A model for calculating the AC losses of second-generation high temperaturesuperconductor pancake coils

Abstract

A model is presented for calculating the AC losses of a stack of second-generation hightemperature superconductor tapes. This model takes as a starting point the model of Clemand co-workers for a stack in which each tape carries the same current. It is based on theassumption that the magnetic flux lines lie parallel to the tapes within the part of the stackwhere the flux has not penetrated. In this paper we allow for the depth of penetration offield to vary across the stack, and use the Kim model to allow for the variation ofJc withB. The model is applied to the cases of a transport current and an applied field. For atransport current the calculated result differs from the Norris expression for a single tapecarrying a uniform current and it does not seem possible to define a suitable averageJc which could be used. Our method also gives a more accurate value for the critical current of thestack than other methods. For an applied field the stack behaves as a solid superconductor with theJc averaged locally over several tapes, but still allowed to vary throughout the stack on alarger scale. For up to about ten tapes the losses rise rapidly with the number of tapes, butin thicker stacks the tapes shield each other and the losses become that of a slab with afield parallel to the faces.