Electric Strength of Polymers Under Vacuum at Cryogenic Temperatures

Abstract The electric strength of solid polymers under high vacuum is investigated at a temperature ranging from 293°K down to that of liquid helium (5°K) under power frequency alternating- and direct-voltages. The electric strength of polymers was found to be very dependent on the pre-breakdown partial discharges when measured in cryogenic medium whether gas or liquid. When helium was used as coolant, discharges may extend very far from the electrodes and thus exerted high electric stresses on large areas of the sample under test. These discharges may produce flash-over and serious troubles. It may also lead to localised breakdown of polymers which may give apparent reduction in dielectric strength. A large increase in the electric strength of polymers was observed when used at cryogenic temperatures under vacuum; the increase was very significant for polyethylene and polyimide specially under direct voltages. The results showed promising properties and it is concluded that the high vacuum will offer the solution to many insulation problems at liquid helium temperatures in cryogenic and superconducting equipment.