资源环境科技发展态势分析平台

Properties of positive and negative leaders developing in air gaps ranging from 4 to 10m that were subjected to 100/7,500-s voltage impulses were examined using a two-frame, high-speed video camera with image enhancement. Abrupt extension (stepping) that culminated in a bright and structured corona streamer burst was observed for both negative (expected for the classical stepping process) and positive (expected for the so-called restrike process) leaders. Selected high-quality images of five negative and four positive leaders with pronounced corona streamer bursts are presented here. The morphology of corona streamer bursts was essentially independent of polarity. Streamer bursts exhibiting nearly spherical symmetry were observed. For the four positive leaders, the newly added channel sections (steps) were almost straight and had lengths ranging from about 50 to over 120cm. For the five negative leaders, most of the steps were curved and their 2-D lengths were some tens of centimeters. It is generally thought that positive leaders in both long sparks and lightning extend continuously or exhibit optically unresolvable steps whose length is comparable to the leader tip size (1cm or less) and that for sparks only when the absolute humidity is relatively high (>10g/m(3) or so) or voltage rise time is relatively long (around 1ms or more) can larger steps occur. In this study, both modes of propagation for different branches of the same positive leader were observed.