Numerical Simulations in Stand-Alone REBCO Coils Under Rotating Magnetic Fields

Large ac loss in the high-temperature superconducting (HTS) armature windings carrying ac under a rotating magnetic field poses a significant challenge for high power-to-weight ratio all-HTS rotating machines. Understanding the loss behavior of HTS coils in rotating magnetic fields and exploring methods for loss reduction are essential. This study numerically investigates the total ac loss (Q_tot) (with current) and magnetization loss (Q_m) (without current) in HTS coils under nonuniform rotating magnetic field. The losses of three 4-turn HTS coils are compared, which are designed with a 4 mm wide tape, a striated tape with two 2 mm filaments, and a striated tape with four 1 mm filaments, respectively. In addition, ac loss in the coils at 77 and 65 K is calculated and evaluated. Simulation results show that, under rotating magnetic fields, the striated structure of the coils significantly reduces both Q_tot and Q_m. At high fields (e.g., above 200 mT for Q_m at 77 K), compared to the loss in the un-striated coil, the loss in the coil designed with the two-filament striated tape decreased by almost half, and the loss in the coil designed with the four-filament striated tape was reduced to nearly a quarter. Interestingly, we also show that under rotating magnetic fields, Q_m and Q_tot can be effectively scaled with the critical current of either the coil or the tape. This suggests that losses at different temperatures can potentially be estimated using their respective critical currents.