X-Ray Free Electron Lasers

Report
X-ray Free Electron lasers
Zhirong Huang
Lecture Outline
XFEL basics
XFEL projects and R&D areas
Questions and Answers
Bright light sources from relativistic electrons
Synchrotron radiation
Undulator radiation
Electrons emit with random phase  radiation intensity  N
(g is Lorentz factor, N is number of electrons ~109)
Free Electron Laser (FEL)
• Produced by resonant interaction of a relativistic electron
beam with EM radiation in an undulator
electron
beam
photon
beam
undulator
e- beam dump
l1
• Radiation intensity  N2
• Tunable, Powerful, Coherent radiation sources
Three FEL modes
Light Source Brightness (Brilliance)
10 orders of
magnitude!
Undulator Radiation
l1
lu
forward direction radiation
(and harmonics)
undulator parameter K = 0.94 B[Tesla] lu[cm]
LCLS undulator K = 3.5, lu = 3 cm, e-beam energy
from 3 GeV to 15 GeV to cover l1 = 30 Å to 1.2 Å
Can energy be exchanged between electrons and copropagating radiation pulse?
Resonant
Interaction of Field
with Electrons
Electrons slip behind EM wave by l1 per undulator period (lu)
+
+
+
+
x
lu
K/g
e-
vxEx > 0
vxEx > 0
l1
vxEx > 0
vxEx > 0
x-ray
z
vxEx > 0
+
+
+
Due to sustained interaction, some electrons lose energy,
while others gain  energy modulation at l1
E
P. Emma
t
e- losing energy slow down, and e- gaining energy catch up
 density modulation at l1 (microbunching)
E
Microbunched beam radiates coherently at l1, enhancing
the process  exponential growth of radiation power
t
FEL Micro-Bunching Along Undulator
electron
beam
photon
beam
undulator
e- beam dump
S. Reiche
log
(radiation power)
distance
What is SASE?
 Shot noise originates from discrete nature of electrons
… .. . ..
SASE
Dz
electron arrival time t is random  spontaneous emission
 amplified by FEL interaction
 quasi-coherent x-rays
SASE FEL Electron Beam Requirements
radiation wavelength (e.g., 1 Å)
transverse emittance: eN < 1 µm at 1 Å, 15 GeV
FEL parameter
peak current
undulator period
relative energy spread:
<0.04% at Ipk = 3 kA,
K  3, lu  3 cm, …
beta function undulator ‘field’ = 0.93∙Blu
FEL gain length: 18LG ≈ 100 m for eN  1.5 µm
We must increase peak current, preserve emittance, and maintain small
energy spread so that power grows exponentially with undulator distance, z,
P(z) = P0 ∙exp(z/LG)
FEL power reaches saturation at ~18LG
SASE performance depends exponentially on e- beam quality ! (challenge)
Slippage leads to coherence length and spiky
structure
Due to resonant condition, light overtakes e- beam by one radiation
wavelength l1 per undulator period (interaction length = undulator length)
+
+
+
+
-
z
x-rays
e-
~1 µm
Nl1
+
+
+
Slippage length = l1 × N undulator periods:
(at 1.5 Å, LCLS slippage length is: ls ≈ 1.5 fs << 100-fs pulse length)
P. Emma
Each part of optical pulse is amplified by those electrons within a
slippage
slippage length (an FEL slice)
Coherence length is slippage over ~2LG (lc ≈ ls/10)
ML ≈ Dz/lc independent radiation sources (modes)
length
Dz
SASE temporal characteristics
• E(t)=j E0(t-tj), tj is the random arrival time of jth eNu l
E0: wave packet of a single e-
l
lc 

  
2c
• lc ~ 500 l1 = 200 as (LCLS 1.5 Å)
• Sum of all packets  E(t)
bunch length Dz
Statistical intensity fluctuation determined by
number of longitudinal modes
Due to noise start-up, SASE is chaotic light with ML coherent modes (i.e.,
