This paper presents a methodology for the estimation of thermal conductances in HVDC MgB2 concentric superconducting cables, with both analytical formulas and finiteelement method (FEM) simulations. A three-conductor thermal network representing the former, the MgB2 layer, and the shield has been developed to quantify the inter-layer heat exchange and assess the impact of temperature-dependent material properties. FEM results for the SCARLET ±25 kV / 20 kA case study show that a variation of thermal conductance with temperature, and that non-reciprocal effects between heating and cooling phases are negligible, allowing a three-conductance simple network to be employed. Consequently, linear analytical models have been proposed to accurately calculate the thermal conductance coefficients with minimal computational cost, enabling fast and reliable parameter identification for circuit-level electrothermal simulations of large-scale superconducting transmission systems.

Filamentisation of superconducting tapes is an effective way to reduce alternating current (AC) losses in REBCO-coated conductors. However, current redistribution in filamented tapes may be a challenge, as local defects in individual filaments may lead to reduced performance. A remedy for this is the introduction of periodically spaced short bridges, or non-filamentized sections in the superconductor layer. The purpose of these segments is to facilitate a rerouting of current to even out the consequences of any defects in individual filaments. Multifilamentary REBCO tapes were fabricated starting from SUBRA's three-dimensionally structured Hastelloy substrate, onto which the buffer and superconductor were deposited by THEVA using large-scale industrial processes. We tested the current-voltage properties and evaluated local voltages by soldering voltage contacts onto individual filaments. A numerical model was developed using COMSOL Multiphysics (COMSOL AB, 2024) to simulate current sharing through bridges and within filamented sections. Our results show that bridge length could be reduced from the current 10 mm to approximately 1 mm without compromising redistribution performance. The simulations suggest that current redistribution over distance alone is less effective than using bridges, even at distances approaching 20 cm. Experimental measurements qualitatively confirmed these predictions, showing improved voltage characteristics due to bridges, although quantitative differences remain and warrant further investigation.

Abstract Superconductivity was discovered more than a century ago, and it has achieved full commercialization for MRI and NMR applications. Superconducting technology has got on spotlight recent years for transportation, power network, and fusion energy, due to the significant advantages offered against its counterpart technologies, including lighter weight, compacter size, lower losses, higher efficiency, and higher power density. Therefore, many superconducting applications are moving towards higher technology readiness levels, with a fast pace. The accelerated research around superconducting applications for modern transportation is due to unique features of this technology towards decarbonisation via electrified systems. Meeting the Net Zero targets to decelerate global warming issue is the main driver of implementing the superconducting technology for aerospace, marine, and railway transport. However, many challenges still remain to be addressed for superconducting devices and applications, which will in turn pave the way for the commercialization of superconducting technology. In this article, a roadmap on electrification of transportation systems for aerospace, marine, and railway application is presented, covering challenges and solutions in design analysis, modelling, monitoring, and operation. A series of short articles are presented to outline the potential applications and solutions. These potential futuristic routes and their materials/technologies are considered/suggested for a 10-20 years time-frame.

The paper discusses the development and application of circuit models for superconducting DC power cables to investigate their interaction with the power system. These models, created as part of the SCARLET project, simulate the behaviour of different cable components (e.g., HTS layers, shield, cryopipes) during power system transients like energization and faults. By integrating these models with power system simulators, accurate current profiles of the cable components are obtained. Additionally, the paper introduces a combined circuit and 2D finite element method (FEM) model to study the electro-thermal behaviour of the cable. The circuit model is developed to account for all the relevant factors like the E-J power-law characteristic, the magnetic field's influence on the superconductor and heat generation. Then the FEM model provides the detailed electro-thermal behaviour of the cable components considering the current profiles obtained from the circuit model as input, enabling efficient and focused simulation of localized effects without the computational complexity of fully coupled simulations. Using this combined model, the performance of a reference HTS cable under a pole-to-pole fault is analysed. Results show a limited temperature increase during transients, including both energization and fault scenarios, with peaks of only fractions of a Kelvin occurring immediately after the pole-to-pole fault. The FEM model further provides detailed temperature maps, illustrating the corresponding temperature distribution throughout the cable components, proving the save operation on post restoration.

