Hoop Stress Tests of an Epoxy-Impregnated REBCO Coil With Fluorine-Coated Polyimide Insulation

Abstract

REBCO coated conductors (CCs) are suitable for high field magnets due to their high in-field critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and high tensile strength. However, in order to apply the REBCO CCs to a high field magnet, it is important not only to evaluate the characteristics of short samples but also to clarify the behavior of the REBCO coil under strong electro-magnetic forces in high magnetic fields. In this study, we investigated the tolerance to the electro-magnetic force by applying hoop stresses to a REBCO double pancake coil in high magnetic fields. The REBCO coil was wound with BHO-EuBCO CCs and impregnated with epoxy resin. In order to prevent a degradation due to thermal delamination stress, fluorine-coated polyimide tapes were wound on the REBCO CCs. When a maximum hoop stress of 608 MPa was applied to the coil, normal voltage was generated and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation was confirmed in one side of double pancake. As a result of the investigation, the degradation occurred only at lap joint part between CCs in the windings, and the cause of the degradation was thought to be stress concentration at the both edges of lap joint. Since there was no <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> degradation except for the lap joint part, it is considered that the REBCO coil can withstand larger hoop stress if we can reduce the stress concentration at the joint part. We therefore proposed a new joint structure and confirmed that it has higher tensile strength than a conventional lap joint structure.