FEL X band issues M Dehler BERF PSI
FEL X band issues M. Dehler, BE/RF & PSI • Swiss. FEL project at PSI • FEL specific RF issues • The CLIC/PSI/ST X band structure
PSI Ost PSI West
Large research facilities Proton Accelerator Spallation Neutron Source SINQ Swiss Light Source SLS
Swiss. FEL – the next big Facility at PSI Slides courtesy H. Braun
Swiss. FEL, the next large facility at PSI Swiss. FEL parameters FEL principle Electrons interact with periodic magnetic field of undulator magnet to build up an extremely short and intense X-ray pulse. Wavelength from 1 Å - 70 Å Pulse duration 1 fs - 20 fs e- Energy 5. 8 Ge. V e- Bunch charge 10 -200 p. C Repetition rate 100 Hz
Time- and length scales of the nano world Swiss. FEL pulselength Swiss. FEL wavelength range Understand dynamics of fundamental processses for chemistry, biology, condensed matter physics and material science
Basic Considerations Swiss. FEL is build as a national facility in a small country Total cost have to fit in a limited framework • Lowest beam energy technically possible • Small period undulators with low K values • Low q. B charge • Normal conducting linac technology
Swiss. FEL baseline S-band & X-band C-band 715 m 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.
Swiss. FEL key parameters Parameters for lasing at 1Å Operation Mode Long Pulses Short Pulses Charge per Bunch (p. C) 200 10 Beam energy for 1 Å (Ge. V) 5. 8 Core Slice Emittance (mm. mrad) 0. 43 0. 18 Peak Current at Undulator (k. A) 2. 7 0. 7 Repetition Rate (Hz) 100 Undulator Period (mm) 15 15 Effective Saturation Power (GW) 2 0. 6 Photon Pulse Length at 1 Å (fs, rms) 13 2. 1
The Operation Modes Charge Wakefield Limited • • Standard operation 200 p. C Maximum FEL pulse energy Longest FEL pulse length Diagnostic Limit Lowest charge operation 10 p. C Short FEL pulse length Single-spike in soft X-ray Special Cases • • • Strong residual energy chirp 200 p. C Large FEL Bandwidth (>1%) for single short Absorption spectroscopy. • • Attosecond FEL pulse 10 p. C Strongest compression Single-spike in hard X-ray Bolko Beutner - FLAC 15. 11. 2010
Swiss. FEL in comparison with the other hard X-ray FEL projects Beam energy min Ge. V Å April 10 2009 ! 13. 6 1. 5 SCSS, Japan 2011 8 1. 0 European X –FEL, Hamburg 2014 17. 5 1. 0 Swiss. FEL 2016 5. 8 1. 0 Project LCLS, USA Start of operation Swiss. FEL 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 Swiss. FEL: 250 Me. V Injector 715 m First beam to dump 9. 8. 2010
Inauguration Swiss. FEL first stage, 24. 8. 2010 Injector building Commissiong crew with first beam Beamline seen from gun end
Swiss. FEL Milestones Gun laser 2010 250 Me. V Injector facility 2014 Building completed Gun laser 2. 1 Ge. V 3. 4 Ge. V 5. 8 Ge. V p 1 Ex ARAMIS FEL 1 -7 Å Exp 2 Ex p 2016 Gun laser 3 Swiss. FEL Phase I Accelerator and hard X ray FEL 2. 1 Ge. V Laser pump 3. 4 Ge. V 5. 8 Ge. V p 1 Ex ARAMIS FEL 1 -7 Å Exp 2 2018 Swiss. FEL Phase II Soft X-ray FEL ATHOS FEL 7 -70 Å Ex p 3 p 1 Ex Seed laser Exp 2 Ex p 3 Laser pump THz pump
RF issues • RF systems with three different frequencies at S -band, C-band X-band • Development of C-band linac module optimized for space and power economy • Extreme phase tolerance specs require sophisticated synchronization and LLRF
Frequencies Swiss. FEL Injektor 2998. 8 MHz 11995. 2 MHz Main C-band LINAC 5712 MHz
Why (not) C Band? 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: the Compression Schemes BC 2 Linac 1 Linac 2+3 Collimator wakes remove chirp double dogleg (slight decompression) over-compression wakes add to chirp double dogleg (slight compression) compression wakes partially remove chirp chicane (compression) • Normal: • Large Bandwidth: • Attosecond: compression Actively making use of single bunch wakes: RF frequency → Aperture → Active length → Gradient Bolko Beutner - FLAC 15. 11. 2010
