HTS CroCo: A Stacked HTS Conductor Optimized for High Currents and Long-Length Production

In order to build high-current cables from second-generation high-temperature superconductor (HTS) rare-earth barium-copper-oxide (REBCO) tapes, numerous approaches were studied, such as conductor on round core, Roebel cable, and several versions of twisted stacked-tape cable types. Based on the work of Takayasu et al. and Uglietti et al., we developed and tested a modified type of stacked HTS tape arrangement optimized for high engineering critical current density. Key aspects were the implementation of a simple and reliable continuous fabrication routine for production of application-relevant lengths of twisted conductors in the range of 100 m and above and the subsequent enveloping by a seamless copper tube. Several samples of this HTS Cross-Conductor (HTS CroCo) were prepared successfully, both partially and fully equipped with REBCO tapes in untwisted and twisted configurations. The critical current of the samples was measured at T = 77 K and self-field conditions. The measurements showed the expected critical currents calculated from the individual tape values if taking into account the enhanced self-field in the tape arrangement. Furthermore, one untwisted partially REBCO-equipped sample was tested in the FBI facility at KIT at T = 4.2 K and in magnetic fields up to B = 12 T, showing good performance with no degradation even at high Lorentz forces. In addition, the mechanical performance of this sample was studied under tensile loads. No degradation could be observed, and the strain dependence was equal to that of single REBCO tapes. Due to the combination of excellent mechanical and electrical performance, the HTS CroCo is a promising candidate as a strand for long-length high-current cables, for example, with I _c (4.2 K, self-field) ≈ 30 kA for power transmission or with I _c (4.5 K, 14 T) ≈ 8 kA as a strand for high-current cables targeting large high-field magnets.