fel x band issues m. dehler, be/rf & psi swissfel project at psi fel specific rf issues the...
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FEL X band issues
M. Dehler, BE/RF & PSI
• SwissFEL project at PSI• FEL specific RF issues• The CLIC/PSI/ST X band structure
PSI West
PSI Ost
Large research facilities
ProtonAccelerator
Swiss Light SourceSLS
Spallation NeutronSource SINQ
SwissFEL – the next big Facility at PSI
Slides courtesy H. Braun
FEL principleElectrons interact with periodic magnetic field of undulator magnet to build up anextremely short and intense X-ray pulse.
SwissFEL parameters
Wavelength from 1 Å - 70 Å
Pulse duration 1 fs - 20 fs
e- Energy 5.8 GeV
e- Bunch charge 10-200 pC
Repetition rate 100 Hz
SwissFEL, the next large facility at PSI
SwissFEL wavelength range
SwissFELpulse-length
Time- and length scales of the nano world
Understand dynamics of fundamental processses for chemistry, biology, condensed matter physics and material science
Basic Considerations
SwissFEL is build as a national facility in a small country
Total cost have to fit in a limited framework
21
2
2
2
KU
4N
nCμm1 BN q
$• Lowest beam energy technically possible
• Small period undulators with low K values
• Low qB charge
• Normal conducting linac technology
Aramis: 1-7 Å hard X-ray SASE FEL, In-vacuum , planar undulators with variable gap.
Athos: 7-70 Å soft X-ray FEL for SASE & Seeded operation . APPLE II undulators with variable gap and full polarization control.
D’Artagnan: FEL for wavelengths above Athos, seeded with an HHG source. Besides covering the longer wavelength range, the FEL is used as the initial stage of a High Gain Harmonic Generation (HGHG) with Athos as the final radiator.
715 m
S-band & X-band C-band C-band C-band
SwissFEL baseline
Parameters for lasing at 1ÅOperation Mode
Long Pulses Short Pulses
Charge per Bunch (pC) 200 10
Beam energy for 1 Å (GeV) 5.8 5.8
Core Slice Emittance (mm.mrad) 0.43 0.18
Peak Current at Undulator (kA) 2.7 0.7
Repetition Rate (Hz) 100 100
Undulator Period (mm) 15 15
Effective Saturation Power (GW) 2 0.6
Photon Pulse Length at 1 Å (fs, rms) 13 2.1
SwissFEL key parameters
The Operation Modes
• Standard operation• 200 pC• Maximum FEL pulse energy• Longest FEL pulse length
Bolko Beutner - FLAC 15.11.2010
• Lowest charge operation• 10 pC• Short FEL pulse length• Single-spike in soft X-ray
• Strong residual energy chirp• 200 pC• Large FEL Bandwidth (>1%) for
single short Absorption spectroscopy.
• Attosecond FEL pulse• 10 pC• Strongest compression• Single-spike in hard X-ray
ChargeWakefield Limited
Diagnostic Limit
Special Cases
Project Start of operation
Beam energy
min
GeV Å
LCLS, USA April 10 2009 ! 13.6 1.5
SCSS, Japan 2011 8 1.0
European X –FEL, Hamburg 2014 17.5 1.0
SwissFEL 2016 5.8 1.0
SwissFEL in comparison with the other hard X-ray FEL projects
SwissFEL has lowest beam energy
Advantages: Compact and affordable on national scale Challenges : More stringent requirements for beam quality, mechanical and electronic tolerances
First existing part of SwissFEL: 250 MeV Injector
715m
First beam to dump 9.8.2010
RF-gun
Cavity #1
#2
#3
Inauguration SwissFEL first stage, 24.8.2010
Commissiong crew with first beam
Beamline seen from gun end
Injector building
Exp3
Exp2
Exp1
Exp2
Exp1
Exp3
Exp2
Laser pump
THz pump
Seed laser
Gun laserARAMIS FEL 1-7 Å
ATHOS FEL 7-70 Å
2.1 GeV 3.4 GeV 5.8 GeV
Exp1
Laser pump
Gun laserARAMIS FEL 1-7 Å2.1 GeV 3.4 GeV 5.8 GeV
2018 SwissFEL Phase IISoft X-ray FEL
2016 SwissFEL Phase IAccelerator and hard X ray FEL
Exp3
2014 Building completed
Gun laser
2010 250 MeV Injector facility
SwissFEL Milestones
RF issues
•RF systems with three different frequencies at S-band, C-band and X-band
•Development of C-band linac module optimized for space and power economy
•Extreme phase tolerance specs require sophisticated synchronization and LLRF
Frequencies
SwissFEL Injektor2998.8 MHz11995.2 MHz
Main C-band LINAC5712 MHz
Active length S-band acceleration 24 m
Active length C-band acceleration 208 m
ARAMIS string of undulators 60 m
Other beam line elements 273 m
Photon beam transport 100 m
Experiment halls 50 m
Total facility length 715 m
No strong motivation for very high gradients !
