Superconductor Science and 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.

Brain imaging MRI comprises a significant proportion of MRI scans, but the requirement for including the shoulders in the magnet bore means there is not a significant size reduction in the magnet compared to whole-body magnets. Here we present a new design approach for brain imaging MRI magnets targeting ±20 kHz B₀ variation over the imaging volume rather than the more usual ±200 Hz making use of novel high-bandwidth MRI pulse sequences and distortion correction. Using this design approach, we designed and manufactured a 1.5 T class ReBCO cryogen-free magnet. The magnet is dome-like in form, completely excludes the shoulders and is <400 mm long. The magnet was wound using no-insulation style coils with a conductive epoxy encapsulant where the contact resistance of the coils was controlled so the emergency shut-down time of the magnet was less than 30 s. Despite acceptable coil testing results ahead of manufacture, during testing of the magnet, several of the epoxy coils showed signs of damage limiting stable performance to <55 A compared to the designed 160 A. These coils were replaced with insulated paraffin encapsulated coils. Subsequently the magnet was re-ramped and was stable at 81 A, generating 0.71 T as several other coils had sustained damage not visible in the first magnet iteration. The magnet has been passive shimmed to ±20 kHz B₀ variation over the imaging volume and integrated into an MRI scanner. The stability of the magnet has been evaluated and found to be acceptable for MRI.

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.

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.

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.

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.

REBa₂Cu₃O_7−x (REBCO) tapes produced by leading manufacturers were tested at UNIGE to characterize the dependence of the critical current on temperature, field intensity and orientation. This measurement campaign was carried out in the frame of international collaborations having the common goal of developing technology for ultra-high field magnets in the 30–50 T range. The examined samples differ in many respects, e.g. processing methods, thickness of the superconducting layer, Rare Earth element in REBCO, and type of artificial pinning centers (3D nanoparticles vs extended 1D nanorods). We measured the transport critical current of full-width tapes at 4.2 K and 20 K in magnetic fields up to 19 T and at various orientations of the field with respect to the tape surface. Additionally, magnetic characterization was conducted over a wider temperature range (4.2–77 K). The highly engineered vortex pinning results in outstanding critical current performance for all examined tapes: the non-copper critical current density, i.e. the critical current divided by the wire cross-section area minus the Cu area, ranges between 1500 and 2000 A mm⁻² at 4.2 K, 19 T and close to 1000 A mm⁻² at 20 K, 19 T in the perpendicular field orientation. We obtained scaling expressions for the critical current surface based on the analysis of the pinning-force curves but the pinning-force shape parameters were found to vary from one manufacturer to another. The results presented in this work may offer valuable information not only to magnet designers but also to manufacturers looking to optimize their tapes and achieve better performance.

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.

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.

The highest magnetic field strength for human-sized magnetic resonance imaging (MRI) currently lies at 11.7 tesla. Given the opportunities for enhanced sensitivity and improved data quality at higher static magnetic fields, several initiatives around the world are pursuing the implementation of further human MRI systems at or above 11.7 tesla. In general, members of the magnetic resonance (MR) research community are not experts on magnet technology. However, the magnet is the technological heart of any MR system, and the MRI community is challenging the magnet research and design community to fulfill the current engineering gap in implementing large-bore, highly homogeneous and stabile magnets at field strengths that go beyond the performance capability of niobium–titanium. In this article, we present an overview of magnet design for such systems from the perspective of MR scientists. The underlying motivation and need for higher magnetic fields are briefly introduced, and system design considerations for the magnet as well as for the MRI subsystems such as the gradients, the shimming arrangement, and the radiofrequency hardware are presented. Finally, important limitations to higher magnetic fields from physiological considerations are described, operating under the assumption that any engineering or economic barriers to realizing such systems will be overcome.

Building upon our previous database of the thermal, electrical, and mechanical properties of commercial REBCO tape, we have constructed a subsequent database focusing on the Cu layer of such tapes from eight distinct manufacturers. This database encompasses information pertaining to the geometry, purity, and grain orientation of the Cu layer. The primary objective of this database is to not only elucidate the material science of the Cu layer across various commercial tapes but also to establish correlations between the thermal, electrical, and mechanical properties and the aforementioned geometry, purity, and grain orientation parameters. After analysis, three significant findings have been validated. Firstly, the non-uniformity of geometry plays a critical role in the electrical resistivity in the radial direction, primarily through altering the actual contact surface area. Secondly, it has been observed that the total grain boundary length per micrometer thickness exhibits a nearly linear correlation with the thermal conductivity in the circumferential direction. Thirdly, the purity of the Cu layer in all the commercial REBCO tapes is lower than anticipated. It is our aspiration that this database will facilitate enhanced comprehension of the Cu layer in REBCO tapes among a broader spectrum of researchers.

