Energy Loss and Regimes of Flux Dynamics in BSCCO Tapes Above the Engineering Critical Current

Time dependent measurements of the electric field in BSCCO tapes carrying DC current and exposed to AC magnetic fields reveal new and rich details about the electric field behavior. In particular, E(t) curves obtained for currents above the engineering critical current exhibit "double peak" feature for each half-cycle of the external magnetic field. For currents lower than Ic, E(t) exhibits a usual sine ("single peak") with amplitude and frequency dependent phase shift relative to the external field. Similar behavior is obtained for currents above I_c for low frequencies and/or high amplitudes of the magnetic field. However, with the decrease of amplitude and/or increase of frequency, a "second peak" emerges and gradually becomes dominant. This "second peak" is in phase with the external magnetic field. We interpret the results by assuming a crossover between two regimes of flux dynamics affecting the mechanism for energy loss generation in the tape. In the "quasi-DC", low frequency - high-field regime, flux lines enter the tape from one side, cross it entirely and leave from the other side, causing a dissipation, which is in phase with the external field. In the high frequency - low-field regime, flux lines enter and leave the tape from the same side and some annihilation of vortex and anti-vortex occurs at the central region of the tape. In this regime the dissipation and its phase are frequency and amplitude dependent.