Transitions between and control of guided and branching streamers in dc nanosecond pulsed excited plasma jets

Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The propagation behavior of those bullets is, in the literature, explained by the formation of an interface between the inert gas and the ambient air created by the gas flow of the plasma jet, which guides these discharges in the formed gas channel. In this paper, we examine this ionization phenomenon in uniform gases at atmospheric pressure where this interface between two gases is not present. By changing electrical parameters and adding admixtures such as oxygen, nitrogen, and air to the gas flow, the conditions for which plasma bullets are present are investigated. Nanosecond time-resolved images have been taken with an ICCD camera to observe the propagation behavior of these discharges. It is argued that the inhomogeneous spatial concentration of metastable atoms and ions, due to the laminar gas flow and the operation frequency of the discharge in the range of a few kilohertz, is responsible for the guidance of the ionization fronts. Furthermore, conditions have been observed at where the branching of the discharge is stable and reproducible over time in the case of a helium plasma by adding admixtures of oxygen. Possible mechanisms for this phenomenon are discussed.