Numerical Model of an Argon Atmospheric Pressure RF Discharge

Radio-frequency discharges are known to operate in two different regimes. The alpha regime of low current density and the gamma regime with higher current density. Our recent simulation results suggest that the formation of filaments observed in an atmospheric pressure argon discharge under RF excitation could be triggered by the regime transition alpha -> gamma. A unidimensional fluid model taking into account the external circuit shows that above 120 mA/cm(2), the differential conductivity of the discharge becomes negative with a rapid increase in density which can lead to the formation of filaments. As the transition to the gamma regime is due to secondary electrons, this threshold value depends on the secondary emission coefficient. In the gamma regime, the instantaneous cathode is sustained by secondary electron emission, which drastically changes the behavior of the discharge. In this paper, we present a numerical analysis of the transition between the two regimes and discuss how this could result in the filamentary mode observed in argon RF discharges.