Superconductor Science and Technology

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.

The attenuation of magnetic fields is crucial for various application fields, including health, space exploration, and fundamental physics, to name just a few. Superconductors are key materials for addressing this challenge. In this review, we mainly focus on the shielding and screening of quasi-static magnetic fields using superconductor-based passive layouts. After providing a brief overview of the principles of magnetic shielding and screening using superconductors, we outline commonly used procedures for measuring the field attenuation. Next, we give an insight into analytical and numerical models able to reproduce experimental results and predict the performances of new designs. Key challenges and achievements in employing low temperature or high temperature superconducting bulk and tape-based structures for reducing a given applied field are then discussed. Additionally, hybrid designs combining superconducting and ferromagnetic materials, aimed at enhancing the shielding ability or fabricating magnetic cloaks, are described. Finally, we highlight future challenges and potential advancements in this technology.

A low-AC loss Rare-earth barium copper oxide (REBCO) cable, based on the VIPER cable technology has been developed by commonwealth fusion systems for use in high-field, compact tokamaks. The new cable is composed of partitioned and transposed copper 'petals' shaped to fit together in a circular pattern with each petal containing a REBCO tape stack and insulated from each other to reduce AC losses. A stainless-steel jacket adds mechanical robustness—also serving as a vessel for solder impregnation—while a tube runs through the middle for cooling purposes. Additionally, fiber optic sensors are placed under the tape stacks for quench detection (QD). To qualify this design, a series of experiments were conducted as part of the SPARC tokamak central solenoid (CS) model coil program—to retire the risks associated with full-scale, fast-ramping, high-flux high temperature superconductors CS and poloidal field coils for tokamak fusion power plants and net-energy demonstrators. These risk-study and risk-reduction experiments include (1) AC loss measurement and model validation in the range of ∼5 T s⁻¹, (2) an IxB electromagnetic (EM) loading of over 850 kN m⁻¹ at the cable level and up to 300 kN m⁻¹ at the stack level, (3) a transverse compression resilience of over 350 MPa, (4) manufacturability at tokamak-relevant speeds and scales, (5) cable-to-cable joint performance, (6) fiber optic-based QD speed, accuracy, and feasibility, and (7) overall winding pack integration and magnet assembly. The result is a cable technology, now referred to as PIT VIPER, with AC losses that measure fifteen times lower (at ∼5 T s⁻¹) than its predecessor technology; a 2% or lower degradation of critical current (I_c) at high IxB EM loads; no detectable I_c degradation up to 600 MPa of transverse compression on the cable unit cell; end-to-end magnet manufacturing, consistently producing I_c values within 7% of the model prediction; cable-to-cable joint resistances at 20 K on the order of ∼15 nΩ; and fast, functional QD capabilities that do not involve voltage taps.

The discovery of rare-earth barium copper oxide (REBCO) materials with high critical temperatures, and the continued advancements in the fabrication of REBCO coated conductors with extremely high critical current densities, has enabled the development of ultra-high-field (>20 T) compact and large-scale thermonuclear fusion devices. At present, around a dozen global commercial manufacturers are able to supply high-quality REBCO coated conductors with excellent performance. Significant advancements have been made for high-temperature, low-field applications such as motors, generators, long-length transmission cables, and so on using REBCO coated conductors. Nonetheless, multiple ongoing critical challenges under low-temperature, high-field conditions, such as irreversible degradation of the critical current, along with insufficient mechanical protection and inadequate reduction of AC losses, remain unsolved, collectively hindering their utilization in high-field thermonuclear fusion reactors. This paper provides a comprehensive theoretical and technical review of the current state-of-the-art, associated challenges, and prospects in the research and development (R&D) of REBCO coated conductors, cables, and magnet systems for high-field fusion. It highlights the significant enhancements in current-carrying capacity, mechanical protection, and AC loss reduction achieved over the past decade. The paper delves into detailed analyses of potential cabling solutions that offer exceptional current-carrying capacity while ensuring an optimal inductance balance for toroidal, poloidal, and central solenoid coils in tokamak devices. This work endeavours to lay the groundwork for the R&D of the next-generation REBCO magnets to facilitate the construction of ultra-high-field compact and large-scale tokamak reactors.

