Sensitive Ion Pump Current Monitoring Using an In-House Built Ion Pump Power Supply

Ion pumps are common vacuum pumps on DC high voltage photoguns and baked-accelerator beamlines. Commercial ion pump power supplies provide a measure of the electrical current drawn by the pump, but typically have resolution to only {approx}0.1 uA, which corresponds to pressure {approx}10{sup -9} Torr, a value considerably higher than the minimum pressure required by photoguns and nearby beamline. This submission describes a very sensitive in-house-built ion pump power supply with current monitoring capability good to less than 1 nA, and corresponding pressure in the low-minus;10{sup -11} Torr range. Besides providing 'free' pressure monitoring on a scale equivalent to the best available commercial pressure gauges, the ion pump power supply also serves as a sensitive diagnostic for detecting field emission from the photogun cathode electrode and bad electron beam orbits that could diminish photogun operating lifetime. Since its inception, this ion pump power supply has become an invaluable tool for operating the CEBAF polarized electron source. It is also a very useful low-cost diagnostic for ultrahigh vacuum studies in the laboratory.