Frequent task in the numerical modeling of superconducting magnets generating AC or pulsing fields is the evaluation of AC loss. Commonly, after solving the electromagnetic problem, the loss is calculated by integrating in time and volume the density of dissipated power, obtained locally as the product of electrical field and current density. However, the loss can also be determined from the product of current and voltage detected at the device terminations, in a way analogous to electrical measurements. Fortunately, this voltage signal can be easily determined in the frequently used T-A formulation. It can be further used for more detailed comparison with experimentin particular, the creation of magnetic hysteresis loops relating the flux linkage to the coil current. Methodology is illustrated on benchmark #3 problem published by the HTS modeling workgroup.

Use of coated conductor tapes from high-temperature superconductors is a possible way for better energy efficiency in large magnetic systems for particle accelerators or fusion tokamaks. Because the hysteresis loss in a time varying magnetic field is proportional to the tape width, a substantial gain is expected by splitting of the REBCO layer into parallel filaments. Usually, a metal layer is added to facilitate stable operation, albeit this conductive connection between filaments introduces a new kind of loss, known as coupling loss. When analyzing the experimental data of samples produced by the scalable technology, where the buffer and the superconductor layer are deposited via standard industrial procedures on a 3D patterned Hastelloy substrate, we have found that the existing models for coupling loss do not provide satisfactory quantitative predictions. This motivated us to develop an improved model for low- and mid-frequency behavior. While preserving the well-known qualitative properties foreseen by previous models, our new model enables quantitative predictions at frequencies up to 1000 Hz, without the need for adjusting the results by fitting the geometric parameters.

Knowledge of the ac loss generated in a superconducting cable during variation of magnetic field is essential when considering its use in pulsed magnets. In round high-temperature superconducting (HTS) cables made from coated conductor (CC) tapes, the hysteresis loss is proportional to the tape width. Then an effective measure for reducing the loss is the division of the superconducting layer into parallel filaments. We investigated the ac loss in short models of round cables, containing in different arrangements two layers of 10 standard helically laid tapes. At magnetic field amplitudes surpassing 0.1 T the loss did not depend on the cable architecture, in agreement with simple analytical prediction. Substantial reduction of magnetization loss was obtained in the cable models from novel coated conductor (CC) tapes with a low-cost filamentized REBCO layer produced in an industrial process utilizing a special 3D patterned metal substrate. Further research should address the improvement of critical current and optimization of metallic layers, allowing a migration of current between filaments without substantial increase of coupling loss.

Variation of critical current along the conductor length is the feature commonly encountered in industrially produced REBCO tapes called coated conductors (CC). Reduction of critical current exceeding several percent in the portions few millimetres long can be observed in the data obtained by reel-to-reel characterisation provided by manufacturers. Metallic layers in the CC architecture can take over some current in such „weak spot”, and help to keep the local temperature stable, preventing its thermal runaway and conversion to a “hot spot”. Understanding the phenomenon is particularly crucial when the space and weight limitations do not allow to add metallic layer with the thickness, that would be sufficient to match the lost transport capability of superconductor. For this purpose, we studied a set of samples, representing both standard as well as infrequent profiles of weak spots identified in direct transport experiments. Analytical theory was then utilised as basic tool for recovering the properties serving as input for numerical modelling. Temperature profiles and current redistribution in the weak spot locations were found, and the effect of cooling conditions and metallic layer thickness on the weak spot resistance against thermal runaway analysed. Quantitative assessment of the possibility to improve the performance of CC tape by adding Cu stabilising layer or improving cooling settings could help to optimise the architecture of coated conductor intended for use in electric transport.

