Superconducting undulator options
for x-ray FEL applications
Soren Prestemon
&
Ross Schlueter
3/1/2010
S. Prestemon
FLS-2010
1
Outline
• Basic undulator requirements for FEL’s
• Superconducting undulators:
– Superconductor: options and selection criteria
– Families by polarization
• Circular
• Planar
• Variable polarization
– Performance comparison/characteristics
• Integration issues
– Spectral scanning rates, field quality correction
– Cryogenics
• R&D needs
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Acknowledgments
Magnetic Systems Group:
Ross Schlueter, Steve Marks, Soren Prestemon,
Arnaud Madur, Diego Arbelaez
With much input from
The Superconducting Magnet Group, Center
for Beam Physics, and
The ALS Accelerator Physics Group
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Basic undulator requirements
for X-ray FELS
• Variable field strength for photon energy tuning
– Beam energy and undulator technology must be matched
to provide spectra needed by users
– Sweep rate, field stability and reproducibility
• Variable polarization (particularly for soft X-rays)
– Variable linear and/or elliptic
– Rate of change of polarization
• Field correction capability
– Compensate steering errors
– Compensate phase-shake
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Beam energy, spectral range,
and undulator performance
Technology-driven
Only for planar undulators
• For any given technology:
– At fixed gap, field increases
with period
– Field drops as gap increases
Regime of interest
=> Choice of electron energy is closely coupled to undulator
technology, allowable vacuum aperture, and spectrum needed
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Superconductors of interest
• Application needs:
•
Critical temperature ~100K
–
12mm wide tape carries ~300A at 77K
–
factor 5-15 higher at 4.5K, depending on applied field
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106
Nb3Sn
5
10
Nb-Ti
104
5
10
15
103
5
10
15
20
20
temperature
(K)
magnetic field
(T)
NbTi
Nb3Sn
~1 micron YBCO layer carries the current
7
10
critical J-H-T
surface
– Hi Jc at low field
– Low magnetization (small filaments)
– Larger temperature margin
•
current2 density
(A/cm )
Arno Godeke,
personal communication
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Superconducting materials
Regime of interest for SCU’s
Plot from Peter Lee, ASC-NHMFL
Superconducting undulators
Ancient history
• The first undulators proposed
were superconducting
Rev. Sci. Instr., 1979
– 1975, undulator for FEL
experiment at HEPL, Stanford
– 1979, undulator on ACO
– 1979, 3.5T wiggler for VEPP
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Bifilar helical
• Provides left or right circular polarized light
• Continuous (i.e. maximum) transverse acceleration of
electrons
• Fabrication
– With or without iron
– Coil placement typically dictated by machined path
D. Arbelaez, S. Caspi
S. Caspi
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S. Prestemon
FLS-2010
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Performance
•
Bifilar helical approaches yield excellent performance:
– applicable for “short” periods, λ>~10 (7?) mm, gap>~3-5mm
• wire dimensions, bend radii, and insulation issues
– well-known technology (e.g. Stanford FEL Group, 1970’s), but not “mature”
– most effective modulator for FEL
• need to consider seed-laser polarization
Assume Je=1750A/mm2, no Iron
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Planar SCU’s
• “Traditional” approach:
– Different methods for coilto-coil transitions
• Can use NbTi or Nb3Sn
Electron beam
– BNb3Sn/BNbTi~1.4
• HTS concept:
– “Winding” defined by
lithography
– Use coated conductors
• YBCO is best candidate
• Use at 4.2K
•
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Current at edges largely cancels
layer-to-layer; result is “clean”
transverse current flow
11
Performance considerations
Motivation for Nb3Sn SCU’s over NbTi
•
Motivation for Nb3Sn
– Low stored energy in magnetic system
• “break free” from Jcu protection limitation
– Take advantage of high Jc, low Cu fraction in Nb3Sn
– “High” Tc (~18K) of Nb3Sn
•
July 26, 2006
provides temperature margin for operation with uncertain/varying thermal loads
Soren Prestemon
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Performance: “Traditional” Planar SCU’s
• Nb3Sn yields 35-40% higher field than NbTi (at 4.2K)
– “Raw” performance has been demonstrated at LBNL, with
a 14.5mm period prototype
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Performance curves (calculated)
•
The HTS short period technology compared to PM and
hybrid devices:
–
Scaling shows regions of strength of different technologies
–
HTS: 2-2.2mm gap
Helical: 3-3.2mm gap, 2kA/mm2
IVID PM: 2-2.2mm gap
HTS low Cu
Assumed Br=1.35 for PM and hybrid devices
–
Data shown for HTS assumes
field
•
for B>~1.5T, scaling needs to be modified to include J(B) relation
J=2x105A/mm2,
HTS baseline
Hybrid PM
independent of
Pure PM
Helical
•
Issues considered:
– Width of current path - assumed ~1mm laser cuts separating “turns”
– Finite-length of straight sections – 83% retained for g=2mm, 12mm wide tape
– Gap-period region of strength – most promising in g<3mm,  mm regime
– Peak field on conductor & orientation - <~2.5T
Gap=2, 3mm
HTS concept
Hybrid
PM
EPU
Variable polarization
• Critical for many experiments, particularly in soft Xrays
– Photoemission, magnetism (e.g dichroism)
• Variety of parameters define polarization capability
– Type and range of polarization control (variable linear,
variable elliptic; spectrum range vs polarization)
– Speed at which polarization can be varied
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Variable polarization capabilities
Existing PM-EPU vs Conceptual SC-EPU
No iron in SC-EPU
-strengths:
-Period doubling
-No moving parts
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Soren Prestemon, LBNL
16
ALS SAC meeting, June 24, 2009
Variable polarization
•
Consider a 4-quadrant array of such
coil-series.
