1Question to be addressed (Tuesday)
CHESS & LEPP

(1) Project overviews(a) JLAB(b) Cornell(c) Daresbury(d) KEK / JAEA
(2) Are there optimal schemes to minimize bunch length and energy spread?
(3) What is the optimal injector-to-linac merger design for ERLs?
(4) What should start to end simulations include?
(5) What are beam abort strategies and beam loss tolerances?
(6) What are diagnostic needs specific to x-ray ERLs?

Georg.Hoffstaettter@Cornell.edu

Preview of WG2 on ERLs of FLS06

15 May 2006

10 kW IR and 1 kW UV• 100 W broadband 2 to 50 cm-1
• 10 kW average power, 1–14 microns• 3 kW average power, .3 to 1 micron
Thomas Jefferson National•Accelerator
Facilityfemtosecond FW @ 75 MHz
all 300
FEL
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

Light Source LandscapeApprox. limit-line for
1 nC, 0.1ps

LCLS XFELJLabIR/UV FEL

JLab
3rd. Gen. Undulators

BNL 4GLS
DipoleAPS, ESRF

DipoleAll data is approximate for illustrative purposes

From Wendy Flavell, Manchester U. UK

Wendy Flavell Manchester University UK

Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

FELHarmonic Radiation while lasing
Tuning movie
Wigglergap

HighReflector

HoleOutcoupler

Beam incontrolroom

Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

FEL

55GeV ERLERL
UpgradeLayoutfor CESRCHESS & LEPP

• Injector prototype under construction•Georg.Hoffstaettter@Cornell.edu

plans for WG2 on ERLs of FLS06

16 May 2006

May 2006

Susan L Smith

Conceptual Layout of 4GLSMay 2006

Susan L Smith

17MeV ERL at JAEA
17MeV ERL-FEL
17MeV SCA FEL

gun

injector

undulator
main acc.
20mwith ERL

beam loading

RF power
without
ERL

R. Hajima et al., Nucl. Instrum. Meth. (2003)

Design, Construction and Operation of the ERL (1999-)

KEK PF Review Committee Meeting 2006

17MeV ERL at JAEA
17MeV ERL-FEL
17MeV SCA FEL

gun

injector

undulator
main acc.
20mwith ERL

beam loading

RF power
without
ERL

R. Hajima et al., Nucl. Instrum. Meth. (2003)

Design, Construction and Operation of the ERL (1999-)

KEK PF Review Committee Meeting 2006Target specifications of ERL

Preliminary design

Beam energy

5 GeV

Average current

100 mA

Normalized emittance

0.1 – 1 mm⋅mrad

Average brilliance (@ 0.1 nm) from ID’s

1021 – 1023
ph/s/0.1%/mm2/mrad2

Average flux

> 1016 ph/s/0.1%

Spectral range

30 eV – 30 keV

Minimum bunch length< 100 fs

Number of ID’s

20 - 30

KEK PF Review Committee Meeting 2006

Questions to Keep In Mind
Method of Compressionz Single Stage (JLab, ERLP)z Stepped/2-Stage (FLASH, XFEL, LCLS etc.)z Progressive/Modular (4GLS)z Large Dispersion/Split Linac Approach (Cornell)
z Velocity Bunching (JAERI)z Sextupoles or 3rd-Harmonic for Linearisation?
z Higher-Order Terms
z Problems in Particular Machines
zCombined Beams (4GLS)z Topology (Cornell)z

12Split Linac Bunch Flattening
CHESS & LEPP

1

1

0.998

0.998

1.9972

0.996

0.996

1.99715

0.994

0.994

0.992

0.992

1.99725

1.9971
1.99705

-10-5

0

510

1.997
-10-505-2

10-1

0

12

1.99725

Nonlinear time of flight

2nd order time of flight

1.9972
1.99715
1.997110.99925

1.99705
0.999

0.998
1.997

0.99875
0.996

0.99850.99825-2-1

012-2-10

12

0.994

0.998
1.99725

0.992

0.99775-2-10

12-10-50510

1.9972
1.99715

4th order time of flight

1.9971
1.99705
1.997-3

Georg.Hoffstaettter@Cornell.edu

plans for WG2 on ERLs of FLS06

16 May 2006

care of emittance dilution in an ERL injector and merger
gun

short-term goals

buncherQ-mag.
solenoid

injection SCA

high-flux = 77pC, 1mm-mradhigh-coh =7.7pC, 0.1mm-mrad
ultrafast = 1nC, 5-10mm-mrad

emittance growth source

locationtransverse space charge

whole the pathemittance compensation by solenoidand Q magnets with avoiding“over bunching”

longitudinal space charge

inside the merger

envelope matching to bunch slice
displacement induced by LSCF

time-dependent RFbuncher, 1st SCA

controlling σr, σz by solenoid, buncher

how to deal withWe cannot eliminate all the emittance dilution simultaneously.
However, numerical simulations give a reasonable design for the emittance requirement.
helpful design tips so far we have:+ changing position and strength of focusing magnets = minimum “εTSCF + εRF”,+ a weak focusing merger shows better emittance compensation.
2step staircase (JAERI type) is not suitable for small emittance due to strong focusingWhat should Start to End Simulations for Light Source ERLs be sure toinclude?

