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 (J_c) 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 J_c and succeed to fabricate 100 m length of MgB₂ wire with Cu addition. J_c 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 (T_c) of the MgB₂ wire using ¹¹B powder was found to be 37 K, and T_c was found to be changed by different boron materials.