1 / 28

Approaches for the generation of femtosecond x-ray pulses

Approaches for the generation of femtosecond x-ray pulses. Zhirong Huang (SLAC). The Promise of X-ray FELs. Ultra-bright. Ultra-fast. Single Molecule Imaging with Intense fs X-ray. R. Neutze et al. Nature, 2000 . Introduction.

tilly
Télécharger la présentation

Approaches for the generation of femtosecond x-ray pulses

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Approaches for the generation of femtosecond x-ray pulses Zhirong Huang (SLAC)

  2. The Promise of X-ray FELs Ultra-bright Ultra-fast

  3. Single Molecule Imaging with Intense fs X-ray R. Neutze et al. Nature, 2000

  4. Introduction • Femtosecond (fs) x-ray pulses are keys to exploring ultra-fast science at a future light source facility • In typical XFEL designs based on SASE the photon pulse is similar in duration to the electron bunch, limited to 100~200 fs due to short-bunch collective effects • Great interests to push SASE pulse length down to ~10 fs and even below 1 fs • A recent LCLS task force studied upgrade possibilities, including short-pulse approaches • I will discuss and analyze several approaches in the next 1800000000000000000 fs!

  5. Outline of the Talk • Temporal characteristics of a SASE FEL • Optical manipulation of a frequency-chirped SASE • Compression • Slicing: single-stage and two-stage • Statistical analysis • Electron bunch manipulation • Spatial chirp • Enhancing undulator wakefield • Selective emittance spoiling (slotted spoiler) • Sub-femtosecond possibilities

  6. Temporal Characteristics of a SASE FEL E(t)=j E1(t-tj), tj is the random arrival time of jthe- E1: wave packet of a single e- after Nu undulator period Nu Coherence time coh determined by gain bandwidth 

  7. bunch length Tb coh • Sum of all e- E(t) • SASE has M temporal (spectral) modes with relative intensity fluctuation M-1/2 • Its longitudinal phase space is ~M larger than Fourier transform limit • Narrower bandwidth for better temporal coherence • shorter x-ray pulse (shortest is coherence time)

  8. 1 % of X-Ray Pulse Length • LCLS near saturation (80 m) • bunch length 230 fs • coherence time 0.3 fs • number of modes ~ 700 statistical fluctuation sw/W ~ 4 % Shortest possible XFEL pulse length is only 300 as!

  9. Optical manipulations of a frequency-chirped SASE

  10. X-ray Pulse Compression • Energy-chirped e-beam produces a frequency-chirped radiation • Pair of gratings to compress the radiation pulse C. Pelligrini, NIMA, 2000 • No CSR in the compressor, demanding optics • Pulse length controlled by SASE bandwidth and chirp

  11. SASE FEL Monochromator X-ray Pulse Slicing • Instead of compression, use a monochromator to select a slice of the chirped SASE ω monochromator short x-ray slice t compression • Single-stage approach

  12. Chicane SASE FEL FEL Amplifier Monochromator Two-stage Pulse Slicing C. Schroeder et al., NIMA, 2002 • Slicing after the first undulator before saturation reduces power load on monochromator • Second stage seeded with sliced pulse (microbunching removed by bypass chicane), which is then amplified to saturation • Allows narrow bandwidth for unchirped bunches

  13. Analysis of Frequency-chirped SASE • Statistical analysis (S. Krinsky & Z. Huang, PRST-AB, 2003) Frequency-chirp • coherence time is indep. of chirp u • frequency span and frequency spike width coh ~ u • A monochromator with rms bandwidth sm passes MF modes

  14. Minimum Pulse Duration • The rms pulse duration st after the monochromator ω u t • Minimum pulse duration is limited to for either compression or slicing • Slightly increased by optical elements (~ fs)

  15. One-stage Approach • SASE bandwidth reaches minimum (~r~10-3) at saturation  minimum rms pulse duration = 6 fs (15 fs fwhm) for 1% energy chirp • st minimum for broad sm choose sm ~ sw to increase MF (decrease energy fluctuation) and increase photon numbers

  16. Two-stage Approach • Slicing before saturation at a larger SASE bandwidth leads to a longer pulse Ginger LCLS run • Synchronization between sliced pulse and the resoant part of chirped electrons in 2nd undulator ~ 10 fs

  17. Electron Bunch Manipulations

  18. Spatially Chirped Bunch P. Emma & Z. Huang, 2003 (Mo-P-52) 200-fs e- bunch 30-fs x-ray Undulator Channel • FEL power vs. y’ offset for LCLS • Gain is suppressed for most parts of • the bunch except the on-axis portion

  19. E = 4.5 GeV, sz = 200 mm, V0 = 5 MV 1.0 m +2sy 0 -2sy y vs. z at start of undulator ? • No additional hardware for LCLS • RF deflector before BC2 less jitter • Beam size < 0.5 mm in linac  FWHM x-ray pulse ~ 30 fs

  20. Courtesy S. Reiche

  21. Using Enhanced Wakefield • Ideal case (step profile) with various materials for the vacuum chamber to control wakefield amplitude 4 fs (FWHM) • Change of vacuum chamber to high resistivity materials (graphite) is permanent, no long pulse operation S. Reiche et al., NIMA, 2003

  22. Where else can we access fs time? Large x-z correlation inside a bunch compressor chicane LCLS BC2 Easy access to time coordinate along bunch 2.6 mm rms 0.1 mm rms

  23. Slotted-spoiler Scheme 1 mm emittance 5 mm emittance 1 mm emittance P. Emma et al. submitted to PRL, 2003 (Mo-P-51)

  24. ParmelaElegantGenesis Simulation, including foil-wake, scattering and CSR

  25. 2 fsec fwhm fs and sub-fs x-ray pulses • A full slit of 250 mm  unspoiled electrons of 8 fs (fwhm) •  2~3 fs x-rays at saturation (gain narrowing of a Gaussian electron pulse) • stronger compression + narrower slit (50 mm)  1 fs e- •  sub-fs x-rays (close to a single coherence spike!)

  26. Statistical Single-Spike Selection Unseeded single-bunch HGHG (8  4  2  1 Å ) I8/ I18 8 Å 1 Å sub-fs spike Saldin et al., Opt. Commun., 2002

  27. Selection Process Set energy threshold to reject multi-spike events (a sc linac helps)

  28. Conclusions • XFEL can open both ultra-small and ultra-fast worlds • Many good ideas to reduces SASE pulse lengths from 100 fs to ~ 10 fs level • Optical manipulations are limited by SASE bandwidth, available electron energy chirp, and optical elements • Electron bunch manipulations and SASE statistical properties may allow selection of a single coherent spike at sub-fs level • Time for experimental investigations

More Related