Critical Current Measurements Research Articles

Experimental Research and Analysis of AC Critical Current in HTS Tapes

High-temperature superconductor (HTS) has found its way into both ac and dc transmission; yet HTS technology was not specially designed for either transmission type. Now critical current of HTS in dc circumstance is defined, while research on ac current-carrying capability of HTS is still scarce. A new critical current criterion of the ac condition should be established for further studies. In the paper, different dc critical current measurement methods were applied to analyze ac critical current. The E–I curves were obtained by the four-probe method and the frequency ranges from 30 to 540 Hz. The ac current carrying capacity of the superconductors is discussed and analyzed by the RMS method, virtual n value method, and the loss concept method. A criterion of ac critical current was proposed. The RMS method and the virtual n value method are no longer applicable in broad frequency band. While the loss concept method is still applied to explain the theory of critical current which varies with frequency; therefore, an ac critical current calculation method is obtained and the calculative equation at different frequencies is established.

Critical current density measurement of striated multifilament-coated conductors using a scanning Hall probe microscope

In this paper the authors succeeded in measuring the critical current density () of multifilament-coated conductors (CCs) with thin filaments as low as 0.25 mm using the scanning hall probe microscope (SHPM) technique. A new iterative method of data analysis is developed to make the calculation of for thin filaments possible, even without a very small scan distance. The authors also discussed in detail the advantage and limitation of the iterative method using both simulation and experiment results. The results of the new method correspond well with the traditional fast Fourier transform method where this is still applicable. However, the new method is applicable for the filamentized CCs in much wider measurement conditions such as with thin filament and a large scan distance, thus overcoming the barrier for application of the SHPM technique on measurement of long filamentized CCs with narrow filaments.

Role of improving the physical properties of Sm-123 phase by adding nano-magnetic MnFe2O4

Superconducting samples of SmBa2Cu3O7−δ (Sm-123) added with various amounts of nanosized MnFe2O4 addition (0.0−0.20wt%) were investigated. The investigated samples prepared by the solid-state reaction method. The phase formation and microstructure of these samples were examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), proton induced X-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS). XRD data indicate that the volume fraction of Sm-123 increases as nanosized MnFe2O4 increases up to 0.02wt%. The elemental distribution and oxygen content were deduced from PIXE and RBS. The oxygen content slightly decreases as MnFe2O4 wt% increases. The superconducting transition temperature (Tc) and critical current density (Jc) of the prepared samples were estimated from electrical resistivity and transport critical current density measurements. It was found that Tc decreases as nanosized MnFe2O4 addition increases, while Jc enhances up to 0.02wt%. Moreover, the temperature dependence of normal state electrical resistivity was studied in view of the pseudogap opening in order to determine the pseudogap temperature T* as a function of nanosized MnFe2O4 addition. T* increases as nanosized MnFe2O4 increased, indicating the enhancement of the pseudogap formation in HTSCs by adding magnetic impurities. The crossover to fluctuation conductivity near the Tc is discussed.

Characterization of Nb–Ti Strands in the Process of Industrial Production for the ITER Poloidal Field Coils

More than 100 tons of Nb-Ti superconducting strands were produced by JSC “ChMP” (RF) for the ITER poloidal field coils (PF1&6) in the period from 2010 to 2014. The strands were manufactured based on the NT-47 alloy with a titanium content within 46.0-48.5 wt%, using the technology developed at Bochvar Institute. Measurements of the temperature and field dependence of the critical currents were carried out at the Low Temperature Physics Department of Atominstitut, Technische Universitat Wien, on strands stemming from the 30%, 60%, and 90% stages of industrial production. The facility for variable temperature critical current measurements was upgraded in order to provide direct measurements of the critical current up to 300 A at relatively high temperatures (T > 5 K) in a gas-cooled mode and up to 900 A in liquid helium. Unique results regarding the field and temperature dependence of the critical current in the range from 2 to 12 T and from 3.5 to 7 K, respectively, are presented and discussed based on singleand two-component pinning models. On the basis of mathematical models, the critical temperature at which the transition to the resistive state occurs (T cs ) was evaluated for the operating conditions (6.4 T and 33 A) of the poloidal field coils. Statistical analysis revealed that both models adequately describe the experimental Ic(T, B) data; however, a comparison of the models based on the Fisher test shows a statistical difference.

