Conductor Architecture and Self-Field of Superconducting Strands

Abstract

Three standard reference material Nb-Ti strands, manufactured for the ITER poloidal field magnets, were extensively characterised using both transport and magnetisation techniques, with a focus on the behaviour of the material in a magnetic field. To quantify the effect of magnetic self-field, the field generated by current flow, the critical current density was measured as a function of the applied magnetic field, temperature, current polarity, and geometry.

A high capacity probe was designed and commissioned for the transport measurements. The characterisation in different measurement geometries was possible with custom-built sample holders (i.e., barrels). As the titanium alloy Ti-6Al-4V used in the standard ITER VAMAS barrel is superconducting at 4.22 K, an alternative titanium alloy (Ti-6Al-2Sn-4Zr-2Mo-0.2Si) was identified that is not superconducting at 4.22 K and used to manufacture the barrels.

The transport measurements at low applied magnetic-fields resulted in high current densities, and the effect of self-field being large. To investigate the self-field both finite element analysis (FEA) and semi-analytic methods were employed. The H-formulation of Maxwell’s equations was implemented using Comsol Multiphysics, a commercial FEA software. The model input for the superconductors properties were defined using a number of experimental J_C (B) relationships. The architecture of the strand was approximated with different degrees of complexity. The FEA models considered the cross-section of the strand as circular, annular, and as three nested cylinders (i.e., tubes-within-tubes). The probability distribution of the magnetic field components in the superconducting domain was calculated and analysed. The changes in the field distribution due to the geometry of the measurement barrels and the current orientation, (resulting in opposite Lorentz force orientation), were used to quantify the magnitude and orientation of the self-field. A semi-analytic method was used to derive a the magnetic field distribution data from the FEA and the experimental data. The resultant piecewise J_C (B) calculated for the Nb-Ti strand, can be considered a universal J_C (B) relationship.