Short Period Undulator Workshop
Review and Personal Perspective
Joseph Bisognano
University of Wisconsin-Madison
Synchrotron Radiation Center
FLS2012
March 6 2012
1
Joe Bisognano, Jonathan Wurtele,co-chairs
Ross Schleuter and Sami Tantawi, undulator technology convenors
Sasha Zholents and Steve Benson, beam dynamics and FEL physics convenors
Bob Beyer, Mike Green, Vladimir Litvinenko, Jamie Rosenzweig co-conspirators
http://cbp.lbl.gov/spu/talks/
2
Outline
 Charge to the Workshop
 Summary of working groups as given at workshop
 Undulator technology
 Beam and FEL physics
 Personal perspective
 We never got around to writing an executive summary
3
4
5
The Context
 The reduction of the undulator period and/or harmonic generation in
free electron lasers can have a significant impact on facility costs
and/or photon energy reach
 Mission
 To explore a variety of technology approaches to reduce undulator
period (< 1 cm our arbitrary reference)
 Magnetostatic devices (conventional, cryo, SC)
 RF devices (room temperature and SRF)
 “Conventional” lasers
 In parallel and then in concert, explore limitations imposed by FEL
and beam physics, engineering constraints, and radiation safety
 Ultimate goals
 Developed holistic schemes and performance measures and tradeoffs for a variety of scenarios; estimate cost savings
 Outline necessary R&D
6
Example:
Cost Breakdown of a Soft X-ray FEL
 Conventional wisdom: ~ 2.5 GeV with few cm period
undulators with cost a good fraction of a billion dollars
or more
 Cost Breakdown
 Linac : 20-25% (less w/ pulsed RT rather than CW SRF)
 Injector, R&D, etc.: 5-10%
 Photon Generation: 20 % (fifty/fifty undulator and
beamline; clearly depends on number of beamlines, say
six)
 Maybe scalable stuff: civil and contingency: 50%
 So, linac energy could drive ~25-50 % of cost
7
Some Fundamental Relationships
u

2
2n
K  Bmax  u ; K  1
l
e normalized £ g
4p
B6 D =
Ne
e nxe nye z
=
gap scaling with u
3D  Ming Xie 3D gain
Ne
e nxe nys zs g


 3 D
PFEL » r3D PBeam
8
Goals
 Developed holistic schemes and performance
measures for a variety of scenarios based on
these technologies that can reduce costs or
improve wavelength reach
 Identify physics and technology constraints
 Estimate cost savings; e.g., is it worth the
trouble
 Outline necessary R&D to assess the benefits
and constraints of short period undulator
FELs and, if attractive, to develop these
technologies
9
Short Period Undulator Workshop
Summary - Working Group #1
Undulator Technologies
21-23 June 2011
Diego Arbelaez, Johannes Bahrdt, Joe Bisognano, Bob Byer, Marco Calvi, Jim
Clark, Jean Delayen, Valery Dolgashev, Rick Donahue, Mike Green, Michael
Hagelstein, Yury Ivanyushenkov, Yong Jiang, Jin-Young Jung, Jim Lawler, Arnaud
Madur, Steve Marks, Jeff Neilson, Finn O’Shea, Soren Prestemon, Don Prosnitz,
Jaime Rosenwieg, Ross Schlueter (co-chair), Sam Tantawi (co-chair), Zach Wolf
10
Laser Driven Devices



Laser Driven Dielectric Accelerators, Robert Byer
Laser Scattering: A path to cost containment? Or better performance/cost? J.E. Lawler
discussion: Laser driver technology and issues - Bob Byer, Jim Lawler, Sami Tantawi
 Bottom lines for Laser Driven Devices:
 Lasers have interesting field strength and energy density, but
clearly we do not yet have the right combination or tech. for a
soft X-ray device at modest electron energy
 Nonetheless, long term, there is great potential for optical
undulators incorporating some combination of Ti:Sapphire and
Photonic Bandgap technology
11
Laser Driven Devices, cont.
Prospects for Optical Undulators in Soft X-Ray FELs:
Two key technologies are mature and have potential Ti:Sapphire lasers provide extraordinary energy density and field
strength in optical pulses with flexible duration, tunability, and
even controlled chirps.
Photonic Bandgap Structures provide substantial control over
dispersion and losses as well as some ability to control field
direction and phase.
A version of the dielectric-based a microstructure (e.g. Plettner &
Byer, Phys. Rev. Special Topics - Accelerators and Beams 11,
030704 (2008)) may provide a short period undulator for a soft XRay FEL.
13
14
RF Undulators



