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“Direct” Injection. D. Douglas, C. Tennant, P. Evtushenko JLab. Acknowledgements. Initial funding provided by ONR Recent work supported by AES under JTO funding

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direct injection

“Direct” Injection

D. Douglas, C. Tennant, P. Evtushenko

JLab

acknowledgements
Acknowledgements
  • Initial funding provided by ONR
  • Recent work supported by AES under JTO funding
  • Initial simulations (sanity check!), useful feedback provided by John Lewellen, discussions with Steve Benson, operational help from Kevin Jordan
direct off axis injection

linac centerline

0.15 m

current sheet or field clamp

“Direct” (off-axis) Injection
  • Rather than merge beams using DC magnetic fields, inject beam into linac at large amplitude and use RF focusing & adiabatic damping to bring orbit into line
  • Can use reverse process for extraction of energy-recovered beam

injected beam

0.075 m

accelerated and recovered beams in linac

recirculated beam, reinjected for energy recovery

direct injection extraction
Direct Injection/Extraction

cross-sectional view of both passes of beam (first = blue, second = pink) looking down linac from injection to dump

issues solutions
Issues & Solutions
  • Concerns
    • Possible emittance dilution from finite phase extent of bunch in RF fields (thanks to Steve Benson for pointing this out…)
    • Potential for HOM excitation/BBU instability
  • Approach
    • Estimates & analysis (emittance, BBU)
    • Simulation (PARMELA, GPT)
    • Beam studies on JLab Upgrade Driver
head tail rf driven emittance dilution
Head-Tail RF-Driven Emittance Dilution
  • Reviewed head-tail issue
    • assumed beam was 8 degrees long (6s, head to tail) (~Jlab injected length)
  • Simulated RF steering of injected beam with simple cavity matrix model

Results:

  • Propagated beam envelopes vary only slightly
  • Differential steering not dramatic
slide8
head/tail (orbit) centroid move ~ ±0.2 mm in position, ±30 microrad in angle.
  • compare to the beam size – for 5 mm-mrad normalized emittance at 100 MeV, with 10 m beta:
    • sx ~ sqrt(be)=sqrt(10*5e-6/(100/0.51099906)) ~0.5 mm
    • sx’ ~sqrt(e/b)=(5e-6/10/(100/0.51099906)) ~50 mrad
  • with stated assumptions about the bunch length get ~ ± ½ sigma motion – over the full (6s) bunch length

Conclusion: emittance dilution may not be too bad; look at more carefully…

detailed study
Detailed Study
  • Performed as part of JTO-funded AES merger study
  • Three part investigation
    • More careful analytic estimates
    • Simulations with space charge
    • Beam study on Jlab IR Upgrade

Conclusions:

emittance growth very modest; tolerable for IR systems

BBU thresholds unaffected; additional power goes into HOM loads

Several cm pass-to-pass possible

results theory simulation
Results – Theory/Simulation
  • Estimates  emittance growth negligible for IR FELs
  • Emittance growth negligible in simulation
    • Beam quality not degraded
  • Analysis  BBU threshold independent of injection offset
    • C. Tennant, JLAB-TN-07-011
  • Power into HOMs depends on injection offset

GPT simulation of beam size in single-module linac (C. Tennant)

slide11

Bunches Traveling Through Linac: Animation

Injected on-axis

Injected 10 mm off-axis

C. Tennant and D. Douglas | July 24, 2008

machine study
Machine Study
  • Measured impact of injection offsets on beam quality in JLab IR Upgrade
    • Aperture limited to ~1 cm offsets
    • Able to run CW @ 1 cm  BBU tests possible
    • Tested at nominal (9 MeV) and low (5 MeV) injection energy

Conclusion: No observable impact on beam quality; BBU-related measurements underway

machine study method
Machine Study: Method
  • Measure injected emittance (multislit)
  • Quad scan emittance measurement after linac
    • On axis & several displacements
  • Tomography in recirculator
  • BBU – look at power into HOMs in 7-cell module
steering
Steering
  • “off-axis” emittance tests: steer off into 1st module, grab at end of module where RF focusing bring (nearly ) to node (no offset downstream)
  • “BBU” tests: steer off into linac, resteer in recirculator to maintain 2nd pass transmission

note path-length/phase/energy effects in arc…

machine study results
Machine Study: Results
  • Transversal beam sizes and profiles largely independent of injected orbit over ±1 cm offsets in H and V
    • Machine drift much higher impact than orbit offset
  • Initial data analysis of emittance data  emittance unaffected by steering (to resolution of measurement)
    • Working through error propagation
  • BBU:
    • set up CW configuration, acquired initial signals, whereupon machine crashed (refrigerator trip);
    • lost rest of run to LCW line break before follow-on shifts
    • will schedule more study time over the summer
beam profile at end of linac
Beam Profile At End of Linac

x=-10 mm x=0 mm x=+10 mm

y=-10 mm y=0 mm y=+10 mm

(some scraping) profile measurement by P. Evtushenko & K. Jordan

transverse emittance 5 mev injection
Transverse Emittance (5 MeV injection)
  • Measured with 3 methods:
    • “multislit” in injector
    • quad scan at end of linac
    • tomography in recirculatorbackleg
  • Results generally consistent and roughly match values w/ full energy injection
emittance data @ 5 mev injection
Emittance Data @ 5 MeV Injection

Quad scan: ~ 12-15 mm-mrad

Multislit: ~ 13 mm-mrad

Multislit courtesy P. Evtushenko

Tomography: ~ 10 mm-mrad

reconstructed phase space

beam spot

tomography

courtesy C. Tennant

direct injection @ 5 mev
“Direct” Injection @ 5 MeV
  • Test of “merger-less” merger
  • Low-loss operation with large (~ cm) injection offsets
  • Beam behavior ~independent of injection orbit
conclusions
Conclusions
  • Direct injection provides possible alternative to traditional merger
  • Beam quality requirements are key
    • likely appropriate for IR systems,
    • may not be quantitatively appropriate for, e.g. shorter wavelength applications
  • Lower frequency better (i.e. “easier”, more available aperture!)
  • Few-several cm separations possible
  • Still need to evaluate emittance data (error analysis) and measure HOM power deposition