Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

Future use of coated conductors in electric power applications like transmission cables, transformers or fault current limiters is sensitive to the amount of dissipation in the AC regime. This paper analyses factors controlling AC loss of coated conductors in typical configurations: the self-field case when transport current generates the magnetic field, and the case of AC applied field where the orientation of magnetic field with respect to the superconducting layer plays a significant role.We illustrate that a high-quality CC tape with non-magnetic substrate follows rather well the models developed for a thin strip. However, to meet an excellent agreement between experiment and theoretical prediction a detailed knowledge of the superconductor properties is necessary and a numerical method must be involved.In the case of a superconducting layer deposited on a ferromagnetic substrate theoretical predictions give only basic directions and one must rely on numerical simulations entirely. We demonstrate that, with the help of a dedicated analysis of experimental data, very good AC loss prediction is also possible for superconductor-ferromagnetic composites. Novel designs of coated conductor architectures can be developed in this way.

Articles you may be interested inMicrostructure dependence of the c-axis critical current density in second-generation YBCO tapes Singular current density in the planar superconductor/normal metal/superconductor junctionIn several superconducting applications, as, for example, in some supercondcuting generators, motors, and power transmission cables, the superconductor experiences a changing magnetic field in a DC background. Simulating the losses caused by this AC ripple field is an important task from the application design point of view. In this work, we compare two formulations, the H-formulation and the minimum magnetic energy variation-formulation, based on the eddy current model (ECM) and the critical state model (CSM), respectively, for simulating ripple field losses in a DC biased coated conductor tape. Furthermore, we compare our simulation results with measurements. We investigate the frequency-dependence of the hysteresis loss predictions of the power law based ECM and verify by measurements, that in DC use, ECM clearly over-estimates the homogenization of the current density profile in the coated conductor tape: the relaxation of the local current density is not nearly as prominent in the measurement as it is in the simulation. Hence, we suggest that the power law resistivity, used as the local relation between the electric field intensity E and current density J in ECM, is not an intrinsic property of high-temperature superconductors. The difference between the models manifests itself as discrepancies in ripple field loss simulations in very low AC fields with significant DC fields or currents involved. The results also show, however, that for many practical situations, CSM and ECM are both eligible models for ripple field loss simulations. V C 2014 AIP Publishing LLC. [http://dx.

Non-uniformity of superconductor properties, e.g. a critical current reduction close to the edge of a coated conductor (CC) tape could degrade its performance in some power applications. Reliable characterization of such non-uniformity and understanding of its mechanism requires investigation of the character and causes of degradation. In this paper spatial distribution of critical current density across the width of a CC tape is studied. Three different experimental methods allowing estimation of the local current density were utilized for this purpose: (i) magnetic field mapping above the tape through which a DC current is flowing, (ii) measurement of the critical current of separate strips prepared by patterning of the CC tape, and (iii) magnetization measurements of the pieces cut from various positions within the tape width. Very good agreement between the results obtained by these methods was found, showing a reduction of the critical current density at the tape edges with respect to its centre. Moreover, structural investigation by scanning electron microscopy revealed a correlation between the morphology and the critical current density across the tape width. Insertion of such real non-uniform distribution of critical current density into AC loss calculation resulted in a dramatic improvement in the agreement with experimental results.

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.The combination of transformation optics with the development of metamaterials has resulted in new ways of controlling electromagnetic waves, allowing the design and fabrication of objects that seemed impossible to make, such as electromagnetic cloaks or perfect lenses [1][2][3]. Cloaks, in particular, have been theoretically devised [2,4,5] and experimentally realized in several regions of the electromagnetic spectrum [6][7][8][9][10][11][12]. However, some problems have prevented the actual realization of fully working cloaks. First, because most cloak designs are based on space transformations, they require fine-tuned values of permittivity ε and permeability µ, often anisotropic, highly inhomogeneous and even singular [13]. Besides, cloaks for electromagnetic 3

A novel substrate design is presented for scalable industrial production of filamentary coated conductors (CCs). The new substrate, called 'two level undercut-profile substrate (2LUPS)', has two levels of plateaus connected by walls with an undercut profile. The undercuts are made to produce a shading effect during subsequent deposition of layers, thereby creating gaps in the superconducting layer deposited on the curved walls between the two levels. It is demonstrated that such 2LUPS-based CCs can be produced in a large-scale production system using standard deposition processes, with no additional post-processing. Inspection of the conductor crosssection reveals that the deposited superconducting layer is physically separated at the 2LUPS undercuts. Filament decoupling is also seen in maps of the remanent magnetic field and confirmed by transport measurements.

Coated conductor (CC) tapes often exhibit critical current, Ic, which fluctuates along the length coordinate, x, leading to an Ic(x) dependence. The need to reduce the price of these tapes does not favor a decrease in the variation of Ic in the near future. Therefore, it is reasonable to develop a method for estimating the possible impact of such fluctuating transport ability on the performance of superconducting devices. Fortunately, providing Ic(x) data together with the delivered conductor is becoming a standard approach among CC tape producers, and in-depth analysis of such data is now possible. Extrapolation of the short sample testing methodology to the case of a device incorporating many meters of CC tape leads to the introduction of an ‘overall critical current’ as a value generating 1 μV cm−1 of electric field over the whole conductor. We show that in the case of Ic(x) fluctuations obeying the Gaussian distribution one can predict the value of the overall critical current from the mean value and the variance of the Ic(x) data set. Deviation from the Gaussian distribution found in real tapes would cause a further reduction of the overall critical current. Conversely, in the case of strong dropouts in the critical current value, a statistical approach is useless and one must analyze the probability of the weakest location developing in a hot spot with a dramatic increase of temperature. Extending the analysis for a single tape to cables and coils results in a rather simple summary: when current sharing is possible, the reduction of overall critical current with respect to its mean value is significantly depressed. This favors the parallel arrangement of tapes with low contact resistance allowing current migration from tape to tape. On the other hand, the insulation of tapes leads to the same overall critical current reduction as observed for a single tape.

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.