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					      ERL Drivers for FELS
- So Easy, Even A Cave Man Could Do IT!
               D. Douglas
                  JLab
     Acknowledgments & Disclaimer
 • 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
• As Engineering Division co- scientists, I don’t get out much, so
    & with many DoD-funded
   rather than trying to give a comprehensive overview of ERL-
    workers – I’m very grateful to
    them FELs, JLab for their
   Drivenand toI’ll primarily speak to experiences (and
    ongoing support and
   misadventures) here at JLab…
    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

                     “oscillator”       E
             E
                                                f       injector
                 f   “amplifier”
                          linac
  dump                                      E
    E
                                                    f
         f


                         wiggler
                 E                  E
                     f                  f
                                              1)   E
         2)   E
                                                            f       injector
                  f
                            linac
                                                       5)       E
  dump
6) E                             4)
                                      E                             f
                  3)   E
     f                       f            f

                           wiggler
      System Design for IR Demo
• 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…
       CSR (Fear)-Driven Design
• 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…
JLab IR Demo Dump
          core of beam off center,
                  even though BLMs showed
                  edges were centered
                         (high energy tail)
        Retrospective on IR Demo
• Answered a number of questions, like “can it be
  done?”
   – BBU control, longitudinal matching, baseline on CSR…
• “polychromatic” source of radiation       Coherent Harmonics
   – 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”…
            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 Scenario
Requirements on phase space:                                E
• high peak current (short bunch) at FEL
     –   bunch length compression at wiggler
         using quads and sextupoles to adjust compactions           f
•   “small” energy spread at dump
     – energy compress while energy recovering                                      E
     – “short” RF wavelength/long bunch,
       large exhaust dp/p (~10%)
      get slope, curvature, and torsion right                                          f
       (quads, sextupoles, octupoles)                                       E
             E
                                                                                f
                                                                E
                      f

                                                                        f
                                         E


                                                  f
   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 g
   Figure 4: quads at -215, but sextupoles 3000 g below design, at 10726 g-cm
   Figure 5: where we left it: trim quads -215 g sextupoles at 12726 g-cm




launch f


             arrival f
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
       BBU – a bump in the road
• 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
            CSR/THz – Bridge Out
• 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
  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 Concept
UV 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
                   Status
• 1st beam through bypass
  – Demonstrated bunch compression, beam quality
                                eps x     3.883392503
                                beta x    7.081035351
                                alpha x   9.527849399
                                eps y     2.386019152
                                beta y    4.412957251
                                alpha y   8.681434968




• 1st CW run 7/29/10: ~1 mA (~100 kW)
• Installing wiggler chamber
• 1st lasing imminent (we hope…)
State of ERL Performance
ERLs provide very high                                                                                             eRHIC
  power/high brightness                         10000
                                                                             ERL
  beams                                                                   ERLSYN                                 ELIC

• FEL drivers                                           High energy path                            Electron-Ion Colliders




                                 Energy [MeV]
                                                 1000
                                                                            4GLS
   –   E: 10s of MeV – few GeV                              CEBAF-ER                        Light Sources

   –   Q: 100s pC – 1 nC                                      10 kW FEL            100 kW FEL
                                                  100
   –   I: mA – 10s mA                                                              ERL Prototype
                                                              1 kW FEL             BNL Electron Cooling
   –    enormalized ~ l/4p
        • 1-10 mm-mrad                                                     High current path
                                                   10
   – Pbeam ~ MW                                         0                      100                    200                  300
• Light sources                                                                Average Current [mA]
   – E: 5 – 10 GeV
                                                        •     ***high power=> halo major issue! Can’t
   – Q: ~10s pC – 100 pC
                                                              lose 10-5 of beam!
   – I: 100(s) mA
                                                              implications: tiny spot size, COTR effects,
   –  enormalized < ~1 mm-                              •

     mrad                                                     6-d systems…
   – Pbeam ~ GW
                  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)?
GERBAL: “Generic Energy-Recovered
           Bisected Asymmetric Linacs”
  – Machine configuration:
                          Multiple wigglers (9.6 GeV beam)
                                     1.2 GeV
                                     Linac

                1.2 GeV ER                                     1.2 GeV
                           10 MeV              1 MW Dump       accel. ER
                4.8 GeV                                        4.8 GeV
                           Injector
                accel. ER
                6.0 GeV                                        6.0 GeV
                                                               accel.

                                      3.6 GeV
                                      Linac
  – Transverse optics

  r         r     r              r                 r       r               r
  e         e     e              e                 e       e               e
  c         c     c              c                 c       c               c
  i         i     i              i                 i       i               i
  r         r     r              r                 r       r               r
  c         c     c              c                 c       c               c




                                                                               29
                          Perspective
• “conventional”or(or – advanced– stone knivesstill–animal
• ERLs – in infancy
                  FELs perhaps not as
  Rings – very“terrible twos”…)advanced, but andvery sophisticated – to
  skins
  like cathedrals bridges             systems equivalent
  nanotechnology or rocket science




But at least ERLs are so easy “even a caveman could do it!”
                     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% dp/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                                         •   ISR
•   BBU                                                       –    emittance, dp/p...
•   other wakes/impedances                               •   Error analysis
     –     linac, vacuum chamber, diagnostic                  –    Alignment
           impedences
                                                                     •   Magnets, cavities, diagnostics
             •   MicrowaveStudio modeling of all
                 components                                   –    Powering
             •   impedance budget, policy, enforcement               •   Excitation, ripple, reproducibility
                 (impedence policing)
     –     resistive wall                                     –    field tolerance
                                                                     •   Homogeniety, calibration
•   vacuum effects
     –     Ions                                               –    timing & synchronism
     –     gas scattering                                     –    phase & gradient
•   intrabeam scattering                                      –    diagnostic errors
     –     IBS                                           •   RF drive
     –     Touschek
                                                              –    transient analysis
•   halo
     –     Formation                                     •   Operational simulations
     –     gas scattering                                     –    threading, orbit correction
     –     beam formation processes                           –    emittance measurement
•   CSR                                                       –    lattice function tuning
     –     CSR basic ("elegant")
                                                              –    longitudinal matching
     –     3-d modeling
                                                                     •   phase transfer function
     –     microbunching instabilities                               •   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 pass
    UV bypass lRF/2 longer (recovers
    bunch train)




                                       Issues:
                                           SYNCHRONISM
                                           UV seed pulse energy,
                                               up-conversion
Synchronization
GERBAL
  – Machine configuration:
                          Multiple wigglers (9.6 GeV beam)
                                     1.2 GeV
                                     Linac

                1.2 GeV ER                                     1.2 GeV
                           10 MeV              1 MW Dump       accel. ER
                4.8 GeV                                        4.8 GeV
                           Injector
                accel. ER
                6.0 GeV                                        6.0 GeV
                                                               accel.

                                      3.6 GeV
                                      Linac
  – Transverse optics

  r         r     r              r                 r       r               r
  e         e     e              e                 e       e               e
  c         c     c              c                 c       c               c
  i         i     i              i                 i       i               i
  r         r     r              r                 r       r               r
  c         c     c              c                 c       c               c




                                                                               43
                          Perspective
• “conventional”or(or – advanced– stone knivesstill–animal
• ERLs – in infancy
                  FELs perhaps not as
  Rings – very“terrible twos”…)advanced, but andvery sophisticated – to
  skins
  like cathedrals bridges             systems equivalent
  nanotechnology or rocket science




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

				
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posted:11/28/2011
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