AC Loss and Coupling Currents in YBCO Coated Conductors With Varying Number of Filaments

Abstract

Abstract. Striation of HTS coated conductors as a way to reduce their magnetization AC losses has been the subject of intense worldwide research in the past years by several groups. While the principle of this approach is well understood, its practical application on commercial material to be used in power application is still far to be implemented due to manufacturing and technological constraints. Recent advances in tape quality and striation technology are now enabling systematic investigations of the influence of the number of filaments on AC loss reduction with a consistency that was not available in the past. In the present work we demonstrate the technological feasibility of importantly reducing the magnetization losses of commercially available coated conductor by striating them into a high number of filaments (up to 120). The loss reduction exceeds one order of magnitude and does not come at the expense of current-carrying capability: samples with 10 and 20 filaments are unaffected by the striation process, while samples with 80 and 120 filaments still retain 80 and 70% of the currentcarrying potential, respectively. We also investigate the transverse resistivity between the filaments in order to understand the paths followed by the coupling currents: we found that the coupling current prevalently flows in the metallic substrate, rather than in and out of the filaments. Finally, we use oxidation as a method to reduce the coupling currents and the corresponding losses. The contribution of this work is three-fold: 1) It describes the know-how to produce a large number of high quality striations in commercially available coated conductor, greatly reducing their losses without extensively degrading their transport properties;2) It provides a comprehensive characterization of said samples (e.g. measurements in a wide frequency range, transverse resistance profiles, influence of oxidation on DC and AC behavior of the sample); 3) It provides new insight on the patterns of the coupling currents.

AC Magnetization Loss and Transverse Resistivity of Striated YBCO Coated Conductors

“…In samples with thinner copper, clean and well defined grooves are found, see Fig. 1(b): the edges are sharp and similar to those found with non-stabilized (Ag) tapes [5]. The FIB cut (Fig.…”

“…The loss results discussed above demonstrate that, in the case of tapes stabilized with 20 μm of copper, it was not possible to obtain the same loss reduction as in non-stabilized samples (as presented in [5]). This is mainly due to the difficulty of obtaining sharply defined filaments and to the melting and redeposition of copper in the grooves between filaments, which effectively couples them together (Fig.…”

“…In the case of 10 filaments tapes, the agreement with FEM is satisfactory for Ag tapes, whereas in case of Cu-tapes we observe a large discrepancy due to the strong coupling. By increasing the frequency from 40 to 130 Hz, the losses of the Ag-sample do not increase much, because the coupling for those samples is not too strong [5]. On the contrary, the increase of frequency causes the losses of the Cu-sample to greatly increase-a sign of the importance of the coupling effect.…”

“…Besides measurements (symbols + dashed lines), the continuous solid line shows the expected decrease of I c due to the mere superconducting material loss in the 20 μm-wide grooves. In the case of 120 filaments, the I c is reduced to 55% of the I c of the original Ag tape (340 A) [5], and 30% of the I c of the original Cu tape (275 A), respectively. And when compared to expectation due to mere loss of HTS material, the relative degradation of I c is 32% and 64%, for the Ag and Cu-tape with 120 filaments, respectively.…”

“…In the case of thin tapes such as HTS CCs, magnetization losses calculated per unit length are proportional to the square of the tape's width, and can therefore be reduced by striating the tapes into thin filaments [1], [2] by different techniques, see references [3]- [5] and the references therein. In our previous work [5], we demonstrated that it is technologically possible to significantly reduce the losses of non-stabilized 12 mm wide tape by more than one order of magnitude without dramatically affecting the tape's current carrying capacity factor of 10 for 10 filament sample and factor of 40 in case of 120 filament tape. This was achieved by striating commercially available tapes using a laser, and additional oxidation of the samples can help further reduce the coupling losses.…”