External Seeding Approaches for Next Generation Free Electron Lasers Erik Hemsing SLAC Greg Penn LBNL

Improving the temporal coherence in the free electron laser (FEL) The Need to Seed FEL output power FEL power FEL spectrum SASE FEL Improving the temporal coherence in the free electron laser (FEL) e-beam energy vs time space (many slices)

The effect of coherent seeding FEL output power FEL power FEL spectrum SASE FEL Dream FEL Seeding would make an FEL an extraordinarily good laser e-beam energy vs time space (many slices)

Seeding Methods Ultimate goal: Seeding to generate transform limited x-ray pulses Several seeding approaches: High Harmonic Generation (HHG) High Gain Harmonic Generation (HGHG) Various Self-seeding techniques (HXRSS and SXRSS) Echo-Enabled Harmonic Generation (EEHG) Echo is a new approach where laser challenges are traded for beam manipulation challenges Has advantage in that bunching is weak function of harmonic number and only small relative energy modulations required Echo (EEHG) demonstration to benchmark critical accelerator and laser physics issues Find optimal combination of high-harmonics and short wavelength seeds Seeding methods potentially relevant to future SLAC programs…

Classical external seeding with HGHG λ Energy modulation in a modulator Energy modulation converted to density modulation with a chicane Coherent radiation at amplified to saturation in a radiator

Limitations on single stage HGHG Low up-frequency conversion efficiency: Modulator exit Chicane exit Current distribution Outcome: Bunching (large ∆E ) OR Gain (small ∆E) But seeded FEL wants: Bunching AND Gain

Echo-Enabled Harmonic Generation (EEHG) G. Stupakov, PRL, 2009; D. Xiang, G. Stupakov, PRST-AB, 2009 First laser generates energy modulation in electron beam First strong chicane stratifies the longitudinal phase space Second laser imprints energy modulation Second chicane converts energy modulation into harmonic density modulation

EEHG FEL: Advantages and Challenges Excellent frequency up-conversion efficiency from small energy modulation UV laser up-converted to soft x-rays in a single stage Tunable through dispersion (Relatively) insensitive to e-beam phase space distortions Challenges Preservation of fine-grained phase-space correlations Sensitive to instabilities, CSR, quantum diffusion, intrabeam scattering, etc. Depends on laser quality, stability, and spectral phase errors

Echo Demonstration at SLAC (starting in 2010) Used existing NLC Test Accelerator for demonstration Rf gun (S), high gradient rf (X) and multiple laser systems pre-installed Added 60 MeV acceleration, 3 chicanes (C0-C2), 3 undulators, and lots of diagnostics in 2010 Subsequently installed two TCAVs, VUV spectrometer and upgraded energy spectrometer ~50 m

Echo Experiment at SLAC’s NLCTA Existing Main echo beam line constructed after 10/2009 C-1 TCAV1 X2 TCAV2 C1 U1 U2 spectrometer

First unambiguous Echo signal with chirped beam 1590 nm laser on 1590 nm laser on (a) 1590 nm laser on H3 (a) (a) H4 795 nm laser on 795 nm laser on H2 H2 (b) 795 nm laser on H2 (b) (b) Both lasers on H4 H2 H3 (c) Echo 350 400 450 500 550 600 Radiation wavelength (nm) D. Xiang et al., PRL 105, 114801 (2010) EEHG and HGHG have different dependence on structures in the e-beam phase space

Pushing EEHG to realistic scenarios The advantage of EEHG lies in efficient upconversion even for Typically a ‘laser heater’ is used to increase beam slice energy spread We use RF TCAV used to increase slice energy spread

7th harmonic Echo (2012) 1590 nm only, V=0 Retuned U3 radiator for resonance at 227 nm 4th to 7th harmonics from HGHG are suppressed with increased beam slice energy spread 7th harmonic reappears with the first laser on Energy modulation is about 2~3 times the beam slice energy spread 1590 nm only, V=85 kV 1590 nm only, V=170 kV 1590 nm only, V=255 kV 795 nm +1590 nm, V=255 kV