spikes in intensity profile):
z = 50 m
temporal spikes appear
bunch length
Dz
ML 

coherence length lc
Longitudinal phase space is ML larger
than Fourier Transform limit
SASE energy fluctuation is…
DW
1

W
M
← 50 % of X-Ray Pulse Length →
L
ML is not constant – reduced by increased coherence during exponential
growth, and increased with reduced coherence after saturation
LCLS near saturation (~50 fs): ML ≈ 200  DW/W ≈ 7 %
FEL startup from e- beam noise
~10 kW
~1 MW
~0.1 GW
spiky temporal structure
~10 GW
All vertical axes are log scale
BW = 0.6%
BW = 0.15%
BW = 0.10%
narrow
bandwidth
BW = 0.08%
FEL Bandwidth set by FEL Parameter,  (~10-3)
LCLS spectrum
spike width
~ l1/(2Dz)
Bandwidth
~ 2
Example, LCLS relative spectral spike width:
Dz = 50 fs bunch length: width = 5×10-6
Dz = 5 fs bunch length:
width = 5×10-5
Dz = 0.5 fs bunch length: width = 5×10-4
Spectral
properties
are similar to
temporal
domain,
except that
everything is
inverted…
SASE 1D Summary
Power gain length:
3.5 m
Exponential growth: P(z) = P0 exp(z/LG)
Startup noise power: P0 ≈  2g mc3/l1
1.5 kW
(spontaneous radiation in two gain lengths)
Saturation power: Psat ≈  × e-beam power
20 GW
Saturation length: Lsat ≈ lu/ ≈ 18LG
60 m
FWHM bandwidth at saturation: ≈ 2
0.1%
Coherence length at saturation: lc ≈ l1/()
0.2 fs
Transverse coherence
Z=25 m
Z=37.5 m
Z=62.5 m
Z=75 m
S. Reiche
Z=50 m
Z=87.5
mm
Single mode dominates  close to 100% transverse coherence
Harmonic Radiation also Generated: ln  l1/n
FEL gain creates e- energy and density modulation at l1
Near saturation, strong bunching at fundamental wavelength
also produces rich harmonics
For example, ~1% of fundamental power in 3rd harmonic
l
E
l
t
linear regime,
before saturation
E
t
non-linear regime,
near saturation
l1
LCLS may produce up to 25 keV in 3rd
harmonic photons at ~100 MW
l3
Peak Brightness Enhancement From
Storage Ring Light Sources To SASE
Undulator in SR
# of
photons
ΩxΩy
ΩZ
B
Ne
(2πex) (2πey)
D   Z 
    10 -3  10 ps
  c
1023
SASE
NeNlc
(l1 2)2
D   Z 
    10 -3  100 fs
  c compressed
1033
Nlc: number of electrons within a coherence length lc
Enhancement
Factor
6
7
Nlc~10 to 10
102
102
1010 to 1011
XFEL accelerator system
emittance
corrector
Linac
rf photocathode
gun
Linac
Linac
SASE Undulator
Pulse compressors
•
Photocathode rf gun
exn ~ 1 m m, Ip ~ 100A
•
Bunch compression
Ip ~ 2-5 kA, Dt ~ 1-100 fs
•
Acceleration
3–20 GeV, l ~ lu/(2g2)
adiabatic damping
•
ex ~ exn/g ~ l/4, g/g <  ~ 10-3
Undulator 100-m long, segmented, a few mm tolerance
Projects undertaken at US, Germany, Japan, Korea, Swiss, Italy…
Linac Coherent Light Source (LCLS) at SLAC
X-FEL based on last 1-km of existing 3-km linac
1.5-15 Å
(14-4.3 GeV)
Proposed by C. Pellegrini in 1992
Injector (35º)
at 2-km point
Existing 1/3 Linac (1 km)
(with modifications)
New e- Transfer Line (340 m)
X-ray
Transport
Line (200 m)
Undulator (130 m)
Near Experiment Hall
Far Experiment
Hall
LCLS: world’s first hard x-ray FEL
1.5 Å
SASE wavelength range: 30 – 1.2 Å
Photon energy range: 0.4 - 10 keV
Pulse length FWHM 5 – 100 fs (5- 500 fs for SXR only)
Pulse energy up to 4 mJ
~95% accelerator availability
Smaller charge, shorter x-rays
L0
TCAV0
heater
3 wires
3 OTR
L1X
3 wires
2 OTR
3 OTR
z1 L2-linac
BC1 stopper
z2
4 wire
scanners
old
screen
L1S
DL1
L3-linac
TCAV3
4 wire
scanners +
4 collimators
m wall
gun
BSY
DL2
vert.
