Distribution Of Critical Current Density Research Articles

Current distribution mapping in insulated (Gd,Y)BCO based stabilizer-free coated conductor after AC over-current test for R-SFCL application

Uniformity of critical current (Ic) over long lengths of (GdY)-Ba-Cu-O ((Gd,Y)BCO)-based high temperature superconducting (HTS) tapes after long periods of AC current excitation is an important criterion in their selection for resistive type superconducting fault current limiter (R-SFCL). The present work describes such critical current (Ic) uniformity measurements performed over 1m long, stabilizer-free (SF), 12 mm wide, 2nd generation (2G) (Gd,Y)BCO based HTS tape. A non-destructive method using a static hall probe (Tapestar®) with moving HTS tape configuration was employed for estimation of Ic uniformity. Scanning Hall probe microscopy (SHPM) was then used to examine the weak superconducting regions (i.e. less Ic) with a static HTS tape. Remanent field distribution on the HTS tape was measured to yield the critical current density distribution. Except for small degradation of Ic at some locations, these studies confirmed near-uniform critical current distribution over meter-long (Gd,Y)BCO tapes, both in virgin state and after exposure to AC over current.

Electromagnetic behaviour of bulk MgB2 determined by numerical modelling using regional supercurrent properties

Previous studies in the literature have shown experimentally that the critical current density in bulk MgB2 depends on the position at which it is measured within the sample, and that the trapped magnetic field saturates above large values of sample diameter. To better understand this behaviour, we have carried out a detailed study of the trapped magnetic field and local critical current distribution in bulk MgB2 using numerical modelling with H-formulation. The properties of bulk MgB2 samples with radius-independent (i.e. uniform) and radius-dependent critical current were modelled, based on experimentally-determined parameters obtained from the literature. Radius-independent samples took properties originally derived from either the central or edge region of the original samples. In our study it was determined that the peak trapped magnetic field value of the sample with the central region properties was higher by 22.4% and 9.20% than the samples with edge region properties and that with radius dependent parameter, respectively. Additionally, to understand why the trapped magnetic field saturates as the diameter of sample increases, the trapped magnetic field was modelled using the radius dependent critical current density Jc (B,r) as input data rather than using a constant bulk critical current density. The peak trapped magnetic field values increase by 8.6% and 6.2% respectively for the central region radius-independent and radius-dependent property bulk MgB2 samples as the bulk diameter increases from 25 mm to 35 mm. This tendency to saturation in the peak trapped field indicates that increasing the bulk diameter alone does not have a significant effect on the value of the trapped magnetic field, unless the bulk superconducting current density is improved uniformly throughout the MgB2 bulk. These results enable us to understand how we can experimentally enhance the trapped magnetic field in bulk MgB2 by producing samples with uniform high critical current density distribution using fabrication methods such as graded doping.

Analysis of Critical Current Density in Bi2Sr2CaCu2O8+x Round Wire with Filament Fracture

Bi2Sr2CaCu2O8+x (Bi-2212) round wires have outstanding electromagnetic behaviors in a high magnetic field. However, the filaments’ fracture or gas bubble is difficult to avoid during the fabrication and application of Bi-2212 round wire, which would affect the critical current density seriously. In this paper, the engineering current density is studied based on Kim model for 7-bundle and 18-bundle wires with filament fracture by considering the effect of bridges between filaments. The distributions of critical current density of filaments are also investigated for different external fields. Finally, we present the relation between the number of cracked filaments and the applied strain.

Critical current density and current distribution in field cooled superconducting disks

Applications of bulk superconductors concern superconducting motors and generators, the levitation of vehicles, the generation of high magnetic fields with small size cryo-magnets, the shielding of magnetic fields and other applications. For all of them, it is essential to determine the critical current density, and to understand the effect of the shape and size of the bulks on the properties of interest. In this contribution, we show how the combination of levitation force and trapped field measurements allow one to determine the characteristics and the potential performances of superconducting disks using analytical modeling. As examples of applications we detail the effects of the magnetizing field and of the bulk sheet critical current density on the levitation force. An important result of the reported measurements is that in field-cooled samples, the shielding currents possibly do not flow along the whole thickness of the disks.

