Plasma plume diagnostics of a LaB₆ hollow cathode with triple Langmuir probes

High-power Hall Effect thrusters (HETs) represent an attractive propulsion option for the next-generation manned and robotic missions. The main challenging feature of these missions is a lifetime in the order of 10⁴ hours required to the propulsion system. One of the most critical components of a HET is the cathode, used to emit electrons to sustain the discharge and to neutralize the ion beam. The main life-limiting factor of hollow cathodes designed for HETs is the material wear due to ion bombardment by the discharge plasma. The aim of this paper is to present the characterization of the electrical parameters and plasma plume properties of a hollow cathode. The effect of the cathode orifice geometry was also investigated, in terms of different length-to-diameter aspect-ratio (AR). Two ARs were analyzed to assess the effect of the orifice geometry on the plasma parameters and onset of unstable modes. The cathode analyzed, named HC20, is a hollow cathode developed for a mid-range power HET under development at SITAEL. The cathode was designed to provide discharge currents between 5 and 30 A, already characterized during previous experimental campaigns. The measurements in the plasma plume of the cathode were performed with triple Langmuir probes, one of which capable of scanning the plume along the cathode axis. The results show that a larger orifice AR can shift the transition to an unstable mode of operation of the cathode, named plume mode, at lower mass flow rate, increasing the cathode operative range.