Why (not) C Band?
Why (not) C Band: the Compression Schemes
• Normal:
• Large Bandwidth:
• Attosecond:
Actively making use of single bunch wakes:
RF frequency → Aperture → Active length → Gradient
Bolko Beutner - FLAC 15.11.2010
Linac 1 BC 2 Linac 2+3 Collimator
compression wakes remove chirpdouble dogleg
(slight decompression)
over-compression wakes add to chirpdouble dogleg
(slight compression)
compression wakes partially remove chirpchicane
(compression)
C-band LINAC Module
Main LINAC #
LINAC modules 26
Modulator 26
Klystron 26
Pulse compressor 26
Accelerating structures
104
Waveguide splitter 78
Waveguide loads 104
Modulator
30.8 MV/m
BOC Pulse-Compressor
50 MW, 3.0 µs max.40 MW, 3.0 µs for operation
120 MW, 0.5 µs
116 MW
30.8 MV/m 30.8 MV/m 30.8 MV/m
LLRF
Courtesy Hansruedi Fitze
2m
C-band development
Linacstructure
2011 2012 2013
Series productionFull scale structureShort structure
2011 2012 2013
BOC Design Fabrication Tests
2012 2013
Powertests
Courtesy Hansruedi Fitze
Klystron
• One E37202 is orderd for startup of test stand• Delivery May 2011• Upgrade Programm in Execution• E37210 to be delivered early 2012
Two Klystrons ordered from ToshibaE37202 E37210
Peak Power 50 MW 50 MW
RF Pulse Width 3 us 3 us
Repetition Rate 60 Hz 100 Hz
Avg. RF Power 7.7 kW 15 kW
Collector Power 35 kW 78 kW
Delivery Date May 2011 Feb 2012
Courtesy Jürgen Alex
Longitudinal phase space manipulations
SwissFEL Injektor2998.8 MHz11995.2 MHz
Main C-band LINAC5712 MHz
X-Band Structure Tasks
1. Removal of quadratic component from RF curvature:
with x-band on-crest – this canbe changed for fine tuning of compression.
2. Compensation of the quadratic contribution to the path length through the chicane
Court.: B. Beutner
S-band X-band
BC Chicane
1
1
2
2
3
3
4
4
First compression stage of SwissFEL
head
tail
Court.: B. Beutner
The Compression Scheme
• Normal:
• Large Bandwidth:
• Attosecond:
Bolko Beutner - FLAC 15.11.2010
Linac 1 BC 2 Linac 2+3 Collimator
compression wakes remove chirpdouble dogleg
(slight decompression)
over-compression wakes add to chirpdouble dogleg
(slight compression)
compression wakes partially remove chirpchicane
(compression)
200pC Mode
Bolko Beutner - FLAC 15.11.2010
Booster 2:
-17 deg16 MV/m
X-Band:
180.13 deg16.98 MV/m
Linac 1:
-20.9 deg26.5 MV/m
4.2 deg 2.15 deg
Linac 2/3:
0 deg26.5 MV/m
355MeV 150A 2.04GeV 3.2kA 3.2kA
head
tail
head
tail
head
tail
200 pC std mode
FEL Performance @ 200 pC
Bolko Beutner - FLAC 15.11.2010
200pC
Saturation 32 m
Esat 0.11 mJ
sp 20 fs
<Psat> 2.1GW
BW 0.065 %
200pC Tolerances
Bolko Beutner - FLAC 15.11.2010
arrival time peak current energy
goals: 20 fs 5 % 0.05 %
S-Band Phase [deg] 0.19 0.23 0.32
S-Band Voltage [rel] 0.001 0.00026 0.0011
X-Band Phase [deg] 30 0.061 0.86
X-Band Voltage [rel] 0.0051 0.0017 0.0058