Within the framework of magnetic confinement fusion, several projects worldwide are demonstrating the possibility of integrating high-temperature superconductors (HTS) in the coil systems. HTS-based technologies are highly attractive for practical applications because they can extend the operating margins of fusion coils in terms of higher temperatures, transport currents and magnetic fields. Based on the results achieved with the twisted-stacked tape cable, we have designed a novel low-loss HTS sector cable-in-conduit conductor, with a target of 60 kA at 4.5 k, 18 T, which is presently of interest for the DEMO Central Solenoid coil. In HTS cables, the AC losses can represent a significant limiting factor, therefore they must be taken into consideration both in the design phase and in the assessment of the overall magnet thermal budget. In this work, to assess the loss behavior and to optimize the cable design, we have explored different aspect ratios and arrangements of the stacked tapes within the cable layout. The magnetization losses are calculated with a 2D finite-element model based on the T-A formulation and analytical approximations based on the Brandt-Halse critical state model. Specifically, we have developed an analytical formulation that allows for the calculation of the instantaneous power losses in HTS stacked cables with a limited number of tapes per stack, achieving sufficient accuracy at high fields. The analytical model enables a sufficiently accurate assessment of the heat deposited on the conductor during those particular instants of a plasma scenario where the variation of the field is very high, such as during the critical initial discharge period of the plasma initiation.

High-temperature superconducting coils are used in various large-scale applications, like rotating machines and high-field magnets. However, modeling these coils is a complicated and time-consuming process, especially due to the non-linearity of the current–voltage characteristics of the superconductors and the complex multiphysics involved. In this work, we used a fast homogenized method to model the coupled electromagnetic and electrothermal properties of racetrack and pancake coils for different applications. For this purpose, various formulations wielding homogenization methods are used and benchmarked with each other, as well as with models considering the detailed structure of the HTS tapes. We observe a very good agreement between different models (homogenized and detailed), and we discuss the pros and cons of the inclusion of insulating layers between the turns in homogenization. This work was performed under the collaboration of the COST action modeling teams and can be used as a review of the state-of-the-art superconductor modeling techniques, and a source for the development and benchmark of future numerical methods.

The world's highest-field dc magnets have, for more than 50 years, consisted of a combination of resistive and superconducting (SC) coils that we refer to as a 'hybrid'. These magnets use SC technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields have been attained than was possible with purely SC magnet technology, and lower lifecycle costs are attained than with a purely resistive magnet. The peak field available has been 45 T for over 20 years in Tallahassee, Florida, USA. There is presently a 'revolution' underway in hybrid magnet development. A second 45 T hybrid was completed in 2022 in Hefei, China that might be upgraded to 48 T in a few years. The high field lab in Grenoble, France is also testing a hybrid magnet intended to reach 43.5 T but which also might be upgraded to 46 T in a few years. In addition, the lab in Nijmegen, The Netherlands is presently assembling a hybrid magnet intended to operate at 46 T. Papers have been presented and published with conceptual designs of hybrid magnets with fields up to 60 T. Given the developments underway, this is an appropriate time to review the history of such systems, with a particular focus on the larger, more expensive part of the magnets: the SC outsert coils. The demands placed on the SC coils of these magnet systems are unique due to their coupling with resistive coils that are operated at very high stress and wear out regularly, resulting in large field transients and fault forces. The evolution of the technology used for the SC coils of these hybrid systems is presented, evolving from ventilated windings to cable-in-conduit to cryogen-free.

Superconducting power supplies commonly known as flux pumps are becoming viable products in various industries. In this work we investigate magnetically switched ($J_\mathrm{c}(B)$) half-wave transformer rectifier flux pumps (TRFPs) via a simulation previously validated against an experimental system. Upgrades to the model are introduced, namely the transition to a fully hysteretic transformer reluctance model and the introduction of a conduction cooling thermal model. The conduction cooling model allows direct comparison between liquid cryogen and conduction cooled systems, highlighting the distinct variations in response. This study focuses on parameter sweeps over some of the most important aspects of a TRFP such as; input power, cooling paths and switch length. The findings from these variable sweeps shed light on how to best design the different aspects of a TRFP for most applications and the conclusions drawn throughout this work can be applied to most TRFP types. Some of the major findings from this work are; i. the need for dedicated cooling busses in TRFP switches, ii. that increased switch lengths increase voltage generation but negatively impact cooling effectiveness iii. that the reduction of the metallic coatings surrounding commercial coated conductors results in larger switch voltages.

