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Lifetime Studies

Lifetime Measurements using the Jefferson Lab (JLAB) Load-Lock Gun J. Grames , P. Adderley, M. Baylac, J. Brittian, D. Charles, J. Clark, J. Hansknecht, M. Poelker, M. Stutzman, K. Surles-Law October 7-9, 2004 PESP 2004 Mainz, Germany. Lifetime Studies.

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Lifetime Studies

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  1. Lifetime Measurements using theJefferson Lab (JLAB) Load-Lock GunJ. Grames, P. Adderley, M. Baylac, J. Brittian, D. Charles,J. Clark, J. Hansknecht, M. Poelker, M. Stutzman, K. Surles-LawOctober 7-9, 2004PESP 2004Mainz, Germany

  2. Lifetime Studies Our job at JLAB is to provide up to 200 mA CW of polarized electrons (~20 C/day). Future photoinjectors will require 10’s of milliAmps of polarized electrons. Photocathode quantum efficiency (QE) degrades with gun operation, correlated with the charge evolved. If QE were initially exceptionally high or laser power were unlimited then degradation of the QE might be tolerable, but neither are practically so. Additionally, non-uniform QE across the illuminated surface may be intolerable for some parity violation experiments.

  3. Presentation Outline A 100 keV load lock gun and beam line have been installed for the studies of QE degradation as a function of gun operation. • Part 1 (Summer-Fall ‘03) • Part 2 (Spring-Summer ’04) • Gun/Beamline Commissioning • QE Degradation Trends • vs. vacuum level • vs. laser spot location • vs. active area • Lifetime Rates • Mask vs. Anodization • Improved vacuum monitoring • Higher current studies • Model construction

  4. 100 keV Load-Lock Electron Gun High Voltage Chamber Load-Lock Chamber Preparation Chamber

  5. 100 keV Beam line for Lifetime Studies Summer-Fall ‘03 Spring-Summer ‘04 780 nm diode, 50 mW, 500 mm Commissioned to 350 mA 780 nm diode, 50 mW, 500 mm 532 nm Nd:YVO4, 11 W, 1000 mm Commissioned to 8 mA

  6. 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)

  7. QE Degradation vs. Gun Pressure (Extractor Gauge) ~2.5E-11 Torr ~5.0E-11 Torr >15.0E-11 Torr

  8. 2.4 mm 1.6 mm 0.8 mm Center QE Degradation Trend vs. Laser Spot Location

  9. Location #1 Location #2 Location #3 QE Degradation Trend vs. Anodization Size 5mm (red) anodized 11 mm (green) unanodized

  10. Lifetime vs. Time (fast & slow process ?) Data Lifetime(s) = QE falls by 1/e Slow Lifetime Fit Fast Lifetime Fit

  11. Part 2: Mask Activation & Vacuum Monitoring A mask to limit activation surface (as demonstrated at Mainz) was installed to the prep chamber (5, 7, 9 mm). Improved UHV on-line vacuum monitoring using nAmp holding ion pump power supplies. Manipulator mis-alignment has resulted in dropped “pucks” and detours…

  12. Bin Samples (~5 min) Lifetime vs. Time (9 mm mask) Goal: Develop self-consistent model.

  13. Lifetime vs. Intensity (7 mm & 9 mm mask) Threshold ? 7 mm 9 mm Beam Off 7 mm 9 mm

  14. Lifetime vs. Laser Spot Location (7 mm & 9 mm mask) Cathode Radius Electrostatic Center 7 mm (’04) 9 mm (’04) 7 mm (’03) Mask Radius Lifetime dependence on spot location can be sensitive to optics.

  15. QE: 300 V vs. 100 kV QE at 300 V 4.15 mA QE at 100 kV (2500,3500)

  16. Transient & Steady State Details 0.1 C 59 C 0.01 C 4 C 19 C 8 C

  17. Summary We are using a 100 keV load-lock gun and beamline to study photocathode lifetime vs. gun operating parameters. Our experimental setup evolves often. Measurements consistently indicate that improved vacuum and limited active area enhance photocathode lifetime. Measurements indicate that photocathode lifetime as a function of laser spot location may depend upon a greater set of parameters, such as beam line optics and laser spot size. In the coming months emphasis will be placed to develop a quantitative lifetime model to better describe collected data and design future measurements.

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