Study On Characteristics Of Streamer Branching In Short Air Gap Discharge

As a method to produce non-thermal plasma, streamer discharge in gases is used for waste gas treatment, combustion ignition and other industrial applications. In many cases, the streamer discharge channel splits into several branches, which is called streamer branching. To study and to understand the streamer branching phenomenon in centimeter-level gap will help to improve the application efficiency of streamer discharge. However, with the limitation of observation conditions and calculation methods, the research on streamer branching phenomenon in short gap is not deep enough. The streamer branching mechanism and process are still not clear. The influence factors of streamer branching characteristics and the role of different factors in the process of branching lack enough exploration. Therefore, it is necessary to conduct further research on streamer branching theory and the influence factors of streamer branching characteristics.At first, this article introduces the streamer branching mechanism and analyses the Laplacian instability of the streamer head's net charge layer during its development. On this basis, the streamer branching process is described. Meanwhile, the coefficient of Laplacian instability ? is proposed to evaluate the net charge layer's instability. This coefficient is the ratio of the net charge layer thickness and the streamer head curvature radius. The net charge layer is closer to planar state when the coefficient is smaller, which makes the layer more unstable and streamer easier to branch.Besides, experimental research on streamer branching phenomenon in short gap is carried out. By constructing a short air rod-plate gap platform, single or repetitive discharge experiments are conducted. In the single discharge experiment, the influences of applied voltage and electrode structure(including the electrode shape and gap distance) on streamer branching characteristics are studied. The results show: when the voltage amplitude increases from 45 kV to 60 kV, the number of streamer branches and the streamer channel optical diameter both increase. Also, when the gap distance decreases from 6cm to 3.5cm, the number of streamer branches and the streamer channel optical diameter increase. Under the same gap distance, the hemisphere-shape rod electrode produces more branches than the cone-shape rod electrode. In the repetitive discharge experiment, the influences of pulse repetition rate on streamer branching characteristics are studied. The results show: when the pulse repetition rate increases from 1Hz to 300 Hz, the discharge becomes more and more intense, the conduction current also increases. When the repetition rate is 7Hz, the number of streamer branches is slightly smaller than the situation where the repetition rate is 1Hz, but there is only a little difference between two situations. Under these two repetition rate, the streamer branching channels generally do not follow the previous discharge's channels.