RF Wigglers – Sami Tantawi
RF Undulators at Ka band – Jiang Yong
discussion: RF wigglers/tech; extendable to CW? – Sami Tantawi, Jean Delayen, Yong Jiang
 Bottom lines for RF Undulators:
 offer potential of both kHz polarization switching & complete
polarization control, including 100% circular polarization at very
reasonable (~5mm) gaps
 Potential for going to much smaller undulator periods (1mm? –
superconducting technology dependent)
 Still unknown – beam dynamics effects such as that of
longitudunal RF fields (possibly mitigated with added focusing?)
 Manufacturing technology for superconducting RF undulators
still needs to be developed
 RF undulators perhaps on a faster track
15
RF Undulators, cont.

proposed offerings:
 cost – typical cost per meter of cryomodule structures
 (but no high cost RF source needed here)
 parameters – 35 GHz, ~1m sections (or longer, if cryomodule so allows),
~5mm gap, 5 mm period (or less if superconductor materials so allow, e.g.
residual resistance or cooper pair breaking in Nb occurs at ~90GHz?)
 performance – K ~ 1 reasonable
 advantages –
 kHz switching with 100% circular or any other desired polarization
 easy phase shifting between sections – fraction of a degree is reasonable
 disadvantages –
 need cryomodules- which are expensive
 risks – requires 10^-4 tolerances; transition sections are difficult
 Next steps/challenges - don’t yet have decent manufacturing technology for
these superconducting RF undulators – this has yet to be addressed
 ETA - full scale SRF prototype in several years; RF earlier……
16
HE1n Modes Scaling Laws
(Tantawi)
For an undulator made of copper at room temperature :

2
Optimal Radius :
u 
Minimum Power :
Stored Energy :
a (m)  0.23 2/3 3 Lx 2/3
P ( MW ) 
0.28K 2 L2/3 x 4/3 J12 ( x)
U ( Jouls) 
u7/6
P~ 1/ λ, so for smaller
wavelength capability one
pays for power (RF) or else
for refrigeration (SRF)
9.22 K 2 L5/3 x 4/3 J1 ( x) 2
u2/3
30867 L
Quality Factor :
Q
Filling Time :
t f (  s )  32.8L u
u
1.02 KxJ1 ( x)
Peak Suface E Field : Es ( MV / m) 
a
3.4 KxJ1. ( x)
Peak Surface B Field : Bs (mT ) 
a
x  {2.40483,5.52008,8.65373,11.7915} for HE11 , HE12 , HE13 , HE14  modes
18
19
PM and Cryo-PM devices




Compact undulators forFELs - Jim Clarke
Short period PrFeB undulators, Johannes Bahrdt
Cryogenic undulators, Finn O’Shea
discussion: PM & Cryo-PM Undulators - Finn O’Shea, Johannes Bahrdt, Jim Clarke
 Bottom lines:
 A variety of PM devices enable variable polarization and K~1
capability down to below 10mm periods
 Cryo-PM devices push performance of PM counterparts by ~40%
(via both enhanced Br, Hc and ability to use new materials)
20
CPMU-9 at UCLA (O’Shea)
 Started as a project to get
~1 keV photons from a
~2 GeV plasma source
electron beam
 Push strong magnets and
short period to get high
performance
 K = 1.7, period = 9 mm,
gap ~1.5 mm and have
low charge help with
wakes
Variable Polarization Undulators
(J.Clarke)
PM vs. SCU (J.Clarke)
 Soft x-ray FEL users want variable polarisation
 Advanced APPLE undulators (APPLE3 or 4) have
significant advantages, but need development
(~15% lower beam energy for NLS)
 In NLS FEL case study over 0.1 to 0.4nm:
 SCU needs 17.5% lower E than PPM
 SCU has 30% shorter saturation length than PPM
 But, 20% lower saturation power than PPM
SCUs




