Review of the European XFEL Bunch Compression System
Summary
Torsten Limberg
Topics and Speakers
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Introduction and Concept
Optic and Tolerances
Simulation Calculations
Tuning
Bunch Compression Options
Diagnostic Overview & FB
Diagnostic Sections Lay Out
Diagnostic Tools and Optical Replica
Vacuum
T. Limberg
W. Decking
M. Dohlus
T. Limberg
M. Dohlus
H. Schlarb
C. Gerth
B. Schmidt & M. Yurkov
N. Mildner,
T. Wohlenberg, K. Zapfe
Design Goals and Considerations
Electron bunches out of the gun: 50 A peak current, small energy spread
BC system has to convert that to:
–
–
–
–
–
–
–
–
–
5 kA peak current
< 25 mm Bunch Length (shorter pulses?)
< 1.4 mm-mrad slice emittance
< 1 MeV slice energy spread (stay about a factor of two below that from
synchrotron radiation in undulator)
Compensate rf structure wake field induced correlated energy spread as
good as possible with rf induced energy chirp for compression (mimimize
laser bandwidth)
avoid high gain for micro-bunch instability
avoid big projected emittance (> 2.5 mm-mrad)
< 10% peak current jitter (SASE jitter <10 %)
arrival time jitter has mainly to be measured and taken care of by the
experiments
Bunch Compression Scheme (TADR)
s = 2 mm
Ipeak : 50 A
s = 0.02 mm
Ipeak : 5 kA
s = 0.1 - 0.15 mm
Ipeak : 0.7 - 1 kA
Injector
Linac
3rd harmonic
RF section
Booster Linac
with 3x4 Modules
R56 = 100 mm
Gun
Main
Linac
Undulator
R56 = 15-25mm
Gun
: L-Band Module
: 3rd harmonic rf Module
: Bending Magnet
0.4 - 0.5 GeV
: Vertical Deflection Cavity Section
: Wire Scanner Section
2.0 GeV
(at 15 MeV/m in booster)
17.5 GeV
Bunch Compressor Beam Line Optics
Dogleg (R56 ≈ - 0.015 m)
Diagnostic
Section
Drift through
shielding
18 deg deflection to
commissioning dump
W. Decking: To Do List Optics and Tolerances
• Include BC Diagnostic Sections in Master Deck
• Increase BC chicane middle dipoles distance to include
diagnostics
• Calculate transverse wakefield effects of 3rd harmonic
cavities
• Adjust phase advance between BC1 and BC2 to n*pi
• Magnet tolerance studies (field quality and alignment of
dipoles)
M. Dohlus: Simulation Calculations
‘laser heater’
(LCLS layout)
 PE exp  jkE dE
slice energy distribution P(E)
rms = 2 keV
(Gaussian)
rms = 10 keV
(from laser heater)
rms = 10 keV
(Gaussian)
E kV
k
gain curves
TDR: gaussian distribution
rms = 10 keV
“real” heater:
rms = 10keV
after dogleg
after BC2
after BC1
entrance mm
shot noise
I noise,rms 
eI

 G d
2
 0
TDR: I noise,rms  29 A
“real” heater: I noise,rms  260 A
dogleg, r56 = 0.84mm  0
 I noise,rms  44 A
energy to current modulation:
“real” heater:
rms = 10keV
TDR: gaussian distribution
rms = 10 keV
M
eV
after dogleg
after BC2
after BC1
entrance mm
ASTRA simulation:
5% modulation at cathode,  = 0.2 mm  injector dogleg (~45m after cathode):
cathode
5%
current / A
energy / eV
0.5%
after 45m (130 MeV)
~2keV
s / mm
Setup Using ‘Multiknobs’
• Make knobs to change independently the first, second and third
derivative of the combined accelerating voltage of Injector Linac and 3rd
harmonic RF, using linac and 3rd harmonic phase and 3rd harmonic
amplitude.
– V(s)
= V1cos(k1s+j1) + V3cos(k3s + j3)
= DV + g ∙ s + x1∙ 1010 ∙s2 + x2∙ 1012 ∙s3 + o(s4)
Use gradient knob for peak current, 2nd derivative to balance beam
distribution in the center region and 3rd derivative knob for adjusting
the tails.
• Linac Amplitude is still used to keep beam energy constant.
Things to Do
• Practical design of multi-knobs for FLASH
• Prepare detailed tuning scheme for FLASH
• Test it and learn…
BC System – Review
Options
● BC2 working point (energy-charge-compr.)
