Undulator options for soft x ray free electron lasers
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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

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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



  • 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



  • 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

Importance of undulator technology

  • 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

Overview technologies

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

Overview comparison of technologies

Overview: Comparison of technologies

SRC electromagnet undulator

LCLS PM undulator

Accel SC undulator

Undulator applications storage ring vs fel

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

Overview ongoing developments undulators only misc applications

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...

Novel concepts issues for soft x ray fel s

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

Performance short period concepts

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)

Performance characteristics

PM Hybrid

SCU helical

Vacuum aperture ~4mm

Performance characteristics

Impact on beam energy selection



Impact on beam energy selection

R d results for planar scu s at lbnl mainly lbnl ldrd funds

A. Madur, F. Trillaud: see SRI 2009, Sydney, Australia


Switch 1

Switch 2





Switch 3

Switch 4


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

Pushing performance limits

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

Other superconducting materials

Other superconducting materials

Plot from Peter Lee, ASC-NHMFL

Consider new materials

  • 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

Performance curves calculated

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


Implications for fel design

Implications for FEL design

A. Zholents





  • 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

Aside micro undulator concept

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

Theoretical performance of ybco micro undulator


Theoretical performance of YBCO micro-undulator

  • Assumptions:

    • 25 layers

    • gap=200 μm

    • 10 μm layer separation

    • no Jc(B) dependence included

Challenges of the hts concepts

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

Summary 1

Summary (1)

“Tabletop” FEL’s

tape micro-undulator

HTS tape undulator

SCU: Planar, helical


Cryogenic in-vac. hybrid




Summary ii critical research needs

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

Technology comparison planar devices

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

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