Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs) have rapidly emerged as a highly promising photon-counting technology for infrared wavelengths. These devices offer high efficiency, low dark counts and excellent timing resolution. In this review, we consider the basic SNSPD operating principle and models of device behaviour. We give an overview of the evolution of SNSPD device design and the improvements in performance which have been achieved. We also evaluate device limitations and noise mechanisms. We survey practical refrigeration technologies and optical coupling schemes for SNSPDs. Finally we summarize promising application areas, ranging from quantum cryptography to remote sensing. Our goal is to capture a detailed snapshot of an emerging superconducting detector technology on the threshold of maturity.
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Chandra M Natarajan et al 2012 Supercond. Sci. Technol. 25 063001
Zachary S Hartwig et al 2020 Supercond. Sci. Technol. 33 11LT01
High-temperature superconductors (HTS) promise to revolutionize high-power applications like wind generators, DC power cables, particle accelerators, and fusion energy devices. A practical HTS cable must not degrade under severe mechanical, electrical, and thermal conditions; have simple, low-resistance, and manufacturable electrical joints; high thermal stability; and rapid detection of thermal runaway quench events. We have designed and experimentally qualified a vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed (VIPER) cable that simultaneously satisfies all of these requirements for the first time. VIPER cable critical currents are stable over thousands of mechanical cycles at extreme electromechanical force levels, multiple cryogenic thermal cycles, and dozens of quench-like transient events. Electrical joints between VIPER cables are simple, robust, and demountable. Two independent, integrated fiber-optic quench detectors outperform standard quench detection approaches. VIPER cable represents a key milestone in next-step energy generation and transmission technologies and in the maturity of HTS as a technology.
Yue Wu et al 2024 Supercond. Sci. Technol. 37 055010
In high-temperature superconducting (HTS) power devices, the presence of iron cores changes the magnetic field profile around the HTS coil windings, potentially affecting their AC loss characteristics. AC loss measurements for HTS coil windings coupled with an iron core using the electrical method can lead to a significant error, owing to the indirect estimation of the iron core loss through using a copper test coil. To investigate the cause of the experimental error and the influence of an iron core on coil AC losses, transport AC losses of REBCO double pancake coil (DPC) assemblies coupled with an iron cylinder were measured. A 40-turn 1DPC and an 80-turn 2DPC assembly wound with 4 mm SuperPower wire were employed in the measurements. To ensure the same iron core loss using the HTS coil assembly and the copper coil, 2D finite element method simulations were conducted iteratively to design the iron core and the copper coil to get the same local magnetic field distributions in the designed iron core for the two cases. The main cause of the error is due to the difference in local magnetic flux densities in the iron core generated by the HTS coil assembly and the copper coil even when the ampere-turns of the coils are identical. We showed that the simulation-guided measurement method can assure accurate AC loss measurement in the HTS coil assemblies coupled with iron cores. Compared with the AC losses in the 1DPC and 2DPC coil assemblies without the iron cylinder, the presence of the iron cylinder significantly increases the coil losses. Frequency dependence is observed in the coil AC losses of the 1DPC and 2DPC assemblies when coupled with the iron cylinder. This is due to the eddy current induced in the iron cylinder generating a magnetic field, which influences the coil AC loss.
Mohammad Yazdani-Asrami et al 2022 Supercond. Sci. Technol. 35 123001
More than a century after the discovery of superconductors (SCs), numerous studies have been accomplished to take advantage of SCs in physics, power engineering, quantum computing, electronics, communications, aviation, healthcare, and defence-related applications. However, there are still challenges that hinder the full-scale commercialization of SCs, such as the high cost of superconducting wires/tapes, technical issues related to AC losses, the structure of superconducting devices, the complexity and high cost of the cooling systems, the critical temperature, and manufacturing-related issues. In the current century, massive advancements have been achieved in artificial intelligence (AI) techniques by offering disruptive solutions to handle engineering problems. Consequently, AI techniques can be implemented to tackle those challenges facing superconductivity and act as a shortcut towards the full commercialization of SCs and their applications. AI approaches are capable of providing fast, efficient, and accurate solutions for technical, manufacturing, and economic problems with a high level of complexity and nonlinearity in the field of superconductivity. In this paper, the concept of AI and the widely used algorithms are first given. Then a critical topical review is presented for those conducted studies that used AI methods for improvement, design, condition monitoring, fault detection and location of superconducting apparatuses in large-scale power applications, as well as the prediction of critical temperature and the structure of new SCs, and any other related applications. This topical review is presented in three main categories: AI for large-scale superconducting applications, AI for superconducting materials, and AI for the physics of SCs. In addition, the challenges of applying AI techniques to the superconductivity and its applications are given. Finally, future trends on how to integrate AI techniques with superconductivity towards commercialization are discussed.
Chukun Gao et al 2024 Supercond. Sci. Technol. 37 065018
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.
