Capacitively coupled discharge as a tunable impedance element for RF systems

A concept of plasma-based reconfigurable radio-frequency (RF) components is studied experimentally. The concept's main idea is to control the real and imaginary parts of impedance experienced through a weak high-frequency signal applied to the RF discharge electrodes by varying the plasma excitation frequency and/or power. To prove the concept, an experimental setup was built that enabled the application of both strong plasma excitation RF and weak RF/microwave signals to the same set of electrodes. Three different plasma cells were examined. The largest system had 5 cm diameter electrodes with 2 cm gap, and the experiments were conducted in argon at 1 Torr. The impedance characteristics of the plasmas as experienced by the probing signal in the range of 270–3000 MHz demonstrated wide variability and a complex non-monotonic behavior at high frequencies. Experiments with a smaller cell that had 1 cm diameter electrodes and 2 mm gap also showed the controllability of the real and imaginary parts of the impedance, including transition from capacitive to inductive behavior. Because of the smaller size comparable with the probing signal wavelength, the high-frequency behavior of this system can be described with a simple lumped-parameter equivalent circuit model. The smallest system studied in this work utilized a commercial sealed gas discharge tube as an RF plasma cell and also demonstrated wide impedance tunability and transition from capacitive to inductive behavior. These results could potentially be useful for plasma-based electronically reconfigurable RF electronics, particularly in applications involving high power where alternative solutions experience problems.