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 (LN2) to discuss breakdown mechanism in cryogenic liquids for superconducting power apparatus. We measured breakdown voltages in LN2 with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that breakdown mechanism changed from area dominant to volume effective region at the larger electrode configurations in LN2. Moreover, we discussed the contribution rate of area and volume effects to the breakdown strength in LN2. It was suggested that mutual contribution of area and volume effects appeared in breakdown characteristics in LN2 under thermal bubble condition, 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.