Numerical simulations of the angular dependence of magnetization AC losses: coated conductors, Roebel cables and double pancake coils

The AC losses in ReBCO coated conductors are large in situations when the conductors are subjected to a considerable magnetic field, as in rotating machines, transformers and high-field magnets. Roebel cables can reduce the AC losses in these cases. However, computer simulations are needed to interpret the experiments, understand the loss mechanisms, reduce the AC losses by optimizing the Roebel cable and design the cryogenic system. In this paper, we simulate and discuss the AC losses due to an applied magnetic field with an arbitrary angle with respect to the cable and taking into account a realistic anisotropic field dependence of the critical current density. We study the AC losses in the superconductor parts for the limits of very high coupling currents and completely uncoupled strands. The simulations for the uncoupled case also describe a double pancake coil with no transport current. For the simulations, we use two different numerical methods with complementary strengths. This serves as a mutual check of the correctness of the simulation results, which agree with each other. As opposed to what was expected, we found that the AC losses do not only depend on the perpendicular component of the applied magnetic field. We also found that the AC losses for applied fields with an orientation below 7° with the strand surface are reduced by more than one order of magnitude as compared with an untransposed cable. Therefore, we recommend to use Roebel cables for windings with important parallel components, such as transformers and high-field magnets.