This review gathers together information important to the understanding of superconducting materials under a.c. conditions, and points out some features which are at present not too well understood. The idea is to try to build a consistent picture of the performance of superconductors rather than to present a historical account of the subject. The basic behaviour of fluxons in the bulk of type II superconductors can be explained by the critical state model, but some of the details of flux pinning and the method of flux motion are in doubt. Flux instabilities under d.c. conditions have received a lot of attention and so have the methods of stabilization, but instabilities under a.c. conditions have not been studied in so much detail, and the problem of stabilization is more difficult than for f.c. Generally surface effects are less well understood than bulk effects, partly because there are a number of possible phenomena to contend with. A.C. loss in the Meissner state can be explained by field en...

A.C. loss and related effects in type II superconductors

Effects of thermal insulation on flux jumps in Nb-50% Ti rods

Electromagnetic phenomena and hysteresis losses in superconductors

System for coil simulation measurements of losses and instabilities in superconducting wires

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.

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

A theory of flux jumps in a slab of a nonideal type-2 superconductor is developed by the use of a phenomenological theory of flux motion To include explicitly in the theory the various quantities which are controllable experimentally, the temperature inside the slab is assumed to be approximately uniform Then a flux jump is characterized by a catastrophically sharp rise of the temperature of the sample The criteria for the occurrence of flux jumps are derived under such an assumption, together with the relation between the value of external magnetic field, at which a flux jump starts, and the sweeping rate of the external magnetic field Various behaviors of flux jumps observed experimentally are reasonably explained by the present theory in a quantitative way

Norio Yano

Papers

Flux Jumps in Nonideal Type-2 Superconductors. I. Theory of Flux Jumps in Slabs