superconducting undulator options for x-ray fel applications · locations of corrections determined...
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Superconducting undulator options for x-ray FEL applications
Soren Prestemon
&
Ross Schlueter
3/1/2010 1S. Prestemon FLS-2010
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
3/1/2010 2S. Prestemon FLS-2010
Acknowledgments
Magnetic Systems Group:Ross Schlueter, Steve Marks, Soren Prestemon,
Arnaud Madur, Diego Arbelaez
With much input fromThe Superconducting Magnet Group, Center
for Beam Physics, andThe ALS Accelerator Physics Group
3/1/2010 3S. Prestemon FLS-2010
Basic undulator requirementsfor 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
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Only for planar undulators
Regime of interest
• For any given technology:– At fixed gap, field increases
with period
– Field drops as gap increases
=> Choice of electron energy is closely coupled to undulatortechnology, allowable vacuum aperture, and spectrum needed
Technology-driven
Superconductors of interest
• Application needs:– Hi Jc at low field
– Low magnetization (small filaments)
– Larger temperature margin
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2015
1015
10
20
5103
5
104
105
106
107
temperature(K)
current density(A/cm )2
Nb Sn3
Nb-Ti
magnetic field(T)
critical J-H-Tsurface
Arno Godeke, personal communication
• ~1 micron YBCO layer carries the current • Critical temperature ~100K
– 12mm wide tape carries ~300A at 77K– factor 5-15 higher at 4.5K, depending on applied field
Nb3Sn NbTi
Superconducting materials
Plot from Peter Lee, ASC-NHMFLRegime of interest for SCU’s
Superconducting undulators
• The first undulators proposed were superconducting– 1975, undulator for FEL
experiment at HEPL, Stanford
– 1979, undulator on ACO
– 1979, 3.5T wiggler for VEPP
Rev. Sci. Instr., 1979
Ancient history
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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
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S. Caspi
D. Arbelaez, S. Caspi
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
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Assume Je=1750A/mm2, no Iron
Planar SCU’s
• “Traditional” approach:– Different methods for coil-
to-coil transitions• Can use NbTi or Nb3Sn
– 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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Electron beam
• Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow
July 26, 2006 Soren Prestemon 12
Performance considerationsMotivation 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
• provides temperature margin for operation with uncertain/varying thermal loads
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)
HTS conceptHybridPMEPU
Gap=2, 3mm
• 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, <10mm regime– Peak field on conductor & orientation - <~2.5T
• The HTS short period technology compared to PM and hybrid devices:
– Scaling shows regions of strength of different technologies– Assumed Br=1.35 for PM and hybrid devices– Data shown for HTS assumes J=2x105A/mm2, independent of
field• for B>~1.5T, scaling needs to be modified to include J(B) relation
HTS low CuHTS baseline
Hybrid PM
Pure PM
Helical
HTS: 2-2.2mm gapHelical: 3-3.2mm gap, 2kA/mm2
IVID PM: 2-2.2mm gap
Variable polarization
• Critical for many experiments, particularly in soft X-rays– 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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Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009
Existing PM-EPU vs Conceptual SC-EPU
No iron in SC-EPU-strengths:-Period doubling-No moving parts
Variable polarization capabilities
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Soren Prestemon, LBNL 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.
IB IA
IC ID
Beam
For IA=IB=IC=ID:
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
BA
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Soren Prestemon, LBNL 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 ALS SAC meeting, June 24, 2009
Broad spectral range of SC-EPU
• Separating the coils in the ⟨ (and ) circuit into two groupings allows for period-halving:
(variable linear, no elliptic)
• Going further… separating the coils in the ⟨1 (and ⟨2, 1, 2) circuit into two groupings allows for period doubling:
Full polarization control
Period-halved linear polarization control
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
3/1/2010 19S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009
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.
Elliptically polarizing undulators
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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
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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
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
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Superconducting switches and shim. The current path can be set by combining the switches.
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
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D. Wollman et al., Physical Review Special Topics-AB, 2008
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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July 26, 2006 Soren Prestemon 26
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• 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
Aggressive spacings:
∆w~0.75mm
∆gv~1mm
∆gv
20-60K
∆w
Yoke
Vacuum chamber
4.2-12K
•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, 2003M. Green et al, Adv. in Cryogenic Eng., Vol. 49
Expected for FEL applications
July 26, 2006 Soren Prestemon 27
Beam heating impact on performance: Example of ALS
0 2 4 6 8 10 12 14 160
1
2
3
4
5
Assumes Asc/Atot=0.25, with no Jc margin. Based on existing Nb3Sn material Jc data.
Performance evaluated for 4.2K, 5K, 6K, 7K, 8K
15mm period
20mm period
25mm period
30mm period
Peak
axi
al fi
eld
[T]
Magnetic gap [mm]
∆gv
20-60K
∆w
Yoke
Vacuum chamber
4.2-12K
Intermediate intercept model
Cold bore model
0T(Q) T +aQ≈
02.51static imQ Q Q Qh
= + = +
Ref: Boris Podobedov, Workshop on Superconducting Undulators and Wigglers, ESRF, June, 2003
2 2 / 3 1/ 3( )05 / 3( )im e
lI sQ Zh lb
α ρλ=
• In synchrotron rings, image current heating impacts design• In FEL’s, low duty-factor typically implies low image currents
→ Other heating sources will dominate
Cold, extreme anomalous skin effect regime:ALS: ~ 2 W/mLCLS: ~ 3.e-4 W/m
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 & micro-undulators, 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