2. Bholat@slac.stanford.edu
Outline
• Scientific motivation for FEL (one example)
• Principal of FEL
• Important parameters of FEL
• Systematic tuning the FEL parameters
• User’s requirements
• Improvement and future plans
• Conclusion and Q&A.
4. t = 0
Bholat@slac.stanford.edu
t = 25 fs
t = 50 fs
…which is being imaged with intense x-rays
BEFORE its destruction
Coulomb Explosion of Lysozyme (50 fs)
J. Hajdu, Uppsala
1-Å spatial
resolution
with hard
x-rays
femtosecond
short pulses to
resolve atomic and
molecular dynamics
5. Bholat@slac.stanford.edu
Experimental Hutches at Linac Coherent Light Source
•AMO: Atomic, Molecular & Optical Science
This instrument will enable the study of the interaction between the extremely intense
LCLS X-ray pulses and the basic constituents of matter: atoms and molecules.
• CXI: Coherent X-ray Imaging
This instrument will take advantage of the extremely bright, ultrashort LCLS pulses of
hard X-rays to allow imaging of non-periodic nanoscale objects, including single or
small clusters of biomolecules at or near atomic resolution.
• MEC: Matter in Extreme Conditions
This proposed instrument will observe matter at temperatures exceeding 10,000 Kelvin
and at pressures 10 million times the earth's atmospheric pressure at sea-level, enabling
unprecedented understanding of exotic states of matter.
•SXR: Soft X-ray Materials Science
This instrument will enable the high brightness and timing capability of the LCLS to be
applied to scattering and imaging experiments that require the use of soft X-rays.
•XCS: X-ray Correlation Spectroscopy
This instrument will observe dynamical changes of large groups of atoms in condensed
matter systems over a wide range of time scales.
• XPP: X-ray Pump Probe
This instrument will predominantly use a fast optical laser to generate transient states of
matter, and the hard X-ray pulse from the LCLS to probe the structural dynamics
initiated by the laser excitation.
•
6. Bholat@slac.stanford.edu
Experimental Hutches at Linac Coherent Light Source
Techniques Electron time-of-flight spectroscopy
Ion time-of-flight spectroscopy
Ion imaging
Ion momentum spectroscopy
X-ray scattering
Sample Environment A variety of pulsed and continuous gas, liquid and particle injectors
pulsed Proch-Trickl, Even-Lavie or Parker gas valves with heating and cooling options
single or double skimmer chambers for gas beams
motorized XYZ paddle for solid samples
liquid jet for biological samples
aerosol jet of particulate samples
Scientific Applications Coherent X-ray imaging on single sub-micron particles
Macromolecular Nanocrystallography
High Fluence X-ray interactions with matter
Time-resolved imaging and scattering with hard x-rays
Techniques and Scattering
Geometry
Forward scattering on target-mounted samples andfree-standing injected particles
Back-scattering
Ion Time-of-flight
7. Bholat@slac.stanford.edu
Scientific Application Time-resolved spectroscopy and scattering with ultrafast and ultraintense soft X-rays at the LCLS
Techniques X-ray Emission Spectroscopy (XES)
X-ray Photoelectron Spectroscopy (XPS)
X-ray Absorption Spectroscopy (XAS)
Coherent Imaging
Fourier Transform Holography
Diffraction
Sample Environment A wide range of experimental setups in UHV and HV conditions
Experimental Hutches at Linac Coherent Light Source
Scientific Applications X-ray PhotonCorrelation Spectroscopy (XPCS)
Coherent X-ray Scattering (CXS)
Scattering Geometries Small Angle X-ray Scattering (SAXS)
Grazing incidence Scattering (GI-SAXS,GID)
Wide Angle X-ray Scattering/Diffraction
Scientific Applications Femtosecond Structural Dynamics
X-ray Interactions with Matter
X-ray Techniques Wide Angle X-ray Scattering/Diffraction
Small Angle X-rays Scattering
X-ray Absorption Spectroscopy
X-ray Emission Spectroscopy
9. Bholat@slac.stanford.edu
Why a Linac-Based Free-Electron Laser* ?
