Real-time detection of outgassing and plasma buildup during the early phase of dielectric surface flashover

Employing a newly developed refractive index sensor, we gather additional information about the processes involved in dielectric surface flashover. The sensor detects gradients in the refractive index above the dielectric surface, time-correlated with other signals of interest, by measuring the deflection of a focused laser beam. It utilizes a 10 mW HeNe laser beam incident on a bi-cell solid state photodetector to provide a signal with angular sensitivity on the order of 0.18 mV/μrad, and temporal resolution of 6 ns. This new diagnostic component is combined with current, voltage, luminosity, and x-ray sensors that have risetimes ranging from 0.4 to 3 ns. To the knowledge of the authors, this is the first application of this diagnostic principle to the problem of dielectric surface flashover. Preliminary measurements with the refractive index diagnostic demonstrate the existence of a time-changing refractive index above the dielectric surface accompanying the exponential rise of the flashover current. These measurements can be interpreted to indicate the formation of a plasma channel a few tens of microns above the dielectric surface, which expands with a velocity on the order of 10⁶ cm/s . A fraction of these measurements display an indication of neutral gas desorption, within the threshold of detection, prior to flashover.