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 a LF voltage having 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 of 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 a LF voltage amplitude lower than 600 V, the ions loss to the cathode is higher than the ions creation related to the secondary electrons. The consequence is a decrease of the plasma density. This density oscillates at a frequency equal to 2LF: it is maximum each time the LF voltage amplitude is equal to 0 and minimum when the LF voltage amplitude is maximum. For a 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

Atmospheric pressure dual RF–LF frequency discharge: transition from α to α – γ -mode

“…As described in more details in a previous article [20], the DBD is a usual plane-to-plane DBD with each electrode powered by two different power supplies, one RF and one LF. These power supplies are synchronized.…”

“…The efficient transport of the ions towards the cathode induces a high secondary electron emission increasing the metastable production and the ionization in the sheath. This was for example observed by applying a 50 kHz voltage to a 5 MHz DBD in Ar/NH3 mixture [20] or 200 kHz on 13.56 MHz [17]. In [20], for an LF voltage amplitude lower than some threshold, the increased ion loss to the cathode is not compensated by increased ionization and the plasma density thus decreases.…”

“…This was for example observed by applying a 50 kHz voltage to a 5 MHz DBD in Ar/NH3 mixture [20] or 200 kHz on 13.56 MHz [17]. In [20], for an LF voltage amplitude lower than some threshold, the increased ion loss to the cathode is not compensated by increased ionization and the plasma density thus decreases. For an LF voltage amplitude above the threshold, when the LF and RF cathode are the same electrode, the ionization in the sheath due to the multiplication of secondary electrons is high enough to counterbalance the ion loss and the discharge is γ-RF during 1/5 of the LF cycle.…”

“…At atmospheric pressure, the transition from α-mode to γ-mode is observed in RF discharges [17]- [19] as well as in dual RF-LF discharges [20]. According to the literature, there are two distinct ways to reach the transition: either increase the electric field into the sheath by increasing the voltage amplitude [17] or increase the secondary electron emission by applying a low frequency voltage able to transport the ions to the cathode [20].…”

“…In this work, we use the same system/tools/model as before [20] to investigate the mechanisms involved in the transition between α-mode and α-γ-mode. For that purpose, the effect of the parameters that strongly influence the ionization rate are studied in the case of a RF-LF dual frequency DBDs in Ar-NH3.…”