Linac 1 Phase [deg] 0.15 0.084 0.43
Linac 1 Voltage [rel] 0.001 0.0041 0.0046
Linac 2 Phase [deg] 5.2e+003 1.6e+002 2.2e+003
Linac 2 Voltage [rel] 0.15 0.87 0.0051
Linac 3 Phase [deg] 4.6e+003 1.8e+002 2.9e+003
Linac 3 Voltage [rel] 0.12 0.19 0.0041
Charge [pC] 19 1.9 47
initial arrival time [fs] 6.2e+002 68 2.9e+003
Initial Energy [rel] 0.00097 0.00031 0.0011
BC1 angle [rel] 0.052 0.0011 0.014
BC2 angle [rel] 0.19 0.0011 0.015
200pC Performance
Bolko Beutner - FLAC 15.11.2010
Expected Perfromance
S-Band Phase [deg] 0.015
S-Band Voltage [rel] 1.2 * 1e-004
X-Band Phase [deg] 0.06
X-Band Voltage [rel] 1.2 * 1e-004
Linac 1 Phase [deg] 0.03
Linac 1 Voltage [rel] 1.2 * 1e-004
Linac 2 Phase [deg] 0.03
Linac 2 Voltage [rel] 1.2 * 1e-004
Linac 3 Phase [deg] 0.03
Linac 3 Voltage [rel] 1.2 * 1e-004
Charge 1%
initial arrival time [fs] 30
Initial Energy [rel] 1e-004
BC1 angle [rel] 5 * 1e-005
BC2 angle [rel] 5 * 1e-005
Tolerance Goal for
Arrival Time [fs]
Peak Current
[%]
Energy Jitter [%]
200pC 20 5 0.05
10pC – Attosecond PulseModification of 10pC mode:
• Fully upright compression• BC1 bending angle: 3.82 deg 4.2 deg • Linac 1 Phase: -16.7 deg -20.8 deg• Reconfiguration of bunch collimator for
additional compression
Bolko Beutner - FLAC 15.11.2010
head
tail
head
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10 pC Performance• Significant enhancement of the current and
thus increase of the FEL parameter.• Single spike operation at one 1 Angstrom
with an RMS pulse length of 60 as!
Bolko Beutner - FLAC 15.11.2010
10pC Tolerances
Bolko Beutner - FLAC 15.11.2010 32
arrival time peak current energy
goals: 5 fs 15 % 0.05 %
S-Band Phase [deg] 0.027 0.027 0.43
S-Band Voltage [rel] 0.00011 0.0003 0.0018
X-Band Phase [deg] 0.12 0.027 0.25
X-Band Voltage [rel] 0.00054 0.0021 0.0099
Linac 1 Phase [deg] 0.13 0.3 1.4
Linac 1 Voltage [rel] 0.00024 0.0065 0.0045
Linac 2 Phase [deg] 2.8e+002 35 5.3e+002
Linac 2 Voltage [rel] 0.0052 0.25 0.0052
Linac 3 Phase [deg] 1.5e+002 36 4.3e+002
Linac 3 Voltage [rel] 0.0041 0.25 0.0041
Charge [pC] 0.92 0.28 4.3
initial arrival time [fs] 81 17 2.8e+002
Initial Energy [rel] 0.00011 0.0012 0.0022
BC1 angle [rel] 0.0015 0.00029 0.0033
BC2 angle [rel] 0.0076 0.001 0.015
10pC Performance
Bolko Beutner - FLAC 15.11.2010 33
Expected Perfromance
S-Band Phase [deg] 0.015
S-Band Voltage [rel] 1.2 * 1e-004
X-Band Phase [deg] 0.06
X-Band Voltage [rel] 1.2 * 1e-004
Linac 1 Phase [deg] 0.03
Linac 1 Voltage [rel] 1.2 * 1e-004
Linac 2 Phase [deg] 0.03
Linac 2 Voltage [rel] 1.2 * 1e-004
Linac 3 Phase [deg] 0.03
Linac 3 Voltage [rel] 1.2 * 1e-004
Charge 1%
initial arrival time [fs] 30
Initial Energy [rel] 1e-004
BC1 angle [rel] 5 * 1e-005
BC2 angle [rel] 5 * 1e-005
Tolerance Goal for
Arrival Time [fs]
Peak Current
[%]
Energy Jitter [%]
