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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.

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slide2

The Promise of X-ray FELs

Ultra-bright

Ultra-fast

slide3

Single Molecule Imaging with Intense fs X-ray

R. Neutze et al. Nature, 2000

slide4

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!

slide5

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
slide6

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 

slide7

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)
slide8

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!

slide9

Optical manipulations of

a frequency-chirped SASE

slide10

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
slide11

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
slide12

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
slide13

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
slide14

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)
slide15

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
slide16

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
slide18

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
slide19

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

slide21

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

slide22

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

slide23

Slotted-spoiler Scheme

1 mm emittance

5 mm emittance

1 mm emittance

P. Emma et al. submitted to PRL, 2003 (Mo-P-51)

slide25

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!)
slide26

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

slide27

Selection Process

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

slide28

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