Limiting current-carrying capacity of Ag-sheathed Bi₂Sr ₂CaCu₂O₈ conductors: Linear approximation

The current-carrying capacity of the Ag/Bi₂Sr ₂CaCu₂O₈ multifilament conductor is studied in detail as a function of the heat transfer coefficient, resistivity of the matrix, and volume fraction of the superconductor. The thermal runaway parameters corresponding to the irreversible superconducting-to-normal transition are derived analytically under the aspect of possible finite temperature rise of the composite conductor before thermal runaway. The static analytical model determines the thermal runaway parameters. The power law describes the voltage-current characteristic of a superconductor with a linear temperature dependence of the critical current (linear approximation). The performed analysis reveals that the allowable magnitude of the current and electric field before the thermal runaway may be higher than those determined by the criterion E_c = 10^-6 V cm^-1 in many practical cases. The condition of the stable state for the over-critical current is formulated and the peculiarities of such operation regimes are discussed. It is shown that the essential stationary overheating of the superconductor may occur before the thermal runaway. The minimum value of the current at which the thermal runaway starts is found if the volume fraction of superconductor in the composite is changed.