Beam Transport for MW Class FEL Drivers

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Beam Transport for MW Class FEL Drivers

• D. Douglas
• Jefferson Lab

2
Homiletics

• A sermon is three points, a poem, and a joke
• drivers for MW class performance are possible
• drivers for MW class performance cant be built
  yet
• ? but the path is clear
• system integration advice its a LINAC
• (change the round hole, not the square peg)

3
System Paradigm (Prejudice, Obsession)

• Low peak, high average power FEL driven by SRF
  ERL
• its elegant
• its in my comfort zone its what I know like
• it might just work
• nobodys publicly admitted to producing kW-level
  CW average power with anything else (yet)
• Consider me the Kings Fool I will tell you the
  truth (hopefully with humor). You may ignore it,
  avoid it, or use it. You may smack me as you
  will, but it will be the truth

4
Machine Concept

• 100 MeV ? 0.5 A ? Pbeam 50 MW
• hFEL 2 ? PFEL 1 MW

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Examples (the usual suspects)

• JLab IR Demo FEL
• 50 MeV ? 5 mA 0.25 MW
• hFEL 0.8 PFEL 0.002 MW
• footprint 45 m ? 6 m

• JLab IR Upgrade FEL
• 150 MeV ? 10 mA 1.5 MW
• hFEL 1 PFEL 0.015 MW
• footprint 65 m ? 6 m

• These systems provide guidance for design of
  higher power devices

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Review of Issues/Requirements

• Management of full 6-d phase space from source to
  FEL, from FEL to dump
• Halo
• Suppression/control of instabilities and other
  collective effects
• Beam quality preservation

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Review Phase Space Management

• Transverse
• include RF focusing effects
• keep envelopes small (instabilities, error
  sensitivities, halo control)
• may need to select/control phase advances to
  suppress instabilities
• CSR control
• Longitudinal
• accelerate long bunch to avoid instabilities
  compress length just before wiggler
• energy compress during energy recovery
• typically must compensate RF waveform curvature
  (both in bunch length and energy compression)
• either magnetic or harmonic RF effective one or
  other may be help in packaging system
• Note cant energy recover even harmonic RF
  (avoid sawtooth waveforms!)
• Note unless you do something special to
  waveform, must use opposite signs of compaction
  to compress bunch length and energy (or recover
  more/less than 180o apart in RF phase)
• Status phase space management is
  straightforward, but to date has required tunable
  system (variable quads, sextupoles) and hasnt
  been demonstrated at high (kW) powers using
  harmonic RF

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Review Halo Management

• Halo is likely a major limitation to very high
  powers
• halo generation complex topic poorly understood
  by ordinary mortals, and thus largely ignored by
  machine designers (like me)
• halo likely largely formed in front end
• chunks of it scrape off and melt stuff, irradiate
  things and make life generally unpleasant. Need
  well less than 1 mA loss at any single point.
• Current loss is worse for big beam envelopes
  (beam large, lattice sensitive), small apertures,
  high currents
• Experience in CEBAF, IR Demo, CEBAF-ER suggests
  C10-6, so in 1 A machine need aperture larger
  than beam envelope (!!??)
• Status unsolved problem, under study - but only
  at most rudimentary level and at low powers
  (signal to noise core beam swamps diagnostic at
  few mA). Much work needed!

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Review Instabilities Collective Effects

• Wakes (beam-induced fields)
• keep bunch long until you need it short
• shield components
• Coherent Synchrotron Radiation (CSR)
• same approach dont compress until you need to,
  in fact, generalize to say
• keep at least some bunch dimensions well beyond
  coherence length (so if its short, make it a
  pancake)
• impact is smaller at larger emittance emittance
  spec loose for IR FELs, so not as critical as in
  UV, X-FEL
• HOM/BBU
• HOM suppression by proper SRF system design
• feedback stabilization
• suppression supported by proper choice of
  betatron phase
• non-zero chromaticity may help - phase
  decoherence across large momentum spread bunch
  (induced by FEL) decorrelates betatron response
  to HOM kicks
• need to worry about power deposition from
  propagating modes!!!

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CSR Simulation
½ nC 10 mm-mrad ? 15 mm-mrad JLAB-TN-00-017
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CSR Simulation JLAB-TN-00-017
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Still Reviewing Instabilities Collective
Effects

• Status
• phenomena are pretty well understood probably
  manageable
• further measurements (esp. CSR, BBU, propagating
  HOM) and benchmarking of codes needed
• expect to see BBU when 3rd module installed in
  JLab 10 kW FEL Upgrade (rich HOM spectrum,
  implying low threshold),
• will be able to more carefully benchmark theory
  simulation
• learn how to build effective feedback systems.
• more work on HOM management, feedback, and
  transport system design needed before very high
  powers will be achieved
• motivates move toward lower frequency structures
  with fewer cells
• effect of power deposition from propagating modes
  is not well understood at high powers

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Review Beam Quality Preservation

• Motherhood statements
• Be sure to suppress collective effects (CSR,
  wakes)
• make bunch short only where it needs to be short
• shielded beamline components
• avoid overly strong bending, focusing
• Control magnitude impact of errors on beam
• magnetic field inhomogeneities have transverse
  and longitudinal emittance dilution effects (DB/B
  generates dx error, couples to (x,x) through
  M12 and M22 couples to (fRF,E) through M52)
• Status
• probably understand magnetostatic effects/seem to
  be able to control them
• IR Demo, IR Upgrade, CEBAF-ER all show
  well-defined beam and rational beam behavior
• these underscore the need to carefully spec out
  system components
• learning about collective effects
• wakes, CSR, BBU, space charge (may become issue
  as bunch charge goes up)

