Quench Properties of a 7-T Force-Balanced Helical Coil for Large-Scale SMES

Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. Based on the FBC design, a superconducting model coil has been developed. The outer diameter of the model FBC is 0.53 m. The model FBC will have 270 kJ magnetic energy with the critical magnetic field of 7.1 T. The critical coil current and self inductance are 552 A and 1.8 H, respectively. The hand-made winding, using NbTi/Cu composite strands with a diameter of 1.17 mm, was finished with 10584 poloidal turns after four months. The helical windings of the model FBC were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Three test runs were conducted with liquid helium cooling at intervals of several months. The number of quench tests was 81 in total. In the third test run, the quench position of the FBC windings was identified using acoustic emission measurements. The first quench current was 293 A, which was 53% of the critical coil current. The training phenomena could be observed even after the coil was warmed up to room temperature. After successive quenches the quench current was improved to 476 A, corresponding to 86% of the critical coil current, and it was successfully excited up to 6.1 T.