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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 l.jpg

“Direct” Injection

D. Douglas, C. Tennant, P. Evtushenko


Acknowledgements l.jpg

  • 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 l.jpg

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 l.jpg
Direct Injection/Extraction

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

Issues solutions l.jpg
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 l.jpg
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


  • Propagated beam envelopes vary only slightly

  • Differential steering not dramatic

Slide8 l.jpg

  • 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 l.jpg
Detailed Study microrad in angle.

  • 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


      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 l.jpg
Results – Theory/Simulation microrad in angle.

  • 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 l.jpg

Bunches Traveling Through Linac: Animation microrad in angle.

Injected on-axis

Injected 10 mm off-axis

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

Machine study l.jpg
Machine Study microrad in angle.

  • 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 l.jpg
Machine Study: Method microrad in angle.

  • 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 l.jpg
Steering microrad in angle.

  • “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 l.jpg
Machine Study: Results microrad in angle.

  • 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 l.jpg
Beam Profile At End of microrad in angle. 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 l.jpg
Transverse Emittance (5 MeV injection) microrad in angle.

  • 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 l.jpg
Emittance Data @ 5 MeV Injection microrad in angle.

Quad scan: ~ 12-15 mm-mrad

Multislit: ~ 13 mm-mrad

Multislit courtesy P. Evtushenko

Tomography: ~ 10 mm-mrad

reconstructed phase space

beam spot


courtesy C. Tennant

Direct injection @ 5 mev l.jpg
“Direct” Injection @ 5 MeV microrad in angle.

  • Test of “merger-less” merger

  • Low-loss operation with large (~ cm) injection offsets

  • Beam behavior ~independent of injection orbit

Conclusions l.jpg
Conclusions microrad in angle.

  • 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