Lateral Inhomogeneity of Critical Current of HTS Conductor Using Hall Sensor Array

A reel-to-reel Hall sensor array system was build and an inverse operation algorithm was applied to obtain the lateral distribution of critical current density by the remanent field around the superconducting tape. Simulation analysis was conducted to verify the validity of the algorithm. Commercial YBCO and Bi2223 tape and YBCO tape with artificial defects were measured. The standard deviation of lateral critical current density on the cross section of conductor was used as a parameter describing the lateral inhomogeneity of critical current density. The result shows that this measurement method is a stable and efficient way to evaluate the lateral inhomogeneity of critical current of high temperature tapes.

Influence of Internal Magnetic Field Distribution on Critical Currents in a Single and Assembled Bi-2223 Tapes

We have investigated the influence of self-field on the critical current density distribution in a Bi-2223 tape and its effect on the current capacity of an assembled conductor. Critical current of a tape is usually measured by four-probe transport method under self-field. On the other hand, the critical current of an assembled conductor cannot be simply summed because the tape experiences different condition of magnetic field due to the interaction among the tapes. To quantify the situation, it is necessary to clarify first how the critical current of a tape is determined at the self-field and then to consider the interaction among the tapes. In this study, it was found that the distribution of critical current density was largely influenced by local magnetic field inside the tape. The results were quantitatively described by a model considering the position dependence and magnetic field dependence of local critical current density. Using such a quantitative model, we numerically investigated the current capacities of assembled conductors with different arrangements of the tapes.

Scanning Hall Probe Microscopy on Laser Striated Multifilament Coated Conductor

A 12-filament laser striated coated conductor is studied using Scanning Hall Probe Microscopy (SHPM). Remanent field is measured and critical current density distribution is calculated according to the measured field map. The results show that the SHPM technique is very sensitive to defects. A point-like defect can be readily discerned in the magnetic field map, even when the drop in critical current density is small. We also found that the choice of a proper filter is important for obtaining accurate critical current density values, especially for the multifilament superconducting tapes.

Spatial and Temporal Flux-Trapping Properties of Bulk High Temperature Superconductors Under Static Magnetization Fields

Magnetized YBaCuO bulks are available to enhance the levitation performance of present superconducting maglev systems due to their high trapped fields. To realize a good YBaCuO bulk magnet, we studied the flux-trapping properties of an YBaCuO bulk sample under different magnetization fields from two aspects. Firstly in spatial view, five Hall sensors were employed to measure the trapped magnetic fields at five typical positions of growth sector regions and growth sector boundaries on the seeded surface of the bulk. And then in temporal view, the trapped magnetic fields of these five positions were continually recorded by a real-time data acquisition device during the magnetization process and the succeeding relaxation processes. Results show that the saturated trapped fields at five surface positions of the YBaCuO bulk magnet exhibit the strong spatial inhomogeneity. The trapped fields at the growth sector boundaries are always higher than those at the growth sector regions, due to different critical current density distributions of the bulk superconductor. And the magnetization field required to saturate the center of the bulk is much higher than that of the edge region. But the much high magnetization field will reduce the final trapped field at the edge of the bulk. Moreover the trapped field variation with time is sensitive to the participation of some external ferromagnetism materials. It is found that the adding of an iron plate above the YBaCuO bulk surface is efficacious to restrain the trapped field relaxation.

Current Density Distribution in 2G HTS Tape in an External Magnetic Field

This paper describes the method of study of the critical current density distribution across a tape in a background magnetic field. We measured the current distribution by the scanning Hall probe method. Then we measured field across a tape by a set of 10 Hall probes placed on a single substrate. By comparison of data from these two experiments we determined positions of Hall probes at a tape. Then we measured the current distribution of current density across a tape inside a magnet in parallel and perpendicular magnetic field of 30 mT. The details of measuring method and results are presented.

