H. S. Ruiz

Crossed-magnetic-field experiments on stacked second generation superconducting tapes: Reduction of the demagnetization effects

The crossed-magnetic-field effect on the demagnetization factor of stacked second generation (2G) high temperature superconducting tapes is presented. The superconducting sample was initially magnetized along the c-axis by the field cooling magnetization method and after achieving the magnetic relaxation of the sample, an extensive set of experimental measurements for different amplitudes of an applied ac magnetic field parallel to the ab-plane was performed. On the one hand, a striking reduction of the demagnetization factor compared with the reported values for superconducting bulks is reported. On the other hand, the demagnetization factor increases linearly with the amplitude of the ac transverse magnetic field confirming the universal linear behavior for the magnetic susceptibility predicted by Brandt [Phys. Rev. B 54, 4246 (1996)]. The study has been also pursued at different frequencies of the ac transverse magnetic field in order to determine the influence of this parameter on the demagnetization factor measurements. We report an even lower demagnetization factor as long as the frequency of the transverse magnetic field increases. Thus, the significant reduction on the demagnetization factor that we have found by using stacked 2G-superconducting tapes, with higher mechanical strength compared with the one of superconducting bulks, makes to this configuration a highly attractive candidate for the future development of more efficient high-power density rotating machines and strong magnet applications.

General critical states in type-II superconductors

The magnetic flux dynamics of type-II superconductors within the critical state regime is posed in a generalized framework, by using a variational theory supported by well established physical principles. The equivalence between the variational statement and more conventional treatments, based on the solution of the differential Maxwell equations together with appropriate conductivity laws is shown. Advantages of the variational method are emphasized, focusing on its numerical performance, that allows to explore new physical scenarios. In particular, we present the extension of the so-called double critical state model to three dimensional configurations in which only flux transport (T-states), cutting (C-states) or both mechanisms (CT-states) occur. The theory is applied to several problems. First, we show the features of the transition from T to CT states. Second, we give a generalized expression for the flux cutting threshold in 3-D and show its relevance in the slab geometry. In addition, several models that allow to treat flux depinning and cutting mechanisms are compared. Finally, the longitudinal transport problem (current is applied parallel to the external magnetic field) is analyzed both under T and CT conditions. The complex interaction between shielding and transport is solved.

Resistive-Type Superconducting Fault Current Limiters: Concepts, Materials, and Numerical Modeling

In recent years, major industrialized countries have begun to be concerned about the need for developing strategies on the integration and protection of the growing power capacity of renewable source energies, attracting back their interest on the development and understanding of superconducting fault current limiters (SFCLs). The reasons for this are simple: An SFCL may offer a rapid, reliable, and effective current limitation, with zero impedance during normal operation, and an automatic recovery after the fault. Nowadays, most of the R&D projects have turned toward the study of resistive-type SFCLs due to their potential to be small and the likely decrease in price of 2G coated conductors. Thus, in this paper, we provide an updated review on the state of the art of resistive-type SFCLs, emphasizing on the different approaches for the numerical modeling of their local physical properties, as well as on the already-tested experimental concepts. Comparison between the properties and characteristics of different resistive-type SFCLs using different superconducting materials is presented.

Investigation of Demagnetization in HTS Stacked Tapes Implemented in Electric Machines as a Result of Crossed Magnetic Field

This paper investigates the practical effectiveness of employing superconducting stacked tapes to superconducting electric machinery. The use of superconducting bulks in various practical applications has been addressed extensively in the literature. However, in practice, dramatic decrease in magnetization would occur on superconducting bulks due to the crossed field effect. In our study, we employed the superconducting stacked tapes in a synchronous superconducting motor, which was designed and fabricated in our laboratory, aiming to lessen demagnetization due to crossed field effect in comparison with superconducting bulks. Applying the transverse AC field, the effects of frequency, amplitude, and number of cycles of the transverse magnetic field are discussed. Furthermore, a stack of 16 layers of superconducting tapes is modelled and the consequences of applying the crossed magnetic field on the sample are evaluated. The confrontation between experiments and simulation allows us to thoroughly understand the crossed field effects on stacked tapes. At the end, a preventive treatment, based on the shielding characteristic of superconductor and materials with high permeability, i.e.

Superconducting wire subject to synchronous oscillating excitations: Power dissipation, magnetic response, and low-pass filtering

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Smooth double critical state theory for type-II superconductors

-metal and metalic glass, is suggested. On the other hand, the shielding feature of aforementioned materials will hinder the penetration of magnetic field and, consequently, reduction of the demagnetization will be attained.

Nature of the nodal kink in angle-resolved photoemission spectra of cuprate superconductors

Numerical simulations of a type-II superconducting wire subject to an ac transport current and oscillating transverse magnetic field are performed using the theory of the critical state. Time-dependent distributions of the current and the density of magnetic flux, the local power dissipation, and cycles of the magnetic moment are displayed. Noticeable inhomogeneous dissipation and field distortions are exposed. Results for hysteretic ac losses are reported too, and significant differences to predictions of available approximate formulae identified. Finally, a distinct low-pass filtering effect intrinsic to the wire’s magnetic response is revealed.

Material laws and related uncommon phenomena in the electromagnetic response of type-II superconductors in longitudinal geometry

Several aspects of the general theory for the critical states of a vortex lattice and the magnetic flux dynamics in type-II superconductors are examined by a direct variational optimization method and widespread physical principles. Our method allows us to unify a number of conventional models describing the complex vortex configurations in the critical state regime. Special attention is given to the discussion of the relation between the flux line cutting mechanism and the depinning threshold limitation. This is done by using a smooth double critical state concept which incorporates the so-called isotropic, elliptical, T and CT models as well-defined limits of our general treatment. Starting from different initial configurations for a superconducting slab in a 3D magnetic field, we show that the predictions of the theory range from the collapse to zero of transverse magnetic moments in the isotropic model, to nearly force-free configurations in which paramagnetic values can arbitrarily increase with the applied field for magnetically anisotropic current–voltage laws. Noteworthily, the differences between the several model predictions are minimal for the low applied field regime.

Experimental Study of the Normal Zone Propagation Velocity in Double-Layer 2G-HTS Wires by Thermal and Electrical Methods

The experimental finding of an ubiquitous kink in the nodal direction of angle-resolved photoemission spectroscopies of superconducting cuprates has been reproduced theoretically. Our model is built on the Migdal-Eliashberg theory for the electron self-energy within the phonon-coupling scenario. Following this perturbative approach, a numerical evaluation of the bare band dispersion energy in terms of the electron-phonon-coupling parameter

Inversion mechanism for the transport current in type-II superconductors

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