Experimental investigation of electron-impact reactions in the plasma discharge of a water-vapor Hall thruster

The axial distribution of the electron density and temperature in the discharge plasma of a water-vapor Hall thruster is measured by a cylindrical Langmuir probe using a fast-moving system. They are evaluated in two ways; by integrating full trace of the measured electron energy distribution function (EEDF) and by fitting a Maxwellian EEDF to the low-energy part of the measured EEDF. The ion current density for each species produced by electron-impact ionization is estimated using the obtained electron density and temperature. It is revealed that OH+ and H+ can be contained other than H2O+ with non-negligible fractions, causing an additional 20%–25% loss of the anode efficiency estimated by the plume diagnostics. It is also suggested that elastic momentum transfer, electronic excitation, and dissociation to the neutral fragments are more dominant processes than ionization, whereas the electron attachment, rotational excitation and vibrational excitation are minor events among the electron-impact reactions. The subsequent ionization processes from the neutral fragments are also expected to play an important role in determining the effective design and operating parameters for further improvement of the mass utilization efficiency.