•“Recent” advances in high-brightness RF photocathode guns
•Longitudinal emittance from linac is ~103 smaller than ring
•Bunch length can be <100 fs and with small energy spread
•Experience from linear collider operation and study (SLC,
TESLA, JLC, NLC, CLIC)
•The last 1-km of the SLAC linac is available
•FEL produces unprecedented brightness and fsec pulses
Use SASE** (Self-Amplified Spontaneous Emission)
no mirrors or seed-lasers at 1-Å wavelengths
* Motz 1950; Phillips 1960; Madey 1970
** Kondratenko, Saldin 1980; Bonifacio, Pellegrini 1984
10. Why a Linear Accelerator (Linac)?
ex ~ g 2
sz 5 mm,
sE/E 0.1% (1 GeV),
gez = szsE/mc2 10000
mm
Storage ring…
Higher energy means larger
emittance
Longitudinal emittance, gez ,
is much too large!
Much smaller gez
Emittance scaling (1/g )!
But lower repetition rate
ex ~ 1/ggez < 10 mm
RF Photocathode
Gun (or SCSS gun) Linac
zyx
e
e
N
B
eee
~
sz 1 mm,
sE/E 0.05% (5 MeV),
gez = szsE/mc2 5 mm
11. Linac Coherent Light Source at SLAC
Injector (35º)
at 2-km point
Existing 1/3 Linac (1 km)
(with modifications)
Near Experiment Hall
(NEH) AMO,SXR,XPP
Far Experiment Hall(FEH)
XCS,CXI,MEC
Undulator (130 m)
X-FEL based on last 1-km of existing 3-km linac
New e- Transfer
Line (340 m)
1.2-25 Å
(15-3.3 GeV)
X-ray
Transport
Line (200 m)
12. BC2
4.3 GeV
BSY
14 GeV
TCAV3
5.0 GeV
BC1
250 MeV
L1S
3 wires
2 OTR
L1X
4 wire
scanners
+ 8 coll’s
L2-linac
L3-linacDL1
135 MeV
L0
gun
TCAV0 old
screen3 OTR
sz1 sz2
3 wires
3 OTR stopper
heater
mwall
DL2
14 GeV
undulator
14 GeV
4 wire
scanners
+ 6 coll’s
vert.
dumpstopper
LCLS Machine Layout
Accelerator is last 1-km of SLAC linac (14 GeV)
RF photocathode gun and off-axis injector
Two bunch compressors + ‘laser heater’
Two transverse RF deflectors for time-resolved beam measurements
X-band (12 GHz) compression linearizer
4 emittance diagnostic stations + 4 spectrometers
Primary and secondary collimation sections
Fixed gap, planar, 132-m undulator at 14 GeV + 1-mm res. RF BPMs
Near and Far Experimental Halls + 500 m of x-ray transport
13. LCLS Accelerator Required for SASE X-Ray FEL
SLAC linac tunnel research yard
Linac-0
L =6 m
Linac-1
L 9 m
rf -20°
Linac-2
L 330 m
rf -36°
Linac-3
L 550 m
rf 0°
BC1
L 6 m
R56 -45 mm
BC2
L 22 m
R56 -25 mm DL2
L =275 m
R56 0
DL1
L 12 m
R56 0
undulator
L =130 m
6 MeV
sz 0.6 mm
s 0.05 %
135 MeV
sz 0.6 mm
s 0.10 %
250 MeV
sz 0.10 mm
s 1.3 %
4.30 GeV
sz 7 mm
s 0.32 %
13.6 GeV
sz 7 mm
s 0.01 %
Linac-X
L =0.6 m
rf= -160
V0 = -20 MV
rf
gun
21-3b
24-6d
X
25-1a
30-8c
...existing
linac
undulator
Most of accelerator existed (1960’s), but new electron source, new
bunch compressors, and new undulator were added
Entire machine is >2000 m long
21-1
b,c,d
14. Gun-Solenoid Assembly Cut-away view of gun (1.6 RF cells)
New Radio Frequency Photo-Cathode Gun
10 cmD. Dowell, E. Jongewaard
copper
cathode
Copper cathode (low QE, but stable)
UV laser illumination extracts electrons
Radio Frequency acc. fields (3 GHz)
