Outgassing and plasma development during the early phase of vacuum surface flashover
Surface flashover at the electricalK stressed interface betsveen a solid dielectnc and the ambient medium is often the limiting factor in a high-voltage system. This work describes the unique application of an improved refractive index sensor to gather additional information about charge-carrier multiplication 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 (e.g., pre-flashover current, visible luminosity, x-ray emission, etc.), 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 of up to 1.5 kV/rad, a risetime of 10 ns or less (depending upon circuit gain), and a spatial resolution of 20 micron. This technique is simple, highly sensitive, inherently quantitative, and has high temporal resolution. Experimental results with the refractive index diagnostic, in this experimental apparatus, demonstrate a plasma channel which generally forms within 5 to 10 micron of the surface near the cathode, and in the range of 75 to 175 micron from the surface near the anode, during the exponential rise of the current to full breakdown. The data indicate that the position at which the plasma channel forms is altered by the application of a magnetic field. Mean electron drift velocities - derived from deflection and current measurements - are used to compute ionization frequencies from published data for those gases that typically adsorb onto dielectric surfaces. Ionization frequencies (of order l ns) computed in this manner correspond to those for Townsend gaseous breakdown, which implies the existence of a substantial neutral gas density in the plasma channel. These results are consistent with the saturated secondary electron emission avalanche (SSEEA) model of surface flashover, which postulates carrier multiplication as a result of gaseous ionization. Gaseous ionization, in the SSEEA model, occurs within a layer of desorbed neutral gas due to interaction with energetic electrons associated with the secondary avalanche. No direct measurement is presently known to exist for neutral gas desorption during the development phase of surface flashover.