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The good news is that Isabel let me come here, but…

Lifetime Measurements using the JLab Load-Lock Gun Joe Grames September 22-26, 2003 Polarized Sources and Targets Workshop Novosibirsk, Russia. Preface. The good news is that Isabel let me come here, but…. the bad news is that she kept most of my talk. Outline.

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The good news is that Isabel let me come here, but…

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  1. Lifetime Measurements using theJLab Load-Lock GunJoe GramesSeptember 22-26, 2003Polarized Sources and Targets WorkshopNovosibirsk, Russia

  2. Preface The good news is that Isabel let me come here, but… the bad news is that she kept most of my talk.

  3. Outline • Lifetime studies: purpose and parameters • Load-Lock Gun Testbed • Lifetime vs. vacuum level • vs. anodization • vs. laser spot location • 4. Trends from Data (preliminary) • 5. Planned Improvements • 6. Conclusions

  4. Lifetime Studies Our job at JLab is to provide the nuclear physics program with 200 mA CW of polarized electrons (this amounts to 5-20 C/day). The electron gun photocathode quantum efficiency degrades with the amount of charge extracted. If the cathode QE were very high to begin with, or laser power were unlimited the degradation would be tolerable, but neither are so. Strong evidence indicates the QE degradation results due to ion back-bombardment. The purpose of these studies is to quantify (and qualify) the manner by which the QE degrades while operating the gun.

  5. Experimental Test Bed:100 kV Load-Lock Electron Gun

  6. Load-Lock Diagnostic Beamline Cathode: Bulk GaAs w/ Cs-NF3 Laser: 780 nm diode, DC 500 mm spot size Beam: 150-180 mA @ 100keV Optics: Copy of CEBAF injector

  7. Leak Fix/Bake NEG’s w/ IP NEG’s w/ Turbo >15.0 4.9 2.3 1.9 (x10-11 Torr) Versus: Vacuum, Anodization, Beam Location Unanodized (11 mm) Anodized (5mm)

  8. Versus Vacuum ~2.5E-11 Torr ~5.0E-11 Torr >15.0E-11 Torr

  9. Versus Spot Location 2.4 mm 1.6 mm 0.8 mm Center

  10. Location #1 Location #2 Location #3 Versus Anodization 5mm (red) anodized 11 mm (green) unanodized

  11. Versus Something Not Understood… Data Slow Lifetime Fit Fast Lifetime Fit

  12. Analysis Plans Include QE surface and ion-pump nA-meter data to improve interpretation.

  13. Improvements to Load Lock Gun Cathode Handling Improve transfer between chambers (dropped, mis-aligned holder) Provide capability to store multiple cathodes (one at a time) Load Chamber Increase ion pump capacity to 20+ L/s (11 L/s capacity too small) Replace H cleaner (inefficient SS resonator, large H plumbing volume) Add N2 vent port toTurbo pump (bleeds to atmosphere) Prep Chamber Add heating/cooling (now done in less clean load chamber) Ground cathode holder (cathode can float during activation) High Voltage Chamber Improve handling and between chambers (dropped, mis-aligned) Provide capability to store multiple cathodes (one at a time)

  14. Conclusions We are performing a series of measurements to study photocathode lifetime vs. operating parameters in our 100 kV load-lock gun and beamline. We have “learned” and added the diagnostics to now make a good series of measurements at typical HV chamber vacuum level. Results of the measurements will be summarized this Fall and prepared for publication. Improvements to the load-lock gun will begin shortly after returning to JLab. The product of that work is planned to be installed in the accelerator in summer, 2004.

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