Facing global environmental problems, such as climate change, global warming, etc., we have to enable a sustainable world as soon as possible. Hydrogen is world-widely garnering attention as a crucial energy resource in achieving carbon neutrality. If liquid hydrogen becomes familiar to our society, superconducting applications operating at 20 K may be able to contribute to saving energy. MgB₂ superconducting wire is promising for a practical conductor at 20 K because it has a high T_c of 39 K, small superconducting anisotropy, no weak-link problem, lightweight, simple chemical composition, possibly to make round wires, expect low production cost, etc. Although MgB₂ multifilamentary wires are already being developed and sold by several companies at present, however, it still remains some R&D issues in order to meet wide practical applications with liquid hydrogen.
We are ongoing R&D of ultra-fine MgB₂ superconducting wires having a very small diameter much less than a human hair. These ultra-fine MgB₂ wires could be bundled and fabricated easily into stranded cables for increasing the current capacity. In principle, the bending strain decreases with decreasing the wire diameter as well as the hysteresis loss. The stranded cables made by these ultra-fine MgB₂ wires would have very flexible mechanical performance and thus react and wind techniques would be applicable. In addition, the coupling loss for the stranded cables is expected to be minimized by increasing the surface and contact resistance, which are relatively easy to be controlled. Therefore, it may be solved both the issues of mechanical brittleness and AC losses at once. Brand-new results for ultra-fine MgB₂ superconducting wires and stranded cables will be presented.
A part of this work is financially supported by NEDO feasibility study program 2022.

Keywords: MgB2, superconducting wire, stranded cable, very small diameter