Probing photo-ionization: Experiments on positive streamers in pure gasses and mixtures

Positive streamers are thought to propagate by photo-ionization whose parameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in nitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as in pure oxygen and argon. Our new experimental set-up guarantees contamination of the pure gases to be well below 1 ppm. Streamers in oxygen are difficult to measure as they emit considerably less light in the sensitivity range of our fast ICCD camera than the other gasses. Streamers in pure nitrogen and in all nitrogen/oxygen mixtures look generally similar, but become somewhat thinner and branch more with decreasing oxygen content. In pure nitrogen the streamers can branch so much that they resemble feathers. This feature is even more pronounced in pure argon, with approximately 10^2 hair tips/cm^3 in the feathers at 200 mbar; this density could be interpreted as the free electron density creating avalanches towards the streamer stem. It is remarkable that the streamer velocity is essentially the same for similar voltage and pressure in all nitrogen/oxygen mixtures as well as in pure nitrogen, while the oxygen concentration and therefore the photo-ionization lengths vary by more than five orders of magnitude. Streamers in argon have essentially the same velocity as well. The physical similarity of streamers at different pressures is confirmed in all gases; the minimal diameters are smaller than in earlier measurements.Comment: 28 pages, 14 figures. Major differences with v1: - appendix and spectra removed - subsection regarding effects of repetition frequency added - many more smaller change

Guided ionization waves: The physics of repeatability

Free to read on publisher website Guided ionization waves, or plasma streamers, are increasingly important for many applications in spanning materials processing and biomedicine. The highly reproducible, repeatable behavior of the most puzzling kind of the streamers–plasma bullets is highly attractive as it promises a high degree of control in many applications. However, despite a dozen years since the discovery of this phenomenon, the exact reasons for such behavior still remain essentially unclear. To understand the dynamics of the guided ionization wave (plasma bullet), a large number of works have been carried out and many interesting results have been reported. Here, we critically examine the available results and generalize the physical mechanisms of the guided ionization waves, which are of particular interest to practical applications of atmospheric-pressure plasma discharges, in general. The critical examination of the fundamental principles will show that, in order to propagate in a repeatable-mode, the plasma bullet must propagate in a channel with a high seed electron density (HSED), which is on the order of 109 cm−3. This review concludes that to distinguish guided ionization waves from traditional positive streamer discharges, it is most appropriate to describe an atmospheric-pressure discharge featuring a plasma bullet behavior as an HSED discharge. When the HSED condition is met, the dynamics of a plasma plume appears to be repeatable. On the contrary, it propagates in an unrepeatable mode and emerges more like a positive streamer discharge when the HSED condition is not satisfied. According to this theory, the transition of the propagation mode of the plasma bullet between the repeatable mode and the stochastic mode can be well explained. Besides by controlling the seed electron density around the transition region between the HSED discharge and the traditional positive streamer, this knowledge will help in better understanding of the positive streamer discharges in air, in cases relevant to practical applications of such plasma discharges in materials processing technologies, industrial chemistry, nanotechnology, and health care