Magnetic shielding in MgB2/Fe superconducting wires

Transport critical current (I_c) was measured for MgB₂/Fe round wires, with the magnetic field oriented perpendicular to the wire and parallel to it. Measurements were made on a wire with a pure MgB₂ core and another wire where the MgB₂ core was doped with nano-size SiC. This doping strongly improved the vortex pinning in MgB₂. The field dependence of I_c was strongly improved due to the presence of the iron sheath. At 30 K, I_c did not depend on the field for fields between 0.09 and 0.7 T. At lower temperatures, I_c increased with the field, after an initial decrease, resembling a "peak effect." This effect was extended to higher fields as the temperature was decreased: at 10 K the peak appeared at 3.5 T. This improvement was not due to mere magnetic shielding by iron, but more likely to an interaction between the iron sheath and the superconductor. Improvement of vortex pinning did not affect the range of fields within which this effect was observed. J_c of SiC doped MgB₂/Fe wires at elevated fields already satisfies the requirements for their use in production of superconducting magnets for particle accelerators.