Relaxation of heavy species and gas temperature in the afterglow of a N2 microwave discharge

Abstract

In this paper we present a self-consistent kinetic model to study the temporal variation of the gas temperature in the afterglow of a 440 Pa microwave nitrogen discharge operating at 433 MHz in a 3.8 cm diameter tube. The initial conditions in the afterglow are determined by a kinetic model that solves the electron Boltzmann equation coupled to the gas thermal balance equation and a system of rate-balance equations for N2 (X 1 ∑ g + , v ) molecules, electronically excited states of N2 , ground and excited states of atomic nitrogen and the main positive ions. Once the initial concentrations of the heavy species and gas temperature are known, their relaxation in the afterglow is obtained from the solutions to the corresponding time-dependent equations. Modelling predictions are found to be in good agreement with previously measured values for the concentrations of N(4 S) atoms and N2 (A 3 ∑u + ) molecules, and the radially averaged gas temperature Tg along the afterglow of a microwave discharge in N2 under the same working conditions. It is shown that gas heating in the afterglow comes essentially from the energy transfer involving non-resonant vibration–vibration (V–V) collisions between vibrationally excited nitrogen molecules, as well as from energy exchanges in vibration–translation (V–T) on N2 –N collisions.