Effect of gas heating on the spatial structure of a traveling wave sustained Ar discharge

Abstract

In this work we report a theoretical and experimental study of the influence of gas heating on the spatial structure of a microwave Ar discharge sustained by a traveling surface wave. The theoretical analysis is based on a discharge model which couples in a self-consistent way electron and heavy particle kinetics, discharge electrodynamics, and gas thermal balance. The set of coupled equations used includes the electron Boltzmann equation, the rate balance equations for the most important excited species and charged particles, the gas thermal balance equation, and the equations describing wave propagation and power dissipation. The principal collisional and radiative processes which determine the populations in the Ar(3p54s) and Ar(3p54p) levels are accounted for. The field strength necessary for steady-state discharge operation is obtained from the balance between total rates of ionization (including direct and step-wise ionization and energy pooling reactions) and of electron loss due to the diffusion to the wall and bulk recombination. The gas thermal balance equation is solved using the experimentally obtained wall temperature as a boundary value. The model determines the axial discharge structure, i.e., the axial variation of the main discharge quantities. An experimental validation of the model predictions is achieved using probe techniques, emission spectroscopy, and radiophysics methods. In particular, spatially resolved measurements of the electron energy distribution function, gas temperature, wave electric field components, and wave attenuation have been carried out. As a result of the nonuniform wave power absorption along the wave path the gas temperature varies along the column. This variation induces axial changes in the neutral density and the reduced electric field which strongly affects the particle kinetics and the discharge electrodynamics, as demonstrated here.