A non-destructive method for detecting turn-to-turn resistivity distribution in NI REBCO coils

Abstract A non-destructive method is proposed for detecting the turn-to-turn resistivity distribution (TTRD) of non-insulation (NI) coils made of REBCO tapes. In conventional designs, TTRD is often estimated to be a constant, while it is actually non-uniform. It is crucial to detect the TTRD of NI coils more accurately as it determines the behaviour of NI coils and may lead to peculiar phenomena such as local reverse currents. The proposed approach involves acquiring the temporal change of voltage distribution during an excitation/demagnetization process, which is subsequently incorporated into a system of ordinary differential equations derived from an equivalent circuit model. A genetic algorithm (GA) is then employed to fit the collected time-varying voltage data and generate the results of the ‘measured’ TTRD. The system of equations can actually be numerically solved. The solved time-varying TTRD results are averaged over the measuring period, which serve as the initial value of GA fitting, and accelerates the fitting process. Virtual measurements were performed on an artificially established mock coil, demonstrating high accuracy in reproducing the predetermined TTRD. Furthermore, an actual measurement was also conducted on a single-pancake coil, however with unknown TTRD, using eight voltage measurement points during the demagnetization process. The measured TTRD was incorporated into the equivalent circuit model to predict the temporal changes in voltage and magnetic field of the coil under additional excitation/demagnetization conditions. By comparing the predicted results with the experimental data, a high level of agreement was observed, thus confirming the potential application of the proposed method.