Magnetic and Transport Characterization of NbTi Strands as a Basis for the Design of Fusion Magnets

We present the results of an extended measurement campaign carried out on available NbTi commercial strands to be used in the design of fusion reactor magnets, including candidate strands for the ITER high field Poloidal Coils, PF1/6, and for the Toroidal Field Coils of the JT-60SA Tokamak. Magnetic and transport measurements have been carried out at variable temperature and magnetic field. From magnetization cycles we were able to extract information about AC hysteretic losses, and to extend the current density determination to lower fields, thus enabling the optimization of numerical fits in a wider magnetic field range. It has been found that the normalized bulk pinning force of the material, though showing good temperature scaling throughout the explored range, cannot be described by a single function of the type ${\rm b}^{\alpha}(1-{\rm b})^{\beta}$. On the contrary, the full summation of two contributions, each dominant in a different magnetic field range, returns a good fit of the data. Extending this 2-components description to an expression for the critical current density, a very good agreement with experimental measurements is obtained over the whole explored B, T range. Collecting a database of available strands performances, especially in the range of relatively high temperatures $({\rm T}>5.5\ {\rm K})$ and magnetic fields $({\rm B}\sim 6\ {\rm T})$, typical of applications in large coils for fusion reactors, constitutes a sound basis for magnets design, which should be based on strand properties measured in the operation-relevant temperature and magnetic field range.