Pre-flashover luminosity along an alumina/vacuum interface

Two preflashover luminosity phases, one near the voltage peak (phase I) and the other just prior to the onset of voltage collapse (phase II), are found to be associated with the surface flashover of alumina in vacuum. The light emission is also observed under the no-breakdown condition (phase III). A new ancillary experiment, performed using three high-sensitivity photomultiplier tubes (two viewing the dielectric surface head-on and the other viewing edge-on) indicates that the plasma corresponding to phases I and III does not expand away from the insulator surface; whereas the plasma of phase II expands away from that surface. The observed results suggest qualitatively that the phases I and III may be attributed to a solid state process whilst the phase II may be a mixed response of both solid state and gaseous processes