Twisted, stacked cable-in-conduit-conductors, including VIPER cables, have emerged as a popular choice for high temperature superconductor fusion applications. The time-varying magnetic fields these cables are exposed to can generate significant AC losses, which are important to quantify for the design of fusion magnets. Although AC loss in twisted tapes and stacks—including VIPER cables—has been the subject of increasing research, many studies focus on higher temperatures and low fields outside the range applicable to fusion, while those considering higher fields tend to provide a less detailed analysis of the loss. This work provides a fundamental examination of the hysteresis loss characteristics of VIPER tapes, stacks and cables under conditions relevant to fusion applications. 3D finite element method models implemented with H-φ formulation are used to simulate the loss at 20, 40 and 77 K, under applied magnetic fields of up to 20 T. Cables with up to 10 tapes per stack are considered. The field-angle dependence of critical current and n-value are accounted for, based on measured data from 4 mm Faraday Factory tape. Results show that hysteresis loss in VIPER strands is independent of pitch length and winding radius, a valuable result for shortening simulation time. A semi-empirical method is proposed to estimate the loss in VIPER cables from 2D simulations of flat stacks, supplementing the established $2/{\pi}$ relationship. It is also shown that hysteresis loss in VIPER geometries can be scaled across temperatures by normalizing with the self-field critical current of a single tape, surprisingly irrespective of cable ${I_{\text{c}}}$.

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.

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.

The vortex dynamics of an undoped uniaxially textured YBa₂Cu₃O_7−δ (YBCO) superconducting film grown onto a MgO (100) substrate was inspected by applying the vortex glass and collective-pinning models. The texture and structural characterization studied by X-ray diffraction revealed a uniaxially (00 l) YBCO layer, which coexists with minor Y₂BaCuO₅ and CuO phases. The temperature dependence of the magnetization in the superconducting state revealed a critical temperature ${T_{\text{C}}}$ = 88 K. By measuring the hysteresis loops ($M\left( H \right)$) at 10–70 K, the critical fields ${H_{{\text{C}}1}},{\text{ }}{H_{{\text{C}}2}},{\text{ }}{H_{\text{P}}}$ and ${H_{{\text{irr}}}}$ were estimated and a vortex matter diagram was sketched. By using the Bean model, the critical current density values ${J_{\text{C}}}\left( {T,H} \right)$ were obtained and the typical peak effect is observed. The vortex dynamics mechanism is discussed taking into account four vortex feature regimes in the double-logarithmical ${J_{\text{C}}}\left( H \right)$ curves. The vortex pinning mechanism is discussed by obtaining the pining force, ${F_{\text{P}}}$, its normalization, $f\left( h \right)$, and magnetic relaxation $M\left( t \right)$ measurements taken in field cooling mode at 10–60 K. The glassy exponent $\mu$ and the characteristic energy $U\left( {J,{\text{ }}T} \right)$ in the vortex glass model were estimated following the Maley method. The collective-pinning model is used to discuss the possible vortex regimes mechanism (individual flux lines, small bundles and large bundles of pinned flux). Eventually, the $E\left( J \right)$ curves, expressed from the swept field and creep measurements, show a power-law behaviour, in agreement with the vortex matter.

In many high-temperature superconducting applications, the advantages of no-insulation (NI) coils, such as self-protecting capability and thermal stability, make them a promising alternative to insulated (INS) coils. Magnetisation loss will be generated when the coil is exposed to time-varying magnetic fields. This loss can vary with the applied field angle, magnitude, and frequency, resulting in parasitic heat loads. In this study, we investigate magnetization loss in NI and INS double-pancake and double-racetrack coils of identical dimensions, experimentally and numerically. Experiments were conducted at 77 K under external AC magnetic fields up to 100 mT, considering various field angles (0°–90°) and frequencies (73–146 Hz). The experimental results are compared with the finite element simulation results of the coils' three-dimensional models. Interestingly, NI coils exhibit no significant angular dependence of loss within a specific field range; however, beyond this range loss increases with increasing field angles. In contrast, the loss in INS coils consistently increases with decreasing field angles across the entire field range. Coil level shielding of the magnetic field is observed in NI coils under parallel fields which is similar to a bulk superconductor. The losses in INS and NI coils are comparable under a perpendicular magnetic field, which can be attributed to the dominance of superconducting currents, as confirmed by the current and field distributions observed in simulations.

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.

Kagome materials possess intriguing properties and have attracted considerable interest. Inspired by the extensive research on kagome superconductors, here, we investigated the superconducting and topological properties of the trilayer kagome metals Rh₃M₂S₂ (M = Pb, In, Tl) via first-principles calculations. The calculated electron–phonon coupling parameters, which are mainly contributed by the in-plane electronic orbitals of Rh and the in-plane atomic vibrations of Rh and M (=Pb, In, Tl), indicate the shandite compounds Rh₃M₂S₂ (M = Pb, In, Tl) are weak superconductors. By solving the Allen-Dynes modified McMillan formula, the superconducting critical temperatures T_c are estimated to be 1.03 (1.43), 2.31 (2.10) and 5.39 (6.88) K for M = Pb, In, Tl without (with) the spin-orbital coupling (SOC) effect, respectively. Based on the computation of topological invariants and surface states, Rh₃M₂S₂ (M = Pb, In, Tl) can be categorized as Z₂ topological metals when the SOCs are considered. This work unveils the superconductivity and nontrivial band topology of Rh₃M₂S₂ (M = Pb, In, Tl), and will facilitate the search for kagome superconductors in shandite-type materials.

