Two Dimensional Measurement of Electron Energy Distribution Function in a Water-vapor Miniature Hall Thruster

The increasing use of small spacecraft in observation and exploration missions has led to an expansion in the application of miniature Hall thrusters. While xenon has traditionally been used as a propellant, there is growing interest in alternatives that offer greater availability and safety. Water has attracted attention as a promising candidate due to its abundance and ease of handling. Experimental studies have been conducted on miniature Hall thrusters operating with water vapor. Nevertheless, water is more difficult to ionize due to its smaller ionization cross section compared to xenon. Furthermore, the increase in neutral particle velocity may promote electron diffusion and cause changes in the potential structure. Previous studies have measured the electron energy probability function (EEPF) along the center axis of the discharge channel, providing insights into ionization processes and electron transport. To better understand ion acceleration region and channel wall losses, it is essential to investigate the radial distribution of the EEPF especially near the channel exit where E × B drift dominates electron transport. In this study, the two-dimensional distribution of EEPF at the region is measured using a Langmuir probe, and the local plasma parameters are evaluated.