Reconnection and merging of positive streamers in air

Abstract

Pictures show that streamer or sprite discharge channels emerging from the same electrode sometimes seem to reconnect or merge though their heads carry electric charge of the same polarity; one might therefore suspect that reconnections are an artefact of the two-dimensional projection in the pictures. Here we use stereo photography to investigate the full three-dimensional structure of such events. We analyse reconnection, possibly an electrostatic effect in which a late thin streamer reconnects to an earlier thick streamer channel, and merging, a suggested photoionization effect in which two simultaneously propagating streamer heads merge into one new streamer. We use four different anode geometries (one tip, two tips, two asymmetric protrusions in a plate and a wire), placed 40 mm above a flat cathode plate in ambient air. A positive high voltage pulse is applied to the anode, creating a positive corona discharge. This discharge is studied with a fast ICCD camera, in many cases combined with optics to enable stereoscopic imaging. We find that reconnections as defined above occur frequently. Merging on the other hand was only observed at a pressure of 25 mbar and a tip separation of 2 mm, i.e. for a reduced tip distance of p · d = 50 µm bar. In this case the full width at half maximum of the streamer channel is more than 10 times as large as the tip separation. At higher pressures or with a wire anode, merging was not observed.

3D properties of pulsed corona streamers

“…The calculations show that the streamer diameter becomes smaller with decreasing oxygen content, which agrees with the measurements [14]. The experiments also observe streamer merging, but only for a shorter distance between the streamers, for details see [21]. …”

“…Figure 2 shows how this is done. The details on how the 3D image is reconstructed from these two images can be found in the recent papers [20,21]. In short, a straight section of a streamer channel is selected in both images, and the end points of these two lines are reconstructed from two images in 2D (xy) to 3D (xyz).…”

“…Figure 3 shows one example where an apparent reconnection is shown to be fake. On the other hand, in our recent preprint [21], we present examples of real reconnections.…”

A time-resolved imaging and electrical study on a high current atmospheric pressure spark discharge

“…[16][17][18][19][20][21] Most of the discharge imaging studies either focus on streamer formation and propagation, or globally analyze the different discharge regimes such as glow, corona, or spark. In the present study, we focus on how the spark channel evolves once it is formed, always under the conditions of ls-long and high intensity current peaks required for NP formation.…”

Percolation Simulation of Laser-Guided Electrical Discharges

“…For example, at an ionized fraction of 0.32, a new path merges with the original path to create a treelike structure. This suggests that bifurcation of streamers occurs not only by splitting of their tips but also by the reconnection of existing paths [24]. As the ionized fraction increases further, the number of paths increases rapidly and the whole region is filled with the channels.…”