Development of Critical Current Measurement System of HTS Tape Using Pulsed Current

The measurement of critical current for HTS tape at various temperature conditions if generally used by the conduction cooling system with dc power supply. However, the continuously increasing dc current affects the temperature rising of metallic parts such as copper terminal for conduction cooling, HTS tape holder, current leads, and contact parts by joule heating. The temperature increasing seriously affects the measuring factors, which are voltage, resistance, or temperature, and then users may get the wrong results. To minimize the heating effect, the authors considered the pulsed current instead of dc current for power supply. A cryostat for HTS tape has cryogenic refrigerant for conduction cooling and metallic current lead for 2500 A. The voltage and temperature variations by pulsed and DC current of the same HTS tape are given in this paper. The test results show the possibility of the measurement method for critical current using pulsed power supply with conduction cooling system.

Soldered joints—an essential component of demountable high temperature superconducting fusion magnets

Demountable superconducting magnet coils would offer significant benefits to commercial nuclear fusion power plants. Whether large pressed joints or large soldered joints provide the solution for demountable fusion magnets, a critical component or building block for both will be the many, smaller-scale joints that enable the supercurrent to leave the superconducting layer, cross the superconducting tape and pass into the solder that lies between the tape and the conductor that eventually provides one of the demountable surfaces. This paper considers the electrical and thermal properties of this essential component part of demountable high temperature superconducting (HTS) joints by considering the fabrication and properties of jointed HTSs consisting of a thin layer of solder (In52Sn48 or Pb38Sn62) sandwiched between two rare-earth-Ba2Cu3O7 (REBCO) second generation HTS coated conductors (CCs). The HTS joints are analysed using numerical modelling, critical current and resistivity measurements on the joints from 300 to 4.2 K in applied magnetic fields up to 12 T, as well as scanning electron microscopy studies. Our results show that the copper/silver layers significantly reduce the heating in the joints to less than a few hundred mK. When the REBCO alone is superconducting, the joint resistivity (RJ) predominantly has two sources, the solder layer and an interfacial resistivity at the REBCO/silver interface (∼25 nΩ cm2) in the as-supplied CCs which together have a very weak magnetoresistance in fields up to 12 T. We achieved excellent reproducibility in the RJ of the In52Sn48 soldered joints of better than 10% at temperatures below Tc of the REBCO layer which can be compared to variations of more than two orders of magnitude in the literature. We also show that demountable joints in fusion energy magnets are viable and need only add a few percent to the total cryogenic cost for a fusion tokamak.

Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapes

In order to elucidate the effect of uniaxial strain on the critical current of practical REBCO tapes (REBa2Cu3O7−d, RE = Y and Gd) fabricated by Superpower and SuNAM, two types of critical current measurements were carried out. In the first, the tape sample was attached directly to a universal testing machine and pulled under a tensile load. In the second, the tape was soldered to a Cu–Be springboard and then attached to the testing machine and then pushed or pulled in order to apply both tensile and compressive strains to the tape sample. An inverse parabolic behaviour was observed for the uniaxial strain dependence of the critical current of both tapes. Using synchrotron radiation, the local strain exerted on the REBCO layer was measured at room temperature under the conditions used for the two techniques for making Ic measurements. On the basis of these room temperature data, the local strain exerted on the REBCO layer at 77 K was numerically evaluated. A one-dimensional chain model for current flow in the REBCO material with fractional lengths of A-domains and B-domains oriented along the uniaxial strain direction is proposed. The model can explain the parabolic strain behaviour of the critical current and shows that the strain at which the peak in Ic occurs, is not only determined by pre-compression or pre-tension on the superconductor at the operating temperature, but also by the ratio of the fractional amounts of the two domains.

Dependence of Critical Current and Quench Energy of BSCCO-2223 Tapes on Bending Diameter

This paper describes the experimental results of a test method for determining the retained critical current after double bending at room temperature for short and straight Ag- and/or Ag alloy-sheathed Bi-2223 superconducting tapes. The critical current measurements were performed after double bending samples of a Bi-2223 tape manufactured by Sumitomo with different bending diameters set to 50, 60, and 80 mm. The results allow one to determine the critical bending diameter of the tape. An experimental technique was also developed for the measurements of the quench energy (QE) of the tapes in boiling liquid nitrogen. The impact of the bending on the QE was experimentally determined through measurements performed on the same samples previously used for the critical current measurements.

Critical Current Measurement in Three-in-One High-Temperature Superconducting Cables

This paper discusses the measurement of critical current I C in three-in-one high-temperature superconducting (HTS) cables. It is difficult to measure the I C of long-length HTS cables by simply connecting a dc power supply to both ends of a single cable core. We propose a go-and-return method, in which the HTS conductor layers of two of the three cores are used to carry current in both directions. A current in the opposite direction is induced in the HTS shield layers. However, the current induction is less than that from ac transmission, and magnetic field leaks from each core. As a result, the measured I C value becomes smaller than the sum of the wires' I C . We used a transient electromagnetic simulation, taking into account the I C -B characteristics of the wires. The estimated I-V waveforms for the 30- and 240-m cable systems are comparable with the measured ones. We have established a practical I C measuring technique that can be used with long-length HTS cable.