100pC 5 15 0.05
Ultra-stable Sync System Requirements
• Most critical issues for sync system:Jitter (RMS, 10Hz..10MHz) between two clients and long term drift (hours)
• Typical FEL client is using ref. (RF, opt.) directly or for locking a PLL• Gun laser: ≈30fs expected (goal: towards 10fs), measure with
BAM (beam arrival time monitor)• Most critical RF stations: goal is “0.02° phase jitter at 3GHz“ for SwissFEL
RF system contributes >10fs (far out) intrinsic jitter, <5fs (diff. mode) req. from sync
• Experiment (pump-probe) lasers: <10fs (optical sync combined w. BAM for sorting of jittery experimental data)
• Seeding laser: <10fs (optical sync combined w. BAM for drift FB)• E/O sampling: <50fs• BAM (opt. sync only): approx. 6fs timing resolution/stability (down to 10pC)• “Differential mode jitter“ betw. stations is critical, “common mode jitter“ (all clients jittering w. ref.)
is less critical. • Actual injector drift requirement: some 100fs over hours, will be tightened in the future
probably down to <10fs.
10fs is equivalent to 3um in air!
Courtesy Stefan Hunziker
Hybrid Layout: High Flexibility, Reasonable Cost
FEL phase reference: generic layout
Want ultimate performance for critical clients pulsed optical ref. signal
Don‘t need ultimate performance everywhere (sub-)distributions withlower cost technologies
electron beam
…
beamlines
RF masteroscillator
opt. syncfront-end(pulsed)
optical reference signals (pulsed and cw)
crit.… … …
electron gun
laser
optical syncfront-end pulsed
uncrit. uncrit.…
uncrit. RF(el. subdistribution)
optical syncfront-end (cw)
clients:
cw
multiple fibers
pulsedlaser
distrib.
cwlasers
cwlasers
mod.mod.
opt. syncfront-end(pulsed)
extremelycrit. RFbal. o-Wdetector
dir. harm. extraction
moderatelycrit. RF
RF ge-neration
Want ultimate performance for critical clients pulsed optical ref. signal
Don‘t need ultimate performance everywhere (sub-)distributions withlower cost technologies
electron beam
…
beamlines
RF masteroscillator
opt. syncfront-end(pulsed)
optical reference signals (pulsed and cw)
crit.… … …
electron gun
laser
optical syncfront-end pulsed
uncrit. uncrit.…
uncrit. RF(el. subdistribution)
optical syncfront-end (cw)optical sync
front-end (cw)
clients:
cw
multiple fibers
pulsedlaserpulsedlaser
distrib.distrib.
cwlaserscwlasers
cwlasers
mod.mod.mod.mod.
opt. syncfront-end(pulsed)
extremelycrit. RFbal. o-Wdetector
dir. harm. extraction
moderatelycrit. RF
RF ge-nerationRF ge-neration
Courtesy Stefan Hunziker
Challenges for FELs( as opposed to linear colliders?)
• Synchronization with electrooptical methods• Photon diagnostics (partially real time, suitable
for feedback and stabilization)• Push for real time 6D phase space diagnostics
for FB• Push for high rep rate NC RF linacs• New RF structures (see next part ...)
