High-Speed Photography of Surface Flashover Across High-Density Alumina in Vacuum under Direct and Impulse Voltages

High-speed streak and framing photography of surface flashover across solid insulators in vacuum is reported. Experiments were performed with direct and impulse voltages. It is shown that the initial propagation of a visible arc across an alumina surface in vacuum occurs at a speed greater than 10⁹ cm . s^-1 . It is shown that previously observed two-stage arcs under impulse voltages are the result of the interaction between the external supply and the arc impedance.