Outgassing and plasma development in the early phase of dielectric surface flashover in vacuum

Surface flashover along dielectric interfaces is the limiting factor in high voltage systems and in particle accelerator technology. The processes leading to flashover are far from being fully understood. With a high sensitivity laser beam deflection method, gradients of gas and plasma densities above the surface are measured, and correlated to high resolution current, x-ray, and luminosity signals, with a temporal resolution of better than 6 ns. Experimental results show a pre flashover phase with linearly rising current accompanied by x-ray emission during which no plasma is observed, followed by an exponential current rise associated with the build-up of a plasma channel several micrometers above the surface. Estimates of the electron drift velocity and the current density in the plasma channel point to gaseous ionization processes responsible for the plasma build-up. The results are consistent with the saturated secondary electron emission avalanche model, with electron induced outgassing as the main process causing surface flashover.