Transient antenna-plasma interaction in inductive plasma thrusters

Abstract

An electric thruster using material sourced from beyond Earth as propellant would unlock new possibilities for the exploration and exploitation of space. Compared to other technologies under consideration, inductive plasma thrusters are a promising candidate due to their high thrust density and proven propellant flexibility. In contrast with conventional space propulsion technologies, inductive coupling can deliver energy to a propellant without direct contact, making it a form of electrodeless electric propulsion. This can increase its lifetime and enable a more flexible propellant choice in terms of composition. A significant hurdle to the development and eventual implementation of inductive plasma thrusters is the lack of detailed information on the discharge behaviour and diagnostic tools that can be applied for various power and propellant operating conditions. These two aspects are critical to advance the understanding of underlying phenomena and hence the potential for both optimisation and in-flight health monitoring of future thrusters. Previous work has identified transient antenna current behaviour within an inductive plasma thruster that points to complex antenna-plasma interactions, including coupling mode transitions. This thesis documents investigations into these phenomena in order to assess, quantify, explain and exploit them. Firstly, it is shown that these phenomena are observable under a range of conditions and are strongly linked to coupling mode transitions. Secondly, the effect is quantified and its relationship with the overall propulsive performance is examined. Thirdly, a model is developed that adequately explains and illuminates the phenomena. Finally, these phenomena are exploited to develop a new diagnostic tool for investigating transient effects based on antenna current measurements. The new diagnostic tool is non-intrusive in nature since it relies on antenna current measurements, making it highly suitable for use in flight to aid monitoring and control of the thruster. It is especially compatible with a propellant-flexible thruster because it can be applied to a wide range of propellants. It can be implemented with a sufficiently high sampling frequency to resolve transient effects and it can be operated in real time to support flight hardware. The combination of diagnostics, modelling approaches and generated results form a powerful resource to understand transient antenna-plasma interactions within a highpower and propellant-flexible inductive plasma thruster. These results are based on existing experimental data that includes various mixtures of oxygen, carbon dioxide, nitrogen and argon at flow rates from 1.66 to 3.74 g/s and input powers from 0.9 to 154.5 kW. Overall, this thesis documents and explains transient antenna-plasma interactions within an inductive plasma thruster. This amounts to the generation of a significant related dataset, associated knowledge and a new diagnostic tool to support the design, development, testing and operation of inductive plasma thrusters.