Thermal Mechanisms of Stable Macroscopic Penetration of Applied Currents in High Temperature Superconductors and their Instability Conditions

Macroscopic mechanisms of applied current penetration in high-Tc superconductors are discussed to understand the basic physical trends, which are characteristic for the stable and unstable formation of the thermo-electrodynamics states of high-Tc superconductors placed in DC external magnetic field. The performed analysis shows that the definition of the current stability conditions of high-Tc superconductors must take into consideration the development of the interconnected thermal and electrodynamics states when the temperature of superconductor may stably rise before instability. As a result, the thermal degradation effect of the current-carrying capacity of superconductor exists. The boundary values of the electric field and the current above which the applied current is unstable are defined taking into account the size effect, cooling conditions, non-linear temperature dependences of the critical current density of superconductor. It is proved that the allowable stable values of electric field and current can be both below and above those determined by a priori chosen critical values of the electric field and current of the superconductor. The violation features of the stable current distribution in high-Tc superconductors cooled by liquid coolant are studied. The necessary criteria allowing one to determine the influence of the properties of superconductor and coolant on the current instability onset are written.