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

TL;DR

This paper analyzes how nonuniform magnetic field fluctuations induce AC losses in type-II superconductors, providing models and simulations to better estimate these losses in practical applications like motors and shields.

Contribution

It introduces a model for AC losses caused by spatially nonuniform magnetic fluctuations, extending beyond uniform field assumptions with analytical and numerical solutions.

Findings

AC losses depend on fluctuation penetration depth.

Analytical solutions are valid in weak penetration limit.

Results applicable to superconducting devices like motors and shields.

Abstract

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. Here we 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.