Development of a One Tenth Sized Model Coil for 100-MJ Class SMES Using Force-Balanced Coil Concept

A one tenth sized model coil based on the force-balanced coil (FBC) design for 100-MJ class SMES has been designed. This coil is a hand-made coil using NbTi superconductors, and the winding of the model FBC is now in progress. The FBC is a helically wound hybrid coil of toroidal field coils and a solenoid. This coil can minimize the mechanical stress and the required mass of the structure for induced electromagnetic forces. The model FBC with an outer diameter of 0.53 m will have the stored energy of 270 kJ at the maximum magnetic field of 7.0 T. The winding form is made of aluminum alloy and slots are cut on its surface with the shape of the helical winding. From the results of the cooling test with liquid nitrogen, the significant deformations of the winding form have not been observed. The winding of the model coil is carried out without reinforcing materials for the NbTi strand. The in-plane curvature of helical coils causes the winding errors. Then after a slight winding tension is applied to the NbTi strand, each winding is adjusted to be fixed in its place. The experiments will be conducted with liquid helium cooling. The DC power supply for the model FBC can simulate various SMES operations. As a first step, the ramp-rate dependence of the superconducting properties of the model coil will be investigated in order to evaluate the mechanical stress in the helical windings without the reinforcing materials for the NbTi strand.