AC losses in type-II superconductors induced by nonuniform fluctuations of external magnetic field

Magnetic field fluctuations are inevitable in practical applications of superconductors and it is often necessary to estimate the ac losses these fluctuations induce. If the fluctuation wavelength is greater than the size of a superconductor, known estimates for an alternating uniform external magnetic field can be employed. The authors consider the opposite case and analyze, using a model critical-state problem, penetration of spatially nonuniform fluctuations into type-II superconductors. Numerical simulation is based on a variational formulation of the Bean model. The analytical solutions, found in a weak penetration limit, are used to evaluate ac losses for two types of fluctuations: the running and standing waves. It is shown that for spatially nonuniform fluctuations the losses are better characterized by the fluctuation penetration depth than by the fluctuation amplitude. The results can be used to estimate the ac losses in flywheels, electric motors, magnetic shields, etc.