Model for Estimating Time-Varying Properties of an Inductively Coupled Plasma

A developing application of inductively coupled plasmas is in the field of electrodeless (propellant-flexible) electric propulsion. A significant issue facing this application is the need for diagnostic techniques that do not disturb the plasma (are nonintrusive), are propellant-agnostic, can resolve time variance, and are suitable for use in-flight. A new technique meeting these criteria is presented in this work. The technique makes use of the transformer model of inductive coupling to estimate the plasma impedance from the antenna current and resonant frequency, both of which can be measured nonintrusively. Having an estimate of the plasma impedance, it is possible to estimate a variety of plasma properties under the assumption of a uniform tubular plasma volume. Starting with a circuit representation of a high-power inductive plasma source, governing equations are derived and a solution method is described. Experimental data from the plasma source showing transient behavior (fluctuations within 300-Hz cycle) in oxygen plasmas with various input powers and flow rates are analyzed to demonstrate the technique and investigate trends. The technique produces results that are self-consistent and align well with previous theoretical work.