Fabrication and Operation of a RHQT Nb3Al Insert Coil Generating 4.5 T at 4.2 K in 15 T Back-Up Field

To demonstrate the RHQT Nb₃Al conductor to be really a candidate for a high-field magnet, we have fabricated 1 km-class Cu stabilized RHQT Nb₃Al flat-wires, manufactured two insert real-sized coils #A and #B, and tested in a superconducting back-up field of 15 T at 4.2 K. Using the coil #A, a thinning of insulator, a winding technique, a new transformation annealing profile in a vacuum with a fast temperature-ramp above 500degC, decarbonization, an impregnation technique used for the Nb₃Sn coil were preliminarily checked to be no matter. The coil #B that was wound with a 764 m piece could produce an additional 4.5 T (total 19.5 T) at 4.2 K in a clear bore of 30 mm without any training, when the coil current achieved the I_c of short samples that were simultaneously transformation annealed with the coil #B.