1. Superconducting undulator options for x-ray FEL applications
Soren Prestemon
&
Ross Schlueter
3/1/2010
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
S. Prestemon FLS-2010

3. 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
3/1/2010
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
S. Prestemon FLS-2010

5. Beam energy, spectral range, and undulator performance
Technology-driven
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
3/1/2010
S. Prestemon FLS-2010

6. Superconductors of interest
Arno Godeke,
personal communication
Application needs:
Hi Jc at low field
Low magnetization (small filaments)
Larger temperature margin
Nb3Sn
NbTi
~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
3/1/2010
S. Prestemon FLS-2010

7. Superconducting materials
Regime of interest for SCU’s
Plot from Peter Lee, ASC-NHMFL

8. Superconducting undulators
Ancient history
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
3/1/2010
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
D. Arbelaez, S. Caspi
S. Caspi
3/1/2010
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
Assume Je=1750A/mm2, no Iron
3/1/2010
S. Prestemon FLS-2010

11. Planar SCU’s “Traditional” approach: Can use NbTi or Nb3Sn
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
Electron beam
Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow
3/1/2010
S. Prestemon FLS-2010

12. Performance considerations Motivation for Nb3Sn SCU’s over NbTi
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
July 26, 2006
Soren Prestemon

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
S. Prestemon FLS-2010

14. Performance curves (calculated)
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: 2-2.2mm gap
Helical: 3-3.2mm gap, 2kA/mm2
IVID PM: 2-2.2mm gap
HTS baseline
HTS low Cu
Hybrid PM
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, λ<10mm regime
Peak field on conductor & orientation - <~2.5T
HTS concept
Hybrid PM
EPU
Gap=2, 3mm

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
S. Prestemon FLS-2010

16. Existing PM-EPU vs Conceptual SC-EPU
Variable polarization capabilities
Existing PM-EPU vs Conceptual SC-EPU
No iron in SC-EPU
strengths:
Period doubling
No moving parts
3/1/2010
S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

17. 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 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
3/1/2010
S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

18. 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
S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

19. Broad spectral range of SC-EPU
(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
Separating the coils in the α (and β) circuit into two groupings allows for period-halving:
3/1/2010
S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

20. 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.
3/1/2010
S. Prestemon FLS-2010
Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

21. Integration issues Field correction Wakefields
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
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
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
3/1/2010
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
Superconducting switches and shim. The current path can be set by combining the switches.
3/1/2010
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
D. Wollman et al., Physical Review Special Topics-AB, 2008
3/1/2010
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
S. Prestemon FLS-2010

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
Expected for FEL applications
M. Green, Supercond. Sci. Tech.16, 2003
M. Green et al, Adv. in Cryogenic Eng., Vol. 49
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
Dgv
20-60K Dw
Yoke
Vacuum chamber
4.2-12K
Aggressive spacings:
Dw~0.75mm
Dgv~1mm
July 26, 2006
Soren Prestemon

27. 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
Dgv
20-60K Dw
Yoke
Vacuum chamber
4.2-12K
Intermediate intercept model
Cold bore model
Cold, extreme anomalous skin effect regime:
ALS: ~ 2 W/m
LCLS: ~ 3.e-4 W/m
Ref: Boris Podobedov, Workshop on Superconducting
Undulators and Wigglers, ESRF, June, 2003
July 26, 2006
Soren Prestemon

28. 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

29. 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)]

30. 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