Probe technique for direct measurement of temporal variation of electron temperature

Abstract

A technique for making direct measurements of electron temperature oscillations in a plasma is discussed. The technique use the relationship between the floating potential, V/sub f/, and the plasma potential, V/sub p/, for a Maxwellian distribution of electrons with temperature T/sub e/, V/sub p/=V/sub f/+(T/sub e//e)ln(m/sub i//0.64 pi m/sub e/)/sup 1/2/. The plasma potential and floating potential oscillations are measured directly, and the temperature oscillation is inferred through this relationship. The technique uses a pair of identical cylindrical probes, each consisting of a tungsten filament stretched between two external leads. One of the probes is operated as an emissive probe, so that it floats near the plasma potential, while the other probe, which is not emitting, is at the floating potential. The difference of these two potentials is monitored with a differential amplifier, giving a signal that is proportional to the electron temperature. Initial measurements were made with argon in a hot-filament discharge plasma confined in a multidipole magnetic field. A perturbation that produced electron temperature variations was introduced by inserting an externally biased plate into the plasmas. With the plate biased DC or with a slowly varying voltage, independent measurements of V/sub p/, V/sub f/, and T/sub e/ were made from the I-V characteristic of a Langmuir probe. By comparison with the direct temperature measurement, both the accuracy of the V/sub p/-V/sub f/ measurement and its relation to the electron temperature were evaluated. >