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

Abstract

MgB 2 is one of the “low activation superconductors”, because the half-life of the induced radio-activity on the MgB 2 is much shorter than that of Nb-based superconductors such as NbTi, Nb 3 Sn and Nb 3 Al. MgB 2 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 Mg 2 Cu was an effective method to enhance J c and succeed to fabricate 100 m length of MgB 2 wire with Cu addition. J c of the 100 m long MgB 2 with Cu addition was comparable to that of a NbTi wire, and the small solenoid coil using Cu added MgB 2 wire reached a central magnetic field of 2.62 T. We found that MgB 2 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 MgB 2 wire using isotope pure boron-11 ( 11 B) powder as the boron source to reduce the radioactivity. The critical temperature ( T c ) of the MgB 2 wire using 11 B powder was found to be 37 K, and T c was found to be changed by different boron materials.