Design and development of a fast-response ionization gauge controller

Fast pressure measurements in the divertor and pump limiter chambers used in magnetic confinement fusion experiments require a system that can respond to a broad range of pressures while maintaining good noise immunity. Available designs of gauge controllers were unable to provide a acceptable noise immunity. A fast ionization gauge controller that can respond to pressure changes in the range from 1 x 10-1 to 2 x 10-5 Torr in less than 10 ms using a Schultz-Phelps gauge assembly has been developed. The Schultz-Phelps gauge provides full coverage of the desired pressure range and is less susceptible to magnetic field interference than other available types of ionization gauges. The gauge controller as designed provides simultaneous readout of three decades of pressure. Major design concerns have been to minimize noise induced by the gauge tube filament drive and noise pickup by the ionization current electrometer. By using DC filament drive, a major shortcoming of available gauge controllers, i.e., 60-Hz pickup caused by filament modulation of the electron current, has been overcome. Second, by locating the electrometer amplifier as close to the Schultz-Phelps gauge as possible, cable-induced noise pickup has been reduced to an acceptable level. Problems encountered in the design and test of the gauge controller and future improvements are discussed. 3 refs., 4 figs