CURRENT, LUMINOSITY, AND X-RAY-EMISSION IN THE EARLY PHASE OF DIELECTRIC SURFACE FLASHOVER IN VACUUM

With high-speed electrical and optical diagnostics, an attempt is made to elucidate the physical mechanisms leading to surface flashover. The experimental device uses a cable discharge to study self-breakdown along the surface of an insulator in vacuum. Preflashover current, breakdown voltage, luminosity, and soft X-ray emission are measured in temporal correlation with a resolution of one nanosecond. The results show a linearly increasing current in the sub-ampere range, and a corresponding linearly increasing luminosity, before an exponential increase of both signals takes over. The linear phase is accompanied by X-ray emission which ceases at the onset of the exponential phase. The strong influence of externally applied magnetic fields, on the linear phase, points to the existence of free electrons above the surface during the early phase of flashover. A linear current rise without magnetic field, and plateau formation of the current with an insulating magnetic field, indicates a saturation of the current amplification mechanism in the early phase.