Microdischarge behaviour in the silent discharge of nitrogen-oxygen and water-air mixtures

In this paper measurements of the behaviour of single microdischarges between metal and glass electrodes for both negative and positive polarities of the metal (designated M(-)G(+) and M(+)G(-) transitions, respectively), as well as the overall discharge behaviour between glass electrodes (GG), will be presented for different nitrogen-oxygen and water vapour-air mixtures. Increasing the oxygen concentration in nitrogen decreases the transferred charge per microdischarge for both polarities (M(-)G(+) and M(+)G(-)), while the total transferred charge per cycle is increased. With increasing water content in air, more charge is transferred per microdischarge for the polarity M(+)G(-), but no significant change in the amount of transferred charge was found for a M(-)G(+) microdischarge. Less total charge is transferred per cycle with increasing water content. This is also true for double-dielectric barrier discharges (GG). The results suggest that water vapour coats the dielectric, reducing the surface resistance and increasing the effective dielectric capacity. This mechanism is supported by analysis of the voltage versus charge plot (Lissajous figure) for the involved capacitances of a double-barrier discharge. The reduction of the surface resistance of the dielectric and the resulting increase in the effective dielectric capacitance, are shown by photographs of spatially isolated microdischarges in a metal-pin dielectric arrangement.