,
however, the BC2 chicane needs to be even stronger and longer, so a compromise has been made
at 4.54 GeV, which is also consistent with operation at a 15-Å FEL radiation wavelength, where
the L3 linac RF is simply switched off.
7-12 ♦ A C C E L E R A T O R