Fedor Gömöry

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.

Computation of Losses in HTS Under the Action of Varying Magnetic Fields and Currents

Numerical modeling of superconductors is widely recognized as a powerful tool for interpreting experimental results, understanding physical mechanisms, and predicting the performance of high-temperature-superconductor (HTS) tapes, wires, and devices. This is particularly true for ac loss calculation since a sufficiently low ac loss value is imperative to make these materials attractive for commercialization. In recent years, a large variety of numerical models, which are based on different techniques and implementations, has been proposed by researchers around the world, with the purpose of being able to estimate ac losses in HTSs quickly and accurately. This paper presents a literature review of the methods for computing ac losses in HTS tapes, wires, and devices. Technical superconductors have a relatively complex geometry (filaments, which might be twisted or transposed, or layers) and consist of different materials. As a result, different loss contributions exist. In this paper, we describe the ways of computing such loss contributions, which include hysteresis losses, eddy-current losses, coupling losses, and losses in ferromagnetic materials. We also provide an estimation of the losses occurring in a variety of power applications.

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.

Theoretical and experimental study of AC loss in high temperature superconductor single pancake coils

The electromagnetic properties of a pancake coil in an 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 tape, 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. In addition, 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.

Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

Low-magnetic-field dependence and anisotropy of the critical current density in coated conductors

Many applications of ReBCO-coated conductors operate at low magnetic fields in the superconductor (below 200 mT). In order to predict the critical current and AC loss in these applications, it is necessary to know the anisotropy and field dependence of the critical current density at low magnetic fields. In this paper, we obtain a formula for the critical current density in a coated conductor as a function of the local magnetic field and its orientation. Afterwards, we apply this formula to predict the critical current of a pancake coil that we constructed. We extract the critical current density of the tape from measurements of the in-field critical current at several orientations. Numerical simulations correct the effect of the self-field in the measurements and successfully predict the critical current in the pancake coil. We found that a simple elliptical model is not enough to describe the anisotropy of the critical current density. In conclusion, the analytical fit that we present is useful to predict the critical current of actual coils. Therefore, it may also be useful for other structures made of coated conductor, like power-transmission cables, Roebel cables and resistive fault current limiters.

Calibration free method for measurement of the AC magnetization loss

A calibration free measurement method for determination of the magnetization loss of superconducting samples exposed to the external AC magnetic field is presented. The idea is based on the measurement of the part of the power which is supplied by the AC source to the AC magnet generating the magnetic field, in which the sample is located. It uses a coil wound in parallel to the AC field magnet as the measurement coil. To achieve the necessary sensitivity, two identical systems are used, each consisting of an AC magnet and a measurement coil, one of them containing the sample and the other left empty. No measurement and/or calculation of the calibration constant is required. To confirm the suitability of this method, the loss of a Cu sample with known dissipation was measured. The applicability to the AC magnetization loss measurements of superconducting tapes is presented.

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.

Low AC loss cable produced from transposed striated CC tapes

In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is exposed to AC magnetic field. The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches. In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis. The overall geometry of the models was identical: each consisted of three tapes 4?mm wide that were placed with a pitch of 50?mm in a single layer on the 8?mm diameter round core. The cable length was designed to reach two complete tape pitches. In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing. The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding. Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments. In particular, at AC fields of 0.1?T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable. Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields.

AC Loss in Pancake Coil Made From 12 mm Wide REBCO Tape

The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density Jc . It is important to understand how much detail of this feature must be taken into consideration in the prediction of maximum achievable current and the expected ac loss. We present the results of investigation performed with a small (ten turns, 60 mm inner diameter) coil made from SuperPower tape of 12 mm width. The knowledge of Jc(B,θ) determined on short sample allowed prediction of the maximum achievable current of the coil and the ac loss behavior. We have also investigated the effect of the tape nonuniformity. Our results confirm that the lateral nonuniformity when Jc at tape edges is lower than in its center leads to significant increase of ac loss. A longitudinal nonuniformity, in particular a reduction of critical current in some portion along the tape length, is hardly observable in the ac loss result. On the other hand, using a piece of tape with lower Jc in the innermost coil turn would significantly reduce the maximum current. We also present calculations showing the change in current-voltage curve and redistribution of ac dissipation in the case of nonuniform tape quality.