Optical Radiation Efficiency of Surface-Wave-Produced Plasmas as Compared to DC Positive Columns

Abstract

The influence of the high-frequency (HF) field (200-900 MHz), gas pressure (0.05–1.0 Torr) and plasma density on the radial density distribution of excited atoms in a cylindrical HF sustained discharge of the surface-wave type is experimentally examined through optical emission spectroscopy, with the use of a tomography technique. These results illustrate the role of the radial inhomogeneity of the wave electric field intensity (the so-called skin effect) on the radial density distributions of excited atoms. This set of data, completed by similar measurements for the positive column of a dc discharge, is further used to investigate the optical radiation efficiency as a function of the field frequency. It shows that there exist discharge conditions for which the surface-wave plasma is more efficient, the optimum frequency then being around 200 MHz. The explanation for such a behavior calls on the frequency dependence of both the radial density distribution of the excited atoms and the electron energy distribution function (EEDF), the latter determining how the power absorbed from the HF electric field is shared between ionization and excitation processes. These various results are compared with the self-consistent model developed for surface-wave discharges in argon by Ferreira and co-workers.