Superconducting Wires Research Articles

Low field anomaly in the critical current of Ba1− x K x Fe2As2 tapes

Ba1−x K x Fe2As2 superconductors have strong potential for magnet applications through their very high upper critical field, relatively high superconducting transition temperature and manufacturability through the powder-in-tube (PIT) route. However, the critical current density in PIT tapes is still low compared to the incumbent technologies, so a greater understanding of the limiting factors is required. We have measured the in-field critical currents (I c) of stainless steel and silver double-sheathed monofilament Ba0.6K0.4Fe2As2 superconductor tapes at elevated temperatures from 15 K to 35 K. At 20 K, the critical current density is up to 140 kA cm−2 in low (optimal) field and 22 kA cm−2 in 8 T. In the low-field region we observe an anomalous and sharp suppression of I c centred at the zero field. This feature is non-hysteretic for lower temperatures and perpendicular fields, but becomes hysteretic for higher temperatures in perpendicular fields and for all temperatures in parallel fields. The low-field suppression is also reflected in the n-values which can otherwise be very high, in excess of 100 in the optimal field. Magnetic-field hysteresis of I c is generally attributed to flux exclusion/flux trapping in granular superconductors and this is likely to be the case also in the present conductors. The low-field I c anomaly also likely has its origin in planar granularity, while magnetic phases in grains or grain boundaries may also play a role.

Analysis of local strain fields around individual threading dislocations in GaN substrates by nanobeam x-ray diffraction

Position-dependent three-dimensional reciprocal space mapping (RSM) by nanobeam x-ray diffraction (nanoXRD) was performed to reveal the strain fields produced around individual threading dislocations (TDs) in GaN substrates. The distribution and Burgers vector of TDs for the nanoXRD measurements were confirmed by prerequisite analysis of multi-photon excited photoluminescence and etch pit methods. The present results demonstrated that the nanoXRD can identify change in the lattice plane structure for all types of TDs, i.e., edge-, screw-, and mixed TDs with the Burgers vector of b = 1a, 1c and 1m + 1c. Strain tensor components related to edge and/or screw components of the TDs analyzed from the three-dimensional RSM data showed a nearly symmetrical strained region centered on the TD positions, which were in good agreements with simulation results based on the isotropic elastic theory using a particular Burgers vector. The present method is beneficial in that it allows non-destructive analysis of screw components of TDs that tend to contribute to leakage characteristics and are not routinely accessible by conventional structural analysis. These results indicate that nanoXRD could be a powerful way to reveal three-dimensional strain fields associated with arbitrary types of TDs in semiconductor materials, such as GaN and SiC.

Influence of pre-overpressure heat treatment on micro-structure and related properties of Bi-2212 round wire

The Bi2Sr2CaCu2O8+x (Bi-2212) material, a high-temperature superconductor, holds significant promise for future high-field applications because of its multifilamentary, twistable production capabilities, coupled with a high upper critical field. Bi-2212 round wire (RW) is made by the powder-in-tube (PIT) technique, and Bi-2212 coils are usually being developed using the wind-and-react method and a high temperature and over-pressure (OP) heat treatment (50 atm, 890℃) is then required to achieve the high current performance. However, over-pressure (OP) heat treatment can lead to a reduction in wire diameter, compromising the stability of the coil structure. Addressing this issue, the pre-overpressure (pre-OP) heat treatment is used and proves effective in mitigating the reduction in Bi-2212 wire diameter. In this paper, to comprehensively investigate the impact of pre-OP heat treatment on the microstructure of Bi-2212 wire, a metallographic analysis of two kinds of Bi-2212 wires was carried out by the scanning electron microscope (SEM) and the ImageJ software. It was found that larger-area filaments experienced a greater reduction in area after pre-OP heat treatment, that pre-OP heat treatment increases the aspect ratio of filaments, and polygonal bundle structure helps maintain filament structure stability during pre-OP heat treatment. Additionally, the impact of pre-OP heat treatment on the performance of Bi-2212 wires was analyzed through critical current testing under high fields.

