FEL Gain length and Taper Measurements at LCLS

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FEL Gain length and Taper Measurements at LCLS
D. Ratner A. Brachmann, F.J. Decker, Y. Ding, D.
Dowell, P. Emma, J. Frisch, S. Gilevich, G.
Hays, P. Hering, Z. Huang, R. Iverson, H. Loos,
A. Miahnahri, H.D. Nuhn, J. Turner, J. Welch, W.
White, J. Wu, D. Xiang, G. Yocky (SLAC, Menlo
Park, California) W. M. Fawley (LBNL, Berkeley,
California) August 25th, 2009
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Gain Length Measurement

• Goal of this talk Characterize FEL as function
  of undulator length

Exp. GAIN
SATURATION

• Gain length studies
• Measure power, radius, position on YAG

• Saturation studies
• Measure energy loss of electrons with dump BPMs

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Gain Length Measurement

• Moving Undulators
• Measure power at YAG screen (50 m from last U)
• Remove undulators sequentially

• Problem 3 minutes per undulator move
• If FEL unstable, too long to measure gain length
• Too long for a routine measurement
• Need faster method!

YAG Screen
X-rays
e-
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Gain Length Measurement

• Shut Off FEL Process

• Sequential dipole correctors kick beam 15mrad

• Strongly suppress short wavelength FEL
• Smears bunching in direction of motion,
  decreases overlap with X-rays

• Full gain length scan in lt 10 minutes

Full FEL
FEL
Bunching from FEL process
YAG Screen
Bunching misaligned, smeared
All undulators remain inserted
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Gain Length Measurement

• Kick Method Problems

• Spontaneous background is larger

• FEL Process can restart, produce additional
  spots
• If spots overlap, distorts measurement

FEL
SASE re-start (New bunching)
FEL
Bunching from FEL process
YAG Screen
All undulators remain inserted
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Gain Length Measurement
Full FEL

• FEL restarts after kick
• Top image shows full FEL
• Bottom image shows second spot from distorted
  orbit
• Cropping image or second kick fix problem

X Kick at Undulator 5
X Kick
0.75 mm
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Gain Length Measurement

• Move Undulator vs. Kick Beam

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Short Wavelength Measurement

• Gain length of 3.3m at 13.6 GeV

gex,y ? 0.4 mm (slice) Ipk ? 3.0 kA sE/E ? 0.01
(slice)
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Long Wavelength Measurement

• Gain length at 4.7 GeV (1.3 nm)
• Kick suppression weak so use undulator-pull
• Stop before saturation to protect YAG

GL 1.62/-0.15 m M. Xie 1.5 m
gex,y ? 0.4 mm (slice) Ipk ? 1 kA sE/E ? 0.01
(slice)
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Gain Length Measurement

• Laser Heater varies energy spread, 0 to 120 keV.

Microbunching Instability? Laser heater improves
gain length
Parameters 1.5Å, 250pC, 3kA, Compression factor
90
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Taper Studies

• Taper undulator K to match e- energy loss

• Linear taper for wakefield and spontaneous
  radiation
• Saturation taper for FEL radiation

Linear Taper
Saturation Taper

• Maximize final power by searching for optimal
  saturation taper

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Taper Studies

• Use same method to study taper
• Dipole corrector kicks, measure electron E-loss
  (YAG saturates)

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Taper Studies

• Saturation taper gives factor of 2 power gain
• Simulation more effective, but sensitive to
  distribution

Experiment
Simulation
Gain factor of 2.4 from taper
Gain factor of 3.3 from taper
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• Thanks to
• LCLS project director J. Galayda, Commissioning
  Team and
• many collaborators and visitors from LBNL, LLNL,
  DESY

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Gain Length Measurement

• Two spots from kick?

X Kick
X Kick
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Longer wavelength measurement

• Two example gain length measurements
• Why pull undulator

4.7 GeV (Undulator pull)
13.6 GeV (X-corr Kick)
Stop before saturation to protect YAG
1.62/-0.15 m gain length
2.85/-0.06 m gain length
gex,y ? 0.4 mm (slice) Ipk ? 1.5 kA sE/E ? 0.02
(slice)
gex,y ? 0.4 mm (slice) Ipk ? 5.0 kA sE/E ? 0.01
(slice)