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 con dition (phase III). A new ancillary experiment, performed using three high-sensitivity photomultiplier tubes (two viewing the dielectric sur face head-on and the other viewing edge-on) indicates that the plasma corresponding to phases I and III does not expand away from the insula tor surface; whereas the plasma of phase II expands away from that sur face. The observed results suggest qualitatively that the phases I and III may be attributed to a solid state process whilst the phase II ma y be a mixed response of both solid state and gaseous processes.