ERL Drivers for FELS

D. DouglasJLab

- So Easy, Even A Cave Man Could Do IT!

Acknowledgments & Disclaimer

• As with many DoD-funded scientists, I don’t get out much, so rather than trying to give a comprehensive overview of ERL-Driven FELs, I’ll primarily speak to experiences (and misadventures) here at JLab…

• Thanks to you all for the opportunity to participate in this happy occasion, and to recognize the contributions and example of our wonderful friend & colleague

• I’ll be relating experience & results from the collective JLab FEL team and our Accelerator & Engineering Division co-workers – I’m very grateful to them and to JLab for their ongoing support and opportunities

Historical Context• ERLs 1st proposed by Tigner (1965), operated at Chalk River

(Schriber, Funk, Hodge and Hutcheon, 1975), identified as potential advance for FEL drivers (use of ER at UCSB & LANL, 1980s, use of same-cell ER at MIT: Flanz and Sargent 1985)

• Successful implementation for high-efficiency/high-duty-factor/high-power FELs depends on two further epiphanies:– Use of SRF technology (Todd Smith, 1980s; Bisognano & Krafft, late

1980s)• Low peak power, high average power by way of high, CW repetition rate

– Longitudinal matching • Bunch compression after acceleration (with correction of higher order effects)

– T. Smith, FEL’85• Energy compression during energy recovery (Larry Doolittle ~1991?;

documented by Piot et al (PRST-AB, 2003))

So, Why Use ERL Drivers for FELs? Great Potential for Cost-Effective 4th GLS!

• Linac quality beam (brightness)• Potential for high duty cycle (CW)

– High average power from high repetition rate, not high instantaneous power• Much easier

• Storage ring wall plug efficiency (cost)• Operational flexibility (robustness)• Entertainment value: numerous beam dynamical effects manifest themselves…

– LSC, BBU, CSR, …

The perfect combination for an FEL driver: great accelerator performance and lots of distractions to keep physicists occupied…

This notion appeared so promising that JLab director Herman Grunder aggressively pursued support for a test system, which led to the IR Demo FEL

"With such a beam, we said `My God, there must be something we can do with it other than fundamental physics'" - H. Grunder, Washington Post,

2 March 1997

Indeed, there was… The Jlab IR Demo FEL

• USN/ONR funded (1995) construction of SRF ERL testbed: “JLab IR Demo FEL”

• Intended to validate a number of concepts– Low peak, high average power paradigm– Use of SRF in “high” CW current application (5 mA)

• BBU management– High brightness CW injector– Beam quality preservation– High average power oscillator-based FEL– Longitudinal matching scenario

• Inject long bunch (alleviate space charge)• Compress length at full energy• Energy compression during energy recovery

Key Concept: Longitudinal Matching in an SRF ERL FEL Driver

E

E

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“oscillator”

“amplifier”

E

E

injector

dump

wiggler

linacE

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injector

dump

wiggler

linac

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E

1)2)

3) 4)

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6)

• Intention was to leverage investment in CEBAF– Use (pilfer) components from inventory, NP DC R&D gun,

…• System needed to accomodate large exhaust energy

spread from FEL – so was intended to be a clone of the large-acceptance MIT-Bates recirculator… but the fates (in the person of Slava…), intervened…

System Design for IR Demo

• Re-worked design to limit bending before wiggler– Risk reduction – Successfully lased CW at various wavelengths with

powers up to 2.1 kW• Validated design paradigms• Investigated BBU & other effects

• Allowed initial work toward THz source• CSR-observant revisions were key to project

success– honestly, we really didn’t have a clue how to do the

longitudinal matching until after we stumbled over it…

CSR (Fear)-Driven Design

JLab IR Demo Dump

core of beam off center, even though BLMs showed edges were centered

(high energy tail)

• Answered a number of questions, like “can it be done?”– BBU control, longitudinal matching, baseline on CSR…

• “polychromatic” source of radiation– THz– IR (+ coherent harmonics)– Compton X-ray source (Krafft et al.)