Accelerate before space-charge blow up
Can produce very bright electron beam
3 GHz RF 120 MV/m
e-
UV
15. Injector Transverse Emittance <0.5 mm
135 MeV
0.25 nC
35 A
gex 0.43 mm
gey 0.46 mm
D. Dowell ,et al.
Exceptional beam
quality from S-
band Cu-cath. RF
gun…
Time-sliced emittance is even less: 0.4 mm
18. Need a very bright, high-energy, bunched electron
beam → linear accelerator provides MUCH brighter
beam than a ring
Electron bunch is injected into a long undulator → an
array of periodic alternating dipole magnets
Stimulated emission is resonantly amplified by
interaction of radiation with electron bunch
What is a SASE* Free-Electron Laser (FEL)?
* Self-Amplified Spontaneous Emission
X-rays
~100 m
19. z
x
Due to sustained interaction, some electrons lose energy,
while others gain energy modulation at 1
e- losing energy slow down, and e- gaining energy catch up
density modulation at 1 (microbunching)
Micro-bunched beam radiates coherently at 1, enhancing
the process exponential growth of radiation power
u
e-
1
x-ray
Electrons slip behind EM wave by 1 per undulator period (u)
Z. Huang
+ - + - + -
- + - + - +
K/g
vxEx > 0
+
-
Resonant Interaction of
Field with Electrons
E t
E t
vxEx > 0 vxEx > 0vxEx > 0 vxEx > 0
undulator
20. FEL Micro-Bunching Along Undulator
SASE*
FEL starts up
from noise
* Self-Amplified
Spontaneous
Emission
log
(radiation power)
distance
electron
beam
undulator
photon
beam
e-beam dump
S. Reiche
Electrons form
micro-bunches at
the radiation
wavelength
(~1.5 Å)
22. + - + - + -
- + - + - +
+
-
Slippage and coherence length
Light overtakes e- beam by one radiation wavelength 1 per
undulator period (interaction length = undulator length)
z
Slippage length = 1 × N undulator periods:
(at 1.5 Å, LCLS slippage is: ls ≈ 1.5 fs << 100-fs pulse length)
Each part of optical pulse is amplified by those electrons within a
slippage length (an FEL slice)
Coherence length is slippage over ~2LG (lc << ls)
ML ≈ Dz/lc independent radiation sources (modes)
N1
e-x-rays
slippage
length
Dz
~1 µm
23. Electron Beam Requirements for SASE FEL
eN < 1 µm at 1 Å, 15 GeV
<0.04% at Ipk = 3 kA,
K 3, u 3 cm, …
18LG ≈ 100 m for eN 1.5
µm
Need high peak current, low emittance, and small energy spread so that the x-ray
radiation 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)
transverse emittance:
(volume occupied in phase space)
radiation wavelength (e.g., 1 Å)
relative energy spread:
peak current undulator period
beta function undulator ‘field’ = 0.93∙Bu
FEL gain length:
FEL parameter
24. sz0
DE/E
z
sz
under-
compressed
V = V0sin(kz)
RF Accelerating
Voltage
Dz = R56DE/E (g >> 1)
Path Length-Energy
Dependent Beamline
DE/E
z
sE/E
DE/E
z
energy
chirp
over-
compressed
chicane
Magnetic Bunch Compression Peak Current ~ q/sz
30. Bholat@slac.stanford.edu
RF Phase scans
After phase scans, LEM (scaling the LINAC lattice) will follow
•L1S phase is set at 22
degree to give the beam
head and tail energy
chirp.
