Acoustic Emission Measurement and Large-Scale Finite Element Analysis of Stress Concentration in Superconducting Coil During Quench Training

Abstract Demand for superconducting coils is growing significantly in medical use, maglev trains, or motors. The quench phenomenon is a significant issue, as it involves the sudden loss of superconductivity. The aim of this study is to elucidate the quench phenomenon and to mitigate its occurrence. We created an ellipse-shaped model coil, and a large-scale finite element method (FEM) analysis was done to calculate details of the stress state caused by thermal stress and electromagnetic force during excitation. We estimated where the concentrated strain energy was and the generation of energy loss. On the other hand, acoustic emission (AE) measurement was applied to this model coil during a process of over-excitation testing called “training.” AE is emitted as a high-frequency sound wave when an elastic body is deformed by an external force. The entire training experiment ended after more than 10 quenches. AE with high amplitude occurred just before a quench, and the location of AE source results frequently appeared at locations identified to be with high internal stresses from analysis. Both the analytical and experimental results of this study indicate that strain energy density concentration during excitation increases the risk of quench.