Going to harmonics >10 In 2012 the entire Echo line (modulators, chicanes) moved upstream by 3m to accommodate new structures Replaced X1 linacs with single RDDS (better alignment, no SLED) Installed chicane bypass (cleaner phase space) Upgraded laser systems and PLCs U2 retuned to be resonant with 2400 nm laser New OPA purchased and commissioned RF undulator installed Spherical mirror installed to enhance spectrometer signal

Going to harmonics >10 The 120 MeV e-beam energy limits the shortest wavelengths that can be detected with Echo 7 infrastructure A ‘gift’ from RF structure testing program: RF undulator lu=1.39 cm, 77 periods, variable tuning with input RF power (K=0-0.7) ~1 Tesla, 30 J of stored energy(!) S. Tantawi, et al PRL 112, 164802 (2014)

15th harmonic Echo (2014) 15th harmonic 2 orders of magnitude higher than incoherent signal HGHG signals also visible, shows double peaked spectrum unless DE2 reduced by 20% (n=-1, m=18) EEHG HGHG (n=0, m=15) 160 nm from 2400 nm DE1=80 keV DE2=65 keV R56(1)=4.8 mm R56(2)=1.0 mm EEHG has 60% higher spectral brightness and narrower bandwidth comparing optimized cases E. H, et al PRST-AB 17, 070702 (2014)

EEHG signal has narrower bandwidth Bandwidth comparison EEHG signal has narrower bandwidth (Dl/l=0.23% vs 0.62 %) 0.38 nm 1 nm Two effects: different dependence of EEHG and HGHG on local phase space distribution and finite length laser pulse Non-linear curvature adds more bandwidth to HGHG by shifting wavelengths across the beam front is compressed, back is decompressed EEHG less sensitive because strong initial R56 removes this smooth variation HGHG EEHG

Central wavelength stability Reduced sensitivity of EEHG to phase space distortions stabilizes central wavelength RF timing drift or jitter in e-beam can change chirp –> shift in central wavelength OR, timing jitter between laser and e-beam (ie, energy jitter) changes laser overlap and selects differently chirped region EEHG HGHG E-beam laser

15th harmonic efficiency at DE/sE~6 Because we are limited by chicane strength (R56<10mm), we use the TCAV to increase the slice energy spread to test harmonic efficiency in realistic regime TCAV increased to 450 kV sE =10 keV EEHG signal persists, HGHG destroyed

Echo experiment status Measured 15th harmonic with modulation ~6 times energy spread EEHG less sensitive to phase space distortions Narrower bandwidth Higher spectral brightness More stable central wavelength Relevant for future seeded FELs in the presence of MBI and wakefields etc that can produce considerable higher order correlations Numerous facility upgrades have been performed to improve beam quality and signal Developing next phase of Echo program…

Toward higher harmonics at shorter wavelengths Boost beam energy to 160 MeV Beginning experimental studies of laser phase errors and stabilization (critical for EEHG and HGHG) Implement a modified zero phasing technique to directly measure laser microbunching on beam compare with spectral measurements Installing 2m VISA undulator to access harmonic undulator frequencies (160 nm, 75 nm and 35 nm) and more photons VISA undulator T105 (+40MeV)

VISA undulator at NLCTA Undulator emission Echo 75 possible at 5th harmonic of undulator 1st harmonic 3rd harmonic 5th harmonic l (Echo harmonic) 164 nm (15) 55 nm (44) 33 nm (73) VISA-II 2m undulator: E=160 MeV lu=1.8 cm K=1.26 Nu=110

Spectral phase measurements with booster Zero-phase crossing Zero phasing technique plus R56 allows measurement of fine scale temporal structures in energy domain Dispersion followed by linear energy chirp rotates phase space so that energy maps directly to time when 1+h R56=0:

Femtosecond visualization of microbunching D. X, E. H et al, submitted to PRL (2014)

Upcoming Plans Move progressively to Echo 75 at ~30 nm by end of FY 2015 Need to continue to study beam parameter space and feasibility of EEHG at ultra high-harmonics Sensitivities to emittance, horizontal dispersion, IBS, etc Install and commission SDL VISA undulators Characterize undulator harmonic emission spectrum Build and commission new VUV spectrometer down to 30 nm Embark on complimentary spectral phase measurements using zero-phasing technique fs 800 nm setup 2.4 um control station