Electron-wall Interaction in Hall Thrusters

Abstract

Electron-wall interaction effects in Hall thrusters are studied through measurements of the plasma response to variations of the thruster channel width and the discharge voltage.The discharge voltage threshold is shown to separate two thruster regimes. Below this threshold, the electron energy gain is constant in the acceleration region and therefore, secondary electron emission (SEE) from the channel walls is insufficient to enhance electron energy losses at the channel walls. Above this voltage threshold, the maximum electron temperature saturates. This result seemingly agrees with predictions of the temperature saturation, which recent Hall thruster models explain as a transition to space charge saturated regime of the near-wall sheath. However, in the experiment, the maximum saturation temperature exceeds by almost three times the critical value estimated under the assumption of a Maxwellian electron energy distribution function.The channel narrowing, which should also enhance electron-wall collisions, causes unexpectedly larger changes of the plasma potential distribution than does the increase of the electron temperature with the discharge voltage. An enhanced anomalous crossed field mobility (near-wall or Bohm-type) is suggested by a hydrodynamic model as an explanation to the reduced electric field measured inside a narrow channel. We found, however, no experimental evidence of a coupling between the electron temperature and the location of the accelerating voltage drop, which might have been expected due to the SEE-induced near-wall conductivity.2

Measurements of Secondary Electron Emission Effects in the Hall Thruster Discharge

“…10,22 The detail analysis of the thruster V-I characteristics is given in Ref. 20. We found that the effect of the channel wall material on the discharge current is stronger than that on the ion flux from the thruster (Fig.…”

“…10,19,20 The 2 kW laboratory Hall thruster 10 was operated with two different channels. One channel was made entirely of boron nitride (grade HP), which is a ceramic material with high SEE yield.…”

“…For a flat carbon, the SEE yield approaches unity at an order of magnitude higher energy of primary electrons than that for boron nitride. 5 In the described experiments two segmented electrodes 18,22 made of low-sputtering carbon velvet material 20 were placed on the inner and outer channel walls. The lengths of the inner and outer electrodes are 4 mm and 6 mm, respectively.…”

“…Field emission from carbon fibers of the segmented electrodes was shown to have a minor effect on the operation of the segmented thruster. 20 The experiments took place in a 28 m 3 vacuum vessel equipped with cryogenic pumps. In each configuration, the thruster was operated at a constant xenon mass flow rate of about 2 mg/s.…”

Particle‐in‐Cell Simulation for the Effect of Segmented Electrodes Near the Exit of an Aton‐Type Hall Thruster on Ion Focusing Acceleration

“…As the SEE yield of a conductive wall is generally much lower than that of a dielectric wall, especially, the SEE yield of a metal wall can be negligible [9,12], segmented electrodes are proposed as a promising way to control the plasma flow and enhance thruster performance [13][14][15]. It has been demonstrated that segmented electrodes with several millimeters in length placed near the thruster exit can remarkably increase electron temperature, reduce discharge current, change electric potential distribution and narrow plasma plume at low gas flow rate (j a 2mg/s) [12,16]. Although the effects of low-emissive segmented electrodes have been well documented, the reason of the plume widening is not fully understood.…”