Velocity-Diameter Relation of Nanosecond Pulsed Streamer Head in Coaxial Electrode

Pulsed discharge plasma in atmospheric pressure gases, a type of non-thermal plasma, has been studied for many years because it is one of the promising technologies for many applications such as removal of the hazardous environmental pollutants, waste-water treatment, and medical application. Due to the high energy electrons that are generated by high voltage pulses, they are capable of the very efficient production of highly reactive radical species and energetic photons. These products can consequently react with, for instance, particles in gas streams (e.g. pollutants, odor and dust), contamination in water. Typically, pulsed discharges consist of the primary streamer and the secondary streamer discharges phases. The tip of primary streamer discharge, which is called the streamer head, is paramount important for highly efficient radical production. In the recent study, a nanosecond (ns) pulsed power generator with a pulse duration of 5 ns has developed at Kumamoto University and then it has confirmed that ns pulsed discharge method has higher treatment efficiency than that of general pulsed discharge methods because the main discharge phase is primary streamer characterized by smaller heat loss. However, the propagation mechanism of a real-time ns pulsed streamer head is not understood yet. The study of streamer dynamics is of crucial importance for the elaboration of various applications of nonthermal plasmas produced by ns pulsed discharge. In the present study, a high-speed imaging system combined with four emICCD cameras was uniquely implemented at Kumamoto University, enabling to capture a real-time four continuous images of ns pulsed streamer. In the experiment, relationships between velocity and diameter of the streamer heads in a coaxial electrode were simultaneously investigated with the four emICCD cameras. As a result, the ns pulsed streamer head propagated with an acceleration between the electrodes, and furthermore the streamer head diameter increased significantly during its propagation. This can be explained from the effect of a significantly fast pulse rise rate peculiar to ns pulse voltage exceeding 10 kV/ns.