An efficient method for AC loss reduction of YBCO pancake coils wound from parallel tapes

We report AC loss data of superconducting pancake coils (PC) wound from several non-transposed YBCO tapes connected in parallel. Due to coupling losses, their magnetization AC losses in AC fields applied parallel to the tape surface are significantly higher than for single-tape PCs. It was experimentally demonstrated that these coupling losses can be effectively reduced by using double-PCs in which the parallel tapes between the two PCs are connected in reversed order. This special connection between PCs imitates a twisting of parallel tapes and results in their decoupling. An AC current applied additionally with an external AC magnetic field to the double-PCs does not impede the decoupling of the tapes. We propose a simple model to calculate the magnetization AC losses of PCs by approximating the coils by face-to-face stacks of YBCO tapes. (paper)