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Short Period Undulator WorkshopReview and Personal Perspective

Joseph BisognanoUniversity of Wisconsin-Madison

Synchrotron Radiation Center

FLS2012March 6 2012

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Joe Bisognano, Jonathan Wurtele,co-chairsRoss Schleuter and Sami Tantawi, undulator technology convenorsSasha Zholents and Steve Benson, beam dynamics and FEL physics convenorsBob Beyer, Mike Green, Vladimir Litvinenko, Jamie Rosenzweig co-conspirators

http://cbp.lbl.gov/spu/talks/

Outline

Charge to the Workshop Summary of working groups as given at workshop

Undulator technology Beam and FEL physics

Personal perspective We never got around to writing an executive summary

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The Context The reduction of the undulator period and/or harmonic generation in free electron

lasers can have a significant impact on facility costs and/or photon energy reach Mission

To explore a variety of technology approaches to reduce undulator period (< 1 cm our arbitrary reference) Magnetostatic devices (conventional, cryo, SC) RF devices (room temperature and SRF) “Conventional” lasers

In parallel and then in concert, explore limitations imposed by FEL and beam physics, engineering constraints, and radiation safety

Ultimate goals Developed holistic schemes and performance measures and trade-offs for a

variety of scenarios; estimate cost savings Outline necessary R&D

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Example:Cost Breakdown of a Soft X-ray FEL

Conventional wisdom: ~ 2.5 GeV with few cm period undulators with cost a good fraction of a billion dollars or more

Cost Breakdown Linac : 20-25% (less w/ pulsed RT rather than CW SRF) Injector, R&D, etc.: 5-10% Photon Generation: 20 % (fifty/fifty undulator and beamline;

clearly depends on number of beamlines, say six) Maybe scalable stuff: civil and contingency: 50%

So, linac energy could drive ~25-50 % of cost

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Some Fundamental Relationships

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22 n

u 1;max KBK u

uwithscalinggap

D3

gain 3D Xie Ming 3 D

Goals Developed holistic schemes and performance

measures for a variety of scenarios based on these technologies that can reduce costs or improve wavelength reach

Identify physics and technology constraints Estimate cost savings; e.g., is it worth the trouble Outline necessary R&D to assess the benefits

and constraints of  short period undulator FELs and, if attractive, to develop these technologies

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Short Period Undulator WorkshopSummary - Working Group #1

Undulator Technologies21-23 June 2011

Diego Arbelaez, Johannes Bahrdt, Joe Bisognano, Bob Byer, Marco Calvi, Jim Clark, Jean Delayen, Valery Dolgashev, Rick Donahue, Mike Green, Michael

Hagelstein, Yury Ivanyushenkov, Yong Jiang, Jin-Young Jung, Jim Lawler, Arnaud Madur, Steve Marks, Jeff Neilson, Finn O’Shea, Soren Prestemon, Don Prosnitz, Jaime Rosenwieg, Ross Schlueter (co-chair), Sam Tantawi (co-chair), Zach Wolf

                                

                                                                

Laser Driven Devices

Laser Driven Dielectric Accelerators, Robert Byer Laser Scattering: A path to cost containment? Or better performance/cost? J.E. Lawler discussion: Laser driver technology and issues - Bob Byer, Jim Lawler, Sami Tantawi

Bottom lines for Laser Driven Devices: Lasers have interesting field strength and energy density, but

clearly we do not yet have the right combination or tech. for a soft X-ray device at modest electron energy

Nonetheless, long term, there is great potential for optical undulators incorporating some combination of Ti:Sapphire and Photonic Bandgap technology

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Laser Driven Devices, cont.

Prospects for Optical Undulators in Soft X-Ray FELs:

Two key technologies are mature and have potential -

Ti:Sapphire lasers provide extraordinary energy density and field strength in optical pulses with flexible duration, tunability, and even controlled chirps.

Photonic Bandgap Structures provide substantial control over dispersion and losses as well as some ability to control field direction and phase.

A version of the dielectric-based a microstructure (e.g. Plettner & Byer, Phys. Rev. Special Topics - Accelerators and Beams 11, 030704 (2008)) may provide a short period undulator for a soft X-Ray FEL.

