Temporal evolution and mechanism of secondary streamers in N2/O2 mixtures at atmospheric pressure under DC voltage

In this work, the temporal evolution of secondary streamers in N-2/O-2 mixtures at atmospheric pressure under DC voltage were investigated using a two-dimensional axisymmetric fluid model. The effects of voltage amplitude, polarity and oxygen concentration were analyzed. The propagation of secondary streamers is divided into two stages: fast propagation followed by slow propagation. The interior of secondary streamers is filled with positive space charges, while the exterior contains negative space charges. The reduced electric field in the secondary streamer is initially linearly proportional to the applied voltage and decreases linearly with the length of secondary streamers. The length of secondary streamers is primarily determined by the applied voltage, regardless of the oxygen concentration. A reversed propagation direction is simulated under negative applied voltage. We propose a different mechanism for secondary streamers. The high-density positive charge layer near the electrode initiates the propagation of secondary streamers, and the extension of positive charge region dominates the propagation. The distribution of electric field along the channel is mainly determined by the distribution of net space charge. The propagation direction is opposite to the direction of electron drift. The secondary streamer eventually stagnates due to the decrease in electron density in high oxygen concentration, while it is caused by the uniform current density at the front of the secondary streamer in low oxygen concentration.