The Spacecraft As A Langmuir Plasma Probe

Abstract

The use of the Langmuir plasma probe model is important in space current collection and spacecraft charging calculations. The model in an ideal geometry, such as a sphere or an infinitely long cylinder, is well understood. However, the realistic geometry of a current collector, or spacecraft, is often non-ideal. An empirical formula for a given non-ideal geometry Langmuir probe would be useful. We derive such a formula for the SCATHA satellite by using the SCIO potential data obtained during electron beam emissions. The satellite is rotating perpendicular to sunlight and the SCIO booms are in the equatorial plane. In one mode of operation, the beam current increases continuously while the beam energy remains constant. The SCIO data, representing the boom-satellite potential difference, responds to the variation of the beam, photoelectron, and ambient currents. We present an algorithm which successfully yields an empirical Langmuir probe formula for SCATHA. By using this formula, we obtain improved results of the ambient electron temperature and Spacc Plasmas Diapostics density. This algorithm may be useful for other non-ideal geometry spacecraft also, if similar types of current-voltage data are available.