Proposed theory for streamer-to-leader-transition in non-uniform field air gaps subjected to positive switching-impulses

On the basis of a recent experimental investigation and an improved knowledge of physical processes in leader discharges, a theory is presented to permit estimation of the field required for streamer-to-leader transition at different humidities. It is clearly shown that electron-impact ionization is the main parameter for leader formation. Two predictions are made: the electric field necessary for leader formation is considerably lower than the field necessary for the first corona streamer formation (i.e., ~30 kV/cm). Secondly, this field decreases with increasing humidity