Mykola Solovyov

Experimental Realization of a Magnetic Cloak

Hidden from Magnetic View An electromagnetic cloak is a device within which electromagnetic fields cannot penetrate, but, more importantly, the device itself does not disturb the electromagnetic fields surrounding it. An article placed in the cloak therefore vanishes from view, creating no shadow or reflection. Such devices have been demonstrated, but only for a particular band of frequencies. Confirming theoretical work that predicts such cloaking should be possible down to zero frequency, Gömöry et al. (p. 1466) designed a cloak for a dc magnetic field. With a composite design of ferromagnetic and superconducting material, together with a relatively simple structure, the device could potentially find immediate application. A ferromagnetic and superconductor composite structure can shield (cloak) a magnetic field without causing any distortion. 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.

AC losses in coated conductors

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.

A quasistatic magnetic cloak

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.

Non-uniformity of coated conductor tapes

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. (Some figures may appear in colour only in the online journal)

Ripple field losses in direct current biased superconductors: Simulations and comparison with measurements

In 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 i...

Magnetization loop modelling for superconducting/ferromagnetic tube of an ac magnetic cloak

From the combination of superconducting (SC) and ferromagnetic (FM) materials, one can prepare composites with unusual magnetic properties, e.g. for the cloaking of a dc or low-frequency ac magnetic field by a shell from a SC/FM composite. In the design and optimisation of such SC/FM structures, numerical modelling is essential. Non-linear magnetic permeability, as well as the hysteresis of both kinds of materials, are to be incorporated in the calculations aimed at achieving reliable estimates. We present a technique that allows the prediction of the ac magnetization loops of SC/FM composites. The critical state model-based approach is used to describe the properties of the superconducting material. The ferromagnetic part is characterized by its (non-hysteretic) nonlinear permeability. With these ingredients, the distributions of the magnetic field are calculated in subsequent instants of the ac cycle and are used to evaluate the preliminary data for the magnetization loop, which is still missing the hysteresis of the FM part. Afterward, the latter component is added to the magnetization loop by an approximation deduced from the known dependence of the hysteresis loss in the FM material on the ac magnetic field. In spite of its approximate nature, this approach demonstrated very good predictability in experimental tests.

Magnetic Cloak for Low Frequency AC Magnetic Field

Metal detection at low frequency (tens or hundreds of Hertz) is usually based on the phase and/or amplitude changes of the magnetic field measured in vicinity of the examined object by a system of coils. We have verified that it is possible to avoid such detection by putting the object into the shell from several layers of superconducting and ferromagnetic materials. When properly designed such structure can act as an invisibility cloak for magnetic field. In our laboratory, a small prototype of magnetic cloak with cylindrical shape has been manufactured. It was tested for hiding small copper and iron cylinders placed in AC magnetic field with the frequency of 72 Hz. The results show a significant reduction of the “visibility” of these cylinders determined by the phase and amplitude detection of voltage induced in a pick-up coil. The cloaking effect better than 80% was observed for the AC fields below 1.5 mT.

Design of Magnetic Cloak for Experiments in AC Regime

The magnetic shielding device causing only negligible distortion of external magnetic field is called the magnetic cloak. We have designed and tested cloaks with a coaxial bilayer structure where a superconducting cylindrical layer is placed inside the tube from a ferromagnetic material. Such object when inserted in the bore of a coil generating ac magnetic field should not change the coils impedance. This experiment is suitable for checking the cloak performance in the ac regime by measuring the voltage on the coil terminations, particularly the change in its amplitude and phase. The impact on signal amplitude comes from the nonzero magnetic moments of the cloak components. Then, the opposite character of ferromagnetic and diamagnetic material can be used to suppress the response, e.g., by modifying the thickness of the ferromagnetic layer. On the other hand, the shift in the phase comes from the hysteresis behavior of both constituents, and their impacts are additive. We achieved decent results using the ferrite powder/epoxy composite for the outer tube and REBCO superconducting tapes for the inner cylinder. Numerical simulations and experimental verification were used to investigate the electromagnetic response at various geometrical arrangements of the superconducting part of the cloak.

Round Conductor With Low AC Loss Made From High-Temperature Superconducting Tapes

The second generation of high-temperature superconductor tape exhibits rapidly progressing electric transport capacity. However, its use as a conductor in magnet technology is somewhat held up by the strip-like geometry. Possible solution is the helical layout of parallel tapes on a flexible round former (recently called the Coated Conductor on Round Core or CORC). We present the study of AC properties of single-layer CORC conductors made from tapes with non-magnetic substrate. When subjected to AC magnetic field, single-layer CORC conductors exhibit negligible coupling currents because the tapes are fully transposed. On top of this the hysteresis loss can be reduced by the tape striation. Numerical simulations of magnetization loss for CORC conductors are in good agreement with experimental data. Prediction of AC loss in a coil made from CORC is compared to that calculated for a classical double pancake one made from the same amount of tape. Result indicate that the CORC-made coil produces lower losses at AC current amplitudes approaching the critical current.

AC loss properties of single-layer CORC cables

In this work, an experimental and theoretical study of the transport and magnetization AC losses of a single layer ReBCO cable is presented. Examined cable consisted of five coated conductor tapes wound in parallel helically on the round core with 40 mm pitch and six full transposition resulting in the 24 cm total cable length. Transport currents up to 600 A amplitude with frequencies 36 Hz and 72 Hz were used in the transport loss measurement. AC fields up to 50 mT were applied during the magnetization loss measurement at 36 Hz and 72 Hz, respectively. Very good agreement between the experimental results and the simulations for magnetization loss was achieved. However, the experimental results for transport loss are markedly greater than the prediction reached from simulation. Nevertheless, they are lower than the transport AC loss theoretically assumed for a straight superconducting tape with the identical critical current.