Schlieren study of the transition to spark of a discharge in air and SF₆

The formation of filaments during the transition to spark in a point-to-plane discharge is analysed in air (250 Torr) and SF 6 (60 Torr). Two separate analyses of the phenomenon are effected. First, the evolution of the electronic population is studied by means of a current probe and photomultiplier, and secondly, the Schlieren technique is used to follow the changes occurring in the density of the neutrals. An image converter is employed to record the Schlieren pictures which permit the observation of the expansion of the channel containing the neutrals. The average temperature of the neutrals is deduced from the rate of radial expansion of the channel and hence the resultant pressure rise at the core of the channel can be estimated. Evidence is given of the formation of local constrictions where intense heating of the neutrals occurs. At these points thermal ionisation is no longer negligible and a large fall in the resistivity of the discharge takes place, giving zones where the initiation process of the thermalisation of the discharge can occur.