–
–
If IC=-IA, Coils A and C generate
additive –fields.
Set IC=-IA, ID=-IB; Independent control
of IA and IB provides full linear
polarization control.
For IA=IB=IC=ID:
IB
IA

Beam
IC
ID
BA
Independent control of IA and IB provides variable linear
polarization control
- If IA=IB, vertical field, horizontal polarization
- If IA=-IB, horizontal field, vertical polarization
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Soren Prestemon, LBNL
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ALS SAC meeting, June 24, 2009
Superconducting EPU
•
•
Add a second 4-quadrant array of such coil-series,
offset in z by  /4 (coil series  and )
•
With the following constraints the eight currents are
reduced to four independent degrees of freedom:
The  and fields are 90° phase shifted,
providing full elliptic polarization control via
C
D
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Soren Prestemon, LBNL
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ALS SAC meeting, June 24, 2009
Broad spectral range of SC-EPU
Full polarization control
•
Separating the coils in the  (and )
circuit into two groupings allows for
period-halving:
Period-halved
linear polarization control
(variable linear, no elliptic)
•
Going further… separating the coils
in the   (and   ) circuit into
two groupings allows for period
doubling:
Period-doubled
full polarization control
(Full polarization control)
NOTE: Two power supplies (A, B) needed for linear polarization control; four needed for full
(linear+elliptic) polarization control; switching network could provide access to the above regimes
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Soren Prestemon, LBNL
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ALS SAC meeting, June 24, 2009
Elliptically polarizing undulators
Nb3Sn superconductor,
24% superconductor in coil-pack cross-section, 90% of Jc, vacuum gap=5 mm
(magnetic gap=7.3 mm for PM-EPU, 6.6 mm for SC-EPU),
Br=1.35 T for PM material; block height and width fixed.
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Soren Prestemon, LBNL
20
ALS SAC meeting, June 24, 2009
Integration issues
• Field correction
– Want no beam steering, no beam displacement
– Must minimize phase-shake
• Wakefields
– What are limitations in terms of bunch stability?
– Image current heating: impact on SCU’s
• Modular undulator sections
– Allows focusing elements between sections
– Requires phase shifters
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Field correction
• PM systems use “virtual” or magnetic shims
• SCU correction methods (proposed):
– Trim “coils”: located on each/any poles
• Amplitude of correction (~1%) has been demonstrated at LBNL
• Individual control is possible, but becomes complex
• Experience with PM devices suggests few “coils” can provide requisite correction =>
locations of corrections determined during undulator testing off-line
• Mechanism to direct current using superconducting switches has been tested
– Passive “shims” (ANKA): use closed SC loop to enforce half-period field integral
• Should significantly reduce RMS of errors
• Some residuals will still exist due to fabrication issues
• Possibility of hysteretic behavior from pinned flux – needs to be measured under
various field cycling conditions
Detailed tolerance analysis is needed to determine amount/type of correction that
may be required. Preliminary data (e.g. APS measurements) suggest fabrication
errors are smaller than typically observed on PM devices
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Superconducting switches
• Allow active control of current (+/-/0) to each shim coil from one common power
supply
– Switch produces negligible heat at 4.K while controlling high currents
– Can be used to control period-doubling in SC-EPU concept
Superconducting switches and shim. The current
path can be set by combining the switches.
