Constructing a High-Gain FEL:
How to Make it Work*
P. Emma
SLAC
August 25, 2009
* assigned title
Many Accelerator/Undulator Challenges
RF Photocathode Gun
Need ≤1-µm normalized emittance all day long
≤10-ps initial bunch length with up to 1 nC charge
Emittance Preservation in LINAC
Transverse wakefields (misalignments)
Dispersion and chromatic effects
Bunch Compression
Coherent synchrotron radiation (CSR)
Micro-bunching instability
Machine Stability
RF phase, voltage, gun-timing, charge
Pointing stability: <10% of beam size
Long Undulator
Need few micron straight trajectory
Field tolerances, absolute alignment
Longitudinal wakefields (small, optically smooth chamber)
Focus of this Presentation
Will not talk about physics or FEL design.
Many here know the physics better.
Will talk about the practical issues of starting
up such a machine, and preparing for this.
Many others here have similar or more
experience (DESY, Spring-8, Frascati, ANL,
BNL, etc).
This is from one point of view (mine at
LCLS), and does not cover all issues (e.g., xray diagnostics and transport).
7 Key Points
0. Good machine design (big issue - not addressed here)
1. Detailed start-to-end tracking simulations from cathode
through undulator, with as much physics as possible.
2. Tolerances studied and tuning strategies well developed
before first beam.
3. Diagnostics well integrated into machine design.
4. High-level applications ready on day #1 (fast, precise,
automated measurement and tuning: ε, σz, BBA, …).
5. Pre-Beam measurement and checkout of all components,
including controls with local (tunnel) verification.
6. Beam-based feedback loops (hold upstream systems steady
while commissioning downstream systems).
7. Adequate budget and time to “do it right”.
1. Detailed start-to-end tracking simulations
from cathode through undulator, with as
much physics as possible
1. Start-to-End Simulations
Start with 2D Longitudinal Phase Space
energy
profile
phase
space
time
profile
after DL1
sz = 830 mm
after L1
after L2
sz = 190 mm
after BC2
sz = 830 mm
after X-RF
sz = 23 mm
after L3
sz = 830 mm
after BC1
sz = 23 mm
at und.
sz = 190 mm
sz = 23 mm
Spikes depend on details of injector distribution
1. Start-to-End Simulations
Micro-bunching Identified During LCLS Simulations*
energy profile
CSR & LSC can
amplify small
current
modulations:
long. space
σδ ≈ 3×10−6
temporal profile
microbunching
230 fsec
Laser heater prior to BC1 increases uncorrelated E-spread and damps instability
σδ ≈ 3×10−5
heater
damps
bunching
* Effect discovered by M. Borland (ANL) using his Elegant tracking code
1. Start-to-End Simulations
Resistive-Wall Wake in Undulator
0.2 nC
1 nC
Cylindrical, Copper, r = 2.5 mm
Bane/Stupakov AC-wake model
1. Start-to-End Simulations
Simulations Suggest
Lower Bunch Charge
1 nC → 0.25 nC
Smaller gun emittance (1.1 µm → 0.5 µm)
Reduced wakefields in undulator
Better control of transverse wakes in linac
Mitigate CSR in compressors
Less micro-bunching
FEL still produces up to ~4 mJ/pulse (50 GW)
1. Start-to-End Simulations
Longitudinal Jitter Simulations*
(Generate full list of stability requirements for drive laser and RF)
energy
0.09%
bunch
length
energy
spread
timing
0.004%
Lg
96 fs
RF phase jitter < 0.1º
Pout
peak
current
10%
pulse
length
2. Tolerances and tuning strategies
developed before first beam
2. Tolerances and Tuning
Transverse Wakefields May Destroy Beam Brightness
HEAD
x'/σx'
σz = 0.02 mm
σz = 0.20 mm
Wakes Reduced with Shorter Bunch
σz = 2.00 mm
N = 3×1010, β = 30 m, L = 3 m, εN = 1 μm, E0 = 1 GeV, Δx = 1 mm
e−
misaligned RF
accelerating structure
Δε
2. Tolerances and Tuning
X-emittance after X-band Cavity (µm)
Steering in RF Structures (X-band here)
11.4 GHz, 3.7-mm iris
Δx = 0.17 mm
Beam position in X-band Cavity (mm)
2. Tolerances and Tuning
Simulations with Random Component Misalignments
trajectory after steering
trajectory
trajectory
corrected
corrected
Large
emittance
growth
γε
γεxx≈≈22μμmm
γε
γεyy≈≈55μμmm
large emittance growth
Elegant by
M. Borland
2. Tolerances and Tuning
Emittance Correction with Trajectory Variations
steering coils
ε meas.