dump
undulator
BL signal
FEL signal
20 pC, photon energy @ 840 eV
Low charge mode developed
by J. Frisch et al.
Simulations* suggest a few fs
electron and x-ray pulse
duration.
A 3-pC bunch is capable of
generating attosecond FEL,
but diagnostics is very
challenging
* Y. Ding et. al, PRL 2009
More to come
Spring-8 SACLA
2011
SASE Wavelength range: 3 – 0.6 Å
Photon energy range: 4 - 20 keV
Pulse length (10 fs FWHM)
Pulse energy up to 1 mJ
more to come:
PAL-XFEL (2015)
SwissFEL (2016)
LCLS-II (2018)
…
European XFEL ~ 2015
25
Also for soft x-ray FELs
FLASH @ DESY
Operates down to 4 nm
Next-Generation Light Source (NGLS), LBL
What comes next for XFELs?
SASE temporal coherence can be drastically improved by
seeding (self or external seeding)
SASE
seeded
Precise control x-ray properties similar to optical lasers
Compact coherent sources
Harmonic generation for seeding
High Gain Harmonic Generation (HGHG)
L.-H. Yu, PRA, 1991
beam
chicane
radiator
modulator
BNL 2003
FERMI FEL, 2011
Echo Enabled HG
G. Stupakov, PRL, 2009
seed
laser 2
seed
laser 1
beam
modulator 1
modulator 2
radiator
Self-Seeding1,2
First undulator generates SASE
X-ray monochromator filters SASE and generates seed
Chicane delays electrons and washes out SASE microbunching
Second undulator amplifies seed to saturation
chicane
1st undulator
2nd undulator
grazing
mirrors
slit
SASE FEL
Seeded FEL
grating
Long x-ray path delay (~10 ps) requires large chicane that take
space and may degrade beam quality
Reduce chicane size by using two bunches3 or single-crystal wake
monochromator4.
1. J. Feldhaus et al., NIMA, 1997.
2. E. Saldin et al., NIMA, 2001.
3. Y. Ding, Z. Huang, R. Ruth, PRSTAB, 2010.
4. G. Geloni, G. Kocharyan, E. Saldin, DESY 10-133, 2010.
Hard x-ray self-seeding @ LCLS
1 GW
25 GW
15 51
16
17
31
Geloni, Kocharyan, Saldin (DESY)
FEL spectrum
after
diamond
crystal
Power dist. after
diamond crystal
Wide-band
power
Monochromatic
seed power
10-5
5 MW
6 mm  20 fs
30
Self-seeding of
1-mm e- pulse
at 1.5 Å yields
10-4 BW with
low charge
mode
Bragg diagnostic with camera
Chicane
magnet
X-rays
Diamond mono chamber
31
J. Amann, P. Emma (LBL)
8.3 keV
SASE spectrum (diamond OUT)
20 eV
Seeded
SASE
0.45 eV
(510-5)
chicane OFF
insert
diamon
d & turn
on
chicane
SASE
diamond IN
A well
seeded
pulse (not
typical)
seeded
Fourier
Transform
limit is 5 fs
0.45 eV
chicane ON
Factor of 40-50 BW reduction
Submitted to
Nature Photon.,
2012
Soft X-Ray Self-Seeding (SXRSS)
Compact grating monochromator and chicane that fit in one
undulator section (4m)
QU08
QU07
(existing
quad)
Grating
B1
(toroidal
VLS)
B2
B3
-0.9°
-0.9°
( plane
mirror)
18 mm
+0.9°
(existing
quad)
M3
B4
+0.9°
3.85 mm
beam direction
M1
= 500 – 1000 eV
Bandwidth ~2×10-4
SXRSS
U1-7
Slit M2
Dtchicane ~ 700 fs
DELTA
HXRSS
U9-15
U17-32 (add 5 more in future?)
