Finite Element Investigation Into the Mechanical Effects of Solder Voids on the Performance of Stacked-Tape REBCO Conductors

Some recent designs for high-field fusion magnets incorporate stacks of high-temperature superconducting (HTS) tapes embedded within a solder matrix. The solder is impregnated into the stack under vacuum at high temperature after winding. However, the solder filling is not always perfect, and tends to leave voids behind. As the HTS stack is subjected to high Lorentz forces during operation, these voids can lead to local deformation in adjacent tapes as they are not supported evenly by the solder. In this study, we use finite element analysis to determine the strain likely to develop in an HTS tape positioned over a void within a stack under transverse compressive loading. We use an experimentally-derived model from literature to estimate the critical current degradation of the tape for various void sizes and shapes, and under varying magnitude of load, with the goal of finding an acceptability criterion for void size. The analysis indicates that voids under 15 mm in length do not damage any of the tapes, as long as they are confined to a single layer. If a void extends across layers of tape, however, the tape will likely experience much higher damage as it is not supported from above or below. This work helps to give a level of quality assurance to a magnet with voids and helps identify the type of void that is most likely to cause significant damage to one or more of the tapes.