Surface flashover in liquid nitrogen

The use of cryogenic high voltage components to achieve higher energy densities is limited by virtually unknown insulator characteristics for cryogenic conditions. Using a fast coaxial setup, the flashover phenomena of dielectric test samples (Lexan, Alumina) immersed in liquid nitrogen are measured with optical and electrical diagnostics with nanosecond time resolution. The flashover voltage reaches a maximum of 30 kV for a 1.75 mm gap after 3 shots, and averages to about 10 kV after conditioning. Three phases in the discharge development can be distinguished: Phase 1 is a rapid current rise to the mA-regime, with several current spikes with amplitudes of less than 5 mA and durations of typically 20 ns. This phase lasts up to several 100 ns. Phase 2 is characterized by a rapid ionization with a current rise to the impedance limited value of several 100 A of phase 3 in a few nanoseconds. Waveforms of the luminosity follow the ones of the current in general. The physical mechanisms leading to this development, and the difference to the flashover phenomena in vacuum, are discussed qualitatively