Critical Current and Current Feeding Test of a 200-Meter High Temperature Superconducting DC Power Cable System

A 200-m high-temperature superconducting (HTS) dc power cable was constructed in 2009 and since then, we have performed six cooling cycle tests to evaluate its performance. Current feeding test is one of the most important issues for this evaluation. This cable has a three-layer coaxial structure. The inner two layers are used as positive (+) polarity, and only one layer in the outmost is used as negative (−) polarity for dc electric energy power, respectively. In the summer of 2014, we started the sixth cooling cycle test on the 200-m cable. After cooling down to liquid nitrogen temperature, we applied a dc transport current to the 200-m cable. In this paper, we will present the current feeding test in the 200-m cable together with the critical current measurements. The critical currents of HTS tapes are enhanced in the assembled cable compared with the manufacturer's specifications. No degradation occurred after the cooling cycles.

Long-Length Critical Current Measurement of MgB2 Wire in a Coil

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> opens the door to viable, cryogen-free, and thermally stable MRI magnets. A multiturn multilayer MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solenoid coil was tested in a variable temperature environment from 5 to 34 K in order to validate the conductor's long-length critical current and n-value performance. The reacted wire was successfully wound and impregnated without imparting noticeable strain damage to the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> filaments as demonstrated by the consistency between measured long-length wire performance of the coil and the short sample data. Expected coil performance was calculated by taking into account ferromagnetic wire matrix and critical current scaling based on current percolation model.

REBCO Performance at High Field With Low Incident Angle and Preliminary Tests for a 10-T Insert

New improvements in high-temperature superconductors (HTS) are encouraging for large-scale applications, particularly for high-field magnets. In this context, the “NOUGAT” project aims at designing and building a 10-T HTS insert operating inside a 20-T resistive magnet. The insert is foreseen as a stack of double pancakes wound with REBCO tapes. The combination of current and magnetic field generate large mechanical stresses. Moreover, as the conductor has strong anisotropic properties, its performances are affected by transverse magnetic fields. Therefore, they have to be studied under high magnetic field at a small disorientation, close to the longitudinal axis of the coil. The objective is to evaluate its limiting critical current. This paper reports critical current measurements on a 4-mm REBCO tape provided by SuperOx. They were performed on the tape full width under a high magnetic field of up to 23 T at 4.2 K at the Laboratoire National des Champs Magnetiques Intenses (LNCMI) at various field angles. The limiting critical current is found to be about 350 A, which represents a current margin of 57% on a basic model. Then, a preliminary test of a prototype double-pancake coil is reportedly tested at up to 16 T.

Magnetization Losses of Roebel Cable Samples With 2G YBCO Coated Conductor Strands

Roebel cable with second-generation YBCO strands is one of the promising high-temperature superconducting solutions of fully transposed high current conductors for high field accelerator magnets. Following the considerable research effort on the manufacturing of Roebel cables in recent years, sample conductors are now available in useful lengths with reproducible performances to allow for detailed characterizations beyond the standard critical current measurements. The ac loss and strands coupling are of significant interest for the field quality of the accelerator magnets. We report a set of systematic ac loss measurements on two different Roebel cable samples prepared for the EuCARD-2 collaboration. The measurements were performed over a wide range of temperature values between 5 and 90 K, and the results were analyzed in the context of strands architecture and coupling. The results show that the transposed bundles are partially decoupled, and the strands in transposition sections behave as an isolated single tape if the strands are insulated.

Toward Superconducting Critical Current by Design.

A new critical-current-by-design paradigm is presented. It aims at predicting the optimal defect landscape in superconductors for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To this end, critical current measurements on commercial high-temperature superconductors are combined with large-scale time-dependent Ginzburg-Landau simulations of vortex dynamics.

Fabrication of FeSe 0 . 5 Te 0 . 5 Superconducting Wires by an Ex Situ Powder-in-Tube Method

We report the fabrication of Cu/Nb-sheathed FeSe0.5Te0.5 superconducting wires by an ex situ powder-in-tube method. After swaging and drawing, the as-drawn wires were heat-treated at different heat conditions. Subsequently, characterizations and transport critical current measurements were performed on the heat-treated FeSe0.5Te0.5 wires. Also, the results of the as-drawn wire are presented for comparison. The FeSe0.5Te0.5 cores reacted with the Nb sheaths when heat-treated at a high temperature or at a low temperature for a long time. Furthermore, according to our results, the highest transport critical current density (J c) at 0.5 and 0.75 T is about 1.6 × 10 4 and 1.5 × 10 3 A/cm 2, respectively.

