Interlayer peeling mechanism and electro-magnetic-thermal behavior of epoxy-impregnated REBCO superconducting coils during quench

Abstract

Superconducting coils made of rare earth-barium-copper-oxide (REBCO) coated conductors have become one of the best candidate materials for high-field magnets due to their excellent electromagnetic properties such as high current and strong magnetic field. However, REBCO superconducting materials have relatively weak adhesion strength and are often operated in complex working environments such as extremely low temperatures, strong magnetic fields, and high current loads. Therefore, they are prone to quenching and generating significant stress concentrations, which can lead to peeling or detachment between coil layers. In this work, based on the finite element idea, the delamination evolution mechanism, heat transfer characteristics, and mechanical behavior of epoxy-impregnated superconducting coils during cooling and operation are systematically investigated. In order to accurately characterize the delamination behavior of the coil, the cohesive zone model is used in the study. A zero-thickness cohesive layer is inserted between the REBCO layer and the Hastelloy substrate layer to simulate the mechanical response at the interface. The results show that during the cooling process, the delamination phenomenon of the coil mainly occurs in the region with a radius less than 1/2. In addition, it is gradually weakened from this position to the inner and outer sides of the coil, while no delamination occurs at the inner and outer ends of the coil. Due to the structural limitations of the two-dimensional axisymmetric model, each turn of the tape at the top of the model is wrapped with a copper stabilizer and insulating material (except for the midplane). As a result, the delamination diminishes along the z axis and eventually no delamination is observed at the top of the coil. When loading and quench occur, the temperature near the heat source region is significantly higher than in other regions, leading to an increase in the local temperature gradient. In turn, a large radial tensile stress is caused, which further increases the cracking distance between the coil layers.