Analysis of collisional and facility effects in a magnetic nozzle plasma expansion

Abstract

An axisymmetric, quasineutral three-fluid model is proposed to study the plasma expansion in a magnetic nozzle under the presence of neutrals coming either from the plasma source or as an homogeneous background. As a difference with other models, electron cooling in the plume is achieved by treating the electron energy flux as mainly convective and without the need to postulate any anomalous resistivity. Solutions are presented for the electron high-magnetization limit, in which the electron main magnitudes can be integrated along magnetic lines. Ionization, elastic and charge-exchange collisions with neutrals do not change the main qualitative features of the plasma expansion, known from previous collisionless models. Ionization enhances the plasma flow in the nozzle, and leads to additional electron cooling, which decreases the electric potential fall along the nozzle. The efficiency of the nozzle is quantified in terms of the gain of magnetic thrust and the plume divergence angle. Two types of boundary conditions are discussed for the electron flow: local current ambipolarity conditions at the nozzle throat and global current-free conditions at the outer boundary (i.e., metallic vacuum chamber walls). These last ones are shown to be physically more reliable: they introduce the influence of the chamber walls on the plasma expansion by shaping the ambipolar electric field; they permit the extrapolation to undisturbed free space conditions; and they approximate better experimental trends with the background pressure.