Triggering of ionization oscillations in hollow cathode discharge by keeper electrode

The keeper electrode was believed to be beneficial for improving the hollow cathode performance. However, this paper shows that it can also be hazardous to the discharge if not treated properly. Interactions between the extracted electron beam and the keeper electrode were revealed in an experiment regarding the hollow cathode discharge in the magnetic field environment. It was found that when the electron beam was wider than the keeper orifice, non-negligible electron current was irreversibly lost on the cold keeper wall, inferring from the escalated floating potential of the keeper with respect to the cathode potential. This loss triggered the high-amplitude ionization oscillations in the cathode plume that were highly coherent with the potential fluctuations of keeper itself. The magnetic field that was parallel to the cathode axis could restrict electron diffusion onto the keeper surface and greatly restrain the triggering. Based on the discovered triggering mechanism, several suggestions regarding the cathode design and thruster-cathode coupling were given.