QUENCH CURRENTS OF AC SUPERCONDUCTOR IN SUPERCRITICAL HELIUM

Supercritical helium is proposed as a coolant for a.c. superconducting fault limiters, because it provides better insulation than convention al pool-boiling However, the effect of SHE cooling on the current carr ying capacity of a.c. devices is presently unknown. Quench currents we re measured for two single-strand NbTi conductors and a tightly-wound two-layer coil at 0-200 Hz frequency and 0-1 T d.c. background magneti c induction in boiling atmospheric helium (LHE) and 6-atm, 4.2-K press urized helium (SHE). The a.c. quench currents for the Cu30%Ni matrix w ire were 20-30% lower in SHE, while the Cu10%Ni matrix composite showe d almost currents in both coolants. analysis suggests that higher temp eratures resulting from the Cu30%Ni smaller thermal diffusivity may be responsible for its reduced current capacity in SHE.