Elongation and branching of stem channels produced by positive streamers in long air gaps

Abstract

The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head.

Optical Signatures Associated With Streamers and Leaders of Laboratory Discharges

“…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.…”

Schlieren techniques for observations of long positive sparks: Review and application

“…In these experiments [69], another high‐speed video camera (Phantom V1212) coupled with a Nikon 50 mm F1.2 lens was set up to capture the optical images with an interval of 5 μs. Note that the exposure operations of two high‐speed cameras in the optical and Schlieren system were not strictly synchronized.…”

“…The statistical results of the charge transfer in the second streamer versus the stem length are plotted in (b). [69]…”

Activation of Abundant Recoil Leaders and Their Promotion Effect on the Negative‐End Breakdown in an Intracloud Lightning Flash

“…One of the notable asymmetries is that the positive leader could reactivate in the form of recoil leader after cooling down or being cutoff, while the negative leader rarely does (Mazur, 2002, 2016; Qie et al., 2017). Lightning positive leader propagation draws more attentions these years, and new insights into its physical characteristics and mechanism are revealed with data in different aspects obtained by comprehensive observations (R. Jiang et al., 2020; Hare et al., 2021; Visacro et al., 2017; Zhao et al., 2021). Yuan et al.…”