Numerical analysis of the electromechanical behavior of high-field REBCO coils in all-superconducting magnets

Despite the excellent improvements in high fields resulting from the adoption of REBa2Cu3O (x) (REBCO)-coated conductors (CCs), the resulting stress and strain due to winding tension, cooling stress, and the screening-current-induced Lorentz force may lead to severe plastic deformation and degradation of superconductivity. Therefore, it is essential to understand the comprehensive electromechanical behavior and deformation mechanism of high-field REBCO magnets. In this paper, multistep finite element analysis is designed to simulate the electromechanical behavior of REBCO coils after the winding, coolingdown, and charging processes. All major constituent materials including the mandrel, CCs, cowinding reinforcement, and over-banding are considered in the mechanical models to emulate contact-separation behavior between them. In addition to contact nonlinearity, an intrinsic elastoplastic property is introduced to predict the plastic deformation observed in experiments. To reveal the screening-current effect, the distributions of the screening current and the magnetic field are analyzed by solving the T-A formulation of Maxwell's equations using a homogeneous technique. Then, with the electromagnetic load acting as a body force, the electromagnetic stress and strain are computed and experimentally validated for a pancake coil, including the transport current and external field. Finally, the electromechanical behavior under a Lorentz load is investigated by considering the accumulation of stress and strain due to winding tension and cooling down. The simulation results reveal that the winding tension and thermal stress affect the final stressstates in the fully charged state and therefore the electromechanical properties. Both plastic deformation and possible degradation are more likely to occur in the outermost tape of several pancakes due to the influence of screening-current-induced stress and strain. In addition, this work further analyzes the effect of the current sweep reversal method, which could be favorable in reducing the screening-current-induced high stress and strain in the steady state.

Automated summary

This study investigated the electromechanical behavior of high-field REBCO coils using a multistep finite element analysis, considering factors such as winding tension and thermal stress. The results showed that plastic deformation and degradation are more likely to occur in the outermost tape due to screening-current-induced stress and strain, emphasizing the importance of using the current sweep reversal method to reduce high stress and strain caused by screening currents.