The discharge plasma in ion engine neutralizers: Numerical simulations and comparisons with laboratory data

Numerical simulations of neutralizer hollow cathodes at various operating conditions and orifice sizes are presented. The simulations were performed using a two-dimensional axisymmetric model that solves numerically an extensive system of conservation laws for the partially ionized gas in these devices. The results for the plasma are compared directly with Langmuir probe measurements. The computed keeper voltages are also compared with the observed values. Whenever model inputs and/or specific physics of the cathode discharge were uncertain or unknown additional sensitivity calculations have been performed to quantify the uncertainties. The model has also been employed to provide insight into recent ground test observations of the neutralizer cathode in NASA's evolutionary xenon thruster. It is found that a likely cause of the observed keeper voltage drop in a long duration test of the engine is cathode orifice erosion.