Compact undulators forFELs - Jim Clarke
Short period SCUs at ANKA - Michael Hagelstein
Short period undulator R&D at LBNL – Soren Prestemon
SCUs, a practical approach – Yury Ivanyushenkov
Bottom lines:
 Various SCU technologies have the capability for K~1 at periods
<10mm to produce down to e.g. 1nm or 0.1nm radiation at modest
(~ 2-4 GeV) electron energy
 On-going R&D
 in both linearly polarized and variable polarized devices
 Using NbTi, Nb3Sn, or HTS
24
SCUs: helical, planar (Hagelstein, Ivanyushenkov, Clarke)
 At RAL: A 4-m helical undulator has been built by RAL team
for the ILC positron source project, 11.5mm period, 0.86T
 At APS: designing and building the first superconducting
planar undulator –SCU0 (18mm period, 9.5mm gap)
 At Karlsruhe-Mainz: an SCU w/ 3.8 mm period, 2mm gap,
855MeV, 100 periods, 100 uA cw (1988)
 At ANKA: SCU14, 14mm period, 100 periods, cryogen-free,
SCU14, (2006)
25
Superconducting Undulators:
Hybrid planar, Helical bifilar, HTS Tape, SC-EPU
Electron beam
S. Caspi
LBNL
•
Current at edges largely cancels
layer-to-layer; result is “clean”
transverse current flow
Key SCU points
 NbTi Prototypes – full scale and subscale, demonstrate performance meets specs
 Phase errors quite low -Key is excellent quality control during fab.
 No real implementation of shimming ; except LBNL proof of principle
 Some variation from device to device; source not evident
 Implementation of Nb3Sn at LBNL; only tentatively investigated elsewhere
 Would benefit from guidance on material and fabrication issues
 Working with conductor vendors for optimal conductors
 Cryogenics:
 Generally using cryocoolers, either with recondensers or via conduction
 General uncertainty on heat load; evidence of unknown source; multiple
calorimeters being designed and fabricated
 Next steps
 First truly successful operation in a storage ring (Anka / APS / other?) needed
 R&D areas
 Need to develop fully functioning measurement system
 Need fully developed shimming approach
 Need to develop sub-10mm period devices
Situating the SCU technology



Performance dominates in the >10mm period range
 Next closest competitor is cryogenic in-vacuum
 Expect SCU’s to be cheaper (ultimately)
 No moving parts, material cheaper; cryogenics more expensive, but
probably not significant if part of large SRF-linac facility
Performance appears strong in the <10mm period regime
 Outperforms hybrid PM devices
 Need to:
 Demonstrate performance
 Understand and control tolerances
Maturity compared to other technologies
 PM>hybrid>in-vac.hybrid>CIVID>NbTi>Nb3Sn>HTS>SRF>plasma
 But… measured SCU(NbTi) devices as APS and ANKA suggest SCU(NbTi)
is in a close race with CIVID (excellent phase errors with SCU’s)
FEL, Beam dynamics, & User needs
 FEL lines can be clearly delineated by…
 Low K (~<1) regime where push is to yield…
 Specific photon energy with lowest beam energy, or…
 Highest photon energy with a given beam energy
 Modes K regime (~1.5-2) where push is to yield…
 Adequate tuning range with minimum beam energy
 Collaboration between undulator, beam dynamics, FEL groups
 tradeoffs between K(gap,period) and
 impact of gap on beam and facility complexity
 Polarization needs:
 Critical for soft X-rays
 Usually of interest along with tuning => goes hand-in-hand with modestK regime above
 Possibly less importance at short wavelengths
S. Prestemon
Slide 30
Magnetic Measurements


Magnetic measurements at SLAC - Zach Wolf
Pulsed/Cold magnetic measurements –Diego Arbelaez
 Bottom lines:
 Pursuing two measurement technologies
 Hall probes in bores as small as 2-3mm diameter and
 Pulsed wires in bores as small as 1 mm diameter
 Key issue: ~25 micron vertical probe positioning in e.g. 10mm
period undulator for δB/B ~ 10-4; By ~ cosh(2y/λu)
 Need Probe movement in straight line to < 25 microns
 Undulator alignment capability to ~ 20 microns
 Quadrupole alignment capability to ~ 10 microns
31
Magnetic Measurements, cont.
 Requirements:
 Space limitations: wire in ~1mm tube; Hall probe in 2-3mm tube
 Trajectories – required straight to within ~ 2 microns
 Field integrals: both Hall probe and Pulsed wire are capable
 Fiducialization <10 microns: Achievable with hall probe
 K – required uniform to 10-4, Achievable with hall probe
 Phase – if required < 10 degrees: Achievable with hall probe
 Cryogenics - cryogenic capability for some undulator technologies
 ETA: cold measurement system capability in ~few years
32
Radiation damage


