Critical Current Distribution Research Articles

Analysis of distribution of critical current of bent-damaged Bi2223 composite tape

Distributions of critical current of damaged Bi2223 tape specimens bent by 0.6, 0.8 and 1.0% were investigated analytically with a modelling approach based on the correlation of damage evolution to distribution of critical current. It was revealed that the distribution of critical current is described by three parameter Weibull distribution function through the distribution of the tensile damage strain of Bi2223 filaments that determines the damage front in bent-composite tape. Also it was shown that the measured distribution of critical current values can be reproduced successfully by a Monte Carlo simulation using the distributions of tensile damage strain of filaments and original critical current.

Changes in the Thermal Stability of 2G HTS Wires by Local Modification of the Stabilization Layer

In order to use 2G HTS wires in superconducting devices, it is important to understand the origin of thermal instabilities in these materials. Previous results showed that inhomogeneities in the critical current distribution are not the only factor that determines the heat generation distribution along the sample when the current exceeds the critical value. In this work, different types of controlled defects have been generated in the Cu-layer using laser ablation. In all cases, laser parameters have been chosen while assuring that the critical current of the superconductor does not deteriorate. It has been obtained that even if the thickness of the stabilizing layer is reduced from 20 μm to less than 10 μm in some regions, the sample response to overcurrents is not modified.

Cabling Method for High Current Conductors Made of HTS Tapes

A small-scale test of a twisted stacked-tape conductor made of coated YBCO tapes was performed using four-tape cable. The critical current degradation and current distribution of this four-tape conductor was evaluated by taking account the twist-strain, the self-field and the termination resistance. The critical current degradation for the tested YBCO tape may be explained by the perpendicular self-field effect solo. The critical currents of the twisted stacked-tape conductor with four-tape cable have been confirmed not to degrade up to 120 mm twist pitch length. This type of conductor design is proposed to make it possible to fabricate highly compact, high current cables from multiple flat HTS tapes.

Computation of Current Distribution in YBCO Tapes With Defects Obtained From Hall Magnetic Mapping by Inverse Problem Solution

The development of superconducting devices based on long-length HTS tapes often requires of these tapes high homogeneity along its length as well as across its width. This implies the absence of significant local defects. Non-destructive characterization techniques to examine critical current distribution for defect detection are of great interest, specially if they could be applied in situ for real-time testing of large lengths of tape. In this work, we continue the adaptation of our method for the computation of critical current maps from Hall measurements of the magnetic field over the tape. We compute the current density distribution in a stretch of a commercial YBCO tape which contains defects by using a specifically designed fast inverse problem solver. The 2-dimensional current map meshes with the current distributions in a cross-section of the tape that we previously computed in real time, so that a map of the critical current circulating on the entire surface of a tape with isolated defects may be obtained, regardless of its length, by running a Hall probe over it. This method is applied to a series of Hall mappings corresponding to several magnetization regimes, produced by applying different current intensities to the tape. Details of the experiments and the calculation method are reported and the applicability to detect the impact of the defects in the tape over the current distribution is discussed.

The Effect of Strand Bending on the Voltage-Current Characteristic of $\hbox{Nb}_{3}\hbox{Sn}$ Cable-In-Conduit Conductors

The primary issue facing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> cable-in-conduit conductors (CICCs) has been degradation, often observed during electromagnetic cycling, in the current sharing temperature, n-index, and critical current, in respect to measured strands values, due to strain effects. In the last years, work performed mostly relating to fusion magnet technology has led to a better understanding of the parameters required to improve the constraints imposed by the brittle nature of the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> filaments, such as cables low void fraction and long twist pitch sequence. On the other hand, experimental campaigns on bent <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> wires, pre-compressed into a stainless-steel jacket, have shown that an appreciable decrease in the n-index values already occurs at the strand level, well below the irreversible mechanical load regime for filament breakage. This result is explained, with the support of simulation results, taking into account the broadening of the critical current distribution on wires cross section due to the presence of the jacket and to bending strain.

