On the Behavior of a Plane Gaseous Cavity in an Insulating Liquid Subjected to an AC Voltage

The visualization of the deformation and division of a flat cylindrical gaseous cavity trapped in various dielectric liquids, subjected to ac fields (at different frequencies) has been made. A great variety of phenomena never previously mentioned is described and interpreted: the dielectrophoretic force (in absence of partial discharges in the gas) produces elongation and branching of the bubble while the coulomb force (when discharges are present) gives rise to vibration of its lateral interface and to spraying. The occurrence of the first partial discharges in the gas takes place at voltages much higher than the Paschen voltage (up to 10 times). Once discharges have started, they produce a progressive conditioning of the liquid and various types of discharges are observed. With a conditioned cell the mean time-lag for the first discharge to occur takes large values, and this is interpreted as the time for appearance by back-ground radiation of an electron able to produce an avalanche. The application of high electric fields, or the illumination of the bubble seems to trigger electron emission by the gas/liquid interface.