Breakdown behavior of cryogenic liquids in the presence of thermal bubbles under ramp stress

V-t characteristics were measured and a thermal-bubble deformation process was observed in liquid nitrogen and helium with a slow-rising voltage simulating the quenching conditions in large superconducting magnet coils. Breakdowns in cryogenic liquid in the presence of boiling are found to be governed by breakdown in locked vapor which is formed by growth of bubbles fixed on the heater surface, which results in the flat region of the V-t curve, and by breakdown in a suspended bubble which is elongated relatively by electric field before breakdown, which occurs in the short time region of the V-t curve. The minimum value of the V- t curve coincides approximately with the breakdown voltage of the saturated gas at boiling temperature