Transition from one-step to stepwise propagation of a positive streamer with varying pulse durations in an argon plasma jet

Atmospheric pressure plasma jet normally behaves as a positive streamer, whose head is honored as a powerful chemical reactor that can produce active species for various applications. Revealing and understanding the streamer dynamics are of great significance to optimize the plasma for various applications. In this Letter, the dynamic transition from one-step propagation to stepwise propagation of the positive streamer is reported through varying duration (Δt) of a pulse voltage. Results indicate that per voltage cycle, there are two discharges occurring at the falling edge and the rising edge of the pulse voltage, respectively, which are separately referred to as Df and Dr. Comparatively, Dr has a higher intensity than Df. Moreover, the integral intensity for Df and Dr increases with increasing Δt. Fast photography reveals that the positive streamer fulfills the propagation in one step with wider Δt, while it undergoes the stepwise propagation with narrower Δt. The stepwise propagation is attributed to the negative ions left by the previous negative streamer. In addition, spatial distributions of streamer velocity and excited electron temperature are also analyzed from the role of negative ions. Moreover, electron density, electron temperature, and gas temperature are compared for the different behaviors of the positive streamer. These results provide a new approach to manipulate the streamer dynamics behaviors.