Non-uniform field gaps partially immersed in liquid coolants are essential components in any cryogenic or superconducting apparatus or equipment. For example, conducting wires are led out of the liquid coolant into thee gaseous phase in normal conditions and a triple junction of electrode, liquid, and gaseous phase is formed at the liquid surface. In abnormal conditions in superconducting magnet coil such as a quench, a local hot spot due to wire movement or vapor lock results in a gaseous bubble in a coil layer and creates essentially a double layer of liquid coolant and the gaseous phase.
We have studied the breakdown behavior of non-uniform field gaps partially immersed in liquid nitrogen, completely immersed in liquid nitrogen, and wholly in the gaseous phase. This leads to a fundamental understanding of the physical processes determining the breakdown voltage at the triple junction of coolant liquid, its gaseous phase and the electrode. We show that a short gap partially immersed in liquid nitrogen gives higher breakdown voltages than a gap wholly in the liquid or wholly in the gas, when the gap has an extremely high stressed electrode with positive polarity.

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