Plasma Particle Analysis on and Near the Surface in Barrier Discharges Using Laser-Based Measurement

In a barrier discharge, often used in a production of microplasma such as plasma display panel, a dielectric surface acts as an electrode, and the characteristics of discharge are determined by surface conditions such as dynamics of excited and charged species on the surface. It is necessary to understand the behavior of plasma particles on and near the surface in order to analyze the discharge mechanisms and surface interaction, which is an interesting subject for a non-equilibrium science. However, the behavior of particles on and near the surface has not been well known. The lack of such data is attributed to experimental difficulties. For further development of discharges, it is necessary to gain knowledge of the dynamics of excited species and accumulated charges near or on the surface. We have proven that a technique using an optical evanescent wave such as laser-induced evanescent-mode fluorescence is very valuable for plasma diagnostics in the vicinity of a dielectric surface. In this paper a laser absorption technique using an optical waveguide is developed to quantitatively measure the density of the metastables in the vicinity of barrier surface. In the experiment, one barrier of discharge electrode was made with a LiNbO₃ crystal which surface had an optical waveguide with a thickness of 0.9x10^-3 mm. A laser transmitted in the waveguide of the barrier surface and was absorbed by Ar metastables existing in the vicinity of the surface. From the absorption spectrum, the metastable density on the surface is directly obtained. Further research work using a laser polarization technique is developed in order to measure directly a barrier wall voltage due to accumulated charges. An electro-optic crystal BSO with a thickness of 0.1 mm was used as a barrier electrode. This was applied to microdischarges with coplanar electrodes of ac PDP-like structure. Temporal and spatial surface charge density was measured and a breakdown voltage on the bar- rier surface is discussed.