Mechanical strain triggering flux jumps of multi-filamentary Nb3Sn wires

Abstract

The composite multi-filamentary Nb₃Sn wire with a high critical current density is a preferred option for fabricating the superconducting magnet beyond the limit of NbTi wire (9–16 T). However, one crucial issue stems from the fact that electromagnetic force in superconducting coils is very strong, and the critical physical properties of Nb₃Sn, such as J_c, are more sensitive to mechanical strain than those of other possible low-temperature superconductors. We theoretically investigated the impact of mechanical strain on the thermomagnetic instabilities such as the flux jump (FJ) and quenching of Nb₃Sn wire exposed to a static magnetic field and transport current. The good agreements with H formulation or H-φ formulation implemented on COMSOL software confirm the validity of our numerical simulations using home-made codes. It is discovered that mechanical strain can trigger flux jumps even in a static magnetic field. Furthermore, the threshold value of mechanical strain to trigger the first flux jump is a monotonic function of the static magnetic field in the case of high transport currents, while it is a non-monotonic function in the case of low transport currents. It is attributed to the fact that flux can be released by the mechanical strain, causing smooth flux penetration before triggering the flux jump. We also present the stable/unstable regions by applying mechanical strain by varying transport current, magnetic field, and working temperature, which helps in avoiding thermomagnetic instabilities while designing the superconducting magnet.