Characteristics of the a.c. losses in Ag-sheathed PbBi2223 tapes

Abstract

A mini review of our latest results on the a.c. losses in Ag-sheathed PbBi2223 tapes is presented with the focus on self-field losses in a variety of situations and external losses with and without a d.c. transport current. Comparative measurements with the thermometric and electric methods have demonstrated the validity of the electric method despite the problem of a distribution of the loss electric field. Using these two methods the effect of a longitudinal distribution of critical current along the tape has been shown. The influence of a transverse distribution of the local J c through the core of the tape is also examined. Studies of the magnetic field dependence of the self-field losses have shown that different regions of the core have a different J c( B) dependence, in contrast to the temperature dependence where all regions of the core show a similar dependence. Measurements at higher frequencies show the importance of flux diffusion where an increased dB dt gives an increased induced critical current. At sufficiently higher frequencies (> 200 Hz) the self-field of the core is shown to induce eddy current losses within the silver sheath of the tape. For the external a.c. field case in parallel fields the losses are well described by the infinite slab geometry. For the case of the perpendicular field the losses are considerably less than those expected from either the thin ellipse or rectangle geometry. Possible mechanisms for this discrepancy include granularity of the tape or partial uncoupling of the filaments. For the combined action of an a.c. external field and a d.c. transport current our results show that the total losses cannot be obtained using only using a pick-up loop to measure the flux changes within the tape. An additional loss sustained by the d.c. current source also needs to be measured via a pair of voltage taps.