● 2BC (rf-rf-bc-rf-bc-rf)
● table: 2BC (rf-rf-bc-rf-bc-rf)
dogleg + 2BC (rf-dog-rf-rf-bc-rf-bc-rf)
n3BC (rf-bc-rf-rf-bc-rf-bc-rf)
3BC (rf-rf-bc-rf-rf-bc-rf-bc-rf)
rollover compression
● laser heater
● cases in detail
peak current
arrival time stability
projected emittance
slice emittance
µ-bunch stability
parameter-sensitivity
uncorrelated
energy spread
remaining chirp
M. Dohlus bc system optimization sheet
compression
factors
r56 knobs
rf knobs
shot noise due
to µ-bunching gain
chirp
absolute tolerances
(amplitude & phase_deg)
minimal relative tolerances
I noise,rms 
µ-bunching gain
eI

G
 0
2
d
Balancing the micro-bunch instability strength vs. the rf jitter sensitivity
inverse tolerance
noise: Irms/A
j L2
deg
r56BC1 mm
noise
Irms/A
inverse tolerance
…continued
nois
e
Irms/A
C1, E1/MeV
20,500
10,500
20,400
10,400
5,400
min(phas_tol) = 0.016 deg
noise: Irms = 260 A
inverse tolerance
E1 = 400 MeV
r56BC1 = 90mm, C1=5
r56BC2 = 75mm, C2=20
jL2 = 10 deg
min(ampl_tol) = 0.1%
min(phas_tol) = 0.023 deg
noise: Irms = 147 A
2BC
rf
(1+3)
dogleg+2BC
-bc-rf-bc-rf-c
rf
(1+3)
rf-d-bc-rf-bc-rf-c
rf
(1+3)
n3BC
3BC
rf-bc-bc-rf-bc-rf-c
(1+3)
rf
rollover compr.
rf
-bc-bc-rf-bc-rf-c
rf(1+3)-bc-rf-bc-rf-c
(1+3)
E=400MeV
2GeV
17.5GeV
E=130MeV
400MeV
2GeV
17.5GeV
E=130MeV
400MeV
2GeV
17.5GeV
E=130MeV
500MeV
2GeV
17.5GeV
E=500MeV
2GeV
17.5GeV
C=5
20
0.98
C=1.2
4.17
20
0.98
C=1.25
4
20
0.98
C=1.45
6.90
10
0.98
C=10
10
0.98
r56=-90mm
-75mm
0.84mm
r56=40mm
-90mm
-87.2mm
0.84mm
r56=30mm
-80mm
-83.7mm
0.84mm
r56=-30mm
-90mm
-45.0mm
0.84mm
r56=-100mm
-200mm
0.84mm
ampl_tol=0.1%
ph_tol=0.023deg
noise= 147 A
ampl_tol=0.11%
ph_tol=0.040deg
noise= 270 A
ampl_tol=0.11%
ph_tol=0.045deg
noise= 93 A
ampl_tol=0.09%
ph_tol=0.048deg
noise= 95 A
ampl_tol=0.2%
ph_tol=0.055deg
small
jL2 = 10 deg
e’=1%@ 130MeV
jL2 = 10 deg
t566_dog=1m
e’=1.6%@ 130MeV
jL2 = 10 deg
t566_dog=1m
e’=2.5%@ 130MeV
jL2 = 10 deg
jL2 = 40.5 deg
Diagnostics overview BC1
• proposed beam line design:
SRF
1.3 GHz
SRF
3.9 GHz
Bunch compressor
TDS Diagnostic
X&Y
section
Standard diagnostics:
SRF 1.3GHz
Spectrometer
Dump
TOR
toroid system for transmission measurements (1,3&4 for interlock)
DC
dark current monitors (upstream BC1, downstream BC1)
BPM
beam position monitor ~ 20 (not yet determined … every quad?)
purpose: orbit correction, transfer measurements, dispersion correction
OTR
optical transition screen (with wire scanners WS?)
Diagnostics overview BC1
• proposed beam line design:
SRF
1.3 GHz
SRF
3.9 GHz
Bunch compressor
TDS Diagnostic
X&Y
section
Special diagnostics:
SRF 1.3GHz
Spectrometer
Dump
TDS
transverse deflecting structure X & Y
EO
electro-optic longitudinal beam profile monitor
BCM
bunch compression monitors (CSR at D4 and CDR/CTR)
SR
synchrotron radiation monitor (energy and energy spread)
BAM
beam arrival time monitor
-> B Schmidt
Diagnostics overview BC1
• proposed beam line design:
SRF
1.3 GHz
SRF
3.9 GHz
Bunch compressor
TDS Diagnostic
X&Y
section
Additional devices:
SRF 1.3GHz
Spectrometer
Dump
COL
collimators (1st & 2nd to remove dark current, 3nd & 4th for kicked e-)
KIC
fast kicker to off-axis screens (2 x and 2 y)
Align
laser for optics alignment
BLM
beam loss monitors (about 8-10 sufficient)
Screen / Kicker arrangement (2)
Horizontal slice emittance / vertical streak
Vertical slice emittance / horizontal streak
45deg
OTR1
OTR2
OTR4
OTR6
45deg
OTR1
OTR2
OTR4
OTR6
HK1
HK1
HK2
HK2
76deg
OTR1
OTR3
OTR4
OTR6
VK1
VK1
VK2
VK2
76deg
OTR2
OTR3
OTR4
OTR5
3 cells = 11.4 m
Horizontal kicker
FODO lattice 6 off-axis OTR screens (y and x)
Vertical
kicker
HK2
HK1
VK1
VK2
OTR1
OTR2
OTR3
OTR4
Bend plane of BCs defines the OTR arrangement
OTR5
OTR6
Diagnostic Section Engineering layout (3)
BAM
T1
VK1
ABCM
EOSD
TDS-x
TDS-y
Alignment
laser
FODO lattice
Lattice can be divided into modules:
3.8 m
HK1
VK2
RES
10 modules
HK2
T2
Booster Linac
7.6 m
5 modules
ABCM
2.5m
Conclusions
Conclusions (1):
For which bunch rep rate, 5MHz or 1MHz, shall the on-line
slice emittance diagnostics be designed in BC1:
• Desired resolution can easily be reached at 1 MHz but is just at
the theoretical limit for 5 MHz.
