The influence of thermal gradients on AC losses in high-temperature superconducting coils

As high-temperature superconducting (HTS) tapes approach commercially acceptable performance levels, attention should be paid to how to utilize these tapes in real applications. One of the most important factors is the AC loss. When wound into a coil the operating conditions of the HTS tape vary in different parts of the coil. The optimum operating current, with respect to the AC losses, increases radially in the coil as the magnetic field decreases. With a constant current in the coil, large parts of the coil will not be optimally operated. The optimum operating current of the HTS tape also varies with temperature. In this paper we propose to use a controlled temperature distribution in the coil so that the coil can be operated at its optimum current in the entire coil. The potential for loss reduction with a controlled temperature distribution in a large AC HTS coil is about 10%.