Flashover mechanisms of bridged vacuum gaps based on cathode electric field measurement

In bridged vacuum gaps, it is believed that the secondary electron emission avalanche (SEEA) and the resultant insulator charging play a crucial role in the progress of discharge leading to the surface flashover. This paper deals with the electric field just before the flashover that includes the accumulated SEEA charge component (E<;sub>sF<;/sub>) and applied geometrical field component (E<;sub>gF<;/sub>) on a cathode and their relation to the flashover voltage. The test gap consists of parallel disc electrodes bridged by an insulating solid cylinder made of Al<;sub>2<;/sub>O<;sub>3<;/sub>. The test insulator changes its length from 5 to 50 mm. As it has been clarified in our previous works that surface roughness of the insulator affects SEEA charge accumulation, we have examined the roughness ranging from 0.2 to 6 μm. Aluminum electrodes are mainly examined, and copper and stainless electrodes are used for comparison. A ring type electrostatic probe embedded near the triple junction on the cathode is employed to measure the electric field. As a result, the total field strength (E<;sub>gF<;/sub> + E<;sub>sF<;/sub>) for various insulator conditions with aluminum electrodes is 6.7 kVmm<;sup>-1<;/sup> as an average. However, the rate (γ) of SEEA charge component that accounts for the total electric field varies according to the insulator length and the roughness. When γ exceeds ca. 15%, the flashover voltage Vb depends on the square root of the insulator length. On the other hand, V<;sub>b<;/sub> is a linear function of the insulator length when the SEEA charging is depressed and thus γ is smaller than 15%. On the condition that γ<;15%, the material of electrode makes an impact on the flashover voltage.