Seeding for Fully Coherent Beams

1. Seeding for Fully Coherent Beams William S. Graves MIT-Bates Presented at MIT x-ray laser user program review July 1, 2003

2. Outline • Bandwidth and pulse length • Terminology • High Gain Harmonic Generation • Facility layout, experimental halls, beamlines • Plans for short wavelength seed generation • Simulations of seeded x-ray performance • Femtosecond timing • Source parameter summary

3. Bandwidth and Pulse Length Data from BNL’s DUV-FEL experiment Seeded beams limited only by uncertainty principle and seed properties. SASE properties determined by ebeam.

4. Terminology • SASE: self amplification of spontaneous emission. Electron beam amplifies initial spontaneous undulator radiation. Transverse coherence, but not longitudinal. • Seeded beams: A coherent laser pulse is introduced at the undulator entrance. The seed power must dominate the initial undulator radiation. • Self-seeding: In a 2-part undulator, radiation from the first section seeds the FEL process in the second section. • HHG: High-harmonic generation. A method of generating pulses of ~10 nm light by focusing a Ti:Sapp laser in a gas jet. • HGHG: High-gain harmonic generation. A method of frequency multiplying an input seed laser to reach a shorter output wavelength. • Cascaded HGHG: Multiple stages of HGHG to reach ever shorter wavelengths. • CPA: Chirped pulse amplification. A time-frequency correlation is introduced in a light pulse so that it may be optically compressed after amplification, greatly increasing the maximum power and decreasing the minimum pulse length. • OPA: Optical parametric amplifier. A method of generating continuously variable wavelength laser light by mixing multiple beams.

5. High Gain Harmonic Generation Method to reach short wavelength FEL output from longer wavelength input seed laser. Input seed at w0 overlaps electron beam in energy modulator undulator. Energy modulation is converted to spatial bunching in chicane magnets. Electron beam radiates coherently at w3 in long radiator undulator. Radiator is tuned to w3. Modulator is tuned to w0. Electron beam develops energy modulation at w0. 3rd harmonic bunching is optimized in chicane.

6. Output at 3w0 seeds 2nd stage Output at 9w0 seeds 3rd stage Final output at 27w0 Input seed w0 3rd stage 1st stage 2nd stage Cascaded HGHG • Number of stages and harmonic of each to be optimized during study. • Factor of 10 – 30 in wavelength is reasonable without additional acceleration between stages. • Seed longer wavelength (100 – 10 nm) beamlines with ~200 nm harmonic from synchronized Ti:Sapp laser. • Seed shorter wavelength (10 – 0.3 nm) beamlines with HHG pulses.

7. Master oscillator Fiber link synchronization UV Hall X-ray Hall Seed laser Pump laser Seed laser Pump laser Undulators 100 nm Undulators 30 nm 1 nm Injector laser 10 nm 0.3 nm SC Linac 1 GeV 2 GeV 4 GeV 10 nm 3 nm 1 nm Undulators Seed laser Pump laser Nanometer Hall

8. UV Hall to master oscillator for timing sync Single HGHG undulator section 100 nm Ti:Sapp + BBO = 200 nm seed Tune wavelength by OPA GW power, .01 – 10 ps FWHM Pump lasers ~10 m length 10 GW peak Seed lasers 30 nm Ti:Sapp + BBO = 200 nm seed Ti:Sapp + HHG = 10-30 nm seed Tune by OPA or harmonic number Direct seeded or cascaded HGHG undulators 10 nm ~20 m length 10 GW peak 1 GeV ebeam

9. Nanometer Hall to master oscillator for timing sync Direct seeded or cascaded HGHG undulators 10 nm Pump lasers Ti:Sapp + BBO = 200 nm seed Ti:Sapp + HHG = 10-30 nm seed Tune by OPA or harmonic number ~20 m length 10 GW peak Seed lasers Cascaded HGHG undulators 3 nm Cascaded HGHG undulators 1 nm Ti:Sapp + HHG = 10-30 nm seed Tune by OPA or harmonic number ~30 m length 4 GW peak 2 GeV ebeam

