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To understand the underlying physical trends of the current instability in a composite high-Tc superconductor, the limiting margin of its current-carrying capacity is derived in dc magnetic fields in the framework of the macroscopic continuum approximation. A static zero-dimensional model was used to formulate the peculiarities of the nonisothermal electric field distribution in a composite in the fully penetrated current states. The power and exponential equations describing the E(J) dependences of a superconductor are used. The boundary of the allowable stable values of the electric field, current, and temperature are investigated using qualitative and quantitative models. Permissible stable values of the electric field and current, which might be lower (subcritical states) or higher (overcritical states) than those determined by the critical voltage criterion, are discussed. It is stated that the subcritical quenching electric states are more probable in the operating regimes, which are observed in the high magnetic field. The overcritical stable quantities of the electric field exist, for example, if the superconducting composite has a relatively small volume fraction of the superconductor in a composite. In the meantime, the stable current modes may be both subcritical and overcritical when the permissible value of the electric field is overcritical. As a consequence of these features, an unavoidable increase in temperature of the composite superconductor occurs before its transition to the normal state. The latter depends on a broad shape of the E(J) dependence of high-Tc superconductor and the current sharing between the superconducting core and the matrix. In the limiting case, a stable value of the composite temperature may equal the critical temperature of the superconductor. For such operating states, the criterion of the complete thermal stability condition is written taking into consideration the nonlinear character of the E(J) dependence. Simultaneously, an allowable change in temperature of the superconducting composite leads to the thermal degradation of its current-carrying capacity. It depends on the critical current density of the superconductor at bath temperature, amount of a superconductor, and cross section of a composite under fixed cooling conditions. In particular, it is shown that the currents corresponding to the instability onset do not increase proportionally with relevant increase of the superconductor’s amount. The estimates presented have general character and may be used to verify the operating states of low-Tc superconducting composite. [ABSTRACT FROM AUTHOR]

From the viewpoint of a cryogen-free high-field superconducting magnet, the thermal stability properties of YBa2Cu3O7 (Y123) coated conductor tapes before thermal runaway are examined under the conduction-cooling condition by a GM-cryocooler. Under such cryocooling condition, Ag-sheathed Bi2Sr2CaCu2O8 (Bi2212) wires with Jc\sim 10^4\ \rm A/cm^2 reveal stable behaviors of 3 times larger current properties before thermal runaway than critical currents, although conventional Nb3Sn wires with Jc\sim 10^4\ \rm A/cm^2 quench almost at their critical currents under the cryocooling condition. In order to determine the maximum current-carrying capacity of the Y123 tape in high fields, we use a zero-dimensional heat balance model. On the basis of the measured basic properties for the Y123 tape, the calculations were done at B=20\ T, bath temperature T0=4.2\ K, Jc\sim 10^7\ \rm A/cm^2, and cryocooling heat transfer coefficient h=10^-3\ W/cm^2K. We found that the Y123 tape with Jc\sim 10^7\ \rm A/cm^2 does not have the over-critical current but rather the sub-critical current before thermal runaway. [ABSTRACT FROM AUTHOR]

Transport current properties of the steady and unsteady operating modes and thermal runaway conditions of the Ag-sheathed Bi2Sr2CaCu2O8 composite superconductors are theoretically studied in DC magnetic fields. It is shown that the temperature of a composite is not equal to the coolant temperature before thermal runaway. As a result, the shape of the voltage-current characteristics of high-Tc superconducting composites has only a positive slope during continuous current charging both before and after thermal runaway. That is why the voltage-current characteristics of high-Tc superconductors do not define the boundary of the thermal runaway. This peculiarity has to be considered during experiments at which the critical or quench currents are defined. [ABSTRACT FROM AUTHOR]

: Magnetic relaxation of the partially penetrated screening current during flux creep has been studied. A theoretical analysis was based on the macroscopic description of the flux creep using the power and exponential equations for current-voltage characteristics of superconductors. The analytical flux-creep solutions were written and compared with numerical simulations of corresponding problems. Equations describing the relaxation of the electromagnetic field, magnetic moment, moving penetration boundary are derived. It is shown that peculiarities of relaxation phenomena are determined by dynamics of the electric field on the surface of the superconductor. The performed analysis allows to formulate nontrivial conjugation conditions that take place on moving boundary of screening current. In accordance with these conditions the electromagnetic field induced inside a superconductor by external perturbations smoothly approaches its undisturbed values. The essential role of the low electric field area of current-voltage characteristics in the flux relaxation and primarily in high-temperature superconductors is shown. [ABSTRACT FROM AUTHOR]