Simulations of plasmas of positive column in rare gases

Abstract

Summary form only given. Positive Column (PC) plasma has found various applications especially in lighting industry. We have applied a comprehensive discharge model to revisit physical phenomena in collisional plasmas of rare gases. The computational model includes an electron Boltzmann solver based on two-term spherical harmonics expansion of the distribution function, continuum model of ions, neutral and excited atoms, and an equation for gas temperature. The Poisson equation is solved to resolve the radial distribution of the electrostatic potential and the sheath structure. The axial electric field is calculated self-consistently for a given discharge current. Simulations are performed in a wide range of gas pressures (pR=0.1-100 Torr cm) and a wide range of discharge currents from diffusion controlled to recombination controlled regimes. Among the physical phenomena studied in this work are specifics of electron kinetics for different operating conditions, radial constriction of plasma with increasing gas pressures and currents, and effects of gas heating on the discharge structure in different gases (Argon, Neon, Helium). We discuss in details the importance of nonlocal effects and Coulomb collisions for electron kinetics under different conditions, and details of chemistry models on plasma structure and composition. In particular, we clarify the relative importance of electron kinetics, metastable atom kinetics and gas heating on discharge constriction in different gases. We also study transition from ambipolar to free electron diffusion at small discharge currents, specifically important for capillar (micro) discharges. We calculate different types of current-voltage characteristics and compare with available experimental data.