After the discovery of ceramic superconductors, research works for realizing high temperature superconducting devices have been advanced, where liquid nitrogen has high possibility to be used as a part of electrical insulation as well as the refrigerant. It is -very important from a point of superconducting device protection to elucidate surface breakdown characteristics along the spacer in gaseous condition.
We measured breakdown voltage of gaseous nitrogen under various condition along the cylindrical small gap spacer sandwiched by parallel plane electrodes. When the spacer has good contact with electrodes, flashover seldom occurred along the spacer surface. In this case, flashover voltage follows Paschen's law and depends only on the product of gas density and gap distance, even though there existed the spacer between electrodes However, when the contact between the spacer and the electrodes is perturbed by a small gap, the flashover voltage decreases and depends on the small gap area directly connected to the spacer edge rather than the total small gap area.
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