• energy spread and emittance are very crucial for FEL operation and have to becarefully observed therefore• most relevant (single bunch) effects with the potential to deteriorate bunch
quality: space charge and CSR fields, cavity wakes, resistive wall and geometricwakes• simulation codes for each of these effects are available but no code that
includes all• for Start-to-End simulations output-input conversion has to be done: best andmost easily the 6D macro particle phase space is used•with parametrizations: correlations between phase space dimensions may not
be neglectedWhat are realistic beam abort strategies and beam loss tolerances
for light source ERLs?
Standard beam trip detection and abort seems to be acceptable for ERLs. Theenergy in the beam appears to be not significantly different from 3rdgeneration light sources. Just don’t put too much energy in sensitive
points.
Beam loss tolerances and the causes of halo are less well characterized andvarious labs are presently taking different approaches. Work on
minimizing sources of such halo and developing methodologies for dealing
with it is desirable.
Halo sources. Beam induced (not usually significant). Field emission from gun and srf cavities. Scattered light in photoinjector (both temporal and transverse halo)
Loss impacts:. Wiggler life
. Vacuum and vacuum failures. Background radiation. Radiation on X-ray beamline. ….
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

FELInjector Diagnostics < 10 MeV
.
.
.
.
.

.

Initial measurement of gun performance should be done in test stand where additional
diagnostics can be used
Shielded Beam Viewers. <10MeV these should be phosphor coated. YAG or ceramic OK but these bloom more than the thin coat of phosphor
Multiple BPMs, Shorted striplines for larger signal than button and low impedance
Multislit or pepper pot for H/V emittanceBeam Current Monitor (BCM). Knowing charge is crucial to setup. Transformer or cavity, advantage with cavity is one can use log-amp on output and thismakes tuning the drive laser E/O cells easy, and it is only ~$5k
Streak camera for longitudinal measurements

Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

FEL

Linac Diagnostics
.
.
.
.
.
.
.

All devices should be shielded to minimize beamline shunt impedance. Including bellows and ion pump drops
BPMs, both single pass and multi-pass
OTR beam viewers. In linac where there are two beams they should have 5mm holes to pass accelerating
beam
Interferometer for bunch length. Source can be from THz and/or OTR
Transverse kicker cavity for longitudinal measurements. Needed where bunches are beyond streak camera resolution
Synchrotron light ports at every bend
Halo monitors. These can be phosphor coated beam viewers with or without holes or forks

Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of EnergyFEL

Energy Recovery Linacs (Thursday)

Susan L Smith
ASTeC, Daresbury Laboratory

Vacuum and Aperture Needs for ERL-LS
conservative design (10-10 loss / turn) = similar to storage rings••

install distributed pumps in bending (and Q) chambers
keep full-aperture > 5mm and vacuum ~10-7 Pa everywhere

progressive design (10-7 loss / turn at controlled locations)•••put scrapers at appropriate locations
allow beam loss of a few watt at eachscraper Æ 10-7 loss / turnvacuum level of 10-4 Pa is enough for
4mm fulll-aperture
keep good vacuum at undulators, ~10-7 Pa,
in any casepay much attention to ion effects

1e-4

beam loss / turn••

1e-5

vacuum (Pa)

1e-610-3

as bendcollision
with gasΔΨ=π/2undulator

as < auβsβu

au

10-4

1e-810-5

1e-9

10-6

1e-10
1e-11

scraper

progressive design

1e-7

conservative design

10-7
0 2 4 6 8 10 12 14 16 18 20
half aperture (mm)beam loss for 5GeV-ERL

Ryoichi Hajima

What are the Advantages and Limits ofMulti Turn ERLs

• Recirculation makes life harder, safety margins smaller,
but may not be limiting• A “value engineering” decision?
• Compression and other niceties should be avoided inlower energy recirculation– Allow emittance cancellations between superperiods– Keeps energy spread reasonable– In general, optimize lower energy arc optics for recirculation,
not bunch manipulations• My favorite issues– Microwave-like CSR instabilities– Cost model

Joe Bisignano

Limits on the Bandwidth forERLs
Resumé& Open Questions
Δf = around 3.6 Hz, QL = mid 108 (Dream?)
Aspects that must be investigated whether high QL operation is possible:● How far can one reduce the microphonic detuning? For around QL = 108-109
require peak detuning between 1 and 10 Hz.
Æ Investigate low-microphonic modules
Æ Given measured RMS microphonics, what peak levels must we designfor?

