Plasma flow and plasma–wall transition in Hall thruster channel

TLDR

In Hall thruster channels, low ion temperature and strong axial acceleration make the presheath comparable to the channel width, turning the channel into an effective presheath. The study develops a model of quasineutral plasma and plasma–wall transition in a Hall thruster channel. The model couples a two‑dimensional hydrodynamic bulk plasma with a one‑dimensional collisionless sheath, incorporates secondary electron emission via an effective coefficient, and self‑consistently determines sheath edge parameters from the bulk. The model predicts that the radial ion velocity at the sheath interface increases from ~0.5 Cs near the anode to the Bohm velocity at the exit, that higher secondary electron emission lowers electron temperature peaks (16 eV for s = 0.95 versus 30 eV for s = 0.8), and that the resulting electron temperature and E×B discharge characteristics agree with experimental measurements.

Abstract

In this paper a model of the quasineutral plasma and the transition between the plasma and the dielectric wall in a Hall thruster channel is developed. The plasma is considered using a two-dimensional hydrodynamic approximation while the sheath in front of the dielectric surface is considered to be one dimensional and collisionless. The dielectric wall effect is taken into account by introducing an effective coefficient of the secondary electron emission (SEE), s. In order to develop a self-consistent model, the boundary parameters at the sheath edge (ion velocity and electric field) are obtained from the two-dimensional plasma bulk model. In the considered condition, i.e., ion temperature much smaller than that of electrons and significant ion acceleration in the axial direction, the presheath scale length becomes comparable to the channel width so that the plasma channel becomes an effective presheath. It is found that the radial ion velocity component at the plasma–sheath interface varies along the thruster channel from about 0.5Cs (Cs is the Bohm velocity) near the anode up to the Bohm velocity near the exit plane dependent on the SEE coefficient. In addition, the secondary electron emission significantly affects the electron temperature distribution along the channel. For instance in the case of s=0.95, the electron temperature peaks at about 16 eV, while in the case of s=0.8 it peaks at about 30 eV. The predicted electron temperature is close to that measured experimentally. The model predictions of the dependence of the current–voltage characteristic of the E×B discharge on the SEE coefficient are found to be consistent with experiment.