Anisotropic Critical Currents Research Articles

Experimental study on the an-isotropic critical current of REBCO tape.

Superconducting magnets are widely used in nuclear fusion reactors, high-energy particle accelerators, steady-state high magnetic fields, etc. Higher magnetic fields and higher operating temperatures are two application trends. High temperature superconducting (HTS) materials are the only choice for high temperature and high field magnets in the future. The first- and second-generation HTS materials have a typical tape structure; their critical performance is magnetic field angle and temperature dependent. A new test facility is developed for an experimental study on the an-isotropic critical current. The field angle can be changed from 0° to 360° with a resolution of 1°. The rotation deviation angle is measured to be 0.2° when the upper part rotates 90°. The temperature can be changed from 4.2 to 80K. The temperature errors are ±50, ±80, and ±135 mK for 4.2-20, 20-40, and 40-80K, respectively. The angle dependence of critical current (Ic) of the tested rare-earth barium copper oxide tape within 0°-30° is strong. From 30° to 90°, the sample Ic almost does not change with the magnetic field angle. The implementation of the project will not only promote the structural optimization of HTS tapes but also promote the miniaturization and economical application of HTS magnets.

Superconductor-ferromagnet bilayer under external drive: The role of vortex-antivortex matter

Using advanced Ginzburg-Landau simulations, we study the superconducting state of a thin superconducting film under a ferromagnetic layer, separated by an insulating oxide, in applied external magnetic field and electric current. The taken uniaxial ferromagnet is organized into a series of parallel domains with alternating polarization of out-of-plane magnetization, sufficiently strong to induce vortex-antivortex pairs in the underlying superconductor in absence of other magnetic field. We show the organization of such vortex-antivortex matter into rich configurations, some of which are not matching the periodicity of the ferromagnetic film. The variety of possible configurations is enhanced by applied homogeneous magnetic field, where additional vortices in the superconductor may lower the energy of the system by either annihilating the present antivortices under negative ferromagnetic domains or by lowering their own energy after positioning under positive ferromagnetic domains. As a consequence, both the vortex-antivortex reordering in increasing external field and the evolution of the energy of the system are highly nontrivial. Finally, we reveal the very interesting effects of applied dc electric current on the vortex-antivortex configurations, since resulting Lorentzian force has opposite direction for vortices and antivortices, while direction of the applied current with respect to ferromagnetic domains is of crucial importance for the interaction of the applied and the Meissner current, as well as the consequent vortex-antivortex dynamics—both of which are reflected in the anisotropic critical current of the system.

Electromagnetics close beyond the critical state: thermodynamic prospect

A theory for the electromagnetic response of type-II superconductors close beyond the critical state is presented. Our formulation relies on general physical principles applied to the superconductor as a thermodynamic system. Metastable equilibrium critical states, externally driven steady solutions, and transient relaxation are all described in terms of free energy and entropy production. This approach allows a consistent macroscopic statement that incorporates the intricate vortex dynamic effects, revealed in non-idealized experimental configurations. Magnetically anisotropic critical currents and flux stirring resistivities are straightforwardly included in three-dimensional scenarios. Starting from a variational form of our postulate, a numerical implementation for practical configurations is shown. In particular, several results are provided for infinite strip geometry: voltage generation in multicomponent experiments, and magnetic relaxation towards the critical state under applied field and transport current. Explicitly, we show that for a given set of external conditions, the well established critical states may be completely obtained as diffusive final profiles.

Finding the Elusive Sliding Phase in the Superfluid-Normal Phase Transition Smeared byc-Axis Disorder

We consider a stack of weakly Josephson coupled superfluid layers with c-axis disorder in the form of random superfluid stiffnesses and vortex fugacities in each layer as well as random interlayer coupling strengths. In the absence of disorder this system has a 3D XY type superfluid-normal phase transition as a function of temperature. We develop a functional renormalization group to treat the effects of disorder, and demonstrate that the disorder results in the smearing of the superfluid-normal phase transition via the formation of a Griffiths phase. Remarkably, in the Griffiths phase, the emergent power-law distribution of the interlayer couplings gives rise to a sliding Griffiths superfluid, with a finite stiffness in the a-b direction along the layers, and a vanishing stiffness perpendicular to it.

Design, construction and performance of an EMS-based HTS maglev vehicle

A laboratory-scale EMS-based HTS maglev vehicle operating over a 1.5 m guideway has been successfully constructed. The fully integrated system consists of a vehicle chassis, four dependent magnetic circuits, four distance sensors, and control and power amplification circuits. As key component of the system, each magnetic circuit includes a U-shape iron core with one HTS coil forming each pole. Eight HTS coils made of Bi-2223 multi-filamentary tape were used to provide the magnetic motive force. Several questions relating to the unique characteristics of the HTS material in a controlled magnetic circuit are discussed. The most important consideration for such applications is that the anisotropic critical current of the Bi-2223/Ag tape depends strongly on the magnetic field. The commercially available FEA software ANSYS was used to simulate the field distribution along the magnetic circuit and HTS coil winding, and thereby identify how the magnetic circuit alters the field distribution in the coil winding and therefore also the critical current. A general optimization process is described for finding the best position in the U-shape iron core to hold the HTS coils. In this process the critical current of the HTS tape and the force–current characteristic of the magnetic circuit are considered synthetically. The results demonstrate the feasibility and stability of HTS material in a typical maglev system and other similar controllability applications.

