Search Results (61)

The integral equation formulation of Maxwell’s equations proposed by Brandt provides an alternative to the H and T-A formulations for modelling high-temperature superconducting (HTS) tapes. A modified version of Brandt’s method in the literature models ferromagnetic domains near the tapes by considering the ferromagnetic domains as equivalent surface current. This paper extends this method by including the effect of external magnetic field acting on the ferromagnetic and HTS domains. The proposed method is used on a benchmark problem, which considers an HTS tape with a ferromagnetic substrate under an external time-varying magnetic field. The results agree closely (error in average ac loss less than 3%) with the widely-used T-A formulation implemented in COMSOL down to 2 mT. In addition, the proposed method is also applied to HTS tapes carrying transport ac current in a slot of a machine’s stator iron core, and HTS tapes in a stator iron slot in a machine under working conditions. It is found that ac loss calculated by the proposed method increases as the discretization size of the ferromagnetic material’s boundary decreases, and overshoots the value calculated by the T-A formulation in COMSOL when using very fine discretization. Full article

This article presents a preliminary study on the potential application of artificial intelligence methods for assessing the durability of HTS tapes in superconducting fault current limiters (SFCLs). Despite their importance for the selectivity and reliability of power networks, these devices remain at the prototype testing stage, and the phenomena occurring in HTS tapes during their operation—particularly the degradation of tapes due to cyclic transitions into the resistive state—are difficult to model owing to their highly non-linear and dynamic nature. A concept of an engineering decision support system (EDSS) has been proposed, which, based on macroscopically measurable parameters (dissipated energy and the number of transitions), aims to enable the prediction of tape parameter degradation. Within the scope of the study, five approaches were tested and compared: Gaussian process regression (GPR) with various kernel functions, k-nearest neighbours (k-NN) regression, the random forest (RF) algorithm, piecewise cubic hermite interpolating polynomial (PCHIP) interpolation, and polynomial approximation. All models were trained on a limited set of experimental data. Despite the quantitative limitations and simplicity of the adopted methods, the results indicate that even simple GPR models can support the detection of HTS tape degradation in scenarios where direct measurement of the critical current is not feasible. This work constitutes a first step towards the construction of a complete EDSS and outlines directions for further research, including the need to expand the dataset, improve validation, analyse uncertainty, and incorporate physical constraints into the models. Full article

In this study, we investigated how to design a tube furnace for the oxygen annealing of a REBa₂Cu₃O_7−x (REBCO, where RE = rare earth) superconducting joint. We confirmed the annealing temperature threshold of REBCO tape I_c degradation, which was 175C. A heat exchange model that included REBCO tape and a tube furnace was established by using this temperature as the boundary condition. At the same time, the temperature distribution of the REBCO tape in a commercial tube furnace was measured for the calibration of the heat exchange model. The feasibility and accuracy of the model were confirmed by comparing the real measurements and the simulation results. We then optimized the furnace design based on the model according to two criteria: a 20 mm length of REBCO tape should be kept at high temperatures for the oxygen annealing of REBCO joints and the length of tape at temperatures over the I_c degradation temperature should be as short as possible. The results of this furnace design investigation could help fabricate shorter REBCO superconducting joints, making the magnet more compact and decreasing the length of the Cu stabilizer layer to be removed. Full article

Embedding stacked HTS tapes into twisted slots is one design approach for constructing fusion conductors. This paper adopts a Cable-in-Conduit Conductor (CICC) structure, utilizing commercially REBCO coated conductors. The cable framework is made of copper and features six helically twisted slots filled with 2G HTS tapes. Two 1 m long samples with twist pitches of 200 mm and 300 mm, respectively, were fabricated. In one slot, copper and superconducting tapes were alternated, while the remaining grooves were filled with copper tapes. The 90 µm thick copper-plated bare tapes provided by Shanghai Superconductor were used for testing. By measuring the critical current of tapes positioned at different locations within the grooves at 77 K, the characteristics of each tape in the stacked arrangement were individually characterized. The study obtained the current degradation patterns of tapes located at different positions within the grooves under various bending radii. This paper will present and discuss the preliminary results of the bending measurements conducted at 77 K under a self-field. Full article

