Feasibility study on high field magnets using stress-minimized helical coils

The force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils (TFCs) and a solenoid. The FBC can minimize the working stresses and enhance the operating current density within the allowable stress of the superconductor by selecting an optimal number of poloidal turns per toroidal turn. In order to demonstrate the feasibility of the FBC concept for high field superconducting magnets, a 7-T force-balanced coil (7-T FBC) has been designed. The 7-T FBC with an outer diameter of 0.53 m will have 270-kJ magnetic energy at the maximum magnetic field of 7.0 T using NbTi superconductors. Although this size of the coil cannot be realized with the TFCs and the solenoid designs due to the induced electromagnetic forces, the maximum working stress of the 7-T FBC is lower than the elastic limit of the Cu matrix of the NbTi strand, so that the 7-T FBC may be excited up to 7.0 T without reinforcing materials for the NbTi strand. In this demonstration, the quench properties of the 7-T FBC will be mainly measured with pool boiling liquid helium cooling in order to evaluate the working stresses in the helical windings without the reinforcing materials for the NbTi strand.