C-band LINAC Module Modulator LLRF 116 MW Main LINAC # LINAC modules 26 Modulator 26 Klystron 26 Pulse compressor 26 Accelerating structures 104 Waveguide splitter 78 Waveguide loads 104 BOC Pulse. Compressor 50 MW, 3. 0 µs max. 40 MW, 3. 0 µs for operation 120 MW, 0. 5 µs 30. 8 MV/m 2 m Courtesy Hansruedi Fitze
C-band development Courtesy Hansruedi Fitze
Klystron Two Klystrons ordered from Toshiba E 37202 E 37210 Peak Power 50 MW RF Pulse Width 3 us Repetition Rate 60 Hz 100 Hz Avg. RF Power 7. 7 k. W 15 k. W Collector Power 35 k. W 78 k. W Delivery Date May 2011 Feb 2012 • • One E 37202 is orderd for startup of test stand Delivery May 2011 Upgrade Programm in Execution E 37210 to be delivered early 2012 Courtesy Jürgen Alex
Longitudinal phase space manipulations Swiss. FEL Injektor 2998. 8 MHz 11995. 2 MHz Main C-band LINAC 5712 MHz
X-Band Structure Tasks 1. Removal of quadratic component from RF curvature: with x-band on-crest – this can be changed for fine tuning of compression. 2. Compensation of the quadratic contribution to the path length through the chicane Court. : B. Beutner
1 S-band 2 X-band 3 1 BC Chicane 4 2 tail head First compression stage of Swiss. FEL 3 4 Court. : B. Beutner
The Compression Scheme Linac 1 • BC 2 • Collimator Normal: wakes remove chirp double dogleg (slight decompression) over-compression wakes add to chirp double dogleg (slight compression) compression wakes partially remove chirp chicane (compression) compression • Linac 2+3 Large Bandwidth: Attosecond: Bolko Beutner - FLAC 15. 11. 2010
200 p. C std mode Booster 2: X-Band: -17 deg 16 MV/m 180. 13 deg 16. 98 MV/m 200 p. C Mode Linac 1: Linac 2/3: -20. 9 deg 26. 5 MV/m 0 deg 26. 5 MV/m 2. 15 deg 4. 2 deg 355 Me. V 150 A 3. 2 k. A 2. 04 Ge. V 3. 2 k. A head tail Bolko Beutner - FLAC 15. 11. 2010
FEL Performance @ 200 p. C 200 p. C Saturation 32 m Esat 0. 11 m. J sp 20 fs <Psat> 2. 1 GW BW 0. 065 % Bolko Beutner - FLAC 15. 11. 2010
200 p. C Tolerances 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. 0046 Linac 2 Phase [deg] 5. 2 e+003 1. 6 e+002 2. 2 e+003 Linac 2 Voltage [rel] 0. 15 0. 87 0. 0051 Linac 3 Phase [deg] 4. 6 e+003 1. 8 e+002 2. 9 e+003 Linac 3 Voltage [rel] 0. 12 0. 19 0. 0041 19 1. 9 47 6. 2 e+002 68 2. 9 e+003 0. 00097 0. 00031 0. 0011 BC 1 angle [rel] 0. 052 0. 0011 0. 014 BC 2 angle [rel] 0. 19 0. 0011 0. 015 Charge [p. C] initial arrival time [fs] Initial Energy [rel] Bolko Beutner - FLAC 15. 11. 2010
200 p. C Performance Expected Perfromance S-Band Phase [deg] 0. 015 S-Band Voltage [rel] 1. 2 * 1 e-004 X-Band Phase [deg] 0. 06 X-Band Voltage [rel] 1. 2 * 1 e-004 Linac 1 Phase [deg] 0. 03 Linac 1 Voltage [rel] 1. 2 * 1 e-004 Linac 2 Phase [deg] 0. 03 Linac 2 Voltage [rel] 1. 2 * 1 e-004 Linac 3 Phase [deg] 0. 03 Linac 3 Voltage [rel] 1. 2 * 1 e-004 Charge 1% initial arrival time [fs] 30 Initial Energy [rel] 1 e-004 BC 1 angle [rel] 5 * 1 e-005 BC 2 angle [rel] 5 * 1 e-005 Tolerance Goal for Arrival Time [fs] Peak Current [%] 200 p. C 20 5 Energy Jitter [%] 0. 05 Bolko Beutner - FLAC 15. 11. 2010
10 p. C – Attosecond Pulse Modification of 10 p. C mode: • • Fully upright compression BC 1 bending angle: 3. 82 deg 4. 2 deg Linac 1 Phase: -16. 7 deg -20. 8 deg Reconfiguration of bunch collimator for additional compression head tail Bolko Beutner - FLAC 15. 11. 2010 head tail
10 p. C 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
10 p. C Tolerances arrival time goals: peak current energy 5 fs 15 % 0. 05 % S-Band Phase [deg] 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. 8 e+002 35 5. 3 e+002 Linac 2 Voltage [rel] 0. 0052 0. 25 0. 0052 Linac 3 Phase [deg] 1. 5 e+002 36 4. 3 e+002 Linac 3 Voltage [rel] 0. 0041 0. 25 0. 0041 0. 92 0. 28 4. 3 81 17 2. 8 e+002 0. 00011 0. 0012 0. 0022 BC 1 angle [rel] 0. 0015 0. 00029 0. 0033 BC 2 angle [rel] 0. 0076 0. 001 0. 015 Charge [p. C] initial arrival time [fs] Initial Energy [rel] Bolko Beutner - FLAC 15. 11. 2010 32
