Distribution of stress in force-balanced coils on virial theorem

Distributions of stress on a device with helical coils for the superconducting magnetic energy storage (SMES) are analyzed experimentally and numerically. In the previous work, we showed a theoretical limit of stored energy in a toroidal SMES system by the virial theorem. To verify the theoretical limit, we designed and construct a small device which is composed of two sets of helical superconducting coils and a toroidal frame of aluminum alloy. Strains by electromagnetic force on the frame were obtained experimentally. Distributions of stress on a toroidal shell with a similar configuration to the device are numerically calculated. Comparing the results of the experiments and the numerical calculations, a qualitative agreement of stress distribution between the calculation and the experiment is obtained in the toroidal direction, while discrepancies of stress in the poloidal direction are not negligible.