Observations of the Channel Illuminations During Dark Periods in Long Positive Sparks

Abstract

No observations show evidence of ionization in long air gap discharge during dark periods. However, for the first time, weak channel illuminations are observed during the so‐called dark periods in this paper. The length of these faint bright channels is almost constant during each dark period and is nearly equal to that of the remaining low‐density channels created by the previous discharge. The phenomenon, that is, the dark period is not really dark, is more likely to occur at the time when there is a greater change rate of the applied voltage. These findings are based on the high‐speed direct imaging and Schlieren photography together, in which the former is used to visualize the bright channels and the latter to detect the low‐density area.

Optical Signatures Associated With Streamers and Leaders of Laboratory Discharges

“…For initial current pulses measured during the voltage rise, we did not observe 777 emissions, similarly to Janda et al (2016), who performed transient discharges in a particular setup. Zhao, Liu, et al (2021) showed millimeter-long stems since the first streamer burst, and that later streamer bursts could branch and elongate stems, driving the propagation of positive leaders.…”

“…Zhao, Liu, et al. (2021) showed millimeter‐long stems since the first streamer burst, and that later streamer bursts could branch and elongate stems, driving the propagation of positive leaders. Their findings provide valuable contributions to the role of stems heating the precursor leader path.…”

“…ICCD cameras have been used for obtaining streak images, taking short‐exposure pictures, or full stroboscopic images that can assess rapid streamers' formation (e.g., Bazelyan et al., 2007; Kochkin et al., 2014; Kochkin et al., 2016; Nijdam et al., 2020). Schlieren images use the observation of changes in temperature (i.e., gas density) to distinguish stem channels from streamer filaments that cause a minimal air density change (e.g., Zhao, Becerra, et al., 2021; Zhao, Liu, et al., 2021).…”

“…ICCD cameras have been used for obtaining streak images, taking short-exposure pictures, or full stroboscopic images that can assess rapid streamers' formation (e.g., Bazelyan et al, 2007;Kochkin et al, 2014;Kochkin et al, 2016;Nijdam et al, 2020). Schlieren images use the observation of changes in temperature (i.e., gas density) to distinguish stem channels from streamer filaments that cause a minimal air density change (e.g., Zhao, Becerra, et al, 2021;Zhao, Liu, et al, 2021). Janda and Macha (2011) and Janda et al (2012Janda et al ( , 2016 conducted several experiments with transient sparks in very short gaps (mm range) for investigating the optical emission spectra associated to streamer-to-spark transition and NOx generation.…”

On the Step Type of Continuous Propagating Positive Leader in Laboratory‐Scale Long Spark Discharges

“…The discharge was momentarily halted and a dark period started. Recent literature (Zhao et al., 2021) detailed the evolution of the stem in the dark period. At about t = 16.5 μs, a continuous current was recorded, indicating that the stable leader began at this moment.…”