Mohammad Yazdani-Asrami et al 2023 Supercond. Sci. Technol. 36 043501
This paper presents a roadmap to the application of AI techniques and big data (BD) for different modelling, design, monitoring, manufacturing and operation purposes of different superconducting applications. To help superconductivity researchers, engineers, and manufacturers understand the viability of using AI and BD techniques as future solutions for challenges in superconductivity, a series of short articles are presented to outline some of the potential applications and solutions. These potential futuristic routes and their materials/technologies are considered for a 10–20 yr time-frame.
Mohammad Yazdani-Asrami et al 2022 Supercond. Sci. Technol. 35 083002
Along with advancements in superconducting technology, especially in high-temperature superconductors (HTSs), the use of these materials in power system applications is gaining outstanding attention. Due to the lower weight, capability of carrying higher currents, and the lower loss characteristic of HTS cables, compared to conventional counterparts, they are among the most focused large-scale applications of superconductors in power systems and transportation units. In near future, these cables will be installed as key elements not only in power systems but also in cryo-electrified transportation units, that take advantage of both cryogenics and superconducting technology simultaneously, e.g., hydrogen-powered aircraft. Given the sensitivity of the reliable and continuous performance of HTS cables, any failures, caused by faults, could be catastrophic, if they are not designed appropriately. Thus, fault analysis of superconducting cables is crucial for ensuring their safety, reliability, and stability, and also for characterising the behaviour of HTS cables under fault currents at the design stage. Many investigations have been conducted on the fault characterisation and analysis of HTS cables in the last few years. This paper aims to provide a topical review on all of these conducted studies, and will discuss the current challenges of HTS cables and after that current developments of fault behaviour of HTS cables will be presented, and then we will discuss the future trends and future challenges of superconducting cables regarding their fault performance.
X Obradors et al 2024 Supercond. Sci. Technol. 37 053001
In this work, we review recent progress achieved in the use of chemical solution deposition (CSD) based on fluorinated metalorganic precursors to grow superconducting REBa2Cu3O7 (REBCO) films and coated conductors (CCs). We examine, first of all, the advances in optimizing the steps related to the solutions preparation, deposition and pyrolysis based on novel low-fluorine metalorganic solutions. We show that a new type of multifunctional colloidal solutions including preformed nanoparticles (NPs), can be used to introduce artificial pinning centers (APCs). We analyze how to disentangle the complex physico-chemical transformations occurring during the pyrolysis with the purpose of maximizing the film thicknesses. Understanding the nucleation and growth mechanisms is shown to be critical to achieve a fine tuning of the final microstructure, either using the spontaneous segregation or the colloidal solution approaches, and make industrially scalable this process. Advanced nanostructural studies have deeply modified our understanding of the defect structure and its genealogy. It is remarkable the key role played by the high concentration of randomly distributed and oriented BaMO3 (M = Zr, Hf) NPs which enhance the concentration of APCs, such as stacking faults and the associated partial dislocations. Correlating the defect structure with the critical current density Jc(H,T,θ) allows to reach a tight control of the vortex pinning properties and to devise a general scheme of the vortex pinning landscape in the whole H–T phase diagram. We also refer to the outstanding recent achievements in enhancing the vortex pinning strength by shifting the carrier concentration in REBCO films towards the overdoped state, where the pinning energy is maximum and so, record values of critical current densities are achieved. This confirms the performance competitiveness of nanocomposite CCs prepared through the CSD route. We conclude with a short summary of the progress in scaling the CC manufacturing using fluorinated solutions.
Kiruba S Haran et al 2017 Supercond. Sci. Technol. 30 123002
Superconducting technology applications in electric machines have long been pursued due to their significant advantages of higher efficiency and power density over conventional technology. However, in spite of many successful technology demonstrations, commercial adoption has been slow, presumably because the threshold for value versus cost and technology risk has not yet been crossed. One likely path for disruptive superconducting technology in commercial products could be in applications where its advantages become key enablers for systems which are not practical with conventional technology. To help systems engineers assess the viability of such future solutions, we present a technology roadmap for superconducting machines. The timeline considered was ten years to attain a Technology Readiness Level of 6+, with systems demonstrated in a relevant environment. Future projections, by definition, are based on the judgment of specialists, and can be subjective. Attempts have been made to obtain input from a broad set of organizations for an inclusive opinion. This document was generated through a series of teleconferences and in-person meetings, including meetings at the 2015 IEEE PES General meeting in Denver, CO, the 2015 ECCE in Montreal, Canada, and a final workshop in April 2016 at the University of Illinois, Urbana-Champaign that brought together a broad group of technical experts spanning the industry, government and academia.
Neil Mitchell et al 2021 Supercond. Sci. Technol. 34 103001
With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.
