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 Proton Accelerator Swiss Light Source SLS Spallation Neutron Source SINQ

SwissFEL – the next big Facility at PSI Slides courtesy H. Braun

FEL principle Electrons interact with periodic magnetic field of undulator magnet to build up an extremely 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 pC Repetition rate 100 Hz SwissFEL, the next large facility at PSI

SwissFEL wavelength range SwissFEL pulse- 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 Lowest beam energy technically possible Small period undulators with low K values Low q B 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-bandC-band SwissFEL baseline

Parameters for lasing at 1Å Operation Mode Long PulsesShort Pulses Charge per Bunch (pC)20010 Beam energy for 1 Å (GeV)5.8 Core Slice Emittance (mm.mrad) Peak Current at Undulator (kA) Repetition Rate (Hz)100 Undulator Period (mm)15 Effective Saturation Power (GW)20.6 Photon Pulse Length at 1 Å (fs, rms)132.1 SwissFEL key parameters

The Operation Modes Standard operation 200 pC Maximum FEL pulse energy Longest FEL pulse length Bolko Beutner - FLAC 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 Charge Wakefield Limited Diagnostic Limit Special Cases

Project Start of operation Beam energy min GeVÅ LCLS, USA April ! SCSS, Japan European X –FEL, Hamburg SwissFEL 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

Inauguration SwissFEL first stage,

Exp3 Exp2 Exp1 Exp2 Exp1 Exp3 Exp2 Laser pump THz pump Seed laser Gun laser ARAMIS FEL 1-7 Å ATHOS FEL 7-70 Å 2.1 GeV 3.4 GeV5.8 GeV Exp1 Laser pump Gun laser ARAMIS FEL 1-7 Å 2.1 GeV 3.4 GeV5.8 GeV 2018 SwissFEL Phase II Soft X-ray FEL 2016SwissFEL Phase I Accelerator and hard X ray FEL Exp3 2014Building completed Gun laser 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 Injektor MHz MHz Main C-band LINAC 5712 MHz

Active length S-band acceleration 24 m Active length C-band acceleration 208 m ARAMIS string of undulators 60 m Other beam line elements273 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 Linac 1 BC 2 Linac 2+3 Collimator compressionwakes remove chirp double dogleg (slight decompression) over-compressionwakes add to chirp double dogleg (slight compression) compressionwakes partially remove chirp chicane (compression)

C-band LINAC Module Main LINAC # LINAC modules26 Modulator26 Klystron26 Pulse compressor26 Accelerating structures104 Waveguide splitter78 Waveguide loads104 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 LLRF Courtesy Hansruedi Fitze 2m

C-band development 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 Toshiba E37202E37210 Peak Power50 MW RF Pulse Width3 us Repetition Rate60 Hz100 Hz Avg. RF Power7.7 kW15 kW Collector Power35 kW78 kW Delivery DateMay 2011Feb 2012 Courtesy Jürgen Alex

Longitudinal phase space manipulations SwissFEL Injektor MHz 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

S-bandX-band BC Chicane First compression stage of SwissFEL head tail Court.: B. Beutner

The Compression Scheme Normal: Large Bandwidth: Attosecond: Bolko Beutner - FLAC Linac 1 BC 2 Linac 2+3 Collimator compressionwakes remove chirp double dogleg (slight decompression) over-compressionwakes add to chirp double dogleg (slight compression) compressionwakes partially remove chirp chicane (compression)

200pC Mode Bolko Beutner - FLAC Booster 2: -17 deg 16 MV/m X-Band: deg MV/m Linac 1: deg 26.5 MV/m 4.2 deg 2.15 deg Linac 2/3: 0 deg 26.5 MV/m 355MeV 150A2.04GeV 3.2kA 3.2kA head tail head tail head tail 200 pC std mode

FEL 200 pC Bolko Beutner - FLAC pC Saturation32 m E sat 0.11 mJ pp 20 fs 2.1GW BW0.065 %

200pC Tolerances Bolko Beutner - FLAC arrival timepeak currentenergy goals:20 fs 5 % 0.05 % S-Band Phase [deg] S-Band Voltage [rel] X-Band Phase [deg] X-Band Voltage [rel] Linac 1 Phase [deg] Linac 1 Voltage [rel] Linac 2 Phase [deg] 5.2e e e+003 Linac 2 Voltage [rel] Linac 3 Phase [deg] 4.6e e e+003 Linac 3 Voltage [rel] Charge [pC] initial arrival time [fs] 6.2e e+003 Initial Energy [rel] BC1 angle [rel] BC2 angle [rel]

200pC Performance Bolko Beutner - FLAC 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 Charge1% 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

10pC – Attosecond Pulse Modification of 10pC mode: Fully upright compression BC1 bending angle: 3.82 deg  4.2 deg Linac 1 Phase: deg  deg Reconfiguration of bunch collimator for additional compression Bolko Beutner - FLAC head tail head tail

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

10pC Tolerances Bolko Beutner - FLAC arrival timepeak currentenergy goals:5 fs 15 % 0.05 % S-Band Phase [deg] S-Band Voltage [rel] X-Band Phase [deg] X-Band Voltage [rel] Linac 1 Phase [deg] Linac 1 Voltage [rel] Linac 2 Phase [deg] 2.8e e+002 Linac 2 Voltage [rel] Linac 3 Phase [deg] 1.5e e+002 Linac 3 Voltage [rel] Charge [pC] initial arrival time [fs] e+002 Initial Energy [rel] BC1 angle [rel] BC2 angle [rel]

10pC Performance Bolko Beutner - FLAC 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 Charge1% 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

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 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: % 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 (3 rd order approach required) Computed resonance frequency: F = GHz (w/o losses) ~ F= GHz (including losses) Design: F= 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 model Each 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 H 2 Metallurgical polishing + etching 75 s in Ammonium peroxodisulfate (NH 4 ) 2 S 2 O 8

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