Breakdown mechanism of liquid nitrogen viewed from area and volume effects

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN/sub 2/) to discuss the breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN/sub 2/ with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that the breakdown mechanism changed from an area dominant to volume effective region at larger electrode configurations in LN/sub 2/. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN/sub 2/. It was suggested that a mutual contribution of area and volume effects appeared in breakdown characteristics in LN/sub 2/ under thermal bubble conditions, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubbles and electrode surface condition for HV insulation of superconducting power apparatus.