Spot-Halo Hexagon Pattern in Dielectric Barrier Discharge

We report on the first observation of a spot-halo hexagon pattern composed of a center spot and an outer hexagonal halo in a dielectric barrier discharge system. It bifurcates from an unstable state with an irregularly shaped halo and finally to hexagon pattern with a bright background with increasing voltage. Instantaneous images taken using high-speed cameras indicate that the hexagonal halos result from surface discharges (SDs) ignited around the center spots and the SDs are induced by volume discharges (VDs). The SDs cannot extend to the midperpendiculars of two neighboring VDs in the spot-halo hexagon pattern while they can be filled with the entire background in the hexagon pattern with a background. The simulation of electric fields of wall charges further clarifies the formation mechanism of the hexagonal halo.