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)