Effect of bubbles on liquid nitrogen breakdown in plane-plane electrode geometry from 100-250 kPa

Liquid nitrogen (LN₂) 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 LN₂. 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. This is particularly illustrated at 100 kPa. After the drop in breakdown strength the breakdown is believed to be due to the formation of a vapor bridge. Also the heater power at which the breakdown strength changes from that of LN₂ to that of gaseous nitrogen increases with increasing pressure. The data can provide design constraints for high temperature superconducting fault current limiters (FCLs) so that the formation of a vapor bridge can be suppressed or avoided.