1 / 24

Undulator options for soft X-ray free electron lasers

Undulator options for soft X-ray free electron lasers. Soren Prestemon, Ross Schlueter Lawrence Berkeley National Laboratory. Outline . Introduction Undulator technologies Permanent magnet & Superconducting Planar, helical, and variable polarization

conan
Télécharger la présentation

Undulator options for soft X-ray free electron lasers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Undulator options for soft X-ray free electron lasers • Soren Prestemon, Ross Schlueter • Lawrence Berkeley National Laboratory

  2. Outline • Introduction • Undulator technologies • Permanent magnet & Superconducting • Planar, helical, and variable polarization • from “robust”, to “near-term development”, to “futuristic” • Performance comparison • Impact on overall facility design • Technical issues to be addressed

  3. Introduction • Strong interest in a soft X-ray FEL facility • High (~MHz) pulse repetition rate (see F. Sannibale) • CW SCRF linac • Multiple (~10) FEL lines feeding independent experiments “R&D for a Soft X-Ray Free Electron Laser Facility” Joint LBNL / SLAC Whitepaper, June 2009

  4. Cryogenics+linacand Undulator lines are dominant cost drivers Importance of undulator technology • Undulator characteristics and beam energy yield photon wavelength • Coupled problem: • Always want tunability • Sometimes want polarization control • Different FEL lines will focus on different spectral ranges, with different timing, synchronization etc. needs

  5. Overview: Technologies ALS EPU50 (1998) Pure permanent magnet technology, Elliptically polarizing capability ALS U50 (1993) Hybrid permanent magnet technology ALS SCU (20??) Nb3Sn superconducting undulator Spring8 IVUN (2000) Small gap In-vacuum device

  6. Overview: Comparison of technologies SRC electromagnet undulator LCLS PM undulator Accel SC undulator

  7. Undulator applications: storage ring vs. FEL • Storage ring: • gap>5mm (sometimes smaller...) • depends on β-function • introduce superimposed matching fodo lattice - very narrow gap? • Requires “large-midplane” • large horizontal dynamic aperture - effectively eliminates bifilar-helical devices • “Transparency” issues: • should be steering and displacement-free • must avoid coupling: skew terms an issue • dynamic multipoles must be compensated • FEL’s: • gap>4mm ->2mm? • limited by wakefields, vacuum, fabrication? • More restrictive trajectory requirements • fewer “knobs” to compensate for tuning/polarization-induced trajectory variations • Multipole fields - needs analysis

  8. Overview: Ongoing developments(undulators only; misc. applications) Schlueter, in “Synch. Rad. Sources”, Ed. H. Winick, 1994 • Diverse spectral properties • quasi-periodicity, variable polarization: EM, PM; SC • In-vacuum permanent magnets • allows reduced gap => higher field • cryogenic PM => leverage increase in Br, enhanced coercivity • Superconducting • short-period helical, for ILC (~12-14mm period) • Daresbury: NbTi • APS: NbTi, considering Nb3Sn • short-period planar (typically ~15mm period) • LBNL: Nb3Sn • APS: NbTi, Nb3Sn • BNL: APS NbTi • ANKA: • NbTi; first device installed in ring, worked with Accel • Now investigating Nb3Sn with CERN • others...

  9. Focus on superconducting devices - Push for highest performance - Leverage material developments Novel concepts: Issues for soft X-ray FEL’s • Approaches: • Focus on “solid-state” technologies • Leverage fabrication techniques: • minimize manual involvement • design for best tolerances • Build-in correction capability • Investigate in-situ tuning • Issues to be addressed: • Performance: • higher field and/or shorter periods • enhanced spectral range • variable polarization • Ease / reliability of manufacturing • quality control: tolerances, reproducibility • measurements, trajectory correction • Integration of electron-bunch controls • focussing magnets, collimation, diagnostics • Minimize overall system cost

  10. D. Arbelaez, S. Caspi S. Caspi, PAC 1995 Performance: short-period concepts • 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 • currently being investigated for positron source production • NbTi (Europe), Nb3Sn (Argonne)

