Development of 2 km-Class Carbon-Doped MgB 2 Wire With Uniform Critical Current Property

Kilometer-scale carbon-doped MgB₂ superconducting wires were fabricated using a three-dimensional vibrating powder-filling powder-in-tube process and systematically evaluated for uniformity and performance. The optimized process enabled fabrication of a 2.1 km-long 18 filament conductor with a critical current density (Jc) of 1.13 × 10⁵ A/cm² at 20 K and 2 T, showing only ∼2% variation along its length. Carbon substitution enhanced high-field performance primarily through impurity scattering, while the n-value demonstrated a strong proportionality with Jc across all measured conditions. This correlation establishes the n-value as not only a research parameter but also a practical quality-control metric for industrial wire production. The combined achievements of process innovation and transport uniformity highlight the feasibility of MgB₂ conductors for MRI magnets, high-field research magnets, and superconducting power transmission.