Several superconducting motor designs with a power density exceeding 20 kW kg⁻¹ have been proposed and are under development. However, maintaining the stable operation of superconducting coils in a rotating environment remains a partially unresolved challenge. The no-insulation (NI) high-temperature superconductor (HTS) winding technique, which deliberately removes insulation materials between turns, has emerged as a potential solution for superconducting motors. NI HTS coils have demonstrated current bypassing characteristics, making them stable under external field disturbances and robust against local critical current drops. However, contact resistivity, which largely governs the 'NI behavior,' has been reported to fall within a broad range in prior studies, making it challenging to consistently obtain specific values. As a result, understanding and managing contact resistivity has become a significant area of research. This is especially critical in motors where the field winding is subjected to periodic external harmonic fields, and maintaining pole-to-pole balance is crucial. In this study, we investigated the influence of contact resistivity on the performance of field windings through simulation as part of our efforts to assess the potential risks associated with NI HTS motors. Torque, voltage, and current were analyzed using the finite element method coupled with a lumped parameter circuit model, and the results were compared.

K-doped BaAs₂Fe₂ (K-Ba122) superconductor is a promising material for applications. However, it has been found challenging to achieve high critical current density (J_c) in untextured bulk sample. In this paper we investigated bulk samples prepared by varying the milling energy density, which affects the grain and grain boundary microstructures, and we investigated their magnetic performance to better understand what causes their different J_c. We found that in our samples, which all have small grain size, T_c does not appear directly correlated to J_c. Moreover, AC susceptibility reveals in at least one case obvious signs of multiscale supercurrents, not caused by granularity but that directly influence the overall J_c performance. Considering the microstructural features and the magnetization response we ascribed the J_c differences to lack of connectivity on a larger scale due to nano-cracks at some grain boundaries, which subdivided the samples into macroscopic regions and inevitably limited the overall performance. We discuss possible routes to overcome those extrinsic current-blocking defects.

The High Magnetic Field Laboratory of China (CHMFL) is developing high-temperature superconducting (HTS) magnets for the upgraded 55–60 T magnet. This is the first Bi2212 cable-in-conduit conductor (CICC) fabricated in the CHMFL with a rectangular-shape to serve the hybrid magnet. The fabrication of the Bi2212 CICC has technical challenges that must be overcome to optimize the critical performance in engineering applications. This paper details the fabrication technology, including cabling, jacketing, heat treatment (HT), and leakage detection, with critical current (I_c) tests at low temperatures. The Bi2212 conductor has the characteristics of rectangular, long twist pitch, low void fraction, and large deformation of the wires and sub-cables. It has no central cooling channel and soft-silver tube, smaller in size and higher in engineering current density (J_e), similar to the real conductor. The rectangular Bi2212 CICC, including 60 superconducting wires, exhibits 34.5 kA at 15 K and exceeds 40 kA at 4.2 K. The current sharing temperature (T_cs) was up to 40 K at 10 kA, and no quench occurred. The resistance of the terminal joints reaches the nΩ-level. This paper presents a detailed fabrication technology of the Bi2212 CICC for the future engineering design of HTS magnets.

The study of irradiation effects in cuprate high temperature superconductors (HTS) has been a topic of interest since their discovery. Enormous progress in the understanding of vortex physics and pinning mechanisms was made in the early 1990s through the irradiation of HTS single crystals with a variety of particles over broad ranges of energies. For YBa₂Cu₃O₇ (YBCO), the overall conclusion was that irradiation could increase the critical current density (J_c) by orders of magnitude. The interpretation of the results was simplified by the fact that the pristine crystals were very clean, with few pinning centers and quite low J_c, thus essentially all pinning in the irradiated crystals could be attributed to the controllably added disorder. The case of the ReBa₂Cu₃O₇ (ReBCO, where Re = Y, a Rare Earth, or combinations of them) epitaxial thin films and coated conductors (CC) is more complex, because the pre-irradiation samples already have high J_c due to the presence of large densities of strong pinning centers, which are fabrication-method and processing dependent. The most popular and efficient method to further increase J_c in CC has been the incorporation of artificial pinning centers (APC) by chemical incorporation of second phases. Efforts by many groups worldwide demonstrated that a diversity of APC can be effective, and it is now clear that mixed pinning landscapes, nanoengineered by the combination of defects of various shapes and sizes, produce the best results. In some cases, particle irradiation is still effective at enhancing J_c in CC, by more modest factors than in the single crystals. Interaction with pre-existing defects cannot be ignored, resulting in both cooperating and competing effects. In this work, I review the vortex pinning generated in YBCO by defects of various geometries (point defects, randomly distributed nanoparticles, aligned or splayed columnar) created either by particle irradiation, incorporation of second phases, or combinations of both routes, and discuss some implications of those results for the design of fusion reactors CC magnets.

