Streamer knotwilg branching: sudden transition in morphology of positive streamers in high-purity nitrogen

Abstract

We describe a peculiar branching phenomenon in positive repetitive streamer discharges in high purity nitrogen. We name it knotwilg branching after the Dutch word for a pollard willow tree. In a knotwilg branching a thick streamer suddenly splits into many thin streamers. Under some conditions this happens for all streamers in a discharge at about the same distance from the high-voltage electrode tip. At this distance, the thick streamers suddenly bend sharply and appear to propagate over a virtual surface surrounding the high-voltage electrode, rather than following the background electric field lines. From these bent thick streamers many, much thinner, streamers emerge that roughly follow the background electric field lines, creating the characteristic knotwilg branching. We have only found this particular morphology in high purity nitrogen at pressures in the range 50 to 200 mbar and for pulse repetition rates above 1 Hz; the experiments were performed for an electrode distance of 16 cm and for fast voltage pulses of 20 or 30 kV. These observations clearly disagree with common knowledge on streamer propagation. We have analyzed the data of several tens of thousands of discharges to clarify the phenomena. We also present some thoughts on how the ionization of the previous discharges could concentrate into some pre-ionization region near the needle electrode and create the knotwilg morphology, but we present no final explanation.

3D PIC-MCC simulations of discharge inception around a sharp anode in nitrogen/oxygen mixtures

“…With 0.2% oxygen, small-scale structure and thin branches are visible in the electron density, see figure 10. Such branches have been observed in several experiments in nitrogen and other pure gases, see for example [15,19,20,53]. In these studies, they were referred to as feathers.…”

Blue Flashes as Counterparts to Narrow Bipolar Events: The Optical Signal of Shallow In‐Cloud Discharges

“…To estimate the remaining factors in Equation 3, we first refer to Briels et al (2008), who observed a ratio of branching length to streamer radius of about 20, so a radius of 2 mm at atmospheric pressure translates into ℓ ≈ 6 × 4 cm = 24 cm (Ebert et al, 2010). Finally, we take the branching number b to be 2, although there are evidences that it may possibly be slightly larger (Heijmans et al, 2013(Heijmans et al, , 2015.…”

A 3D numerical study of positive streamers interacting with localized plasma regions

“…These phenomena are shown in figure 4 where the streamer length versus time is given. As previously used in experiments and simulations [36][37][38], here the streamer length is defined asL 2 , where (x max , y max , z max ) are the maximum electric field coordinate.…”