This work presents a comparison of different commercial tapes belonging to the second-generation High-Temperature Superconductors (2G HTS) produced by SuNAM Co., Ltd., SuperOx, and Shanghai Superconductors Technology Co., Ltd. (SST) companies. The aim is to investigate pinning mechanisms responsible for best performances, looking at the anisotropy of the irreversibility field and of the flux pinning energy. The irreversibility line states the upper limit of current-carrying capacity, whereas the flux pinning energy explores the ability of material defects to act as weak collectively or strong single vortex pinning centers. All investigated samples have artificial pinning centers (APCs) included in the superconducting matrix: BHO-doped EuBCO for SST, ${\mathrm{Y}}_2{\mathrm{O}}_3$ in YBCO for SuperOx, and ${\mathrm{Gd}}_2{\mathrm{O}}_3$ particles trapped in GdBCO for SuNAM. Resistive transition curves were measured in high magnetic fields up to 16 T for magnetic field orientations parallel and perpendicular to the tape surface. We found that the anistropy of SST tape shows an overall independence both on temperature and magnetic field, while the other two samples show a more complex behavior. This leads to the conclusion that properly engineered APC optimization in coated conductors can further reduce anisotropy of superconducting properties. Full article

New aerodynamic aircraft concepts enable the storage of volumetric liquid hydrogen (LH2). Additionally, the low temperatures of LH2 enable technologies such as the superconductivity of electrical fan drives and power distribution components. An increased power density of the onboard wiring harness and the electrical machine can be expected. The highest system efficiency and the smallest fuel and tank weight will be achieved with a highly efficient energy conversion by the fuel cell from LH2 to electrical energy. This publication shows a comprehensive study for cryogenic fan drives based on experimental-driven tape superconductor investigations, mission profile-based considerations, design analyses of superconducting electrical machines, and studies of the cooling concepts. A cryogenic system cannot be considered without a feasible cooling concept. Here, an approach with a safe He-based cooling system is proposed, using the LH2 flow to the fuel cell as a heat sink for the losses in the electrical system. Full article

Iron-based superconductors have strong potential for magnet applications through their very high upper critical field, low anisotropy and manufacturability through the powder-in-tube (PIT) route. The engineering critical current density (J_e) is a key parameter for measuring the maximum current density that superconducting materials can withstand in practical applications. It serves as a bridge between theoretical research and practical applications of superconductors and has great significance in promoting the development and application of superconducting technology. In this study, Ag sheathed Ba_0.6K_0.4Fe₂As₂ (Ba-122) iron-based superconducting tapes were prepared by using the process of drawing, flat rolling and heat treatment by hot pressing (HP). For the first time, the filling factor of the tapes increased to about 40%, leading to a reduction in the volume fraction of Ag, consequently lowering the overall cost. The optimal parameters for achieving high transport J_e were obtained by comparing the effects of different HP pressures on the properties and micro-morphology of the tapes. The prepared mono-filament tapes are capable of carrying the transport J_e of 4.1 × 10⁴ A/cm² (I_c = 350 A) at 4.2 K, 10 T, marking the highest J_e reported for Ba-122 wires and tapes to date. Our results show that high transport J_e can be obtained in Ba-122 superconducting tapes, and iron-based superconductors have a promising future in practical applications. Full article