The study of irradiation effects in cuprate high temperature superconductors (HTS) has been a topic of interest since their discovery. Enormous progress in the understanding of vortex physics and pinning mechanisms was made in the early 1990s through the irradiation of HTS single crystals with a variety of particles over broad ranges of energies. For YBa₂Cu₃O₇ (YBCO), the overall conclusion was that irradiation could increase the critical current density (J_c) by orders of magnitude. The interpretation of the results was simplified by the fact that the pristine crystals were very clean, with few pinning centers and quite low J_c, thus essentially all pinning in the irradiated crystals could be attributed to the controllably added disorder. The case of the ReBa₂Cu₃O₇ (ReBCO, where Re = Y, a Rare Earth, or combinations of them) epitaxial thin films and coated conductors (CC) is more complex, because the pre-irradiation samples already have high J_c due to the presence of large densities of strong pinning centers, which are fabrication-method and processing dependent. The most popular and efficient method to further increase J_c in CC has been the incorporation of artificial pinning centers (APC) by chemical incorporation of second phases. Efforts by many groups worldwide demonstrated that a diversity of APC can be effective, and it is now clear that mixed pinning landscapes, nanoengineered by the combination of defects of various shapes and sizes, produce the best results. In some cases, particle irradiation is still effective at enhancing J_c in CC, by more modest factors than in the single crystals. Interaction with pre-existing defects cannot be ignored, resulting in both cooperating and competing effects. In this work, I review the vortex pinning generated in YBCO by defects of various geometries (point defects, randomly distributed nanoparticles, aligned or splayed columnar) created either by particle irradiation, incorporation of second phases, or combinations of both routes, and discuss some implications of those results for the design of fusion reactors CC magnets.

The attenuation of magnetic fields is crucial for various application fields, including health, space exploration, and fundamental physics, to name just a few. Superconductors are key materials for addressing this challenge. In this review, we mainly focus on the shielding and screening of quasi-static magnetic fields using superconductor-based passive layouts. After providing a brief overview of the principles of magnetic shielding and screening using superconductors, we outline commonly used procedures for measuring the field attenuation. Next, we give an insight into analytical and numerical models able to reproduce experimental results and predict the performances of new designs. Key challenges and achievements in employing low temperature or high temperature superconducting bulk and tape-based structures for reducing a given applied field are then discussed. Additionally, hybrid designs combining superconducting and ferromagnetic materials, aimed at enhancing the shielding ability or fabricating magnetic cloaks, are described. Finally, we highlight future challenges and potential advancements in this technology.

High-T_c superconducting (HTS) flux pumps are capable of wirelessly powering HTS magnets, and are becoming promising alternatives of driven mode excitation which requires thermally inefficient current leads. HTS transformer-rectifiers, also considered as a type of HTS flux pumps, have drawn broad attention in recent years, since they enabled a number of novel HTS magnet applications. Compared to other types of HTS flux pumps, these devices are clear in physics and circuit topologies, easily controllable, and superior in some key performances. In this work, we aim to give a comprehensive overview on the thriving field of HTS transformer-rectifiers, especially those unconventional ones which do not involve superconducting-to-normal state transition. The work starts with explaining the working principle, including the underlying physics of induction-rectification effect, circuit topologies, and switching methods; followed by introducing design methods and construction considerations for engineering devices; and ends with summarizing research and development status, as well as potential applications of HTS transformer-rectifiers.

The high-temperature superconducting (HTS) closed-loop coil, characterised by shorted coil terminals and the low resistance of HTS conductors, can sustain a persistent DC current with minimal decay. These coils enable the generation of a DC magnetic field without the need for current leads or a power supply during operations, offering several advantages: (i) the development of compact, lightweight and portable DC magnet systems; (ii) the elimination of heat leakage and ohmic losses associated with current leads; and (iii) the removal of magnetic field harmonics caused by current supply. Recent advancements have revealed promising applications for HTS closed-loop coils, including maglev trains, nuclear magnetic resonance, scientific instruments, and energy storage systems. This paper firstly reviews various HTS closed-loop coils constructions, focusing on their distinctive characteristics. Then, the key research aspects of HTS closed-loop coils are overviewed, highlighting the latest advancements in persistent-current joint technologies, excitation methods, current control methods, current decay mechanisms and suppression techniques, simulation models, and quench detection and protection methods. Next, the applications of HTS closed-loop coils are analysed, emphasising their current status and future challenges. A detailed account is provided of our group's progress in developing an electrodynamic suspension train in Changchun, China, where all onboard magnets exclusively utilise HTS closed-loop coils. Finally, suggestions for future research directions are proposed.

