Degradation-Free Impregnated YBCO Pancake Coils by Decreasing Radial Stress in the Windings and Method for Evaluating Delamination Strength of YBCO-Coated Conductors

A radial stress is generated in the winding when an impregnated YBCO pancake coil is cooled from room temperature to 77 K or less. If this radial stress locally exceeds the delamination strength of the YBCO-coated conductor under transverse tensile stress, the coil will be degraded. Although several methods for evaluating the delamination strength of YBCO-coated conductors have been reported, the delamination strength determined by these methods has shown considerable variation. In order to fabricate an impregnated coil that does not suffer from degradation, it is important to determine the exact delamination strength. The delamination strength was evaluated using small impregnated test coils with an inner diameter of 30 mm and an outer diameter from 36 mm to 60 mm, since the radial stress depends on the ratio of inner to outer diameter regardless of the coil size. The delamination strength can be obtained by investigating the ratio of inner to outer diameter that does not result in degradation. On the other hand, for a larger ratio of inner to outer diameter, it is necessary to make the radial stress less than the delamination strength of the YBCO-coated conductor. To achieve this, the radial stress was decreased by dividing the coil into a number of parts in the radial direction.