Ambient electron density derived from differential potential measurements

The potential difference, AV, between a satellite and an electric field probe is well correlated with the ambient electron density, n e , in tenuous plasmas where the potential of the biased electric field probe can be kept close to that of the plasma nearly independently of n e . Hence, A V is approximately equal to the satellite potential V s . On the other hand, V s is very sensitive to n e as long as the satellite is positive with respect to the plasma potential. Therefore A V measurements provide a quite accurate diagnostic method for determining the bulk electron density in tenuous plasmas (i.e., below a few times 10 cm -3 ). We present a numerical analysis of this technique under various plasma conditions. In particular we investigate how well n e is determined from AV measurements when the ambient electron temperature, T e , is not well known. It turns out that this relationship can be described by AV = 0.9 T ph ln(0.15 (a 2 √T ph /I b )n e ) (where I b is the bias current, a is the probe radius, and T ph is the photoelectron temperature).