Eliminating the influence of the ion current on probe measurements of the electron energy distribution function in plasmas

An experimental method is proposed and tested for compensating the ion current and its derivatives in probe measurements of the electron energy distribution function and temperature. This method is based on detecting a difference signal from two cylindrical probes with different diameters under conditions such that the hydrodynamic approximation for the ion drift to the probe is valid. Experiments conducted on an afterglow plasma confirm the effectiveness of this technique. It is shown that at high pressures the theory of the ion current gives a satisfactory description both of the current as a function of the probe current and of its second derivative.