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Magnetic Field in the Winding of an YBCO Pancake Coil: Experiments and Calculations

The AC loss is an important issue for many applications containing coils of ReBCO coated conductors, specially transformers. In order to reduce the AC loss, it is necessary to understand the loss mechanisms. We measured and computed the radial magnetic field component in a pancake coil with a net transport current, as this component of the magnetic field plays a crucial role in the AC loss. For this purpose, we prepared a small pancake coil wound with 12 mm wide YBCO tape with non-magnetic substrate and measured the radial component of the magnetic field in the space between turns at both 77 K and room temperature. We measured at six radial positions inside the coil winding during the first AC cycle and the following ones. Additionally, we numerically calculated the magnetic field in all the pancake coil, taking into account the hysteretic nature of the supercurrents. The agreement of the calculated and measured profiles of radial component proves that the calculation method is correct. The radial component at room temperature, when the YBCO is not superconducting, is not hysteretic. In conclusion, the comparison of the experimental values of the radial field component at room temperature and 77 K provide information on the hysteretic currents in the superconductor. Moreover, our simulation method for the calculations is useful to predict the AC loss and the hysteretic effects of pancake coils.