The physics governing the propagation of lightning leaders and long spark leaders is still not well understood. Positive and negative leaders seem to behave differently. Negative leaders develop in a step manner, guided by the separate luminous structures termed space stems and space leaders. Positive leaders, on the other hand, are generally thought to have no separate luminous structure involved in their propagation. However, a separate luminous structure observed in a positive leader discharge had been reported in recent literature, suggesting that positive leaders may similarly do steps to negative leaders under certain conditions. Here we report the observation of the positive leader step led by a separate luminous structure at high humidity in laboratory lightning-like discharges. We also found the streamer-like common zone connecting the primary leader channel with the separate luminous structure, as well as the bi-directional development of the separate luminous structure. We hope that these findings would contribute to a better understanding of the nature underlying positive long spark leaders and lightning leaders.

Accurate quantitative detection for trace gas has long been the center of failure diagnosis for gas-insulated equipment. An absorption spectroscopy-based detection system was developed for trace SF6 decomposition SO2 detection in this paper. In order to reduce interference from other decomposition, ultraviolet spectrum of SO2 was selected for detection. Firstly, an excimer lamp was developed in this paper as the excitation of the absorption spectroscopy compared with regular light sources with electrodes, such as electrodeless lamps that are more suitable for long-term monitoring. Then, based on the developed excimer lamp, a detection system for trace SO2 was established. Next, a proper absorption peak was selected by calculating spectral derivative for further analysis. Experimental results indicated that good linearity existed between the absorbance and concentration of SO2 at the chosen absorption peak. Moreover, the detection limit of the proposed detection system could reach the level of 10−7. The results of this paper could serve as a guide for the application of excimer lamp in online monitoring for SF6-insulated equipment.

High‐speed video frames and discharge current records of 34 long laboratory sparks were examined. Two consecutive frames of the breakthrough phase, depicting the evolution of the common streamer zone, were obtained in five events, all presented in this study. Two types of luminous structures were observed inside the common streamer zone just before the return stroke phase: the floating luminous segment and the luminous channel linking the opposite polarity leaders. The luminous structure served as a selected route for the return stroke channel. These results demonstrate that the transformation of high‐impedance common streamer zone into a hot plasma channel may involve two main features: the common streamer zone shrinking and the dominant development of the selected route.

No abstract