Superconducting undulator options for x-ray FEL applications. Soren Prestemon & Ross Schlueter

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

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

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

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

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

6 Superconductors of interest Application needs: Hi J c at low field Arno Godeke, personal communication critical J-H-T surface current density (A/cm 2 ) Nb3Sn Nb-Ti Low magnetization (small filaments) Larger temperature margin temperature (K) magnetic field (T) Nb 3 Sn 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

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

8 Ancient history 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., /1/2010 S. Prestemon FLS

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

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 J e =1750A/mm 2, no Iron 3/1/2010 S. Prestemon FLS

11 Planar SCU s Traditional approach: Different methods for coilto-coil transitions Can use NbTi or Nb 3 Sn B Nb3Sn /B NbTi ~1.4 Electron beam HTS concept: Winding defined by lithography Use coated conductors YBCO is best candidate Use at 4.2K Current at edges largely cancels layer-to-layer; result is clean transverse current flow 3/1/2010 S. Prestemon FLS

12 Performance considerations Motivation for Nb 3 Sn SCU s over NbTi Motivation for Nb 3 Sn Low stored energy in magnetic system break free from J cu protection limitation Take advantage of high Jc, low Cu fraction in Nb 3 Sn High T c (~18K) of Nb 3 Sn provides temperature margin for operation with uncertain/varying thermal loads July 26, 2006 Soren Prestemon 12

13 Performance: Traditional Planar SCU s Nb 3 Sn 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

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=2x10 5 A/mm 2, 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/mm 2 IVID PM: 2-2.2mm gap Helical HTS low Cu HTS baseline Hybrid PM Pure PM 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 Gap=2, 3mm HTS concept Hybrid PM EPU

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

16 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 Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

17 Variable polarization Consider a 4-quadrant array of such coil-series. If I C =-I A, Coils A and C generate additive fields. Set I C =-I A, I D =-I B ; Independent control of I A and I B provides full linear polarization control. I B I C I A Beam I D For I A =I B =I C =I D : B A Independent control of I A and I B provides variable linear polarization control - If I A =I B, vertical field, horizontal polarization - If I A =-I B, horizontal field, vertical polarization 3/1/2010 S. Prestemon FLS 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 /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 3/1/2010 S. Prestemon FLS Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

19 Broad spectral range of SC-EPU Separating the coils in the (and ) circuit into two groupings allows for period-halving: Full polarization control Period-halved linear polarization control (variable linear, no elliptic) Going further separating the coils in the 1 (and 2, 1, 2) 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 3/1/2010 S. Prestemon FLS Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

20 Elliptically polarizing undulators Nb 3 Sn superconductor, 24% superconductor in coil-pack cross-section, 90% of J c, vacuum gap=5 mm (magnetic gap=7.3 mm for PM-EPU, 6.6 mm for SC-EPU), B r =1.35 T for PM material; block height and width fixed. 3/1/2010 S. Prestemon FLS Soren Prestemon, LBNL ALS SAC meeting, June 24, 2009

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 S. Prestemon FLS

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

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

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, /1/2010 S. Prestemon FLS

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

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 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 w K Vacuum chamber Aggressive spacings: 20-60K g v w~0.75mm g v ~1mm July 26, 2006 Soren Prestemon 26

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 Yoke Q im I 2 l = α s Z 2/3 ( ρλ ) 1/3 e 5/3 0 h ( lb) Cold, extreme anomalous skin effect regime: ALS: ~ 2 W/m LCLS: ~ 3.e-4 W/m w K Vacuum chamber 20-60K g v Ref: Boris Podobedov, Workshop on Superconducting Undulators and Wigglers, ESRF, June, Assumes A sc /A tot =0.25, with no Jc margin. Based on existing Nb 3 Sn material Jc data. Cold bore model Intermediate intercept model Peak axial field [T] Performance evaluated for 4.2K, 5K, 6K, 7K, 8K 30mm period 25mm period 20mm period T(Q) T +aq Q= Qstatic + Qim = + Q0 h mm period Magnetic gap [mm] July 26, 2006 Soren Prestemon 27

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 & microundulators, 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 Nb 3 Sn 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