Generation of Ultrafast Mid-IR pulses using a 100 MeV ERL-FEL (Drivers for tunable HHG based coherent X-Ray sources ?)

T. Popmintchev, nature photonics | VOL 4 | DECEMBER 2010 Phase matched HHG using mid-IR lasers (Experiments)  Idea (A.Foehlisch): Can we drive HHG by a compact ERL(FEL)?  requirements imposed on drive lasers : - HHG (phase matched) needs preferably few cycle to ~10 cycle drive laser pulses in NIR/MIR and intensities in the range of 1-5x10 14 W/cm 2 (noble gas filled hollow waveguide apertures: ~100m-200m ) Generation of coherent X-Ray pulses by HHG OPCPA’s NIR sub-10 fs with 70  J energy at 100kHz. NIR sub-10 fs multi-kHz, multi-mJ Mid-IR (~3  m) sub-100 fs with a few micro-Joule energy at 100kHz 3.9  m sub-100 fs with 6 mJ at 10-20Hz

Outline :  short term: carrying out the HHG experiments on an existing FEL facility that meets the requirements set on the mid-IR drive laser, verifying the theory throughout the mid- IR (and beyond 10  m if necessary) (JLab ???)  long term: mid-IR ERL-FELs should be able to perform better than atomic lasers in terms of : tunability (throughout the nir/mid IR and beyond) - rep rate (MHz) in generating mJ(s) of ultrafast pulses with high average power (problems in CEP stabilization???) simulation study has been and still is mainly focused on the latter and on the question: What system requirements will be imposed on a compact ERL, (particularly concerning timing jitter budget)

Chirped pulse generation in a FEL oscillator using a chirped electron beam and pulse compression (JLab) Mode-locking techniques in FELs -Active mode-locking (multiple OK sections used in a cavity) - Passive mode-locking (JAERI, lasing at ~22  m) (single spike, high gain superradiant FEL osc.) Generation of short electron pulses (JLab) Ultrashort Pulse Generation in (Mid IR) FELs

E ~ 60 MeV (NIR/MIR) E ~ 13 MeV (FIR) 135 pC pulses  z ~ 0.5 – 4 ps 10.7 MHz (21.4 MHz FIR) Parameter NIR FEL MIR FEL FIR FEL Wavelength (μm) 2.5 to 27 8 to > to 1100 Wawenum (cm −1 ) 400 to 4000 < 70 to to 100 FSU-NHMFL NIR/MIR/FIR (&broadband THz) FEL Proposal X MIR/FIR FIR NIR inclusion of a HHG based coherent X-Ray source ?

Trim Quads reading Beam parameters FEL (1.6  m ) Units Beam Energy115MeV Bunch charge110 (135)pC  _ z rms 150fs Peak current~300A  _ e rms (uncorrelated) 0.1%  _ e rms (correlated) 0.5% nor. trans. Emit.8  rad rep. rate~75MHz Coherent OTR interferometer autocorrelation scans for bunch length measurements system parameters BERLinPro JLab IR FEL

stretcher compressor PLE dielectric mirror NIR/MIR FELO mode matching telescope - Beam Energy: 100 MeV - Bunch Charge: 80 pC - Rep rate: 40 MHz - Outcpl.Pls. Energy:  J - Cav. Enhancement: Pulse width: ~ fs (fwhm) -I L ~ 1x10 14 – 3.5x W/cm 2 - high-Q enhancement cavity (EC) smoothes out power and timing jitter of the injected pulses inherent to FEL interaction. - allows fs ( ?) level synchronization of the cavity dumped mid-IR pulse with the mode-locked switch laser. Mode-locked NIR Laser - Depending on the recombination time of the fast switch, sequence of micropulses with several ns separation can be ejected from the EC ! Suggested (3-6  m) MIR FEL & Pulse Stacker Cavity

Brewster W. vacuum vessel Opt. Switch mount Folded cavity FEL Input Coupler High Reflector T. Stanford IR-FEL achieved enhancement of ~ using an external pls stacker cavity (1996) Q ~ 40 (Finesse ~ 300 ) enhancement :~90 Q~ 50 enhancement :~ estimated JLab ~ 100 Enhancement JLab