So at what phase and
Amplitude of L1X does
Cs=-Cx;
where Cs is Beam
curvature after L1S, and
Cx is
Beam curvature caused
by L1X RF.
LiTrack simulation is
used, set L1X need to
be
almost back phase to ~-
160 degree.
L1X setting
32. SLAC linac tunnel research yard
Linac-0
L =6 m
Linac-1
L 9 m
rf -25°
Linac-2
L 330 m
rf -41°
Linac-3
L 550 m
rf 0°
BC1
L 6 m
R56 -39 mm
BC2
L 22 m
R56 -25 mm DL2
L =275 m
R56 0
DL1
L 12 m
R56 0
undulator
L =130 m
6 MeV
sz 0.83 mm
s 0.05 %
135 MeV
sz 0.83 mm
s 0.10 %
250 MeV
sz 0.19 mm
s 1.6 %
4.30 GeV
sz 0.022 mm
s 0.71 %
13.6 GeV
sz 0.022 mm
s 0.01 %
Linac-X
L =0.6 m
rf= -160
21-1
b,c,d
...existing
linac
rf
gun
21-3b
24-6d
X
25-1a
30-8c
undulator
Bholat@slac.stanford.edu
Injector match guiSector 21 match guiSector 28 match guiLTU match gui
These match
Quads talk
heavily to
FEL power
Undulator match gui
39. e-
sz
sy
RF
‘streak’
V(t)
S-band (2856 MHz)
transverse RF deflector
off-axis
screen
single-shot, absolute
bunch length
measurement
e- Bunch Length Measured with Transverse RF Deflector
Deflector used to measure:
absolute bunch length,
time-sliced x-emittance,
time-sliced energy spread,
electron arrival time jitter
deflector OFF deflector ON
time
…one deflector at 135 MeV & another at 5 GeV
40. And slotted Foil Control gui
Bunch length GUI
Bunch length Measurement gui
41. P. Emma, M. Cornacchia, K.
Bane, Z. Huang, H. Schlarb
(DESY), G. Stupakov, D. Walz
Narrow or Double X-Ray Pulses from a Slotted Foil
PRL 92, 074801 (2004).
time (fs)
Power(GW)
0
10
5
0-150 fs
2 fs
0-6 mm
0.25 mm
pulses not
coherent
42. OTR screen in BC2
(30 cm up-beam of foil)
FELX-rayPulseEnergy(mJ)
Scan only the
single-slot section…
FEL power is proportional to slot width (short pulses?)
3-mm thick
moveable
Aluminum foil
43. Few ways to change energy of the machine
Energy ramp, Energy change and Charge change GUI
Charge change gui
44. 132-m Long Undulator with 5-DOF Motion Control Girders + IN/OUT
33 undulator
segments and
room for a second
undulator (LCLS-
II)
cavity beam
position
monitor (BPM)
quadrupole
magnet
3.4-m
undulator
magnet
cam-based
5-DOF motion
control
slider
stage
hydrostatic
leveling monitor
support
girderstretched wire
system
45. Beam-Based Undulator Alignment
Measure undulator trajectory at 4 energies (4, 7, 9, & 14 GeV)
Scale all upstream magnets each time
Do not change anything in the undulator
Calculate quadrupole and BPM alignment… (Matlab GUI)
Move quads and adjust BPM offsets for dispersion free
trajectory
Iterate…
RESULT: vary energy by factor of 3 trajectory changes by <10 mm
H. Loos
47. Also monitor
and calibrate
gas detector
readings
vary FEL power with oscillations & record e- energy loss
1.5 Å
FEL e- Energy-Loss Shows >2 mJ per X-ray Pulse
Horizontal
Dog-Leg
BPMs
Dumpline
BPMs
Vertical
Bend
10 MeV/e- (2.4 mJ)
~100 meters
initial DEi
final DEf
DE = DEf - DEi
48. FELON
Electron Energy Loss & Spread Induced by FEL Gain
4.3 GeV, r = 15 Å, Ipk = 500 A
FEL OFFFEL ON
suppress FEL with oscillationallow FEL with straight orbit
FELOFF
0.05%
50MeV
Undulator, Dump-Line, and Screen
10 MeV
loss/e-
2.6 mJ @
830 eV
50. Undulator Gain Length Measurement at 1.5 Å
gex,y = 0.4 mm (slice)
Ipk = 3.0 kA
sE/E = 0.01% (slice)
(25 of 33 undulators installed)
Lg = 3.3 m
Saturation length
of 60 m in 112-m
und.