• Brought the issue of beam quality preservation to the forefront– CSR nonfatal, but very much an issue

• Led immediately to “do it again… with MORE power”…

Retrospective on IR Demo

Coherent Harmonics

Jlab IR Upgrade FEL

• “That was easy”… so power scale-up (by 10x) was an obvious next step

• Double current, raise FEL extraction efficiency, triple energy to get to 10 kW

• “CSR is your friend”– leverage IR Demo design to provide more flexible

longitudinal match, including curvature and torsion correction

(not just survivable, but a funding source)– Include THz beamline

Longitudinal Matching ScenarioRequirements on phase space:• high peak current (short bunch) at FEL

– bunch length compression at wigglerusing quads and sextupoles to adjust compactions

• “small” energy spread at dump– energy compress while energy recovering– “short” RF wavelength/long bunch,

large exhaust p/p (~10%) get slope, curvature, and torsion right

(quads, sextupoles, octupoles)

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E

E

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E

Nonlinearity Control Validated By Measurement: Harmonic RF Unnecessary (and

Expensive!)Figure 1: Inner sextupoles to 12726 g-cm and trim quads to -215 g Figure 2: trim quads at -185 g with same sextupoles Figure 3: trim quads at -245 gFigure 4: quads at -215, but sextupoles 3000 g below design, at 10726 g-cmFigure 5: where we left it: trim quads -215 g sextupoles at 12726 g-cm

arrival

launch

Injector to Wiggler Transport

If you do it right linac produces stable ultrashort pulses

Can regularly achieve 300 fs FWHM electron pulses

~150 fsec rms

Injector to Reinjection Transport

• Schedule constraints led to use of “The Admiral” – a high gradient prototype SRF module with light HOM damping– Predictions => BBU threshold at 2.5 mA– How to fix?

• By this time, Slava had arrived at Jlab, and had thoroughly inculcated us all with the outlook that phase space is phase space, not a bunch of disconnected orthogonal transverse and longitudinal subspaces – so it was natural to adopt a fully coupled solution

• Rand & Smith, 5 quad rotator interchanging transverse phase spaces; BBU completely stabilized

BBU – a bump in the road

• Successfully generating a short bunch at the wiggler lead to a short bunch in the return arc, with significant CSR generation in each location– 10s of W of THz dumped onto FEL outcoupler… resulting in

distortion & power limitations• Initial 10 kW run at 25% duty cycle: 1 second on, 3 seconds off (cool

mirrors)• “The JLab Occasionally 10 kW FEL (2004)”

• Installed “de (actually, over)-bunching” chicane after wiggler; “THz traps”, cryo-cooled OC, thereby alleviating effect

• 14.3 kW in November 2006

CSR/THz – Bridge Out

Retrospective on the IR Upgrade

• Learned how to manage BBU• Encountered CSR as an unanticipated limit:

– Not beam quality dilution – POWER DEPOSITION!

• Had 1st look at halo, other collective effects– Wakes, LSC, RF heating…

Next Step: JLab UV FEL• IR Demo validated

– SRF ERL driver– Low peak/high average power paradigm

• IR Upgrade validated– Power scaling– BBU control– Role of CSR as performance limit

• Issue is not just “beam quality preservation”, its also “power in the wrong place”

• Short wavelengths more challenging– Test of beam brightness & beam quality preservation, mirror

design, power-flow management, …

System ConceptUV FEL “bypass”• ~150 MeV• 60 pC x 37 MHz

– (5 mA)

Tighter beam quality required at shorter wavelength– Test of beam

brightness– Check beam quality

preservation

• 1st beam through bypass– Demonstrated bunch compression, beam quality

• 1st CW run 7/29/10: ~1 mA (~100 kW)• Installing wiggler chamber• 1st lasing imminent (we hope…)

Status

eps x 3.883392503beta x 7.081035351alpha x 9.527849399eps y 2.386019152beta y 4.412957251alpha y 8.681434968

