8/30/2010 FLS 2010 1

Advanced insertion devices -practices and concepts

Ross Schlueter & Soren Prestemon

8/30/2010 FLS 2010 2

Outline

• Abridged undulator tour: – the work horses, the up-and-coming, the future

• Basic undulator requirements for FEL’s

• Undulator choices & potential performances

• Issues to resolve– Integration issues

• Spectral scanning rates, field quality correction• Cryogenics

– R&D needs

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Undulators : Today’s statusEstablished technology

•Planar permanent magnet undulators for storage rings are routine•Some types of polarizing undulators (EPUs) are also routine• In-vacuum undulators - standard at many light sources

Presently under development and new(er) (apply to all machines)•New varieties of polarizing undulators•Quasiperiodicity is being included in IDs•Cryogenic permanent magnet undulators•Superconducting undulators/wigglers – planar designs and beyond

Designs specific for FELs & ERLs that wouldn’t work for storage rings•Designs with poles close to beam horizontally: vertically-polarizing planar und, some designs for variable polarization (Delta, Apple-III), superconducting designs (e.g., helical windings on round beam tube)• Smaller/smallest-gap devices - high-Tc tape SCUs•Designs for small emittance: crossed undulators – capable of fast polarization switching

8/30/2010 FLS 2010 4

Established technology–the work horses

• Planar PM or hybrid (PM+iron)– Slowly tunable via variable gap

• EPUs (Apple-II)– Adjustable gap (tuning) and shifting axially

(polarization) >> changing forces– Dynamic multipoles– Slow (~1Hz) polarization switching

• In-vacuum PM hybrids– Tuning via variable gap; beam loss issue?

Planar PM and Hybrid (PM+iron) undulators

ALS Hybrid 1993

KLAUS HALBACH SHOWN IN 1986 WITH KWAN-JE KIM DISCUSSING A MODEL OF AN UNDULATOR HALBACH DESIGNED

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APPLE II device for PETRA III (5m long) –

small gap (11mm), huge forces (73kN),sophisticated mechanical layout and features

J Bahrdt

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In-vacuum undulators,Tanaka, et al. 27th IFELC, Stanford 2005

In-vacuum undulators (IVU’s)

Helical IVU

25m IVU

IVU

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Presently under development andthe new(er) insertion devices

• New varieties of polarizing undulators– fixed gap EPUs, Apple III’s, Delta EPUs

• Quasiperiodic EPUs– no “integer” higher harmonics – slowly adj. gap & shifting, dynamic multipoles

• Cryogenic PM undulators• Superconducting undulators – planar & beyond• crossed undulators – fast polarization switching

8/30/2010 FLS 2010 9

Apple -3, Fixed gap EPU; Quasiperiodic EPU

Many variants of Quasiperiodic devices, including…

• PM quasiperiodic devices at Elettra, Walker, DiViacco, SRN 13 1, 01/2000• Electromagnet quasiperiodic EPU (UE212) at SLS, T Schmidt, EPAC 2002

Hybrid quasiperiodic at Bessy – Bahrdt NIM A 467 P1, J uly 2001,130-133

APPLE-3 APPLE-2 EPU

J Bahrdt, FEL04, p610 For reference

Delta EPU- compact, no gap adjustment - 30% higher field vs. Apple-II- FEL/ERL applications- on-axis vacuum? OK- rad damage? OK, w/ low-T

(1) magnetic arrays, (2) baseplates, (3) rails for longitudinal motion, (4) plates forming rigid frameALEXANDER B. TEMNYKH Phys. Rev. ST Accel. Beams 11, 120702 (2008)

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Cryogenic PM Undulator (CIVID)

• Proposed – Hara, Phys. Rev. ST Accel. Beams 7, 050702 (2004)• R&D - Many groups: Spring-8, ESRF, Brookhaven, others

– Some industrial efforts (e.g. ADC)– Prototypes have been built and tested– Prototypes typ. Do not use highest remanance material

• Motivation:– Increases Remanance by as

much as ~12%– Increase in Coercivity allows

use of higher remanance material

=>Theoretical increase of ~30% - motivates research

T. Tanaka, New Frontiers in ID’s, ELETTRA, Nov. 2006

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CIVID Issues• Key concerns:

– Phase error correction: does room temperature correction apply at cryogenic temperatures?

• Latest data from SPrING8: yes,…– if backing beam deflections are compensated

• Can enhanced coercivity be leveraged? • Cannot bake-out devices; will devices “Cryopump” at 150K?

Can sufficient pumping be provided without baking?

