Magnetization Loss and Its Angular Dependence in a No-Insulation HTS REBCO Double-Pancake Coil

Abstract

No-insulation (NI) high-temperature superconductor (HTS) coils possess inherent advantages compared with their insulated (INS) counterparts, including enhanced current density, improved mechanical integrity, thermal stability, and self-protecting properties, making them a more appealing technological choice for high-field magnets, rotating machines, and linear propulsion applications. In many such applications, the coils are exposed to time-varying magnetic fields which results in magnetization loss. This loss, which creates a heat load that could significantly impact the cryogenic system, has yet to be well characterized and understood for NI coils. In this study, magnetization loss in a double-pancake, solder-impregnated NI coil wound HTS-coated conductor is investigated both experimentally and numerically at 77 K. The amplitude of the applied ac field is varied between 10 and 100 mT with frequency varying from 72 to 145 Hz and field orientation varying between parallel to perpendicular with respect to the normal vector of the conductor surface of the coil. The experimental results showed no frequency dependence and no clear angular dependence with the loss essentially equal between parallel and perpendicular fields, in striking contrast to INS coils. Numerical analysis carried out in a 2D configuration for parallel fields was able to reproduce the relevant experimental data. The current density and magnetic field distributions calculated from the analysis show that the NI coil can completely shield its inner turns, attributed to the effective shielding provided by the radial and screening currents. As such the magnetization loss characteristics of NI coils are better understood as those of an HTS bulk, rather than of a coil.