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RF Undulators RF Wigglers – Sami Tantawi RF Undulators at Ka band – Jiang Yong discussion: RF wigglers/tech; extendable to CW? – Sami Tantawi, Jean Delayen, Yong Jiang

Bottom lines for RF Undulators: offer potential of both kHz polarization switching & complete

polarization control, including 100% circular polarization at very reasonable (~5mm) gaps

Potential for going to much smaller undulator periods (1mm? – superconducting technology dependent)

Still unknown – beam dynamics effects such as that of longitudunal RF fields (possibly mitigated with added focusing?)

Manufacturing technology for superconducting RF undulators still needs to be developed

RF undulators perhaps on a faster track

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RF Undulators, cont. proposed offerings:

cost – typical cost per meter of cryomodule structures (but no high cost RF source needed here)

parameters – 35 GHz, ~1m sections (or longer, if cryomodule so allows), ~5mm gap, 5 mm period (or less if superconductor materials so allow, e.g. residual resistance or cooper pair breaking in Nb occurs at ~90GHz?)

performance – K ~ 1 reasonable advantages –

kHz switching with 100% circular or any other desired polarization easy phase shifting between sections – fraction of a degree is reasonable

disadvantages – need cryomodules- which are expensive risks – requires 10^-4 tolerances; transition sections are difficult

Next steps/challenges - don’t yet have decent manufacturing technology for these superconducting RF undulators – this has yet to be addressed

ETA - full scale SRF prototype in several years; RF earlier……

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HE1n Modes Scaling Laws (Tantawi)

For an undulator made of copper at room temperature :

2/3 2/33

2 2/3 4/31

7/6

2 5/3 4/3 2

2/3

2

1

2

: ( ) 0.23

0.28 ( ): ( )

9.22 ( ): ( )

30867:

: ( ) 32.8

: (

u

u

u

u

f u

s

Optimal Radius a m Lx

K L x J xMinimumPower P MW

K L x J xStored Energy U Jouls

LQuality Factor Q

FillingTime t s L

Peak Suface E Field E M

1

1.

11 12 13 14

1.02 ( )/ )

3.4 ( ): ( )

{2.40483,5.52008,8.65373,11.7915} HE ,HE ,HE ,HE

s

KxJ xV m

aKxJ x

Peak Surface B Field B mTa

x for modes

P~ 1/ λ, so for smaller wavelength capability one

pays for power (RF) or else for refrigeration (SRF)

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PM and Cryo-PM devices

Compact undulators forFELs - Jim Clarke Short period PrFeB undulators, Johannes Bahrdt Cryogenic undulators, Finn O’Shea discussion: PM & Cryo-PM Undulators - Finn O’Shea, Johannes Bahrdt, Jim Clarke

Bottom lines: A variety of PM devices enable variable polarization and K~1

capability down to below 10mm periods Cryo-PM devices push performance of PM counterparts by ~40%

(via both enhanced Br, Hc and ability to use new materials)

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CPMU-9 at UCLA (O’Shea)

Started as a project to get ~1 keV photons from a ~2 GeV plasma source electron beam

Push strong magnets and short period to get high performance

K = 1.7, period = 9 mm, gap ~1.5 mm and have low charge help with wakes

Variable Polarization Undulators

(J.Clarke)

PM vs. SCU (J.Clarke)

Soft x-ray FEL users want variable polarisation Advanced APPLE undulators (APPLE3 or 4) have

significant advantages, but need development (~15% lower beam energy for NLS)

In NLS FEL case study over 0.1 to 0.4nm: SCU needs 17.5% lower E than PPM SCU has 30% shorter saturation length than PPM But, 20% lower saturation power than PPM

SCUs Compact undulators forFELs - Jim Clarke Short period SCUs at ANKA - Michael Hagelstein Short period undulator R&D at LBNL – Soren Prestemon SCUs, a practical approach – Yury Ivanyushenkov

Bottom lines: Various SCU technologies have the capability for K~1 at periods

<10mm to produce down to e.g. 1nm or 0.1nm radiation at modest (~ 2-4 GeV) electron energy

On-going R&D in both linearly polarized and variable polarized devices Using NbTi, Nb3Sn, or HTS

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SCUs: helical, planar (Hagelstein, Ivanyushenkov, Clarke)

At RAL: A 4-m helical undulator has been built by RAL team for the ILC positron source project, 11.5mm period, 0.86T

At APS: designing and building the first superconducting planar undulator –SCU0 (18mm period, 9.5mm gap)

At Karlsruhe-Mainz: an SCU w/ 3.8 mm period, 2mm gap, 855MeV, 100 periods, 100 uA cw (1988)