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Developing the Technology

• probably wont successfully run initial high
  power (100 kW) FELs without a tunable driver
  accelerator
• probably will be able to run offspring high power
  systems with a precast compact driver
  particularly if you commission using a blue-tip
  wrench (recut pole-pieces, move stuff around)
• suggests that FEL and driver evolve along
  matrixed developmental tracks
• have a separate operationally flexible tunable
  laboratory (testbed) driver for each generation
  of FEL (10 kW (exists), 100 kW, 1 MW). When it
  works, move FEL to a deployable field driver
• reduce flexibility of each subsequent field
  driver (compact 100 kW, very compact 1 MW)
• allows separate, controlled development of
  source, driver accelerator, FEL, and system
  integration/packaging process

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Example System Family Tree
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Technology Choices

• Lower RF frequency with fewer cells
• better HOM spectrum impedances
• bigger apertures
• requires lower compaction
• allows use of harmonic RF correction of RF
  waveform
• simplifies magnetic transport
• coax couplers?
• magnets electromagnetic for laboratory driver,
  permanent magnet (PM) for field drivers

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The Minimalist Machine

• Parameters
• Einjection 7 MeV ? 0.5 A (500 MHz, 1 nC)
• ? Pinjection 3.5 MW
• Efull 100 MeV ? 0.5 A ? Paccel 46.5 MW,
  Pfull 50 MW
• hFEL 2 ? PFEL 1 MW,
• dp/pout 10 (specifies energy recovery
  transport)
• Precovered 46.5 MW ? Pdumped Pfull - PFEL -
  Precovered 2.5 MW
• ? Edumped Pdumped/I 5 MeV
• you recover power, not energy!
• should figure out something to do
• with the 2½ MW!

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Features of the Minimalist Machine

• Linearized RF
• 500 MHz fundamental, 1500 MHz 3rd harmonic SRF
• 20 MV/m at 500 MHz ? 5 m active fundamental,
  probably 8 m real estate
• 25 MV at 1500 MHz ? 1 m active harmonic, probably
  2 m real estate
• ? 10 m of SRF
• run 20o off crest to provide enough energy
  compression
• Injection somehow
• Beam materializes on linac axis miraculously
  matched to rest of system
• beam envelopes linac acceptance (use RF
  focusing)
• long bunch/low momentum spread
• RF curvature corrected
• Simplistic phase space management
• Accelerator serendipitously provides beam
  transversely matched (via RF focusing) to
  mirror-bend achromat (with chicane for bunch
  length compression)
• nose-pieces to fix T566 of chicane?
• Two quads, properly placed, match beam to wiggler
• Two quads, properly placed, match beam to return
  arc mirror bend, which, through some undetermined
  feat of parlor magic, provides proper transverse
  match to cryomodule for energy recovery whilst
  its compaction sets the longitudinal match

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System Concept

• Major Components
• injector
• 100 MeV linac (four 500 MHz cavities, two 1500
  MHz cavities)
• 6 dipoles (10 kG, PM)
• 4 quads (PM)

• Integration Overview
• footprint 13 m ? 2 m
• weight ??????? lbs
• cost if you have to ask

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3rd harmonic RF
fundamental
fundamental 1/9th 3rd harmonic in phase
fundamental 1/9th 3rd harmonic 120o out of
phase (increases required compaction, can be used
to match to compaction)
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3rd harmonic RF
fundamental 1/4th 3rd harmonic 120o out of
phase (makes flat region away from crest)
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Parting Salvo

• Its front end loaded. The injector is definitely
  not easy
• required performance is orders of magnitude
  higher than prior art
• combination of current-charge/bunch-longitudinal
  emittance, desired cathode lifetime,
• injector footprint integration (how large,
  where to locate, how to inject)
• operability (come visit JLab, and see what a real
  mans injector is all about!)
• View it as long and skinny. Do not consider a
  spherical, cubical, conical, ellipsoidal, or
  otherwise blob-shaped FEL. Its called a linac
  for a reason.
• review integration options (put vacuum pipes
  through bulkheads, etc)
• dont expect technology to conform to prior
  notions of usage for shipboard volume transcend
  the paradigm of the 5 gun mount!
• And while were talking about linacs do not be
  fooled about available real estate gradients
• e.g. SLAC 17 MV/m (50 GeV/3 km) pulsed by
  SLEDing. Transients and all. And you dont want
  the transients Dont expect vastly more from SRF
  
• Dump requires work 100 kW level CW dump at JLab
  not trivial, 2.5 MW dumps may be very challenging
• Think carefully about operational cycle
• existing systems take 10s of minutes to go from
  off to full on
• may need to run in an idle (e- on, FEL off) mode
  when laser ops anticipated

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Altar Call

• drivers for MW class performance are possible
• drivers for MW class performance cant be built
  yet
• - but the path to MW levels is clear
• its a LINAC!
• I hope we have some converts!
• Supported by ONR, NAVSEA, AFRL, DOE

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Everything Ive Said is Wrong

• Based on paradigm of conventional mirror bend
  with harmonic RF
• Can now probably make compaction managed mirror
  bend no harmonic RF or higher order corrections
  needed

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Constraints

• Path length same at all momenta
• Footprint same at all momenta
• Solve, at each momentum, for angle and drift
  length thereby specify location of poles

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