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.

The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density J_c . 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 J_c(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 J_c 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 J_c 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.

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.

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 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.