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Electron Driver Capabilities and Proposed JLab Test Facility

Electron Driver Capabilities and Proposed JLab Test Facility. Reza Kazimi JLab. Outline. Existing CEBAF injector capabilities and limitations. JLAB FEL Injector Injector Test Facility * Thanks to Carlos Hernandez, Joe Grames, and Matt Poelker for discussions and some of the slides.

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Electron Driver Capabilities and Proposed JLab Test Facility

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  1. Electron Driver Capabilities and Proposed JLab Test Facility Reza Kazimi JLab

  2. Outline • Existing CEBAF injector capabilities and limitations. • JLAB FEL Injector • Injector Test Facility * Thanks to Carlos Hernandez, Joe Grames, and Matt Poelker for discussions and some of the slides

  3. INJECTOR

  4. Layout of the CEBAF Injector

  5. Measured Bunch length Along the Injector

  6. CEBAF Injector Beam Parameters

  7. What limits the high current? • High average current More RF power needed for acceleration • High average current Less Cathode life • High charge/bunch High space charge beam blow up in transverse and longitudinal

  8. INJECTOR The JLab FEL is an Energy Recovery Linac, Fourth Generation Light Source.

  9. Electron bunches are generated when the GaAs photocathode is illuminated by pulses of green light from a drive laser

  10. The JLab FEL is driven by a 350 kV DC GaAs electron gun

  11. JLab FEL photo-injector operational performance: • 135pC bunch charge • 9 MeV/c • Measured Normalized transverse emittance ε=8 π mm mrad • Average current up to 9 mA • Measured rms bunch length 3.4 ps • Measured rms energy spread 18 keV • Longitudinal Emittance 61 ps-keV • Photocathode lifetime operating at 5 mA CW and 135pC bunch charge is about 550 Coulombs or 50 hours per re-cesiation

  12. Challenges for High Current electron driver • Longer Cathode life time: 1 mA beam requires 86 C/day 10 mA, 860 C/day • Higher voltage electron gun. • Need shorter distance between the gun and first acceleration

  13. Injector Test Facility

  14. What does Test Cave have? • Room: 70’ long,12’ wide 10’-12’ high • Thick concrete walls for shielding

  15. Other resources at Test Cave • Some of the other resources at test Cave • LCW, e.g., cooling the Faraday cup or magnets • Compressed air, e.g., pneumatic viewers • Compressed LN2 room temperature boil-off, e.g., vacuum work • 240V electrical power, e.g., for heater power supplies • 120V electrical power, e.g., wall power items • laser room interlock w/ magnetic locks, keycode access, • Interlock to fire system • PSS room interlock (for low energy operations) • two CARMS • two RF waveguide feedthroughs • high voltage shed and cable for 500 kV feedthrough • laser clean room • two trim card magnet racks and cabling • two iocs, support for CAMC • it's own fiefdom (server), ITS • storage cage • basic shop

  16. Lifetime versus Laser Spot Size • Imperfect vacuum limits photocathode lifetime - damage from ion backbombardment • Can we increase operating lifetime by merely increasing the laser spot size? Same number electrons, same number ions, but distributed over larger area. • Exceptionally high charge lifetime, >1000C at beam current to 10mA! • Lifetime scales with laser spot size but simple scaling not valid. Factor 10 instead of factor 20.

  17. Experimental Setup Faraday Cup Laser (1 W @ 532 nm) High Voltage (100 kV) NEG pipe Activation (Cs/NF3, 5 mm) Spot Size Adjustment 350 mm 1500 mm Load lock (GaAs on puck)

  18. 2 1500 Expectation: ≈ 18 350 5 15 SMALL vs. LARGE Laser Spot (BP vs. LL)

  19. Side View • High Voltage Chamber • “Side ceramic” design • load chamber at ground potential • No moving parts at HV • Activation Chamber • Mini-stalk heater • Mask selects active area • UHV IP supplies gauge activation • Keyed & eared pucks Load Locked Gun

  20. A possible next generation gun design • Will be based on CEBAF/Cornell load-lock systems • Will explore CEBAF’s new approach of inverted insulator… Picture courtesy of Matt Poelker

  21. Compact Injector • The aim is to put together an accelerator which produces ~10 MeV, few mA CW electron beam. • Among many other uses, it could be used as a driver for the positron production.

  22. 10 MeV Teststand (option 1) 100/350 keV 10 MeV 1 mA 10MeV ¼ Cryo Diagnostics: Spectrometer, Mott, FC, … Photo-Cathode Gun Buncher Warm cavity

  23. 10 MeV Teststand (option 2) 10 MeV 100/350 keV 1 mA 10MeV Small Chicane ¼ Cryo Diagnostics: Spectrometer, Mott, FC, … Photo-Cathode Gun Buncher Warm cavity

  24. Standard five cell cavity

  25. Modified five cell cavity for low energy entrance

  26. Summary • The CEBAF injector optics has been tested during the G0 operation up to 1.4 pC/bunch which is ~0.7 mA @ 0.5 GHz and 2.1 mA @ 1.5 GHz • Jlab FEL injector has operated up to 135 pC/bunch, 9 mA average current at 9.1 MeV/c. • 1 mA polarized beam with 200 Coulomb cathode life time has been achieved in the test cave. • 10 mA unpolarized beam with life time of thousands of Coulomb has been achieved in the test cave. • Inverted gun design can provide higher voltage gun for CEBAF load locked system. • A Compact injector could provide an independent driver for positron. • Production and transport of 1 mA polarized beam in CEBAF machine needs to be demonstrated before it could be used for Positron production.

  27. CHL-2 Production at 11 GeV Convert 1.1 GeV Electron to Positron Compact High Current Electron Injector

  28. CHL-2 12 GeV CEBAF

  29. The End

  30. Synchronous Photoinjection

  31. Photocathode lifetime operating at 5 mA CW and 135pC bunch charge is about 550 Coulombs or 50 hours per re-cesiation

  32. The quantum efficiency drops during average current operation when the electron beam ionizes residual gas in the gun vacuum chamber.

  33. Wafer 25 mm dia Active area 16 mm dia Drive laser 8 mm dia A single GaAs wafer delivered over 7000 Coulombs and over 900 hours of CW beam at currents ranging from 1 to 8 mA. This wafer was activated into a photocathode a total of 9 times in 36 months of operation with an average of 6 re-cesiations per activation. Front-end view of the GaAs photocathode being illuminated by the drive laser while delivering 5 mA of CW electron beam

  34. Incoming drive laser beam Anode/mirror plate 12 inches The FEL and the GTS guns are identical in design and dimensions except for two features • The anode plate in the GTS gun is used as a mirror for reflecting off the drive laser and illuminating the photocathode at a 40 degree angle.

  35. Three Proposed plans

  36. Layout of the Injector Make ~10 MeV Electrons Convert to Positrons Insert Before the Modules

  37. Layout of the Injector Make ~10 MeV Electrons Accelerate to ~65 MeV Convert to Positrons 500 keV dump

  38. 10 MeV Teststand 10 MeV 100/350 keV 1 mA 10MeV Small Chicane ¼ Cryo Diagnostics: Spectrometer, Mott, FC, … Photo-Cathode Gun Buncher Warm cavity

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