Plasma flow and plasma–wall transition in Hall thruster channel

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 ͑SEE͒, s. In order to develop a self-consistent model, the boundary parameters at the sheath edge ͑ion velocity and electric field͒ 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 0.5C s ͑C s is the Bohm velocity͒ 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 sϭ0.95, the electron temperature peaks at about 16 eV, while in the case of sϭ0.8 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 EϫB discharge on the SEE coefficient are found to be consistent with experiment.

Space charge saturated sheath regime and electron temperature saturation in Hall thrusters

“…In the commonly accepted model of electron-wall interaction, the electron energy loss at the walls for the Maxwellian electrons can be expressed as 8,9 ( ) (…”

“…1,3 A general description of the secondary electron emission (SEE) effects 1,2 on the plasma flow has been recently included in fluid models of Hall thrusters. [8][9][10][11] A Hall thruster is a plasma discharge device with crossed fields, which is used for spacecraft propulsion. In a Hall thruster, the axial electric and radial magnetic fields are applied in an annular ceramic channel.…”

Measurements of secondary electron emission effects in the Hall thruster discharge

“…In this brief communication we demonstrate how the maximum electron temperature depends on SEE properties of the channel wall material. Although the effects of the wall material on the thruster discharge are well documented in the literature, 5,9,17,18 this work presents new experimental results that show how SEE affects the electron temperature in the bulk plasma of the thruster discharge.…”

“…At this critical point the fluxes of secondary electrons from the wall and primary electrons from the plasma are approximately equal, and the wall acts as an extremely effective electron energy sink. The SCS sheath has been the subject of many studies due to its relevance to plasma applications such as, for example, fusion devices, 2 emissive walls, 3 dusty plasmas, 4 and Hall thrusters [5][6][7][8][9][10][11] (so-called stationary plasma thrusters or SPTs).…”

Operation of a segmented Hall thruster with low-sputtering carbon-velvet electrodes

“…6,8 Without SEE, electron energy losses at the walls are lower and contribution of electron-wall collisions to the electron-crossed field mobility (so-called near-wall conductivity 1 ) is negligible. [9][10][11] For a constant discharge voltage, the accelerating voltage drop inside the channel with segmented electrodes is increased, 6 which may lead to erosion of the electrodes due to ion-induced sputtering. In addition, ion-induced sputtering of the electrodes causes the backflow of contamination, which produces a conductive coating on the ceramic part of the thruster channel.…”