A novel joint-less second-generation high-temperature superconducting toroidal coil: Promise for fabricating compact toroidal magnetic fields

This paper presents a novel joint-less toroidal magnet made of second-generation high-temperature superconducting (2G-HTS) tapes. This approach effectively resolves the closed-loop issue for 2G-HTS magnets and has the potential to provide higher and more stable magnetic fields. Compared with traditional 2G-HTS toroidal magnets, while the current loops of the joint-less magnets have no resistance, a decrease in magnetic field still occurs, especially in coils with insulation. Therefore, this work focuses on the decrease of the magnetic field of this novel magnet using experimental and microanalytical methods. For the first time, it was verified that, by winding two coil groups, insulated and no-insulated, parallel charging does not cause interference between them. Furthermore, the magnetic field area was expanded by finite element analysis, and simulations showed that the magnetic field converged with increasing number of coils. Besides, we found that the decrease of the magnetic field was related to the damage of the tape during slitting and winding, where insulated coils were more susceptible to damage during winding. The damage usually occurred at the starting point of the tape slitting, because the copper layer was separated due to adhesion during the winding of insulated coils, which further caused the superconducting layer to detach, resulting in a decrease in the critical current. From our perspective, benefiting from the high critical field of the 2G-HTS tapes, this novel toroidal coil structure has significant implications for the construction of compact toroidal magnets.

23 Tesla high temperature superconducting pocket magnet

We present a compact 23 T no-insulation (NI) magnet that was wound with 60 m of 10 mm wide high temperature superconducting (HTS) tape. The magnet consists of only one pocket-sized double pancake (DP) coil with an inner diameter of ∼6 mm, a height of 20 mm, and an outer diameter of 41.6 mm. Another NI coil of similar size but with a larger inner diameter of 8 mm reached a slightly lower magnetic field of 21 T. We also present a smaller coil which was wound with only 20 m of HTS tape and still achieved a magnetic field of 16 T. During the experiments in liquid helium, each coil was charged to a current between 690 A and 850 A, corresponding to a high current density of 1500–1900 A mm−2. The small bore size and high current density contributed to the high fields generated by these coils. We present the fabrication details, helium tests and repeatability analysis of these ‘pocket’ magnets.

Enhancing electric field calculation in HTS tape simulations for currents exceeding the critical limit using full HTS tape modeling

Superconducting devices are an interesting technological option for improvement of efficiency of energy systems. High-temperature superconducting tapes are among the best choices for power applications. Because of their high non-linearity, the design of devices with superconducting tape requires specific simulation models and methods to correctly represent the superconducting behavior. The J-A formulation has been investigated as an efficient tool to represent the electromagnetic behavior of such devices. It is based on the current density (J) and magnetic vector potential (A), and able to represent both superconducting materials and ferromagnetic materials. With regards to the tapes, the use of thin-film approximation can be successful with the J-A formulation. Usually, only representations considering the superconducting layer of tapes have been investigated. This becomes a problem for operations where the current in the superconductor surpasses the critical current, for example in transient analysis. This paper presents an analysis of the effects the inclusion of all of the tapes’ layers have on simulations with the J-A formulation with thin-film approximations. By considering all layers of the tape, one represents the transition between superconducting and high-loss states of the tape. A simple system, consisting of a single tape with applied current, has been studied as benchmark and simulated with J-A and the more established T-A formulation in two cases: considering all layers; and only the superconducting layers. Results show that results for the J-A and T-A formulations are compatible and the inclusion of all layers improves the stability of the simulation and provides correct assessment of the electric field in the tapes.

Density dependent constitutive model for Bi-2212 powder compression deformation

Bi-2212 HTS materials are fabricated into multi-filamentary wires via powder-in-tube (PIT) method followed by proper heat treatment to obtain superconductivity, but how to predict the large compression deformation behaviors of the Bi-2212 powder is critical to design the processing of the Bi-2212 HTS wire. Drucker Prager/Cap (DPC) model was the most commonly used model for powders including Bi-2212 with soil-like mechanical behavior to consider its shear failure as well as hydrostatic compression. However, the parameters for DPC Cap evolve with densities change and the original model is inadequate to precisely describe the densification process of Bi-2212 powder with large strain. In this study, the modified DPC model with density dependent parameters was introduced for Bi-2212 powder compressions by measuring the failure strength and hydrostatic compressive behavior under different density states. The DPC yield surface was plotted with an evolution trend of non-linear outward expansion with density increased. FEM model of uniaxial compression based on the as-introduced model was built with subroutine VUSDFLD applied. The distribution of Mises stress and relative density were analyzed. The axial stress-density curve for FEM and experimental results were normalized and quantitively evaluated by Mean Square Error (MSE). The introduced model shows good convergence and could match the experimental results well with normalized MSE of 0.000207 and Root Mean Square Error (RMSE) of 0.0144, indicating the mean error percentage of 1.44%. The model introduced in this article provides supports toward large strain deformation simulation of Bi-2212 powder.

