Structural Modeling of HTS VIPER Cable for High-Field Magnet Applications

A structural model has been developed for the REBCO-based VIPER cable [1], which is designed for high magnetic field applications. In this work, structural finite element analysis (FEA) is used to simulate the stresses incurred within the REBCO tape stacks under the following conditions: (1) bending to 1 m diameter (2) cooling down to 77 K and (3) repeated transverse electromagnetic Lorentz loadings up to 1600 kN/m. The effect of solder impregnation onto the stress state of the tapes was studied via a set of friction coefficients of 0.02 (perfect-slip), 0.2 (metal-to-metal contact), and 1 (no-slip). For bending, high friction between the tape-stack and solder results in high axial strain in the tape-stack which may lead to I _c degradation. During thermal cooldown, the stress on the tape-stack is negligible. However, the differential in thermal contraction between the copper and stainless-steel jackets leads to the formation of a small gap that may cause mechanical instability. Under cyclic Lorentz loading, a higher friction coefficient reduces the deformation of the cable by restricting the relative sliding between the neighboring tapes, which results in a more uniform stress distribution and a lower peak stress. A lower friction coefficient results in larger deformation of the solder and stack and a higher peak stress. In addition, due to the accumulated plastic deformation and stress concentration in the solder, both the averaged von Mises stress and the peak stress of the outermost four tapes increase with each of the first three loading cycles. These results may explain the early-stage degradation observed in the experiments conducted on VIPER cables (Z. S. Hartwig et al. , 2020).