Nonlinear approximation for limiting current-carrying capacity of Ag-sheathedBi2Sr2CaCu2O8 conductors

Abstract

The thermal runaway phenomenon inAg/Bi2Sr2CaCu2O8 composite conductors is studied for the nonlinear temperatureand magnetic field dependences of their critical current densityJc(T,B) and matrixresistivity ρm(T,B) (nonlinear approximation). The influence of the applied magnetic field, heat transfercoefficient, matrix resistivity and volume fraction of the composite superconductor on thestatic thermal and electric states were investigated. The used model is basedon steady heat and current balance equations. It allows us to investigate thenon-isothermal voltage–current characteristic of composite superconductors, that rootsthe static description of their operating regimes and allows us to determine thelimiting quantities of the thermal runaway parameters. It is found that there existstable jumps of the electric field, current and temperature without the transitionof the superconducting composite into the normal state. The presence of thesestatic states is a result of the additional stable branches on the voltage–currentcharacteristics, which appear according to the variation of the differential resistivity of acomposite. The latter may have wide range fields due to the coupled thermo-magneticand current-sharing mechanisms changing the quantities and . This variation takes place when the applied magnetic field or operating parameters of acomposite exceed some characteristic values. As a result, the non-monotonic temperaturedependence of the Joule heat release will take place. Besides, it is shown that thestable voltage (without thermal runaway) may exist when the temperature of theAg/Bi2Sr2CaCu2O8 composite stably increases from the coolant temperature up to the critical temperature of asuperconductor during current charging. The thermal runaway parameters as a function ofthe applied magnetic field are numerically derived accounting for the additional stablebranches of the voltage–current characteristic.