HTS 2G tapes used in Superconducting Fault Current Limiters (SFCLs) have properties that allow for the effective limitation of short-circuit currents; however, due to the specificity of the device operation, they should be characterized by the high stability of the parameters when repeatedly leaving the superconducting state. During the operation of SFCLs, a situation may occur in which the parameters of the HTS tapes used will change several times as a result of the action of short-circuit currents that exceed the critical current I_C of the superconductor of the tape used. This paper presents the results of microstructural tests of 2G HTS tapes intended for SFCLs, subjected to surge currents corresponding to prospective short-circuit currents with values higher than their critical currents I_C and for which I_C changes were observed. The HTS tapes were examined using a JEOL 7600F field emission scanning electron microscope (SEM), and their chemical composition was analyzed using Energy-Dispersive X-ray Spectroscopy (EDS). The test results indicate the possibility of micro-damage in the form of cracks in the superconductor layer, as well as the interruption of the buffer layers and the oxidation of the silver layers. The analysis of the chemical composition of the HTS tape layers may indicate the occurrence of diffusion processes. Full article

Bulk high-temperature superconductors (HTSs) such as REBa₂Cu₃O_7−x (REBCO, RE = Y, Gd) are commonly used in rotationally symmetric superconducting magnetic bearings. However, such bulks have several disadvantages such as brittleness, limited availability and high costs due to the time-consuming and energy-intensive fabrication process. Alternatively, tape stacks of HTS-coated conductors might be used for these devices promising an improved bearing efficiency due to a simplification of manufacturing processes for the required shapes, higher mechanical strength, improved thermal performance, higher availability and therefore potentially reduced costs. Hence, tape stacks with a base area of (12 × 12) mm² and a height of up to 12 mm were prepared and compared to commercial bulks of the same size. The trapped field measurements at 77 K showed slightly higher values for the tape stacks if compared to bulks with the same size. Afterwards, the maximum levitation forces in zero field (ZFC) and field cooling (FC) modes were measured while approaching a permanent magnet, which allows the stiffness in the vertical and lateral directions to be determined. Similar levitation forces were measured in the vertical direction for bulk samples and tape stacks in ZFC and FC modes, whereas the lateral forces were almost zero for stacks with the REBCO films parallel to the magnet. A 90° rotation of the tape stacks with respect to the magnet results in the opposite behavior, i.e., a high lateral but negligible vertical stiffness. This anisotropy originates from the arrangement of decoupled superconducting layers in the tape stacks. Therefore, a combination of stacks with vertical and lateral alignment is required for stable levitation in a bearing. Full article

In recent years, the scientific and industrial interest regarding alternative technologies for transmission cables has increased. These conductors should efficiently transmit significant amounts of power between grid nodes, which are expected to be particularly congested due to the projected global increase in electricity production. Superconducting cables are considered a promising solution in this context, offering the potential to transmit large amounts of energy with minimal losses and compact dimensions, thereby potentially benefiting the environment. To evaluate the feasibility of integrating superconducting cables into existing grids, techno-economic approaches should be adopted. Such techniques enable the conceptual design of a specific cable structure, allowing users to explore a wide range of operating parameters to derive optimal designs. This paper reports a comprehensive techno-economic analysis of High Voltage Alternating Current (HVAC) cables realized with High-Temperature Superconducting (HTS) tapes, with the aim to transmit extremely high-power level. The optimal coaxial design is selected using Optimization Tool for Superconducting Cable Research (OSCaR) by implementing a graded approach to the critical current of the HTS tapes used for the different phases. This optimization aims to achieve the most effective balance between the cost of the coated conductors and their electrical properties. The whole set of model equations, the user-defined parameters, and the applied constraints are detailed. The OSCaR tool is then applied to assess the impact on the optimized design of the cable system and the corresponding cost indexes of several crucial parameters, such as the maximum transmitted power, the voltage level, and the line length. Full article

This paper presents the influence of superconducting tape insulation on the recovery time of superconducting fault current limiters. The analysis is based on the experimental results of short-circuit tests. The reduction in the thermal and dynamic effects of the passage of a fault current can be achieved by limiting the short-circuit time and the value of the surge current. An ideal fault current limiter is required to have almost zero impedance at operating currents and significant impedance at fault conditions. A superconducting fault current limiter (SFCL) meets these requirements under certain conditions. The recovery time—a very important parameter—shows the ability of the limiter to return to the superconducting state to be ready to limit the subsequent short circuit. The experimental results show that the recovery time can be significantly reduced with the application of thin-film insulation and an appropriate design of the conduction cooling of the HTS tape. Full article

