Helium breakdown near the critical state

Liquid or supercritical helium is the basic insulant for superconducting devices. Many efforts have been taken to find out its dielectric strength under various conditions. Paschen´s law has been found valid in helium gas down to very low temperatures, as far as the field strength does not exceed 15 to 20 kV/mm¹. In liquid and supercritical helium the situation is more complex. Often a large statistical scattering up to nearly one order of magnitude has been found. Most work was concentrated to evaluate a minimum breakdown strength level which can be taken as a base for reliable insulation systems. The minimum breakdown strength of liquid helium is correlated with the dielectric strength of the saturated vapour.