Fabrication of high-entropy alloy superconducting wires by powder-in-tube method

High-entropy alloys (HEAs) are novel functional materials that exhibit excellent mechanical and chemical properties. Recent discovery of self-healing superconductivity of HEAs under ion irradiation suggests their great potential applications under extreme conditions such as in nuclear fusion reactors or space environments. Here, we report a fabrication of the HEA superconducting (SC) wires using the ex-situ powder-in-tube (PIT) technique for the first time, a crucial stepping stone to achieve various industrial and technological applications. The Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 HEA SC powder prepared by planetary ball milling was filled into Fe tubes, drawn into wires, and then sintered in evacuated quartz tubes at various temperatures. The HEA SC wire sintered at 700 °C for 1 h exhibits the highest critical current density (Jc) among the HEA SC wires sintered in the temperature range of 600 – 1000 °C. At 4.2 K, the Jc of this wire exceeds the common benchmark value of Jc = 100 kA cm−2, indicating the suitability of the HEA wire for large-scale applications, such as in high-field SC magnets. The SC transition temperature (Tc) of the HEA wire significantly increases from 5.10 K for the as-cast wire to 7.58 K with the sintering time (ts) of 30 min at 700 °C. The scaling of the flux pinning force as a function of the magnetic field indicates that normal point pinning plays a major role in achieving a high in-field Jc in HEA SC wires. These results underscore the significant potential of HEA superconductors for practical applications in high-field SC magnets, SC electric machines, and next-generation nuclear fusion reactors.

The SPARC cryogenic system

The SPARC project at Commonwealth Fusion Systems (CFS) is a tokamak system designed to demonstrate commercially relevant fusion energy and achieve net fusion power output during the first operating campaign, with eventual fusion pulse energies exceeding 1 GJ. The SPARC tokamak includes eight magnet systems, three that use high-temperature superconducting (HTS) tapes. The SPARC cryogenic system (CRYO) consists of three supercritical helium cryogenic loops at nominal temperatures of 8 K, 15 K, and 80 K. CRYO 8 K and 15 K loops cool the HTS magnets to maintain thermal stability and prevent quench, while magnets cooled to 80 K are normally conductive. CRYO provides cold helium using a hybrid system that includes a Brayton-cycle-based cryoplant supporting all CRYO temperature loops, and a fixed volume 8 K blowdown system to remove heat and maintain temperature stability of the toroidal field (TF) magnets during and immediately following fusion pulses. CRYO 4.5 K equivalent peak cooling power is 17 kW for the cryoplant and 2.9 MW for the blowdown system during a 10-second fusion pulse. Primary cryoplant mechanical equipment includes screw compressors, turbo-expanders, heat exchangers, and circulation pumps, while the blowdown system consists of a series of warm and cold helium storage tanks operating at independent temperatures and pressures, with make-up compressors to reset the blowdown system between pulses. CRYO also includes a distribution valve box and multiple vacuum-jacketed (VJ) process lines ranging in nominal diameter from 20 to 600 mm. This paper investigates the various sub-elements which in combination represent SPARC CRYO, and attempts to address some of the technical challenges identified by the team.

