Vacuum surface flashover: A high-pressure phenomenon

Desorbed gases from insulator surfaces may be much more important in flashover initiation than properties of the dielectric surface. Ambient neutral densities, immediately above the insulator surface, in the initial stages of flashover have been calculated to lie in the range from 2×1017/cm3 to 2×1021/cm3 with a mean of about 1×1019/cm3, whether flashover occurred on surfaces in air at atmospheric pressure or in vacuum as low as 10−6 Torr. The evidence presented indicates that, for dc and microsecond pulse voltages, surface flashover is a local, relatively high-pressure phenomenon. Comparison of field strengths of surface flashover for various insulating materials perpendicular to the electrodes, operating pressures, and voltage waveforms appears to indicate that flashover values range well within an order of magnitude (∼18–80kV/cm). A filamentary nature of the flashover discharge is shown and is consistent with gas breakdown phenomena. Ionization wave front velocities are examined and these compare favorably with those obtained from gaseous breakdown studies.