Effects of Secondary Electron Emission on the Plasma Sheath and Local Electron Energy Distribution with Application to Hall Thrusters.

Abstract

The nature of plasma transport across the magnetic field in crossed-field (CF) devices such as Hall effect thrusters (HETs) remains largely an unsolved problem. This can be further complicated by the presence of secondary electrons derived from the thrusters channel wall due to the impact of photons and electrons. The role of these secondary electrons in the operation of HETs has been a subject of investigation in recent years. Under normal operating conditions of a HET, several physical phenomena occur simultaneously and the interaction of the plasma with the channel walls of the thruster play an important role in its effective operation. These plasma wall interactions produce secondary electrons that have a non-linear coupling effect with the bulk plasma and affect the performance of crossed field devices by changing the sheath potential as well as the electron energy distribution. This influence is not yet fully understood in the community and thus the computational models are based on assumptions that are not highly accurate. Experimentally, there is little available data on the SEE yield in plasma and its effects to environments similar to that of a Hall thruster, which could be used to validate existing numerical models. A test-bed apparatus is needed to understand these effects that could serve as a tool to validate and improve existing numerical models by providing the appropriate boundary conditions, secondary yield coefficients and variation of plasma parameters to aid the future design of HETs.

In this work, a bench-top apparatus is developed to elucidate the role that secondary electrons play in regards to crossed field transport and energy flow to the walls. An electron beam which simulates energetic electrons in Hall channel is used to generate a secondary electron plume at the surface of various targets (Cu, C, BN) which simulates channel wall. The response of the plasma to these secondary electrons is assessed by measuring changes to the potential distribution in the sheath of the irradiated target and the measured electron energy distribution. An attempt is made to relate phenomena and trends observed in this work with those in Hall thrusters.