Effects of electron spatial inertia and magnetic field gradient on transport across magnetic filter

Abstract

This work investigates, via particle-in-cell with Monte Carlo collision simulations, electron transport across magnetic filter (MF) in obstructed-drift plasma sources. The studied source consists of three stages, namely: the ionization stage where power is coupled to the plasma, the magnetic filter stage where the magnetic field strength is maximal and impedes electron transport, and the diffusion stage where most of the charged particles come from the first stage. The plasma source is schematized by a square whose walls (sides) are dielectric, except for the wall at the exhaust plane which is metallic and simulates a biased screen grid. As expected, our results show that as the plasma passes through the MF, electron density and temperature decrease. The electron flux and fluid velocity spatial distributions present an S-shaped path through which these plasma properties are significant. These high flux and fluid velocity are due to the superimposition of E×B and diamagnetic drifts that are enhanced by the walls. Moreover, at the entrance and exhaust of this S-shaped path, i.e., in the vicinity of the walls, the magnitude of the spatial inertia is significant in comparison to acceleration induced by an electric field and pressure gradient. The analytical analysis of this inertia shows that its magnitude and direction are mainly controlled by the gradients of the electric field, pressure force, electron density, and magnetic field strength. Therefore, the control of the spatial inertia, and thus, of electron flux across the magnetic filter, can be achieved by controlling the spatial distribution of the magnetic field strength.