10. X-ray Hall to master oscillator for timing sync Cascaded HGHG undulators 1 nm Pump lasers ~30 m length 6 GW peak Seed lasers Cascaded HGHG undulators Ti:Sapp + HHG = 10-30 nm seed Tune by OPA or harmonic number 0.6 nm Cascaded HGHG undulators 0.3 nm ~60 m length 4 GW peak 4 GeV ebeam (also 0.1 nm at 1% of 0.3 nm intensity)

11. 100 mJ - 1 mJ @ 800 nm XUV @ 3 – 30 nm h = 10-8 - 10-5 t Propagation Recombination 0 wXUV x tb -Wb Ionization Energy Laser electric field High-Harmonic Generation Noble Gas Jet (He, Ne, Ar, Kr)

12. High Harmonic Generation Layout Courtesy of M. Murnane and H. Kapteyn, JILA W.S. Graves, MIT Bates Laboratory

13. HHG enhancements Pulse shaping of drive laser can enhance a single harmonic line. Quasi-phase matching in modulated hollow-core waveguide. Courtesy of M. Murnane and H. Kapteyn, JILA How much improvement do we get with additional phase control for the very high harmonics in the water window < 4nm ?

14. HHG enhancements • HHG has produced wavelengths from 50 nm to few angstroms, but power is very low for wavelengths shorter than ~10 nm. • Best power at 30 nm. • Improvements likely to yield 10 nJ at 5 nm. • Rapidly developing technology. HHG spectra for 3 different periodicities of modulated waveguides. Courtesy of M. Murnane and H. Kapteyn, JILA

15. Initial GINGER simulations at 0.3 nm • What is included • Fully time dependent…includes short pulse effects. • Accurately models interaction of seed power with electron beam. • Includes all electron beam effects: energy spread, time structure, beam size and divergence. • What is not yet included • Modeling of HGHG process from long wavelength seed to short wavelength output. • Cascaded HGHG sections.

16. SASE properties Time profile Time profile (log plot) Spectrum GINGER simulation of SASE FEL at 0.3 nm. For simulation speed. True bunch length will be longer.

17. Seeding for short pulse Output time profile Time profile (log plot) Spectrum GINGER simulation of seeded FEL at 0.3 nm. Note: does not include earlier HGHG stages Same ebeam parameters as SASE case.

18. Seeding for narrow linewidth Spectrum Output time profile Time profile (log plot) GINGER simulation of seeded FEL at 0.3 nm. Note: does not include earlier HGHG stages Same ebeam parameters as SASE case.

19. Seeded and SASE comparison Seeded and SASE time profiles and spectra. Different schemes require different undulator length.

20. frequency FEL bandwidth slippage time time Chirped pulse amplification (CPA) FEL bandwidth of ~1.0E-3 limits minimum pulse length, while induced energy spread limits peak power. These limits can be stretched by overlapping seed pulse that has time/frequency correlation (chirp) with matching electron beam. Compress optical beam with grating or crystal following amplification. energy Seed optical pulse Electron pulse

21. CPA FEL speculation Theoretical pulse length and peak power assuming 50 fs seed pulse with 6% chirp (3% FWHM ebeam chirp). Output is sub-femtosecond at TW peak power. Caveat: compression ratio up to 5000 depends upon no distortion of optical phase during FEL amplification. (Conventional lasers routinely exceed 104 compression.)

22. Femtosecond synchronization • Goal is to synchronize multiple lasers and electron beam to level of 10 fs. • MIT has locked multiple independent lasers together with sub-fs accuracy using an optical heterodyne detector (balanced cross correlator). • Optical clock community developing fs timing synchronization over longer distances. • Our timing requirements are considered quite challenging in the accelerator community.

23. Cr:Fo and Ti:Sapp lasers in Kaertner lab

24. Independent Cr:Fo and Ti:Sapp lasers synchronized with sub-fs timing jitter by F. Kaertner. Error signal from optical double balanced mixer.

25. Source comparison