The combination of liquid hydrogen and superconducting cables presents a unique opportunity to distribute both hydrogen and bulk electricity in the same infrastructure. In particular, liquid hydrogen around 20 K is ideally suited for cooling the MgB 2 superconductor, resulting in a compact power cable that also leaves sufficient place for the hydrogen flow. Such a hybrid system operating in the MVDC range at 25 kV and 20 kA constitutes one of the main goals of the European project SCARLET. After a description of the rationale and benefits of the electricity – hydrogen system, various possible applications and a first distribution system are presented. Furthermore, the different cable components already designed are discussed along with the research challenges and general strategy for the development.

With a focus on magnetic shielding techniques, this article discusses the compensation and control of magnetic fields based on superconducting and ferromagnetic materials. The magnetic cloak combines superconducting and ferromagnetic materials to achieve internal magnetic field suppression without notable deformation of the magnetic flux lines in surrounding space. Magnetic shielding deflects unwanted magnetic fields away from shielded objects, although complete blocking of magnetic flux is not possible. One of the cases of shielding is a magnetic cloak. Demagnetization of such cloak involves using the effect of dynamic magnetoresistance to reduce the remnant magnetic field in the shielded space by applying an axial alternating magnetic field. The demagnetization process consists of alternating demagnetization and relaxation periods. The results show that optimizing measurement parameters improves demagnetization efficiency, with a notable reduction in the remnant magnetic field. However, the process may be affected by various factors, and future steps aim to improve its reproducibility.

In this work, the changes in the microstructural and DC transport properties of coated conductor tape, deformed in helical form during the manufacturing of a round cable, were studied. The superconducting layer experienced both outward (‘OUT’) and inward (‘IN’) bending with respect to the round core (rod) at various lay angles and former diameters. The microstructure of a rare-earth barium copper oxide (REBCO) surface was observed using a scanning electron microscope. Direct transport measurements in liquid nitrogen were used to investigate the influence of the bending parameters on the DC transport properties. In the OUT configuration, cracks on the REBCO surface were formed at a diameter of 9 mm or less at a lay angle of 45°. The critical current of the tape started to degrade at a diameter of 10 mm or less. The investigation showed that cracks are formed in the direction following the rod axis. In the IN configuration, the measurements were performed at lay angles of 25°, 30°, 38°, 45°, 52° and 60°. The highest critical current retention and the lowest degree of damage on the REBCO layer were observed at a lay angle of 30°; in particular, the critical current visibly degraded at diameters as small as 3 mm, and defects were visibly observed at diameters of 2 mm. At lay angles higherthan 30°, the critical current degraded sooner (at higher diameters), and an increased density of defects in the form of ‘protrusion lines’ was observed. We found that the protrusion lines followed the preferential cleavage direction at approximately 80° to the tape length, independently of the lay angle used. By using a lay angle lower than 30°, the critical current degraded sooner; no protrusion lines were observed, but cracks were formed at the tape edges. For both bending configurations, the lower former diameters led to a higher density of defects on the REBCO surfaces accompanied by the degradation of superconducting properties.

The patterns of longitudinal I c fluctuations in 12 mm wide manufactured high-temperature superconductor tapes are compared with those obtained after slitting the tapes to widths of 6 mm, 4 mm and 3 mm, respectively. In the statistical analysis of I c ( x ) data, the overall critical current, I c , ovrl , was first calculated. In the case of fluctuating critical current, I c , ovrl is lower than the average critical current, I c , aver . Also, the dissipation concentrates in ‘weak spots’ with reduced critical current. A simple model allows us to estimate the thermal runaway current, I tr , at which the weakest location would convert to a ‘hot spot’ with rapid increase in temperature. In the prediction of I tr , the absolute minimum of the I c value, I cmin , is essential. Then, by comparing the I cmin / I c , aver ratio in the slit tapes to that established previously for the manufactured tape, one can deduce if some new defects have been introduced during slitting. It is also interesting to check if the relation between I tr and I c , ovrl remains unchanged, because the lower of these two quantities represents the limit of transportable current. We have found a reduction of both I cmin / I c , aver and I tr / I c , ovrl ratios in those tapes containing the edges of the originally manufactured 12 mm tape. Contrary to our expectations, the most serious drops in slit tapes were not in the same location as in the original tape. However, the systematically better performance of the central strips (the tapes with both edges cut) is a strong indication that the most serious defects are created during manufacturing, close to the tape edges. We conclude that tape cutting by femtosecond laser has not introduced additional non-uniformity. The analysis also shows that the criticality of defects in applications is largely governed by the width of said defects, the cooling conditions and the thickness of the stabilizing layer.

