Anomalous conductivity and secondary electron emission in Hall effect thrusters

Abstract

This paper is devoted to the study of the effects of electron-wall interactions on cross magnetic field electron momentum and energy losses in Hall effect thrusters. By coupling a semianalytical model of the wall sheath similar to models used by several authors in this context, with a two-dimensional hybrid simulation of a Hall effect thruster, we find that the cross magnetic field conductivity enhanced by electron-wall collisions and secondary electron emission is not sufficient to explain the conductivity deduced from experiments. Calculated current-voltage curves including electron-wall collisions from a standard sheath model as the sole “anomalous” conductivity mechanism do not reproduce the measurements, especially at high discharge voltages, and for various wall ceramics. Results also show that a one-dimensional description of electron-wall collisions with a constant radial plasma density profile as used by many authors leads to an overestimation of the contribution of electron-wall interactions to cross magnetic field conductivity.

Power dependence of the electron mobility profile in a Hall thruster

“…This is a problem common to most Hall thruster models and stems from the fact that the mechanism for electron transport is anomalous and not well-understood. [11][12][13][14][15][16][17][18] In order to accurately simulate performance, it is necessary to specify the magnitude of the electron anomalous collision frequency as a function of position in the thruster. Since this is not known a priori, the correct profile can only be arrived at by iterating on possible solutions in the code and attempting to match the simulation to measured plasma parameters and thruster performance.…”

Low frequency azimuthal stability of the ionization region of the Hall thruster discharge. I. Local analysis

“…12 Currently, there is no agreement within the Hall thruster community about the mechanism of the anomalous diffusion, but the most accepted explanations are two: plasma oscillations, referred to as Bohm-type or turbulent diffusion, based on the fact that correlated azimuthal oscillations of density and electric field can induce a net axial electron current; 13,14 and near-wall conductivity, where secondary electrons emitted by the walls can induce a net axial current. 15 However, near-wall conductivity does not seem to explain the anomalous diffusion since many simulation codes [16][17][18] that include a near-wall conductivity model still need a Bohm-type diffusion contribution to match the electron conductivity measured experimentally. On the other hand, several experiments have confirmed with various techniques the presence of azimuthal oscillations.…”

Device convolution effects on the collective scattering signal of the E × B mode from Hall thruster experiments: 2D dispersion relation

“…It may be due to wall conductivity 7,12 or to micro-turbulence. 1,6,10,18 Micro-turbulence at the millimeter scale has been observed in PIC code simulations, 1,3 predicted by linear kinetic theory 5 and observed experimentally by the collective light scattering technique.…”