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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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