A Monte Carlo simulation on critical current distribution of bent-damaged multifilamentary Bi2223 composite tape

It was attempted to reproduce the measured critical current (Ic) distributions of the Bi2223 composite tape bent by 0–0.833% by simulation. Simulation was carried out with a Monte Carlo method in combination with a model that correlates the critical current to damage evolution. Two variables that differ from specimen to specimen were input in the simulation. One was the damage strain parameter, with which the difference in extent of damage among specimens was expressed. Another was the original critical current (Ic0) values at zero bending strain. With the present simulation approach, the measured distributions of critical current at various bending strains, and the measured variations of average and coefficient of variation of critical current values with increasing bending strain were reproduced well.

Lateral critical current density distributions degraded near edges of coated conductors through cutting processes and their influence on ac loss characteristics of power transmission cables

The critical current density (Jc) near the edges of coated conductor could impact the ac loss characteristics of the cables comprising coated conductors. A wide coated conductor is often cut into narrower coated conductors in fabrication processes, and the Jc might degrade near the edges of coated conductors through the cutting process. This possible degradation of Jc could depend on cutting technology. Lateral Jc distributions of coated conductors cut by using a mechanical slitter as well as a laser were measured by the magnetic knife method. The measured coated conductors are fabricated by PLD process on clad type textured-metal substrates with reduced magnetism. Based on the degradation of Jc near the edges of coated conductors which was determined experimentally, ac losses of multi-layer cables as well as mono-layer cables were calculated numerically to evaluate the impact of the Jc degradation near the edges of coated conductors on the their ac loss characteristics.

Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy

We have carried out 2-dimensional (2D) measurement of local critical current in a Trifluoroacetates-Metal Organic Deposition (TFA-MOD) processed YBCO coated conductor using scanning Hall-probe microscopy. Recently, remarkable R&D accomplishments on the fabrication processes of coated conductors have been conducted extensively and reported. The TFA-MOD process has been expected as an attractive process to produce coated conductors with high performance at a low production cost due to a simple process using non-vacuum equipments. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the coated conductors are definitely very important for practical applications. According to our measurements, we can detect positions and spatial distribution of defects in the conductor. This kind of information will be very helpful for the improvement of the TFA-MOD process and for the design of the conductor intended for practical electric power device applications.

Ac loss reduction of multilayer superconducting power transmission cables by using narrowcoated conductors

The ac loss characteristics of coated conductors are dominated by the magnetic fieldcomponent normal to their superconductor layer. Multilayer cables as well asmonolayer cables consisting of 4 mm-wide coated conductors (named 4 mm cables)and those consisting of 2 mm-wide coated conductors (named 2 mm cables) weredesigned, and numerical electromagnetic field analyses were performed in their crosssections to calculate their ac losses. Trapezoidal lateral critical current densityJc distributions with shoulders as well as uniform ones were assumed incoated conductors for the analyses. The former models the degradedJc near the edges of coated conductors. In the case of the monolayer, thecalculated ac losses of the 2 mm cables were comparable to those of the 4 mmcables. In the cases of the multilayers, the calculated ac losses of the 2 mmcables were obviously less than those of the 4 mm cables. The degradedJc near the edges of coated conductors more seriously affects the ac loss characteristics of the 2 mmcables than those of the 4 mm cables. However, even if we consider the influence of the degradedJc near the edges of coated conductors, 2 mm-wide coated conductors are more profitable than4 mm-wide coated conductors in multilayer cables from the viewpoint of ac loss reduction.

Current Sharing and Critical Current Distribution in Bi-2223 Tapes

For the applications of technical superconductors, for example in superconducting high-field or NMR magnets, a detailed knowledge of the superconducting-to-normal transition is essential. Usually, this transition is described by a power law leading to the critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and the n-value. I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and n are global variables which ignore the microstructure of a superconductor. In contrast, the assumption of a distribution of local critical currents provides a statistical approach to the superconducting-to-normal transition taking into account the non-uniformity of the material. In this paper, the model of local critical currents is applied to describe the measured voltage-current characteristics of the superconducting transition for several types of Bi-2223 tapes. The probability density of the distribution is evaluated and in addition the current sharing occurring in the transition region between the superconducting phase and the matrix material of the composite is discussed.

