Investigation of plasma detachment mechanisms in a magnetic nozzle

Summary for only given. Electrodeless and gridless plasma thrusters are currently considered for micro propulsion applications. These thrusters, such as the Helicon thruster, are appealing because they do not require a neutralizer, and are insensitive to grid erosion. However, they use for plasma confinement and acceleration magnetic nozzles. Magnetized electrons follow the diverging magnetic stream tubes, which leads via ambipolar coupling to the plasma acceleration and radial expansion. Eventually, the plasma detaches from the magnetic field line. An important issue is then to understand the detachment of the quasi-neutral plasma from this nozzle, which affects the beam divergence and thus the thruster specific impulse. Currently, detachment analysis have been addressed either in the case of paraxial approximation, or with limiting assumptions such as local ambipolarity (j=0). Despite these limitations, these works have shown that three mechanisms contribute to plasma detachment: electron inertia, collisions with the neutral background, and distortion of the static confining magnetic field by the induced magnetic field. In this paper, we seek to have a better understanding of the relevance of these mechanisms for plasma detachment, while trying to relax some limiting assumptions found in previous work. For this purpose, a full kinetic (Particle-In-Cell) modelling of the neutral plasma plume is performed, to test inertia- and collision- induced detachment; induced magnetic field effects are not modeled in this case. Consequently, this model is supplemented by a fluid model to assess the effect of induced magnetic field. This analysis is performed for a range of plasma conditions typical of what is found in Helicon thrusters.