Traditionally, the critical current, Ic , of a superconducting wire, considered as the limit of DC operation, is experimentally verified on a sample few centimeters long. This approach could fail in case of high-temperature superconductor wires, where significant changes of Ic along the conductor length, x , are commonly observed. Then, the quantity that could replace the uniform critical current as the entry data in designing of a superconducting device is to be found. We have analysed the properties of 6 coated conductor tapes provided by 4 different manufacturers. First, the overall critical current of the full tape length, Ic,ovrl , has been evaluated from the Ic(x ) data. However, the dissipation in a non-uniform conductor with fluctuating critical current concentrates in the “weak spots” with the lowest Ic values. Then, depending on the conditions of heat removal, thermal runaway in some location could happen before reaching Ic,ovrl during conductor testing or operation. Taking into account the conductor architectures and assuming the cooling in liquid nitrogen bath, we computed the thermal runaway current, Itr , at which the weakest location in each of the analysed conductors would convert to a “hot spot” with rapid increase of temperature. Then, comparing Itr to Ic,ovrl one can identify which of these two values is lower, i.e., represents the upper limit of transportable current. We discuss how the result of such analysis would be influenced by a metallic stabilisation that exhibits significant impact on cooling.

MgB 2 is one of the most promising materials for superconducting bulk applications. However, thermomagnetic instabilities can arise in the material because of its low heat capacity and thermal conductivity as well as its high critical current density. Being able to predict these phenomena, can guide and optimize MgB 2 -based devices for magnetic flux shielding or trapping applications. In this work, the flux-jump occurrence in an MgB 2 cup-shaped shield is numerically studied using the finite element method by means of the commercial software COMSOL 6.0 Multiphysics®. To this aim, we developed a 2D axial-symmetric model coupling the heat diffusion equation and the magnetic equations based on a magnetic vector-potential ( A ⃗ ) formulation. The comparison of the computed shielding curves with the experimental ones evidenced a good agreement between the two sets of data at different temperatures and positions along the shield’s axis. The as-validated model was then exploited to investigate possible optimization routes via the improvement of both the thermal conductivity of the material and the thermal exchange between the device and the cooling stage.

Superconducting cables with a circular cross-section are attractive for various applications, such as power transmission lines, current leads, and large magnetic coils. In the case of coated conductor (CC) tapes, such an arrangement can be achieved by wrapping the tapes in a helical fashion around a circular former. While a braid of Cu wires serves as the core for the CORC ® cable, in the conductor-on-round-tube the CC tapes are laid in several layers on a tube. Traditionally, the tapes in the neighbouring layers are wound following an alternating sense of helicity. Then, the tape edges are raised due to the Poisson effect, causing local surface irregularity and pressing into subsequent layers. We have found that this effect is less deteriorating in the case of coinciding lay angles because then the tape edges create imprints that are parallel to the tape length. Here, we present the results of investigating the AC loss performance of this innovative arrangement and its comparison with the traditional architecture. Both verification possibilities—experimental testing of small laboratory models and numerical modelling— were used for this purpose.

