Study of coupling loss on bi-columnar BSCCO/Ag tapes by a.c. susceptibility measurements

摘要

Coupling losses were studied in composite tapes containing superconductingmaterial in the form of two separate stacks of densely packed filamentsembedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometryis quite favorable for studying the coupling currents and in particular therole of superconducting bridges between filaments. By using a.c. susceptibilitytechnique, the electromagnetic losses as function of a.c. magnetic fieldamplitude and frequency were measured at the temperature T = 77 K for two tapeswith different matrix composition. The length of samples was varied bysubsequent cutting in order to investigate its influence on the dynamics ofmagnetic flux penetration. The geometrical factor $\chi_0$ which takes intoaccount the demagnetizing effects was established from a.c. susceptibility dataat low amplitudes. Losses vs frequency dependencies have been found to agreenicely with the theoretical model developed for round multifilamentary wires.Applying this model, the effective resistivity of the matrix was determined foreach tape, by using only measured quantities. For the tape with pure silvermatrix its value was found to be larger than what predicted by the theory forgiven metal resistivity and filamentary architecture. On the contrary, in thesample with a Ag/Mg alloy matrix, an effective resistivity much lower thanexpected was determined. We explain these discrepancies by taking into accountthe properties of the electrical contact of the interface between thesuperconducting filaments and the normal matrix. In the case of soft matrix ofpure Ag, this is of poor quality, while the properties of alloy matrix seem toprovoke an extensive creation of intergrowths which can be actually observed inthis kind of samples.