Study of Nb3Sn Superconducting Coil for AC Use

We developed Nb3Sn superconductors for AC use and fabricated a coil. The Nb3Sn superconductors were manufactured using the internal tin diffusion process. To reduce AC losses, the spacing between Nb filaments was designed to be 0.5μm, sequentially, the space factor of Nb filaments was 6%. The diameter of a strand is 0.2mm, and the diameter of a Nb filament is 0.4μm. AC losses in the strand were 180kW/m3 at 0.5T, and were 18kW/m3 at 0.1T (60Hz, peak value). A coil was made by the Wind and React method using conductors composed of 7×7 strands. Specifications of the coil are as follows. The inner diameter is 156mm, outer diameter is 188mm, height is 34mm, and the number of turns is 17 turns×4 layers. To reduce wire motions, the coil is impregnated with epoxy resin. The first quench current for DC operation was 1, 130A. The third quench current was 1, 280A, and the maximum magnetic field of conductors was 1.6T. The magnet was tested under AC 60Hz operation. The quench current was 340A (rms). The cause of quenching is supposed to be the temperature rise of conductors due to coupling losses among the strands.