Surface breakdown on insulators at cryogenic temperatures
Butcher, Michael David
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Developments of high power systems have lead to an increased use of cryogenic components. Since virtually all high-voltage electrical systems use solid dielectrics as insulators it is important to understand the basic characteristics of how these insulators will perform at cryogenic temperatures. A fast coaxial setup is used to test dielectric samples in vacuum, at cryogenic temperatures of less than 100 K. Combined with fast, nanosecond region, electrical and optical diagnostics the very early flashover development processes can be observed. There are three stages in the development process of self-breakdown. Phase I represents a current rise to mA amplitudes within a few nanoseconds. This rise is probably associated with field emission. Phase n is a slower current rise to ampere amplitudes over a period of up to a few hundred nanoseconds. Phase II is most likely associated with saturated secondary electton emission avalanche. Finally, Phase III represents the fin2d breakdown process, where the current rises to amplitudes of several hundred amperes. This is associated with a rapid gaseous ionization just above the surface of the sample caused by electton induced outgasing. The temperature dependence of these three stages is examined, in detail, experimentally for Lexan, a polymer, and Alumina, AI2O3, a ceramic. Phase I shows a current amplitude for the lower temperature that is 4 to 5 times as high as that of room temperature for both Lexan and Alumina. Also there is a tendency for the cooler samples to have higher initial breakdown voltages for the first few shots. Specttoscopic measurements of the flashover event were taken to try and determine the species of gases released from the surface of the sample and electtodes. Visible images of the early phase of breakdown were taken to examine the discharge in the early phase of flashover.