Plasma formation and cross-field electron transport induced by azimuthal neutral inhomogeneity in an anode layer Hall thruster

Abstract

The fluctuation of the azimuthal electric field caused by the electron drift instability or the rotating spokes in the E×B plasma is known to enhance the electron cross field transport. The increased electron current, observed during the operation of a Hall thruster with a nonuniform propellant supply in azimuthal direction, also appears to be related to the azimuthal electric field. In this paper, we experimentally investigate how neutral inhomogeneity in azimuth affects the plasma structure formation, and how this self-organized structure influences the electron cross field transport. We observed an axial-azimuthally varying space potential structure, which results in an alleviated effective axial potential profile and induces the azimuthal electric field. From the distributions of the light emission intensity and plasma density, we show that the azimuthal profile of plasma is skewed in the direction of Ez×Br drift of the magnetized electrons and that the spatial scale of the structure matches the spatial scale of the input neutral variation. The plasma structure reveals that the axial electron drift transport from the induced azimuthal electric field is the most dominant factor due to its equivalent mobility 1/B, which is two orders of magnitude greater than the classical collisional mobility perpendicular to the magnetic field. This indicates that neutrals contribute to the electron cross field transport not only directly through the elastic collisions but also indirectly through their influence on the formation of the plasma structure, which enhances the cross field transport. Lastly, we show that the effective electron mobility rides on 1/16Br line when deviating from the classical mobility line and that the effective Hall parameter is greatly reduced where Eθ develops.