Current instability of high temperature superconducting tapes in the AC modes

The basic physical reasons rooted in the current instability phenomenon of AC regimes are discussed. To explain their stable formation, the conduction-cooled high-temperature superconductors (HTS) are investigated studying mutually dependent thermal and electrodynamics states of a tape. It is proved that there exist characteristic times defining time windows of stable development of overloaded AC regimes. They lead to the states for which the limiting peak values of charged current and induced electric field before instability onset much more than the corresponding critical values of a superconductor. Moreover, the limiting stable peak values of the temperature, electric field and current are higher than the corresponding quench ones calculated at DC modes. It is shown that the stability boundary of overloaded AC modes slowly depends on the current frequency. As a result, the high-T _c superconducting magnets may stably operate under very high AC losses even at the conduction-cooled conditions. The obtained results should be taken into consideration when the range of stable losses is defined in HTS during AC regimes.