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1 Daniel Ratnerdratner@slac.stanford.edu
1Gain Length and TaperAugust, 2009
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
2 Daniel Ratnerdratner@slac.stanford.edu
2Gain Length and TaperAugust, 2009
Gain Length Measurement Goal of this talk: Characterize FEL as function of undulator length
Gain length studies: Measure power, radius, position on YAG
Exp. GAIN
SATURATION
Saturation studies: Measure energy loss of electrons with dump BPMs
3 Daniel Ratnerdratner@slac.stanford.edu
3Gain Length and TaperAugust, 2009
Gain Length Measurement
Moving Undulators: Measure power at YAG screen (50 m from last U)
Remove undulators sequentially
YAGScreen
X-rays
e-
Problem: 3 minutes per undulator move If FEL unstable, too long to measure gain length
Too long for a routine measurement
Need faster method!
4 Daniel Ratnerdratner@slac.stanford.edu
4Gain Length and TaperAugust, 2009
Gain Length Measurement
Shut Off FEL Process
All undulators remain inserted
FEL
Bunching misaligned,
smeared
Bunching from FEL process
YAGScreen
Full FEL
Sequential dipole correctors kick beam ~ 15rad Strongly suppress short wavelength FEL
Smears bunching in direction of motion, decreases overlap with X-rays
Full gain length scan in < 10 minutes
5 Daniel Ratnerdratner@slac.stanford.edu
5Gain Length and TaperAugust, 2009
Gain Length Measurement
Kick Method Problems
Bunching from FEL process
SASE re-start(New bunching)
YAGScreen
All undulators remain inserted
FEL
FEL
Spontaneous background is larger
FEL Process can restart, produce additional spots If spots overlap, distorts measurement
6 Daniel Ratnerdratner@slac.stanford.edu
6Gain Length and TaperAugust, 2009
Gain Length Measurement
FEL restarts after kick Top image shows full FEL
Bottom image shows second spot from distorted orbit
Cropping image or second kick fix problem
Full FEL
X Kick atUndulator 5
X Kick
+0.75 mm
7 Daniel Ratnerdratner@slac.stanford.edu
7Gain Length and TaperAugust, 2009
Gain Length Measurement
Move Undulator vs. Kick Beam
8 Daniel Ratnerdratner@slac.stanford.edu
8Gain Length and TaperAugust, 2009
Short Wavelength Measurement
Gain length of 3.3m at 13.6 GeV
x,yx,y 0.4 0.4 m (slice)m (slice)
IIpkpk 3.0 kA 3.0 kA
EE//EE 0.01% (slice) 0.01% (slice)
9 Daniel Ratnerdratner@slac.stanford.edu
9Gain Length and TaperAugust, 2009
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 mM. Xie: ~1.5 m
x,yx,y 0.4 0.4 m (slice)m (slice)
IIpkpk 1 kA 1 kA
EE//EE 0.01% (slice) 0.01% (slice)
10 Daniel Ratnerdratner@slac.stanford.edu
10Gain Length and TaperAugust, 2009
Gain Length MeasurementLaser Heater varies energy spread, 0 to 120 keV.
Microbunching Instability? Laser heater improves
gain length
Parameters: 1.5Å, 250pC, 3kA, Compression factor = 90
11 Daniel Ratnerdratner@slac.stanford.edu
11Gain Length and TaperAugust, 2009
Taper Studies
Taper undulator K to match e- energy loss
Linear Taper
Saturation Taper120
20
)(2
]2/1[
Kusat
Linear taper for wakefield and spontaneous radiation
Saturation taper for FEL radiation
Maximize final power by searching for optimal saturation taper
12 Daniel Ratnerdratner@slac.stanford.edu
12Gain Length and TaperAugust, 2009
Taper Studies
Use same method to study taper Dipole corrector kicks, measure electron E-loss (YAG saturates)
13 Daniel Ratnerdratner@slac.stanford.edu
13Gain Length and TaperAugust, 2009
Taper Studies
Saturation taper gives factor of 2 power gain Simulation more effective, but sensitive to distribution
Gain factor Gain factor of 2.4 from of 2.4 from
tapertaperGain factor of Gain factor of 3.3 from taper3.3 from taper
Experiment Simulation
14 Daniel Ratnerdratner@slac.stanford.edu
14Gain Length and TaperAugust, 2009
Thanks to:
LCLS project director J. Galayda, Commissioning Team and
many collaborators and visitors from LBNL, LLNL, DESY
15 Daniel Ratnerdratner@slac.stanford.edu
15Gain Length and TaperAugust, 2009
16 Daniel Ratnerdratner@slac.stanford.edu
16Gain Length and TaperAugust, 2009
Gain Length Measurement
Two spots from kick?
X Kick X Kick
17 Daniel Ratnerdratner@slac.stanford.edu
17Gain Length and TaperAugust, 2009
Longer wavelength measurement
Two example gain length measurements
Why pull undulator
1.62+/-0.15 mgain length
2.85+/-0.06 mgain length
4.7 GeV (Undulator pull) 13.6 GeV (X-corr Kick)
Stop before saturation to protect YAG
x,yx,y 0.4 0.4 m (slice)m (slice)
IIpkpk 1.5 kA 1.5 kA
EE//EE 0.02% (slice) 0.02% (slice)
x,yx,y 0.4 0.4 m (slice)m (slice)
IIpkpk 5.0 kA 5.0 kA
EE//EE 0.01% (slice) 0.01% (slice)
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