The world's highest-field dc magnets have, for more than 50 years, consisted of a combination of resistive and superconducting (SC) coils that we refer to as a 'hybrid'. These magnets use SC technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields have been attained than was possible with purely SC magnet technology, and lower lifecycle costs are attained than with a purely resistive magnet. The peak field available has been 45 T for over 20 years in Tallahassee, Florida, USA. There is presently a 'revolution' underway in hybrid magnet development. A second 45 T hybrid was completed in 2022 in Hefei, China that might be upgraded to 48 T in a few years. The high field lab in Grenoble, France is also testing a hybrid magnet intended to reach 43.5 T but which also might be upgraded to 46 T in a few years. In addition, the lab in Nijmegen, The Netherlands is presently assembling a hybrid magnet intended to operate at 46 T. Papers have been presented and published with conceptual designs of hybrid magnets with fields up to 60 T. Given the developments underway, this is an appropriate time to review the history of such systems, with a particular focus on the larger, more expensive part of the magnets: the SC outsert coils. The demands placed on the SC coils of these magnet systems are unique due to their coupling with resistive coils that are operated at very high stress and wear out regularly, resulting in large field transients and fault forces. The evolution of the technology used for the SC coils of these hybrid systems is presented, evolving from ventilated windings to cable-in-conduit to cryogen-free.

Magnetic impurities connected to superconductors reservoir result in bound states within the superconducting gap, so called Yu–Shiba–Rusinov (YSR) state. In the past few years, this field has gained much attention since it is crucial for engineering novel superconducting many-body states, with the perspective of manufacturing Majorana Fermions. The underlying physical picture of YSR state depends closely on the form of the impurities connected to the leads, the manner in which the impurities are organized, and also the diverse local interactions, which is always disclosed with the aid of quantum impurity models. This article provides a comprehensive overview of the progress achieved by previous studies, focusing on the issues demonstrated by quantum impurity structures. The physical mechanisms and the related phenomena assisted by different interactions are discussed in detail. Furthermore, a comprehensive overview of recent experimental achievements is presented, using various metal phthalocyanine molecules as illustrative examples, thereby establishing a robust foundation for future inquiries in this domain.

The recently discovered kagome superconductors AV₃Sb₅ (A= K, Rb, Cs) provide a new platform to explore intertwined symmetry-breaking orders. However, great controversies exist to date, including the origin of charge density wave (CDW), the unconventional or conventional nature of superconductivity, and the presence or absence of time-reversal symmetry breaking. A thorough understanding of the fundamental electronic structure is crucial for addressing these disputes. In this review, we provide an extensive summary of the key structural and electronic properties of AV₃Sb₅ compounds and evaluate the current research on their unconventional electronic order, especially the superconductivity and CDW, with a particular focus on insights from angle-resolved photoemission spectroscopy studies. We expect this review to be timely due to the convergence of various experimentally observed phenomena related to the CDW and superconducting order parameters in AV₃Sb₅ compounds. Our goal is to guide future investigations aimed at uncovering the microscopic origins of these unconventional electronic properties in kagome superconductors.

The highest magnetic field strength for human-sized magnetic resonance imaging (MRI) currently lies at 11.7 tesla. Given the opportunities for enhanced sensitivity and improved data quality at higher static magnetic fields, several initiatives around the world are pursuing the implementation of further human MRI systems at or above 11.7 tesla. In general, members of the magnetic resonance (MR) research community are not experts on magnet technology. However, the magnet is the technological heart of any MR system, and the MRI community is challenging the magnet research and design community to fulfill the current engineering gap in implementing large-bore, highly homogeneous and stabile magnets at field strengths that go beyond the performance capability of niobium–titanium. In this article, we present an overview of magnet design for such systems from the perspective of MR scientists. The underlying motivation and need for higher magnetic fields are briefly introduced, and system design considerations for the magnet as well as for the MRI subsystems such as the gradients, the shimming arrangement, and the radiofrequency hardware are presented. Finally, important limitations to higher magnetic fields from physiological considerations are described, operating under the assumption that any engineering or economic barriers to realizing such systems will be overcome.

