Speaker
Description
The lifetime of the Penning ion source at iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) is strongly influenced by the discharge voltage and current, yet the mechanisms responsible for source degradation remain incompletely understood. While the discharge current is controlled during operation, the discharge voltage varies dynamically in response to the demand current and hydrogen-gas ionisation rate, making it a sensitive indicator of changes within the plasma reservoir. In particular, fast voltage fluctuations may reveal plasma instabilities that are not readily detected through current monitoring and could contribute to enhanced sputtering of plasma-facing components. To investigate these effects, a high-speed cathode-to-anode voltage monitoring system is being developed in collaboration with the ISIS Facility of the Rutherford Appleton Laboratory. The diagnostic is intended to characterise transient voltage behaviour during continuous-wave operation and to determine whether short-duration excursions exceed the energy thresholds required for proton-induced sputtering. Preliminary considerations indicate that the typical discharge voltage of approximately 130 V is below the calculated hydrogen-sputtering thresholds for molybdenum, tungsten, and tantalum. However, X-ray fluorescence measurements of a post-operation lanthanum hexaboride cathode revealed tungsten, tantalum, and molybdenum deposits, suggesting that impurity-driven processes or transient plasma events may contribute to cathode poisoning. The proposed voltage monitor will therefore provide a means of correlating discharge-voltage instabilities with source performance, sputtering, and lifetime, improving understanding of the degradation mechanisms in Penning ion sources.