Effect of Eu Substitution in Low Density Bi (Pb)-2223 High Temperature Superconductors

High temperature superconductor ceramic of low density Bi1.6Pb0.4Sr2(Ca2-xEux)Cu3Oy both doped and non-doped, have been produced by the method of conventional solid state reaction synthesis. The effects of rare earth doping on the low density superconducting compound have been studied by considering Eu substitution at Ca site to stoichiometric of Bi (Pb)-2223 with x ranging from 0.000 to 0.2000. The experimental works in this study consist of direct-current (DC) electrical resistance-temperature and critical current density measurements for both electrical and superconducting properties, and powder X-ray diffraction (XRD) for phase analyses and lattice parameters while Field Emission Scanning Electron Microscopy (FESEM) for surface morphology. All the Eu-doped samples consist of both low-Tc Bi (Pb)-2212 and High-Tc Bi (Pb)-2223 phases. The substitution of Eu at the Ca site induced the decrease of volume fraction for the Bi (Pb)-2223 phase and the increase of the volume fraction for Bi (Pb)-2212 phase. Eu doping does not enhance the Tc zero but gradually decrease its value and become semiconducting at higher dopant concentration with x=0.200. From FESEM investigation, the grain size started to degrade from x=0.000 to x=0.0100 and form smaller plate-like grain which belong to Bi (Pb)-2212 phase and the grain particles are seem to be coagulated to form clusters for x=0.200.

Hysteresis Losses and Effective $J_{\mathrm{c}}(B)$ Scaling Law for ITER Nb3Sn Strands

Hysteresis losses of five Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn International Thermonuclear Experimental Reactor reference strands were investigated by means of magnetization loop measurements in a vibrating sample magnetometer in a perpendicularly applied magnetic field. The magnetization loops were recorded while continuously sweeping the applied field between the extreme values ±B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , covering a wide range of maximum applied fields (0.2-10 T). In this paper, we compare the directly determined hysteresis losses based on the area of the smaller measured loops and the losses calculated by the integration of the width ΔM of the B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> = 10 T magnetization loop. A suitable fitting function is proposed to describe the ΔM(B) dependence, which leads, for each strand, to an excellent agreement with the experimentally determined hysteresis losses, magnetization, and pinning force. Transport critical current measurements in a perpendicularly applied magnetic field were also performed for all the strands, and on the basis of the comparison with the transport measurements, the critical current densities were determined from the magnetization data.

A new scaling approach and quantitative angular critical current measurement using magnetization

This is a viewpoint article that discusses fast track communication by Mishev et al (2015 Supercond. Sci. Technol. 28 102001)

Modeling of trapped fields by stacked (RE)BCO tape using angular transversal field dependency

Stacks of superconducting (RE)BCO tape are gaining popularity as a potential alternative for superconducting bulks for trapped field applications. This is partly due to versatility and uniformity of the starting material, allowing for more deterministic prediction of field profile and magnitude. However, most FEM models of trapped field magnets do not incorporate parameters such as critical current and n-value dependence on the angle of applied magnetic field, leading to only qualitative modeling results. More quantitative results can be obtained from incorporating more data for superconductivity and thermal properties of the material. Such models can be used as a starting point for most geometries and both trapped field and current transport modeling problems. An FEM model of a stack of tapes was constructed using the H formulation, incorporating goniometric critical current and n-value measurements. The modeling results were compared to field cooling experiments for stacks of different heights. The experiment and modeling show good agreement.

International round robin test of the retained critical current after double bending at room temperature of Ag-sheathed Bi-2223 superconducting wires

An international round robin test was carried out in order to establish a test method for retained critical current after double bending at room temperature of Ag-sheathed Bi-2223 superconducting wires. Tests for commercial Bi-2223 tape were conducted by six laboratories using the same guidelines. The standard uncertainties (SUs) of measurands were evaluated for these four quantities: IC0, IC/IC080, IC/IC060, IC/IC050, where, IC0 is initial critical current and IC /IC0XX is critical current after XX mm bending. Using an F test to determine where the most scatter was generated in the test results it was found that the greatest scatter in the normalized critical current measurements came from inter-laboratory scatter. In a type-B uncertainty evaluation, the major contribution was from the bending diameter and measuring temperature. The relative SU tended to increase as the bending diameter decreased. A specific mandrel diameter corresponding to a retained critical current of 95% could be determined with a relative SU of 1.3%. In order to reduce the overall scatter, the temperature difference between the critical current measurements before and after bending should be small.