Local plastic deformation effects on the critical current of high-J_c Nb₃Sn strands under uniaxial strain

Abstract

In order to meet the target operating parameters of the toroidal field coils (TFCs) for the next-generation Chinese compact burning plasma tokamak, high critical current density (J c) Nb3Sn strand will be applied to the high-field winding-package of the TFC. To improve the transverse stiffness of the cable in withstanding the huge Lorentz force and avoiding conductor performance degradation, the short-twist-pitch and copper-wound-strand cable patterns were taken into consideration. In the processes of cabling and compaction of the conductor, the tight cable configurations lead to severe local plastic deformation (LPD) within the strands. The strands in the conductor are subjected to strain caused by thermal contraction and Lorentz force during conductor cooling down and operation. So far it is unknown, whether the LPD could impact the critical current (I c) versus uniaxial applied strain behavior of high-J c Nb3Sn strand. Aiming to investigate the effect of LPD on the I c of strands under uniaxial strain, three types of high J c Nb3Sn strand with different indentation depths were tested on a U-shaped bending spring. The axial strain ranges from −0.9% to +0.4% at 14 T and 4.2 K. The three types of strands showed more strain sensitivity and lower tensile irreversible strain limit with increasing LPD, while even irreversible degradation of the I c could be observed in the compressive strain region. The sample preparation, test process, test results and analysis are reported.