At ANKA: SCU14, 14mm period, 100 periods, cryogen-free, SCU14, (2006)

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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

LBNL

Key SCU points NbTi Prototypes – full scale and subscale, demonstrate performance meets specs

Phase errors quite low -Key is excellent quality control during fab. No real implementation of shimming ; except LBNL proof of principle Some variation from device to device; source not evident

Implementation of Nb3Sn at LBNL; only tentatively investigated elsewhere Would benefit from guidance on material and fabrication issues Working with conductor vendors for optimal conductors

Cryogenics: Generally using cryocoolers, either with recondensers or via conduction General uncertainty on heat load; evidence of unknown source; multiple

calorimeters being designed and fabricated Next steps

First truly successful operation in a storage ring (Anka / APS / other?) needed R&D areas

Need to develop fully functioning measurement system Need fully developed shimming approach Need to develop sub-10mm period devices

Situating the SCU technology Performance dominates in the >10mm period range

Next closest competitor is cryogenic in-vacuum Expect SCU’s to be cheaper (ultimately)

No moving parts, material cheaper; cryogenics more expensive, but probably not significant if part of large SRF-linac facility

Performance appears strong in the <10mm period regime Outperforms hybrid PM devices Need to:

Demonstrate performance Understand and control tolerances

Maturity compared to other technologies PM>hybrid>in-vac.hybrid>CIVID>NbTi>Nb3Sn>HTS>SRF>plasma But… measured SCU(NbTi) devices as APS and ANKA suggest SCU(NbTi)

is in a close race with CIVID (excellent phase errors with SCU’s)

FEL, Beam dynamics, & User needs FEL lines can be clearly delineated by…

Low K (~<1) regime where push is to yield… Specific photon energy with lowest beam energy, or… Highest photon energy with a given beam energy

Modes K regime (~1.5-2) where push is to yield… Adequate tuning range with minimum beam energy

Collaboration between undulator, beam dynamics, FEL groups tradeoffs between K(gap,period) and impact of gap on beam and facility complexity

Polarization needs: Critical for soft X-rays Usually of interest along with tuning => goes hand-in-hand with modest-

K regime above Possibly less importance at short wavelengths

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S. Prestemon

Magnetic Measurements

Magnetic measurements at SLAC - Zach Wolf Pulsed/Cold magnetic measurements –Diego Arbelaez

Bottom lines: Pursuing two measurement technologies

Hall probes in bores as small as 2-3mm diameter and Pulsed wires in bores as small as 1 mm diameter

Key issue: ~25 micron vertical probe positioning in e.g. 10mm period undulator for δB/B ~ 10-4; By ~ cosh(2y/λu) Need Probe movement in straight line to < 25 microns Undulator alignment capability to ~ 20 microns Quadrupole alignment capability to ~ 10 microns

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Magnetic Measurements, cont.

Requirements: Space limitations: wire in ~1mm tube; Hall probe in 2-3mm tube Trajectories – required straight to within ~ 2 microns Field integrals: both Hall probe and Pulsed wire are capable Fiducialization <10 microns: Achievable with hall probe K – required uniform to 10-4, Achievable with hall probe Phase – if required < 10 degrees: Achievable with hall probe Cryogenics - cryogenic capability for some undulator technologies ETA: cold measurement system capability in ~few years

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Radiation damage

Radiation damage in FELs – Rick Donahue discussion: Rick Donahue, Jim Clarke, Finn O’Shea, …

PM Radiation Damage FLASH experiment suggested that only 104Gy gave 0.5% loss in B

field of PM undulator SCU Radiation Damage

is in epoxy used for potting coils, but much less susceptable than PMs

Should still be protected with collimation scheme Commonly accepted dose limit for epoxies is 107 Gy - used in

ITER, Fusion Technology Institute, Wisconsin

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Radiation damage can also be an issue for SCUs

(refers to SR induced damage rather than neutrons):

Beam dynamics and FEL physics subgroupConveners - Steve Benson and Sasha Zholents

Attendees:Brian AustinSiva DarbhaDaniele FilippettoPunit GhandiRyan LindbergAtosa MeseckPhillippe PiotJi QiangMatthias ReinschRobert RyneJonathan Wurtele

Mini-Workshop on Short Period Undulators for FELs, Berkeley, June 21-23, 2011

How short is it reasonable to go in period?