• Kickers with the required kick strength for 1MHz are in operation
in several machines at DESY (‘off-the-shelf’).
5 MHz would requires new design and prototype development.
• If standard FEL operation will be 5 MHz slice emittance
diagnostics cannot be operated parasitically if designed for 1 MHz
(or might not be used if resolution is not sufficient).
• If standard FEL operation will be 1 MHz one would lose at least a
factor of 1.6 in resolution if designed for 5 MHz
Conclusions
Conclusions (2):
Dump defines the horizontal streak direction in BC2.
If the BCs are installed vertically slice emittance could be
measured in the bend plane of BCs.
Number of quads in current layout
BC1 was 22 now 22
BC2 wsa 13 now 19
New lattice layout requires slightly more space
BC1: 1.5 m in BC + 0.9 m in diag section = 2.4 m
BC2: 1.0 m in BC + 1.5 m in diag section* = 2.5 m
*Additional FODO cell for 45 deg lattice requires 7.6 m more space
Layout of the dignostics sections can be arranged in
modules. Components can be prealigned and tested.
This saves time during installation and commissioning.
Layout of BC1 diagnostic section almost finalized.
After beam dynamic and sensitivity studies (2 months) the
vacuum and engineering layout could be started
Coherent
radiation
Status :
- spectrally resolving single shot instrument developed
(multi stage grating spectrograph with parallel read out)
-Advanced prototype running at FLASH (THz beam-line)
- Existence of spectroscopic fingerprints shown down to µm scale
To be done :
- develop compact monlithic version
- explore and establish feedback capabilities
- detailed planning of station lay-out
existing detector unit
Zur Anzeige wird der QuickTime™
Dekompressor „TIFF (LZW)“
benötigt.
Potential layout for
4-stage spectrograph
Electro-optical monitors
Status :
- different methods under study at FLASH
- integrity and validity of data largely explored
- spectral decoding method proven to be sufficiently simple
- dedicated fiber-laser version under construction
To be done :
- step from ‘experiment’ to ‘on-line tool’
- more robust and reliable laser system (fiber-laser)
- fast (parallel) read-out system (line camera)
- direct (optical) coupling to optical timing system
Requirements / implications :
- EO crystals inside beam pipe (r ~ 2-5 mm), retractable
- optical ports for laser in/out
~0.6 m
laser
beam
space underneath beam pipe : ~ 2 m2 optical
table (laser +spectrometer + camera).
T. Wohlenberg: Bunch compressor section BC1 and BC2
General remarks
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Lengths of the vacuum system BC1 and BC2:
BC1: total length ~ 69m → chicane length ~ 27m → deflection of the
chicane ~ 0.68m
BC2: total length ~ 90m → chicane length ~ 25m → deflection of the
chicane ~ 0.33m
• Vacuum requirements:
 Pressure needs to be in the range of 10-10 mbar (next to cold sections)
 Pump system: sputter ion pumps and titan sublimations pumps
• Both sections are particle free :
 The design of all vacuum components needs to be according to the
particle free conditions. Early discussion of concept of all components
including beam diagnostic is necessary!
 All vacuum components have to be cleaned under particle free condition
(clean room).
 Installations needs to be done under local clean room conditions.
Bunch compressor section BC1 and BC2
General remarks
 From the point of view of vacuum technology both BC sections should be

treated similar. This should be valid for the aspect of material choice,

joining technology, support for the chambers etc..
 The design concept for the flat chamber in the chicane is similar to FLASH!
Bunch compressor section BC1 and BC2
Schedule
•
Draft:
 Components layout + girder and frames concept including
electronics/diagnostics units concept ~ 1 year
 Design of BC1 and BC2 ~ 1 year
 Fabrication of all components ~ 1.5 years
 2007, A rough concept should be settled for the girders/frames concept
including electronics and diagnostics as well as part of the layout of the
components.→ layout for the arrangement of the components should be
available!
 2008, The detailed concept for the layout of the components, electronic
concept and the girder and frames concept should be finished.
Bunch compressor section BC1 and BC2
open issues
 Do we have the BC‘s chicane to be installed vertically or
horizontally? → we prefer vertical installation!
 Do all components need to be copper coated in both BC’s?
 Can the RF-shielding remain the same as for FLASH or
do we have to design a new concept for the flange connections,
bellows, valves and pump connections?
 Is a massive lead shielding necessary ?
→ need to be included into the girder and frame design!
 How does the dump section for BC1 and BC2 look like?
 What diagnostic installations will be needed next to the beam
line?