Nb3Sn superconducting coil for AC use

Nb₃Sn superconductors were developed for AC use, and a coil was fabricated. The Nb₃Sn superconductors were manufactured using the internal diffusion process. To reduce AC losses, the spacing between Nb filaments was designed to be 0.5 μm; consequently, the space factor of Nb filaments was 6%. The diameter of a strand was 0.2 mm, and the diameter of a Nb filament was 0.4 μm. AC losses in the strand were 180 kW/m³ at 0.5 T (60 Hz, peak value). A coil was made using the wind-and-react method using conductors composed of 7×7 strands. The Specifications of the coil were an inner diameter of 156 mm, an outer diameter of 188 mm, a height of 34 mm, and a number of turns of 17 turns×4 layers. To reduce wire motion, the coil was impregnated with epoxy resin. The quench current for DC operation was 1280 A, and the maximum magnetic field of the conductors was 1.6 T. Coil degradations were not observed. The magnet was tested under AC 60-Hz operation. The quench current was 340 A (r.m.s.). The cause of quenching is thought to be the temperature rise of the conductors due to coupling losses among the strands