AC dielectric performance of liquid helium impregnated multi-layer plastic film insulation

Several types of thin-film plastic insulation are being evaluated for possible use in ac superconducting underground transmission cables. The materials include polyolefins, polycarbonates, polysulphones, and nylons, in thicknesses ranging from 25µ to 125µ. Multiple-layer samples of the films are placed between 9.5 mm diameter, spherical stainless steel electrodes, and the film and electrodes encapsulated in epoxy. The epoxy casting is arranged to allow injection of liquid or gaseous helium between the plastic layers, at pressures up to 5 atm and a temperature of 4.2 K. Each casting contains eight pairs of electrodes.