~ 3 m /~ 4%-5% 100fs (fwhm) 200fs (fwhm) 3  m - 6  m Short Pulse FEL (cavity detuning) /~ 4%-5% - low time jitter - low peak to peak power deviations - Outcoupled Pulse Enegies: ~ J ~ 10 cycle pulses (HHG drive laser) ~ 6 m Talk in Nov. 2010

High Gain (superradiant) FEL Oscillator operating at cavity synchronization Synchrotron Osc. Freq. nearly an order of magnitude higher outcoupled pulse intensity (despite low outcoupling ratios) FEL efficiency in superradiance mode more than doubled fs (fwhm) l c ~ 45fs Talk in Nov. 2010

Comparison between two FEL simulation methods 3D (semi-)frequency domain 1½D - SVEA time domain (superradiant) FEL synchr.' case ‘FEL oscillator-cav. detuning' case  good agreement between the models in 'FEL oscillator with cavity detuning' case (in terms of outcoupled pulse energy, temporal and spectral pulse profiles)  Disagreements in the 'superradiant operation at cavity synchronism' in obtaining self similar pulses following saturation, differences in temporal and spectral pulse profiles.

 : timing jitter L : cavity length  L: cavity length detuning f : bunch rep. frequency (perfectly synchronized to L)  : cavity roundtrip time ( 2L/c)  /  =  L/L +  f/f e- bunch FEL Osc. sensitivity to temporal jitter  Bunch time arrival variation effectively has the same effect as cavity length detuning.  effect of the timing jitter on the FEL performance In slippage dominated short pulse FEL oscillators cavity detuning is necessary to optimize the temporal overlap between optical and e- pulses (Lethargy effect).Timing jitter induces fluctuations on the operational cavity detuning.

~ 6 m Peak power fluctuations ~4-5% rms Pulse width fluctuations limited to a few % timing jitter ~ ±20 fs (optical pulse) Jitter 5 fs rms Jitter 10 fs rms w/o initial Jitter FEL Osc. sensitivity to temporal jitter ~ 6 m Simulation using BERLinPro parameters, 'FEL oscillator with cavity detuning'

P~2% rms 100fs (fwhm) Pulse width fluctuations limited to a few % rms Timing jitter ~ ±20 fs (optical pulse) ~ 3 m Peak power fluctuations ~8 -10% rms jitter 5 fs rms w/o initial jitter FEL Osc. sensitivity to temporal jitter Simulation using BERLinPro parameters, 'FEL oscillator with cavity detuning'

Timing jitter JLab IR-FEL phase noise spectra measured in the vicinity of the wiggler-entrance (behind the bunch compressor) e- bunch length: 150 fs rms average current : 0.5 mA to 4.5 mA (bunch charge ~135 pC kept constant, bunch rep rate varied) measured timing jitter : ~25 fs 1.5 mA - ~80 fs 4.5 mA estimated FEL spec (to keep pp-power fluct. below 10 l = 1.6  m ) on arrival time jitter :  L/L < 3.8x10 -8 ( P. Evtushenko, ELECTRON BEAM TIMING JITTER AND ENERGY MODULATION MEASUREMENTS AT THE JLAB ERL ) (Beam Current Monitor (cavities) and Signal Source Analyzer employed for power spectrum measurements at harmonics to characterize phase noise)

FEL Osc. sensitivity to temporal jitter ~ 6 m jitter 2.5 fs rms w/o initial jitter jitter 2.5 fs rms 1D-SVEA Simulation using BERLinPro parameters, 'superradiant operation at cavity synchronism'

~5E~5E  r  ~  E  r  8% -10% spent beam momentum spread (full) generated by the FEL interaction large energy spread acceptance is required for beam transport/energy recovery (JLab IR Upgrade acceptance :~15 %) Calculated spent beam energy saturation =3 m =6 m