Comments from LCLS FAC Meeting (April 2004):

1. Laser-Heater Modulation P. Emma, Z. Huang, J. Wu Comments from LCLSFAC Meeting (April 2004): J. Roßbach: “How do you detect weak FEL power when the gain is very low (few hundred)?” K. Robinson: “Can you modulate the laser heater in some way to help FEL signal detection?”

2. LCLS with poor gain (large emittance, jitter, M. Xie model) 1.5-Å LCLS, but gex,y 3.6 mm Gain 470 P 370 kW (not 10 GW)

3. Laser Heater at 135 MeV modulate laser power (1.2 to 19 MW @ 7 Hz) Kem Robinson idea… laser heater modulated slice energy spread (0.01% to 0.04% rms at 14 GeV)

4. Simulate LCLS (Linac + M. Xie) with linac jitter, large emittance (3 mm), spontaneous radiation background, and laser-heater modulated at 7 Hz (or other unique frequency << 120 Hz) • 1.5-ÅLCLS, except gex,y= 3.6 mm • 2% rms charge jitter • 0.5 ps rms gun-timing jitter • 0.1-deg rms RF phase jitter (each of 4 linacs) • 0.1% rms RF amplitude jitter (each of 4 linacs) • 2% rms emittance jitter • includes small emittance growth in laser-heater • 10-MW spontaneous power (1% BW cut)* • 10% rms radiation energy measurement noise fast, accurate 2nd-order linac model in Matlab, with jitter

5. Modulate Slice Energy Spread with Heater (Square-wave suggested by Gennady Stupakov) 7 Hz

6. bunch length energy spread peak current bunch arrival time e- relative energy Effects of linac jitter & modulation

7. Use ‘Ming Xie’ model to calculate gain length, Lg, then calculate power as: P < Psat: P > Psat: (none) 1-D startup power: Assume 1% BW cut to get 10-MW spontaneous power

8. Red: Total power + meas. noise Magenta: Spontaneous power Green: FEL power

9. 14.7 ± 4.1 m 0.029 ± 0.013 % 4.2 ± 0.6 kA Lg 13-29 m gex,y = 3.6 mm 21 ± 3.0 mm 0 ± 0.088 % 1.0 ± 0.02 nC

10. FFT of total power + noise 7 Hz

11. Enhanced Self-Amplified Spontaneous Emission A. Zholentsb P. Emmaa, W. Fawleyb, Z. Huanga, S. Reichec, G. Stupakova, a)SLAC, b)LBNL, c)UCLA • First proposed by A. Zholents (LBNL) • Submitted to PRL • LCLS study published in FEL’04 proc.

12. ESASE: “nuts and bolts” Modulation Acceleration Bunching 20-25 kA Energy modulation in wiggler at 4 GeV 50 fs laser pulse lL= 2 microns Peak current, I/I0 z /lL Only one optical cycle is shown • Electron beam after bunching at optical wavelength • Laser peak power ~ 10 GW • Wiggler with ~ 10 periods

13. ~200 as -100 0 -200 100 200 Theoutput x-ray radiation from a single micro-bunch • Each spike is nearly temporally coherent and Fourier transform limited • Pulses less than 100 attoseconds may be possible with 800 nm laser

14. A schematic of the LCLS with ESASE 250 MeV z  0.19 mm 4.3 GeV z  0.022 mm 13.6 GeV z  0.022 mm 6 MeV z  0.83 mm 135 MeV z  0.83 mm rf gun Linac-0 BC1 BC2 undulator L =130 m Linac-1 Linac-2 Linac-3 X ...existing linac DL1 wiggler LTU laser research yard SLAC linac tunnel

15. Beta and dispersion functions in the LCLS 3-m wiggler existing buncher

16. Peak current, emittance and energy spread after linac gex gey

17. At undulator entrance: E = 14 GeV

18. Genesis Ginger FEL Simulations laser wavelength of 0.8 or 2.2 nm

19. Summary • Shorter gain length, higher power, ~same average power • Adjustable x-ray pulse duration by changing laser pulse • Nearly temporally coherent and transform limited radiation within spike with random carrier phase between spikes - solitary attosecond x-ray pulse • Absolute synch. between laser pulse and x-ray pulse • Relaxes emittance requirement • Shorter x-ray wavelengths possible • Better with smaller b (and closer und. quad spacing) • Work on impedance issues ongoing (see FEL’04 proc.)