Three-dimensional structure of electron density fluctuations in the Hall thruster plasma: E¯×B¯ mode

Abstract

Collective scattering measurements have been conducted on the plasma of a Hall thruster, in which the electron density fluctuations are fully characterized by the dynamic form factor. The dynamic form factor amplitude distribution has been measured depending on the k-vector spatial and frequency components at different locations. Fluctuations are seen as propagating waves. The largest amplitude mode propagates nearly along the cross-field direction but at a phase velocity that is much smaller than the E¯×B¯ drift velocity. Refined directional analysis of this largest amplitude mode shows a thin angular emission diagram with a mean direction that is not strictly along the E¯×B¯ direction but at small angles near it. The deviation is oriented toward the anode in the (E¯,E¯×B¯) plane and toward the exterior of the thruster channel in the (B¯,E¯×B¯) plane. The density fluctuation rate is on the order of 1%. These experimentally determined directional fluctuation characteristics are discussed with regard to the linear kinetic theory model and particle-in-cell simulation results.