Plasma instability in the presence of negative ions

Abstract

Ion density fluctuations are studied in a diffusion-controlled argon plasma containing, as a dilutant, two electron-attaching species, carbon dioxide, and sulfur hexafluoride. A glow discharge tube is used in this experiment; electrical probes and digital spectral analysis are our principal diagnostic and analytical tools. It is found that the system becomes increasingly unstable as the concentration of the electron-attaching species is increased. Nonlinear mode-mode couplings have been identified and the coupling coefficients for these interactions have been computed. Turbulent fluctuations are observed to have pronounced three dimensionality with distinct axial and azimuthal behaviors. The power spectra of these fluctuations are composed of many discrete modes and follow a P(w)∝$w^{\mathrm{-}n}$ trend with 3<n<5. The importance of changes in the total discharge pressure is dependent on the electron-attachment cross section of the dilutant species. From these results we show that the ionization instability is probably responsible for the observed phenomenon.