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C Fiamozzi Zignani et al 2024 Supercond. Sci. Technol. 37 075009
The superconducting magnet system of the Divertor Tokamak Test (DTT) facility, composed of 18 toroidal field (TF) coils, 6 poloidal field coils and a central solenoid, has been designed and many procurements have been launched. Some manufacturing aspects and some conductor features require characterization under relevant close-to-operative conditions. To confirm the design choices in all details, cryogenic tests in qualified facilities have been foreseen. In this work, the results of the TF samples characterization at the SULTAN facility at the Swiss Plasma Centre (SPC, EPFL) are presented. The 3 week test campaign started on July the 8th, 2022. The DTT TF SULTAN sample was made of two Nb3Sn cable-in-conduit conductor 'legs', namely 'TF-A' and 'TF-B', made with wires produced by Kiswire Advanced Technology, differing for the cabling twist pitch sequence only, and designed to work in DTT at 42.5 kA at 11.9 T peak field. The extensive characterization comprised 3000 electro-magnetic (EM) cycles and two warm-up-cool-down (WUCD) steps, and in detail it included: AC measurements on the virgin conductors, on cyclic loaded conductors and after WUCDs; DC tests at 10.85 T/42.5 kA with intermediate EM cycles at 10.85 T/45 kA before and after WUCDs; DC tests using partial Lorentz force loads, and Minimum Quench Energy tests at 9 T/42.5 kA after cycles and WUCDs. The results of the DC measurement analysis verified the design, in terms of current sharing temperature (Tcs) and critical current (Ic), as both samples are over the minimum acceptance values. In particular, the 'TF-A' sample, characterized by a so-called 'long twist pitch' cabling sequence, showed higher performance without any degradation with loading and WUCD cycles, whereas sample 'TF-B' presented an initial Tcs reduction that afterwards substantially remained unchanged. In terms of strain acting at the Nb3Sn filaments level, this result can be described by a lower effective strain in the 'TF-A' sample. AC losses were measured with a calorimetric method as a function of frequency for each series of AC sinusoidal pulsing measurements, and the characteristic coupling time constants were determined.
Shahin Alipour Bonab et al 2024 Supercond. Sci. Technol. 37 075008
The advent of superconducting bulks, due to their compactness and performance, offers new perspectives and opportunities in many applications and sectors, such as magnetic field shielding, motors/generators, NMR/MRI, magnetic bearings, flywheel energy storage, Maglev trains, among others. The investigation and characterization of bulks typically relies on time-consuming and expensive experimental campaigns; hence the development of effective surrogate models would considerably speed up the research progress around them. In this study, we first produced an experimental dataset containing the levitation and lateral forces between different MgB2 bulks and one permanent magnet under different operating conditions. Next, we have exploited the dataset to develop surrogate models based on Artificial Intelligence (AI) techniques, namely Extremely Gradient Boosting, Support Vector Regressor (SVR), and Kernel Ridge Regression. After the tuning of the hyperparameters of the AI models, the results demonstrated that SVR is the superior technique and can predict levitation and lateral forces with a worst-case accuracy scenario 99.86% in terms of goodness of fit to experimental data. Moreover, the response time of these models for the estimation of new datapoints is ultra-fast.
A Prudnikava et al 2024 Supercond. Sci. Technol. 37 075007
In order to determine optimal parameters of vacuum thermal processing of superconducting radiofrequency niobium cavities exhaustive information on the initial chemical state of niobium and its modification upon a vacuum heat treatment is required. In the present work the chemical composition of the niobium surface upon ultra-high vacuum baking at 200 ∘C–400 ∘C similar to 'medium-temperature baking' and 'furnace baking' of cavities is explored in-situ by synchrotron x-ray photoelectron spectroscopy (XPS). Our findings imply that below the critical thickness of the layer () niobium starts to interact actively with surface impurities, such as carbon and phosphorus. By studying the kinetics of the native oxide reduction, the activation energy and the rate-constant relation have been determined and used for the calculation of the oxygen-concentration depth profiles. It has been established that the controlled diffusion of oxygen is realized at temperatures 200 ∘C–300 ∘C, and the native-oxide layer represents an oxygen source, while at 400 ∘C the pentoxide is completely reduced and the doping level is determined by an ambient oxygen partial pressure. Fluorine (F to Nb atomic ratio is 0.2) after the buffered chemical polishing was found to be incorporated into the surface layer probed by XPS (), and its concentration increased during the low-temperature baking (F/Nb = 0.35 at 230 ∘C) and depleted at higher temperatures (F/Nb = 0.11 at 400 ∘C). Thus, the influence of fluorine on the performance of mid-T baked, nitrogen-doped and particularly mild-baked (120 ∘C/48 h) cavities must be considered. The possible role of fluorine in the educed reaction under the impact of an x-ray beam at room temperature and during the thermal treatment is also discussed. The range of temperature and duration parameters of the thermal treatment at which the niobium surface would not be contaminated with impurities is determined.
