Electrode material dependence of vacuum breakdown initiation process
The development of high-voltage vacuum circuit breakers requires a fundamental knowledge of the vacuum breakdown phenomena, which are caused by plasma flare expansion initiated on cathode and/or anode surfaces. Therefore, the information on which electrode plays a dominant role in the development of the plasma flares is quite important for the improvement of dielectric strength in the vacuum circuit breakers and the achievement of the optimized spark conditioning. However, the relationships between experimental conditions and plasma flare development have never been identified. In this paper, high speed photography with a temporal resolution of 10ns was conducted to measure the electrode material dependence of the plasma flare development under a rod-to-rod electrode configuration with a gap length of 0.3mm. The cathode material was fixed at oxygen-free copper, and the anode materials selected in this study were oxygen-free copper, a mixture of 65% copper and 35% chromium in weight and a mixture of 25% copper and 75% chromium in weight. The plasma flares were always initiated on the cathode surface and the cathode flares were developed from the cathode surface to the vacuum gap in the present experimental conditions. In case of the anode containing 35% chromium, only the cathode flares were generated and they bridged the vacuum gap in the breakdown process. On the other hand, anode flares were also generated after the cathode flare initiation, and both flares encountered around the gap centre for the pure copper anode and the 75%-chromium anode. Further, the gap length dependence of the plasma flare development was also investigated by setting the gap lengths at 0.3, 0.5 and 0.75mm and using the 35%-chromium anode. The plasma flares were always initiated on the cathode surface for the gap lengths in this study. The volumetric ratios of the anode flares occupying the vacuum gap increased from 0 to 70% with increasing the gap length.
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