Thermal Peculiarities of Stable Macroscopic Distribution of Applied Current in High-Temperature Superconductors
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The influence of the stable temperature rise induced by current charging on the current-carrying capacity of a high-temperature superconductor without stabilizing matrix is investigated. Two characteristic values of the electric field at which the convective and conductive heat transfer mechanisms mostly affect the formation of current charging states in the superconductor are defined. It is shown that its stable temperature preceding the onset of current instability not only is always above the coolant temperature but also increases with decrease in these characteristic electric fields. As a result, the current stability conditions are modified due to the thermal peculiarities of the current charging formation of stable electrodynamics states of the superconductor. Namely, the allowable stable values of electric field and current can be below or above those determined by a priori chosen critical parameters of the superconductor. The criteria allowing one to estimate the boundary of existence of these stability areas (sub-critical and over-critical) are written taking into account the size effect. The performed analysis reveals that the temperature-dependent heat capacity of superconductor leads to such shape of its voltage-current characteristics, which has only the positive slope during continuous current charging both before and after current instability onset. It is also proved that the thermal degradation of the current-carrying capacity of superconductor occurs in the sub-critical area. Due to this effect, the currents flowing stably in the superconductor are not only obviously less than the corresponding critical current, but also not increase proportionally to increasing cross-sectional area of superconductor or to its critical current density. These effects must be considered in investigations of current-carrying capacity of superconductors
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