Quench currents of AC superconductor in supercritical helium

Supercritical helium (SHE) is proposed as a coolant for AC superconducting fault current limiters because it provides better electrical insulation than conventional pool-boiling helium. However, the effect of SHE cooling on the current carrying capacity of AC devices is presently unknown. Quench currents were measured for two single-strand NbTi conductors and a tightly-wound two-layer coil at 0-200 Hz frequency and 0-1 T DC background magnetic induction in boiling atmospheric helium (LHE) and 6-atm, 4.2 K pressurized helium (SHE). The AC quench currents for the Cu30%Ni matrix wire were 20/spl sim/30% lower in SHE, while the Cu10%Ni matrix composite showed almost identical quench currents in both coolants. Analysis suggests that higher temperatures resulting from the Cu30%Ni smaller thermal diffusivity may be responsible for its reduced current capacity in SHE.<>