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The current-voltage characteristics of superconductors are commonly determined by means of a standard 4-probe DC measurement technique. In coated conductors, however, one often encounters problems due to weak or no stabilization and in such case, when the risk of burning the tape is high, the 4-probe approach can fail. In other cases such as in HTS cable structures, besides vulnerability and high risk of damage, it may also be difficult to achieve a high critical current level with a power supply that is normally used to characterize single strands. We therefore investigated an alternative method to determine the DC I-V curve, based on measuring the AC transport loss as a function of frequency in overcritical current range. In the AC regime, both the hysteretic loss component and the joule dissipation are present for applied transport currents exceeding the critical current Ic. In this work, AC loss measurements at different frequencies are related to different electric fields in DC and a curve similar to the DC I-V is obtained. From this curve, the power index N and Ic values are estimated. AC regime operation can be more beneficial for two reasons: firstly, because very high currents can be achieved easily by using a transformer; secondly, because the effective joule heat dissipation with overcritical AC currents is much lower than that with DC of the same amplitudes. In the experiment, we used a 4 mm wide YBCO reference tape with 20 {\mu}m of Cu-stabilization. For the comparison with AC transport loss, the sample was measured with the standard 4-probe technique and remained stable even in overload condition. To support our study, numerical simulations, reproducing both transient and stationary operation conditions, were performed. A similar relation between the AC current transport loss and the I-V curves was also observed in the simulations., Comment: EUCAS 2015

The optimization of a HTS cable design with respect to AC-losses is of crucial importance for the economic viability of the respective concept. However the experimental determination of AC-losses is not straightforward since for short cable samples the distribution of current among the super-conducting tapes is mainly determined by the contact resistances of the individual tapes. The resulting inhomogeneous current distribution definitely falsifies the results. To solve this experimental problem we present a new experimental technique. The setup is a 2m-long three phase concentric cable model for which, within each phase, the superconducting tapes (up to 30) are connected in series. The Cu-braid backwards conductors were assembled in a rotational symmetric cage type arrangement, such that their self fields at the cable cancel. If experimental peculiarities of this setup, as the strong inductive coupling between the phases and the suitable positioning of the voltage contact leads, are correctly taken into account, the currents can be controlled independently and the electrical properties of the cable can be measured unambiguously. In this paper preliminary results are presented. The work is part of the German government funded cable project AMPACITY (1 km / 20 kV/ 2 kA)

Striation of HTS coated conductors (CCs) as a way to reduce their magnetization AC losses has been the subject of intense 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 this work we demonstrate the technological feasibility of reducing the magnetization losses of commercially available CC 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 current-carrying potential, respectively. We also investigate the transverse resistivity 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 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 CCs, 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); 3) It provides new insight on the patterns of the coupling currents.

First experimental observations of the ferromagnetic shielding effect in high-Tc 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., Comment: 4 pages, 3 figures, 1 table