Positive streamer propagation due to background or photo ionization: Experiments and theory

Positive streamers in air are generally believed to propagate against the electron drift direction due to the nonlocal photo-ionization reaction. Photo-ionization is the ionization of O2 molecules by UV radiation from excited N2 molecules; therefore this reaction depends on the ratio between oxygen and nitrogen. Another possible source of free electrons in front of a positive streamer is background ionization that can remain from previous discharges, or it can be created by cosmic rays or by radioactive species like radon. We study the effects of both photo- and background- ionization on propagation and morphology of positive streamers by changing the ratio between nitrogen and oxygen and by changing the repetition frequency. We also study streamers in pure nitrogen with a small addition of radioactive 85Kr to increase background ionization. While streamer velocities are amazingly insensitive to these changes, their overall morphology largely depends on gas composition, repetition rate and radioactive admixtures. Essential observations can are explained theoretically

Feather-like structures in positive streamers interpreted as electron avalanches

In experiments positive streamers can have a feather-like structure, with small hairs connected to the main streamer channel. These feathers were observed in pure nitrogen (with impurities of 1 ppm oxygen or less) but not in air. We hypothesize that these hairs are individual electron avalanches moving towards the streamer channel. Based on results of numerical simulations, we provide a theoretical explanation why these hairs are visible in nitrogen, but not in air

Feather-like structures in positive streamers.

In experiments positive streamers can have a feather-like structure, with small hairs connected to the main streamer channel. These feathers were observed in pure nitrogen (with impurities of 1ppm oxygen or less) but not in air. Based on results of numerical simulations, we provide a theoretical explanation for the emergence of these hairs as well as why the hairs are visible in nitrogen, but not in air