FEL X band issues M. Dehler, BE/RF & PSI SwissFEL project at PSI FEL specific RF issues The...

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


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:


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


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

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tail

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200 pC std mode

FEL Performance @ 200 pC


200pC

Saturation 32 m

Esat 0.11 mJ

sp 20 fs

<Psat> 2.1GW

BW 0.065 %

200pC 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 0.0041 0.0046

Linac 2 Phase [deg] 5.2e+003 1.6e+002 2.2e+003


Linac 3 Phase [deg] 4.6e+003 1.8e+002 2.9e+003


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


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





Charge 1%

initial arrival time [fs] 30

Initial Energy [rel] 1e-004

BC1 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


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tail

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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!


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 2 Phase [deg] 2.8e+002 35 5.3e+002


Linac 3 Phase [deg] 1.5e+002 36 4.3e+002


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







Charge 1%

initial arrival time [fs] 30

Initial Energy [rel] 1e-004



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.


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


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.


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 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

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

FEL X band issues M. Dehler, BE/RF & PSI SwissFEL project at PSI FEL specific RF issues The...

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