Electrical Breakdown of Ice at Cryogenic Temperatures
An ice or ice-silica compound system is attractive for electrical insulation at cryogenic temperatures. A sample of ice without silica was found to have a breakdown strength of 0.3 MV/cm for ac voltages, and 0.6 MV/cm for impulse voltages. The breakdown strength of an ceicesilica compound system was 0.7 MV/cm for ac voltages. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids may exist, since partial discharges were observed at about half the value of the ac breakdown voltage. The relationship between partial discharges and microscopic voids, and the voltage-time characteristics of this insulation system were studied.
Effect of ice attached to electrode on pre-breakdown current in liquid nitrogen
The effect of ice on DC prebreakdown events was investigated using a needle-to-plane electrode system in liquid nitrogen kept at 77.3 K. It was found that there were remarkable features in the effect of ice on prebreakdown current, corona onset voltage, and breakdown voltage. The breakdown voltage at atmospheric pressure depends on microscopic cracks in the ice.
Dielectric Breakdown of Liquid Helium
The dielectric strength of liquid ${\mathrm{He}}^{4}$ under its saturated vapor pressure has been determined from 1.2 to 4.2\ifmmode^\circ\else\textdegree\fi{}K. With $\frac{3}{8}$-inch diameter spherical steel electrodes 0.15 mm apart the average breakdown field, ${\overline{E}}_{b}$, is approximately 1 Mv/cm, nearly independent of temperature. At spacings of 0.5 and 1 mm, ${\overline{E}}_{b}$ is lower, and appears to decrease with decreasing temperature below 2.5\ifmmode^\circ\else\textdegree\fi{}K. The mechanism of breakdown is not clear, but field emission of electrons from the cathode is probably involved, because the average breakdown voltage ${\overline{V}}_{b}$, obtained with a point and plane electrode system when the point is negative, is about half that obtained when the point is positive, both above and below the $\ensuremath{\lambda}$-point. Pre-breakdown currents were never detected, and were probably less than ${10}^{\ensuremath{-}13}$ amp, implying an electrical resistivity for liquid helium of ${10}^{18}$ ohm-cm.