Alternate Parameters and Setup C. Limborg-Deprey Oct. 25, 2006

1. LCLS Alternate Parameters and Setup C. Limborg-DepreyOct. 25, 2006 • Laser pulse • Longitudinal (Stacker vs “Square”) • Transverse • Reduced gun voltage • 110 MV/m , 100MV/m • Solenoid Alignment

2. Laser Longitudinal Pulse shape • Difficult to meet specifications • Rise time 1 ps • Uniformity 5% rms ( ~50 -300m) • To suppress micro-bunching instability • Mitigation: Pulse stacker • Pulse built with 2-3 gaussians give satisfactory performances • Lower risk of space-charge induced microbunching • “Twin Peak” even better for compression (core flatter at 135 MeV ) = 1.7 ps = 1.15 ps

3. 3.4 kA 3.4 kA After Compression from nominal “Square” Laser pulse from “Twin Peak “ Laser pulse built with stacker of 3 Gaussians Spikes reduced Core has higher peak current Courtesy P.Emma

4. Transverse Uniformity Newport Simulations Newport shaper has ~20% ptp Exceeds specifications of 10% ptp Simulations of that particular frequency show acceptable performances Newport will keep working on meeting the specs

5. Gun Peak Field • 120MV/m peak gun field might be difficult to run reliably (breakdowns, large dark current) • 110 MV/m gives performances similar to 120 MV/m • 100 MV/m give acceptable performances for commissioning

6. 100 MV/m, 2 Gaussians, 0.5 nC < {10,90} > ~ 0.7 mm-mrad Tuning is even better if using a 0.6 mm radius beam 80 ~ 0.78 mm-mrad < {10,90} > ~ 0.6 mm-mrad

7. Solenoid Alignment • Identified as important for reaching emittance levels required for FEL operation • Tight tolerances (i.e. 250 rad for nominal parameters) • Requires Beam Based Measurements • Very non-linear problem • Studied in collaboration with FLASH team • benchmark of Matlab code against V-code • recommendation of remotely controlled movers • bring expertise (M.Krasilnikov,PITZ) for experiment

8. Conclusions • Identified some risks (laser, gun field, solenoid alignment) • Mitigation • Laser longitudinal pulse shape • stacker might even give better performances • Laser transverse uniformity • Close to specs, might still improve • Gun Peak field: 100MV/m ok for commissioning • Solenoid Movers requested for Fall’ 07 down

9. BACK-UP

10. Micro-Bunching Instability • J.Wu et al. LCLS-TN-04-6 • Tolerance longitudinal modulation < 5% rms Longi. Phase Space at entrance undulator Initial modulation 5% rms with ~150 m No laser heater Same but with laser heateron

11. Longitudinal Space Charge Instability LSC observed at the DUVFEL Courtesy of Timur Shaftan Also observed at TTF Current Density Energy Simulations and theoretical studies Z.Huang et al. PhysRev. SLAC-PUB-10334 J.Wu et al. LCLS Tech Note , SLAC-PUB-10430 G.Geloni. Et al. DESY 04-112 The self-consistent solution is the space charge oscillation

12. ASTRA/ PARMELA Simulations , Amplitude = +/- 5%,  = 100 mm GUN EXIT 6 MeV ENERGY CURRENT

13. End L0b 135 MeV ENERGY CURRENT Microstructure at the end of the injector Laser Heater provide enough energy spread (40keV) for “Landau damping” preventing -further amplification of the microbunching - the increase an energy spread (as it needs to remain < the FEL parameter)

14. Risk: Solenoid Alignment • Tolerance : 250 m, 250rad w.r.t to gun electrical axis • Requires beam based alignment • 1) Determine center of cathode • 2) Determine error in solenoid position with as few steps of solenoid motion • F(X, rf, Vrf, Bsol, Xsol) = Xf • 4 unknowns Xsol = (x,x’,y,y’) • Center BPM/Screen cannot be determined with beam • Angle resolution from BPM2-BPM3 > 100 rad • Code • Single Particle tracking in Matlab for on-line modeling • gun to L0a (including misalignment of components + earth magnetic field) • to be extended to DL1 (i.e linacs + quads) Solenoid SC0 SC1 SC2 BPM2 BPM3 BPM5 Gun L0a

15. Solenoid Risk: Solenoid Alignment • 1) Center of cathode • Steer laser centroid on 2D grid • Scan Gun RF phase YAG02 Gun Centroid on cathode Centroid on YAG01, rf [24,36]

16. Risk: Solenoid Alignment • 2) Mispositioning Solenoid • (Position, Angle ) does not vary with strength when the Solenoid aligned Vary Solenoid strength • Algorithm • A- Assume center cathode known to better than 50 m • B- Assume axis gun on screen is known within 50 m • C- Requires at least 8 motions of solenoid • Issues: • Too little light on single pulse when solenoid field small Only explore small Bz range • Model based needs to use variation of phase

17. Risk: Optimization • Scan parameters : (rf, Vrf , B solenoid) • Large Variation of betatron function while varying Bsolenoid • Rematching necessary for emittance measurements • 3 screen emittance : best resolution for perfect parabola (with 2 = ½ 1 = ½ 3 )

18. Tuned to matching of -0.6% case Tuned to matching of 1.8% case Matching performed for each point

19. Emittance Compensation 0.3m 10 Courtesy J.Schmerge Theory (perfect surface) ~ 0.3 mm.mrad /mm radius Measured ~ 0.6 mm.mrad /mm radius

20. Modeling Transverse Profile • Case of ~ 30% peak-to-peak • Optimization: scan solenoid 1 • Minimum SOL at +1% , p ~1.2 mm-mrad , 80 ~1.0 mm-mrad uses all the error budget • But, <10,90> ~0.92 mm-mrad

21. Modeling Transverse Profile • Case of 60% peak-to peak • 80 just hardly meets 1.1 mm-mrad • < (10,90) > hardly meets 1mm-mrad

22. Modeling Transverse Profile • Case of 20% peak-to peak • 80~ 0.94 mm-mrad , < (10,90) > ~ 0.85 mm-mrad

23. Summary Transverse profile