Thermal Plasma Measurements in Space Using Direct Measurements of Derivatives of Probe Current-Voltage Characteristics

Abstract

In-situ measurement of the thermal plasma environment of space platforms dates back to the beginnings of the space age and continues at the present time. Of particular interest is what is referred to as thermal plasma, the generally cool, high density plasma which comprises planetary ionospheres and, in modified form, provides a partially conducting and emitting environment to large, manned platforms because of their rather high outgassing rate. This paper will summarize a technique for making measurements on Maxwellian plasma for more than 30 years, and has applications to identify and characterize the non-Maxwellian components of plasmas. The technique is based on the use of two alternating voltage signals applied to a probe immersed in the local plasma, then using the intermodulation between the two applied frequencies to extract a signal proportional to the second derivative of the probe current-voltage characteristic. The theoretical basis for the technique will be described, its practical implementation in a spherical retarding potential analyzer and some results from the technique on both unmanned satellites and the Space Shuttle will be presented. Finally some suggestions for development of the instrumentation and suggestions for future use are discussed.