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					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
 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!
                 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
   Temporal Characteristics of a SASE FEL

E(t)=j E1(t-tj), tj is the random arrival time of jth e-


E1: wave packet of a single e- after Nu undulator period
                        Nu 




   Coherence time coh determined by gain bandwidth 
                                           coh
 Sum of all e-  E(t)




                                    bunch length Tb

 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)
                            • 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!

1 % of X-Ray Pulse Length
Optical manipulations of

a frequency-chirped SASE
                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
                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


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




                        SASE FEL                      FEL Amplifier
                                   Monochromator

 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
        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 m passes MF
modes
              Minimum Pulse Duration
 The rms pulse duration t after the monochromator
                                             ω
                                                      
                                              / u
                                                      u
                                                           t



 Minimum pulse duration is limited to     /u
for either compression or slicing

 Slightly increased by optical elements (~ fs)
                One-stage Approach
 SASE bandwidth reaches minimum (~r~10-3) at saturation
  minimum rms pulse duration (  ) min / u ~ r / u = 6 fs
(15 fs fwhm) for 1% energy chirp




 t minimum for broad m  choose m ~  to increase MF
(decrease energy fluctuation) and increase photon numbers
                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
Electron Bunch Manipulations
                   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
E = 4.5 GeV,
                   1.0 m
z = 200 mm,
 V0 = 5 MV
                                   +2y

                                    0


                                   -2y




                                  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
Courtesy S. Reiche
                  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
   Where else can we access fs time?
Large x-z correlation inside a bunch compressor chicane




 LCLS BC2
       2.6 mm rms




                                         Easy access to
                                         time coordinate
                                         along bunch


                       0.1 mm rms
                      Slotted-spoiler Scheme

                                                  1 mm emittance




                                                  5 mm emittance




                                                   1 mm emittance


P. Emma et al. submitted to PRL, 2003 (Mo-P-51)
Parmela  Elegant  Genesis Simulation, including foil-wake, scattering and CSR
              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)




                 2 fsec fwhm




• stronger compression + narrower slit (50 mm)  1 fs e-
 sub-fs x-rays (close to a single coherence spike!)
            Statistical Single-Spike Selection
 Unseeded single-bunch HGHG (8  4  2  1 Å )




                            I 8 / I 18
  8Å                                                1Å




Saldin et al., Opt. Commun., 2002        sub-fs spike
                      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

				
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posted:10/17/2012
language:English
pages:28