This paper investigates iodine as an alternative propellant for space plasma propulsion. Measurements are taken in a low-pressure inductively-coupled plasma chamber used as the ionisation stage of a gridded ion-engine. Langmuir probes are used to measure the electron density and the electron energy distribution functions spatial variations between the inductive coil and the extraction grids for several radiofrequency (RF) powers and mass flow rates. Measurements in iodine are compared to xenon, krypton and argon in order to evaluate performances of these various propellants for ionization (and therefore power) efficiency. At low mass flow rates, iodine is found to be the most efficient propellant, however, as the mass flow rate increases, the ionization cost in iodine increases rapidly due to both its molecular and electronegative nature. The ratio of negative ion to electron density is measured using laser-induced photodetachment in order to quantify the effect of iodine electronegativity. Finally, all measurements are compared to a previously published global (volume-averaged) model. The agreement between model and experiments is acceptable, however several modelling improvements are proposed.

A model for electrons in partially ionized plasmas that self-consistently captures non-Maxwellian electron energy distribution function (EEDF) effects is presented. The model is based on the solution of scalar and vectorial velocity moments up to the contracted fourth-order moment. The set of fluid (macroscopic) equations is obtained with Grad's method and exact expressions for the collision production terms are derived, considering the electron–electron, electron–gas, and electron–ion elastic collisions as well as for electron–gas excitation and ionization collisions. A regularization of the equations is proposed in order to avoid spurious discontinuities, existing in the original Grad's moment model, by using a generalized Chapman–Enskog expansion that exploits the disparity of mass between the electrons and the heavy particles (ions and atoms) as well as the disparity of plasma and gas densities, typical of gas discharges. The transport model includes non-local effects due to spatial gradients in the EEDF as well as the impact of the EEDF in the calculation of the elastic and inelastic collision rates. Solutions of the moment model under spatially homogeneous conditions are compared to direct simulation Monte Carlo and a two-term Boltzmann solver under conditions that are representative of high plasma density discharges at low-pressure. The moment model is able to self-consistently capture the evolution of the EEDF, in good quantitative agreement with the kinetic solutions. The calculation of transport coefficients and collision rates of an argon plasma in thermal non-equilibrium under the effect of an electric field is in good agreement with the solutions of a two-term Boltzmann solver, largely improving models with a simplified Bhatnagar–Gross–Krook collisional operator.