Variation in time lags of vacuum surface flashover utilizing a periodically grooved dielectric

In a wide variety of high-power pulsed devices, vacuum surface flashover plays an important role in the system performance. The capability of grooved dielectric to hold off high-voltage pulse is investigated experimentally by using a long pulse electron beam accelerator, which is capable of generating a quasi-square wave with pulse duration of 180 ns and output voltage of 100-400 kV. It is found that both concavely and convexly grooved dielectrics increase promisingly the flashover time lags. With groove width of 2 mm, depth of 2 mm, period of 4 mm, electrode gap spacing of 100 mm, and diode voltage of 265 kV, the flashover time lag for polycarbonate is found to increase from 93.0 ns to 182.6 ns. While for polytetrafluoroethylene, its flashover time lag is increased from 45.0 ns to 81.2 ns, and for high-density polyethylene, its flashover time lag is increased from 72.4 ns to 100.5 ns. Electrical and optical diagnoses on temporal evolution of flashover channel confirm that surface flashover in vacuum is characterized by three-phase development, namely the secondary electron emission avalanche (SEEA), impact ionization in desorbed gas, and the final plasma driven breakdown. Based on this analyzing, it is found that the increase of flashover time lags is mainly due to the suppressing of SEEA process by using grooved structures.