The vanadium-based kagome metals AV₃Sb₅ (A = K, Rb, and Cs) host a superconducting ground state that coexists with an unconventional charge density wave (CDW). The CDW state exhibits experimental signatures of chirality, electronic nematicity, and time-reversal-symmetry-breaking, raising the questions whether the superconductivity (SC) in AV₃Sb₅ may also be unconventional, how SC interplays with CDW, and how the two orders evolve upon tuning. This article reviews studies of the superconducting pairing symmetry, and the tuning of SC and CDW in the AV₃Sb₅ compounds. Various experimental techniques consistently find that CsV₃Sb₅ exhibits nodeless SC, which remains robust regardless whether the CDW is present. Under hydrostatic pressure, SC in AV₃Sb₅ becomes enhanced as the CDW is gradually suppressed, revealing a competition between the two orders. In CsV₃Sb₅, a new CDW state emerges under pressure that competes more strongly with SC relative to the CDW at ambient pressure, and results in two superconducting domes that coexist with CDW. After the CDW in AV₃Sb₅ is fully suppressed with hydrostatic pressure, a further increase in pressure leads to a nonmonotonic evolution of the superconducting transition temperature driven by lattice modulations. Thickness is shown to be a powerful tuning parameter in AV₃Sb₅ thin flakes, revealing the evolution of CDW and SC upon dimensional reduction, and can be combined with hydrostatic pressure to shed light on the interplay between SC and CDW. Based on results reviewed in this article, we discuss outstanding issues to be addressed in the AV₃Sb₅ systems.

A fully three-dimensional multi-physics model to simulate quench propagation in HTS conductors for fusion applications is presented. It accounts for thermal, electric and fluid dynamics throughout the entire transient. The need for high-fidelity models for quench simulations comes from the bulky layouts of many HTS conductors that are being proposed. The model is then validated against experimental data, showing a good agreement on all the relevant quentities (local voltage and temperatures). It is shown that the detailed model improves the quality of the agreement with the measured data with respect to more simplified models. It also allows an insight on the temperature distributions in the conductor cross-section, which can be relevant for the interpretation of experimental data as well as to support the design of quench detection strategies which rely on local temperature variations.

The process to obtain a superconducting joint between Coated Conductors (CCs) involves the simultaneous application of temperature and transverse compressive pressure to promote the joining of the two adjacent REBCO layers. We performed experiments to simulate this procedure by subjecting samples from commercial CCs to different combinations of pressure, and temperature. The objective was to understand the effects of the thermomechanical cycles on the critical current (Ic) and, therefore, determine the upper limit for the achievable current in a superconducting joint between CCs. We observed a reduction of the Ic across the investigated parameter range that is accelerated at higher temperatures and pressures. For instance, the average reduction of Ic measured at 77 K in self-field varied from 45% to 90% when increasing the pressure for samples heated at 820 °C, as compared to samples heated at the same temperature without applied pressure. As a complement to electrical transport measurements, we carried out TEM and EDX investigations to study the relation between the degradation of Ic and alterations in the microstructure of REBCO. These analyses indicate that the concurrent application of temperature and pressure accelerates the decomposition of the REBCO phase. The EDX data suggests that the decomposition products could correspond with a peritectic reaction of the REBCO occurring at lower temperatures, accelerated by the presence of an external pressure. This early decomposition occurs in localized areas of the REBCO layer, constricting the available cross-section for current flow and thereby diminishing the critical current.

Single crystals of (Ca1-xLax)KFe4As4 (0 ≤ x ≤0.16) have been grown by using the self-flux method, and the evolution of physical properties including the critical current density (Jc) with La-doping has been investigated. Tc decreases monotonically with increasing x, while Jc at the same temperature and magnetic field increases initially and reach its maximum at x = 0.082. The increase in Jc is more obvious at low temperatures and high fields. At T = 5 K and H = 40 kOe, Jc reaches 0.34 MA/cm2, which is ~4 times larger than that for pure crystals. It is also found that anomalous temperature dependence of Jc in CaKFe4As4 is wiped away as the La content is increased. However, Jc shows non-monotonic field dependence (peak effect) at high fields in crystals with large x. In addition, we found that despite weak anisotropy of Hc2, there is extremely large anisotropy of Jc up to ~15, which is most likely caused by novel planar defects in the crystal, similar to CaKFe4As4. Jc characteristics in (Ca1-xLax)KFe4As4 with disorder outside FeAs planes is compared with that in CaK(Fe1-xCox)4As4 with disorder within FeAs planes.

