Dependence of current carrying capacity and AC loss on current distribution in coaxial multi-layer HTS conductor

We had developed a simulation method of current distribution in coaxial multi-layer HTS conductor and investigated influence of the nonlinear voltage-current characteristic of HTS tape on current distribution. It had been reported that homogeneous current distribution, especially the same layer current, is effective in terms of reducing AC loss. There are, however, many sets of cable parameters to achieve homogeneous current distribution in such the coaxial multi-layer cables. Therefore, using our developed evaluation method, we numerically investigated the relationship between AC loss and the cable parameters such as twisting pitch, radius, and direction in coaxial three- and four-layer conductors. We evaluated both hysteresis loss and flux flow loss as AC loss using the Norris's model and V-I characteristic of HTS tape, respectively. The critical current of whole cable and current density of each tape are key parameters in terms of reducing AC loss. The larger twisting pitch is better for increasing the critical current of cable due to the greater number of usable tapes and the shorter tape-length per unit length of cable in longitudinal direction. Alternate twisting pitch, however, is ineffective for increasing the critical current due to small twisting pitch and small number of tapes for realizing homogeneous current distribution. There is no effect of the degradation of the critical current caused by magnetic field generated by the other layers on AC loss in the cable with the current carrying capacity of the order of at least 1 kA.