Novel Insulator Designs for Superior DC Hold-Off in Bridged Vacuum Gaps

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Abstract
The dielectric strength of insulator-bridged vacuum gaps has been investigated under dc stresses. Contrary to many previous investigations, careful study undertaken here reveals that negative-angled conical frustrums withstand high er conditioned withstand voltages than positive frustrums and cylindrical insulators. The superior voltage perform ance of negative-angled conical frustrums over positive angled ones stems from negative charging of the former which supresses electron emission from the cathode triple junction X-ray emission within the bridged vacuum gap, and electron interaction with the insulator surface, due to emission from the cathode triple junction, are both shown to lower the voltage hold-off significantly. Novel insulator profiles were tested which exhibited significantly superior voltage hold-off over conventional cylinders and conical frustrums. The design strategy for such designs relied on: (1) providing stress relief at the cathode triple junction by facilitating the self-generation of negative charge on the insulator region near the critical junction, (2) minimizing X-ray production from the anode due to the impact of elec trons, and (3) minimizing electron interaction with the in sulator surface by moving the cathode triple junction away from the main body of the insulator.