Instabilities in Technical Superconductors upon AC Input

This article discusses the issue of stability of AC input to a high temperature superconductor during its non-intense cooling. The conditions for the occurrence of instability are formulated depending on interrelated changes in its electrodynamic state with temperature. It has been demonstrated that, before the occurrence of unsteady states, the peak values of electric field intensity, current, and temperature of superconductor exceed respective values that determine the thermal electrodynamic boundary of stability of currents continuously running over superconductor (the so-called quench currents). It has been established that, upon AC input to superconductor, intense steady energy dissipation can occur, which is not taken into account in the existing theory of losses. These steady modes can be determined as overloaded. The performed analysis demonstrates that there exist characteristic times determining intervals of steady existence of AC in the overloaded mode. The main thermal electrodynamic mechanisms of their existence are formulated. They explain the existence of high steady overheating of superconductor and induced electric field before the occurrence of instability upon AC input. The existence of steady overloaded modes significantly expands the scope of practical application of high temperature superconductors.