Characterization of a Hollow Cathode Plasma with Magnetic Fields

The relationship between the magnetic field strength and the plasma parameters of a hollow cathode was investigated experimentally. The ion energies, discharge currents, and ion currents were measured under a magnetic field, ranging from 0 mT to 12 mT of the magnetic field strength. We evaluated the ion energies using the most probable ion potentials Vmp from the ion energy distribution functions. The experimental results show that the Vmp increased with increasing magnetic field strength for plume mode conditions, but was nearly constant for spot mode conditions. Therefore, in plume mode conditions, the erosion of the hollow cathode may increase progressively with increasing magnetic field strength due to the increased ion energy. The discharge current oscillations for the plume mode were minimized at a magnetic field strength of 1.5 mT, but rose monotonically for the spot mode. In this study, we evaluated potential fluctuations using the fluctuations of ion saturation currents. Through ion saturation current measurements, we found that the potential fluctuations increased above 1.5 mT, and the feature of the potential fluctuation spectrum is different between the plume mode and spot mode. We also discuss the effect of potential fluctuations on Vmp and the discharge current oscillation.