optimize
optimizexxand
andyy
emittances
emittanceswith
with
two
- and
twoxxandtwo
twoyysteering
steeringcoils…
coils…
emittance
growth
now <10%
γε
γεxx≈≈1.02
1.02μμm
m
γε
γεyy≈≈1.09
1.09μμm
m
2. Tolerances and Tuning
‘Tweaker’ quads allow critical chicane dispersion correction
4000-μm beam size!
50-μm
beam size
50-μm beam size
LEUTL chicane (ANL)
…and same at LCLS
with two quads…
correct η and (αη + βη′), orthogonally
ΔΔψψx ≈≈ππ/2/2
x
2. Tolerances and Tuning
X emittance after BC1 Chicane (µm)
Bipolar Tuning Quad in BC1 Chicane
Quad = 0
set quad here
Quadrupole Integrated Gradient (kG)
2. Tolerances and Tuning
P. Emma, J. Wu,
EPAC’06, p. 151.
Sources of Transverse Beam Jitter
Steering coil
current
regulation
Wakefield
kicks with
charge
jitter
Quadrupole
magnet
transverse
vibration
CSR kicks
with charge
or bunch
length jitter
Quadrupole
current
regulation with
misalignments
Drive laser
pointing
jitter
GOAL:
Ax,y < 10% rms
Smaller ε means
tighter tolerances
2. Tolerances and Tuning
Undulator Alignment Strategy
Beam-based alignment using large energy changes (4 to 14 GeV)
Quadrupole magnets are well aligned to dnstr. end of undulator (CMM)
Beam-finder wires used to align upstream end to beam
Beam
Finder
Wire
calculate
Undulator
BPM
Undulator
BPM
e−
Beam
Finder
Wire
Quad
Magnet
move
Quad
Magnet
Beam
Undulator
Quadrupole
Quads
Vary
-finder
electron
Full
and
ends
system
wire
BPMs
and
energy
already
BPM
needed
now
areand
position
not
aligned
precisely
torecord
aligned
align
corrections
after
aligned
upstream
BPM
well
~3 readings
enough
iterations
tonow
end
quadrupoles
initially
applied
to(BBA)
beam
H.-D. Nuhn, et al
14
GeV
7 GeV
4 GeV
3. Diagnostics built into machine design
3. Diagnostics
Electron Beam Diagnostics
YAG screens
OTR screens
Wire scanners
Phase monitors
heater
3 wires
3 OTR
TCAV0
3 wires
2 OTR
σz1
BC1 stopper
135 MeV 250 MeV
3 OTR
σz2
BC2
TCAV3
4.3 GeV 5.0 GeV
4 wire
scanners
+ 8 coll’s
4 wire
scanners
+ 6 coll’s
μ wall
gun
14 GeV 14 GeV
vert.
stopper dump
undulator
14 GeV
2 Transverse RF cavities (135 MeV & 5 GeV)
7 YAG screens (at E ≤135 MeV)
12 OTR screens at E ≥ 135 MeV
15 wire scanners (each with x & y wires)
4 beam phase monitors (2805 MHz)
CSR/CER pyroelectric bunch length monitors at BC1 & BC2
Gun spectrometer line, injector spectrometer line, BC1 stopper
~200 BPMs and toroids
3. Diagnostics
Many Emittance Measurement Options
OTR screens and wire-scanners both available
Multi-screen or quad-scan with beam waist at screen
Quad scan (invasive), multi-wire is non-invasive
Transverse RF for time-resolved measurements
Measurement stations in each critical section
γεy = 1.06 μm
OTR screen
3. Diagnostics
Wire Scanner Emittance Measurements Near End of Linac
σy = 49 μm
σy = 41 μm
σy = 57 μm
σy = 35 μm
Individual wire-scanner profiles
Emittance measured at 10 GeV with 10 µm rms bunch length
γεy = 0.79 μm
γεy = 0.79 μm
E ≈ 10 GeV
Q = 0.25 nC
ξξ==((ββ00γγ−−22αα00αα++γγ00ββ)/2
)/2
45º
per
wire
3. Diagnostics
Wire Scanner Vibration
σy = 97 μm
σy = 45 μm
Scan before addition of
10× reducer gear
Scan after addition of
10× reducer gear
J. Frisch
3. Diagnostics
OTR Screens Compromised by Coherent Radiation
(especially after bunch compression)
Bright beam becomes a
bigger problem?
