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 eE1: wave packet of a single e- after Nu undulator period
Nu 
Coherence time coh determined by gain bandwidth 
 Sum of all e-  E(t)
coh
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  
for either compression or slicing
/u
 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
1.0 m
E = 4.5 GeV,
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
2.6 mm rms
LCLS BC2
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 )
8Å
I 8 / I 18
Saldin et al., Opt. Commun., 2002
1Å
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