Characterization of the background pressure effects on high power Hall thrusters

Nowadays, the Hall Thruster technology has experienced an increased use for spacecrafts propulsion and attitude control. This opened the market to larger payloads in space and also to new kinds space exploration and trasportation missions. Indeed, the recent developments in solar panel technology together with power distribution allowed to increase the available power on board latest spacecrafts. As a consequence, the use of high power Hall thruster is increasingly expected for high-powered platforms. However, in order to develop and qualify such propulsion systems, large vacuum facilities, with satisfactory levels of vacuum, are needed. The aim of this thesis is to characterize the effect of the background pressure on the behaviour of high power Hall Thrusters and assess which physical parameters change due to pressure. In the present work, we start by introducing a simplified performance model that also consider the ingested mass flow rate coming from the residual neutrals inside chamber, after that, we performed a sensitivity analysis and isolated the terms that are affected by the change in the pumping speed of the chamber. It turns out that the change in performance can be due to a ingestion of the neutrals together with the change in the plasma proprieties. Moreover, the two most common characterization strategies for Hall thrusters have been formalized and presented, relating them to the background pressure problem. In parallel, a simple one dimensional model model, summarizing the neutral dynamics at steady state inside the vacuum chamber, has been introduced and commented. The model, coming from literature, has been reformulated and takes into account the pumps positions and the main dimensions of the vacuum chamber. It is then possible to estimate the neutrals particle density in the proximity of the thruster exit plane and to calculate the ingested mass flow rates levels. After the introduction of the test items and plasma diagnotics, we performed and exstensive experimental campaign on different 20 kW class Hall thruster models, all featuring the magnetic shielding of the ceramic channel. The results of the campaign showed that the measured performance levels were mainly related to a change of the physics of the plasma discharge, these aspects are better studied in the last part of the work. The conclusive part of the work consist of the analysis of the plasma discharge measurements on a 5 kW class Hall thruster operating at the same operating point in two different vacuum chambers. We first estimated the values of the electron mobility on the two cases and then we used the gathered data to calibrate a non-stationary one dimensional Hall thruster plasma discharge model. The results of this work highlight that, in order to have a predictive tool to estrapolate the performance in space, the future research effort should be focused on a better study and uderstanding of the change of the electron mobility as a function of the neutrals surrounding environment.