Qualification and test of space compatible superconducting current leads (REBCO) designed for adiabatic demagnetization refrigerators

Many astrophysics observations require space telescopes, either to reduce atmospheric perturbation or simply to make these detections possible (in the X-Ray spectrum for example). One of these missions, Athena, is led by the European Space Agency (ESA), with additional international contributions, dedicated to X-Ray observation. Two instruments will be part of this mission and among them, X-IFU, will use Transition Edge Sensors (TES) to detect and precisely measure the energy of X-Ray photons. These sensors require a temperature of 50 mK to reach their ambitious sensitivity goals. In space, this temperature can be reached using Adiabatic Demagnetization Refrigeration (ADR) and such a cooling system is currently being developed for the X-IFU instrument. ADR utilizes magnetocaloric materials which, upon variation in magnetic fields, can produce a cooling effect. The magnetic field of the order of 1 T in a volume of 10s of cm3 is produced by a superconducting coil with high winding number and current limited to approximately 2 A. Even though this current is low compared to most earth-based systems, metallic current leads to link the high- and low-temperature stages would cause high thermal loads, unacceptable for the limited capacity of the cryogenic cooling chain of the spacecraft. Therefore, a harness consisting of superconducting current leads is planned to reduce the thermal loads at the low-temperature stage. As part of an ESA contract, our team designed, built and tested such a space-compatible harness. This harness includes the electrical interfaces at both ends as well as mechanical support. Its development is capable of operating between interfaces at 80 K and 4 K. The harness is based on industrially available Rare-Earth-Barium-Copper-Oxide (REBCO) High-Temperature Superconductor (HTS) tapes. The tapes were laser-cut by our group to fulfill our specifications, Parylene coated and reinforced with Kapton laminate tape for mechanical and insulating purposes. After characterization of the single tapes, the assembled harness has been subjected to an extensive qualification sequence including thermal cycling and mechanical testing based on launch loads requirements. This paper will summarize the technical design choices for this HTS harness. It will discuss the test results and propose some perspectives for the next iteration of the development.

Study on High Yield Strength Monolith NbTi Superconducting Wire for MRI Magnets in WST

Study on High Yield Strength Monolith NbTi Superconducting Wire for MRI Magnets in WST

3D modeling and measurement of HTS tape stacks in linear superconducting magnetic bearings

Superconducting magnetic bearings (SMBs) are among the possible new technologies to be incorporated in maglev vehicles. Stacks of high-temperature superconductor (HTS) tapes can be used as an alternative to bulks, because stacks offer better mechanical properties, a better thermal conductivity and a simpler production process. Numerical modeling has been employed as a cost-effective, fast and reliable tool for improving the performance of SMBs. Several scenarios can be simulated with fast and relatively simple 2D models; however, in some cases using 3D models is inevitable. In this study, we use a full 3D model to solve the problem of magnetization of the tape stacks and obtaining the hysteresis force loop between a permanent magnet and the tape stacks. For this purpose, we employ an energy minimization-based method called minimum electromagnetic entropy production in 3D, combined with a homogenization technique and the Jc(B,θ) dependence of the HTS tape as input. The modeling results agree very well with the experiment both in the zero-field cooled and field-cooled conditions. The presented approach offers significant computational advantages, delivering faster and more efficient results compared to previously proposed 3D methods.

Optimizing Vermicomposting Methods Involves Improving Efficiency, Maximizing Nutrient Retention and Minimizing Environmental Impacts: A Review

Vermicomposting is a sustainable and eco-friendly approach to organic waste management and fertilizer production. Earthworms are fundamental to the delivery of a range of soil ecosystem services such as nutrient cycling and water management, which in turn affects crop development and yield.In fact, one study states the presence of earthworms in soils can increase crop yields by an average of 25% and total crop biomass by 21%. Comparing the production efficiency between the upper pit and windrow method for vermicomposting involves considering several factors including processing capacity, labor requirements, processing time, and resource utilization.

Influence of HTS tape arrangement on the transverse compression performance of copper former CORC cables

