Characterizing streamer branching in N₂-O₂ mixtures by 2D peak-finding

The stochastic nature of streamers and the manual identification of features in 2D discharge images together cause great ambiguities when analysing streamer branching characteristics. Here we present the development of streamer image diagnostics by a 2D peak-finding method to obtain accurately quantified extensive statistics on streamer branching. And we present quantitative results on the growth of the streamer head number as a function of time in N₂-O₂ mixtures at 100 and 200 mbar. Decreasing the oxygen concentration decreases the nonlocal photoionization, and hence allows for local instabilities and more branching. The oxygen concentration in N₂-O₂ mixtures affects streamer branching not only by smoothening the electron number density in front of streamer heads but also by the creation of an inception cloud. Streamers in pure nitrogen have no noticeable inception cloud, which gives the nitrogen streamers a longer effective propagation time during a voltage pulse of 550 ns; they branch more both as a function of space and of time. However, the statistical results show that the number of streamer heads in high purity N₂ is less than in mixtures with 0.1% O₂, and it depends on pressure.