• Note: enhanced coercivity may nevertheless be useful for applications where demagnetization due to thermal / radiation loads is a concern

Superconducting Undulators:Hybrid planar, Helical bifilar, HTS Tape, SC-EPU

Electron beam

S. Caspi

• Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow

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Crossed Undulators

• Orthogonal transverse fields in upstream and downstream sections separated by a phase shifter

• Especially suited to seeded FEL applications• “Orthogonality” also w.r.t. undulator technology used

K-J Kim, NIMA 445, p329

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Basic undulator requirementsfor 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 (esp. for soft X-rays)

– Variable linear and/or elliptic – Rate of change of polarization

• Field correction capability– Compensate steering errors

C t t j t h h k

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Beam energy, spectral range, and undulator performance

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 - coupled to undulator technology, allowable vacuum aperture, and spectrum needed

Technology-driven

8/30/2010 FLS 2010 17

Fig 2, Fig 3, & Fig 4 from Undulator options for soft X-ray free electron lasers, S.O.Prestemon, R.D.Schlueter, FEL09

Figure 3: Performance comparison between the in-vacuum PM-hybrid, superconducting bifilar, and the HTS tape concept. g_vac=2mm. For calculations tape 1” assumes 100m thick tape. ”HTS tape 2” assumes 50m thick tape. In both cases the device operates at 4:2K.

HTS tape excels for g_vac<3mm, per<10mm

Figure 2: Performance comparison between APPLE EPU,Delta-EPU, in-vac PM-hybrid, superconducting bifilar, and SCU-Nb3Sn, for a vacuum aperture of 4mm. Calculations of the bifilar helical SCU data assume an iron-free system with JE = 1500A/mm2, neglect Jc(B) dependence, and are only reasonably valid for B < 2T. PM-based devices assume Br = 1:35T.

Nb3Sn excels for g_vac>3mm, period > 10mm

IMPACT: Undulator capability can drive required machine energy

1 relative vertical position of bands shows sensitivity of req’d E to und. tech. 2 For each technology, width of band shows E sensitivity to vac gap3 slope of a band shows required E sensitivity to tunability capability

Electron energy needed to produce radiation in the range 1nm < λ < λ2 for various undulator technologies. Planar modes assumed. Bands represent vacuum apertures of 4 to 5mm to illustrate the gap-dependence.

From Undulator options for soft X-ray free electron lasers, S.O. Prestemon, R.D.Schlueter, FEL09

1nm < λ < λ2

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Superconducting insertion devices

• Many superconducting wigglers are being installed (Canadian Light Source, Brazilian Light Source,…; ALBA, SRRF planning SC wigglers)

• ANKA has detailed performance data for first NbTi undulator– First spectral data (Rossmanith, ASC 2006)– Thermal load measurements

• EU funded collaboration (ANKA, MAXLAB, ESRF, ELETTRA) (Rossmanith, New Frontiers in ID’s, ELETTRA, Nov. 2006)– Cryogenic systems– Magnet measurements

• ANKA proceeding with procurement of a second superconducting undulator; and Nb3Sn long-term

• LBL: successful tests of several Nb3Sn prototype• APS: several NbTi prototypes; continuing Nb3Sn research

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Superconductors of interest• Application needs:

– Hi Jc at low field– Low magnetization (small filaments)– Larger temperature margin

2015

1015

10

20

5103

5

104

105

106

107

temperature(K)

current density(A/cm )2

Nb Sn3

Nb-Ti

magnetic field(T)

critical J-H-Tsurface

Arno Godeke, personal communication

• ~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

Nb3Sn NbTi

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Performance considerationsMotivation for Nb3Sn SCU’s over NbTi

• Motivation for Nb3Sn– 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

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Superconducting materialsPlot from Peter Lee, ASC-NHMFLRegime of interest for SCU’s

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

S. Caspi

D. Arbelaez, S. Caspi

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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, 1970’s), but not “mature”– effective modulator for FEL

Assume Je=1750A/mm2, no Iron

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Planar SCU’s• “Traditional” approach:

– Different methods for coil-to-coil transitions

• Use NbTi or Nb3Sn– BNb3Sn/BNbTi~1.4

• HTS concept:– “Winding” defined by

lithography– Coated conductors

• YBCO - best candidate• Use at 4.2K

Electron beam

• Current at edges largely cancels layer-to-layer; result is “clean” transverse current flow

8/30/2010 FLS 2010 26

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

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LBL Superconducting undulator prototype• Third LBL prototype – reached

“short sample”– Jeng=1760A/mm2

– 14.5mm period; would yield B~1.6T for a magnetic gap of 6mm

0

100

200

300

400

500

600

700

800

900

0 1 2 3 4 5 6 7 8Bcoil

Ic, I

[A]

Load-lineShort-sample, corrected (1mT/A)Test quench data

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Variable polarization superconducting undulators

• Multiple design concepts have been proposed• Typically do not provide significant field enhancement

over permanent magnet devices• Advantages

– No moving parts – Possibly enhanced spectral control– Possible enhanced spectral range (period doubling/halving)– Possibly mitigated adverse beam dynamics effects

• Disadvantages– Superconductors not conducive to rapid polarization change – Phase-error correction & field measurement need be addressed

0

1

2

3

4

-2

-1

0

-2

-1

0

1

2

-2

-1

0

1

Polarization control: LBNL SC-EPU conceptGenerating variable elliptic polarization

