Case study of a 20 T-φ400 mm room temperature bore superconducting outsert for a 45 T hybrid magnet

The High Field Laboratory for Superconducting Materials (HFLSM) and the Tsukuba Magnet Laboratory (TML) conducted in collaboration a case study on development of a 50 T-class hybrid magnet. To construct a high magnetic field magnet with compact and energy-saving design as well as with easy operation and maintenance, one has to develop high-strength Nb₃Sn strand cables, with maximized superconducting characteristics and which can withstand a large electromagnetic force over 500 MPa. For this purpose, the HFLSM has proposed and investigated the effect of repeated bending treatment (prebending) on Nb ₃Sn strands internally reinforced with CuNb stabilizer leading to significant enhancement of the critical current density. In this report we present our results on application of the prebending effect to the development of high-strength strand cables. The designed prebent-strand cables are composed of three CuNb/Nb³Sn strands (3 × φ = 1.73 mm) and four stainless steel strands (4 × φ = 1.73 mm). High-strength CuNb/Nb ₃Sn strand cables have shown a stress limit of 552 MPa at 0.4% strain, and a critical current of I_c = 1000 A at 18.5 T and 2.0 K. For such high-strength strand cables, a 20 T superconducting magnet with a room temperature bore (φ = 400 mm) consisting of five layers made of CuNb/Nb ₃ Sn and two layers of NbTi was designed. The coil parameters are: inner diameter φ = 440 mm, outer diameter φ = 1332 mm, coil height 1321 mm, inductance 350 H and magnetic stored energy 144 MJ at 908 A of the operation current. Winding of the coil was experimentally successfully simulated using dummy 3 + 4 strands cable composed of three Cu strands and 4 stainless steel strands with a similar design to the 3 + 4 strands superconducting cable presented above. The 20 T superconducting coil will be used as a 20 T outsert for a 25 T water-cooled resistive insert to obtain a 45 T hybrid magnet.