Recent progress in a development of Nb3Sn internal tin strand for fusion application

Abstract

The layout optimization of ITER-type Nb3Sn internal tin (IT) superconducting strand for magnet systems of future fusion reactors was carried out aiming on the large scaled industrial production. The new approach to a formation of the initial composite billets for fabrication of IT superconducting strand has been proposed. This approach assumes the use of round shaped instead of hexagonal shaped Cu-Nb subelements covered by tin layers. The positive influence of the use of round shaped multifilamentary Cu-Nb subelements on the uniformity of filaments array in the final strands has been shown. The investigation on the use of composite filaments consisting of multiple niobium rods and rods made from NbTi alloy was carried out. It was shown that the use of proposed composite filaments enabled to effectively regulate the grain size of niobium in Cu-Nb subelements before the cold deformation by drawing. The experimental data on the influence of the proposed composite design of the filaments on the kinetics of Nb3Sn layer formation process are given. The relations of microstructure and properties of Nb3Sn IT strand with modified design have been analyzed. The superconducting properties attained met the main ITER requirements for the TF strands with non-copper critical current density higher than 800 A/mm2 (4.2 K; 12 T) and hysteresis losses (±3 T) less than 500 mJ/cm3.