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
Gain Length and Taper
August, 2009
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Daniel Ratner
dratner@slac.stanford.edu
Gain Length Measurement
Goal of this talk:
Exp. GAIN
Characterize FEL as
SATURATION
function of undulator
length
Gain length studies:
Measure power, radius,
position on YAG
Saturation studies:
Measure energy loss of
electrons with dump BPMs
Gain Length and Taper
August, 2009
2
2
Daniel Ratner
dratner@slac.stanford.edu
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
YAG
Need faster method!
Screen
X-rays
eGain Length and Taper
August, 2009
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Daniel Ratner
dratner@slac.stanford.edu
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 < 10 minutes
Full FEL
FEL
Bunching
from FEL
process
Gain Length and Taper
August, 2009
YAG
Screen
Bunching
misaligned,
smeared
All undulators
remain inserted
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4
Daniel Ratner
dratner@slac.stanford.edu
Gain Length Measurement
Kick Method Problems
Spontaneous background is larger
FEL Process can restart, produce additional spots
If spots overlap, distorts measurement
SASE re-start
(New bunching)
FEL
FEL
Bunching
from FEL
process
All undulators
remain inserted
Gain Length and Taper
August, 2009
5
5
YAG
Screen
Daniel Ratner
dratner@slac.stanford.edu
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
X Kick at
Undulator 5
+0.75 mm
Gain Length and Taper
August, 2009
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Daniel Ratner
dratner@slac.stanford.edu
Gain Length Measurement
Move Undulator vs. Kick Beam
Gain Length and Taper
August, 2009
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7
Daniel Ratner
dratner@slac.stanford.edu
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)
Gain Length and Taper
August, 2009
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8
Daniel Ratner
dratner@slac.stanford.edu
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)
Gain Length and Taper
August, 2009
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9
Daniel Ratner
dratner@slac.stanford.edu
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
Gain Length and Taper
August, 2009
10 10
Daniel Ratner
dratner@slac.stanford.edu
Taper Studies
Taper undulator K to match e- energy loss
Linear taper for wakefield
and spontaneous radiation
Saturation taper for FEL
radiation
sat
Linear Taper
[1  K 0 / 2]
 u
 1
2
2(g 0  g )
2
Saturation Taper
Maximize final power by searching for optimal
saturation taper
Gain Length and Taper
August, 2009
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Daniel Ratner
dratner@slac.stanford.edu
Taper Studies
Use same method to study taper
Dipole corrector kicks, measure electron E-loss
(YAG saturates)
Gain Length and Taper
August, 2009
12 12
Daniel Ratner
dratner@slac.stanford.edu
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
Gain Length and Taper
August, 2009
13 13
Daniel Ratner
dratner@slac.stanford.edu
Thanks to:
LCLS project director J. Galayda,
Commissioning Team and
many collaborators and visitors from
LBNL, LLNL, DESY
Gain Length and Taper
August, 2009
14 14
Daniel Ratner
dratner@slac.stanford.edu
Gain Length and Taper
August, 2009
15 15
Daniel Ratner
dratner@slac.stanford.edu
Gain Length Measurement
Two spots from kick?
X Kick
Gain Length and Taper
August, 2009
X Kick
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Daniel Ratner
dratner@slac.stanford.edu
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)
gex,y  0.4 mm (slice)
Ipk  1.5 kA
sE/E  0.02% (slice)
Gain Length and Taper
August, 2009
Ipk  5.0 kA
sE/E  0.01% (slice)
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Daniel Ratner
dratner@slac.stanford.edu