Plasma discharge modeling of a Hall-effect thruster

Space technology applications are increasingly present and essential in our everyday lives. While satellite launch costs are currently falling sharply thanks to the development of low-cost reusable launchers, and new applications are emerging from the new space elan, the costs of using and operating satellites must be drastically reduced. This can be achieved, among other things, by using electric propulsion systems that are more efficient than their traditional chemical equivalents. Indeed, thanks to a higher gas exhaust speed, these thrusters offer a much better efficiency. Although many electric space propulsion systems exist, the present work concentrates on one type of thruster: the Hall effect thruster, also known as Hall current thruster or stationary plasma thruster. This type of thruster is one of the most popular and successful thrusters available today. However, these propulsion systems remain complex and relatively little understood. Indeed, some key questions remain open, whether about anomalous electron transport or plasma-wall interactions. The answers to these two questions are based on kinetic mechanisms, and therefore cannot be studied through fluid simulation codes. Moreover, the characteristic time and space scales of these mechanisms make them difficult to experimentally and exhaustively study. Thus, in order to answer these questions and complete the experimental work carried out so far, a two-dimensional kinetic simulation code has been developed from scratch. The development of this simulation code has been characterized by a particular attention given to the reliability of the results thanks to the use of well documented test cases. In addition, this simulation tool has been designed to make efficient use of the computational powers available today. This effort has led to a massively parallelized architecture of the IT tool, allowing numerous parametric studies. Thanks to the use of a simplified Hall effect thruster model, we were able to study the different physical phenomena taking place in the plasma discharge of a thruster. Based on the recent results of a kinetic theory developed specifically to describe the plasma discharge of Hall effect thrusters and the abnormal transport of electrons, it has been possible to highlight the importance of the electron drift instability concerning the abnormal transport of electrons in the discharge channel. Then, by refining the model used to simulate plasma discharge in a more realistic way, it was possible to quantify the contribution of the walls to the anomalous transport of electrons. The effect of the walls on the plasma discharge and its behavior could be confirmed, and its involvement in the electron drift instability was highlighted. Thanks to an exhaustive parametric study, the impact of the emissivity from the walls on the plasma discharge has been quantified, allowing the identification of three distinct discharge regimes. Finally, numerous parametric studies have been carried out on the impact of dielectric walls, secondary emission models, and alternative propellants. These parametric studies were made possible by the flexibility and performance of the simulation tool developed during this work. Thanks to realistic models (autonomously and coherently modeled electrical models, collision processes derived from experimental measurements), these studies confirmed the previous results. In addition, the various parametric studies conducted have led to the development of a global model allowing for a better understanding of the operation of plasma discharge in a Hall effect thruster. Thanks to this global model, we have been able to quantify more precisely the effects of the emissivity of the walls, as well as the mass of the propellant used, on the discharge and the performance of the thruster. However, limitations are present in the simulations conducted during this work. While these limitations do not allow for the actual modeling of a Hall effect thruster, since this tool is not used as an aid to the design of thrusters, they do not prevent the study of physical mechanisms at work. On the contrary, they open up a new field of possibilities for future studies and simulations. (author)