Beam pipe is
flooded with light
Beam only
passes by this
off-axis screen
Image is unusable
Coherent Transition,
Diffraction, Synchrotron, and
Edge Radiation corrupt image
Transverse RF Deflectors for
Time-Resolved Measurements
3. Diagnostics
RF off-axis screen
‘streak’
V(t)
e−
σz
σy
S-band (2856 MHz)
single-shot, absolute bunch
transverse RF deflector
length measurement
Deflector used to measure:
absolute bunch length,
time-sliced emittance,
time-sliced energy spread,
electron arrival time jitter
deflector OFF
deflector ON
3. Diagnostics
Measuring Bunch Arrival Time Jitter
e−
S-band (2856 MHz)
BPM
V(t)
slope = −2.34 mm/deg
BPM Y Position (mm)
Q = 0.25 nC
Now measure BPM jitter both
with transverse RF OFF, and
then ON (at constant phase)
TCAV OFF
Δt ≈ ±0.6 ps
9 μm rms
TCAV ON
110 μm rms
Timing Jitter (w.r.t. RF) = (110 μm)/(2.34 mm/deg) = 0.047 deg ⇒ 46 fsec rms
3. Diagnostics
Estimating X-ray Pulse Energy with Well Placed BPMs
2 BPMs placed
2π apart and
with opposite
sign dispersion:
δ = (x1 – x2)/2η
10
e−−(2.4
)
10MeV/
MeV/e
(2.4mJ
mJ)
One BPM here
has very large ydispersion and
very small βy:
δ ≈ y/ηy
initial ΔEi
Dumpline
BPMs
← ~100 meters →
Vertical
Bend
Horizontal
ΔE = ΔEf − ΔEi
Dog-Leg
BPMs
vary FEL power with oscillations & record e− energy loss
final ΔEf
4. High-level physics applications ready
on day #1 (fast, precise, automated
measurement and tuning)
4. High-Level Applications
Correlation Plot GUI in Matlab
Vary any device
Watch profiles
as scan
proceeds
Measure bunch
Measure length?
emittance?
Add devices
to read
Quickly
minimize
parameter by
scanning any
device
H. Loos
4. High-Level Applications
Laser Heater Spatial Alignment
IR
Calculate and re-alignpoor
laserheating?
time
energy
e−
good heating
One button click
(~1 minute)
H. Loos
4. High-Level Applications
Fast, Precise RF Phasing with Beam
e.g.,
calibrate gun voltage
vary RF phase & read BPMs
5. Pre-Beam measurement and checkout
of all components, including controls
with local (tunnel) verification
5. Measurement and Checkout
Component Measurements (e.g., RF Gun)
D. Dowell, et al
3.5
I-sol=172.76amps
I-sol=147.5958 amps
3
I-sol=197.93amps
2.5
Bz (kG)
2
1.5
1
0.5
0
solenoid field
-0.5
0.008
0.004
0.002
0.000
-180
-0.002
-120
-60
0
60
120
focal length(m)@5MeV/c
quadrupole kGauss-m
1000000
180
-0.004
-0.006
-0.008
quadrupole component of RF
rf phase
focal length (m)
γβr/mm
racetrack cavity with
d=0.124"
racetrack cavity with
d=0.14"
racetrack cavity with
d=0.134"
0
0.2
correction coils built
into the design
quadrupole component of solenoid
100000
8.E-05
7.E-05
6.E-05
10000
5.E-05
4.E-05
1000
3.E-05
100
2.E-05
10
1.E-05
0.E+00
1
-0.4
0.4
z(m)
cylindrical cavity
0.006
-0.2
-0.3
-0.2
-0.1
0
z(m)
0.1
0.2
0.3
0.4
quadupole field (kG-m)
Race-track cavity design
Bucking coil
-0.4
5. Measurement and Checkout
Undulator Measurements
Undulator
Undulator K
K measured
measured and
and
recorded
over
±6
mm
x
recorded over ±6 mm x range
range
Undulator
Undulator fields
fields integrals
integrals
measured
measured over
over ±6
±6 mm
mm xx range
range
Z. Wolf,
Y. Levashov,
H.-D. Nuhn, et al
5. Measurement and Checkout
All Beamline Components Get Local Checkout
100+ page checkout lists
Check all
component
locations, and…
…magnet
polarities
…wire scanners
Check using real controls as well…
…BPM cables & orientations …collimator travel
etc.
