superconducting undulator options for x-ray fel applications
DESCRIPTION
Superconducting undulator options for x-ray FEL applications. Soren Prestemon & Ross Schlueter. Outline . Basic undulator requirements for FEL’s Superconducting undulators : Superconductor: options and selection criteria Families by polarization Circular Planar - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/1.jpg)
S. Prestemon FLS-2010 1
Superconducting undulator options for x-ray FEL applications
Soren Prestemon &
Ross Schlueter
3/1/2010
![Page 2: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/2.jpg)
S. Prestemon FLS-2010 2
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
![Page 3: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/3.jpg)
S. Prestemon FLS-2010 3
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
![Page 4: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/4.jpg)
S. Prestemon FLS-2010 4
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
3/1/2010
![Page 5: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/5.jpg)
S. Prestemon FLS-2010 5
Beam energy, spectral range, and undulator performance
3/1/2010
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 undulator technology, allowable vacuum aperture, and spectrum needed
Technology-driven
![Page 6: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/6.jpg)
S. Prestemon FLS-2010 6
Superconductors of interest
• Application needs:– Hi Jc at low field– Low magnetization (small filaments)– Larger temperature margin
3/1/2010
2015
1015
10
20
510 3
5
10 4
10 5
10 6
10 7
tem perature(K )
current density(A /cm )2
N b Sn3
N b-Ti
m agnetic fie ld(T)
critical J-H -Tsu rface
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
![Page 7: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/7.jpg)
Superconducting materials
Plot from Peter Lee, ASC-NHMFLRegime of interest for SCU’s
![Page 8: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/8.jpg)
S. Prestemon FLS-2010 8
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
3/1/2010
![Page 9: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/9.jpg)
S. Prestemon FLS-2010 9
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
3/1/2010
S. Caspi
D. Arbelaez, S. Caspi
![Page 10: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/10.jpg)
S. Prestemon FLS-2010 10
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
3/1/2010
Assume Je=1750A/mm2, no Iron
![Page 11: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/11.jpg)
S. Prestemon FLS-2010 11
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
3/1/2010
Electron beam
• Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow
![Page 12: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/12.jpg)
Soren Prestemon 12July 26, 2006
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
![Page 13: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/13.jpg)
S. Prestemon FLS-2010 13
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
3/1/2010
![Page 14: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/14.jpg)
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, λ<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
![Page 15: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/15.jpg)
S. Prestemon FLS-2010 15
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
3/1/2010
![Page 16: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/16.jpg)
S. Prestemon FLS-2010 16Soren 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
3/1/2010
![Page 17: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/17.jpg)
S. Prestemon FLS-2010 17Soren 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:
ψ
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
3/1/2010
![Page 18: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/18.jpg)
S. Prestemon FLS-2010 18Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009
Superconducting EPU• Add a second 4-quadrant array of such coil-series,
offset in z by λ/4 (coil series α and β)• With the following constraints the eight currents are
reduced to four independent degrees of freedom:
• The α and β fields are 90° phase shifted, providing full elliptic polarization control via C
D
3/1/2010
![Page 19: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/19.jpg)
S. Prestemon FLS-2010 19Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009
Broad spectral range of SC-EPU
• Separating the coils in the α (and β) circuit into two groupings allows for period-halving:
(variable linear, no elliptic)
• Going further… separating the coils in the α1 (and α2, β1, β2) 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
![Page 20: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/20.jpg)
S. Prestemon FLS-2010 20Soren 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
3/1/2010
![Page 21: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/21.jpg)
S. Prestemon FLS-2010 21
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
3/1/2010
![Page 22: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/22.jpg)
S. Prestemon FLS-2010 22
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
3/1/2010
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
![Page 23: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/23.jpg)
S. Prestemon FLS-2010 23
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
3/1/2010
Superconducting switches and shim. The current path can be set by combining the switches.
![Page 24: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/24.jpg)
S. Prestemon FLS-2010 24
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
3/1/2010
D. Wollman et al., Physical Review Special Topics-AB, 2008
![Page 25: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/25.jpg)
S. Prestemon FLS-2010 25
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?
3/1/2010
![Page 26: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/26.jpg)
Soren Prestemon 26July 26, 2006
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:
Dw~0.75mm
Dgv~1mm
Dgv
20-60K
Dw
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
![Page 27: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/27.jpg)
Soren Prestemon 27July 26, 2006
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 Nb
3Sn material Jc data.
Performance evaluated for 4.2K, 5K, 6K, 7K, 8K
15mm period
20mm period
25mm period
30mm period
Pea
k ax
ial f
ield
[T]
Magnetic gap [mm]
Dgv
20-60K
Dw
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 λ
• 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
![Page 28: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/28.jpg)
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
![Page 29: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/29.jpg)
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)]
![Page 30: Superconducting undulator options for x-ray FEL applications](https://reader034.vdocument.in/reader034/viewer/2022051317/568164a6550346895dd69857/html5/thumbnails/30.jpg)
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