Retuning Effects and Dynamic Instability of a Radio-Frequency Capacitive Discharge

A discharge plasma is established in air at pressures of 0.2–20 Torr in the rf electric field between parallel-plate electrodes in a resonant circuit at 14 MHz. By adjusting a calibrated tuning capacitor in parallel with the electrodes, one can observe a wide range of plasma conditions extending from normal extinction at low rf potential and power to an apparent dynamic instability at high potential and power. The loading effect of the plasma is found to be that of a lossy capacitance. Net input rf power to the plasma-loaded circuit at the tuning condition just below the threshold for dynamic instability is found to be ∼10 times that for a plasma maintained at the tuning condition for the resonant circuit without plasma. The retuning effects are discussed on the basis of the nonlinearity of the plasma medium, and a possible mechanism for the dynamic instability is proposed.