Low field critical current density of titanium sheathed magnesium diboride wires

Magnesium diboride (MgB2) is replacing some of the conventional superconductors due to its low cost and availability in kilometer lengths. MgB2 has also been considered for AC applications. In order to model the AC losses and the critical currents of the applications, intrinsic Jc(B)-dependence is an important factor also at low fields. In this work Jc(B)-dependence of an MgB2 sample is extracted from the standard in field voltage-current measurements. The proposed method is applied to a non magnetic titanium sheathed sample at 16 and 20 K and a simple formula for Jc(B) aligns with the measurements. In the fitting process, the critical current distribution inside the wire is numerically simulated in order to take the self field of the sample into account. Moreover, the same formula aligns with measurements of a different sample. These critical current measurements, performed at 4.2 K, were based on magnetization. In the self field computations, the superconducting cross section must be determined accurately. Therefore, we tailored an image processing tool for MgB2 wires to obtain the geometry from a photograph.

Nondestructive Measurement of Critical Current Distribution of SmBCO Coated Conductor Using Hall Probe

The Hall probe measurement system was used to measure the critical current distribution of superconducting coated conductor. The system consists of reel to reel moving apparatus, 7 array Hall probe, a rotary encoder and permanent magnet. The magnetic field profile across the width of superconducting coated conductor using Bean's critical state model was calculated. The effect of various parameters of the formulas on the magnetic field distribution and the effect of shape and size of artificial defects, which were formed on the surface of SmBa2Cu3O7-d(SmBCO) coated conductor using laser marking system, on the Hall probe magnetic field signal of the Hall probe measurement system was investigated.

Modeling analysis of irreversible bending strain distribution and critical current distribution at low bending strains of Bi2223-composite tape

Under applied strain, the damage behavior of bent (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi2223) composite tape is different from specimen to specimen. Accordingly, the irreversible bending strain and critical current differ among specimens. The present work attempted to reveal the irreversible bending strain distribution and the critical current distribution in low bending strain region. For analysis, a damage model, in which the shape of the core where the current transporting Bi2223 filaments are bundled into Ag, and the damage strain parameter given by the difference between the intrinsic tensile damage strain and residual strain of the Bi2223 filaments, were incorporated to describe the damage evolution in the core and its relation to critical current, was employed. The model was applied to the experimental results measured at limited bending strains. With the present approach in combination with a statistical analysis, the irreversible bending strain distribution and the variation of the critical current distribution with bending strain in low bending strain range were revealed successfully.

Study on critical current distribution in CT-OP Bi-2223 tape based on the scanning Hall probe magnetic microscopy

Critical current (Ic) of a Bi-2223 tape fabricated by controlled-overpressure (CT-OP) sintering method has been attained up to 200 A-class in recent years. However, current limiting factors have not been fully clarified yet. In this study, we have investigated local current distributions in a CT-OP Bi-2223 tape by using the scanning Hall probe magnetic microscopy. As a result, it has been found that large sheet current flows in the center of the sample. Even though the influence of self-field is taken into account, there is a clear difference of local Ic across the width. The maximum sheet current value in the center of the tape is 400–450A/cm when we applied the bias current of 100A, which is equal to the Ic at around 87K. This value is equivalent to 800–900A/cm at 77K taking into account the value of Ic. Furthermore, such current distribution continues uniformly in length direction. From these results, we have concluded that improvement of current uniformity across the tape width is the effective way to enhance critical current for the present CT-OP Bi-2223 tape. Our results show the feasibility for 300 A-class Ic at 77K.

AC Alternating-Current Loss Analyses of a Thin High-Temperature Superconducting Tube Carrying AC Transport Current in AC External Magnetic Field

Numerical simulations on the AC loss characteristics in a thin high-temperature superconducting (HTS) tube are presented. Geometry of the HTS conductor is modeled as a tube with negligible thickness, and assumed to carry a transport current with the same phase as an AC externally applied magnetic held perpendicular to the axis. Based on the classical theory of AC loss with the Bean critical current model, the distribution of critical current density jc and AC loss Q are obtained by means of numerical analysis. The results are in very good agreement with experiments. A double-peak profile is observed in the curve of the critical current density distribution along the azimuth angle. This numerical simulation method is suitable for a thin HTS tube, which may be applicable on a thin tube configuration consisting of coated superconductors.

