Theory of ac loss in type II superconductors in the Meissner state

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Abstract
The problem of ac loss in the Meissner state of type II superconductors has been examined theoretically by considering the field enhancement and consequent flux penetration at surface irregularities. A rough estimate of the loss can be made with little difficulty, but to study the problem in more detail the author equates the Lorentz force, (- phi 0*curl H(B,x)), with the pinning force on the fluxons. This gives the flux gradient as a function of flux density and position. Numerical integration is necessary to obtain the flux distribution and hysteresis loss, but the final results are in good agreement with experiment, and show that the loss is dependent upon the shape, the purity, and to a lesser extent the metallurgical state of the surface region. The low frequency motion of small quantities of flux trapped in the superconductor perpendicular to the surface has also been examined, and the contribution to ac loss determined.

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