Self-rotating dc atmospheric-pressure discharge over a water-surface electrode: regimes of operation

A dc atmospheric-pressure glow discharge produced between a metallic electrode and a water electrode is studied in this experiment. The discharge is characterized by means of visualization, high-speed imaging, voltage-current measurements, mass spectrometry and temperature measurements. Under certain conditions, the discharge exhibits a distinctive rotating motion in which the cathode spot remains stationary and the anode spot traces a circular pattern. Regimes of rotation occur in general at lower currents, at larger discharge gap lengths and when the water surface is the anode. Temperature measurements made in the rotating and stationary regimes show similar trends. Various metallic electrode materials, electrode geometries and discharge gases are investigated to determine the conditions under which rotation occurs. Rotation is only observed with a smooth cathode and a non-oxidizing anode material, such as water (or gold surface) that is either flat or otherwise provides no hindrances to the movement of the anode spot. Rotation is observed to occur in air and N-2-H-2 mixtures but not in pure N-2, H-2 or He; this suggests chemical mechanisms resulting in the formation of electronegative species as a possible cause for the rotation. Finally, measurements of the frequency of rotation of the discharge with respect to discharge length and current are made. These qualitative and quantitative results are used to evaluate various types of interactions as potential causes of this behavior.