Ion Collection by a Sphere in a Flowing Plasma: 1. Quasineutral

The spatial distribution of collisionless ions collected by a spherical object of radius much larger than the Debye length, in a flowing plasma, is calculated using a particle-in-cell code. The results provide the first rigorous theoretical calibration of a “Mach probe” in a plasma with negligible magnetic field. They are also applicable, for example, to spacecraft-plasma interactions. Ion to electron temperature ratios 0.1 < Ti/ZTe < 10 are explored. Qualitative differences are observed between the results for low and high Ti/ZTe, arising from ion dynamics in the wake. However, a practically universal form for the upstream to downstream current density dependence on flow velocity is observed for Ti/ZTe <∼ 3.