Influence of bolt positions and electrode structure in yoroi-coil structure on stress distribution in an HTS coil winding

Large hoop stress causes the deterioration of the transport properties of an high-temperature superconducting (HTS) coil winding. A high-strength pancake coil structure called “Yoroi coil” has been suggested. Yoroi-coil structure is expected to decrease the electromagnetic force applied to the winding and the volume of the coil frame at the outside of the winding. The effect of Yoroi coil on the hoop stress reduction has been investigated in an ideal “Yoroi-coil” structure without bolts and electrodes. In a practical “Yoroi-coil” structure, however, there are electrodes and bolts for fixing frames and reinforcing outer plates to the coil. In this study, the effect of the stress reduction of an HTS coil winding using the practical “Yoroi-coil” structure with the bolts and the electrodes was numerically investigated by 3-D-finite element method (FEM) analysis. Numerical results showed that the circumferential strain and hoop stress generated in the winding depended on the bolt location. Local large stress was applied to the winding, especially around the electrode. In order not to apply the local large stress to the winding, the mechanical strength of the electrode was very important. Adding bolts to the electrode was effective as the counterplan of stress concentration at the electrode. The optimal number of bolts at the electrode, however, largely depended on the thermal contraction during the cooling process of the coil.