Low-pressure gas breakdown in dual-frequency (27/2 MHz) RF electric fields in nitrogen

We present measured breakdown curves for dual-frequency (27.12 /2 MHz) discharges in nitrogen. RF voltages at frequencies of 27.12 MHz (HF) and 2 MHz (LF) were fed to the same powered electrode whereas another one was grounded. The inter-electrode gap was 20.4 mm. The 27 MHz breakdown curve is shifted to higher voltages and gas pressures when a 2 MHz voltage ($<$300 V) is added, due to the increased loss of electrons due to the drift in the LF field. At LF voltages above 300 V the LF field contributes to gas ionization. Positive ions oscillating in the enhanced LF field can collide with the electrode surface and produce secondary electrons. Therefore the HF breakdown voltage is decreased and approaches zero (a self-sustained LF discharge is ignited). Adding an HF voltage always leads to a decrease in the LF breakdown voltage because of the reduction of electron losses due to oscillations in the HF field.