Breakdown in liquid nitrogen in the presence of thermally generated bubbles for different electrode geometries

Liquid nitrogen is used as the cryogen and dielectric for many high temperature superconducting high voltage applications. When a quench in the superconductor occurs, bubbles are generated which can affect the dielectric properties of the liquid nitrogen. An experiment has been set up to generate bubbles in liquid nitrogen. Bubbles were generated using a kapton heater. Three different electrode geometries were applied: plane-plane, sphere-sphere, and sphere-plane. Breakdown measurements were made in both open bath liquid nitrogen and in a pressurized dewar at pressures up to 200 kPa absolute. The voltage applied was 60 Hz AC. For sphere-plane it was observed that the breakdown did not always occur at the minimum gap and this was likely due to the actual location of the bubble when breakdown was initiated. For plane-plane geometry where bubbles were generated in the plane electrode, breakdown voltages dropped at a certain heater power and remained low thereafter. The heater power at which the drop occurred increased with pressure. Breakdown data for subcooled liquid nitrogen will also be presented.