6. Beam-based feedback loops
(hold upstream systems
steady while commissioning
downstream systems)
6. Feedback
The Importance of Feedback Loops
(beam-based and other)
The main function of feedback is usually thought of
as jitter reduction…
But there are more important functions…
De-coupling of machine sections, minimizing (for example)
steering in the undulator during a ‘quad-scan’ in the injector
De-coupling of effects, holding (for example) the beam position
constant at a wire-scanner while running a ‘quad-scan’
Machine recovery after a maintenance day is much more
automatic
Long term drift control (termperature and day/night especially)
Feedback Loop Examples in the LCLS
9 electron trajectory loops (BPMs and steering coils)
Drive laser pointing
Gun launch angle
Injector trajectory
X-band RF structure X & Y position
Post-BC1 launch
Post-BC2 launch
End-of-linac launch
Linac-to-undulator trajectory
Undulator launch
Bunch charge loop (toroid and laser waveplate)
6x6 longitudinal loop (energy at 4 locations + peak current at
each compressor – very important)
Many RF-based phase and amplitude loops (set points are
then adjusted with beam-based loops)
Feedbacks run 24/7 enabling other work by holding FEL steady
6. Feedback
Feedback Systems - Bunch Length & Energy (6×6)
Read 4 BPMs & 2 bunch
DL1length
energy monitors…
Control 4 RF voltages & 2 phases
BC1 energy
J. Frisch
Mesh Filter
Mesh
Mesh Filter
Filter
Mesh Filter
J.Pyro
Frisch
Detector
Pyro
Pyro Detector
Detector
Paraboloid
Paraboloid
Paraboloid
Paraboloid
0.02% rms
0.08% rms
BC2 energy
DL2 energy
0.14% rms
0.06% rms
250 ± 10 A
BC1 peak current
3000 ± 350 A
BC2 peak current
Beam Splitter
Beam
Beam Splitter
Splitter
Beam Splitter
Beam
Beam
Beam
Beam
Pyro Detector
CSR-based bunch
length monitor
Edge Radiation
Edge
Edge Radiation
Radiation
Edge Radiation
Charge feedback: Q = 0.25 nC
bunch 〈ΔQ2〉1/2/Q = 1.5%
charge
6. Feedback
Transverse Stability of LCLS Injector
one of 50 jittering trajectories
11-σσ
beam
beam
size
size
50 shots at 10 Hz
(250 MeV, after BC1)
Ax = 3.9% rms
Q = 0.25 nC
Ay = 3.4% rms
6. Feedback
Transverse Stability of LCLS Linac
one of 50 jittering x-trajectories
11-σσ
beam
beam
size
size
one of 50 jittering y-trajectories
50 shots at 10 Hz
(14 GeV, near end of linac)
Q = 0.25 nC
Ax = 14.2% rms
(needs work)
Ay = 9.5% rms
goal <10%
goal <10%
7. Budget and Time to “Do It Right”
7. Budget and Time
Adequate Budget and Time
LCLS was a 420 M$ project
17 years of design, simulations, and
optimization (not full time)
4 years of construction
Hundreds of people involved
2.5 years of commissioning
…and with an existing linac
A Few More Points to Make…
A good electron gun solves many problems…
Soon after planning how to complete some machine
setup task, consider how to automate that process
Be prepared to spend 80% of commissioning time on
controls (amazing but true)!
Then get ready for the next 15%... safety system
interlock testing (5% allowed for FEL physics).
Finally, this slide was shown many times over the years…
LCLS requires very bright electron beam (emittance)…
εN = 1.2 μm
εN = 2.0 μm
P ≈ 10 GW
P ≈ 0.1 GW
Finally, these worries
have been relieved
courtesy S. Reiche
SASE FEL is not forgiving — instead of mild luminosity
loss, power nearly switches OFF
electron beam must meet brightness requirements
Thanks for your attention