CORC cables are subject to large transverse compression electromagnetic forces in fusion projects. Unfortunately, the electromagnetic force exceeding its critical transverse compression load will cause an irreversible decrease in its critical current. Therefore, it is particularly important to enhance the critical transverse compression load to ensure that the critical current does not decrease during operation. The winding method of high temperature superconducting (HTS) tape on the central former is variable. So the experimental study on how to increase the critical transverse compression load of CORC cable by changing the winding method of HTS tape is carried out in this paper. Firstly, the influence law of parameters of the number of HTS tapes per layer and the number of HTS tape layers on their transverse compression performance are analysed independently. The results indicate that increasing the number of HTS tapes per layer and the number of HTS tape layers can both improve the transverse compression performance of CORC cables. Whereas, in the case of a cable with a certain critical current demand (the same total number of HTS tapes), increasing the number of HTS tape layers necessarily reduces the number of HTS tapes per layer. Therefore, in order to compare the degree of influence of the above two parameters, we conducted transverse compression experiments on multiple groups of CORC cables with different winding methods (more layers with few tapes per layer or few layers with more tapes per layer) under the same critical current demand. The results show that under the same critical current demand, choosing the winding method that reduces the number of HTS tape layers and increases the number HTS tapes per layer can effectively improve the transverse compression performance of CORC cables. A 3D multilayer CORC cable transverse compression finite element model is also established to explain the inherent reasons for the differences in transverse compression performance of CORC cables under different HTS tape winding methods.

Controlled commutation modes in a switch based on HTS composites

This paper presents the results of experimental studies of switching processes of high-temperature superconducting tapes under the action of a pulse current load of microsecond duration. The reversible, stable and irreversible switching of the superconductor to the normal state has been realized. To demonstrate the opportunity of controlled generation of transients in superconducting composites, the characteristics of a superconducting switch capable of cyclic switching between flux flow and ohmic modes with preservation of switching parameters were investigated. The characteristics of a superconducting switch have been calculated based on experimental results.

Investigation of the effect of conformal pinning array on critical current density in infinitely long superconductors

In this study, the influence of conformal pinning centers on enhancing the critical current density in superconductors has been investigated. To increase the critical current density, a square-pinned array with dimensions of 10λ by 10λ under two appropriate conformal transformations is considered. Initially, by specifying the forces acting on the vortices, the relevant Langevin equation for the vortices is solved. By solving this equation, the vortex positions versus time are determined, and using this information, the critical current density is calculated as a function of the magnetic field. Then, the critical current density is calculated for various conformal function parameters in a superconductor tape. The results indicate that after applying two conformal transformations with appropriate parameters, the critical current density could be increased at higher magnetic fields. Simulations demonstrate a 25% increase in critical current density in the optimal case.

Performance degradation due to thermal cycling in soldered Faraday and SuperPower HTS tape stacks

Soldering stacks of high temperature superconducting (HTS) tapes results in a current carrying matrix with high electrical, thermal, and mechanical stability. In some applications, these tape stacks may be exposed to elevated temperatures during thermal heat cycles. Previous research on thermal degradation of high temperature superconductors has been performed with isolated crystals or individual HTS tapes, showing that oxygen-out diffusion from the rare-earth barium copper oxide (REBCO) results in a reduction of the superconducting performance. In this work, the performance of Pb37Sn63 soldered HTS tape stacks with thermal cycling to 170∘ C is quantified through determination of the critical current Ic , the n-value, the linear resistance R in the pre-transition region of the tape stack current–voltage (I–V) traces, and the diffusion coefficient D associated with oxygen-out diffusion. The effect of tape manufacturer and nickel electroplating were considered; three soldered stacks each of unplated Faraday, nickel electroplated Faraday, unplated SuperPower, and nickel electroplated SuperPower tapes were fabricated. Across 20–30 one hour thermal cycles, there are no significant differences in degradation profiles observed between the unplated and nickel electroplated tape stacks for a given manufacturer. There is however a marked difference in degradation profiles observed between the SuperPower and Faraday tape stacks. For the unplated and plated SuperPower tape stacks, the degradation in Ic generally follows the model to describe oxygen-out diffusion from the superconducting REBCO layer. Significantly less degradation is observed in the unplated and plated Faraday tape stacks, and in some sections Ic even increases slightly across cycles. Further, cases of increasing Ic accompanied with decreasing n are observed, a decoupling between the two parameters. Across all samples, the linear resistance was highest in current-redistribution regions near the leads. Consistent with the model of oxygen-out diffusion and the formation of an increasingly wide oxygen deficient layer in the REBCO, the linear resistance increased with cycle number.

