Development of MgB2 superconducting wire for the low activation superconducting magnet system operated around core D-T plasma

MgB2 is one of the “low activation superconductors”, because the half-life of the induced radio-activity on the MgB2 is much shorter than that of Nb-based superconductors such as NbTi, Nb3Sn and Nb3Al. MgB2 wire would become an alternative material for Nb-Ti wire for the Poloidal Field and correction feedback coils, if the critical current density (Jc) around the magnetic field of 4 T could be improved. We found that addition of Cu using the compound Mg2Cu was an effective method to enhance Jc and succeed to fabricate 100 m length of MgB2 wire with Cu addition. Jc of the 100 m long MgB2 with Cu addition was comparable to that of a NbTi wire, and the small solenoid coil using Cu added MgB2 wire reached a central magnetic field of 2.62 T. We found that MgB2 wire is an alternative material to commercial NbTi wires under the hard neutron irradiation in a fusion reactor. Furthermore, we also fabricated a Cu added MgB2 wire using isotope pure boron-11 (11B) powder as the boron source to reduce the radioactivity. The critical temperature (Tc) of the MgB2 wire using 11B powder was found to be 37 K, and Tc was found to be changed by different boron materials.