Two-dimensional supersonic plasma acceleration in a magnetic nozzle

A two-dimensional model of the expansion of a collisionless, electron-magnetized, low-beta, current-free plasma in a divergent magnetic nozzle is presented. The plasma response is investigated in terms of the nozzle/plasma divergence rate, the magnetic strength on ions, and the Hall current at the nozzle throat. Axial acceleration profiles agree well with those estimated from simple one-dimensional models. A strong radial nonuniformity develops downstream. There is a separation between ion and electron/magnetic streamtubes which leads to the formation of, first, a longitudinal electric current density, which indicates that current ambipolarity is not fulfilled, and, second, a small ion azimuthal current that competes negatively with the electron azimuthal (Hall) current. The analysis of the mechanisms driving thrust, ion momentum, and ion energy unveils the dual electrothermal/electromagnetic character of the magnetic nozzle. In general, the thrust includes the contributions of volumetric and surface Hall currents, this last one formed at the plasma-vacuum interface. Plume efficiency, based on radial expansion losses, is computed. Plasma detachment and the transonic matching with the upstream plasma are not addressed.