Investigation of vacuum gap breakdown under microsecond pulses

Experiments on centimeter scale vacuum gap breakdown between plate electrodes under microsecond pulses were carried out. The images of vacuum gaps formed by plate titanium electrodes were obtained in each period of vacuum discharge by an optical diagnostic system. The images showed that the plasma formed initially on the cathode surface; the anode plasma formed in several hundred nanoseconds due to the bombardment of electron-beam; the plasma expanded along with the development of vacuum discharge, and the average horizontal and vertical velocities of the cathode plasma were about 1.8 cm/μs and 4.2 cm/μs, respectively. Based on the results, a theoretical model considering the space-charge-limited effect and the transmission line theory was established. The calculation results corresponded to the experimental counterparts. Through comparing the breakdown strengths of the vacuum gaps formed by titanium cathodes and the anodes made of titanium alloy, stainless steel and brass, respectively, experimental results showed that there was no significant difference in the initial breakdown field strength between the vacuum gaps with different anode materials and the completely breakdown field strengths of vacuum gaps showed positive correlation with the melting point and the specific heat capacity of the anode material.