One-dimensional model of a Hall effect thruster and anomalous electron transport

Hall effect thrusters (HETs) are electrostatic devices which exploit the reduction of the local cross-field electron transport due to an externally applied magnetic field, in order to properly shape the electrostatic accelerating potential. Although great efforts have been dedicated to the understanding of the physics behind the operation of these devices, an accepted description of the mechanism behind cross-field electron transport across a magnetic field has yet to emerge. Experimental results show a cross-field electron mobility higher than the one predicted by the classical collisional theory. This higher than expected electron mobility is related to the so called anomalous diffusion mechanism. In literature plasma oscillations have been identified as the most likely explanation for anomalous electron transport. Even if advancements have been made in understanding the mechanisms responsible for the anomalous transport, the latter is still an unanswered question affecting the development of accurate models. Actually, numerical simulations of HET plasma discharge typically depend on the adoption of semi-empirical models of the mobility and usually require a calibration with experimental data. First, in order to investigate the role of anomalous diffusion in HETs, an overview of classical electron transport theory and of the anomalous transport problem is presented, together with an analysis of experimental and theoretical research on plasma oscillations in HETs. Then, a one-dimensional model of the HET plasma discharge was developed. The model, which includes the effects of plasma-wall interactions, neutral dynamic and plume expansion, provides a reliable description of the plasma proprieties inside the channel. A sensitivity analysis of the effects on the plasma dynamic of electron pressure and anode ion velocity was performed, allowing for a reduction of the model complexity and for a significant simplification of the numerical integration procedure. The model was validated using experimental results of the Alta HT-5k and a first attempt to relate anomalous mobility and thruster operating condition was made. Furthermore, a remarkable agreement between computed and experimental thruster performance was observed over a wide range of operating conditions