Experimental study of satellite wakes in a simulated ionospheric plasma

Abstract

Wakes of simple bodies (discs, strips) were investigated using an electrostatically accelerated stream of argon ions and electrons. Typical conditions are: beam ion energy is 80 eV, ion density is 10^7-10^8 cm^(-3), electron temperature 1-3 eV, ion thermal speed very small compared to mean ion velocity. The dimensionless parameters closely approximate satellite flight conditions, with the exception of the electron-ion temperature ratio, which is near unity in flight and large in these experiments. The dependence of principal near wake features (such as the large ion current peak on the centerline behind the body) on the shape of the body was investigated systematically. All trends can be explained qualitatively by recognizing the dominant role of those portions of the sheath where the free stream ion velocity is tangential to the body. The far wake of a strip (downstream of the ion current peak) displays a decaying radial distribution on the scale of the body size, somewhat similar to what is expected from a neutral gas. For axial symmetric models, the far wake displays a small structure on the scale of the ion current peak. The evolution of these disturbances is qualitatively explained by a simple, linearized two fluid theory. These features are initiated by the interaction of the inwardly deflected ion streams behind the body. At least in certain regions of the flow field, this interaction involves two-humped ion distribution functions, which may play a role in the further development of the far wake. The effects on the simulation of varying the vacuum chamber background pressure was also examined in detail.