Steady and Unsteady Current Modes and Thermal Runaway Conditions of High-Tc Composite Superconductors

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.