Abstract
This paper theoretically deals with electrical breakdown phenomena under a simulated quenching condition of high T superconducting coils. Bubble motion in liquid nitrogen under different electric fields is calculated and breakdown mechanism on the basis of the estimated bubble behavior is discussed. The basic electrode system is a Cylinder (Ø 10mm)-to-plane and applied voltage is changed between the breakdown voltage of liquid nitrogen and the one of nitrogen gas at the normal boiling temperature. The gap length is in the range of 0.5-40mm considering the practical space between coil turns, coil layers and coil wire to vessel wall in superconducting coils. The calculation results of bubble motion suggest that bubble behavior is influenced markedly by the gap length and applied voltage. Then the effect of thermally induced bubble on the breakdown voltage is suggested to depend on the gap length. Moreover it is found that distorted bubbles around the shortest gap may cause the reduction of breakdown voltage.
