Impact of strain and field ramp functional form on thermomagnetic instabilities in composite Nb3Sn wires with multi-filaments inside the superconducting coil

Abstract

Superconducting Nb3Sn magnets have composite structures consisting of Nb3Sn multi-filaments, cooper, and epoxy resin. The complicated strain states due to different thermal expansion coefficients for Nb3Sn and matrix materials and high Lorentz force will cause significant degradation of critical current density Jc of Nb3Sn wires, thereby having a substantial impact on the thermomagnetic instabilities in the superconducting magnets. So, it is highly needed to explore the effects of strain on thermomagnetic instabilities with numerical simulations. We theoretically analyze the effects of Cu/SC (superconductor) ratio, strain, and the field ramp functional form on flux jumps in composite Nb3Sn wires inside the superconducting coil to obtain more accurate results. Considering the composite multi-filamentary structures, we find that a lower Cu/SC ratio leads to higher temperature peaks, and a larger proportion of Cu causes higher voltage peaks. Moreover, the strain significantly causes a higher frequency of flux jumps and higher voltage peaks. The temperatures recover to working temperature more difficultly, and SC wires quench earlier in the presence of strain. For the ramping magnetic field with a jagged ramp form, it is interesting that few flux jumps and temperature jumps occur at the decreasing branch, whereas frequent flux jumps can be observed promptly again when the applied current exceeds the pre-existing peak value. Our simulated results agree very well with experimental observations in Nb3Sn coils. Additionally, unlike the pulsed flux jumps observed in linear ramp cases, giant and prolonged flux jumps are observed at the increasing branch under linear field ramp with additional sinusoidal field oscillations, when the applied current is sufficiently large. Reverse voltage signals are also observed during a decreasing branch, which can be revealed by the variations of current, magnetic field, and temperature distributions. The findings in this paper provide new insights into understanding and exploring the complex flux jumps in composite Nb3Sn wires with multi-filaments.