Shengchen Xue et al 2024 Supercond. Sci. Technol. 37 075006
Current sharing between RE–Ba–Cu–O (REBCO, RE = rare earth) tapes within a high-temperature superconducting coil or cable is important to avoid damage from uncontrolled quench of superconducting devices operating at high currents. Current sharing between REBCO tapes is found to be limited by the contact resistivity between adjacent tapes, which is about 20x higher in the REBCO-facing-substrate (face-to-back) configuration that is commonly used in devices compared to a REBCO-facing-REBCO (face-to-face) configuration. Double-sided REBCO tapes always offer face-to-face contacts between adjacent tapes, and this benefit of excellent current sharing has been validated in experiments wherein an artificial defect is introduced in one tape in a 2-ply tape stack. Additionally, current sharing between the two REBCO layers within one double-sided REBCO tape has also been investigated. Slotting of the double-sided tapes, wherein slots through the insulating buffer stack are filled with a conductive material, has been found to significantly enhance the current sharing from one REBCO layer to the opposite layer.
Kai Zhang 2024 Supercond. Sci. Technol. 37 070501
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X Obradors et al 2024 Supercond. Sci. Technol. 37 053001
In this work, we review recent progress achieved in the use of chemical solution deposition (CSD) based on fluorinated metalorganic precursors to grow superconducting REBa2Cu3O7 (REBCO) films and coated conductors (CCs). We examine, first of all, the advances in optimizing the steps related to the solutions preparation, deposition and pyrolysis based on novel low-fluorine metalorganic solutions. We show that a new type of multifunctional colloidal solutions including preformed nanoparticles (NPs), can be used to introduce artificial pinning centers (APCs). We analyze how to disentangle the complex physico-chemical transformations occurring during the pyrolysis with the purpose of maximizing the film thicknesses. Understanding the nucleation and growth mechanisms is shown to be critical to achieve a fine tuning of the final microstructure, either using the spontaneous segregation or the colloidal solution approaches, and make industrially scalable this process. Advanced nanostructural studies have deeply modified our understanding of the defect structure and its genealogy. It is remarkable the key role played by the high concentration of randomly distributed and oriented BaMO3 (M = Zr, Hf) NPs which enhance the concentration of APCs, such as stacking faults and the associated partial dislocations. Correlating the defect structure with the critical current density Jc(H,T,θ) allows to reach a tight control of the vortex pinning properties and to devise a general scheme of the vortex pinning landscape in the whole H–T phase diagram. We also refer to the outstanding recent achievements in enhancing the vortex pinning strength by shifting the carrier concentration in REBCO films towards the overdoped state, where the pinning energy is maximum and so, record values of critical current densities are achieved. This confirms the performance competitiveness of nanocomposite CCs prepared through the CSD route. We conclude with a short summary of the progress in scaling the CC manufacturing using fluorinated solutions.
Joshua Feldman et al 2024 Supercond. Sci. Technol. 37 033001
Construction of high-temperature superconducting magnets typically involves impregnation of a coil in a liquid medium, such as epoxy, which is then solidified. This impregnation provides mechanical integrity to the magnet and facilitates heat transfer. The choice of material used for impregnation requires careful consideration of the material properties and the performance requirements in order to ensure optimal magnet operation. This paper offers a comprehensive educational resource on this topic, reviewing the literature available on materials for magnet impregnation. A detailed explanation of considerations for selecting an impregnation material are presented, along with a review of several types of materials and their characteristics as reported in the literature. The materials are compared, and their suitability to different applications is discussed. Topics for future research are suggested.
Zhuoran Geng et al 2023 Supercond. Sci. Technol. 36 123001
We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on the fabrication and characterization of Al/EuS/Cu and EuS/Al/Co-based detectors, along with their noise analysis. Additionally, we suggest some approaches for multiplexing such self-biased detectors.
Arno Godeke 2023 Supercond. Sci. Technol. 36 113001
The steadily increasing magnetic fields that can be generated with superconducting magnets are reaching the limits of what is achievable with low-temperature superconductors (LTS). At the same time, a reduction of fossil-fuel extraction will amplify the already limited availability of helium as a coolant for superconducting magnets in the near future. Hence, manufacturers of commercial applications that rely on superconducting magnets have become increasingly interested in exploring technologies that enable a move beyond the magnetic-field limitations posed by LTS conductors, and/or enable higher operating temperatures to allow for cryogen-free operation. High-temperature superconductors (HTS), such as (REBCO), (Bi-2212), and BiPbxSr2Ca2Cu3O (Bi-2223) have all matured to a certain commercial extent, and have thereby become enablers for such technologies. The emergence of various new commercial magnet-systems that utilize HTS, suggests that we are at the dawn of a wider commercial implementation. A review of which HTS properties are critical for these magnets, what is currently available, and what is missing, is therefore considered timely and appropriate in this context.