Radiation damage in FELs – Rick Donahue
discussion: Rick Donahue, Jim Clarke, Finn O’Shea, …
 PM Radiation Damage
 FLASH experiment suggested that only 104Gy gave 0.5% loss in
B field of PM undulator
 SCU Radiation Damage
 is in epoxy used for potting coils, but much less susceptable than
PMs
 Should still be protected with collimation scheme
 Commonly accepted dose limit for epoxies is 107 Gy - used in
ITER, Fusion Technology Institute, Wisconsin
33
Radiation damage can also be an issue for SCUs
(refers to SR induced damage rather than neutrons):
Beam dynamics and FEL physics subgroup
Conveners - Steve Benson and Sasha Zholents
Attendees:
Brian Austin
Siva Darbha
Daniele Filippetto
Punit Ghandi
Ryan Lindberg
Atosa Meseck
Phillippe Piot
Ji Qiang
Matthias Reinsch
Robert Ryne
Jonathan Wurtele
Mini-Workshop on Short Period Undulators for FELs, Berkeley,
June 21-23, 2011
How short is it reasonable to go in period?
•
If one restricts the design of the FEL so that the emittance limit is met
[often missed a little, jjb] we find that the minimum energy is given by
g min =
4pe N
l
• One can substitute this into the resonance equation to get a minimum
wiggler wavelength:
32p 2e N2
lmin =
2
l (1+ K rms
)
• Thus the minimum wavelength is critically dependent on the
emittance.
• A simple way to reduce emittance is to reduce charge. The energy is
also low. How do we make up low bunch energy? For the soft X-ray
range, going to high repetition rate looks like a good way.
• The short wavelength undulators may be limited to small charge and
therefore short bunches with relatively large bandwidth.
• Going to shorter period almost always results in lower K and smaller
gap. Both of these may be more of a limitation than the emittance.
How Small Can We Make
the Undulator gap?
• Wake fields trajectory and chamber position tolerances
Lsat
X
Wz  rms
2
me c / e
• The value of X should be less than unity for an FEL.
The value of the wake potential has to be calculated for
a variety of pulse lengths, shapes and chamber sizes.
• Can we shape the bunch to minimize?
• Radiation damage. How can we collimate?
• Can define a wiggler acceptance as acc = g g 2 (8Lsat )
• Ion trapping for high duty cycle
Transverse and longitudinal wakefield
 LCLS ultra-short beam and double-horn start to
sample the short-range resistive-wall wake
Horns : 10 fs  3 microns;
Overall: 100 fs  30 microns
Example:
transverse
3 kA
2 kA
1.5 kA
500 A
Aug. 06, 2010
LBL Compact XFEL
Juhao Wu
jhwu@SLAC.Stanford.ED
For 250 pC charge
38/20
Longitudinal with FEL: example
(cont'd)
FEL at 1.5 Å, electron energy 13.64 GeV, 3 kA
current
– Similarly, by comparing the energy loss scan
without kicking the electron bunch to that when
kicking the electron bunch, the additional FEL
induced energy loss is found
– Measurement: the wakefield loss is about 40 MeV
(0.3 %), the spontaneous radiation is about 16 MeV
(0.1 %) for 25 undulator sections, and the FEL is
about ~ 10 MeV (0.07 %) [about 2.5 mJ FEL for this
250 pC case].
Aug. 06, 2010
LBL Compact XFEL
Juhao Wu
jhwu@SLAC.Stanford.ED
39/20
How small is too small for K?
• Calculated parameters for 1 nm operation with 20 pC
of charge with 0.1 mm-mrad emittance and 5 keV-psec
longitudinal emittance. Longitudinal match optimized
for each point.
• If Krms<0.5 there is essentially no tunability.
• The gain is decreasing so rapidly for If Krms<0.5 that
the wiggler length actually increases for decreasing
wavelength.
• The acceptance of the wiggler is getting close to 100
times the emittance which means the collimator may
cause large wakes.
• Mattias has system for looking at cost savings to
compare 5 and 10 mm wavelength systems.
How Small can we make K (cont.)?
Polarization
• Polarization control is extremely important in the soft Xray range. Not as important in the hard X-ray range.
• Three options - Crossed undulators, Delta, RF
undulator
– Ding and Huang indicate moderate polarization in
simulations with crossed polarizers. Variations from
pulse to pulse are unacceptable so this may not
work. This also argues against use in SASE system.
– kHz switching speed is highly desired. Possible with
crossed undulators and RF undulator.
– All three have fairly fine control of the ellipticity.
– Only the Delta and RF undulator have good
polarization purity (but only at the fundamental)
RF Undulator
• Advantages
– Fast polarization switching and wavelength tuning.
– Less affected by radiation.
• Disadvantages
– Can’t get as high a field
– Relatively immature technology
•
•
•
•
•
How much does the input and output taper hurt you?
How uniform does the field have to be?
How is the beam focused? (external focusing?)
Can segments be driven in parallel?
Can you still fast switch an SRF cavity?
Laser Undulators
• Very intriguing technology
• Requires extremely small emittance (1
nm)
• Possible approach to gamma ray laser
Personal Perspective
 Undulator technology developments point to 1.5 cm or less as a
reasonable period, with “defensible” gap, for next generation
proposals
 Superconducting undulators now fully competitive or better
 K much less than unity not good gambit
 For CW and/or shorter periods with small gaps, halo is great
unknown
 “Conventional” lasers
 Power requirements still too high for even modest rep rate
unless something novel is done
 Dielectric interesting, but will require much R&D
 RF structures help gap limitations and give rapid tunablility
 Tests of pulsed copper may be near at hand
 SRF versions would be wonderful; SRF wall losses vs
frequency may be problem to go sub centimeter
45