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.
The properties of a small pancake coil made with a 10mm wide copper-stabilized YBa2Cu3O7−x (YBCO) coated conductor were investigated. The radial component of the magnetic field was mapped at the coil edge in both the dc and ac regimes and differs significantly from that calculated assuming a uniform current distribution. The observed hysteresis indicates the strong influence of the ferromagnetic properties of the substrate. The ac losses of the coil were measured for ac frequencies between 60 and 1000Hz. The differences in properties of the YBCO coil and a similarly prepared copper coil are discussed.
It is well known that the separation of thin (RE)BCO superconducting films into electrically isolated stripes (striation process) leads to significant reduction of the magnetization losses. However, in practice, achieving the theoretically predicted loss reduction is quite complicated, due to imperfect separation of the stripes: techniques used for striation leave resistive bridges between the stripes, and coupling currents are free to flow. Very little is known about the precise paths of the coupling currents, other than the fact that the transverse resistivity may play a major role. In this paper, we investigate the magnetization ac loss and the transverse resistivity profile on samples with different numbers of filaments and with different thicknesses of the stabilization layer. The reduction of stabilization layer thickness leads to better control of the laser grooves and substantially suppresses coupling loss. The total loss in those tapes was reduced significantly and is very close to the theoretical expectation.
First experimental observations of the ferromagnetic shielding effect in high-T c superconducting coated conductors were carried out. Experimental results were compared to simulations calling upon finite-element calculations based on the H-formulation of Maxwell equations to model superconducting strips with ferromagnetic shields. Samples of copper-stabilized coated conductors were electroplated with nickel shields and afterwards characterized. Both externally applied oscillating transverse magnetic fields as well as transport currents were studied. Having observed promising gains with respect to the reduction of ac losses in both cases, we further investigated the potential of ferromagnetic shielding. The numerical model was able to reproduce and also predict experimental results very well and will serve as an indispensable tool to determine the potential of soft ferromagnetic materials to significantly reduce hysteretic losses.
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