D J Gameiro Carvalho et al 2023 Supercond. Sci. Technol. 36 103001
Different electromagnetic formulations were proposed and implemented in finite element (FE) software to model high-temperature superconductors-coated conductors (HTS-CCs) and HTS tape topologies. However, their modelling can be notably demanding in computational resources, particularly computation time. Mixed formulations such as , T − A, and were proposed and used, proving to be considerably faster than conventional ones, although these formulations present different performances and characteristics depending on the modelled conditions and geometry. This paper reviews the electromagnetic formulations proposed in the literature for FE simulation of HTS-coated conductors and HTS tape topologies. Implementation aspects, which are lacking in the literature, are presented, especially for T − A and formulations developed for most relevant tape topologies, for example, HTS CC stacked (CCS) tape and HTS twisted tapes. Simulation results are analysed, alongside the consequent conclusions regarding the accuracy, as well as advantages and limitations of each formulation, all made taking into account each tape geometry and its operating conditions. Their implementation review will be straightforward in the case of formulation and formulation. In advance, the T − A formulation is shown to be the most efficient FE formulation for HTS-CC topologies, being, among the studied, the most efficient computational resource. Moreover, its inherent approximation of the HTS tape as a thin sheet has delivered accurate results, specifically regarding current density distributions in the HTS layer and AC losses when compared with the formulation. Correspondingly, FE multiphysics simulations are shown for three HTS-CC topologies: a single HTS tape, an HTS CCS tape, and an HTS twisted-stacked tape cable.
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Abdioglu et al
This paper presents a study of the current carrying capacity and AC loss of high-temperature superconducting (HTS) stacks to be used in busbar applications for all-electric aircraft. A 2D model was developed using COMSOL Multiphysics with a T-A formulation for detailed analysis. The study began by applying a stable 20 kA DC offset current to the HTS stacks, to simulate practical operating conditions. Firstly, the behaviour of the critical current was studied under self-field conditions for stacks with different number of HTS tapes and spacing. Secondly, AC ripple currents were introduced together with DC offsets and the effects of 3rd and 5th harmonic distortions (HD) were studied.
The results show that configurations with 40 tapes and gaps of more than 2 mm are considered suitable for safe current transport under DC conditions. On the other hand, increasing the tape spacing leads to an increase in the safe transport current ripple, due to the reduced magnetic field interaction within the stack. In addition, the transport loss decreases as the air gap increases due to the reduction in the self-field, whereas it increases as the number of strips increases. The influence of the 3rd HD on the transport loss is minimal at a ripple current of 1% and slightly noticeable at 2%. However, it becomes more obvious as the ripple current approaches the critical value. Remarkably, even cases with equivalent total harmonic distortion (THD) show significantly higher transport losses when characterised by higher 5th HD than their counterparts with 3rd HD.
This comprehensive analysis provides valuable information on the performance characteristics of HTS stacks in all-electric aircraft busbar applications and offers important insights for the development and optimisation of these systems in practical aerospace applications.
Yin et al
Superconducting nanowire single-photon detectors (SNSPDs) have been widely applied in quantum information and deep-space exploration owing to their high detection efficiency, low dark count rate, and wide spectral response. In particular, the heat transfer in SNSPDs largely affects their performance parameters (e.g., quantum efficiency, count rates and recovery time), which can be modulated to optimize the performance and develop novel devices. Considering the potential of SNSPDs and the significance of heat transfer, the most recent progress toward understanding the mechanism and the modulation of heat transfer in SNSPDs is critically reviewed, with particular emphasis on the macroscopic correlation with device performance parameters and the microscopic analysis of existing theories, especially at interfaces. Furthermore, representative novel devices inspired by the need for heat transfer modulation in SNSPDs are discussed in detail. Finally, the development of heat transfer in SNSPDs is placed in perspective with the aim of highlighting future theoretical directions and practical applications.
Asaduzzaman et al
We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using muon spin rotation (μSR). Using thin Ag foils as energy moderators for the implanted muon spin-probes, we "profile" the vortex penetration field μ0Hvp at sub-surface depths on the order of ~10 μm to ~100 μm. In a coated sample [Nb3Sn(2 μm)/Nb], we find that μ0Hvp is depth-independent with a value of 234.5(35) mT, consistent with Nb's metastable superheating field and suggestive of surface energy barrier for flux penetration. Conversely, in a surface-treated sample [Nb baked in vacuum at 120 ○C for 48 h], vortex penetration onsets close to pure Nb's lower critical field μ0Hc1 ≈ 170 mT, but increases with increasing implantation depth, consistent with flux-pinning localized at the surface. The implication of these results for technical applications of superconducting Nb, such as superconducting radio frequency (SRF) cavities, is discussed.
