Improving critical current density of Nb3Sn by optimizing pinning potential of grain boundary and grain size

Abstract

Nb3Sn superconductor is of significant interest for applications in constructing high-field magnets beyond the limit of NbTi. However, its critical current density decreases rapidly at high magnetic fields (12 T) and the state-of-the-art level of Nb3Sn superconductors still cannot meet the requirements of the planned future accelerator magnets. The primary flux pinning centers for Nb3Sn wire mainly arise from the grain boundaries (GBs). In the present paper, we theoretically investigate, through time-dependent Ginzburg-Landau theory and with graphics-processing unit parallel technique, the vortex pinning and the critical current density in large-scale polycrystalline Nb3Sn superconductor by varying the pinning potential of GB and grain size at various magnetic fields. Unlike the conventional dot-like pinning systems, it is found that the critical current is not a monotonous function by suppressing the superconductivity of the GBs. The optimal pinning potential of GB for maximum critical current density strongly depends on magnetic fields. Furthermore, we find that the critical current density can be significantly enhanced by reducing grain size at low magnetic fields, while increase of critical current density cannot always be observed at high magnetic fields. Actually, critical current density even decreases by reducing grain size, which depends on the superconductivity of GBs. The findings in the paper provide theoretical foundations to achieve further improvement of Nb3Sn with optimizing the flux pinning.