Electric propulsion technologies for enabling the use of molecular propellants.

The focus of this research is to explore technologies for enabling the use of molecular propellants in the field of electric propulsion. The research presented in this thesis addresses three main areas of development: plasma physics modelling of hollow cathode neutralisers, experimental investigation of a neutraliser able to operate with alternative propellants and experimental investigation of plume composition of a Hall Effect Thruster operated with molecular propellant. A zero-dimensional plasma model has been developed which overcomes some assumptions implemented in prior literature as well as incorporates a revised formulation of the terms in the equations of conservation. The model predicts the power deposited on internal walls due to electron and ion bombardment in both insert and orifice regions, enabling the estimation of the power budget and performance metrics of a hollow cathode based on a lanthanum hexaboride emitter. Conventional neutralisers rely on thermionic materials whose performance is highly degraded if not-noble gases are employed. Within the research covered in this thesis, a novel neutraliser whose working principle is based on a ExB fields discharge, rather than thermionic emission, is presented. The working principle, scaling methodology and the experimental investigation of three neutraliser configurations are here reported and discussed. An empirical model has also been implemented with the aim to predict the performance of neutralisers based on ExB fields discharges. In the past, Hall Effect Thrusters have been tested and operated with molecular gases. However, little is known about the ion plume composition and how molecular cracking due to electron impact influences the discharge and, consequently, the thruster performances. In this research, the ion plume of a Hall Effect Thruster is analysed by means of a ion velocity selector, known as Wien filter or ExB probe, which allows the estimation of the ion species ejected by the thruster. Moreover, an extension of previous methods used to analyse Wien filter spectra is proposed. The method allows the quantitatively estimation of the thruster performance when operated at various operating conditions and molecular gases. Finally, a novel diagnostic architecture in which a Wien filter is coupled with homodyne detection has been suggested and investigated