Transmission-Type Impedance Probes for Use in Complex Plasmas

Abstract

Impedance probes measure electron density (ne ) and electron temperature (Te) by interpreting frequency-dependent transmission or reflection spectra (S21 or S11) of radiofrequency (RF) signals of low power (10−5 W). This diagnostic method is promising for use in complex plasmas because it is less disruptive to plasma and dust than Langmuir probes, and it is easier and cheaper to implement than many common spectroscopic measurements, such as laser-induced fluorescence (LIF). Transmission-type impedance probes offer some further advantages over reflection-type probes because they can resolve plasma conditions near the probe vs farther away in the chamber, and because they also may be used as a DC double Langmuir probe. One potential application is to measure the charge of a dust cloud by performing RF and DC measurements to measure n e and ni , and then to determine the charge on the dust via the quasineutrality condition, ni − ne − zd nd ≈ 0.

This work explores the topic of transmission-type impedance probes in three ways. Firstly, a theoretical model is developed to understand transmission-type impedance probe spectra. Secondly, experiments are performed in which transmission spectra are recorded using these probes in RF glow discharge plasmas. And lastly, a computer model is developed to simulate the probe-plasma system, treating the continuous plasma medium as a series of discrete circuit elements and using SPICE algorithms to simulate the transmission of RF signals through the plasma. This computer model is then used to determine ne and Te from experimental transmission spectra. We end by discussing ways of implementing this diagnostic technique in dusty plasma experiments, both for ground-based experiments and for microgravity experiments.