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Experimental research on electrical breakdown strength of long-gap vacuum-insulated coaxial line under microsecond pulses

Experiments to test the breakdown voltage (Ub) of centimetre-scale vacuum gap using the coaxial electrodes under microsecond pulses are carried out. The vacuum gaps are set as 5.75 cm, 7.67 cm and 10.2 cm, respectively. The ratio of the outer diameter to the inner diameter of the coaxial electrodes for each gap is set as constant to produce a fixed field enhancement factor. The experimental results show that Ub increases while the electric breakdown field (Eb) decreases as the vacuum gap increases. In addition, the increasing and the decreasing tendencies are nonlinear. Since both the gap distance (g) and the electrode area (A) change for each set of experiments, both the two factors are taken into account together to obtain a unified formula to describe the Eb dependencies on g and A. The fitting results show that Eb is linearly dependent on g−α and A−β, where α is about 0.3 and β is about 1/5.6. Both the gap effect and the area effect are consistent with those of the plane-plate electrodes. The mechanism responsible for the coaxial long-gap vacuum breakdown under microsecond pulses is also discussed.