The discovery of rare-earth barium copper oxide (REBCO) materials with high critical temperatures, and the continued advancements in the fabrication of REBCO coated conductors with extremely high critical current densities, has enabled the development of ultra-high-field (>20 T) compact and large-scale thermonuclear fusion devices. At present, around a dozen global commercial manufacturers are able to supply high-quality REBCO coated conductors with excellent performance. Significant advancements have been made for high-temperature, low-field applications such as motors, generators, long-length transmission cables, and so on using REBCO coated conductors. Nonetheless, multiple ongoing critical challenges under low-temperature, high-field conditions, such as irreversible degradation of the critical current, along with insufficient mechanical protection and inadequate reduction of AC losses, remain unsolved, collectively hindering their utilization in high-field thermonuclear fusion reactors. This paper provides a comprehensive theoretical and technical review of the current state-of-the-art, associated challenges, and prospects in the research and development (R&D) of REBCO coated conductors, cables, and magnet systems for high-field fusion. It highlights the significant enhancements in current-carrying capacity, mechanical protection, and AC loss reduction achieved over the past decade. The paper delves into detailed analyses of potential cabling solutions that offer exceptional current-carrying capacity while ensuring an optimal inductance balance for toroidal, poloidal, and central solenoid coils in tokamak devices. This work endeavours to lay the groundwork for the R&D of the next-generation REBCO magnets to facilitate the construction of ultra-high-field compact and large-scale tokamak reactors.

Carbon doping has always played an irreplaceable and important role in improving the performance of MgB2 superconductors. However, the excessive suppression of Tc by carbon element and its excessive accumulation at grain boundaries make it difficult to realize its potential to enhance the performance of MgB2 in the preparation of practical materials. In addition, introducing magnetic flux pinning centers that are more effective than interface pinning is another challenge faced by MgB2. In this study, a mixture of carbon nanoparticles and trace rare earth elements (CNPs/REEs) obtained from high-temperature calcination of dicranopteris pedate waste was used as a dopant, which can effectively introduce an appropriate amount of carbon into the MgB2 lattice in short-term low-temperature reactions to enhance the irreversibility field (Hirr) of MgB2. Moreover, the residual carbon nanoparticles and rare earth impurities that have not entered the lattice can be highly dispersed in MgB2, serving as point pinning centers, and at the same time, limiting the growth of MgB2 particles, which further strengthens the role of grain boundary pinning force. The synergistic consequence of these doping effects leads to a significant increase in Jc throughout the entire magnetic field range (0-10 T) at 4.2 and 20 K. As a result, Jc@3T at 4.2, 10 and 20 K increased from 90, 41, and 10 kA/cm2 of the undoped MgB2 to 230, 87, and 20 kA/cm2 of the C-005 sample, respectively. At 20K, the Hirr value reached more than 5T (doping amount x=0.05). It was also found that the irreversibility field is governed by the giant flux creep characteristic as the dopant concentration increases and the main pinning mechanism of the doped samples is still the grain boundary pinning.

Research on high-temperature superconductors has primarily focused on hydrogen-rich compounds, however, the need for extreme pressures limits their practical applications. The X₂MH₆-type structure Mg2IrH6 stands out because it exhibits superconductivity at 160 K under ambient pressure. This study investigates methods to increase the superconducting transition temperature of this structure via atomic substitution and low-pressure treatment and assess the mechanical, thermodynamic, and dynamic stability of structures obtained by substituting Mg and Ir atoms in Mg2IrH6 with elements from the same groups using first-principles calculations. The findings identify 11 stable ternary compounds, 10 of which exhibit superconducting transition temperatures, with three compounds, Mg2CoH6, Mg2RhH6, and Mg2IrH6, exceeding 100 K, classifying them as high-temperature superconductors. Their superconducting figure of merit S values are 2.71, 3.35, and 3.83, respectively, suggesting strong practical application potential. The analysis results indicate that mid-frequency hydrogen phonons significantly enhance superconducting properties via electron-phonon coupling. The band structure study highlights the importance of van Hove singularities near the Fermi level. In addition, electron localization function and Fermi surface topology analyses reveal that the Fermi surface shape and density of states are crucial for increasing superconducting transition temperatures.
Keywords: high-temperature superconductors, electron-phonon coupling, first-principles calculations, hydrogen-rich metallic compounds

Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are extensively utilized in applications subjected to cyclical electromagnetic loads, which can lead to significant degradation of the critical current () due to damage within the superconducting layer. Although previous research has investigated the degradation mechanisms and critical current levels in REBCO CC tapes under mechanical deformation, the mechanical failure behavior under cyclic fatigue loading and its impact on remain insufficiently explored. This study addresses this gap by developing a comprehensive fatigue damage model for REBCO CC tapes, focusing on the mechanisms governing their transport properties under repeated loading cycles and varying stress amplitudes. We propose an improved nonlinear fatigue damage accumulation model to predict the fatigue lifetime and damage progression. Additionally, a phenomenological model incorporating the Weibull distribution is introduced to evaluate degradation. Material parameters for the models were determined from experimental data, and critical current tests were performed at liquid nitrogen temperature following cyclic loading. The results demonstrate that the proposed damage model exhibits pronounced nonlinear behavior, characterized by three distinct stages of damage evolution. Extensive cyclic loading experiments validated the model's reliability and precision. This non-invasive analytical framework provides a robust methodology for quantifying and predicting degradation in REBCO CC tapes under cyclic stress or strain conditions. The findings advance the understanding of fatigue-induced degradation and offer a predictive tool for assessing the performance of REBCO CC tapes in practical applications, contributing to the optimization of their operational lifespan and reliability.

AC losses in the high temperature superconducting (HTS) toroidal field (TF) magnets of the STEP tokamak are analysed, focusing on the transient electromagnetic response of the centre column to the charging and discharging of the central solenoid (CS) and poloidal field (PF) magnets during a plasma initiation scenario. An innovative H-H0-Φ simulation method is proposed to quantify the distribution of hysteresis losses and eddy current losses in the TF limb formed from HTS cables. The study examines the impacts of the peak shift present in the HTS tape critical current data and current sharing within the tapes. Furthermore, the magnitude of the coupling losses is estimated both by theoretical analysis and simulation, and the temperature rise caused by the AC losses under adiabatic conditions is evaluated.

The results indicate that hysteresis losses dominate, with eddy current losses contributing an additional 40% on top of the hysteresis losses. Consideration of the peak shift in the tape's Jc (B,θ) curve reduces hysteresis losses by 13%, while current sharing in the copper stabiliser has no significant impact. Coupling losses add only about 3% on top of the hysteresis loss. The most notable local temperature rise of approximately 7 K occurs towards the ends of the CS and the S1 PF coil, a plasma shaping coil positioned in close proximity to the centre column.

Through detailed simulation and analysis, this study demonstrates the rationality of the AC loss design of the STEP magnet system and provides theoretical support for the application of HTS materials in large fusion magnets, offering insights for future tokamak design and optimization.


We study the Josephson junction where nonreciprocal critical current was induced by the interplay of the $4\pi$-periodic and $2\pi$-periodic current-phase relation of the junction. We take the model of a topological junction which serves as a Josephson diode with nonreciprocal critical currents. For this Josephson diode, we demonstrate an inverse ac Josephson effect where an effective dc voltage is induced by a pure ac driving current. We show that this inverse ac Josephson effect originates from the voltage rectification by the nonreciprocal critical current of the system. We explore the dependence of the induced dc voltage on the amplitude and frequency of the ac driving current and reveal the optimized condition for the inverse ac Josephson effect.

K-doped BaAs₂Fe₂ (K-Ba122) superconductor is a promising material for applications. However, it has been found challenging to achieve high critical current density (J_c) in untextured bulk sample. In this paper we investigated bulk samples prepared by varying the milling energy density, which affects the grain and grain boundary microstructures, and we investigated their magnetic performance to better understand what causes their different J_c. We found that in our samples, which all have small grain size, T_c does not appear directly correlated to J_c. Moreover, AC susceptibility reveals in at least one case obvious signs of multiscale supercurrents, not caused by granularity but that directly influence the overall J_c performance. Considering the microstructural features and the magnetization response we ascribed the J_c differences to lack of connectivity on a larger scale due to nano-cracks at some grain boundaries, which subdivided the samples into macroscopic regions and inevitably limited the overall performance. We discuss possible routes to overcome those extrinsic current-blocking defects.

AC losses in the high temperature superconducting (HTS) toroidal field (TF) magnets of the STEP tokamak are analysed, focusing on the transient electromagnetic response of the centre column to the charging and discharging of the central solenoid (CS) and poloidal field (PF) magnets during a plasma initiation scenario. An innovative H-H0-Φ simulation method is proposed to quantify the distribution of hysteresis losses and eddy current losses in the TF limb formed from HTS cables. The study examines the impacts of the peak shift present in the HTS tape critical current data and current sharing within the tapes. Furthermore, the magnitude of the coupling losses is estimated both by theoretical analysis and simulation, and the temperature rise caused by the AC losses under adiabatic conditions is evaluated.

