Development of a compact 18 T MF-(NbTi)3Sn coil

The Nb3Sn superconducting coil which can generate 18.4 T under the backup magnetic field of 13.2 T in the liquid helium at 4.2K was developed by cooperation with Japan Atomic Energy Research Institute. The generation of the magnetic field of 18.4 T at 4.2 K is the world highest record. This coil has the effective space of 32 mm diameter at the center, and the winding part is graded with Nb3Sn superconducting wires with two different wire diameters and the multi-core of tube type. The mean current density of this coil is 70 A/mm2 in the high magnetic field region of 18.4 T and 107 A/mm2 in the low magnetic field region of 16.7 T. The stored energy of the whole 18 T coil system including the backup coil is mere 1.48 MJ at the time of 18.4 T, accordingly, this is a very compact system. The time required for exciting up to 18 T in this system is as very short as 12 min. The success of this coil is the very significant result for the development of the high performance superconducting wires for the high magnetic field, large coils for nuclear fusion reactors in future. The design and manufacture of the BC18 coil and its excitation test are reported. The performance of critical current density is the most fundamental factor which exerts large effect to the design of the coil system. (K.I.)