Gyrofluid Model of Plasma Expansion in a Diverging Magnetic Field

The gyrofluid equations for electrons and for ions are used to model the acceleration of plasma from a source region of high magnetic field to a region of lower field. The model includes a source term for plasma generation, a self-consistent electric field, nonzero ion temperature, plasma acceleration by the mirror force, and the reduction in density from the diverging flow. The model is applicable to plasma experiments with helicon sources, ion engines for spacecraft, and some plasma processing configurations. The ion flux from the source region is found to be approximately the Bohm flux and the downstream ion flux density scales with the magnetic field. The downstream flow is determined by the conversion of upstream thermal energy in both electrons and ions into kinetic energy in flow speed. For large expansion ratios, the final speed approaches 1.5 times the ion sound speed.