CRITICAL CURRENT IN THE HIGH-Tc GLASS MODEL

The high-Tc glass model can be combined with the repulsive tt'–Hubbard model as microscopic description of the striped domains found in the high-Tc materials. In this picture, the finite Hubbard clusters are the origin of the d-wave pairing. In this paper we show that the glass model can also explain the critical currents usually observed in the high-Tc materials. We use two different approaches to calculate the critical current densities of the high-Tc glass model. Both lead to a strongly anisotropic critical current. Finally we give an explanation of why we nonetheless expect a nearly perfect isotropic critical current in high-Tc superconductors.

Flux penetration into flat rectangular superconductors with anisotropic critical current

Superconductors with anisotropic critical-current density ${j}_{c}$ exhibit characteristic anisotropic flux-density patterns during penetration of magnetic flux. We investigate this anisotropic flux penetration in detail by observations using the magneto-optical Faraday effect and by first-principles calculations which describe the superconductor as a nonlinear anisotropic conductor. Our samples are thin plates of DyBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ into which anisotropic pinning is introduced by oblique irradiation with 340-MeV Xe ions creating linear defects. Excellent agreement between experiment and theory is obtained. In particular, we find that in rectangular plates with ${j}_{c}$ anisotropy equal to the side ratio, the intrinsic and shape anisotropies may compensate such that the flux pattern looks like that in an isotropic square stretched to the rectangular shape. This means the current streamlines are concentric rectangles which shrink to a point rather than to a line, and the discontinuity lines where the current bends sharply, coincide with the diagonals of the rectangle rather than forming the usual double-Y structure.

Pinning mechanisms in directionally solidified YBa 2Cu 3O 7: Influence of Y 2BaCuO 5 concentration

In this work we report a detailed study of the inductive critical currents (up to 10 5 A/cm 2 at 77 K) of high quality melt textured YBa 2Cu 3O 7 as a function of the Y 2BaCuO 5 (211 phase) content. The study of the temperature and field dependence of the anisotropic critical currents J c, together with a very accurate examination of the microstructure, by HRTEM, indicates that interface pinning is the dominant pinning mechanism in the low field regime with a crossover to a different mechanism in the high field region, where an almost field-independent J c is observed below about 70K.

Calculation of the magnetization of anisotropic superconductors with cylindrical geometry in transverse fields

Abstract Expressions for the magnetization of a monolithic superconductor with anisotropic critical currents and a cylindrical geometery have been found. A transverse field geometry, along with field independent critical currents are assumed. Additionally, we restrict ourselves to the case of full field penetration, and thus demagnetization effects are ignored. Similarly to the calculation for the slab geometry, two regimes are found; (1) sample length is greater than a certain “transfer length”, and (2) sample length is shorter than the transfer length. Analytical expressions for the magnetization in both regimes, as well as linear approximations to these expressions, are found. These results are directly compared to the equations used to describe the finite slab geometry.

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

We have extended Bean’s critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu-O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu-O planes is about 30 times smaller than flow parallel to these planes.

Magnetization jumps and critical currents in HoBa 2Cu 3O 7 single crystals, up to 18 T

The magnetization of a large single crystal of HoBa 2Cu 3O 7 exhibits several new features associated to the anisotropy (and sample size) : at low temperature large magnetization jumps are observed which depend on the field variation rate and thermalization of the sample. At high temperature the hysteresis loops, M(H), are convex lens shaped which gives a maximum of the current density J c(H). The anisotropic critical currents can be compared with values obtained in small crystals. The anisotropic critical currents can be compared with with value obtained in small superconducting state. A large crystal field anomaly is observed around 80 K for H applied along the c direction.

Anisotropic critical currents in heterogeneous lead–sodium alloys

Measurements of the transverse critical current densities ic are reported in type II superconducting Pb-8.4 at% Na alloys containing precipitates of a Pb3Na phase. Small precipitates are randomly distributed, larger ones are aligned in the 〈100〉 directions. The ic versus H curves depend on the crystallographic orientation of the external magnetic field H and show a peak-effect which can be explained by a geometrical matching of the regular precipitate and flux line arrangements. It appears necessary to take into account proximity effects between the precipitated phase and the matrix. Die kritische Stromdichte ic von Typ II-supraleitenden Pb-8,4 At%Na-Einkristallen wurde im transversalen Magnetfeld H gemessen. Die Legierungen enthielten Ausscheidungen einer Pb3Na Phase verschiedener Größe und Anordnungen. Insbesondere waren kleine Teilchen statistisch verteilt, während große Teilchen in 〈100〉 Richtungen aneinandergelagert waren. Die ic(H)-Kurven hängen von den kristallographischen Orientierungen im Magnetfeld ab und zeigen einen Peak-Effekt, der durch die Passung zwischen Fluß-faden- und Teilchenanordnung erklärt werden kann. Dabei muß der Proximity-Effekt zwischen Ausscheidung und Matrix berücksichtigt werden.