3 Hz peak microphonics?
Æ QL < 2E8

Æ Stabilize the helium system down to the 0.01 mbar levels!
Æ Use microphonic compensation. This is in its infancy, but promising.
● What spread in microphonics should one expect● Must allow for a safety factor when dimensioning RF system● At present, factors of 3 in microphonics are not unreasonable● Installed RF power must be greater by factor of 3 + coupler adjustability● Can the RF field be stabilized down to the 0.01-0.02 deg and 10-4 level?
Æ Requires a high-gain system (500)Æ Cornell/JLAB measurements demonstrated this can be done at
QL = 1.2 x 108 with a “quiet” moduleÆ Coupling optimized for δfpk = 6 σmicJens KnoblochÆ Even if QL = 1E8,
some cavities will need torun at QL = 3E7. RF
system must be designedfor thisÆ QL = 1.2E8 demonstratedEven higher values may bepossible provided low
microphonics and low
pickup noiseLimits on the Bandwidthfor ERLs
Resume& Open Questions ctd

Aspects that must be investigated whether high QL operation is possible:● How much uncompensated beam current can one expect in 100 mA ERLs?

Æ QL < 1E8?

Æ changes in beam loading may prove to limit the QL. Even QL = 108 may be tough, but moremeasurements are needed● How important is it to RF process the cavities?
● For light sources this may be a big reliability issueÆ QL < 1E8● For RF processing require QL values around 106● Coupling ranges of x 100 will be tough to achieve Æ Maximum QL would be 108
Æ Given present status, QL values much above 108 do not appear feasible (??). Possibly one will
have to stay below this.

Jens Knobloch

What are the Optimal Parameters forSuperconducting Cavities of an ERL Light Source?

• L-Band technology sufficient for proposed sources• Accelerator gradient < 20MV/m – determined by CW
technology. Cost optimisation with both capital and runningcosts of equipment pushes gradient down
• Qo should be as high as possible > 1010
• Operation at 1.8K – provided not limited by RBCS
• Trip levels should be no worse than storage ring source• HOMs damped to better than Q =104, power into either
beampipe absorbers of wavequide loads at 80K• Active control of microphonics

Mike Dykes

What are good beam stabilization
strategies and their limits for ERLs?
• Most users will come from storage ring based 3rd generation
light sources, which are extremely stable (submicron position,permille beamsize)– Combination of passive source suppression and– Active feed-forward, feedback• ERLs with their smaller beamsizes will be more challenging(need fixed beamsize fraction for stability). They need toincorporate stability in design from beginning.
– Ground plate/site, temperature, cooling water, girders, magnet
mounting, …
– Orbit feedback for ‘slow’ (100 Hz) effects seems well in reach withtechnology extrapolation (except in linac where two beams arepresent and out of phase)– Fast feedback more similar to multibunch feedbacks in rings, easierin return line configs than straight FELs, but resolution needsimprovement– Beamsize stability might involve gun feedback– Overall there are additional challenges for ERLs!

Christoph SteierUndulator Issues for ERLs
• Round beam – round vacuum chamber so alternative undulator
geometries possible• Electron losses – need to be ~ 1 in 1010 to be equivalent to 3GLS
– Losses on undulator can be controlled e.g. FLASH• Short bunches and narrow vessel gaps are bad combination– e.g. 4GLS 50fs bunch; resistive wall electron loses ~80keV in 10m longcopper vessel with 7mm diameter• Vessel roughness demanding (0.1 to 1 μm) in all ERLs at narrowgaps (to avoid energy spread growth)– These levels are available from industry• Multi-user facility will have undulator cross talk due to energy loss ofelectron vs undulator gap– Feedforward schemes envisaged– Variability of dipole CSR emission needs investigation• Energy spread and emittance growth effects small• Electron timing affected by undulator gap change– Important for low energy ERLs (long wavelengths)

Jim Clarke

Critical Issues Common To All ERLs•

Source issues– Cathode heating– Cathode lifetime– Laser power on cathode– Halo production•Injector issues–•High power couplers

Linac issues––––––––Multi turn ERLs
BBU and very large current
Beam stabilization (!!!)
Electron background calculations
Accumulation
Halo propagation
CW high voltage cavity operation with low trip rate
Accelerator protection issues

Georg Hoffstaetter

Thanks

Participants in the ERL working group
AndSession Experts
Ryoichi Hajima
Joe BisignanoJens Knobloch
Christoph Steier
Jim Clarke
Georg Hoffstaetter

The end