In this work, we have investigated two series of Nb/Cu samples deposited by HiPIMS technique, and differing in Nb deposition conditions, Nb film thickness and Cu substrate polishing techniques. All the films were additionally irradiated by Nd:YAG laser to smooth their surfaces. The impact of the magnetic field enhancement at the surface defects on the premature start of magnetic field penetration into the superconducting film was studied, combining experiments and numerical calculations. Compared to previous study, improved numerical calculations by using the Finite Element Method (FEM) served to calculate the maximum field enhancement factor β m , reflecting impact of the most crucial surface defects found in the samples. Magnetization measurements at 4.2 K in DC magnetic field, oriented parallel to the film, were employed to determine the start of the field penetration H en . The SEM and AFM analyses served to investigate the Nb surface morphology. In some samples, deviations from the H en (β m ) dependence were observed. It was found that the magnetic field penetration could start from the Nb/Cu interface rather than from the free Nb surface due to visibly better quality of the free Nb surface observed by SEM analysis, and that could lead to the deterioration of H en (β m ) dependence.

Superconducting cables have been proven in a variety of pilot projects and utility installations, demonstrating several of their advantages, including compact size and low energy losses, which can make the technology economically attractive for certain applications. It is clear though that different applications impose different requirements and challenges, but also opportunities for the cables. An interesting application is high-power DC transfer at medium voltage (MVDC). The high-current capability of the superconductor allows for a reduction in voltage while maintaining or increasing the power transfer level. In this way, one MVDC superconducting cable can replace one or more conventional high-voltage DC cables. In the European project Superconducting cables for sustainable energy transition (SCARLET), two types of MVDC cables will be developed, one based on HTS and one on MgB 2 materials. Additionally, protection requirements will be considered, including the development of a modular DC fault current limiter for 10 kA. A main motivation for the development is the elimination of costly high-voltage converter stations when going from high to medium voltage, e.g., for offshore wind power plants. Another feature is the combined hydrogen and electricity transmission from generation sites to industry or mobility end users. This paper describes the superconducting MVDC cable concept as well as the main challenges and research needed to develop and type test the cables.

Critical current ( Ic ) variations along the coated conductor (CC) have a significant impact on the conductor stability and quench behavior. In the direct current (DC) transport case, localized Ic minimum can lead to the formation of a region with a higher electric field. Joule heating in such a region can disable the operational functionality of the device. Then a sufficiently detailed knowledge of the longitudinal profile of the conductor Ic is in the first place necessary in assessing the conductor eligibility for a specific application. In the second place, it serves to design appropriate electro-thermal stabilization, securing an effective operation, and reducing the probability of conductor degradation during the quench. We report on the experimental investigation of longitudinal Ic homogeneity of CC observed by means of transport and magnetization measurements. Basic information is the CC data from the producer on Ic profiles obtained with ∼1-milimeter spatial resolution by Tapestar TM XL. We compare it with the result obtained by multi-channel DC transport measurement with 3-millimeter spatial resolution between neighboring contacts on a multiprobe device. Then the whole sample is characterized by a complex AC susceptibility experiment. We analyze possible causes of observed discrepancies between respective methods and discuss the relevance of various techniques for the determination of longitudinal Ic profiles in commercial CC.

The aim of this work is to study delamination promotion of the coated conductor (CC) tape under the influences of the scratches on the Ag surface when bending. For this purpose, the microstructural and superconducting properties of the CC tape bent in helical fashion with different former diameters were analyzed. Such CC tape that exhibits visible damage along its length in the form of "scratch line" caused on the final product was used. The CC tape was bent at a constant lay angle of 45 degrees with a superconducting layer positioned inward to the former core experiencing compression. Transport current measurements in liquid nitrogen were performed to study superconducting properties of the CC tape. The results showed that delamination of the REBCO (Rare-Earth Barium Copper Oxide) layer started to form exclusively on the scratch location at tube diameter of 7 mm (corresponding to strain epsilon = -0.71 %) while the rest of the REBCO surface started to delaminate at tube diameter of 5 mm (epsilon = -0.99 %). As the superconducting properties of the CC tape are related with the REBCO delamination, the critical current degraded when first signs of REBCO delamination appeared as expected. Thus, it was observed that the scratches induce REBCO delamination during the bending process resulting in increase of critical bending radius.