Plasma On the Rocks: DC Atmospheric-Pressure Normal Glow Plasma Enhanced by Natural Basalt Microdischarges

DC normal glow discharges at atmospheric pressure in air and other gases are of interest in plasma processing, since they eliminate the need for low-pressure technology and vacuum-compatible materials. We have found that a DC normal glow discharge in air is stabilized and enlarged by passing it through a thickness of low-porosity rock such as basalt or granite. We observe a stable positive column in air up to 15 mm long with stable striations that depend on current. The airborne portion of the discharge shows characteristics of a normal glow discharge, including relatively constant voltage as current varies. A 13-kV, 5 mA discharge between a tungsten electrode 30 mm away from the surface of a basalt sample enlarges to over 15 mm diameter at the surface. We will present still and motion photography, spectra, and I-V measurements of this phenomenon, along with a simplified theory.