Alternative Propellants for Hall Thrusters

Hall thrusters are devices that rely on a complex combination of plasma phenomena to ionize and accelerate the propellant to produce thrust. Since the early applications in the 60s, xenon has been the propellant of choice, given its good ionization properties, which reduce the plasma generation losses, and its high atomic mass, which leads to higher thrust-to-power ratios. Additionally, it permitted acceptable storage densities and easy mass flow metering. However, its scarcity and highly variable price have led to increase the interest in finding alternative elements to be used as propellants. The present dissertation is aimed at studying the topic of propellants alternative to xenon. The first part presents a comprehensive analysis of the different aspects that may be affected by the use of a different propellant, and how the physical-chemical properties of a given element can modify the behaviour of the thruster. A summary of the desired properties for an ideal propellant is introduced. The second part is dedicated to the use of iodine in low power thrusters. A simplified 1-D model of the plasma within the channel is introduced to compare the ionisation process of the different propellants. The model takes into account the chemistry of the different elements, in particular the chemical reactions of iodine, as well as interactions with the walls. The model allowed observing that the anode temperature has an important impact on the behaviour of an iodine-fed Hall thruster, as the dissociation degree of the injected propellant is highly influenced by it, with consequences in the thrust-to-power ratio, as a large proportion of molecular ions can be present if the anode is colder. Following on, the development of an iodine feeding system is presented. A thermal-fluid model of the gas generation, and flow control and metering process is introduced to guide the design. Test using air as a working gas permitted to verify the thermal throttle model and allowed to better understand the flow along the capillary pipe. An experimental characterisation of the iodine feeding system prototype is presented and its results are used to calibrate the model. The third part is dedicated to the study of krypton as an alternative propellant for high power magnetically shielded Hall thrusters. Several magnetic configurations and operative points were analysed. The plasma properties within the channel and near-plume were measured using a fast triple Langmuir probe, and the near-wall plasma was characterised using flush-mounted single Langmuir probes. Data analysis methodologies, based on the Bayesian approach were developed to infer the plasma properties profiles. It was observed that magnetically shielded topologies are less effective when krypton is the working fluid, probably because of the lower electron-neutral collisionality in the near-anode region. The plasma probe measurements were combined with global measurements (current and thrust) to perform a rough estimate of the electron cross-field mobility. The mobility results were later compared to theoretical values. Bohm diffusivity theory showed poor agreement, while the values predicted assuming mobility is induced by an electron cyclotron drift instability showed better qualitative (and in some cases quantitative) agreement, both for xenon and krypton. Krypton presented higher cross-field mobility when compared to xenon. An analysis of the current oscillations showed that progressive shielding of the thruster reduces the amplitude of the breathing mode, while the frequency remains almost constant.