Power- and Spatial-Dependent Characterization of Reflection and Transmission Electric Parameters of an Argon Microwave Plasma

In this article, the power- and spatial-dependent macroscopic electrical transmission and reflection properties of an argon microwave plasma are analyzed. A custom two-port coaxial prototype is built with a variable spatial gap between two electrodes. All measurements are based on the extension of the large-signal scattering parameter (Hot-S) measuring environment and are proof-of-concept for the determination of plasma properties without an external probe. The coaxial topology of the prototype has a fixed reference impedance so that the reference planes can be located at the tip of both electrodes. An equivalent lumped element circuit valid over a wide bandwidth is investigated to simulate the linear macroscopic electromagnetic reflection and transmission properties of the plasma. A series-resonance circuit is chosen since its representation is capable of covering the measurements from 10 to 40 W of input power and gaps from 1 to 3 mm. With this circuit, a continuous relationship between the impedance, power, and the spatial dimensions of the plasma is found.