Modeling of atmospheric-pressure plasma columns sustained by surface waves.

A self-consistent two-dimensional fluid-plasma model coupled to Maxwell's equations is presented for argon discharges sustained at atmospheric pressure by the propagation of an electromagnetic surface wave. The numerical simulation provides the full axial and radial structure of the surface-wave plasma column and the distribution of the electromagnetic fields for given discharge operating conditions. To describe the contraction phenomenon, a characteristic feature of high-pressure discharges, we consider the kinetics of argon molecular ions in the charged-particle balance. An original feature of the model is to take into account the gas flow by solving self-consistently the mass, momentum, and energy balance equations for neutral particles. Accounting for the gas flow explains reported discrepancies between measured and calculated plasma parameters when assuming the local axial uniformity approximation. In contrast to the low-pressure case, the latter approximation is shown to be of limited validity at atmospheric pressure. The gas temperature is found to be a key parameter in modeling surface-wave discharges sustained at atmospheric pressure. It determines the radial and the axial structure of the plasma column. The calculated plasma parameters and wave propagation characteristics using the present two-dimensional fluid model are in good agreement with our set of experimental data.

Electron density and gas temperature from line broadening in an argon surface-wave-sustained discharge at atmospheric pressure.

We have used the collisional broadening of neutral argon lines to determine the electron density and gas temperature of a microwave discharge at atmospheric pressure. The gas temperature can be obtained from the Van der Waals broadening, provided that the Stark broadening is negligible. This can be achieved by using lines from low-lying levels (close to the ground state). On the other hand, lines corresponding to transitions from high-lying levels, which are more sensitive to Stark (quadratic) broadening, can be utilized to determine electron density. The electron density values obtained from the quadratic Stark broadening of argon atoms are in reasonable agreement with those derived from the linear Stark broadening of the H(beta) line. The proposed method ensures perturbation-free access to plasma parameters, which is not the case when adding hydrogen to the discharge, even in a small amount, to observe the Balmer series lines.

Modeling of microwave-sustained plasmas at atmospheric pressure with application to discharge contraction.

The modeling of microwave-sustained discharges at atmospheric pressure is much less advanced than at reduced pressure (<10 Torr) because of the greater complexity of the mechanisms involved. In particular, discharge contraction, a characteristic feature of high-pressure discharges, is not well understood. To describe adequately this phenomenon, one needs to consider that the charged-particle balance in atmospheric-pressure discharges relies on the kinetics of molecular ions, including their dissociation through electron impact. Nonuniform gas heating plays a key role in the radial distribution of the density of molecular ions. The onset of contraction is shown to depend only on radially nonuniform gas heating. The radial nonuniformity of the electric field intensity also plays an important role allowing one, for instance, to explain the lower degree of contraction observed in microwave discharges compared to dc discharges. We present a numerical fluid-plasma model that aims to bring into relief the main features of discharge contraction in rare gases. It calls for surface-wave discharges because of their wide range of operating conditions, enabling a closer check between theory and experiment.