Effect of thermally induced bubbles on the electrical breakdown characteristics of liquid nitrogen

In any practical large-scale superconducting magnet system, abnormal high voltage across the coil layer and thermally induced bubbles in the cryogenic liquid coolant appear, during the process of quenching, with the spread of the normal zone. The electrical breakdown characteristics of liquid nitrogen are investigated under a simulated quenching condition, i.e. in the presence of thermally induced bubbles and an electrical field with a rise time of 1-30 ms. It is found that the application of an electrical field will promote the formation of bubbles fixed on an electrode and the production of vapour locking between the electrodes, and the time required for the vapour locking is at least 1 ms. To analyse quantitatively the deformation of a bubble in the presence of a time-varying electric field, a simplified theory is proposed to explain the motion of the bubble. In the theory, the existence of a thermal layer around the bubble and the use of the Clausius-Clapeyron equation, as the relationship between vapour pressure in the bubble and ambient temperature, are assumed. The theory suggests that a partial discharge inside the bubble occurs, although the light pulse from the discharge was not detected in the experiments. A bubble breakdown mechanism in the absence of thermally induced bubbles is also discussed, on the basis of the bubble deformation process under a time-varying electric field.<>