Theoretical and experimental study of AC loss in HTS single pancake coils

The electromagnetic properties of a pancake coil in AC regime as a function of the number of turns is studied theoretically and experimentally. Specifically, the AC loss, the coil critical current and the voltage signal are discussed. The coils are made of Bi2Sr2Ca2Cu3O10/Ag (BiSCCO) tape, although the main qualitative results are also applicable to other kinds of superconducting tapes, such as coated conductors. The AC loss and the voltage signal are electrically measured using different pick up coils with the help of a transformer. One of them avoids dealing with the huge coil inductance. Besides, the critical current of the coils is experimentally determined by conventional DC measurements. Furthermore, the critical current, the AC loss and the voltage signal are simulated, showing a good agreement with the experiments. For all simulations, the field dependent critical current density inferred from DC measurements on a short tape sample is taken into account.Comment: 22 pages, 15 figures; contents extended (sections 3.2 and 4); one new figure (figure 5) and two figures replaced (figures 3 and 8); typos corrected; title change

Magnetic flux penetration and AC loss in a composite superconducting wire with ferromagnetic parts

The current distribution and the AC loss in a composite superconducting tape containing a layer from magnetic material is calculated and compared with experiments, showing a very good agreement. The situations of an alternating uniform applied field or a transport current are studied. The newly developed numerical model is an approximation to the critical state model, adapted for the applicability to commercial finite elements codes that solve the vector potential. Substantial feature of this procedure is that it can be carried out in the case when the critical current density in superconductor depends on the magnetic field and the magnetic layer material is non- linear. Additionally, the hysteresis loss in the magnetic material is estimated, based on its measured magnetization loops. Measurements on Bi-2223 multifilamentary tapes covered on edges by nickel confirmed our predictions, showing a substantial ac loss reduction in both the investigated regimes.Comment: 33 pages, 12 figure

AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

Many applications of ReBCO coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this article, we measure and simulate the AC loss and the critical current, I_c, in stacks of pancake coils ("pancakes"). We construct stacks of up to 4 pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of J_c from I_c measurements of the tape. This J_c is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double width but a stack of more pancakes is very different. Finally, we found that a 2-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, the AC loss in stacks of pancakes is too high. However, our simulations are useful to predict the AC loss and optimise the coil design, reducing the AC loss.Comment: 34 pages, 18 figures. All figures are modified; figures 3, 7 and 10 are new. Text thoroughly revised and extende

Influence of Ferromagnetic Layer on Critical Current of a Superconducting Wire

We investigated how a 10-50 μm layer from soft ferromagnetic material, incorporated in the architecture of a composite superconducting tape, could influence the capability of the tape to transport electrical current. Numerical procedure was developed to calculate the local distribution of magnetic field and current in the condition when all the superconductor is carrying its critical current density. It was found that the concentration of magnetic flux in ferromagnetic layers allows to reduce the local value of magnetic flux density felt by the superconductor. In this way, the critical current of the tape in low applied magnetic fields can be improved. This theoretical prediction is confirmed by experimental observation

Influence of DC Magnetic Field on AC Loss of YBCO Coated Conductor with Ferromagnetic Substrate

Most of YBCO tape conductors are prepared on a ferromagnetic substrate. The ferromagnetic material is then exposed to the self magnetic field of the superconductor and also to external magnetic field. In AC magnetic field hysteretic and eddy current losses are generated in the substrate. In this work the AC transport loss of an YBCO coated conductor on a ferromagnetic substrate was measured using electromagnetic method. There are two contributions to total loss - a loss in the superconductor and a loss in the substrate. To suppress the hysteretic loss in the substrate, it is possible to saturate it using DC magnetic field. In this regime significant decrease in total loss was observed

DC Characterization of the Coaxial Superconducting Cable

Coaxial cable model with superconducting core and superconducting shield conductor was constructed and tested in DC regime. While the core was already examined in our previous works, in this contribution the detailed study of the superconducting shield conductor in DC conditions is presented. It consists of 16 ReBCO coated tapes with critical current 35 A each connected in parallel. Using shunts with known values placed in series the currents in individual tapes were possible to measure. Distribution of the total cable current into the individual tapes was monitored and its influence on critical current of the cable is discussed