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This article reviews a method for characterizing the local critical current density (Jc) distribution in superconducting tapes and wires based on scanning Hall-probe microscopy (SHPM). This method is very powerful for (1) finding the bottleneck that limits the global performance of a superconductor, (2) investigating the local inhomogeneity that may become the origin of local quench, especially in HTS applications, (3) establishing the processes for narrow and/or multifilamentary HTS conductors for reducing the magnetization in magnet applications and the AC losses in power applications. The principle and the functions of this technique are introduced by referring to characterization examples of REBCO coated conductors and an MgB2 wire.

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.

Lateral Distribution of Critical Current Density in Coated Conductors Slit by Different Cutting Methods

We have investigated the lateral distributions of critical current densities in narrow coated conductors slit by different cutting methods. The objective was to find an optimal slitting process to make a narrow conductor that should be effective for ac loss reduction of assembled conductors such as power transmission cables. In-plane 2-D distribution of critical current density was characterized by means of scanning Hall-probe microscopy. From an analysis along the width direction, we could estimate the degradation in critical current density by a slitting process at both tape edges. We determined an equivalent width for the slit tapes in order to discuss such degradation with a high spatial resolution. Furthermore, the 2-D scanning allows us to evaluate the effective tape width as a function of longitudinal coordinate. In other words, we could evaluate the equivalent width of a slit conductor together with its statistics along the longitudinal direction. As a result, it was found that coated conductors slit by laser cutting processes were clearly better than those by mechanical ones from the viewpoint of less degradation at their edges. In addition, an optimal condition among laser slitting processes was also presented.

Critical Current Properties in $\hbox{REBa}_{2} \hbox{Cu}_{3}\hbox{O}_{y}$ Films With Nanorods Depending on Growth Conditions

Growth temperature (Ts) dependent microstructures of nanorods were clarified for BaNb2O6 (BNO)-doped YBa2Cu3Oy (Y123) films. The morphology of BNO in the Y123 matrix was changed from fluctuated nanorods to uniform nanorods with the increase in Ts. Reflecting this change in the nanorod morphology, spatial distribution of local critical current density (Jcl) determined from the percolation transition model became sharp with the increase in Ts. The sharp distribution of Jcl led to a steep slope of electric field vs. current density curve and a sharp Jc peak under B//c in the magnetic field angular dependence. The evaluation of the distribution of Jcl is a strong tool to understand quantitatively the relationship between the microstructure of nanorods and critical current properties for RE123 films with nanorods. To optimize the critical current properties under the magnetic field for RE123 coated conductors with nanorods, precise control of Ts is very important in the production process.

Relationship between vortex pinning properties and microstructure in Ba–Nb–O-doped YBa2Cu3Oy and ErBa2Cu3Oy films

In-field Jcs were improved by introducing Ba–Nb–O (BNO)-nanorods in YBa2Cu3Oy (Y123) and ErBa2Cu3Oy (Er123) films. Retention of Jc against the magnetic field for the BNO-doped Er123 film was superior to that for the BNO-doped Y123 film. Sharp distribution of local critical current density originating from vortex pinning by nanorods with uniform morphology was demonstrated in the Er123 film. On the other hand, fluctuating microstructures of nanorods formed in the Y123 film prepared by the same deposition conditions. Moreover, different growth temperature dependences of nanorod morphology between the Y123 and Er123 films were clarified.

Effect of infiltration temperature on the properties of infiltration growth processed YBCO superconductor

The importance of optimizing the fabrication of the Y2BaCuO5 (Y-211) preform in achieving high current densities to high magnetic fields has recently been established. We report the effect of the choice of infiltration temperature 1040°C (sample A) and 1100°C (sample B) on the microstructure, magnetic properties and mechanical strength. Both the samples showed [103] texture after slow cooling through peritectic temperature. Infiltration at higher temperature is found to yield highly dense composites with minimal macrodefects and higher hardness of 18.73GPa in sample B. Both the samples show uniform distribution of Y-211and comparable zero-field critical current density. High current densities are retained to a higher field of 7T in sample B, unlike 2T in sample A. The occurrence of [103] texture promoting higher hardness, simultaneous with retention of considerably high current density to high fields in sample B has definite advantages for trapped field applications.