• 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:

,

,C A D B

C A D B

I I I II I I I

α α α α

β β β β

= − = −

= − = −• The α and β fields are 90° phase

shifted, providing full elliptic polarization control via

( , ; ), ( , ; ) :A B A BB I I z B I I zα α α β β β

( ) ( )( ) ( )

( ) ( )( ) ( )

Cos Cos 2SinSin Sin

Cos Cos 2Sin2Sin Sin

x A

y B

x A

y B

B I zB I

B I zB I

α α

α α

β α

β α

ψ ψ πηλψ ψ

ψ ψ π πηλψ ψ

− =

− = −

A

B

C

D

3, ; ,

1

2Note: sin ; typically 2%x y n x yn

anxB aa

α πλ

= <

∑

8/30/2010 FLS 2010 30

A conceptual design for the SC-EPU• Four-quadrant, iron-free design• Performance limited by AC losses (dB/dt-induced heating) of coil• Period halving/doubling requires “switchyard” – superconducting switch to be demonstrated

8/30/2010 FLS 2010 31

1017

2

3

456

1018

2

3

456

1019

2

3

45

Ph/s

/0.1

%bw

/mm

2 /mr2

2 3 4 5 6 7 8 9100

2 3 4 5 6 7 8 91000

2 3 4 5

Photon Energy

λ=28mm horiz. pol. SC-EPU λ=56mm horiz. pol. (28mm period-doubled) SC-EPU λ=28mm circ. pol. SC-EPU λ=56mm (28mm period-doubled) circ. pol. SC-EPU λ=32mm horiz. pol. PM-EPU λ=32mm circ. pol. PM-EPU

Spectral range and Brightness of example SC-EPU λ=28mm device and PM-EPU λ=32mm

Circular

λ linear polarization

2λ linear polarization

2λ

λ

Beam Parameters:I=0.5Aβx/y=13.65 / 2.25mεx/y=6.3 / 0.03nm0.06 disp. in xEnergy spread not included

Limited by aperture

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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

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 w/ undulator testing off-line• Mechanism to direct current using superconducting switches has been tested

– Passive shims (ANKA): closed SC loop enforces 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 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

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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.

A. Madur

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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

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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?

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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

Aggressive spacings:

∆w~0.75mm

∆gv~1mm

∆gv

20-60K

∆w

Yoke

Vacuum chamber

4.2-12K

•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, 2003M. Green et al, Adv. in Cryogenic Eng., Vol. 49

Expected for FEL applications

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Beam heating impact on performance: Example of ALS

0 2 4 6 8 10 12 14 160

1

2

3

4

5

Assumes Asc/Atot=0.25, with no Jc margin. Based on existing Nb3Sn material Jc data.

Performance evaluated for 4.2K, 5K, 6K, 7K, 8K

15mm period

20mm period

25mm period

30mm period

Peak

axi

al fi

eld

[T]

Magnetic gap [mm]

∆gv

20-60K

∆w

Yoke

Vacuum chamber

4.2-12K

Intermediate intercept model

Cold bore model

0T(Q) T +aQ≈

02.51static imQ Q Q Qh

= + = +

Ref: Boris Podobedov, Workshop on Superconducting Undulators and Wigglers, ESRF, June, 2003

2 2 / 3 1/ 3( )05 / 3( )im e

lI sQ Zh lb

α ρλ=

• In synchrotron rings, image current heating impacts design• In FEL’s, low duty-factor typically implies low image currents

→ Other heating sources will dominate

Cold, extreme anomalous skin effect regime:ALS: ~ 2 W/mLCLS: ~ 3.e-4 W/m

8/30/2010 FLS 2010 39

Principal SCU challenges/Readiness

• Principal challenges – Fabrication of various SCU design types– vacuum, wakefields, heating -> acceptable gap?– Shimming/tuning– Cold magnetic measurements

• Readiness– SCU Prototypes: three at LBNL; several at ANL– SCU Concepts: for SC-EPU, stacked HTS undulator

& micro-undulators, Helical SCU’s

8/30/2010 FLS 2010 40

Undulator R&D plan(initial cut- undulator R&D list)

• SCU – NbTi and subsequntly Nb3Sn-based planar and bifilar helical– demonstrate reliable winding, reaction, & potting process

for Nb3Sn– develop trajectory correction method– cold 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)

8/30/2010 FLS 2010 41

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

• FEL/ERL-long-undulator, misc:– (fast) shifters/modulators between FEL sections,– cryo-PM undulator development and prototyping– other concepts?…for undulators?

8/30/2010 FLS 2010 42

Acknowledgments

Magnetic Systems Group:Diego Arbelaez, Arnaud Madur, Steve Marks,

Soren Prestemon, Ross Schlueter

With much input fromThe Superconducting Magnet Group, Center

for Beam Physics, andThe ALS Accelerator Physics Group