State of ERL Performance ERLs provide very high

power/high brightness beams

• FEL drivers– E: 10s of MeV – few GeV– Q: 100s pC – 1 nC– I: mA – 10s mA– normalized ~ /4

• 1-10 mm-mrad– Pbeam ~ MW

• Light sources– E: 5 – 10 GeV– Q: ~10s pC – 100 pC– I: 100(s) mA– normalized < ~1 mm-mrad– Pbeam ~ GW

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1000

10000

0 100 200 300

Average Current [mA]

Energ

y [

MeV

]

Light Sources

Electron-I on Colliders

eRHI C

ELI CERLSYN

ERL

4GLS

1 kW FEL

10 kW FEL

BNL Electron Cooling

ERL Prototype

100 kW FEL

CEBAF-ER

High current path

High energy path

10

100

1000

10000

0 100 200 300

Average Current [mA]

Energ

y [

MeV

]

Light SourcesLight Sources

Electron-I on CollidersElectron-I on Colliders

eRHI C

ELI CERLSYN

ERL

4GLS

1 kW FEL

10 kW FEL

BNL Electron Cooling

ERL Prototype

100 kW FEL

CEBAF-ER

High current path

High energy path

• ***high power=> halo major issue! Can’t lose 10-5 of beam!

• implications: tiny spot size, COTR effects, 6-d systems…

The Future• Higher powers

– Higher charge/bunch, shorter bunches => extraction efficiency for (and power from) CSR rivals (exceeds) that of FEL

• High rep rates at shorter wavelengths– JLAMP– Hard X-FEL

• Multiple FELs driven by single ERL – RF separation as in CEBAF (with recombination)

The Late, Great JLAMP• IR -> IR Upgrade -> UV…. Where next?• JLAMP – yet another upgrade

– 2 pass x 300 MeV linac; seeded amplifier reaching ~10 nm

– XFELO test

ERL-Driven X-FELS with apologies to Paul Emma and other people that actually have X-FELs!

• Higher energies => longer linacs => higher cost• Recirculation/energy recovery are palliative

measures: make systems more affordable• Will require extensive study and creative

design to ensure beam quality preserved, optimum cost/benefit achieved– More FELs/unit linac is better… – Multiplicity by way of RF separation (a la CEBAF)?

– Transverse optics

29

– Machine configuration:

GERBAL: “Generic Energy-Recovered Bisected Asymmetric Linacs”

10 MeV Injector

1.2 GeV Linac

3.6 GeV Linac

Multiple wigglers (9.6 GeV beam)

1.2 GeV accel.4.8 GeV ER

1 MW Dump1.2 GeV ER

6.0 GeV accel.

4.8 GeV accel.6.0 GeV ER

recirc

recirc

recirc

recirc

recirc

recirc

recirc

• Rings – very advanced systems – equivalent to nanotechnology or rocket science

Perspective• “conventional” FELs – perhaps not as advanced, but still very sophisticated –

like cathedrals or bridges

But at least ERLs are so easy “even a caveman could do it!”

• ERLs – in infancy (or “terrible twos”…) – stone knives and animal skins

Observations

As we’re way too early in the game to draw conclusions…• 35 years of ERL operation experience

– Chalk River, MIT, LANL, JLAB, JLAB, JLAB, JAERI, Novosibirsk, JLAB, Daresbury, JLAB, …

• Successful trend toward shorter & shorter wavelengths and higher & higher powers

• Many unresolved issues, but thanks to great leadership – by our guest of honor and those he’s influenced – there’s good reason to expect excellent outcome!

