Gyrofluid Model of Plasma Expansion in a Magnetic Nozzle

Abstract

The gyrofluid equations are a fluid model for plasma that includes the magnetic mirror force and the reduction in density that occurs in a diverging field.1 These equations are applied to collisionless electrons and ions that flow from an upstream region of high magnetic field to a downstream region of lower field. The model is applicable to ion engines for spacecraft, helicon plasma sources, and some plasma processing devices. The ion flux from the source region is found to be approximately the Bohm flux and the continuity equation requires that the particle flux within a magnetic flux tube be constant. The mirror force converts upstream thermal energy of both species into energy in the ion flow speed downstream. The downstream ion flow speed is about 1.5 times the ion sound speed for large expansion ratios. The downstream density is about half the upstream density scaled additionally by the magnetic field ratio. Quasineutrality is assumed hence the possibility of additional ion acceleration in a double layer is not considered.