Cathode electric field preceding flashover of a bridged vacuum gap

This paper reports the characteristic of flashover voltage and its relation to the electric field strength on a cathode. The test gap consisted of parallel disc electrodes bridged by an insulating solid cylinder made of Al₂O₃. The electric field just before the flashover, which included an applied geometrical field component E_g and a surface charge component E_s, was measured by using a ring type electrostatic probe embedded in the cathode made of aluminum. As a result, we have found, although the rate of each field component varies, that the total electric field strength (E_g +E_s) is in the rage between 5.4 to 7.5 kVmm^-1 and is almost constant irrespective of neither the insulator length nor its surface roughness, which ranges from 5 to 50 mm and 0.2 to 6 μm, respectively.