  11. PM Hybrid SCU helical Vacuum aperture ~4mm Performance characteristics

  12. λ1=1nm gap=4-5mm Impact on beam energy selection

  13. A. Madur, F. Trillaud: see SRI 2009, Sydney, Australia B Switch 1 Switch 2 I A C Iload Switch 3 Switch 4 D R&D results for planar SCU’s at LBNL (mainly LBNL LDRD funds) Supported by APS WFO contract • Demonstrated performance • supports predictions of B(gap,period) • Demonstrated shim field • sufficient amplitude based in anticipated tolerances • Demonstrated “switch network” • reliably switch currents +, -, or off for each shim • allows fast commissioning of SCU • relevant for period-doubling schemes

  14. Question: can a new approach simultaneously address: - performance issues - tolerance issues - cost issues Pushing performance limits • Technologies missing to reach ~5-8mm regime (or shorter!) • gap/period dictates performance • wire-based technologies reach limit due to: • finite wire size - winding becomes impractical • insulation does not scale • effective current density decreases • Need a new approach: • leverage microfabrication techniques • leverage strengths of new materials

  15. Other superconducting materials Plot from Peter Lee, ASC-NHMFL

  16. Consider stacked YBCO tapes • use micro-machining or lithography to produce current path • highly reproducible periodicity • low labor cost • stacked tapes operate in series • joints at ends • 20-30 tapes sufficient • large tolerance to stack errors • Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow Consider new materials • Material (YBCO) is in the form of a “tape” • ~1 micron YBCO layer carries the current • does not scale: benefit drops with increased thickness • Critical temperature ~100K • 12mm wide tape carries ~300A at 77K • factor 5-10 higher at 4.5K, depending on applied field

  17. 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 • 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, l<10mm regime • Peak field on conductor & orientation - <~2.5T HTS: 2-2.2mm gap Helical: 3-3.2mm gap, 2kA/mm2 IVID PM: 2-2.2mm gap HTS low Cu HTS baseline Hybrid PM Pure PM Helical

  18. Implications for FEL design A. Zholents K=1.4 K=1.2 K=1.0 K=0.8 • High fields at small periods allows access to 1nm with ~1.3GeV beam • Multiple routes for technology enhancements: • reduce tape thickness - work with vendor • increase YBCO layer thickness - collaborate with LANL / vendor

  19. Aside: Micro-undulator concept • Push stacked-tape concept to a new level: • Concept: • Layered YBCO using micro-fabrication • Layers separated by ~5-10 microns • Series connection (i.e. joints) integrated in process • Eliminate almost all manual assembly • Period ~0.5-2mm • Gap ~200 microns Trial mask/deposition underway, in collaboration with LANL groupMPA-STC

  20. A/mm2 Theoretical performance of YBCO micro-undulator • Assumptions: • 25 layers • gap=200 μm • 10 μm layer separation • no Jc(B) dependence included

  21. Challenges of the HTS concepts • Shielding device from synchrotron radiation • Image current impact due to narrow gap • Beam dynamics / image current interaction with superconductor • Field errors are key challenge for all technologies: • Pros for HTS concept: • Possibly reduced longitudinal “assembly” tolerances • Laser/lithography cuts define periodicity • Layer-to-layer longitudinal placement not critical • No difficulty with longer lengths • Basic assembly very simple • Error calculations analytic • Cons for HTS concept: • Small gap requirement calls for integrated vac. Chamber • Vertical placement of layers must be consistent • Magnetization effects may cause unwanted field kicks • homogeneity of YBCO layer dictates reproducibility of current path • needs to be measured, controlled

  22. Summary (1) “Tabletop” FEL’s tape micro-undulator HTS tape undulator SCU: Planar, helical ??? Cryogenic in-vac. hybrid Hybrid SC-EPU PM EPU

  23. Lei Zang, Cockcroft Institute presentation Advanced undulators could address two dominant cost drivers of an FEL: the linac and the undulators Summary II: Critical research needs • SCU - planar and bifilar helical: • demonstrate reliable winding, reaction, and 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 (e.g. ANKA passive shim) • 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

  24. Field strength of planar devices: Nb3Sn~40% higher than NbTi NbTi~best theoretical CIVID (assume same vacuum aperture) Technology comparison: Planar devices Calculations for 15mm-period devices

More Related