erl drivers for fels d. douglas jlab - so easy, even a cave man could do it!
TRANSCRIPT
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ERL Drivers for FELS
D. DouglasJLab
- So Easy, Even A Cave Man Could Do IT!
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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
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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))
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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
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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
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Key Concept: Longitudinal Matching in an SRF ERL FEL Driver
E
E
E
“oscillator”
“amplifier”
E
E
injector
dump
wiggler
linacE
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E
E
E
injector
dump
wiggler
linac
E
E
E
1)2)
3) 4)
5)
6)
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• 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
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• 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
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JLab IR Demo Dump
core of beam off center, even though BLMs showed edges were centered
(high energy tail)
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• 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
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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
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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)
E
E
E
E
E
E
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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
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Injector to Wiggler Transport
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If you do it right linac produces stable ultrashort pulses
Can regularly achieve 300 fs FWHM electron pulses
~150 fsec rms
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Injector to Reinjection Transport
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• 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
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• 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
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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…
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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, …
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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
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• 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
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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
10
100
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…
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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)
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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
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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)?
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– 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
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• 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
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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!
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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
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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$
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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
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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…
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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
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JLAMP Recirculator Beam Dynamics
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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)?
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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
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Synchronization
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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
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• 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
![Page 45: ERL Drivers for FELS D. Douglas JLab - So Easy, Even A Cave Man Could Do IT!](https://reader036.vdocuments.net/reader036/viewer/2022081519/56649e2e5503460f94b1dde7/html5/thumbnails/45.jpg)
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!