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

Improvement in the transport critical current density and microstructure of isotopic Mg11B2 monofilament wires by optimizing the sintering temperature

Superconducting wires are widely used in fabricating magnetic coils in fusion reactors. In consideration of the stability of ¹¹B against neutron irradiation and lower induced radio-activation properties, MgB₂ superconductor with ¹¹B serving as boron source is an alternative candidate to be used in fusion reactor with severe irradiation environment. In present work, a batch of monofilament isotopic Mg¹¹B₂ wires with amorphous ¹¹B powder as precursor were fabricated using powder-in-tube (PIT) process at different sintering temperature, and the evolution of their microstructure and corresponding superconducting properties was systemically investigated. Accordingly, the best transport critical current density (J_c) = 2 × 10⁴ A/cm² was obtained at 4.2 K and 5 T, which is even comparable to multi-filament Mg¹¹B₂ isotope wires reported in other work. Surprisingly, transport J_c vanished in our wire which was heat-treated at excessively high temperature (800 °C). Combined with microstructure observation, it was found that lots of big interconnected microcracks and voids that can isolate the MgB₂ grains formed in this whole sample, resulting in significant deterioration in inter-grain connectivity. The results can be a constructive guide in fabricating Mg¹¹B₂ wires to be used as magnet coils in fusion reactor systems such as ITER-type tokamak magnet.