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Vacuum research at the Thomas Jefferson National Accelerator Facility has focused on characterizing the vacuum system of polarized photoelectron guns and on quantifying the effects of vacuum improvements for new guns including electropolishing, vacuum firing and NEG coating the chamber walls as well as studying optimal NEG activation protocol. Pressure in both the original and improved photoguns, as measured by extractor gauges corrected for hydrogen, is near 1x10-11 Torr, which is significantly higher than calculations would suggest based on measured pump speed and outgassing rates. The ultimate test of vacuum in a DC photogun is the photocathode operational lifetime, where ionized residual gas between the cathode and anode electrodes degrades the photocathode yield. This talk will investigate correlations between expected gun vacuum conditions and photocathode lifetimes. |