Analysis of critical current distribution of bent Bi2223 composite tapes by unifying parameterapproach and its application to the description of average critical current–bending strainrelation near the average irreversible strain

When the critical current of bent Bi2223 composite tape is measured for manyspecimens, the critical current value is different from specimen to specimen, beingdistributed from low to high values. We need to characterize the distribution ofcritical current at various applied strains and also to predict the distribution ofirreversible strain for reliability and safe design. The present work attempted todescribe them from the statistical viewpoint. In the analysis, the unifying parameterapproach proposed recently by the authors was employed, in which the differencein damage evolution and critical current among the specimens was correlatedwith the distribution of the unifying parameter that refers to the tensile damagestrain of Bi2223 filaments in composite tapes. By application of this approachto the measured critical current values under various bending strains for threeBi2223 composite tapes from different suppliers (VAM1, VAM2 and VAM3 usedas the common test samples in the round robin test of VAMAS/TWA16), theunifying parameters were estimated and were formulated by the Weibull distributionfunction. With the formulated distribution function of the unifying parameter,the distributions of the critical current at various strains, the distribution of theirreversible bending strain and the average critical current—bending strain relationnear the average irreversible bending strain could be described satisfactorily.

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

The distribution of the normalized critical transport current (critical current normalized with respect to the original value) of Bi2223/Ag/Ag alloy composite tape under bending strain of 0%–0.833% was studied experimentally and analytically. The experimental results were analyzed by a modeling approach based on the relation of the heterogeneous damage evolution to the distribution of the critical current. The main results are summarized as follows. (1) The measured distribution of the critical current values varying with bending strain was described well by the present approach. (2) When all specimens were damaged at high bending strains (0.338%–0.833% in the present work), the distribution of the critical current of the bent-damaged specimens was expressed by the three-parameter Weibull distribution function, the reason for which was revealed. (3) The distribution of the irreversible strain was estimated, with which the influence of the increase in the fraction of damaged specimens on the variation of critical current distribution in the low bending strain range (0%–0.35%) was elucidated.

Statistical analysis of the distribution of critical current and the correlation ofn value to the critical current of bent Bi2223 composite tape

Distributions of local and overall critical currents and correlation ofn value to the critical current of bent Bi2223 composite tape were studied from thestatistical viewpoint. The data of the local and overall transport critical currents andn valuesof the Bi2223 composite tape specimens were collected experimentally for a wide range of bendingstrain (0–1.1%) by using the specimens, designed so as to characterize the local and overall critical currents andn values. The measured local and overall critical currents were analyzed with various types ofWeibull distribution function. Which of the Weibull distribution functions is suitable forthe description of the distribution of local and overall critical currents at eachbending strain, and also how much the Weibull parameter values characterizingthe distribution vary with bending strain, were revealed. Then we attempted toreproduce the overall critical current distribution and correlation of the overalln valueto the overall critical current from the distribution of local critical currents and the correlation of the localn valueto the local critical current by a Monte Carlo simulation. The measured average values of critical currentand n value at each bending strain and the correlation ofn value to critical current were reproduced well by the present simulation, while thedistribution of critical current values was reproduced fairly well but not fully. The reasonfor this is discussed.

Accuracy of AC Transport Current Loss in HTS Tape Measurements Using a Pick-Up Loop

AC transport current loss in high temperature superconducting (HTS) tapes can be measured without a direct electrical connection by using a pick-up loop method. This method requires a correction factor, which is related to the ratio of the linkage magnetic flux determined by the distribution of the transport current in the HTS tape and pick-up loop. In this study, to determine the accuracy of the AC transport current loss measurement using a pick-up loop method, we measured the critical current and AC transport current loss distribution in the tape axis. We found the pick-up loop voltage of the deteriorated section of the HTS tape to be higher than in the normal section with the same transport current. We concluded that the pick-up loop method accuracy is sufficient to measure the AC transport current loss in HTS tapes.

Detection of Current Distribution in Bulk Samples With Artificial Defects From Inversion of Hall Magnetic Maps

A two-sided inverse Biot-Savart algorithm for the computation of critical current distributions in high temperature superconductor (HTS) pellets is introduced. It is based on simultaneous measurement of the magnetic field on top and bottom of the bulk sample with a double Hall probe, and the authors' discretization and QR inversion scheme. It yields current maps which are detailed in the ab-plane for each half of the bulk sample along the c-axis. The computed current is a O(1/z)-weighted average of the real current along the c-axis over each slice. This is the best possible resolution for current maps along the c-axis, regardless of inversion scheme. We demonstrate the new procedure by analyzing a non-c-homo- geneous simulation and a real HTS pellet of cylindrical geometry with asymmetrically drilled holes on both faces.