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Generation of Sub-fsec, High Brightness Electron Beams for Single-Spike SASE FEL Operation

Generation of Sub-fsec, High Brightness Electron Beams for Single-Spike SASE FEL Operation. J.B. Rosenzweig UCLA Dept. of Physics and Astronomy for SPARC(X)-UCLA collaboration. Ultra-short FEL pulses. Investigations at atomic electron spatio-temporal scales

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Generation of Sub-fsec, High Brightness Electron Beams for Single-Spike SASE FEL Operation

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  1. Generation of Sub-fsec, High Brightness Electron Beams for Single-Spike SASE FEL Operation J.B. Rosenzweig UCLA Dept. of Physics and Astronomy for SPARC(X)-UCLA collaboration

  2. Ultra-short FEL pulses • Investigations at atomic electron spatio-temporal scales • Angstroms-nanometers (~Bohr radius) • Femtoseconds (electronic motion, Bohr period) • 100 femtoseconds using standard techniques • Many methods proposed for the fsec frontier • Slotted spoiler; ESASE; two stage chirped pulse • Unsatisfactory (noise pedestal, low flux, etc.) • Still unproven • Use “clean” ultra-short electron beam • Myriad of advantages in FEL and beam physics

  3. A new path: ultra-low charge electron beam • Excellent phase space ( and ||) • Very low emittance • Highly compressible • Ultra-short beam • Very high brightness • Bunch ~ cooperation length; super-radiantsingle spike • Short cooperation length; femtosecond pulse • Clean, ultra-short pulse • Mitigate collective effects dramatically • CSR instability • Undulator beam-pipe wakes

  4. Working backwards: FEL requirements • 1D dimensionless gain parameter • 1D gain length • Cooperation length • Single spike operation

  5. Numerical example: SPARX • Take 2 GeV operation, “standard undulator”, =3 nm • Peak current I=2 kA, • Estimate single spike condition: • Note: with ultra-small Q,  is enhanced • Spike is a bit shorter… • FEL gain better (watch diffraction)

  6. Compression scaling • Beam momentum distribution in , ignoring slice spread • Quadratic term no longer dominant • Include uncorrelated term • Moments:

  7. Compression with chicane • Needed chicane essentially unchanged • Final bunch length/initial • “Thermal” spread from velocity bunching • In low Q limit

  8. Example: SPARX (preliminary) • Compression at 2 GeV before undulator • Need nm • Choose to accelerate 23° forward of crest • Deduce upstream beam of • Must produce from velocity buncher • Check consistency with energy spread • We have • Final (full) energy compression OK • not in LCLS case

  9. Photoinjector scaling • Change beam Q while keeping plasma frequency (n and aspect ratio) same • Dimensions scale • Shorter beam… • Emittances: • At low Q, x,th dominant in Ferrario WP • With low Q vel. bunching (1st attempt)

  10. Low charge working point • Velocity bunching gives • Space charge limit on long. Dynamics • Depends on transverse focusing • Need • Not that short… factor of 10 below present • Work at Q=1 pC (factor of 103) • Emittance > compensated value • Some growth in velocity buncher (?) • Higher brightness beam in the end!

  11. Velocity bunching • Proposed by Serafini, Ferrario; tool for SASE FEL injector, avoids magnetic compression • Inject emittance-compensated beam at 5-7 MeV into vf≤c linac • Effectively compresses at low energy — good for energy spread control • Perform one-quarter of synchrotron oscillation to compress beam • Gentle, low gradient option • Hamiltonian model (forward wave) Longitudinal phase space schematic for velocity bunching from Hamiltonian picture

  12. Beam simulations for original 1 pC case (UCLA PARMELA) Summary

  13. Beam envelope evolution (cm) Very small beam sizes

  14. Emittance evolution Still very small, other optimizations possible

  15. Compression at SPARX • SPARX example, compress at 2 GeV, • Compressor: • Analytical est. of growth in , p: • With I=260 A, and • Two orders of magnitude enhanced

  16. Genesis simulation of SPARX • Standard case • Start-to-end from PARMELA/Elegant • Do not take advantage of lower  by changing  to evade diffraction • Single spike operation

  17. FEL peak power, profile evolution z  Saturation in <30 m Extremely clean pulse No sideband instability

  18. Power profile • 220 MW peak power (/10 from standard) • <1 femtosecond rms pulse! • Narrower than electron beam

  19. Spectral properties • Nearly Fourier transform limited at onset of saturation

  20. Push to shorter pulses at SPARX Q=1 pC case • Study 1 and 10 pC case. Use LANL PARMELA (standardization) • Relax focusing during velocity bunching, low emittance growth • For 1 pC, sz only 4.7 mm after velocity bunching • Use most June 2008 version of SPARX lattice • compression no longer at end, at 1.2 GeV (Final 2.1 GeV) • Much higher currents, • some CSR emittance growth, • Longitudinal tails, higher peak brightness

  21. FEL performance at 1 pC • Single spike with some structure • > 1 GW peak power at saturation (30 m) • 480 attosecond rms pulse s (mm)

  22. Higher Q SPARX case: 10 pC • Put back beam power, X-ray photons • Velocity bunch to 10.3 mm rms (~Q1/3) • Emittance growth

  23. SPARX FEL at 10 pC s (mm) • 10 GW peak FEL power • Saturation at 20 m (v. high brightness) • Quasi-single spike

  24. Extension to LCLS case • 1 pC does not give quasi-single spike operation • Beam too long, need to scale with l/r • Use 0.25 pC (1.5M e-), obtain en=0.033 mm-mrad • Yet higher brightness; saturation expected in 60 m Over 350A peak

  25. LCLS Genesis results Deep saturation achieved Minimum rms pulse: 150 attosec Quasi-single spike, 2 GW

  26. Not all shots single spike z 

  27. Alternative scenario: new undulator for short l operation • Can also use high brightness beam to push to short wavelength • Shorter period undulator

  28. Other injector scenarios X-band simulation • X-band (higher field, >200 MV/,m) • Higher field can yield higher brightness • Scaling requires new structure: e.g.hybrid SW-TW • Lower peak field, ~eliminate RF reflection • Inherent velocity bunching • S-band high power test @UCLA (w/LNF-Univ.Roma) with modular “split” structure

  29. Conclusions; future work • Very promising option • Excellent emittance and gain; can allow shorter  • Marry to “blowout regime” injector; shorter beams • Excellent beam scenario, but… • All measurements change • Low energy similar to electron diffraction scenario • Coherent optical signals at high energy • Clean up noise • Dark current has much more charge • Natural focusing; may use dual deflector/collimator • Tests at SPARC (velocity bunching only) • Beam dynamics of 1st stage compression (1 pC) • Scaled FEL operation: see Ferrario poster • First operation at LCLS at 18 pC shows excellent e • Need to add compression

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