The results indicate that hysteresis losses dominate, with eddy current losses contributing an additional 40% on top of the hysteresis losses. Consideration of the peak shift in the tape's Jc (B,θ) curve reduces hysteresis losses by 13%, while current sharing in the copper stabiliser has no significant impact. Coupling losses add only about 3% on top of the hysteresis loss. The most notable local temperature rise of approximately 7 K occurs towards the ends of the CS and the S1 PF coil, a plasma shaping coil positioned in close proximity to the centre column.

Through detailed simulation and analysis, this study demonstrates the rationality of the AC loss design of the STEP magnet system and provides theoretical support for the application of HTS materials in large fusion magnets, offering insights for future tokamak design and optimization.


The critical current of superconducting materials, such as magnesium diboride (MgB2) bulk superconductors, is a key parameter influencing their performance in various applications, including magnetic field shielding, MRI, and Maglev systems. Spark Plasma Sintering (SPS) is one of the most efficient methods to fabricate high-quality MgB2, significantly saving fabrication time and controlling grain growth. The fabrication conditions, including temperature, pressure, and dwell time, can affect the critical current density. Traditional methods for estimating critical currents are time-consuming and costly. This study explores the use of advanced artificial intelligence (AI) techniques to develop accurate and efficient models for predicting the critical current in MgB2 bulks with respect to 10 different influential fabrication properties and physical conditions. By using AI algorithms such as Gaussian Process Regression, Extremely Gradient Boosting, and Generalized Regression Neural Network (GRNN) an extremely high accuracy in predictions against the actual experimental data was achieved. By defining and studying the extrapolation scenario, this study goes beyond of simple AI-based estimator model that performs well only within the training range of data. The developed AI models not only reduce the need for extensive experimental campaigns but also offer real-time prediction capabilities, paving the way for faster advancements in the research and development of superconducting technology. Overall, GRNN model demonstrated a good performance for both interpolation and extrapolation tasks with an R-squared of 0.999958 and 0.99521, respectively.

Twisted, stacked cable-in-conduit-conductors, including VIPER cables, have emerged as a popular choice for high temperature superconductor fusion applications. The time-varying magnetic fields these cables are exposed to can generate significant AC losses, which are important to quantify for the design of fusion magnets. Although AC loss in twisted tapes and stacks—including VIPER cables—has been the subject of increasing research, many studies focus on higher temperatures and low fields outside the range applicable to fusion, while those considering higher fields tend to provide a less detailed analysis of the loss. This work provides a fundamental examination of the hysteresis loss characteristics of VIPER tapes, stacks and cables under conditions relevant to fusion applications. 3D finite element method models implemented with H-φ formulation are used to simulate the loss at 20, 40 and 77 K, under applied magnetic fields of up to 20 T. Cables with up to 10 tapes per stack are considered. The field-angle dependence of critical current and n-value are accounted for, based on measured data from 4 mm Faraday Factory tape. Results show that hysteresis loss in VIPER strands is independent of pitch length and winding radius, a valuable result for shortening simulation time. A semi-empirical method is proposed to estimate the loss in VIPER cables from 2D simulations of flat stacks, supplementing the established $2/{\pi}$ relationship. It is also shown that hysteresis loss in VIPER geometries can be scaled across temperatures by normalizing with the self-field critical current of a single tape, surprisingly irrespective of cable ${I_{\text{c}}}$.

Upon reviewing the published version of our work, we identified misprints in the equations, which are corrected here. Additionally, we re-evaluated the oxygen concentration depth profiles. This re-evaluation does not affect the validity of the paper's statements or conclusions.

Stochastic computing (SC) is a form of probabilistic computation that encodes information in the probability of a '1' occurring within a finite-length binary sequence. SC has been investigated for applications in various fields that do not require deterministic and precise computation. A superconducting single-flux-quantum (SFQ) circuit is considered a promising candidate for implementing SC hardware due to its high-speed operation and probabilistic behavior. In this study, we propose a novel large fan-out signal splitter to enable large-scale SFQ-based stochastic arithmetic circuits, addressing the issue of computation accuracy degradation caused by correlations between binary sequences. The proposed signal splitter generates uncorrelated output binary sequences by utilizing superconductor random number generators frequency-synchronized to the input binary sequence. The fan-out can be easily increased by simply adding more superconductor random number generators. We implemented a four-output stochastic number signal splitter using the 10 kA cm⁻² Nb four-layer superconducting circuit fabrication process. Its operation was successfully demonstrated by measuring the average voltage at the input and outputs under continuous high-speed binary sequence input. High-speed operation up to 33.2 GHz was confirmed. The proposed signal splitter uniquely leverages the properties of superconducting circuits, where flux quanta determined by fundamental physical constants serve as the information carrier. We believe this development will significantly advance the realization of practical SFQ-based SC systems.

