Dielectric strength of liquid helium impregnated plastic tapes

The dielectric for a flexible superconducting power transmission cable will necessarily be in the form of multi-layered tapes, impregnated by supercritical helium, wound with butt gaps to provide flexibility.1 Major consideration for the choice of an appropriate dielectric are that the dielectric should posses suitable mechanical properties at low temperatures (5–10°K), the material should have very low dielectric loss (< 20 μ radian) and it should possess high enough dielectric strength. Economic considerations dictate that the cable should operate at very high electric stress levels. For example, the Brookhaven National Laboratory superconducting cable design2 specifies an operating stress 10 MV/m, free of any significant partial discharge up to a stress of 20 MV/m. So far no dielectric material simultaneously satisfying all the mechanical and electrical requirements cited above has been found and the search for a suitable dielectric tape is still in progress. In order to arrive at an appropriate material it is necessary to evaluate various materials in terms of their mechanical and electrical characteristics. It is in this connection that we undertook a program of measurement of dielectric strength and partial discharge inception levels of polymeric insulating tapes at cryogenic temperatures.