Evolution of the Stem During the Dark Period in Long Air Gaps

The stem is an essential object of the streamer to leader transition and leader propagation processes. While it is not well observed especially during the dark period. The Schlieren technique was chosen to obtain the stem evolution process during the dark period in this paper. Experiments were carried out under positive switching impulses, and the Schlieren images of the stem were recorded. The results show that the structure of the first corona stem can be classified into the cylinder type and conical frustum one; the evolutions of the two types during the dark period are of different characteristics: the thermal diameter of the cylindrical type is smaller than that of the conical frustum one, while the channel length is opposite. Besides, the variation of the thermal diameter and channel length of the cylindrical stem can be divided into two stages, but they change linearly for the conical frustum type; the different types of stems have a great effect on the initiated path of the second corona stem.