Evaluation and Improvement of Uniformity in km-Class MgB₂ Superconducting Wires

An 18-multifilamentary MgB ₂ wire, classified within the 250 m-class, was produced utilizing a technique to improve the uniformity of powder filling by employing three-dimensional vibrators. A comprehensive evaluation of the fabricated wire revealed that the uniformity of the critical current ( I _c ) and fill factor, determined through the coefficient of variation, were 2 and 4%, respectively, along the wire direction. Building upon this achievement, a scaled-up MgB ₂ wire was fabricated, with dimensions of approximately 3 km in length and 0.83 mm in diameter. At 20 K and 2 T, the I _c was measured to be 191 A. The corresponding critical current density ( J _c ), determined to be 190,000 A/cm ² , represents a significant improvement when compared to that yielded by the conventional gravity-based powder filling method.