• If one restricts the design of the FEL so that the emittance limit is met

[often missed a little, jjb] we find that the minimum energy is given by

• One can substitute this into the resonance equation to get a minimum

wiggler wavelength:

• Thus the minimum wavelength is critically dependent on the

emittance.• A simple way to reduce emittance is to reduce charge. The energy is

also low. How do we make up low bunch energy? For the soft X-ray

range, going to high repetition rate looks like a good way.• The short wavelength undulators may be limited to small charge and

therefore short bunches with relatively large bandwidth.• Going to shorter period almost always results in lower K and smaller

gap. Both of these may be more of a limitation than the emittance.

How Small Can We Makethe Undulator gap?

• Wake fields trajectory and chamber position tolerances

• The value of X should be less than unity for an FEL. The value of the wake potential has to be calculated for a variety of pulse lengths, shapes and chamber sizes.

• Can we shape the bunch to minimize?

• Radiation damage. How can we collimate?• Can define a wiggler acceptance as

• Ion trapping for high duty cycle

rmsze

sat Wecm

LX

/2

LCLS ultra-short beam and double-horn start to sample the short-range resistive-wall wake

Transverse and longitudinal wakefieldTransverse and longitudinal wakefield

Horns : 10 fs Horns : 10 fs 3 microns; 3 microns;Overall: 100 fs Overall: 100 fs 30 microns 30 microns

500 A500 A

2 kA2 kA

3 kA3 kA

1.5 kA1.5 kA

For 250 pC chargeFor 250 pC chargeJuhao Wu jhwu@SLAC.Stanford.ED

U

38/20Aug. 06, 2010LBL Compact XFEL

Example: transverse

FEL at 1.5 Å, electron energy 13.64 GeV, 3 kA current

– Similarly, by comparing the energy loss scan without kicking the electron bunch to that when kicking the electron bunch, the additional FEL induced energy loss is found

– Measurement: the wakefield loss is about 40 MeV (0.3 %), the spontaneous radiation is about 16 MeV (0.1 %) for 25 undulator sections, and the FEL is about ~ 10 MeV (0.07 %) [about 2.5 mJ FEL for this 250 pC case].

Longitudinal with FEL: example Longitudinal with FEL: example (cont'd)(cont'd)

39/20Aug. 06, 2010LBL Compact XFEL

Juhao Wu jhwu@SLAC.Stanford.ED

U

How small is too small for K?

• Calculated parameters for 1 nm operation with 20 pC of charge with 0.1 mm-mrad emittance and 5 keV-psec longitudinal emittance. Longitudinal match optimized for each point.

• If Krms<0.5 there is essentially no tunability.

• The gain is decreasing so rapidly for If Krms<0.5 that the wiggler length actually increases for decreasing wavelength.

• The acceptance of the wiggler is getting close to 100 times the emittance which means the collimator may cause large wakes.

• Mattias has system for looking at cost savings to compare 5 and 10 mm wavelength systems.

How Small can we make K (cont.)?

Polarization• Polarization control is extremely important in the soft X-

ray range. Not as important in the hard X-ray range.• Three options - Crossed undulators, Delta, RF undulator

– Ding and Huang indicate moderate polarization in simulations with crossed polarizers. Variations from pulse to pulse are unacceptable so this may not work. This also argues against use in SASE system.

– kHz switching speed is highly desired. Possible with crossed undulators and RF undulator.

– All three have fairly fine control of the ellipticity.– Only the Delta and RF undulator have good

polarization purity (but only at the fundamental)

RF Undulator

• Advantages– Fast polarization switching and wavelength tuning.

– Less affected by radiation.

• Disadvantages– Can’t get as high a field

– Relatively immature technology

• How much does the input and output taper hurt you?

• How uniform does the field have to be?

• How is the beam focused? (external focusing?)

• Can segments be driven in parallel?

• Can you still fast switch an SRF cavity?

Laser Undulators

• Very intriguing technology

• Requires extremely small emittance (1 nm)

• Possible approach to gamma ray laser

Personal Perspective Undulator technology developments point to 1.5 cm or less as a reasonable

period, with “defensible” gap, for next generation proposals Superconducting undulators now fully competitive or better K much less than unity not good gambit For CW and/or shorter periods with small gaps, halo is great unknown “Conventional” lasers

Power requirements still too high for even modest rep rate unless something novel is done

Dielectric interesting, but will require much R&D RF structures help gap limitations and give rapid tunablility

Tests of pulsed copper may be near at hand SRF versions would be wonderful; SRF wall losses vs

frequency may be problem to go sub centimeter

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