Theory of a satellite electrostatic probe

Abstract

A theoretical study is presented of the properties of a circular planar electrostatic probe, flush-mounted in the skin of a satellite. This geometry results in a two-dimensional potential distribution. The current-voltage characteristics of the external aperture grid and of an internal repelling collector (for attractive aperture-grid potentials) are investigated by means of several techniques involving detailed analysis of particle trajectories. An approximate theory is found to give useful insight into the properties of the probe. More exact numerical calculations are made by following trajectories in the field of the probe when the Debye length is infinite (Laplace field) and when the Debye length is finite (Poisson field). The Poisson field is computed by an iterative technique in which the charge densities in the Poisson equation are evaluated by summing charge contributions over trajectories. It is found that when the internal collector is repelling with respect to the plasma, its current-voltage characteristic is similar to that of a repelling probe, regardless of the fact that the external aperture grid is attractive. This has important consequences for satellite experiments.