Anode Fall Formation in a Hall Thruster

Abstract

As was reported in our previous work, accurate, non-disturbing near-anode measurements of the plasma density, electron temperature, and plasma potential performed with biased and emissive probes allowed the first experimental identification of both electron-repelling (negative anode fall) and electron-attracting (positive anode fall) anode sheaths in Hall thrusters. An interesting new phenomenon revealed by the probe measurements is that the anode fall changes from positive to negative upon removal of the dielectric coating, which appears on the anode surface during the course of Hall thruster operation. As reported in the present work, EDS analysis of the chemical composition of the anode dielectric coating indicates that the coating layer consists essentially of an oxide of the anode material (stainless steel). However, it is still unclear how oxygen gets into the thruster channel. Most importantly, possible mechanisms of anode fall formation in a Hall thruster with a clean and a coated anodes are analyzed in this work; practical implication of understanding the general structure of the electron-attracting anode sheath in the case of a coated anode is also discussed. I. Introduction N a gas discharge, there can be either an increase or a drop in the plasma potential toward the anode, generally referred to in the literature as the “anode fall”. When the anode is at a higher potential than the near-anode plasma, the anode fall is called “positive”, and when it is at a lower potential – “negative”. The positive and negative anode falls are essentially associated with formation of the electron-attracting and electron-repelling anode sheaths, respectively. The anode sheath is a thin space-charge layer adjoint to the electrode. It is a non-linear structure that was first observed and studied by Langmuir and Mott-Smith in glow discharges. 1