51. <1 mm Undulator Quadrupole Remote Position Control
±4 mm
3-parameter fit
to 20 BPMs
along undulator
(y0, y0, and Dy)
Dy = 30 nrad kick due to quad
(y0, y0)
0.7 mm
backlash
<0.5 µm res.
53. Undulator Taper Control Gui.
Bholat@slac.stanford.edu
Taper control
options, Set e- beam
parameter
54. Undulator Taper more than Doubles Power
1%
1.5 Å
5-mrad pole
cant angle
unmeasured range (>5 mm)
measured range (|x| 5 mm)
Wake (3 kA) 40 MeV
Spont. (13.6 GeV) 20 MeV
Post Sat. Taper 70 MeV
H.-D. Nuhn, THOA02
55. FEL Energy (YAG)
FEL-induced
Energy Loss (BPMs)
~100 meters
4.6 MeV
Vary the FEL power and record e- energy loss
Total x-ray screen
signal vs. undulator
K-taper (yaw)
4.6 MeV at 0.25 nC
= 1.1 mJ or 0.8×1012
photons/pulse (15
GW at 75-fs FWHM
pulse length)
1.5 Å
Dumpline
BPMs
Undulator ‘Taper Scan’ Shows 1.1 mJ per X-ray Pulse
Dog-Leg
BPMs
56. FEL Intensity Jitter 9.6% rms at 1.5 Å over >1 hr
1.87 ± 0.18 mJ
30 Hz beam rate,
5 Hz sample rate,
10000 data
points,
SASE saturation
klystrontrips
> 1 hr
57. LCLS Parameter Changes
Machine Pulse Rate (1, 10, 30, 60, 120 Hz & one-shot)*
changed in seconds with one button push.
Wavelength (25 - 1.2 Å) changed in 5-60 minutes.
Pulse Energy (0 - 4 mJ) easily lowered, but may take 1-2 hrs
to achieve >2.5 mJ (depends on wavelength, etc).
Pulse Length (60 - 500 fs FWHM – for soft x-rays) easily
changed in 1 minute (closed loop control).
Peak FEL Power (0 - 40 GW) set by pulse length (increases
with peak current – see next slide).
Ultra-Short Pulse Length (<10 fs FWHM?) 20 pC - requires
1-2 hours to establish (starting from nominal 250-pC
conditions).
* Single shot and burst mode can be control by the users
58. Pulse Length Easily Adjusted (500-60 fs)*
X-Ray Pulse Energy
vs. Pulse Length
(2.5 – 3.8 mJ)
e- Peak
Current
vs. Pulse
Length
(500-4000 A)
Peak FEL
Power
vs. Pulse
Length
(5-40 GW)
e- bunch length is quickly adjustable (<1 min)
from 60 to 500 fs (hard x-rays: 60 to 100 fs)
1.7 keV, 250 pC, 23 of 33 undulators inserted
* for soft x-rays (0.5-2 keV)
59. Conclusions.
• Routine tuning can be and is done during beam
“delivery”. User’s consent usually required.
• Energy changes take away from “delivery” time.
Smaller energy changes are accomplished by
Energy ramp gui. Larger energy change require
multiple steps,
Load known good configuration close to desired
energy.
Activate Klystrons on the beam to match the
configuration energy.
LEM (scale LINAC lattice)
Run energy ramp gui to get to the desired energy.
Bholat@slac.stanford.edu
60. Conclusions.
Steer to “gold” orbit.