10 p. C Performance Expected Perfromance S-Band Phase [deg] 0. 015 S-Band Voltage [rel] 1. 2 * 1 e-004 X-Band Phase [deg] 0. 06 X-Band Voltage [rel] 1. 2 * 1 e-004 Linac 1 Phase [deg] 0. 03 Linac 1 Voltage [rel] 1. 2 * 1 e-004 Linac 2 Phase [deg] 0. 03 Linac 2 Voltage [rel] 1. 2 * 1 e-004 Linac 3 Phase [deg] 0. 03 Linac 3 Voltage [rel] 1. 2 * 1 e-004 Charge 1% initial arrival time [fs] 30 Initial Energy [rel] 1 e-004 BC 1 angle [rel] 5 * 1 e-005 BC 2 angle [rel] 5 * 1 e-005 Tolerance Goal for Arrival Time [fs] Peak Current [%] 100 p. C 5 15 Energy Jitter [%] 0. 05 Bolko Beutner - FLAC 15. 11. 2010 33
Ultra-stable Sync System Requirements • Most critical issues for sync system: Jitter (RMS, 10 Hz. . 10 MHz) between two clients and long term drift (hours) • • Typical FEL client is using ref. (RF, opt. ) directly or for locking a PLL Gun laser: ≈30 fs expected (goal: towards 10 fs), measure with BAM (beam arrival time monitor) Most critical RF stations: goal is “ 0. 02° phase jitter at 3 GHz“ for Swiss. FEL RF system contributes >10 fs (far out) intrinsic jitter, <5 fs (diff. mode) req. from sync Experiment (pump-probe) lasers: <10 fs (optical sync combined w. BAM for sorting of jittery experimental data) Seeding laser: <10 fs (optical sync combined w. BAM for drift FB) E/O sampling: <50 fs BAM (opt. sync only): approx. 6 fs timing resolution/stability (down to 10 p. C) “Differential mode jitter“ betw. stations is critical, “common mode jitter“ (all clients jittering w. ref. ) is less critical. Actual injector drift requirement: some 100 fs over hours, will be tightened in the future probably down to <10 fs is equivalent to 3 um in air! • • Courtesy Stefan Hunziker
FEL phase reference: generic layout Hybrid Layout: High Flexibility, Reasonable Cost 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 6 D phase space diagnostics for FB • Push for high rep rate NC RF linacs • New RF structures (see next part. . . )
Multi purpose X band structure 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 …
Special considerations for FEL • Operating structure at relatively low beam energies (PSI injector: 250 Me. V) • 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 Me. V 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 H 75 without damping manifolds!
NLC type H 75 • 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) |E| r z |B| r z
Constant gradient design • 72 cells, active length 750 mm • Relatively open structure: mean aperture 9. 1 mm • Average gradient 40 MV/m (30 Me. V 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 performance: 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
Eigenvalues with ACE 3 P The accelerating mode • 66 cell substructure: • Omit power couplers, matching cells • 500’ 000 elements, 10’ 000 unknowns (3 rd 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 (more to come in an up coming CLIC structures meeting. . )
Mechanical model Each two structures for PSI (Swiss. FEL) and ST (Sincrotone Trieste) with wakefield monitors under fabrication Wakefield monitor details 48 (court. D. Gudkov)
Short test stack done with diffusion bonding Bonding at 1040°C for 90 minutes under H 2 Metallurgical polishing + etching 75 s in Ammonium peroxodisulfate (NH 4)2 S 2 O 8 49
(Court. : Markus AICHLER) Site of Interest 1: Outer side of disc stack • Grains grew down across the joining plane Joining plane 50
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 ACE 3 P: A. Candel, K. Ko, R. Lee, Z. Li
- Slides: 55