Atmospheric pressure dual RF-LF frequency discharge: influence of LF voltage amplitude on the RF discharge behavior

Abstract

This work is a contribution to a better understanding of dual frequency discharge at atmospheric pressure. Based on experiments and numerical modeling, it is focused on radio frequency (5 MHz)—low frequency (50 kHz) plane/plane dielectric barrier discharge in a Penning mixture (Ar-NH3). The discharge is in the α-RF mode, biased by an LF voltage with an amplitude ranging from 0 to 1300 V. When the LF amplitude increases, there is a threshold (around 600 V for a 2 mm gap) from which the light intensity (experiment) and the ionization level (modelling) drastically increase. In this work the physics of the RF-LF DBD below and above this threshold is studied. Depending on the respective RF and LF polarity, the net voltage applied to the gas is alternatively enhanced or reduced which induces an increase or a decrease in the ionization level. In all cases the ion drift to the cathode due to the LF voltage results in an ion loss and a production of secondary electrons. For an LF voltage amplitude lower than 600 V, the ion loss to the cathode is higher than the ion creation related to the secondary electrons. The consequence is a decrease in the plasma density. This density oscillates at a frequency equal to 2LF: it is at maximum each time the LF voltage amplitude is equal to 0 and at minimum when the LF voltage amplitude is at maximum. For an LF voltage amplitude higher than 600 V, when the LF and RF polarity are the same, the secondary electrons emission is high enough to counterbalance the ion loss, to enhance the bulk ionization and the discharge becomes a γ-RF. The gas voltage is controlled by the dielectric charge like a low frequency DBD. Around the gas voltage maximum, on each RF cycle, the discharge is alternatively an α-RF and a γ-RF discharge. When the discharge is in the γ mode, the ions flux at the cathode is increased by a factor of 40.