Shao et al
We performed a comprehensive investigation of the relationship between the maximum operating frequency of the NbN superconducting frequency divider and the key parameters of the NbN Josephson junction. We designed a superconducting frequency divider that uses a single NbN Josephson junction as a pulse generator, several NbN Josephson transmission lines (JTLs), an NbN Toggle Flip-flop (TFF), and a load. By comparing the bias voltage Vin of the pulse generator with the output voltage Vout of the load, we determined if the circuit was working correctly and calculated the maximum operating frequency fmax. Additionally, we employed JSICsim software for simulations to analyze the impact of key parameters of the NbN Josephson junction, such as critical current density Jc, gap voltage Vg, characteristic voltage Vc, quality factor Q, and specific capacitance Cs, on fmax. The simulation results demonstrate that fmax increases with increasing Jc and Q. Specifically when Jc exceeds 300 kA/cm2 and Q surpasses 4, the superconducting frequency divider can achieve a fmax of 1 THz. Furthermore, we successfully fabricated a superconducting frequency divider using a 10 kA/cm2 process and tested a fmax of 260 GHz, with a deviation of approximately 6% from the simulation results.
Zheng et al
The global shift towards sustainable development and technological advancements has propelled the energy transition trend. Recognizing the substantial environmental impact of conventional commercial airplanes, there is a growing urgency to develop a sophisticated superconducting motor system for commercial aviation. The advent of high-temperature superconducting motors presents a transformative leap, offering significant advantages in power density and efficiency when compared to traditional motors. To validate the issues that future liquid-hydrogen superconducting electric airplanes may encounter, a kilowatt-class aerospace high-temperature superconducting motor is designed. Based on the requirements of airborne applications, critical parameters such as electromagnetic characteristics, operating characteristics, and AC losses have been analyzed. Furthermore, extensive research and testing have been conducted on the superconducting motor magnet, leading to the successful assembly of a prototype. The superconducting motor has a rated output power of 2.7kW and a rated speed of 5000rpm . Rigorous ground operation performance tests have also been conducted to ensure the feasibility and reliability of the motor in practical applications. Benefiting from the topological structure design, the superconducting motor has an excellent sealing performance at low temperatures. The superconducting motor can maintain low temperature and high vacuum for a long time, when the vacuum pump is removed and the liquid nitrogen inlet is closed after the motor is completely cooled. The culmination of these endeavors is the realization of a successful flight validation of an unmanned aerial vehicle (UAV) equipped with a high-temperature superconducting motor, demonstrating a sustained flight of nearly one hour.
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C Fiamozzi Zignani et al 2024 Supercond. Sci. Technol. 37 075009
The superconducting magnet system of the Divertor Tokamak Test (DTT) facility, composed of 18 toroidal field (TF) coils, 6 poloidal field coils and a central solenoid, has been designed and many procurements have been launched. Some manufacturing aspects and some conductor features require characterization under relevant close-to-operative conditions. To confirm the design choices in all details, cryogenic tests in qualified facilities have been foreseen. In this work, the results of the TF samples characterization at the SULTAN facility at the Swiss Plasma Centre (SPC, EPFL) are presented. The 3 week test campaign started on July the 8th, 2022. The DTT TF SULTAN sample was made of two Nb3Sn cable-in-conduit conductor 'legs', namely 'TF-A' and 'TF-B', made with wires produced by Kiswire Advanced Technology, differing for the cabling twist pitch sequence only, and designed to work in DTT at 42.5 kA at 11.9 T peak field. The extensive characterization comprised 3000 electro-magnetic (EM) cycles and two warm-up-cool-down (WUCD) steps, and in detail it included: AC measurements on the virgin conductors, on cyclic loaded conductors and after WUCDs; DC tests at 10.85 T/42.5 kA with intermediate EM cycles at 10.85 T/45 kA before and after WUCDs; DC tests using partial Lorentz force loads, and Minimum Quench Energy tests at 9 T/42.5 kA after cycles and WUCDs. The results of the DC measurement analysis verified the design, in terms of current sharing temperature (Tcs) and critical current (Ic), as both samples are over the minimum acceptance values. In particular, the 'TF-A' sample, characterized by a so-called 'long twist pitch' cabling sequence, showed higher performance without any degradation with loading and WUCD cycles, whereas sample 'TF-B' presented an initial Tcs reduction that afterwards substantially remained unchanged. In terms of strain acting at the Nb3Sn filaments level, this result can be described by a lower effective strain in the 'TF-A' sample. AC losses were measured with a calorimetric method as a function of frequency for each series of AC sinusoidal pulsing measurements, and the characteristic coupling time constants were determined.
