Estimation of cross-field electron mobility in a water-vapor Hall thruster

The use of Hall thrusters is expanding with the rise of space industries and advanced science missions. Since the supply of the conventional propellants such as xenon is becoming unstable, alternative propellants that can be sustainably used need to be investigated. Water is one of the candidates owing to its inexhaustible presence on the earth and potential for future procurement in space. A water-vapor Hall thruster, which uses pure water vapor for the propellant, has been proposed and experimentally demonstrated. The primary issue is the efficiency loss associated with ionization and acceleration. Electron confinement within E×B field should be optimized to improve that. As a first step, it is necessary to understand the electron transport mechanisms. In this study, we estimate the cross-field electron mobility in a discharge plasma by coupling the experimentally obtained plasma distribution and steady-state fluid modeling. The electron temperature and electric field data are used in the neutral and ion fluid equations to estimate the ion velocity; then, the electron mobility is estimated using the drift-diffusion approximation. The estimated mobility suggests that the collisional events play a dominant role in electron diffusion even near the channel exit, which is a different characteristic from the conventional xenon type Hall thrusters.