In situ detection of delamination and critical current degradation caused by the thermal stress in epoxy impregnated REBa2Cu3O7−δ coils

Epoxy impregnated superconducting coils have better structural integrity and thermal stability. However, for REBa2Cu3O7−δ (REBCO, RE=Rare earth) coils, the mismatch of thermal expansion coefficients between epoxy and REBCO tapes is a serious problem. In this work, the temperature distribution, stress evolution during the cooling process, critical current distribution, and delamination sites inside REBCO coils impregnated using Stycast 2850FT have been studied. We measured the temperature distribution and the hoop strain in the penultimate turn of impregnated coils during the cooling process and analyzed the thermal stress evolution. No damage was observed for coils with the ratio between outer and inner diameter Ro/Ri<1.93. The delamination behavior occurred in coils with Ro/Ri>2.37, where the coils even exhibited a two-stage delamination. The delamination mechanism of REBCO coils was proposed from three aspects: the mechanical analysis, the critical current degradation, and the microscopic analysis. In this work, we found that the actual delamination behavior may appear earlier than the steady-state temperature, and temperature distribution will push the radial stress peak toward the inner radius of the coil, making the inner turn more susceptible to delamination. Multiple delamination locations were accurately predicted and confirmed. The measured compressive hoop strain first increased from −4212 με to −4684 με with the increase in Ro/Ri and then decreased to −3835 με obviously due to delamination. This work reveals in detail the delamination mechanism in impregnated REBCO coils, which is of great significance for the development of damage-free coils.