Criticality of External Circuit in Simulating Atmospheric Pressure Direct Current Microglow Discharge

The effects of the external circuit on discharge conditions are not explicitly considered in most numerical simulations of nonthermal plasma discharges. The effects of including and excluding the external circuit on the simulation of atmospheric pressure reactive microglow discharges are addressed. 2-D simulations of direct-current (dc) atmospheric pressure H₂/CH₄ microglow discharges were conducted using a hybrid model. The external circuit consisted of a ballast resistance (in series) and a parasitic capacitance (in parallel) connected to the discharge electrodes. Simulations were conducted over a broad discharge current range (by varying ballast resistance). The parasitic capacitance values were also varied to examine the effects on discharge behavior. Depending on the value of the ballast resistance, the discharge operated in the subnormal (large ballast resistance) and normal glow (small ballast resistance) regimes. When the external circuit is not considered, the discharge was found to operate in the Townsend like regime as a dark discharge. The simulations further indicated that for higher values of the parasitic capacitance, the discharge (even with a dc power supply) was self-oscillatory, indicating an unstable regime. The oscillations were found to arise as a result of comparable time scales between the ion transit time (τ_ion) within the interelectrode separation and the circuit response time (τ_RC). The predicted results were found to be in agreement with experimental observations.