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

Marcy Stutzman IVC-17 Jefferson Lab CEBAF: Nuclear physics electron accelerator laboratory 100 kV DC polarized electron gun Recirculating linear accelerators Three experimental halls running simultaneously User community of physicists Operates 310 days/year, 24 hours/day FEL program at JLab A B C

Marcy Stutzman IVC-17 Existing CEBAF photoguns GaAs Photoelectron gun –100 kV –200 μA –85% polarization –delivers beam simultaneously to three experimental halls CEBAF pressures ~1.2x Torr Guns pumped with combination of NEG and ion pumps Gun chambers vented and baked each time new photocathode installed Activation chemicals in HV chamber Full NEG activation may harm photocathode

Marcy Stutzman IVC-17 Vacuum Characterizations 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 Ultimate Pressure Outgassing Rate

Marcy Stutzman IVC-17 Pump speed measurement Hydrogen Pump speed measurements –SAES getter ST707 –WP950 modules Throughput method High speed >5x 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 ST707 Getter Ti/Zr/V coating

Marcy Stutzman IVC-17 Photocathode lifetime to diagnose vacuum GaAs photocathode activated with cesium and NF 3 or O 2 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 residual gas cathode electron beam OUT anode laser light IN

Marcy Stutzman IVC-17 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

Marcy Stutzman IVC-17 Prototype CEBAF LL gun 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

Marcy Stutzman IVC-17 New CEBAF load-locked gun 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 Small bake Load region HV chamber Heat/activation chamber Additional Benefits Photocathode activation through mask Four photocathodes can be exchanged in 12 hours

Marcy Stutzman IVC-17 Vacuum measurements Significant design improvement Extractor gauge readings no better –Gauge characterization (x-ray limit) –Gauge utilization (electrometer, cables) New LL Tunnel prototype LL

Marcy Stutzman IVC-17 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

Marcy Stutzman IVC-17 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 operational gun new load lock Reference: J. Grames et. al PAC’07 proceedings

Marcy Stutzman IVC-17 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…