W.S. Graves1 Seeding for Fully Coherent Beams William S. Graves MIT-Bates Presented at MIT x-ray laser user program review July 1, 2003

W.S. Graves2Outline 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

W.S. Graves3 Seeded beamSASE beam Output wavelength FEL param  FEL  t min (fs) at max BW  E min (meV) at 1 ps FWHM SASE  t min (fs) SASE  E min (meV) 100 nm9.e nm4.e nm1.5e nm0.2e Bandwidth and Pulse Length Seeded beams limited only by uncertainty principle and seed properties. SASE properties determined by ebeam. Data from BNL’s DUV-FEL experiment

W.S. Graves4Terminology 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.

W.S. Graves5 High Gain Harmonic Generation Modulator is tuned to  0. Electron beam develops energy modulation at  0. 3 rd harmonic bunching is optimized in chicane. Energy modulation is converted to spatial bunching in chicane magnets. Input seed at  0 overlaps electron beam in energy modulator undulator. Electron beam radiates coherently at  3 in long radiator undulator. Radiator is tuned to  3. Method to reach short wavelength FEL output from longer wavelength input seed laser.

W.S. Graves6 Cascaded HGHG Input seed  0 1 st stage2 nd stage 3 rd stage Output at 3  0 seeds 2 nd stage Output at 9  0 seeds 3 rd stage Final output at 27  0 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.

W.S. Graves7 UV HallX-ray Hall Nanometer Hall 4 GeV2 GeV1 GeV 1 nm 0.3 nm 100 nm 30 nm 10 nm 3 nm 1 nm Master oscillator Pump laser Seed laser Pump laser Fiber link synchronization Injector laser Undulators SC Linac

W.S. Graves8 to master oscillator for timing sync Pump lasers Ti:Sapp + BBO = 200 nm seed Tune wavelength by OPA GW power,.01 – 10 ps FWHM Ti:Sapp + BBO = 200 nm seed Ti:Sapp + HHG = nm seed Tune by OPA or harmonic number 100 nm 30 nm 10 nm Single HGHG undulator section Direct seeded or cascaded HGHG undulators 1 GeV ebeam UV Hall Seed lasers ~10 m length 10 GW peak ~20 m length 10 GW peak

W.S. Graves9 to master oscillator for timing sync Pump lasers Ti:Sapp + BBO = 200 nm seed Ti:Sapp + HHG = nm seed Tune by OPA or harmonic number 10 nm 3 nm 1 nm Direct seeded or cascaded HGHG undulators 2 GeV ebeam Nanometer Hall Seed lasers Ti:Sapp + HHG = nm seed Tune by OPA or harmonic number Cascaded HGHG undulators ~20 m length 10 GW peak ~30 m length 4 GW peak

W.S. Graves10 to master oscillator for timing sync Pump lasers 1 nm 0.6 nm 0.3 nm 4 GeV ebeam X-ray Hall Seed lasers Ti:Sapp + HHG = nm seed Tune by OPA or harmonic number Cascaded HGHG undulators ~30 m length 6 GW peak ~60 m length 4 GW peak (also 0.1 nm at 1% of 0.3 nm intensity)

W.S. Graves11 High-Harmonic Generation Noble Gas Jet (He, Ne, Ar, Kr) 100  J nm 3 – 30 nm  = Recombination Propagation -W b  XUV Energy  x bb 0 Laser electric field Ionization

W.S. Graves12 High Harmonic Generation Layout Courtesy of M. Murnane and H. Kapteyn, JILA W.S. Graves, MIT Bates Laboratory

W.S. Graves13 Pulse shaping of drive laser can enhance a single harmonic line. Courtesy of M. Murnane and H. Kapteyn, JILA Quasi-phase matching in modulated hollow- core waveguide. HHG enhancements How much improvement do we get with additional phase control for the very high harmonics in the water window < 4nm ?

W.S. Graves14 HHG spectra for 3 different periodicities of modulated waveguides. Courtesy of M. Murnane and H. Kapteyn, JILA 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 enhancements

W.S. Graves15 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.

W.S. Graves16 SASE properties GINGER simulation of SASE FEL at 0.3 nm. Time profileTime profile (log plot)Spectrum Electron beam parameters Energy4.0 GeV Peak current (amp)2000 A RMS emittance 0.8  m RMS energy spread.01 % Charge80 pC Beam power8.0 TW Bunch FWHM40 fs Laser beam parameters Pulse FWHM35 fs (~ebeam length) Saturation power~3.0 GW Energy0.2 mJ FWHM linewidth7.0E-4 Saturation length59 m For simulation speed. True bunch length will be longer.

W.S. Graves17 Seeding for short pulse Output time profileTime profile (log plot)Spectrum Seed laser parameters FWHM0.5 fs Power10.0 MW Pulse energy5 nJ FEL output parameters Saturation FWHM0.75 fs Saturation power~2.0 GW Saturation energy 1.5  J FWHM linewidth6.0E-4 Undulator length20 m GINGER simulation of seeded FEL at 0.3 nm. Note: does not include earlier HGHG stages Same ebeam parameters as SASE case.

W.S. Graves18 Seeding for narrow linewidth Output time profileTime profile (log plot) Spectrum Seed laser parameters FWHM50 fs Power0.1 MW Pulse energy5 nJ FEL output parameters Saturation FWHM30 fs Saturation power~2.0 GW Saturation energy0.1 mJ FWHM linewidth1.0E-5 Saturation length28 m GINGER simulation of seeded FEL at 0.3 nm. Note: does not include earlier HGHG stages Same ebeam parameters as SASE case.

W.S. Graves19 Seeded and SASE comparison Seeded and SASE time profiles and spectra. Different schemes require different undulator length.

W.S. Graves20 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. time frequency FEL bandwidth slippage time energy Seed optical pulseElectron pulse

W.S. Graves21 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 10 4 compression.)

W.S. Graves22 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.

W.S. Graves23 Cr:Fo and Ti:Sapp lasers in Kaertner lab

W.S. Graves24 Independent Cr:Fo and Ti:Sapp lasers synchronized with sub-fs timing jitter by F. Kaertner. Error signal from optical double balanced mixer.

W.S. Graves25 Source comparison APSMIT Bates Und. ASASE FEL Min bandwidth seeded FEL Min pulse length seeded FEL X-rays per pulse (0.1% max BW) 1.E+083.E+11 6.E+09 Peak brilliance (p/s/0.1%/mm 2 ) 3.E+221.E+333.E+357.E+33 Peak flux (p/s/0.1%)1.E+186.E+24 1.E+23 Avg. flux (p/s/0.1%)7.E+143.E+14 6.E+12 Average brilliance (p/s/0.1%/mm 2 ) 4.E+195.E+221.E+253.E+23 Degeneracy parameter0.14.E+093.E+116.E+09 Pulse length (fs) Photon beamlines Wavelength (nm) Pulse frequency (Hz)7.E