Physico-Chemical and Microbiological Changes in Degradation of Agri-residue under Pit Compositing

Management of crop residues has remained a major challenge due to lack of easily adaptable and affordable interventions at farmer’s field. Cellulolytic bacteria and fungi have been recommended for recycling lignocellulosic-rich organic matter and to convert it into nutrient rich compost. However, these inoculants need to be generated on commercial scale for handling large volume of biomass. Also, the microbial formulation should also be capable of degrading diverse type of agri-residues. Seven fungal strains, namely Aspergillus awamori (F-18), Aspergillus nidulans, Trichoderma viride, Trichoderma longibrachitum, Drechslerahalodes, Eupenicilliumcrustaceumand Paecilomycesvariotiiwere characterized based on shelf life, functional activity to make consortium for degradation of various agro and horticultural residues using pit method of composting. An experiment was laid in cemented pits using four different substrates-maize cobs and stalks, paddy straw, kinnow fruit waste and flower waste for rapid degradation. The results revealed a maximum increase in spore count with Paecilomycesvariotii followed by Eupenicilliumcrustaceum and Drechslerahalodes. Out of four different additives, glycerol was found to be most suitable additive for incubation of selected fungi. The results on microbial population revealed highest bacterial population in flower waste and highest actinobacterial population in kinnow waste. It was also observed that inoculated organic waste degraded faster than un-inoculated wastes. As the degradation process progressed, total N also increased with time and varied from 1.1 to 1.5 percent. Flower and kinnow waste degradation was completed in 45 days while that of maize and paddy straw in 75 and 90 days, respectively. The pH range was 6.5-8.0 in all the substrates but in inoculated substrates, the range of pH was 7.2-7.6, while electrical conductivity (EC) was 1.4-3.6. The highest humus% was found with paddy straw followed by maize, flower and kinnow. The final product also showed variation in K% and S% which were found to be 0.6% -1.8% and 0.1% -0.20%, respectively.

The Effect of the Interfacial Resistance of the Superconducting-stabilizer Film on the Typical Sector Diffusion Pace for 2G HTS Tapes

High temperature superconducting (HTS) tapes of the second generation have been widely used as energy storage materials, such as in superconducting magnetic energy storage (SMES) devices. In order to enhance the current-carrying characteristics, these systems are typically run close to the critical currents of the coated conductors; as a result, hot spots may develop, which could cause the superconductor to become quenched. In order to prevent the start of hot spots and to reduce the amount of faulting, efforts have been made to raise the normal zone propagation velocity (NZPV) in this manuscript. The interfacial resistance between the superconductor and stabilizer layer, which can act as a current flow diverter during fault circumstances, has been shown to be the key to producing massive NZPV. The tape's architecture has been slightly modified by the addition of a high resistive layer between the superconducting and stabiliser layers, where various interfacial resistances have been utilised to forecast the temperature distribution between 10 cm lengths of HTS tape. A 2D numerical model was created using COMSOL to assess the NZPV and temperature distribution of the 2G superconducting tape. It has been concluded that larger NZPVs can be achieved by using substantial interfacial resistances to prevent superconducting tape quenching.

Investigation of Layered Structure Formation in MgB2 Wires Produced by the Internal Mg Coating Process under Low and High Isostatic Pressures.

Currently, MgB2 wires made by the powder-in-tube (PIT) method are most often used in the construction and design of superconducting devices. In this work, we investigated the impact of heat treatment under both low and high isostatic pressures on the formation of a layered structure in PIT MgB2 wires manufactured using the Mg coating method. The microstructure, chemical composition, and density of the obtained superconductive wires were investigated using scanning electron microscopy (SEM) with an energy-dispersive X-ray spectroscopy (EDS) analyzer and optical microscopy with Kameram CMOS software (version 2.11.5.6). Transport measurements of critical parameters were made by using the Physical Property Measurement System (PPMS) for 100 mA and 19 Hz in a perpendicular magnetic field. We observed that the Mg coating method can significantly reduce the reactions of B with the Fe sheath. Moreover, the shape, uniformity, and continuity of the layered structure (cracks, gaps) depend on the homogeneity of the B layer before the synthesis reaction. Additionally, the formation of a layered structure depends on the annealing temperature (for Mg in the liquid or solid-state), isostatic pressure, type of boron, and density of layer B before the synthesis reaction.