Paper VT-WeM4
Working toward XHV: Characterization and Improvements of the Vacuum System for GaAs Photoemission Electron Sources

Wednesday, October 20, 2010, 9:00 am, Room Laguna

The operational lifetime of a DC electron source using GaAs photocathode material is limited primarily by the system vacuum; the residual gasses ionized by the electron beam are accelerated into the photocathode where they cause damage and limit photocathode yield. Though we operate in the deep-UHV range, improvements to the vacuum should increase lifetime for today's electron sources, and is essential for proposed future accelerators needing considerably higher average current. This talk describes our efforts to improve vacuum in the Jefferson Lab polarized electron source, including efforts to characterize NEG and ion pumps in the deep-UHV range, carefully determine the x-ray limit of our Leybold extractor gauges, and quantify the reduction in outgassing from stainless steel chambers after a long 400°C heat treatment. The goal of these studies is to determine which factors primarily limit our ultimate pressure, to find ways to lower the ultimate pressure for future electron sources, and to quantify these improvements.