Life expectancy and failure mechanisms of lapped synthetic tape impregnated with supercritical helium

Investigations into the life expectancy and failure mechanisms of cable models comprising layers of wrapped polymer tape impregnated with supercritical helium are described. Over the limited times of the tests the life law Ertt = constant is seen to apply, and, for the materials comprehensively tested, i.e high-density polythene, Valeron, polypropylene and polysulphone, n was found to be in the range 7 to 11. Valeron and polysulphone do not shatter when cold, and the extrapolated 30-year life stress was in the same order as the 1 pC inception stress, indicating that failure was due to the gradual erosion of the tapes by the butt-gap discharges. With polypropylene and polythene, small local shattering was observed in the mechanically highly stressed tapes examined immediately prior to breakdown. For those latter materials the life is determined by discharge erosion and shattering, and the 30-year life and inception stresses are not of the same order. Valeron is recommended as the best tape material for superconducting cable insulation; however, its AC working stress (1 pC) is less than the 10 MV m¿¿2 often used for dielectric design of superconducting cables.