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Experience with GaAs Cathodes at BNL

Experience with GaAs Cathodes at BNL. Triveni Rao Gatling Gun Review June 28, 2012. Outline. Objective Description of system 1 Results from system 1 Description of system 2 Results from system 2 Summary and Future Plans. Objective.

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Experience with GaAs Cathodes at BNL

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  1. Experience with GaAs Cathodes at BNL Triveni Rao Gatling Gun Review June 28, 2012

  2. Outline • Objective • Description of system 1 • Results from system 1 • Description of system 2 • Results from system 2 • Summary and Future Plans

  3. Objective • Establish 10-11-10-12 Torrvacuum in fabrication chamber • Fabricate GaAs cathodes • Optimize fabrication process • Cs source • Oxygen source • yo-yo vs co-deposition • Cleaning temperature • QE, surface roughness, elemental properties • Basic Physics studies • Effect of high intensity, high charge, high current on QE and polarization

  4. System 1- Erdong Wang System 2- Riehn and Rahman

  5. System 1 • Built to test GaAs in SRF gun • Load-lock transfer system • GaAs cleaned mostly by radiant heat 10” diameter sphere, baked to 400 C Pumped by 270 l/s ion pump and 1000 l/s TSP RGA, Gauge, Cs, O2, anode Base pressure of 2x 10-11 Torr Stalk heater temp 720 C, cathode temp 630 C Max

  6. Cathode: P doped (~ 3x 1018 Zn) GaAs • Laser: 543 nm, ~ 300 µm diameter on cathode • Anode Bias: 50 V • Detector: Picoammeter • Source: • Alvatec-pure Cs, too high evaporation, can not be exposed to air once opened, possible indium contamination • SAES Getter-Cs2CrO4 with ZrAl, activate to release Cs, typical activation current 4A, temp. 550 C • Activation: yo-yo and saturation

  7. Long life time at 2 A/cm2 current density • yo-yo: • Relatively simple • Longer time • Lower Life time • Saturation: • Shorter activation time • Longer Life time • Sensitive to oxygen flow rate

  8. System 2 Stalk heater similar to Jlab, Substrate temperature up to 640 C Ion pump and TSP Base pressure < 6x10-12Torr Quick turn around time (~1 wk/sample)

  9. Comparison of Oxygen Sources- High Purity Oxygen Bottle, Manganate in Ag Boat Bottle chosen due to availability

  10. Typical cathode activation • Sample to sample variation in QE • QE(T) Normalized to QE (560 C) does not depend on T • Variation in QE for 780 nm is indicative of spot to spot variation of QE

  11. SEM pictures: Sample heated to 610 C At the Boundary Inside the boundary

  12. Area inside the boundary is affected differently from that outside the boundary Elemental analysis of the features and surface profile with AFM to be done soon Samples heated from 560 C to 600 C to be analyzed soon Outside the boundary

  13. Summary and Future Plans Summary • Built two XHV systems • Third under construction • Fabricated bulk GaAs cathodes • Optimized the activation process • Started Basic science experiments Future Plans • Build prep chamber compatible with Gatling Gun • Fabricate cathodes for polarized electrons • Conduct systematic basic science studies: Changes in surface morphology, QE, Polarization using polarimeter, SEM, AFM, x-ray beams • Study performance in Gatling Gun

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