Approaches for the generation of femtosecond x-ray pulses

Zhirong Huang (SLAC)

Ultra-bright Ultra-fastThe 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 1800000000000000000 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 E1(t-tj), tj is the random arrival time of jth e-

Temporal Characteristics of a SASE FEL Temporal Characteristics of a SASE FEL

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

Nu

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 Tb

Sum of all e- E(t)coh

1 % of X-Ray Pulse Length1 % 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 FELMonochromator

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 FELMonochromator

FEL Amplifier

ChicaneC. 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 SASEAnalysis of Frequency-chirped SASE

A monochromator with rms bandwidth m passes MF modes

Minimum Pulse DurationMinimum 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

u/

u/

One-stage ApproachOne-stage Approach SASE bandwidth reaches minimum (~) at saturation minimum rms pulse duration = 6 fs (15 fs fwhm) for 1% energy chirp

uu /~/)( min

t minimum for broad m choose m ~ to increase MF (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 2nd undulator ~ 10 fs

Electron Bunch Manipulations

• FEL power vs. y’ offset for LCLS

• Gain is suppressed for most parts ofthe bunch except the on-axis portion

Spatially Chirped BunchSpatially Chirped Bunch

30-fs 30-fs xx-ray-ray

200-fs 200-fs ee bunch bunch

Undulator ChannelUndulator 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

?

+2y

0

2y

EE = 4.5 GeV, = 4.5 GeV,zz = 200 = 200 m,m,

VV00 = 5 MV = 5 MV

1.0 m1.0 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 WakefieldUsing Enhanced Wakefield

Large x-z correlation inside a bunch compressor chicaneLarge x-z correlation inside a bunch compressor chicane

Where else can we access fs time?

2.6

mm

rm

s2.

6 m

m r

ms

0.1 mm rms0.1 mm rms

Easy access to Easy access to timetime coordinate coordinate along bunchalong 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)

ParmelaParmela ElegantElegant GenesisGenesis Simulation, including foil-wake, scattering and CSR Simulation, including foil-wake, scattering and CSR

2 fsec fwhm2 fsec fwhm

fs and sub-fs x-ray pulsesfs 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 SelectionStatistical Single-Spike Selection

I8 / I18

8 Å 1 Å

Unseeded single-bunch HGHG (8 4 2 1 Å )

sub-fs spikeSaldin et al., Opt. Commun., 2002

Selection ProcessSelection ProcessSet energy threshold to reject multi-spike events (a sc linac helps)

ConclusionsConclusions 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