Diamagnetism and neutrals depletion in a plasma

Abstract

Recent experimental and theoretical findings [Shinohara et al., Phys. Plasmas 23, 122108 (2016)] regarding the pressure balance between a cylindrical plasma, an axial magnetic field, and neutral gas are explored further theoretically. The length of the cylinder is assumed much larger than its radius, so that axial losses are small and cross-field transport is dominant. Conditions for either magnetic pressure or neutral pressure balancing the plasma pressure and an associated coupling parameter, which were identified in the above-mentioned recent study, are examined further. In addition, a second coupling parameter is identified which determines which is larger, the relative change in the magnetic field or the relative change in neutral density. An unexpected nonmonotonic variation of the plasma density with the plasma particle flux is demonstrated. It is shown that for plasma beta close to unity, as plasma generation and plasma particle flux increase, the plasma density surprisingly decreases. This decrease follows a decrease in plasma confinement due to an increased plasma diamagnetism. The effect of the magnetic field on neutral depletion is examined. It is shown that an increase in the magnetic field as the plasma density is kept constant results in a decrease in neutral depletion, while an increase in the magnetic field as the plasma particle flux is kept constant results in constant neutral depletion.