Frequency Conversion in the Sheath Capacitance of a Glow Discharge Plasma

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Abstract
A study is made of large-signal, high-frequency phenomena occurring in a slightly ionized plasma contained within a pair of metallic coaxial cylinders. An rf equivalent circuit is developed wherein the sheath is represented by a voltage-dependent capacitor in parallel with a voltage-dependent resistor, the presheath by a series resistor, and the plasma region by a series resistor and inductor. It is found that when the sheath is thin compared to the radius of the inner cylinder, the voltage dependence of the sheath capacitor reduces to a form identical with that of an abrupt junction varactor diode, so that the plasma capacitor is properly called a plasma varactor. As an application of the plasma varactor, frequency doubling using standard semi-conductor varactor techniques is attempted. The resultant peak conversion efficiency is found to be 31.6% at a drive frequency of 142 MHz and input power of 0.42 W. It is shown that the sheath conductance is responsible for this low conversion efficiency relative to that obtainable from a semiconductor varactor.