Critical Current and Stability of MgB$_2$ Twisted-Pair DC Cable Assembly Cooled by Helium Gas

Long length superconducting cables/bus-bars cooled by cryogenic gases such as helium operating over a wider temperature range are a challenging but exciting technical development prospects, with applications ranging from super-grid transmission to future accelerator systems. With limited existing knowledge and previous experiences, the cryogenic stability and quench protection of such cables are crucial research areas because the heat transfer is reduced and temperature gradient increased compared to liquid cryogen cooled cables. V-I measurements on gas-cooled cables over a significant length are an essential step towards a fully cryogenic stabilized cable with adequate quench protection. Prototype twisted-pair cables using high-temperature superconductor and MgB2 tapes have been under development at CERN within the FP7 EuCARD project. Experimental studies have been carried out on a 5-m-long multiple MgB$_2$ cable assembly at different temperatures between 20 and 30 K. The subcables of the assembly showed similar critical current, consistent with the I$_c$ of single twisted-pair MgB$_2$ cable tested previously. The assembly was shown to operate stably with flowing helium gas cooling in a moderate vicinity of the critical current. A low and reproducible specific contact resistance of ~200 nΩ cm2 was obtained.