Abstract:
To understand the physical regularities underlying the current instability onset in composite high-Tc superconductors， the limiting margin of its current-carrying capacity is derived under DC magnetic fields in the framework of the macroscopic continuum approximation. The current sharing and temperature effects were considered. A static zero-dimensional model was used to formulate the criteria defining the peculiarities of the non-isothermal electric field distribution in a composite during fully penetrated states. Power and exponential E（J）-dependences that describe the creep states of superconductors are considered. The boundary of the allowable stable values of the electric field， current and temperature are investigated under qualitative and quantitative considerations. Permissible stable modes of the electric field and current， which might be lower and higher than those determined by the critical voltage criterion， and the reasons leading to both sub-critical and over-critical regimes are discussed. In particular， it is proved that the sub-critical electric states are more probably at operating regimes， which are observed in the high magnetic field. The over-critical quantities of the electric field exist， for example， if the superconducting composite has relatively small volume fraction of superconductor. In the meantime， the stable currents may be both sub-critical and over-critical when an permissible value of the electric field is over-critical. In consequence of these features， the unavoidable temperature rise of the composite superconductor before its transition to the normal state takes place. The latter is a result of a broad shape of E（J）-dependence of high-Tc superconductors and the current sharing between a superconducting core and a matrix. As a result， an allowable temperature change of a composite depends on amount of a superconductor in a composite at the given E（J) -dep endence. In the limiting case， the stable value of composite temperature may equal the critical temperature of a superconductor. For such operating states， the criterion of the thermal stability condition is written when the charging current will flow stably only in a matrix considering the non-linear character of E（J）-dependence.