Influence of a Coil Bobbin on Transient Thermal Stress in a REBCO Pancake Coil

It has been often reported that a superconducting property of a coil wound by a coated conductor is degraded by epoxy impregnation. The degradation is considered that a superconducting layer or a buffer layer is peeled from a metal substrate by thermal stress. The steady thermal stress in the winding at the operating temperature is examined frequently. However, the transient thermal stress in a cooling process becomes larger than the steady thermal stress due to the difference of heat contraction of each material and temperature distribution, since heat contraction is dependent on a temperature. In this study, by using finite-element analysis, we examined the transient temperature distribution and the transient thermal stress distribution inside a coated conductor's coil winding with epoxy impregnation, when the cooling process with liquid nitrogen was considered. The transient one at the edge of the winding with a glass fiber rein-forced plastic bobbin is more than twice as large as the steady thermal tensile stress and is greater than the cleavage stress strength. Moreover, we have found that it is desirable to use a material with a smaller thermal expansion coefficient and larger heat transfer as the coil bobbin in order to reduce the transient thermal stress in the winding. The effectiveness of the stainless steel bobbin to reduce the transient thermal stress in the winding was showed. Furthermore, it was shown that sufficient precooling was required for reduction of transient thermal stress when a bobbin material with smaller thermal conductivity is used.