A CORC ® cable is composed of several layers of helically wound high-temperature superconducting (HTS) tapes on a round core with the winding direction reversed in each successive layer. The cable is flexible but the flexibility is limited by the critical strain value causing breakage of the HTS layer when this strain level is exceeded. The cables for magnets in fusion reactors experience large mechanical and electromagnetic loads. These loads arise from the cabling, coil manufacturing, cooling, and magnet operation. In order to optimize the manufacture and operating conditions, the mechanical behavior of CORC ® cables must be understood for the different relevant loading conditions. The cable configuration with many contact interactions between tapes and the non-linear behavior of the materials during the production and operating conditions makes the modeling challenging. Detailed finite element (FE) modeling is required to account for these complexities. The FE modeling allows an accurate calculation of the stress-strain state of the cable components under various loads and avoids time-consuming large-scale experimental optimization studies. This work presents a detailed FE modeling of the 3D stress-strain state in a CORC ® wire under bending load. The elastic-plastic properties of the individual tape composite materials and its temperature dependence are taken into account. The FE model is experimentally validated by a multi-layer CORC ® bending test performed by Advanced Conductor Technologies LLC. A critical intrinsic tensile strain value of 0.45% is taken as the threshold where the individual tape performance becomes irreversibly degraded. The proposed FE model describes the bending test of the CORC ® wire adequately and thus can be used to study other types of loads. A parametric study is ongoing with dependent variables to pursue a further optimization of CORC ® cables and wires for various applications.

Cuprate high-temperature superconductors (HTS), such as RE-Ba 2 Cu 3 O 7−δ (REBCO, RE = rare earth), (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10−x and Bi 2 Sr 2 CaCu 2 O 8−x , have enabled the development of high-field superconducting magnets capable of generating magnetic fields far exceeding 20 T. The brittle nature of HTS requires elaborate means to protect them against the high stresses and strains associated with high-field magnet operation, and so far, has prevented reliable high-field HTS magnets from becoming a reality. Here we report a more than tenfold increase in the irreversible strain limit under axial tension (ε irr ) to over 7% in optimized high-current conductor on round core (CORC ® ) conductors, compared to the REBCO tapes from which the CORC ® conductor is wound. Minimizing the tape winding pitch of the helical wind mechanically decouples the brittle REBCO film from the overall conductor. The REBCO tapes behave as springs, limiting the rate at which applied strain is transferred to the ceramic film. In addition, high-strength alloy cores allow the critical stress (ε crit ) under axial tension at which initial degradation of CORC ® conductors occurs to exceed 600 MPa, making them one of the strongest superconductors available. Mechanically decoupling the ceramic REBCO films from the overall CORC ® conductor allows effective protection against the high operating stresses in high-field magnets. This breakthrough presents a monumental shift for HTS magnet technology, bringing reliable high-field superconducting magnets for compact fusion machines, the next generation of particle accelerators, and 40-60 T research solenoids within reach.

Many high-temperature superconductor (HTS) applications require superconducting cables with high currents while operating in an alternating magnetic field. HTS cables should be composed of numerous superconducting tapes to achieve the required current capacity. Alternating current and magnetic fields cause AC losses in such cables and can provoke conductor instability. AC losses and contact resistances were measured of several cable designs based on commercially available REBCO tapes at the University of Twente. The AC loss was measured under identical conditions for eight REBCO conductors manufactured according to three types of cabling methods—CORC® (Conductor on Round Core), Roebel, and stacked tape, including a full-size REBCO CICC (cable in conduit conductor). The measurements were done at T = 4.2 K without transport current in a sinusoidal AC magnetic field of 0.4 T amplitude and frequencies from 5 to 55 mHz. The AC loss was measured simultaneously by calibrated gas flow calorimeter utilizing the helium boil-off method and by the magnetization method using pick-up coils. Also, the AC loss of two CORC® conductors and a Roebel cable was measured at 77 K. Each conductor was measured with and without background field of 1 T. The measured AC coupling loss in the CORC® and Roebel conductors is negligible at 4.2 K for the applied conditions while at 77 K coupling loss was observed for all conductors. The absence of coupling loss at 4.2 K can be explained by shielding of the conductor interior; this is confirmed with measurement and calculation of the penetration field of CORC® and Roebel cables. The inter-tape contact resistance was measured for CORC® and stacked tape samples at 4.2 and 77 K. It was demonstrated that a short heat treatment of CORC® conductor with solder-coated tapes activates tape-to-tape soldering and decreases the contact resistance. The reduction of contact resistance by two orders in magnitude to tens of nΩm is comparable with the interstrand contact resistance in ITER Nb3Sn type conductors.

CORC® cables and wires are composed of spiraled HTS REBCO tapes, wound in multiple layers, and can carry very high currents in background magnetic fields of more than 20 T. They combine isotropic flexibility and high resilience to electromagnetic and thermal loads. The brittle nature of HTS tapes limits the maximum allowable axial tensile strain in superconducting cables. An intrinsic tensile strain above about 0.45% will introduce cracks in the REBCO layer of straight HTS tapes resulting in irreversible damage. The helical fashion at which the REBCO tapes are wound around the central core allows tapes to experience only a fraction of the total axial tensile strain applied to the CORC® wire. As a result, the critical strain limit of CORC® wires can be increased by a factor of more than 10 that of REBCO tapes. Finite element (FE) and analytical models are developed to predict the performance of CORC® wires under axial tensile strain. A parametric analysis is carried out by varying the winding angle, the Poisson’s ratio of the CORC® wire core, the core diameter, and the tape width. The results show that a small variation in winding angle can have a significant impact on the cable’s axial tensile strain tolerance. While the radial contraction of the helically wound tapes in a CORC® wire under axial tensile strain depends on its winding angle, it’s mostly driven by the Poisson’s ratio of the central core, affecting the tape strain state and thus its performance. Contact pressure from multiple layers within the CORC® wire also affects the CORC® wire performance. The FE model can be used to optimize the cable design for specific application conditions, resulting in an irreversible strain limit of CORC® cables and wires as high as 7%.