Shahin Alipour Bonab et al 2024 Supercond. Sci. Technol. 37 075008
The advent of superconducting bulks, due to their compactness and performance, offers new perspectives and opportunities in many applications and sectors, such as magnetic field shielding, motors/generators, NMR/MRI, magnetic bearings, flywheel energy storage, Maglev trains, among others. The investigation and characterization of bulks typically relies on time-consuming and expensive experimental campaigns; hence the development of effective surrogate models would considerably speed up the research progress around them. In this study, we first produced an experimental dataset containing the levitation and lateral forces between different MgB2 bulks and one permanent magnet under different operating conditions. Next, we have exploited the dataset to develop surrogate models based on Artificial Intelligence (AI) techniques, namely Extremely Gradient Boosting, Support Vector Regressor (SVR), and Kernel Ridge Regression. After the tuning of the hyperparameters of the AI models, the results demonstrated that SVR is the superior technique and can predict levitation and lateral forces with a worst-case accuracy scenario 99.86% in terms of goodness of fit to experimental data. Moreover, the response time of these models for the estimation of new datapoints is ultra-fast.
A Prudnikava et al 2024 Supercond. Sci. Technol. 37 075007
In order to determine optimal parameters of vacuum thermal processing of superconducting radiofrequency niobium cavities exhaustive information on the initial chemical state of niobium and its modification upon a vacuum heat treatment is required. In the present work the chemical composition of the niobium surface upon ultra-high vacuum baking at 200 ∘C–400 ∘C similar to 'medium-temperature baking' and 'furnace baking' of cavities is explored in-situ by synchrotron x-ray photoelectron spectroscopy (XPS). Our findings imply that below the critical thickness of the layer () niobium starts to interact actively with surface impurities, such as carbon and phosphorus. By studying the kinetics of the native oxide reduction, the activation energy and the rate-constant relation have been determined and used for the calculation of the oxygen-concentration depth profiles. It has been established that the controlled diffusion of oxygen is realized at temperatures 200 ∘C–300 ∘C, and the native-oxide layer represents an oxygen source, while at 400 ∘C the pentoxide is completely reduced and the doping level is determined by an ambient oxygen partial pressure. Fluorine (F to Nb atomic ratio is 0.2) after the buffered chemical polishing was found to be incorporated into the surface layer probed by XPS (), and its concentration increased during the low-temperature baking (F/Nb = 0.35 at 230 ∘C) and depleted at higher temperatures (F/Nb = 0.11 at 400 ∘C). Thus, the influence of fluorine on the performance of mid-T baked, nitrogen-doped and particularly mild-baked (120 ∘C/48 h) cavities must be considered. The possible role of fluorine in the educed reaction under the impact of an x-ray beam at room temperature and during the thermal treatment is also discussed. The range of temperature and duration parameters of the thermal treatment at which the niobium surface would not be contaminated with impurities is determined.
Md Asaduzzaman et al 2024 Supercond. Sci. Technol.
We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using muon spin rotation (μSR). Using thin Ag foils as energy moderators for the implanted muon spin-probes, we "profile" the vortex penetration field μ0Hvp at sub-surface depths on the order of ~10 μm to ~100 μm. In a coated sample [Nb3Sn(2 μm)/Nb], we find that μ0Hvp is depth-independent with a value of 234.5(35) mT, consistent with Nb's metastable superheating field and suggestive of surface energy barrier for flux penetration. Conversely, in a surface-treated sample [Nb baked in vacuum at 120 ○C for 48 h], vortex penetration onsets close to pure Nb's lower critical field μ0Hc1 ≈ 170 mT, but increases with increasing implantation depth, consistent with flux-pinning localized at the surface. The implication of these results for technical applications of superconducting Nb, such as superconducting radio frequency (SRF) cavities, is discussed.
Jianfeng Huang et al 2024 Supercond. Sci. Technol.
The lap-type twin-box joints are integral components in ITER fusion magnets, with profound implications for magnet stability based on their electro-magnetic, thermal, and mechanical properties. Throughout the extensive R&D process, rigorous qualification tests are conducted to meet stringent standards. However, existing tests often prioritize global performance, which lack of strand-level details due to inherent limitations in test setups. Furthermore, as the referencing test facility of SULTAN falls short in replicating relevant ITER operating conditions, numerical methods that offer both accuracy and the requisite level of detail for comprehensive magnet and component analysis and development are necessary.
This paper introduces the utilization of the JackPot-AC/DC code, developed at the University of Twente, as a fundamental tool for achieving strand-level precision in handling CICCs and joints, which encompasses copper and solder components. The primary focus of this study is to obtain precise input parameters, emphasizing their role in conducting a quantitative analysis using JackPot-AC/DC. The investigation centers on an ITER PF5 joint (PFJEU6), where contact resistances and AC losses were measured under parallel magnetic fields. Given the constraints in the measured results, an enhanced parameterization is performed to derive precise resistivity and solder-related parameters. Additionally, sensitivity analyses of individual parameters and cable compact configurations are thoughtfully evaluated. With the optimal input parameters acquired, systematic simulations of the joint exposed to transverse magnetic fields, mimicking SULTAN and ITER operating conditions, are processed and validated against experimental results. This research establishes a comprehensive foundation for the analysis of lap-type twin-box joints, including DC, AC, and stability properties. The outcomes will significantly contribute to advancing the understanding of the intricate behavior of these joints in the context of fusion magnet applications.