At present, HTS magnets cannot operate in the real closed-loop persistent current mode due to the existence of joint resistance, flux creep, and AC loss of the HTS tape. Instead of using a current source, HTS flux pumps are capable of injecting flux into closed HTS magnets without electrical contact. This paper presents a practical superconducting DC dynamo for charging a conduction-cooled HTS magnet system based on a flux-pumping technique. To minimize heat losses, the rotor is driven by a servo motor mounted outside the vacuum dewar by utilizing magnetic fluid dynamic sealing. Different parameters, such as air gap and rotating speed, have been tested to investigate the best pumping effect, and finally, it successfully powers a 27.3 mH HTS non-insulated double-pancake coil to the current of 54.2 A within 76 min. As a low-cost and compact substitute for the traditional current source, the realization of a contactless DC power supply can significantly improve the flexibility and mobility of the HTS magnet system and could be of great significance for the technological innovation of future HTS magnets used in offshore wind turbines, biomedical, aerospace, etc. Full article

As the second-generation high-temperature superconducting conductors, rare earth–barium–copper–oxide (REBCO) coated conductor (CC) tapes have good potential as high-field and high-energy superconductors. In superconducting applications, several joints are required for conjugating comparatively short REBCO CC tapes. Soldering lap joints are the simplest and most commonly applied REBCO CC joints. In addition to joint resistance, the mechanical behavior and electromechanical properties are also crucial for superconducting applications. In this paper, the electromechanical properties and mechanical behaviors of soldering lap joints at 77 K under a self-field were studied. The mechanical behavior was addressed by using a full three-dimensional multilayer elastic–plastic finite element model (FEM) with REBCO CC tape main layers and solder connecting layers. Then, the electromechanical properties were analyzed by using Gao’s strain-$I_c$ degradation general model on the basis of the FEM results. Both the mechanical behavior and electromechanical properties were verified by experimental results. The effects of soldering lap conditions including lap length, soldering thickness and lap style on the electromechanical properties and mechanical behaviors were discussed. The results indicate that shorter overlap lengths and a thinner solder can reduce the premature degradation of I_c due to stress concentrations nearby the joint edges; moreover, the irreversible critical strain is significantly higher in the back-to-back joint approach compared to the widely used face-to-face joint approach. Full article

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility ${\chi }_1^{″}$ related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density J_c and the pinning energy U of the sample have been extracted at 77 K by using the ${\chi }_1^{″}$ measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the ${\chi }_1^{″}$ at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the ${\chi }_1^{″}$ as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the ${\chi }_1^{″}$ with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications. Full article

In order to describe the dependence of critical current on specimen length and crack size distribution in the superconducting tape with cracks of different sizes, a Monte Carlo simulation and a model analysis were carried out, employing the model specimens of various lengths constituted of multiple short sections with a crack per each. The model analysis was carried out to evaluate the effects of the two factors on the critical current of a specimen. Factor 1 is the size of the largest crack in a specimen, and Factor 2 is the difference in crack size among all sections at the critical voltage of critical current. Factors 1 and 2 were monitored by the smallest ligament parameter among all sections constituting the specimen and by the number of sections equivalent to the section containing the largest crack at the critical voltage of the critical current of the specimen, respectively. The research using the monitoring method revealed quantitatively that the critical current-reducing effect with increasing specimen length is caused by the increase in the size of the largest crack (Factor 1), and also, the critical current-raising effect is caused by the increase in the difference of crack size (Factor 2). As the effect of Factor 1 is larger than that of Factor 2, the critical current decreases with increasing specimen length. With the present approach, the critical current reducing and raising effects under various crack size distributions were evaluated quantitatively as a function of specimen length, and the specimen length-dependence of critical current obtained by the Monte Carlo simulation was described well. Full article