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External Seeding Approaches for Next Generation Free Electron Lasers

External Seeding Approaches for Next Generation Free Electron Lasers. Erik Hemsing SLAC Greg Penn LBNL. The Need to Seed. FEL output power. FEL power. SASE FEL. FEL spectrum. Improving the temporal coherence in the f ree electron laser (FEL).

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External Seeding Approaches for Next Generation Free Electron Lasers

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

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

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

  4. 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

  5. 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

  6. 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

  7. 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

  8. EEHG FEL: Advantages and Challenges Advantages • 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

  9. Echo Demonstration at SLAC (starting in 2010) ~50 m • 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

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

  11. 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 795 nm laser on (b) 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

  12. 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

  13. 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

  14. Going to harmonics >10 3 m • 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

  15. Going to harmonics >10 • The 120 MeVe-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)

  16. 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)

  17. 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 EEHG HGHG

  18. 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

  19. 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

  20. 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…

  21. 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)

  22. VISA undulator at NLCTA Undulator emission • Echo 75 possible at 5th harmonic of undulator 1st harmonic 3rd harmonic 5th harmonic

  23. 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:

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

  25. 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

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