Hall-Effect Thruster-Cathode Coupling, Part II: Ion Beam and Near-Field Plume

This is the second part of a two-part paper in which the effect of cathode position and magnetic field configuration on Hall-effect thruster performance is explored. The effect magnetic field topology has on the coupling between a Hall-effect thruster and its cathode has been studied. With two Hall-effect thruster configurations, each with a different external field topology, the cathode is positioned across a range of radial distances and the effect on performance, ion beam, and near-field plasma is investigated. The importance of the magnetic field separatrix, a surface which divides the magnetic field lines into internal and external regions, is shown. In particular, total efficiency improvements of up to seven percentage points are seen when placing the cathode near the separatrix as opposed to locations further away of the thruster. Analysis of the thruster telemetry, ion beam current distribution, and ion energy distribution functions properties show the effect of the cathode position on the efficiency loss mechanisms of beam divergence, current utilization, ion velocity distribution, voltage utilization, and cathode coupling. Measurements of near-field plasma potential, electron temperature, and electron density provide help to explain why these efficiency improvements come about and add insight into the cathode coupling processes. As the cathode is moved radially away from the thruster up to 250 mm, the cathode coupling efficiency decreases by up to 10 percentage points. Furthermore, the near-field plasma potential increases by up to 30 V, and this is correlated with a decrease in beam divergence efficiency of up to 15 percentage points.