Michael J. Wolf

High-Current HTS Cables: Status and Actual Development

Second-generation high-temperature superconductors (HTS) REBCO are promising for high-field application due to the excellent Jc(B) performance at low temperatures and for power transmission at liquid nitrogen temperature. For power transmission with HTS, the formation of high-current HTS cables from single HTS tapes is desirable for cable currents of several 10 kA up to more than 100 kA. On the other hand, to avoid high inductances, which would cause high-voltage problems in case of quench or fast shutdown, high-current HTS cables are needed for larger high-field magnets, too, typically operated at 4.5 K or lower. In the last years, several HTS cable designs have been proposed, such as twisted stacked cable, CORC cable, and Roebel cable. This talk will give an overview of such proposals and highlight actual developments in HTS cable design and REBCO tape optimization, e.g., for highest field. In addition, an optimization of round TSTC strand will be introduced, which is designed for simple fabrication in long lengths for versatile use either for the production of cables to be used at high fields or for power transmission.

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 Ic(4.2 K, self-field) ≈ 30 kA for power transmission or with Ic(4.5 K, 14 T) ≈ 8 kA as a strand for high-current cables targeting large high-field magnets.

Toward a High-Current Conductor Made of HTS CrossConductor Strands

Over the last decade, several approaches for the fabrication of high-current cables from second-generation high-temperature superconductor (HTS) REBCO tapes were investigated, among them Roebel cable, conductor on round core, and several twisted stacked tape cable types. We introduced a novel type of stacked HTS tape arrangement, which is called HTS CrossConductor or HTS CroCo, aiming for high engineering critical current densities je that can be scaled to application-relevant lengths of conductors. Measurements of the critical current of a triple-HTS CroCo superstrand at T = 77 K and self-field conditions and at T = 4.2 K, as well as in fields up to B = 12 T, show the expected current calculated from the individual tape critical currents. Twisting experiments of the triple-HTS CroCo superstrand indicate a critical bending strain of around 0.6%. Based on these results, a Rutherford cable design for a cable with a critical current of 80 kA is suggested.