Simulation of Striations in DC Glow Discharges in Nitrogen

Plasma Stratification that results in a self structured successive bright and dark region is a most widely observed instability phenomenon in dc glow discharges [1]. Striations have been observed over a wide range of currents and pressures for different discharge configurations. Despite the large number of experimental studies reported in this topic, numerical modeling of the phenomenon is mostly limited to rare gases [2]. In this study, we have developed a self-consistent fluid model of standing striations in nitrogen with detailed kinetics of vibrationally excited molecules. The model enabled us to simulate self-excited standing striations observed in Nitrogen dc glow discharges operating at moderate pressure (i.e., few Torrs) at the low current regime (0.02 – 0.35 mA) in a tube of radius 0.18 cm and length 8.5 cm. The one-dimensional model resolves the structure of discharge in the axial direction with the radial diffusion losses of charged species taken into account via an approximate loss term. The reaction kinetics having multiple vibrational excitation states consists of 26 species participating in 76 reactions. Simulations are conducted for a range of discharge current density (0.2 – 3.5 mA/cm
²
), and the Pressure x discharge radius (P
gas
R) values (0.2 – 1.5 Torr cm). The model successfully predicts the development of plasma stratification in the positive column. Results from the parametric studies indicate a strong dependence of this phenomenon on discharge current and pressure - an increasing number of strata (i.e., number of alternating bright and dark regions) observed at higher currents and pressures. Furthermore, kinetic analysis suggests that the vibrationally excited molecules associated with the lower energy levels (0.29 – 2.7 eV) strongly dictates the striation pattern as well as the phenomenon itself.