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Use of superconductors for the shielding of the magnetic field is a common solution nowadays. In case when the magnetic signature of the shielding device matters, forming a magnetic cloak by combining superconductor with ferromagnetic material is a viable option. When considering the operation in alternating magnetic fields the AC loss linked to the delay in phase of the shielding currents represents an important topic. It is well known that the coated conductor (CC) tapes represent a valid alternative to bulk superconducting material when an AC loss reduction appears as a significant requirement. However, the commercially available coated conductors are usually limited to 12 mm width, therefore finding a suitable arrangement of tapes plays a significant role in the magnetic cloak design. We made a set of experiments on two possible arrangements in the superconducting part (helically wound tapes or straight pieces of the tape) and with the total number of layers up to six. For understanding the mechanism of magnetic field penetration inside the cloak the 2D and 3D numerical models were built. Presented numerical models helped in the localization of the cloak component with high energy dissipation. Analysis of the experimental and computation results allow us to formulate some recommendations for the construction of the superconducting part of the magnetic cloak. [ABSTRACT FROM AUTHOR]

Cloaking a three-dimensional object in free space from electromagnetic waves has recently become a theoretical possibility, although practical implementations can only be made in reduced schemes. If static fields are involved, requirements are less restrictive and some practical realizations have been possible. Here we present a third regime between the full wave and the static cases. We experimentally demonstrate that a cloak constructed under the dc conditions can keep cloaking properties for applied magnetic fields oscillating at low frequencies (up to hundreds of Hz). Because electromagnetic technology works at these frequencies, applications of our ideas to present technology are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Uniformity of superconducting layer can play important role in various applications of coated conductor tapes. For example a degradation of critical current density at the tape edges causes significant elevation of transport AC loss compared to the uniform tape with the same critical current. Then the information about tape uniformity is essential when a valid AC loss prediction should be obtained by a numerical simulation. For this purpose we investigated experimentally the distributions of magnetic field above the tape surface obtained by Hall probe mapping. An advanced inversion procedure has been developed to resolve the critical current distribution across the tape width. The results obtained for SuperPower̈ CC tapes with non-magnetic substrate are presented and the implications on the AC loss behaviour are discussed. [Copyright &y& Elsevier]

Covering the edges of a Bi-2223/Ag tape conductor by a layer of soft ferromagnetic material has positive impact on its two basic performance characteristics in low magnetic fields: the critical current is improved by ∼20%, and both the magnetization and transport AC loss are reduced. Reasonable theoretical prediction of the critical current in the tape is obtained by a numerical finite-element method: the whole superconducting portion is saturated by the critical current density assumed to be anisotropic and dependent on the magnetic field. However, for the prediction of AC loss the intermediate states with partial saturation of superconductor are also significant. We studied experimentally the development of magnetic field above a common multifilamentary Bi-2223/Ag tape with edges covered by nickel during stepwise change of the transport DC current. Indirect information about the distribution of electrical current and hence the process of magnetic flux penetration and dissipation at changing the value of current can be extracted from these experiments. [ABSTRACT FROM AUTHOR]

We present the numerical procedure suitable for computing the distribution of electrical currents in a superconductor with geometry extending in all three spatial dimensions when it is exposed to a magnetic field changing in time. Its main advantage is that it solves the problem in terms of vector potential, of the magnetic field. Such an A-formulation is usually the default option for magnetic field calculations in commercial finite element codes. We have used it in the past when a two-dimensional (2D) approximation was found to be sufficient to predict AC losses of single wires, cables, and coils with circular turns. Incorporation of superconductor properties based on the critical state model is straightforward in 2D because the flow of current is restricted to one direction only. The main challenge in extending the method to three dimensions is to cope with the situation when currents could flow in any direction. We have found that assuming that the local current density in a superconductor is always parallel to the local electrical field, and that its modulus is limited by the critical current density, provides sufficient input for accomplishing a new working equation, linking the vector of current density to the time derivative of the magnetic vector potential. Our method successfully passed testing on the benchmark problem of superconducting cube magnetization. It is also used to interpret the magnetization measured on two cylindrical superconducting objects in the transverse field: one was a tube made from bulk BSCCO and the other a REBCO coated conductor tape helically wound on a non-metallic tube. Computed magnetization loops in both cases show good agreement with experiments and confirm the validity of the method. [ABSTRACT FROM AUTHOR]