Approaches for the generation of femtosecond x-ray pulses

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Approaches for the generation of femtosecond
x-ray pulses
Zhirong Huang (SLAC)
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The Promise of X-ray FELs
Ultra-bright
Ultra-fast
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Single Molecule Imaging with Intense fs X-ray
R. Neutze et al. Nature, 2000
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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 100200 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!
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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

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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
??
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?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)

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• 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!
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• Optical manipulations of
• a frequency-chirped SASE

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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

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X-ray Pulse Slicing

• Instead of compression, use a monochromator to
  select a slice of the chirped SASE

?
monochromator
short x-ray slice
t
compression

• Single-stage approach

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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

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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

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Minimum Pulse Duration

• The rms pulse duration st after the
  monochromator

?
u
t

• Minimum pulse duration is limited to
• for either compression or slicing
• Slightly increased by optical elements ( fs)
  

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One-stage Approach

• SASE bandwidth reaches minimum (r10-3) 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

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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

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Electron Bunch Manipulations
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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

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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 lt 0.5 mm in linac

? FWHM x-ray pulse 30 fs
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Courtesy S. Reiche
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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
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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
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Slotted-spoiler Scheme
1 mm emittance
5 mm emittance
1 mm emittance
P. Emma et al. submitted to PRL, 2003 (Mo-P-51)
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Parmela ? Elegant ? Genesis Simulation, including
foil-wake, scattering and CSR
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fs and sub-fs x-ray pulses

• A full slit of 250 mm ? unspoiled electrons of 8
  fs (fwhm)
• ? 23 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!)

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