Transverse mechanical load effects on superconducting Nb3Sn current-carrying performance

Superconducting Nb3Sn strands are used for making high field magnets such as ITER (an experiment to prove fusion as an energy source), and the next upgrade to the Large Hadron Collider. In manufacturing process and operations, the strands are subjected to transverse compression that could lead to significant critical current degradations due to the fact that superconducting properties of Nb3Sn are very sensitive to mechanical strain. Therefore, to optimize the magnet design, it is extremely important to be able to predict load effects on critical current behavior.;Typically the electromechanical behavior of Nb3Sn strands under transverse load is studied experimentally; however, even when small sub-sized cable tests are performed in a desired high magnetic field, these experiments are very expensive and challenging to perform. Additionally, every single Nb3Sn strand is composed of thousands of micrometer-size filaments, and strains of individual Nb3Sn filaments determine the critical current behavior, so it is necessary to obtain the strain states of the filaments which experiments cannot measure.;In this work we developed a novel method to calculate the critical current degradation of Nb3Sn strands under transverse compression. We conducted finite element analysis of strands evaluating strains of the filaments. The finite element models have the ability to consider the twisting of the filaments and obtain the principle strains of the Nb3Sn filaments for the configuration considered. Those principle strains were used in an available scaling law to calculate the critical current degradation of the strands.;Based on the calculation results it was found that the critical current behavior of Nb3Sn strands is sensitive to the Young's modulus of Nb3Sn filaments but not to the plastic tangent modulus of copper. The novel calculation method taking into account the strain states of the filaments has been found to agree well with the experimental measurements of both a triplet using internal-tin strands and a single powder-in-tube strand. These results indicate this new method is very promising although more work is necessary to include the electrical coupling of the filaments and ultimately use a similar approach to predict the behavior of larger cables used in magnets.