Insight into the dynamic evolution behavior of subsonic streamers in water and their voltage polarity effect

Due to the complex interaction between liquid, gas, and plasma, the pre-breakdown process in water under quasi-static moderate electric fields, namely the development of subsonic streamers, was unclearly understood so far. In this paper, the dynamic evolution behavior of subsonic streamers and their voltage polarity effects were investigated. It was indicated that the whole streamer development process can be divided into two successive stages: bottom-up period characterized by root spherical expansion and OH (309 nm) emission line; top-down period characterized by head burst expansion and Hβ (486 nm), Hα (656 nm), and O (777 nm) emission lines. Further analysis revealed that the magnetic pinch effect on the internal plasma distribution determines the expansion mode of the streamer. The low capture energy of the solvated electron and local space charge accumulation make the positive streamer propagate faster at a low voltage level. However, the limited carrier resource and relatively divergent internal plasma distribution (weak magnetic pinch effect) hinder the propagation acceleration of the positive streamer with the applied voltage. Thus, the voltage polarity effect variation can be observed at high voltage levels. Finally, a novel framework model was proposed to depict the dynamic evolution behavior of subsonic streamers. Our results can provide a deeper insight into the electrohydrodynamics of dielectric fluid and promote the relevant industry applications.