LCLS

1

• CSR micro-bunching in compressors
• Superconducting wiggler can reduce effect
• FEL gain evaluation after system tracking

LCLS Tracking Studies
Paul Emma January 17, 2002
CSR Workshop
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LCLS Accelerator and Compressor Schematic
7 MeV ?z ? 0.83 mm ?? ? 0.2
150 MeV ?z ? 0.83 mm ?? ? 0.10
250 MeV ?z ? 0.19 mm ?? ? 1.8
4.54 GeV ?z ? 0.022 mm ?? ? 0.76
14.35 GeV ?z ? 0.022 mm ?? ? 0.02
Linac-X L 0.6 m ?rf180?
rf gun
Linac-1 L 9 m ?rf -38
Linac-2 L 330 m ?rf -43
Linac-3 L 550 m ?rf -10
new
Linac-0 L 6 m
undulator L 120 m
25-1a 30-8c
21-1b 21-1d
21-3b 24-6d
X
...existing linac
BC-1 L 6 m R56 -36 mm
BC-2 L 22 m R56 -22 mm
DL-1 L 12 m R56 ?0
DL-2 L 66 m R56 0
SLAC linac tunnel
undulator hall
(12/01/01)
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LCLS Linac Layout
BC2 5.4 GeV
BC1 0.25 GeV
undulator 14.3 GeV
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Compression Evolution using Elegant
before BC1
after BC1
E 250 MeV sz ? 190 mm
E 250 MeV sz ? 830 mm
under-compression of head tail from 3rd-order
term
L2 wakes induce new 3rd-order correlation
over-compression of head and tail
before BC2
after BC2
E 4.5 GeV sz ? 190 mm
E 4.5 GeV sz ? 22 mm
written by M. Borland, ANL/APS
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3rd-order energy-time correlation from injector
E0 150 MeV
1st, and 2nd-order correctable with S-band and
X-band RF
3rd-order term generated with space charge and
l?(s) in gun-to-linac drift (C. Limborg)
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Smoothed Parmela dist. input to LCLS
N 2?106
plus temporal smoothing
at 150 MeV, before any bends
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LCLS bunch tracked to start of BC2
cubic correlation term generates head/tail spikes
at end of chicane
CSR ON in upstream bends (BC1, etc)
CSR OFF in upstream bends (BC1, etc)
CSR in BC1 causes microbunching at entrance to BC2
otherwise have smooth dist. at BC2 entrance
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Superconducting Wiggler in LCLS
Add Super-Conducting wiggler prior to BC2 to
increase incoherent energy spread (10)
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Superconducting wiggler OFF
Using Stupakov CSR model (based on Saldin et.
al.) and Elegant tracking
CSR may still be over-estimated in present
tracking
10
LCLS Distribution After BC2 Chicane
(SC-wiggler ON, CSR ON in upstream bends)
sE/E0 0.726
wiggler ? 3?10-5 incoherent energy spread
? bunch head
ss 24.8 mm
E0 4.54 GeV
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LCLS Distribution After BC2 Chicane
(SC-wiggler OFF, CSR ON in upstream bends)
sE/E0 0.736
3?10-6 incoherent energy spread
? bunch head
ss 24.8 mm
E0 4.54 GeV
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LCLS Distribution After BC2 Chicane
(SC-wiggler ON and CSR OFF in upstream bends)
sE/E0 0.716
wiggler ? 3?10-5 incoherent energy spread
? bunch head
ss 24.8 mm
E0 4.54 GeV
no CSR in upstream bends
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LCLS Distribution After BC2 Chicane
(SC-wiggler OFF, CSR OFF in upstream bends)
sE/E0 0.736
3?10-6 incoherent energy spread
? bunch head
ss 24.8 mm
E0 4.54 GeV
no CSR in upstream bends
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Final x-x? Phase Space (LCLS input)
e/e0 ? 2.5
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Slice Mismatch and Oscillation (LCLS Input)
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Slice Emittance (LCLS)
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LCLS BC2 CSR-integrated-wake (tracked dist.)
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Transverse RF deflector used to diagnose slice
emittance after BC2
screen at 6.2 MeV after BC2 with transverse-RF OFF
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Sliced Beam After Injector and Linac Tracking
gex
gey
osc-amp/s
mismatch
zx
zy
Ming-Xie method
LG lt 4 m
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Summary

• New awareness of CSR micro-bunching and new
  design optimization
• Projected emittance doubles over linac, but slice
  parameters allow LG lt 4 m
• Projected emittance growth presents a diagnostics
  and tuning challenge (use transverse RF)
• Stability studies and Genesis results ?
  (see M. Borland talk)