Jingnan Cai et al 2024 Supercond. Sci. Technol.
We consider, for the first time, the effects of strong capacitive and inductive coupling between radio frequency Superconducting Quantum Interference Devices (rf SQUIDs) in an overlapping metamaterial geometry when driven by rf flux at and near their self-resonant frequencies. The equations of motion for the gauge-invariant phases on the Josephson junctions in each SQUID are set up and solved. Our model accounts for the high-frequency displacement currents through capacitive overlap between the wiring of SQUID loops. We begin by modeling two overlapping SQUIDs and studying the response in both the linear and nonlinear high-frequency driving limits. By exploring a sequence of more and more complicated arrays, the formalism is eventually extended to the N × N × 2 overlapping metamaterial array, where we develop an understanding of the many (8N2-8N+3) resulting resonant modes in terms of three classes of resonances. The capacitive coupling gives rise to qualitatively new self-resonant response of rf SQUID metamaterials, and is demonstrated through analytical theory, numerical modeling, and experiment in the 10-30 GHz range on capacitively and inductively coupled rf SQUID metamaterials.
Paul Huslage et al 2024 Supercond. Sci. Technol.
We designed and constructed two non-planar coils with high-temperature superconductors (HTS) based on shapes from the Wendelstein 7-X stellarator. 
Tape track orientation of the HTS was optimized to reduce the coil size as much as possible while staying within the strain limits of the Gadolinium Barium Copper Oxide (GdBCO) superconductor. This resulted in average coil radii of 0.23 m and 0.48 m at strain limits of up 0.45 % to for the coil shapes that were chosen.
The coils were produced by winding the GdBCO tapes onto 3D-printed plastic frames. 
We confirmed the integrity of the superconducting layer after winding by spatially resolved measurement of the critical current and by energizing the coils in liquid nitrogen. Coil 1 showed a resistance of 1.75 µΩ and did not have any critical current degradation, while coil 5 had a resistance of 195 µΩ and showed only one dropout, attributable to a handling error. We measured the magnetic field of the coil with a 3-axis Hall probe system and found good agreement with predictions. This work demonstrates the manufacturing of small-scale, non-planar magnetic coils from commercially available HTS.
Kai Walter et al 2024 Supercond. Sci. Technol. 37 075002
The superconducting properties of SmBa2Cu3O7−δ (SmBCO) thin films are predominantly influenced by the oxygen deficiency δ. Yet, the established methods to determine δ such as iodometric titration or thermogravimetry cannot be applied to thin films due to their very small volume. Therefore, an alternative way to determine δ for SmBCO thin film samples using x-ray diffraction (XRD) is presented. Main point of this analysis is the structural relationship between the a, b and c lattice parameters and δ. A linear relationship between c and δ is found in SmBCO powder samples for both the orthorhombic and tetragonal phase. Furthermore, an attempt is made to quantify the chemical composition using time-of-flight secondary ion mass spectrometry. This attempt was inconclusive because of drastically changing ion yields due to δ influencing the valence state of the analyzed ions. The crystal structural relationship gathered from the powder samples is applied to thin film samples. Thereby, it becomes clear that thermal strain is affecting the crystal structure of the thin films. A simple correction model is used to correct for thermal strain and a good match between powder, literature, and thin film data is achieved and thus a non-destructive way for the determination of δ using XRD.
G Aubert et al 2024 Supercond. Sci. Technol. 37 075001
Gradient-magnet interactions increase with higher field magnets and stronger gradient coils. Perhaps the most sensitive aspect is magnet quench, which can be induced by a loss of superconductivity of the main coil caused by a rise of the temperature of the He bath with gradient activity. Predicting power depositions thereby can be a very valuable tool to avoid dangerous frequency zones but also eventually correct design flaws. In this work, we report model predictions compared to measurements of power deposition in the He bath of the Iseult 11.7 T magnet for the Z gradient coil axis.
Nicholas M Strickland et al 2024 Supercond. Sci. Technol.
Ba1-xKxFe2As2 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 in-field critical currents (Ic) 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/cm2 in low (optimal) field and 22 kA/cm2 in 8 T. In the low-field region we observe an anomalous and sharp suppression of Ic centred at zero field. This feature is non-hysteretic for lower temperatures and perpendicular field, but becomes hysteretic for higher temperatures in perpendicular field and all temperatures in parallel field. The low-field suppression is reflected also in the n-values which can otherwise be very high, in excess of 100, in optimal field. Magnetic-field hysteresis of Ic 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 Ic anomaly also likely has its origin in the planar granularity, while magnetic phases in grains or grain boundaries may also play a role.