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DC photogun vacuum characterization through photocathode lifetime studies

DC photogun vacuum characterization through photocathode lifetime studies. Marcy L. Stutzman, Philip Adderley, Joseph Grames, Matthew Poelker, Ken Surles-Law Thomas Jefferson National Accelerator Facility Newport News, Virginia, USA. Jefferson Lab. A. C. B.

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DC photogun vacuum characterization through photocathode lifetime studies

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  1. DC photogun vacuum characterization through photocathode lifetime studies Marcy L. Stutzman, Philip Adderley, Joseph Grames, Matthew Poelker, Ken Surles-Law Thomas Jefferson National Accelerator Facility Newport News, Virginia, USA

  2. Jefferson Lab A C B • CEBAF: Nuclear physics electron accelerator laboratory • 100 kV DC polarized electron gun • Recirculating linear accelerators • Three experimental halls running simultaneously • User community of 2000+ physicists • Operates 310 days/year, 24 hours/day • FEL program at JLab

  3. GaAs Photoelectron gun 100 kV 200 μA 85% polarization delivers beam simultaneously to three experimental halls CEBAF pressures ~1.2x10-11 Torr Guns pumped with combination of NEG and ion pumps Existing CEBAF photoguns • Gun chambers vented and baked each time new photocathode installed • Activation chemicals in HV chamber • Full NEG activation may harm photocathode

  4. Vacuum Characterizations Outgassing Rate Ultimate Pressure • Ultimate Pressure discrepancy • Outgassing rates • Accumulation with spinning rotor gauge • throughputmethod • good agreement • Untreated 304 SS as expected • EP/vacuum fire (900ºC, 4 hrs) • slightly better • fewer bakes • fits to guide the eye • Outgassing rate does not explain ultimate pressure discrepancy

  5. Pump speed measurement Ti/Zr/V coating ST707 Getter • Hydrogen Pump speed measurements • SAES getter ST707 • WP950 modules • Throughput method • High speed >5x10-11 Torr • Activation protocol increases pump speed • Decrease at low pressures • Pressure measurement? • Artifact of measurement? • Homemade NEG coating • Higher pressures • Same behavior • Still no explanation for ultimate pressure discrepancy

  6. Photocathode lifetime to diagnose vacuum • GaAs photocathode activated with cesium and NF3 or O2 • Quantum efficiency (QE) falls during operation • Electrons out per unit photons in • Mechanism for QE decay: • Surface chemistry corruption • Crystal damage • Both mechanisms dependent on vacuum laser light IN electron beam OUT anode residual gas cathode

  7. Load-locked future of JLab gun • Successfully run program so far with “vent and bake” gun • Upcoming experiments with very high current (300 μA) will require frequent cathode maintenance • Load-locked designs allow • Faster cathode change • Separation of surface chemistry from • High voltage electrodes • High voltage (HV) chamber vacuum • Allow full activation of NEG cartridges in HV chamber

  8. 304 Stainless Steel Electropolished, not vacuum fired Cartridge getter pumps on 8 inch ports Conductance limitations 220 L/s ion pump Does fast pump speed imply good ultimate pressure? Photocathode heating in relatively poor vacuum of load chamber Prototype CEBAF LL gun

  9. New CEBAF load-locked gun Heat/activation chamber • To be installed July, 2007 • High voltage chamber • 304 SS • Electropolished • vacuum fired • NEG coated • Smaller surface area • NEG modules surround cathode/anode gap • Full NEG activation possible Suitcase HV chamber Small bake Load region Additional Benefits • Photocathode activation through mask • Four photocathodes can be exchanged in 12 hours

  10. Vacuum measurements • Significant design improvement • Extractor gauge readings no better • Gauge characterization (x-ray limit) • Gauge utilization (electrometer, cables) New LL Tunnel prototype LL

  11. Photocathode lifetime improvement • Lifetime improves with better vacuum design • factor >2 • Damage analysis: TEM, SIMS • Improve vacuum design regarding most damaging species Bulk GaAs, green light Ref: J. Grames et. al SPIN 2006 Kyoto

  12. High Polarization Superlattice Lifetime • Tunnel: Lifetimes at high polarization typically ~100 Coulombs running ~100 μA • New LL gun: Lifetimes over 200 Coulombs at 250 μA or 1 mA on test beamline • We anticipate (hope for!) improved lifetime during operation with new load lock new load lock operational gun Reference: J. Grames et. al PAC’07 proceedings

  13. Conclusions • Vacuum improvements incorporated during construction of New LL gun • Little improvement seen in ultimate pressure measurement • Significant improvement in photocathode lifetime • Future work • Better materials • 316 LN electropolished, then baked at 400ºC for ~1 week - JLab FEL • Ion pump limitations? • Anticipate positive operational experience with new LL gun in CEBAF machine • Future machines: ILC, EIC, UV FELs…

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