Electric breakdown in liquid nitrogen

The phenomenology of breakdown in liquid nitrogen is investigated with high-speed electrical and optical diagnostics (temporal resolution down to several 100 ps). The discharge apparatus uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen. Transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps), and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. First results on self-breakdown with a gap width of 1 mm and electrodes with 5 mm radius of curvature (breakdown voltage 42 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. Detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiating/assisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.