| Based on a two-dimensional equilibrium fluid model with a local field approximation, atmospheric pressure glow discharges (APGs) in helium with nitrogen impurities are studied. The model self-consistently solves the Poisson equation for the electric field and the continuity equations for the species' densities. The momentum equations are simplified by the drift-diffusion flux. The electrons, helium atomic and molecular ions, helium metastables, and nitrogen molecular ions are included in the simulation.; The model reveals a Townsend phase, a fast ionization phase, and a diffusion phase in the discharge initiation processes. The diffusion phase is the longest in the discharge initiation process. If the non-uniformities, which formed during the fast ionization phase, decay completely in the diffusion phase, a uniform discharge appears. Otherwise, a filamentary discharge is obtained. A periodic steady state is reached 8-10 ac cycles after the first gas breakdown.; The study of the formation mechanism shows that the high pre-ionization level and the large seed electron density are essential for the formation of the APGs. It is demonstrated that Penning ionization is the dominant ionization mechanism in the discharge. The existence of Penning ionization not only reduces the gas breakdown voltage, but also increases the effective ionization frequency at lower electric field and boosts the pre-ionization level at the beginning of the gas breakdown.; The model also predicts the formation of self-organized filaments in the discharge gap. The numerical results show that the number of filaments increases with the increasing frequency and voltage amplitude. The coalescence and overlapping of the large number of filaments lead to a uniform appearance of the discharge. A smaller number of filaments form for a dielectric material with lower permittivity. A uniform discharge is obtained with the continuous reduction of the filaments. Although a uniform discharge can be obtained both ways, the transition mechanisms are different. Continuously increasing the number of the filaments and the coalescence of the filaments give rise to a glow discharge, whereas reducing the filaments by using a material with lower permittivity leads to a Townsend-like discharge. |
