Quench Property of Twisted-Pair Superconducting Cables in Helium Gas

CERN´s twisted-pair superconducting cable is a novel design which offers filament transposition, low cable inductance and is particularly suited for tape conductors such as 2G YBCO coated conductors, Ag-sheathed Bi2223 tapes and Ni/Monel-sheathed MgB₂ tapes. A typical design of such twistedpair cables consists of multiple superconducting tapes intercalated with thin copper tapes as additional stabilizers. The copper tapes are typically not soldered to the superconducting tapes so that sufficient flexibility is retained for the twisting of the tape assembly. The electrical and thermal contacts between the copper and superconducting tapes are an important parameter for current sharing, cryogenic stability and quench propagation. Using an MgB₂ twisted-pair cable assembly manufactured at CERN, we have carried out minimum quench energy (MQE) and propagation velocity (v_p) measurements with point-like heat deposition localized within a tape. Furthermore, different contacts between the copper and superconductor around the hot spot have also been studied, including the co-twisted assembly in Kapton wrapping and locally separated tapes. The measurements have been performed in cooling helium gas at temperatures between 20 K and 35 K, with different current fractions with respect to the thermal runaway current. The results suggest a potential optimization strategy, compromising between: a higher stabilization with better contact between the copper and superconducting layers; and a faster propagation velocity and easier quench detection with a higher contact resistance.