Splitting and merging behaviors of the positive streamer propagating in an atmospheric pressure argon plasma jet

Atmospheric pressure plasma jet (APPJ) offers extensive potential applications and has become a hot topic in low-temperature plasma research. The streamer dynamics are crucial to the distribution of reactive species in APPJ. To shed light on the streamer dynamics, the behaviors of splitting and merging are reported in this work for the positive streamer propagating in an argon APPJ. Results indicate that two discharges (the negative and the positive streamers) occur at the voltage rising edge and the falling edge, respectively. Their discharge intensities vary with changing pulse amplitude (Va) and bias voltage (Vb). High-speed photography reveals that the track of the positive streamer is filamentary, whose structure evolves with increasing Va from a cone, a fork, and to a fork and void. Additionally, the fork-and-void structure becomes a two-void shape with decreasing Vb. Moreover, the positive streamer demonstrates various behaviors, including linear propagation, splitting, merging, double splitting, and double merging. Spatial distributions of streamer velocity, electron excitation temperature, electron density, and molecular vibration temperature are also investigated. It is deduced that splitting and re-splitting are attributed separately to the negative ions and the Penning effect in the interfacial layer. If the negative ions in the interfacial layer are insufficient or the Penning effect at the axis is strong enough, merging and re-merging occur, respectively. The results mentioned above are of great significance for improving the performance of APPJ.