Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB{sub 2}

The intrinsic properties of MgB{sub 2} form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB{sub 2} is limited by the upper critical field H{sub c2} and its anisotropy. In wires or tapes, the MgB{sub 2} grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting current path and not by depinning as in high-temperature superconductors. Based on a percolation model, we demonstrate how the changes of the upper critical field and its anisotropy and how the changes of flux pinning will influence the critical currents of a wire or a tape. These predictions are compared to results of neutron irradiation experiments, where these parameters were changed systematically.