Flux-pinning and inhomogeneity in MgB2 /Fe wires

Transport critical current densities J c and irreversibility fields B irr of undoped and nanoparticle doped (10 wt% SiC) Fe-sheathed MgB2 wires were measured from 2 to 40 K in magnetic field B ≤ 16 T . For the best segments of wires (≤1 cm) both the magnitude and field variations of J c and the pinning force density F p = J c B depend only on the magnitude of B irr , hence the strength of flux-pinning. B irr of doped wire for T ≤ 30 K is ∼1.4 times larger than that of undoped and reaches that of NbTi (10 T at 4.2 K) already at 20 K. Accordingly, its high-field J c s and F p s are large, typically three times larger than the best literature results, and are limited by the porosity and inhomogeneity of the superconducting cores in present-day MgB2 wires.