D. Cocco, Y. Feng, J. Hastings et al.,
in collaboration with NGLS (LBL)
Taper to enhance FEL efficiency
FEL saturates due to significant E-loss
Tapered undulator keeps FEL resonance and increase power
x-rays
e-beam
Taper works much better for a seeded FEL than SASE
400 GW
Taper seeded
Taper SASE
Notaper SASE
LLNL microwave FEL
T. Orzechowski et al. PRL (1986)
W. Fawley, Z. Huang et al. NIMA (2002)
LCLS-II simulation
8.3 keV -- 1.5 Å (13.64 GeV)
4 kA, 0.3 um emittance
LCLS low charge parameters
Optimized tapering starts at 16 m
with 13 % K decreasing to 200 m
1.3 TW over 10 fs
~1013 photons
1.0 x 10-4
FWHMBW
After self-seeding crystal
W. Fawley, J. Frisch, Z. Huang, Y. Jiao, H.-D. Nuhn, C. Pellegrini, S. Reiche, J. Wu, FEL2011
C. Schroeder, FLS2012
Laser Plamsa Accelerator (LPA)
Transverse gradient undulator (TGU)*
FEL resonant condition
By canting the undulator poles, generate a linear field gradient
Sort e-beam energy by dispersion h so that
y
g+
g-
x
Resonance can be satisfied for all energies if
* T. Smith et al., J. Appl. Phys. 1979
Compact XFELs driven by LPA
1GeV, 10kA, 10 MeV energy spread;
0.1um emittance; 5 fs (50 pC)
5-m SC undulator lu = 1 cm, au = 1.41 (G. Fuchert, NIMA 2012)
Radiation wavelength l1 = 3.9 nm
For TGU, dispersion h = 0.01 m, x = 100um, y = 15um
cFLASH (compact FLASH)?
Z. Huang, Y. Ding, C. Schroeder, submitted to FEL2012
Compact XFEL with TGU (3.9 nm)
Single-shot spectrum
1 GW
TGU
TGU
no TGU
no TGUx100
TGU is insensitive to energy jitters (energy jitters 
transverse position jitters), no change in l1.
Good for laser plasma accelerators (currently at a few %
energy jitters)
Good for a seeded FEL when wavelength is fixed
Summary
Driven by development of accelerator science
and technology, fourth-generation x-ray source
based on FEL mechanism has become a reality
LCLS is opening up a new world of ultrasmall
and ultrafast.
The high demands from the x-ray community
will drive continuous growth of such sources and
innovative R&Ds.
Thank you for your attention!
Quiz 1
An experimenter places a monochromator with 1 eV
bandwidth centered at 10 keV photon energy after the LCLS
beam. If the SASE pulse length is estimated to be 10 fs fwhm.
What is the expected rms intensity fluctuation for the filtered
radiation?
Quiz 2
How many 10 keV photons per pulse for 2 mJ hard x-ray FEL?
Assuming SASE has 100% transverse coherence, the fwhm
pulse duration is 50 fs. What is the number of photons per mode
(the degeneracy parameter)?
In this context, discuss what is the benefit of seeding?

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