The attenuation of magnetic fields is crucial for various application fields, including health, space exploration, and fundamental physics, to name just a few. Superconductors are key materials for addressing this challenge. In this review, we mainly focus on the shielding and screening of quasi-static magnetic fields using superconductor-based passive layouts. After providing a brief overview of the principles of magnetic shielding and screening using superconductors, we outline commonly used procedures for measuring the field attenuation. Next, we give an insight into analytical and numerical models able to reproduce experimental results and predict the performances of new designs. Key challenges and achievements in employing low temperature or high temperature superconducting bulk and tape-based structures for reducing a given applied field are then discussed. Additionally, hybrid designs combining superconducting and ferromagnetic materials, aimed at enhancing the shielding ability or fabricating magnetic cloaks, are described. Finally, we highlight future challenges and potential advancements in this technology.

Vacuum thermal treatments (baking) are known to improve the superconducting properties of the RF surface layer of niobium cavities, and are employed as a last processing step to increase their efficiency determined by intrinsic quality factor Q₀. A specific method to perform the baking has been used. It consists in annealing of an evacuated cavity with the local heaters installed on its outer surface in a cryostat which ensures an exterior vacuum and protects the outer cavity surface from oxidation. Such a set-up has a number of advantages as it does not require to cool the cavity flanges during baking, and allows to perform the 'cold' RF characterization of the cavity in situ, immediately after the thermal treatment without disassembly of heating elements. Moreover, the air exposure that causes partial degradation of Q₀ by surface reoxidation is avoided. The heat treatment of a single-cell 1.3 GHz niobium cavity at 230 ^∘C for 24 h demonstrated the doubling of Q₀ at $E_\mathrm{acc}$ = 10 MV m⁻¹ (from $1.20\times10^{10}$ to $2.4\times10^{10}$) and retained the maximal accelerating field of 35 MV m⁻¹ without quenching. The selection of treatment parameters is based on our previous XPS studies. This treatment ensures incomplete dissolution of the native oxide by oxygen diffusion, thereby preventing interaction of niobium surface with external contaminants. We propose to bake the cavities directly in a cryomodule, which would allow to use the treatment to improve their performance. The potential impact of material parameters on the components of surface resistance has been briefly examined.

Bulk high-temperature superconductors (HTSs) are capable of generating very strong magnetic fields while maintaining a compact form factor. Solenoids constructed using stacks of ring-shaped bulk HTSs have been demonstrated to be suitable for low-resolution nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). However, these stacks were magnetised via field cooling, which typically requires a secondary superconducting charging magnet capable of sustaining a high magnetic field for a long period. A more economical alternative to field cooling is pulsed-field magnetisation (PFM), which can be carried out with an electromagnet wound from a normal conductor, such as copper. In this work, we present an additional advantage of PFM, where the trapped field in a stack of ring-shaped bulk HTSs is iteratively homogenised by manipulating the spatial profile of the applied pulsed field. Compared to field cooling with a uniform applied field, the variation in field along the central 10 mm of the solenoid is halved using this technique. If PFM of HTS rings could be advanced further to trap higher fields, this technique could be instrumental in magnetising HTS solenoids for NMR/MRI applications.

In many high-temperature superconducting applications, the advantages of no-insulation (NI) coils, such as self-protecting capability and thermal stability, make them a promising alternative to insulated (INS) coils. Magnetisation loss will be generated when the coil is exposed to time-varying magnetic fields. This loss can vary with the applied field angle, magnitude, and frequency, resulting in parasitic heat loads. In this study, we investigate magnetization loss in NI and INS double-pancake and double-racetrack coils of identical dimensions, experimentally and numerically. Experiments were conducted at 77 K under external AC magnetic fields up to 100 mT, considering various field angles (0°–90°) and frequencies (73–146 Hz). The experimental results are compared with the finite element simulation results of the coils' three-dimensional models. Interestingly, NI coils exhibit no significant angular dependence of loss within a specific field range; however, beyond this range loss increases with increasing field angles. In contrast, the loss in INS coils consistently increases with decreasing field angles across the entire field range. Coil level shielding of the magnetic field is observed in NI coils under parallel fields which is similar to a bulk superconductor. The losses in INS and NI coils are comparable under a perpendicular magnetic field, which can be attributed to the dominance of superconducting currents, as confirmed by the current and field distributions observed in simulations.