Plasma Streamer Propagation in the Gas Phase, Catalyst Pores and Surface DBD*

    Streamer discharges at atmospheric pressure have been widely used for various environmental applications, such as hydrocarbon reforming, air pollution control, greenhouse gas conversion, and nitrogen fixation. However, the propagation mechanism and the interactions between the plasma streamer and dielectrics/catalysts are rather complicated and still far from being well understood. On one hand, depending on the polarity of the applied voltage, either a negative or positive streamer can be generated, and its propagation behavior and interactions with material surfaces are very distinct. On the other hand, the discharge structure could be very different, including the gas phase, packed dielectric-barrier discharge (DBD) with catalysts, internal space in catalysts and surface DBD (SDBD).

    We have executed multiple simulations based on a two-dimensional particle-in-cell/Monte Carlo (PIC/MC) collision model, to pursue a comprehensive understanding of the plasma streamer propagation. It is found that photoionization (background electrons) plays a vital role in the positive streamer evolution and its branching nature. The Debye length is an important criterion for plasma penetration into catalyst pores, i.e. a plasma streamer can penetrate into pores if their diameter is larger than the Debye length. The surface charging plays an important role in the streamer propagation and discharge enhancement inside catalyst pores, and in the plasma distribution along the dielectric surface, whose role greatly depends on the dielectric constant of the material. 

    Both a negative and a positive streamer can be exited simultaneously on the two sides of a twin SDBD, which leads to a large discharge volume. The streamer polarity (negative/positive) can be converted when the rf voltage polarity changes. This can further promote the streamer to spread into a larger area and interact more with the dielectric/catalyst surface.