Influence of the magnetic field configuration on the performance of Helicon thrusters

The Helicon thruster (HT) is an innovative concept of electric propulsion system designed for in-space applications. The thruster relies on a Radio frequency antenna that effectively ionizes the propellant and excites helicon waves in the plasma. The antenna is thus able to heat the plasma, which then expands through a divergent magnetic field. Since no moving components or electrodes are needed to accelerate the propellant, the HT design significantly improves the thruster lifetime with respect to other electric propulsion systems. However, a low power efficiency was measured during the experimental campaigns carried out on HT prototypes. The present Thesis investigates the influence of the magnetic field configuration on the HT performance and analyses technological solutions to significantly increase the efficiency of a low-power HT. The typical thruster configuration comprises two different region: the plasma source (where the plasma is generated) and the magnetic nozzle (where the internal energy of the flow is transformed into kinetic energy). In order to reduce the power losses to the rear surface of the plasma source, in the present Thesis an axial magnetic shield is added to the standard magnetic configuration. An accurate description of the plasma behaviour is presented for each of the three region of the Helicon Thruster. The model equations, which assume that the plasma is ideal, perfectly magnetized and non-inductive, are then numerically solved. Scaling laws of an ideal Helicon Thruster are derived to analyze the effects of the magnetic field intensity. The power efficiency of the Helicon Thruster is finally evaluated as a function of the magnetic field configuration. The performed numerical investigations show that the magnetic shield improves the thruster performance, offering the possibility to reach a power efficiency higher than 50%