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

Ultra-brightUltra-fast The Promise of X-ray FELs

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

 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 Introduction in the next fs!

 Temporal characteristics of a SASE FEL Outline of the Talk  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

E(t)=  j E 1 (t-t j ), t j is the random arrival time of j th e - Temporal Characteristics of a SASE FEL E 1 : wave packet of a single e - after N u undulator period N u Coherence time  coh determined by gain bandwidth  

 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) bunch length T b  Sum of all e -  E(t)  coh

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  w /W ~ 4 % Shortest possible XFEL pulse length is only 300 as!

Optical manipulations of a frequency-chirped SASE

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

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

 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 SASE FEL Monochromator FEL Amplifier Chicane C. Schroeder et al., NIMA, 2002 Two-stage Pulse Slicing

 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 Analysis of Frequency-chirped SASE  A monochromator with rms bandwidth  m passes M F modes

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

One-stage Approach  SASE bandwidth reaches minimum (~   ) at saturation  minimum rms pulse duration = 6 fs (15 fs fwhm) for 1% energy chirp   t minimum for broad  m  choose  m ~   to increase M F (decrease energy fluctuation) and increase photon numbers

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

Electron Bunch Manipulations

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

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 ? +2  y 0 2y2y E = 4.5 GeV,  z = 200  m, V 0 = 5 MV 1.0 m  FWHM x-ray pulse ~ 30 fs

Courtesy S. Reiche

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

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

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

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

2 fsec fwhm fs and sub-fs x-ray pulses A full slit of 250  m  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  m)  1 fs e -  sub-fs x-rays (close to a single coherence spike!)

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

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

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