Thermal instability of superconducting composites at magnetic flux creep

Abstract

The conditions of the onset and development of thermal instabilities occurring under the action of thermal disturbance in superconducting composites with voltage-current characteristic described by a power equation are discussed. The investigation is done in the framework of the quasi-linear model, as in the Stekly approximation. The critical energies, which lead to the transition of the superconducting state to the normal one, and the velocities of irreversible propagation of the thermal instability along the composite are determined. It is shown that the largest current stably flowing in a superconducting composite can be either lower or higher than a priori defined critical current of a superconductor with a real (continuously increasing) voltage-current characteristic. The condition of the complete thermal stabilization for the superconducting composites with a power voltage-current characteristic is formulated. It is shown that the commonly accepted Stekly condition of the complete thermal stabilization, according to which the superconducting composite is completely stable to the thermal perturbations with any energy, does not allow to determine correctly the conditions providing the complete thermal stabilization of superconducting composites with real voltage-current characteristics. The results of the numerical simulations express that the existing theory of the thermal stabilization in the framework of which a jump transition from a superconducting state to a normal one is supposed leads to a limitation of the currents, at which a stable superconducting state exists.