Effects of UV Illumination on Surface Flashover Under Pulsed Unipolar Excitation

Summary form only given. Dielectric surface flashover in an atmospheric environment is an important consideration in the design of insulating support structures for pulsed, high voltage applications. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring at atmospheric conditions. We have previously shown qualitatively that UV illumination of the surface, either externally or possibly generated by the developing discharge itself, affects the distance between flashover path and surface for small gaps with a nonnegligible field component normal to surface. By studying the effects of UV illumination on the flashover behavior, information was gained about the underlying mechanisms of dielectric surface flashover. Utilizing a solid-state UV source with a much faster turn-off time than gas tubes along with the flashover testing apparatus\´ generally high temporal resolution enabled us to measure applied UV pulse, voltage, current and flashover self luminosity with high temporal precision. For all experiments, the dielectric flashover sample is placed inside an environmentally controlled chamber with a constant gas flow. Using advanced field simulations the electrode/dielectric geometry was designed to produce an optimized field shape for the testing of surface effects. Besides reducing the flashover delay times by up to 50%, the application of a 1 mW/cm², 20 mus UV pulse, prior to the voltage pulse, forces the flashover discharge in nitrogen to "hug" the surface rather than develop a few millimeters away from the surface along the field lines. Increasing the time delay between UV and voltage pulse application, it was revealed that the impact of the UV pulse on the flashover path becomes weaker with a time constant, tau, of ~ 3.1 mus. That is, after roughly 10 mus (3 times tau), the flashover path develops as without any UV application at all. In addition to experimental - data of dielectric surface flashover with varying degrees of UV illumination, we present a simple model that describes the observed behavior as well as a more advanced analysis utilizing a Monte Carlo type code for the electron collision dynamics.