Exploration of correlation between the material characteristics of the copper layer on the electrical and thermal properties of REBCO tape and coil

To achieve the approriate trade-off between the low charing delay and enough self-protection ability in the NI REBCO coil, the controllable Cu layer structure in REBCO tape is essential, which can cause required electrical and thermal properties in both the radial and circumferential direction. In this paper, the correlation between the material characteristics of the copper layer on the electrical and thermal properties of REBCO was explored by investigating the REBCO tape with different Cu layer thicknesses. The influence of charging decay, self-protection, and cold shrinkage stress on the REBCO coil performance was also evaluated based on three tapes. Three new findings showed the possible dominant factor on electrical and thermal properties in either radial or circumferential direction. Unlike the traditional electroplated layer, the Cu layer in the SP3 sample has piecemeal Cu grains, which may be the primary factor in significantly increasing the electrical resistivity and decreasing the thermal conductivity significantly. In the radial direction, the nonuniform top, bottom, and even side thickness of the Cu layer could increase the electrical resistivity and thermal conductivity. And the bad corner contact quality between the Cu and Ag/REBCO could be an important factor. Furthermore, the Cu layer purity may be significantly lower than previously thought. Using the electrical resistivity and thermal conductivity values of pure Cu as the Cu layer in the simulation could result in significant errors. This study provides possible directions for finding a method to control the properties of the Cu layer of REBCO commercial tape, which can better fit different application scenarios.