Standardize all the bend magnets
• Known problems:
Correctors don’t scale properly.
Muon Shield wall generate multipole
fields.
BX01/BX02 correctors.
50B1 degauss.
A4 BSY correctors remnant fields.
Bholat@slac.stanford.edu
61. Conclusions.
• One way to speed up large energy change
would be to establish every 500MeV “safe
zone” configuration file during Machine
Development (MD)
• Going to low charge ultra short pulse take
away time from delivery. There are many
steps involved, some are automated others
can’t be automated. (1-2hrs)
• There is an ongoing effort to create new tools
to configure LCLS machine efficiently and
repeatable manner so as to satisfy user
requirements.
Bholat@slac.stanford.edu
62. Acknowledgements
R.Akre, A.Brachmann,FJ. Decker, D. Dowell,
P.Emma, S. Giliveich, Z. Huang, S.Khan, H.
Loos, A.Novohatsky, HD. Nuhn, H.Smith,
M.Stanek, S. Tantawi, S. Weathersby, and
ALL THE OPERATORS.
Bholat@slac.stanford.edu
63. Thank you for your attention.
bholat@slac.stanford.edu
END
64. More slides
• FEL Summary
• LASER Heater
• LCLS II
Bholat@slac.stanford.edu
65. SASE 1D Summary
Radiation wavelength:
Power gain length:
Exponential growth: P(z) = P0 exp(z/LG)
Startup noise power: P0 ≈ Kr2gmc3/1
(spontaneous radiation in two gain lengths)
Saturation power: Psat ≈ r × e-beam power
Saturation length: Lsat ≈ u/r ≈ 18LG
FWHM bandwidth at saturation: ≈ 2r
Coherence length at saturation: lc ≈ 1/(2pr)
Z. Huang
3 m
2 kW
20 GW
60 m
0.1%
0.3 fs
1.5 Å
-
66. EM radiation (radio waves) is
produced by electrons running up
and down the antenna wire…
If we accelerate the
electrons to near light
speed we get a Doppler
shift producing x-rays
(instead of radio waves),
where here the up and
down electron motion is
caused by the FEL
wiggler magnets.
1-Å Wavelengths? Doppler Shifted Radiation
accelerator
/g 2
Huge boost from high energy, E = gmc2
67. Micro-bunching Instability (can ruin e- beam)
Initial e- bunch current modulation induces energy modulation
through impedance, Z(), converted to more current modulation by
bunch compressor, R56
growth of slice energy spread (and emittance)
time
projection
t
t
gain = 10
R56
Current
t
Energy
t
1% 10%initial current modulation
energy modulation more bunching
amplified bunching
Z. Huang
Z() impedance
FEL-like effect
68. Laser Heater Spatial Alignment
e-
IR Calculate and re-align laserpoor heating?
good heating
One button click
(~1 minute)
time
energy
After three iterations
Dave is smilingP. Emma
H. Loos
69. Micro-Bunching on LCLS Electron Beam
(measurements with and without laser heater)
Heater ONHeater OFF m-bunching on
dump screen
(after FEL) in
over-
compressionbunch
length
Heater’s energy spread Landau-damps micro-bunching before it can
degrade the electron beam brightness (better FEL performance)
70. undulator
X
L1 L2 L3BC1 BC2
LCLS-II - Second Injector/Linac
RF
gun-1
L0
3-15 GeV
sector-11 sector-21 sector-24sector-14
existing
und-hall
L3
HXR4-14 GeV bypass line
4-14 GeV
X
RF
gun-2
L1 L2BC1 BC2
L0
FACET wall
Second injector & linac allows 2 independent FELs serving 2+
experiments simultaneously with completely flexible param’s (plus
1-km linac remains).
New hard X-ray FEL with ~2 – 13 keV photons (+ self-seeding?)
SXR
New soft X-ray FEL with ~0.24 – 2 keV photons
Just one step on the way to “LCLS-2025” with ~10 x-ray
beamlines…
one more
km of linac
