1. Z. Huang LCLS Lehman Review May 14, 2009
FEL Study Plans
Z. Huang
LCLS Lehman Review
May 14, 2009

2. Questions to be addressed here
What is the plan for detailed study and understanding of FEL performance and its sensitivity to the beam brightness, matching, (energy and phase errors), LSC, CSR, etc.?

3. What has been done?
FEL gain length in nom. conditions (3 kA, 0.25 nC, 0.4 µm)
FEL-induced electron energy loss (4-8 MeV, or >1012)
Effects of Laser heating
Post-saturation taper
FEL bandwidth & central wavelength using Ni K-edge
Different compression configurations
Transverse coherence test (diffraction on finder-wires)
20 pC run (fs pulses?)
Drive Laser spatial shaping study

4. What is planned
Gain length dependence on Ipk (CSR?) & energy spread (LSC?) to understand our slice parameters
Gain length dependence on projected emittance, b-matching, different average beta in undulator
Emittance spoiling studies (using UV laser, OTR)
FEL wavelength tuning (1.5 nm – 1.5 Å)
Look for nonlinear third harmonic (in FEE?)
BC2 over-compression for chirped SASE
More extensive 20 pC studies
Other charge configurations (1 nC, 100 pC?)

5. Gain length and saturation level
At nominal machine conditions (250 pC, 3 kA under-compression, LH at 7 mJ, 13.6 GeV, 1.5 Å)
gex,y = 0.4 mm (slice)
Ipk = 3.0 kA
sE/E = 0.01% (slice)
4.6 MeV
DK/K = 0.16%
FEL Gain Length
= (3.3 ± 0.3) m
Saturation energy ~ 1 mJ
Most studies are done with these nominal parameters, will explore more beam parameter space

6. 12 undulators inserted here
Laser Heater Studies
Heater has shown its effectiveness
More studies to determine optimal heating and slice energy spread
12 undulators inserted here
Laser optimal
heater on
heater off
Laser heater OFF

7. Dependence on peak current
Laser heater =76 uJ
Laser heater =11 uJ
12 undulators inserted
(FEL not saturated here)

8. FEL bandwidth Used Ni K-edge (w/ 12 undulators), Ni replaced by B4C
May use YAG K-edge with saturated FEL at 11.3 GeV + enough attenuation
Error function fit yields
s=0.1% (FEL BW)

9. Post-saturation Taper
> 8 MeV E-loss
(2 mJ)
More taper studies with 8 more undulators to be installed

10. Preliminary 20 pC results
Lased at 20 pC and saturated with 60 mJ energy
Do not know bunch length (must be short to support lasing)
More extensive 20 pC studies
Try to determine how short is the pulse?
Photodiode signal after BC2 suggests bunch length < 1 mm (Ding et al., SLAC-PUB-13525)
20 pC FEL results

11. Drive Laser shaping study
Laser spatial shaping to determine sensitivity of slice emittance and FEL gain on drive laser transverse uniformity (effects of transverse space charge)
20 mesh
50 mesh
2 x 50 mesh
180 mesh
‘Bagel’
‘Donut’
‘Airy’
‘Gaussian’
Slice emittance and FEL not very sensitive to spatial shaping, worse case 20 MG with emittance ~ 2 mm

12. Other studies
FEL dependence on laser temporal uniformity (effects of LSC)
FEL gain length at higher peak current, and in over-compression mode (effects of CSR)
Look for third harmonic with fundamental strongly attenuated (presence of strong third harmonic is another indication of FEL saturation)