Self-excited standing striations in moderate pressure dc nitrogen glow discharge*

Despite the many experimental studies reported on plasma stratification, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed electron and vibrational state kinetics is employed to simulate moderate-pressure (i.e. a few Torrs) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model includes ambipolar diffusion of ions and electrons to the wall. The model predicts self-excited standing striations in nitrogen for a range of discharge currents (~0.018 – 0.080 mA cm^-2). The impact of electron induced reaction rates obtained from numerical solution of a local two-term and a multi-term Boltzmann equation on the predictions are assessed. Our analysis indicates that the striations result from the undulations in electron mean energy (temperature) caused by an interplay between ionization and vibrational excitation processes. The vibrationally excited molecules associated with the lower energy levels are found to significantly influence nitrogen plasma stratification and the striation properties. Parametric studies show that the striation length inreases linearly with increasing the tube radius but increases in a non-linear fashion with increasing discharge current. The predictions from the model agree favorably with our experimental measurements.