AC loss and flux instabilities in single-component and double-component type II superconductors

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Abstract
Calculations are presented for AC loss and flux instabilities under AC and DC conditions in type II superconductors in fields that are greater than Hc1 but not so large that surface currents, the annihilation energy of fluxons and other effects associated with Hc1 can be neglected. The calculations are particularly relevant to the design of AC power transmission cables, and attention is paid to a two-component superconductor (eg a Nb/high-field superconductor) under fault conditions; that is, with current flowing in both the niobium and the high-field superconductor. In general, inclusion of surface currents and other effects associated with a finite value of Hc1 leads to a decrease in the AC loss and a decrease in the probability of flux jumping. It also shows that the energy of annihilation of fluxons is only important for flux jumping when the peak field is less than about 2Hc1. It is the stored energy in the flux distributions that gives more flux jumping with AC than with DC. In the two-component superconductor an abrupt change in flux occurs when the flux density at the boundary between the two materials falls to zero. This gives larger losses than the equivalent superconductor without the niobium layer, and makes flux jumping more likely.

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