Characteristics and mechanisms of transition from filament to homogeneous glow in atmospheric helium dielectric barrier discharges under variation of the applied voltage amplitude

In this paper, a 2D fluid model is employed to investigate the radial evolution of the discharge structures in a helium dielectric barrier discharge with 100 ppm nitrogen impurity. By elevating the applied voltage amplitude (V-am), the discharge exhibits some distinctive radial evolution features that comprise three aspects: (1) the lateral migration of the peripheral filament, i.e. outward filament migration nearby the electrode edge; (2) the reduced intervals between two successive filaments; (3) the growth in the number of filaments. It is revealed that the radial position of the peripheral filament is basically consistent with that of the initial local intense discharge, whose location is closely related to the surface charge distribution during the initial breakdown. An increase in V-am reduces the duration of the current pulse, and hence, the displacement of space charges is restrained. When more charges are restrained in the gap rather than being attached to the dielectric surfaces, the surface charge distribution becomes more uniform, which contributes to the lateral migration of the peripheral filament. Meanwhile, the lateral uniformity of Penning ionization rate is improved with V-am increasing, and the seed electron level in the intervals becomes comparable to that in the filamentary channels, leading to a more uniform radial seed electron profile that attenuates the electric field distortion. As a result, the intervals between two adjacent filaments are shortened. With the lateral migration and reduced intervals as V-am increases, we observe the growth in the number of filaments, and the improvement of the radial discharge uniformity.