Neutral heating in glow to spark transition in air and nitrogen

The heating of the neutral molecules in the glow to spark transition in air and nitrogen has been theoretically studied. A hydrodynamic first-order model has been used for electrons (density, momentum) and a second-order model (density, momentum, temperature) for neutral molecules. The interaction between the electron dynamics and the neutral dynamics has been compared in two kinds of discharges: a fast discharge (170 ns) in an overvolted plane parallel gap in nitrogen, and a discharge of longer duration (1.15 mu s) in a positive point-to-plane gap in air. The kinetics of the discharge, which is linked to the possible feedback of the neutral gas on the electron cloud, is mainly a function of the duration of the energy injection. In the case of the fast parallel gap discharge, the background gas plays a passive role; it is an energy absorber, so its temperature increases continuously. In the case of the point-to-plane discharge, lasting longer, the dynamics of the interaction between the electron cloud and the background gas constitutes the fundamental process of the discharge. The gas heating gives rise to a neutral depopulation in the core of the discharge. This leads to an increase of the reduced electric field E/N, thus to an increase of the electron production and thus of the current. In this kind of discharge, the heterogeneity of the neutral gas leads to the breakdown, owing to a constriction of the current core induced by the depopulation mechanisms.