Double-layer formation and propulsive assessment for a three-species plasma expanding in a magnetic nozzle

The quasi one-dimensional expansion of a collisionless plasma with a hot-electron tail in a gentle convergent-divergent nozzle is studied. A parametric investigation of the plasma response is carried out in terms of the relative density and temperature of the hot-electron population. The formation of a steepened layer is shown to be due to the anomalous thermodynamic behavior of the plasma, which creates a local minimum of the Mach number. The change from a quasineutral to a non-neutral steepened layer occurs when this minimum goes below one and several sonic points appear. The non-neutral double layer does not introduce further changes in the plasma response. All gain in plasma momentum and thrust is related to the supersonic expansion in the divergent nozzle, with zero contribution of the double layer. A comparative analysis of thrust efficiency of plasmas with and without hot electrons does not find any gain in the presence of hot electrons; instead, a small penalty in the expansion efficiency seems to exist. The study is limited to Maxwellian electron populations and finite nozzles.