Effects of vortex line shape on critical current density in high Tc superconducting film with nano-rod pinning centers under applied magnetic field of various orientations

We measured magneto-optical images (MOIs) on a coated conductor comprised of (Gd,Y)1Ba2Cu3O7−δ-BaZrO3 film under applied magnetic field of various orientations. MOIs showed systematic changes of asymmetric distribution of magnetic flux density and critical current density. TEM measurements showed that nano-rod pinning centers (RPCs) tilt by ∼13° from the c-axis. We were able to explain the results of MOI measurements using a simple model calculation about vortex line shapes, which change according to the magnitude and the orientation of applied magnetic field. In this model the critical current density, which is due to the vortex pinning, is determined by the geometrical relationship of the curved vortex lines and the tilted RPCs.

Influence of Critical Current Density Distribution on Transport AC Losses for Round Superconducting Wire

The AC loss per cycle per unit length of a round superconducting wire has been theoretically analyzed based on the critical state model and numerically calculated from the power-law model. By using Bean’s critical state model, it is found that the distribution of critical current density J c which increases from the center to the edge along a radial direction lowers the loss value. And the sharper the J c varies, the more significant the loss reduction. After considering the coupling of thermoelectric interaction, the results show that the temperature in superconducting wires gradually increases and finally becomes stable. The temperature of stable state has a dependence on the J c distribution and the rise of temperature has an important effect on the AC loss behavior.

Measurement of in-plane magnetic relaxation in RE-123 coated conductors by use of scanning Hall probe microscopy

We have investigated electric field criterion of in-plane critical current density in a coated conductor characterized by scanning Hall-probe microscopy (SHPM). From remanent field distribution and its relaxation measurements, we could obtain critical current distribution and induced electric field simultaneously by considering the Biot-Savart law and the Faraday’s law, respectively. These results lead us to evaluate a distribution of local critical current density and the corresponding criterion of electric field. As a result, it was found that the electric field criterion for the SHPM analysis was several orders lower than that used in the conventional 4-probe resistive method. However, the data point obtained by the SHPM shows good agreement with E–J curve analytically extended from the measurements by the 4-probe method. This means that we could characterize in-plane distribution of critical current density in a coated conductor at an electric field criterion quantitatively by this method in a nondestructive manner. These findings will be very important information since the uniformity of local critical current density in a coated conductor at extremely low electric fields is a key issue (1) especially for DC applications, (2) for quality control of coated conductors, and (3) for the standardization of the characterization of critical current among different methods.

Ac loss analyses of superconducting power transmission cables considering their three-dimensional geometries

Numerical electromagnetic field analyses of superconducting power transmission cables consisting of coated conductors were made considering their three-dimensional spiraled structures. Thin strips of superconductor were wound spirally around a round former to represent a layer in a cable. The analyses were made for two-layer cables where the spiral pitch of the inner layer and that of the outer layer are different. A non-uniform lateral critical current density distribution, a trapezoidal distribution with shoulders, was assumed across each coated conductor. The calculated ac losses were compared with measured ones. The influence of the relative positions between the inner-layer coated conductors and the outer-layer coated conductors on the current distribution across each coated conductor as well as ac losses was discussed. The influence of the current distribution between layers, determined by the spiral pitches, on the ac losses was also studied.

Effects of the vortex line shape on the critical current density in high Tc superconducting film with nanorod pinning centers

We studied the critical current density distribution in a coated conductor comprised of (Gd,Y)1Ba2Cu3O7−δ–BaZrO3 film. Transmission electron microscopy measurements showed that nanorod pinning centers tilt by ∼13° from the c-axis. Magneto-optical image (MOI) measurements showed interesting asymmetric distributions of magnetic flux density. From MOIs we calculated the asymmetric distributions of the critical current density, which is associated with the properties of vortex pinning. We were able to explain these results through the geometrical relationship of the tilted rod pinning centers and the curved vortex lines.