An Experimental Study of the Breakthrough‐Phase and Return‐Stroke Processes in Long Sparks

Abstract

The return stroke in lightning or long spark is a wave of electric potential and current propagating from the ground over several kilometers (for lightning) or from the grounded electrode over several meters (for a long spark) along the channel created by the leader. A high current generated during the return stroke process is the source of most damaging effects of lightning. Therefore, the return-stroke process in lightning and in the laboratory long spark discharge simulating lightning (at least at early times) has attracted attention of many researchers.Many theoretical models of return stroke have been developed and tested, and the results of these studies have been published (Сooray, 2010;Rakov & Uman, 2006). In all models [(except for the one developed by Gorin (1985)], the evolution of the return-stroke current wave was considered assuming an instantaneous contact of the main downward leader channel with the ground or with the upward connecting leader (UCL) channel. As a result of this assumption, all parameters of the current pulse were determined by the parameters of the leader channels only. Actually, the conductivity of the common streamer zone formed during the breakthrough phase increases gradually, by several orders of magnitude, from a small value at the beginning of the breakthrough phase to a large value at its end, and leads to a significant increase of the current (Bazelyan & Raizer, 1998). Therefore, the front of the current pulse is largely formed during the breakthrough phase, and it can be expected that the parameters of the common streamer zone also affect the maximum discharge current. Current views of the breakthrough phase, which are still not fully understood, are summarized by Rakov and Tran (2019). For more recent finding, we refer the reader to Jiang et al. ( 2021).The collision of downward leader with the UCL usually occurs not high above the ground and is usually not seen in lightning images (e.g., Idone & Orville, 1982;Rakov & Uman, 2006), except for very tall strike objects, for which UCL length can be of the order of hundreds of meters (e.g., Rakov & Uman, 2006;Yokoyama et al., 1990). The UCL colliding with the downward leader was first observed in rocket-triggered lightning (Wang et al., 1999). In those experiments, the velocities of the main and UCLs and the average velocities of the return stroke waves propagating up and down from the junction point of the leaders were measured. The average speeds of the downward dart leader and of the UCL were found to be 4 × 10 7 and 2 × 10 7 m/s, respectively. The speed of the return stroke was found only for the upward waves, 2 × 10 8 and 3 × 10 8 m/s for two cases. First images of common streamer zone were obtained by Hill et al. (2016) andRakov (2017) for triggered and natural lightning, respectively. However, the detailed spatial and temporal variations of the channel luminosity were not obtained. In experiments with a long sparks, which are often viewed as a laboratory analog of lightning, the velocities of the main negative and conn...

Observations of the Breakthrough Phase of Attachment Process in Laboratory Long Sparks

“…All the discharges considered in this letter involved a positive leader developing from the high‐voltage electrode and the negative connecting leader extending from the grounded rod electrode, which is similar to Bogatov et al. (2021).…”

“…The two consecutive frames of the evolution of the breakthrough phase in Figures 2 and 3 show the shrinking of the common streamer zone after its establishment, which can also be found from two snapshots and streak-camera images of the breakthrough phase in laboratory sparks obtained by Kostinskiy et al (2016) and Bogatov et al (2021), respectively.…”

“…A lower rod electrode (0.8 and 1.8 m in length) was placed directly below the upper rod electrode on a grounded plane. All the discharges considered in this letter involved a positive leader developing from the high-voltage electrode and the negative connecting leader extending from the grounded rod electrode, which is similar to Bogatov et al (2021).…”

“…They interpreted the conversion of the high-impedance common streamer zone to a hot plasma channel as the dominant development of the selected route rather than the shrinking of the common streamer zone. Recently, Bogatov et al (2021) documented the breakthrough phase in long laboratory sparks using a streak camera and also estimated the average velocities of two approaching leaders in the breakthrough phase. The streak-camera images indicated the head-to-head development of opposite polarity leaders leading to the shrinking of the common streamer zone.…”

“…Recently, Bogatov et al. (2021) documented the breakthrough phase in long laboratory sparks using a streak camera and also estimated the average velocities of two approaching leaders in the breakthrough phase. The streak‐camera images indicated the head‐to‐head development of opposite polarity leaders leading to the shrinking of the common streamer zone.…”