Plasma induced erosion phenomena in a Hall thruster

Specific features of Hall thrusters, which are advanced electric propulsion devices for space applications, in terms of thrust, specific impulse, efficiency and lifetime make them suitable candidates for missions like geo-stationary satellite orbit correction and station keeping. A large research program devoted to the investigation of physical processes connected to the working of Hall thrusters has been carried out in France for many years. One of the current research topic concerns the erosion phenomena induced by the created plasma. Indeed, erosion of the ceramic insulators of a Hall thruster is always considered as a critical point as the main limiting factor for the thruster lifetime. It is clear that a better understanding as well as a better monitoring of wear processes of the insulators is necessary in order to employ Hall thrusters for long term missions like interplanetary journeys. By comparing experiments and numerical outcomes, it appears that present models describing erosion mechanisms, solely based on ion bombardment, do not explain the overall losses of insulator material and the periodic pattern observed at the channel outlet. The peculiar wear of the insulators could find its origin in the so-called "anomalous erosion" phenomenon. It is most likely that electrons play a major role in such an erosion process, however, the way they act and the deep reason for the formation of a network of longitudinal streaks are still to be clarified. In terms of erosion monitoring, to observe the light emitted by atoms sputtered from insulators appears to be a promising way of real-time controlling the wear of the ceramic walls.