Development of a modelling tool for performance optimization in pulsed plasma thrusters

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The overall aim of the project is to develop a numerical model for pulsed plasma thrusters that will allow their performanceto be optimized. This model involves several parts including, ablation of the solid Teflon propellant and subsequentionization of the resulting vapour creating a plasma, through which a discharge current flows between the two electrodes.The interaction of this current with the self-induced magnetic field produces JxB forces, which accelerate the plasma to ahigh velocity. In order to develop the numerical otimisation tool a plasma model is required and this is envisaged as themost challenging part of the modelling. The academic contribution lies in how this plasma is to be modelled and inparticular three key aspects of this modelling:1) how the current sheet attaches at the electrodes2) how the geometry of the current sheet changes as electrons tends to diffuse away3) what assumptions are made in terms of the thermodynamic state of the plasmaThe former two are closely related to the calculation of the dimensions of the current sheet whilst the latter deals with thefact that the plasma is unlikely to be in a state of equilibrium (LTE) but in a highly non-equilibrium state with the electronsfar from being Maxwellian. If one couples these three together one can arrive at the plasma resistance, which is a keyinput to the overall numerical model ( a modified snowplow model) which represents the PPT as an RLC circuit but withparameters that vary in both space and time.The novelty of the university contribution to the overall project goal of a numerical optimization tool is in the approach to theplasma modelling, in particular in allowing for a non-equilibrium distribution for the electrons, examining the currentemission of electrons from the cathode together with current attachment at the cathode and non-uniform distribution ofelectrons density in the sheet, which have never been investigated before and the effects that these will have on the overalloptimisation of the performance using the numerical tool.The first step will be to critically examine the previous plasma modelling approaches that have been published in theliterature. This will allow us to identify exactly where the gaps are and crystallize our detailed methodology. Neverthelessour current view is that the key aspects seem to lie in cathode emission and current attachment and the non-equilibriumnature of the electrons.Our approach will be to start with the simplifying assumption of a given gas mass flow from the ablating solid surface,giving us the upstream boundary and avoiding solving for the ablation of the Teflon. This then reduces the problem in effectto one of a gas fed PPT on which there has been significant fundamental research at Princeton University and allowing us to use these results. For the non-equilibrium electron distribution, we will begin with the existing drift-diffusion numericalmodel for a dielectric barrier discharge, which assumes a swarm distribution, and modify the electron distribution (initialideas include using bi-Maxwellian and/or primary plus a Maxwellian) or solving the conservation equations forconcentration and energy of electrons . For the electron emission, it will be assumed that two mechanisms are possible,field emission and ion bombardment although a third one of explosive spots will also be looked at. To estimate the currentsheet thickness, previous modelling approaches will be used and also a semi-empirical approach based onmeasurements.