The Late, Great JLAMP• IR -> IR Upgrade -> UV…. Where next?• JLAMP – yet another upgrade

– 2 pass x 300 MeV linac; seeded amplifier reaching ~10 nm

– XFELO test

Design Requirements

Requirements• Generate, accelerate, and deliver properly configured drive

beam to FEL– 1 mm-mrad x 50 keV-psec x 200 pC– Ipeak ~ 1 kA (200 fsec FWHM x 0.1% p/p)

• Recover (degraded) exhaust beam• Preserve beam quality, manage losses, avoid instabilities, etc

etc• Fit in vault (an upgrade)• Cost < 100 M$

Design Parameters (F. Hannon, IPAC2010)

2010 2012

Bunch charge (pC) 135 200

Bunch rep. rate (MHz) 75 4.68

Average current, max (mA) 10 1

Norm. transverse emittance at FEL (µm) 10 1

Longitudinal emittance at FEL (keV ps) 60 50

Energy spread at FEL (% rms) 0.4 0.1

Bunch length at FEL, rms (fs) 150 80

Bunch energy (MeV) 100 600

Reality Check

• As defined by these requirements, JLAMP will– Be a low cost user facility meeting significant

scientific need– Test numerous concepts critical to next

generation light sources• High brightness/high duty factor sources• Beam quality preservation in SRF environment

– LSC, CSR, MBI, … • Multi-pass recirculation/energy recovery

• Very high risk, very high return…

Beam Dynamics Issues• space charge• BBU• other wakes/impedances

– linac, vacuum chamber, diagnostic impedences

• MicrowaveStudio modeling of all components

• impedance budget, policy, enforcement (impedence policing)

– resistive wall• vacuum effects

– Ions– gas scattering

• intrabeam scattering– IBS– Touschek

• halo – Formation– gas scattering– beam formation processes

• CSR– CSR basic ("elegant")– 3-d modeling– microbunching instabilities

• ISR– emittance, p/p...

• Error analysis– Alignment

• Magnets, cavities, diagnostics

– Powering• Excitation, ripple, reproducibility

– field tolerance• Homogeniety, calibration

– timing & synchronism– phase & gradient– diagnostic errors

• RF drive– transient analysis

• Operational simulations– threading, orbit correction– emittance measurement– lattice function tuning– longitudinal matching

• phase transfer function• bunch length compression tuning• energy compression tuning

JLAMP Recirculator Beam Dynamics

ERL-Driven X-FELS with apologies to Paul Emma and other people that actually have X-FELs!

• Higher energies => longer linacs => higher cost• Recirculation/energy recovery are palliative

measures: make systems more affordable• Will require extensive study and creative

design to ensure beam quality preserved, optimum cost/benefit achieved– More FELs/unit linac is better… – Multiplicity by way of RF separation (a la CEBAF)?

FEL-Seeded ERL-Driven XFEL

Two bunch trains UV seed, XFEL drive

RF separation in 1st passUV bypass RF/2 longer (recovers bunch train)

Issues:SYNCHRONISMUV seed pulse energy,

up-conversion

Synchronization

43

– Machine configuration:

GERBAL

10 MeV Injector

1.2 GeV Linac

3.6 GeV Linac

Multiple wigglers (9.6 GeV beam)

1.2 GeV accel.4.8 GeV ER

1 MW Dump1.2 GeV ER

6.0 GeV accel.

4.8 GeV accel.6.0 GeV ER

recirc

recirc

recirc

recirc

recirc

recirc

recirc

– Transverse optics

• Rings – very advanced systems – equivalent to nanotechnology or rocket science

Perspective• “conventional” FELs – perhaps not as advanced, but still very sophisticated –

like cathedrals or bridges

But at least ERLs are so easy “even a caveman could do it!”

• ERLs – in infancy (or “terrible twos”…) – stone knives and animal skins

Observations

As we’re way too early in the game to draw conclusions…• 35 years of ERL operation experience

– Chalk River, MIT, LANL, JLAB, JLAB, JLAB, JAERI, Novosibirsk, JLAB, JLAB, Daresbury,…

• Successful trend toward shorter & shorter wavelengths and higher powers

• Many unresolved issues, but thanks to great leadership – by our guest of honor and those he’s influenced – there’s good reason to expect excellent outcome!