A study of coupling loss on bi-columnar BSCCO/Ag tapes through ac susceptibility measurements

Abstract

Coupling losses were studied in composite tapes containing superconducting material in the form of two separate stacks of densely packed filaments embedded in a metallic matrix of Ag or Ag alloy. This kind of sample geometry is quite favorable for studying the coupling currents and in particular the role of superconducting bridges between filaments. By using a.c. susceptibility technique, the electromagnetic losses as function of a.c. magnetic field amplitude and frequency were measured at the temperature T = 77 K for two tapes with different matrix composition. The length of samples was varied by subsequent cutting in order to investigate its influence on the dynamics of magnetic flux penetration. The geometrical factor χ0 which takes into account the demagnetizing effects was established from a.c. susceptibility data at low amplitudes. Losses vs frequency dependencies have been found to agree nicely with the theoretical model developed for round multifilamentary wires. Applying this model, the effective resistivity of the matrix was determined for each tape, by using only measured quantities. For the tape with pure silver matrix its value was found to be larger than what predicted by the theory for given metal resistivity and filamentary architecture. On the contrary, in the sample with a Ag/Mg alloy matrix, an effective resistivity much lower than expected was determined. We explain these discrepancies by taking into account the properties of the electrical contact of the interface between the superconducting filaments and the normal matrix. In the case of soft matrix of pure Ag, this is of poor quality, while the properties of alloy matrix seem to provoke an extensive creation of intergrowths which can be actually observed in this kind of samples.

Eddy current modeling in linear and nonlinear multifilamentary composite materials

“…We consider an HTS multifilamentary untwisted tape submitted to an external magnetic field oriented normal to its large surface (width). The results of the eddy current losses are compared to coupling losses (Qc) given by the Campbell's model [8], expressed as J/m 3 /cycle in (10), involving a time constant of the magnetic flux penetration (τ) and a geometrical factor (χ) related to the demagnetizing factor [9].…”

“…For the comparison, we use in (10) both theoretical and experimental values of the parameters (τ, χ). The experimental ones are given by D. Zola et al, obtained through measurements for the specified geometry [9]. Figure 3 shows a comparison between the analytical and numerical results of the eddy current losses.…”

“…Figure 3 shows a comparison between the analytical and numerical results of the eddy current losses. The comparison is performed in the limits of the validity of the analytical model (feeble fields) [8,9]. The results present constant discrepancy over the entire interval of calculation between the numerical and analytical values, which is of 2.8% when theoretical values of the parameters (τ, χ) are used and about 11% when the measured ones are used.…”

AC magnetization loss in striated YBCO conductors

“…In untwisted flat superconducting cables the CCAC loss (Q c ) for a sinusoidal external field was calculated using Campbell's model [4]: …”

“…Hysteresis AC loss in striated conductors decreases with the number of filaments but additional coupling current AC loss (CCAC) appears [2] due to finite resistivity between the filaments. The CCAC loss is well known for multifilament low-T c superconductors (SC) and BSCCO tapes [3,4] and can be reduced by twisting of the superconducting filaments. In untwisted flat superconducting cables the CCAC loss (Q c ) for a sinusoidal external field was calculated using Campbell's model [4]:…”