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Passive shimming
• Passive scheme – does not have/need external control
– Will compensate errors independent of error source
– Assumes “perfect conductor” model for superconductor
• Pinned (i.e. trapped) flux may yield some hysteresis – needs
measurements
D. Wollman et al., Physical Review Special Topics-AB, 2008
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Measurements
• Any field correction depends on ability to measure fields with
sufficient accuracy
– “traditional” Hall probe schemes not applicable
– Need system compatible with cryogenic temperatures:
• System must work with integrated vacuum chamber
• Hall probe “on a stick” or “pull”:
–
–
–
–
most common and basic approach;
suffers from uncertainty in knowledge of Hall probe location
Could use interferometry to determine location
Could use Hall probe array to provide redundancy to compensate spatial uncertainty
• Pulsed wire:
– need to demonstrate sufficient accuracy
– benefits from vacuum for reduced signal noise
• In-situ:
– Use electron beam=>photon spectrum as field-quality diagnostic
– Fourier-transform – loss of spatial information – recoverable?
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Cryogenic design options
•
Can use liquid cryogens or cryocoolers
–
–
Liquid cryogen approach requires liquifier + distribution system or user refills
Cryocoolers require low heat load and (traditionally) incur temperature gradients through conduction
path and impose vibrations from GM cryocooler
•
•
•
Expected for FEL applications
Limits operating current due to current-lead heat load (despite HTS leads; typical limit is <1kA)
Solution: heat pipe approach (C. Taylor; M. Green)
Need to know the heat loads under all operating regimes
•Vacuum chamber and magnet can be
thermally linked; magnet and
chamber operate at 4.2-8K
•Vacuum chamber and magnet can be
thermally isolated; chamber operates at
intermediate temperature (30-60K);
magnet is held at 4.2K
M. Green, Supercond. Sci. Tech.16, 2003
M. Green et al, Adv. in Cryogenic Eng., Vol. 49
Yoke
Dw
4.2-12K
Vacuum
chamber
Dgv
20-60K
Aggressive spacings:
Dw~0.75mm
Dgv~1mm
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Soren Prestemon
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Beam heating impact on performance: Example of ALS
• In synchrotron rings, image current heating impacts design
• In FEL’s, low duty-factor typically implies low image currents
→ Other heating sources will dominate
Qim  
I 2l s
Z 2 / 3 ( e )1/ 3
0
5
/
3
h (l b)
Cold, extreme anomalous skin
effect regime:
ALS: ~ 2 W/m
LCLS: ~ 3.e-4 W/m
Yoke
Dgv
Dw
Vacuum
chamber
4.2-12K
20-60K
Ref: Boris Podobedov, Workshop on Superconducting
Undulators and Wigglers, ESRF, June, 2003
Cold bore model
Intermediate intercept
model
5
Assumes Asc/Atot=0.25, with no Jc margin.
Based on existing Nb3Sn material Jc data.
Peak axial field [T]
4
Performance evaluated for
4.2K, 5K, 6K, 7K, 8K
3
T(Q)  T0 +aQ
2
30mm period
Q  Qstatic  Qim  1  Q0
25mm period
1
20mm period
15mm period
0
0
2
4
6
8
10
12
14
16
Magnetic gap [mm]
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27
2.5
h
Principal SCU challenges/Readiness
• Principal challenges
– Fabrication of various SCU design types
– vacuum, wakefields, heating -> acceptable gap?
– Shimming/tuning
– Cold magnetic measurements
• Readiness
– Prototypes: three SCU LBNL prototypes; ANL prototypes
– Concepts: for SC-EPU, stacked HTS undulator & microundulators, Helical SCU’s
Undulator R&D plan
• SCU – NbTi and subsequently Nb3Sn-based planar and bifilar helical
– demonstrate reliable winding, reaction, & potting process for
Nb3Sn
– develop trajectory correction method
– magnetic measurements
• Stacked HTS undulator :
– demonstrate effective J (i.e. B)
– evaluate image-current issues
– determine field quality / trajectory drivers
– current path accuracy, J(x,y) distribution
– accuracy of stacking
– develop field correction methods [consider outer layer
devoted to field correction (ANKA passive shim)]
Undulator R&D plan, cont.
(initial cut- undulator R&D list)
• Stacked HTS Micro-undulator
– demonstrate ability to fabricate layers
– demonstrate effective J (i.e. B)
– evaluate image-current issues
• SC-EPU
– develop integrated switch network
– Demonstrate performance
• All SCU concepts:
– Detailed tolerance analysis
– Need reliable measurements