On the positive upward leader in response to downward stepped leader in a 10m rod-to-rod long air gap

Abstract

Long negative sparks have been widely used to investigate the lightning attachment process of grounded structures. The rod-to-rod air gap is one of the most common configurations used in laboratory experiments to investigate the characteristics of the positive upward leader in response to the negative descending leader steps. In the research presented in this paper, experimental observations have been carried out in a 10 m rod-to-rod air gap. Many discharge parameters were recorded using a high speed camera, wideband current measurement devices and Pockels electric field sensors. It was found that the average linear charge density of a negative leader is about 142μC/m, which is close to the charge density of a natural lightning downward leader delivering a stroke current of about 7kA. The maximum upward streamer and leader lengths are 2.24 m and 0.64 m, respectively. The space E-field En in negative long sparks consists of a series of electric field steps. Although the dEn/dt is relatively high during the formation of each negative leader step, the dEn/dt decreases significantly and the upward leader stops elongation during the time interval of two negative leader steps. As a consequence, the velocity and length of the upward leader initiation by negative long sparks might only be close to the initiation stage under natural lightning conditions.