Breakdown Oscillations of the ISIS H− Penning Ion-Source Hydrogen–Cesium Discharge

Abstract

The ISIS Penning ion source is a pulsed dc discharge surface plasma source delivering a 55-mA beam of negative hydrogen ions (H−) in 250- ${ mu }\text{s}$ pulses to the ISIS accelerator for neutron and muon physics at 50-Hz pulse repetition rate. The H− ions are extracted from the hydrogen–cesium discharge sustained by a current-regulated pulsed power supply delivering a current of 50–60 A. The discharge exhibits an ignition transient that requires the pulselength to be prolonged to 700 ${ mu }\text{s}$ to extract a clean H− beam pulse. The current and voltage fluctuations during the breakdown oscillation enhance the sputtering damage of the ion-source cathodes, thus contributing to the lifetime limitation of the device. It has been hypothesized that the breakdown oscillations are due to cesium dynamics causing the surface work function and electron emission from the cathode surfaces to fluctuate periodically. Experiments with the ISIS H− Penning ion source, reported here, reveal that the breakdown oscillations are primarily driven by the discharge power supply and its auxiliary circuits, whereas cesium dynamics have only a minor effect on the ignition transient. The conclusion is derived from careful characterization of the discharge current–voltage characteristics, optical emission measurements combined with a 0-D model, and comparison of different discharge geometries. Engineering solutions to mitigate the breakdown oscillations are presented and the potential effect of the applied modifications on the molybdenum cathode erosion by cesium sputtering is estimated.