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.

The formation and propagation of post-discharge blast waves are studied theoretically and the minimum rate of energy transfer between electrons and neutral particles is deduced in the case of a positive point-to-plane discharge in air, described and numerically analysed in a previous paper by the authors. In this type of discharge, the injection of energy in the discharge is spread in time. The blast waves generated by the discharge constitute a set of waves with a complex structure. They first propagate separately, then merge in a single wave. The core of the discharge remains perturbed for a long time (i.e. low neutral densities and high temperatures) after the discharge crossing and thus constitutes a privileged path for possible recurring discharges. The classical analysis of blast waves was adapted to the experimental situation in which the energy injection is spread in time and space. The influence of the injected energy on the expansion radius of the wave has been studied and the authors show that the minimum rate of energy transfer fmin between electrons and neutral particles in the glow to spark transition can be deduced from the Schlieren records. In the experiments analysed, the rate of energy transfer f is greater than 20+or-6%.

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