Evaluation of a Plasma Impedance Probe in a Time-varying Non-uniform Plasma

Abstract

Numerical simulations and experimental measurements were combined to determine the ability of a PIP to measure plasma density and electron collision frequency in a plasma containing spatial gradients as well as time-varying oscillations in the plasma density. Collision frequency was to be measured by the width of the parallel resonance in the real part of plasma impedance-vs.-frequency, while plasma density was to be measured using the zero-crossing of the imaginary impedance-vs.-frequency at parallel resonance. Simulations of the probe characteristic in a plasma gradient indicated that the broadening the real part of plasma impedance due to the spatial gradient obscured the broadening due to electron collision frequency. Simulation results also showed that it may be possible to measure relative changes in electron collision frequencies in a spatial plasma gradient, but the broadening effect of the time-varying oscillations made collision frequency measurements impossible. The time-varying oscillations had the effect of causing multiple zero-crossings in the imaginary part of impedance at parallel resonance. Results of experiments and simulations indicated that the lowest-frequency zero-crossing represented the lowest plasma density in the oscillations and the highest-frequency zero-crossing represented the highest plasma density in the oscillations, thus the PIP probe was found to be an effective tool to measure both the average plasma density as well as the max and min densities due to temporal oscillations.