Demonstration of the stress-minimized force-balanced coil concept for SMES

Strong electromagnetic force caused by high magnetic field and coil current is a serious problem in SMES systems. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) which is a helically wound toroidal coil. Based on the virial theorem, the FBC can minimize structure requirements for energy storage by selecting an optimal number of poloidal turns. We designed and fabricated a small experimental device which is composed of inner and outer helical coils mutually wound in opposite toroidal directions using NbTi superconductors. The distribution of the working stresses in this device can be changed by selecting the optimal current ratios between inner and outer coil currents. From the experimental results, we demonstrated the validity of the FBC concept.