A CERN/PSI/ST collaboration
• Motivation for CLIC:• Another data point in high gradient test program• Validation of design and fabrication procedures• A true long term test in another accelerator facility
• Motivation for the FEL projects:• An X band structure to compensate long. phase space nonlinearities• High gradient/power requirements of CLIC = a design for safe
operation at the more relaxed parameters of the PSI X-FEL• RF design (mostly) by PSI, engineering design, fabrication,
assembly & LL RF test at CERN, mechanical support & other parts by FERMI
…. Possibly create a general purpose structure for other applications …
Multi purpose X band structure
Special considerations for FEL
• Operating structure at relatively low beam energies (PSI injector: 250 MeV)
• High sensitivity to transverse wakefields!• Strategy:
– Passive: Try to have open structure while maintaining good efficiency and breakdown resilience
– Active: Wake field monitors• See offsets before they show up as emittance dilution• Possibly measure higher order/internal misalignments
(tilts, bends ….)
A priori specifications
• Beam voltage 30 MeV at a max. power of 45 MW• Mechanical length <1017 mm• Iris diameter > 9 mm• Wake field monitors• Operating temperature 40 deg. C• Constant gradient design, no HOM damping• Fill time < 1 usec• Cooling assuming 1 usec/100 Hz RF pulse
The strategy• Use 5π/6 phase advance:
– Longer cells: smaller transverse wake
– Intrinsically lower group velocity: Good gradient even for open design with large iris
– Needs better mechanical precision• Long constant gradient design
(efficiency!)• No HOM damping• Wake field monitors to insure optimum
structure alignment• Do a castrated NLC type H75
without damping manifolds!
NLC type H75
• Well optimized design (iris aperture, thickness and ellipticity varying along structure)
• Original design gives 65 MV/m for 80 MW input power
• Sucessfully tested up to 100 MV/m with SLAC mode launcher (below)
r
z
r
z
|E|
|B|
Constant gradient design
• 72 cells, active length 750 mm• Relatively open structure: mean
aperture 9.1 mm• Average gradient 40 MV/m (30 MeV
voltage) with 29 MW input power• Group velocity variation: 1.6-3.7%• Fill time: 100 nsec• Average Q: 7150
HOM coupler a la NLC DDS
• TE type coupling minimizes spurious signals from fundamental mode and longitudinal wakes
• Need only small coupling (Qext<1000) for sufficient signal
• Minor loss in fundamental per- formance: 10% in Q, <2% in R/Q
• Output wave guides with coaxial transition connecting to measurement electronics
• Two monitors replacing cells 36 and 63 for up- and downstream signals
Electric short on one side
Axial signal output wave guides
Output signal spectra
Signal envelopes of wake monitors
Signal at time t is correlated with frequency – is correlated with cell number…..
Can we learn something about internal misalignments?
Structure tilt
Beam axis
Tilted
Ref. - offset
The accelerating mode• 66 cell substructure:• Omit power couplers, matching
cells• 500’000 elements, 10’000’000
unknowns (3rd order approach required)
• Computed resonance frequency:
• F = 11.99235 GHz (w/o losses)• ~ F=11.9912 GHz (including
losses)• Design: F=11.991648 GHz
Accuracy of design approach exceeds mechanical
precision!
|E| of 5 π/6 mode
Below: monitor at cell 36
Eigenvalues with ACE 3P
(more to come in an up coming CLIC structures meeting ..)
Mechanical modelEach two structures for structures for PSI (SwissFEL) and ST (Sincrotone Trieste) with wakefield monitors under fabrication
Wakefield monitor details
48 (court. D. Gudkov)
Short test stack done with diffusion bonding
49
Bonding at 1040°C for 90 minutes under H2
Metallurgical polishing + etching 75 s in Ammonium peroxodisulfate (NH4)2S2O8
50
Joining plane
Site of Interest 1: Outer side of disc stack• Grains grew down across the joining plane
(Court.: Markus AICHLER)
RF check of assembled structure
(court. J. Shi)
Assembly structure before bonding
(court. S. Lebet)
Sub stack ready for bonding
(court. S. Lebet)
Straightness check after bonding
(court. S. Lebet)
Big thanks to:
• Design work: A. Citterio, G. D‘Auria, M. Dehler, A. Grudiev, J.-Y.
Raguin, G. Riddone, I. Syratchev, W. Wuensch, R. Zennaro
• Mechanical design and production team of G. Riddone: M.
Filipova, D. Gudkov, S. Lebet, A. Samoshkine, J. Shi & ... & ... & ...
• Access & support for ACE3P: A. Candel, K. Ko, R. Lee, Z. Li