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Streamer discharges are a primary mode of electric breakdown in thunderstorms and high voltage technology; they are generally believed to grow along electric field lines. However, we here give experimental and numerical evidence that streamers can propagate nearly perpendicularly to the background electric field. These streamers are guided by pre-ionization that is orders of magnitude lower than the ionization density in a streamer channel, hardly affecting the background field. Positive streamers could be guided in nitrogen with 0.5% of oxygen or less, but not in air. This observation also tests the role of photo-ionization in gas mixtures with varying nitrogen–oxygen ratio. [ABSTRACT FROM AUTHOR]

A recently developed ultrafast camera that can acquire 108 frames per second was used to investigate positive streamer discharge. It enabled single-shot evaluation of streamer evolution without the need to consider shot-to-shot reproducibility. This camera was used to investigate streamers in argon. Growing branches, the transition when a streamer forms a return stroke, and related phenomena were clearly observed. [ABSTRACT FROM AUTHOR]

Because positive streamers propagate opposite to the electron drift velocity, their growth not only depends on the local electric field, but also on the electron density ahead of them. We have recently demonstrated the importance of this electron density, by showing that positive streamers could be guided by weak preionization from a laser. Here, we study streamer guiding in nitrogen–oxygen mixtures in more detail, to better understand the propagation of positive streamers. First, the conditions required for guiding are investigated, both experimentally and with three-dimensional numerical simulations. Particular focus is on the role of oxygen and photoionization, which tend to inhibit guiding. Then the position of the guided streamers is investigated, and successfully related to the electron drift. [ABSTRACT FROM AUTHOR]