Liquid nitrogen breakdown due to thermally generated bubbles in plane-plane electrode geometry

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 breakdown properties of the liquid nitrogen. Experiments were performed using plane-plane electrode geometry where bubbles were introduced into the gap through a pinhole in the ground electrode. Bubbles were generated using one or more kapton heaters producing heater powers up to 30 W. Pressure was varied from 100-250 kPa. Breakdown strength was found to be relatively constant up to a given heater power and pressure at which the breakdown strength drops to a low value depending on the pressure. After the drop the breakdown strength continues to drop gradually at higher heater power. At low heater power the breakdown strength increases with pressure nonlinearly saturating at around 200 kPa, while the breakdown strength at high heater power increases according to Paschen law indicating a gas gap breakdown which is believed to be due to the formation of vapor bridge in liquid nitrogen. The heater power at which the breakdown strength changes from that of liquid nitrogen to that of gas nitrogen increases with increasing pressure. The data can provide design constraints for high temperature superconducting fault current limiters so that the formation of a vapor bridge can be suppressed.