Experimental characterisation of the External Discharge Plasma Thruster

Hall thrusters have high thrust-to-power ratios and robust designs. However, plasma losses to the discharge channel walls can reduce their efficiency and lifespan at low power levels. To solve this problem, concepts have relocated the ionisation and acceleration regions away from the walls. The External Discharge Plasma Thruster (XPT) has been proposed as a solution to this problem by relocating these regions outside the cavity, potentially extending the lifetime of Hall thrusters and decreasing the mass of the thruster head. Previous research has shown the XPT has a low mass utilisation efficiency and a large plume divergence. This work investigates the XPT operated on xenon and krypton to identify plasma properties and ion beam characteristics using invasive and non-invasive diagnostic tools.

Optical Emission Spectroscopy (OES) is a non-invasive diagnostic tool used to estimate plasma properties such as electron temperature and density via the use of collisional-radiative models. However, the estimation is prone to uncertainty due to intrinsic factors, and the measurement is non-local. Langmuir probes are used to measure electron temperature, plasma density and potential at different near-field locations. To measure the multiply charged ions and map the ion current density in the XPT, a Wien filter and a Faraday probe are used respectively. These diagnostics provide a comprehensive understanding of the plasma composition and propulsive performance, enabling design improvements and performance optimisation.