Characterization of the Active Zone for Leader Propagation in SF6

The propagation of step-leader discharges in SF6 depends on the inception of electronic avalanches in the streamer phase that occur in the region where the electrical field strenght exceeds a critical threshold E cr Such region surrounds the propagating discharge leader tips and may be considered as an active zone since it is the locus where new streamers and leaders take place. The active zone is crucial for the propagation of leader discharges since it determines the main directions and the branching of the discharge channels. Its shape and local properties are strictly related to those of the propagating leader path, the pattern. In the context of physical and chemical sciences it has been shown1 that the characteristic scaling laws of frozen structures generated by diffusion-limited aggregation or chemical reactions may be related to the scaling exponents of the growing active zone which, in turn, depend on the frozen pattern. In that case some conditional probabilistic laws are used to find out the characteristics of the active zone given those of the pattern. In the leader discharge the new leader tips come from the active zone but their location may not be simply deduced from the preceding leader pattern because of the underlying physical processes. In fact, the streamers propagate within the active zone following statistically the background electrical field distorted by the space charge. Some streamers only change into leaders after a sufficiently intensive charge injection, usually occurring if a critical tip charge amount Q cr is excedeed. Moreover, during the discharge propagation some leaders may not propagate and become inactive because either the local electrical field does not exceed the threshold E cr or they are shielded by faster neighbouring leaders.