Capacitively coupled radio-frequency discharge in alpha-mode as a variable capacitor

摘要

In many cases, the capacitance of a plasma discharge is determined by the sheath capacitance, and therefore controlling the sheath thickness can result in a novel plasma-tunable capacitor. In this work, experiments were conducted with capacitively-coupled RF discharge in the alpha regime using parallel-plate electrode geometry in air at 1 Torr with an RE amplifier operating at a constant voltage in a wide range of frequencies, 20-120 MHz. The impedance characteristics and the sheath and plasma parameters were inferred from the current and voltage measurements supplemented with optical imaging. The experimental data show that the sheath thickness is approximately inversely proportional to the excitation RF frequency, so that the capacitance is proportional to the frequency, in agreement with a simple theory. In applications, tuning the sheath thickness by varying the excitation RE frequency could be used to control the impedance variation in a different range of probing RF frequencies, including control of the transition from capacitive to inductive behaviour.