The threshold magnetic field is a key parameter for evaluating the current decay caused by dynamic resistance in superconducting windings and magnets. For a DC-carrying superconducting slab under an AC parallel magnetic field, the analytical theory clearly shows that there is only one electric central line (ECL) across the slab width at the onset of dynamic-resistance. However, threshold magnetic fields in superconducting strips and coils have not been fully investigated. Based onthe one-ECL criterion, this paper first presents a method for numerically determining the threshold magnetic field via the evolving internal magnetic field in superconducting strips and coils. By probing transient electromagnetic behaviours, interestingly, we found a distinctive feature of superconducting strips in which a wide region of zero electrical field is observed when dynamic resistance/loss initially occurs. With increasing magnetic fields, this region gradually shrinks and eventually become the ECL. More importantly, this numerical method can analyse the local threshold magnetic field in a targeted coil turn. The ability to quantify threshold magnetic field provides a clear guidance on the acceptable level of ripple and harmonic magnetic fields for coil windings in superconducting maglev trains and field windings of superconducting machines operating at persistent current mode.&#xD;

Vortex motion can lead to significant energy dissipation, resulting in hot spots and thermomagnetic instabilities that are detrimental to the application of superconductors. This paper presents a theoretical examination of thermomagnetic instabilities triggered by vortex motion within a Nb$_{3}$Sn-I-Nb cavity featuring a multilayer structure. The investigation is conducted using Ginzburg-Landau theory in conjunction with the heat diffusion equation. The numerical simulations align well with experimental data from Nb$_{3}$Sn superconducting cavities. Given that the performance of SRF cavities is highly sensitive to various defects, this study also considers the interaction between vortices and these defects. It reveals the impact of edge cracks and impurities on temperature rise and the quality factor. The findings indicate that edge cracks significantly reduce the threshold field for thermomagnetic instability in superconducting radio-frequency (SRF) cavities. The performance of SRF cavities is influenced not only by the RF field amplitude and frequency but also by the length and number of edge cracks. These results offer valuable insights for evaluating the performance of SRF cavities subjected to RF fields.

Within the framework of magnetic confinement fusion, several projects worldwide are demonstrating the possibility of integrating high-temperature superconductors (HTS) in the coil systems. HTS-based technologies are highly attractive for practical applications because they can extend the operating margins of fusion coils in terms of higher temperatures, transport currents and magnetic fields. Based on the results achieved with the twisted-stacked tape cable, we have designed a novel low-loss HTS sector cable-in-conduit conductor, with a target of 60 kA at 4.5 k, 18 T, which is presently of interest for the DEMO Central Solenoid coil. In HTS cables, the AC losses can represent a significant limiting factor, therefore they must be taken into consideration both in the design phase and in the assessment of the overall magnet thermal budget. In this work, to assess the loss behavior and to optimize the cable design, we have explored different aspect ratios and arrangements of the stacked tapes within the cable layout. The magnetization losses are calculated with a 2D finite-element model based on the T-A formulation and analytical approximations based on the Brandt-Halse critical state model. Specifically, we have developed an analytical formulation that allows for the calculation of the instantaneous power losses in HTS stacked cables with a limited number of tapes per stack, achieving sufficient accuracy at high fields. The analytical model enables a sufficiently accurate assessment of the heat deposited on the conductor during those particular instants of a plasma scenario where the variation of the field is very high, such as during the critical initial discharge period of the plasma initiation.

The world's highest-field dc magnets have, for more than 50 years, consisted of a combination of resistive and superconducting (SC) coils that we refer to as a 'hybrid'. These magnets use SC technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields have been attained than was possible with purely SC magnet technology, and lower lifecycle costs are attained than with a purely resistive magnet. The peak field available has been 45 T for over 20 years in Tallahassee, Florida, USA. There is presently a 'revolution' underway in hybrid magnet development. A second 45 T hybrid was completed in 2022 in Hefei, China that might be upgraded to 48 T in a few years. The high field lab in Grenoble, France is also testing a hybrid magnet intended to reach 43.5 T but which also might be upgraded to 46 T in a few years. In addition, the lab in Nijmegen, The Netherlands is presently assembling a hybrid magnet intended to operate at 46 T. Papers have been presented and published with conceptual designs of hybrid magnets with fields up to 60 T. Given the developments underway, this is an appropriate time to review the history of such systems, with a particular focus on the larger, more expensive part of the magnets: the SC outsert coils. The demands placed on the SC coils of these magnet systems are unique due to their coupling with resistive coils that are operated at very high stress and wear out regularly, resulting in large field transients and fault forces. The evolution of the technology used for the SC coils of these hybrid systems is presented, evolving from ventilated windings to cable-in-conduit to cryogen-free.

Superconducting power supplies commonly known as flux pumps are becoming viable products in various industries. In this work we investigate magnetically switched ($J_\mathrm{c}(B)$) half-wave transformer rectifier flux pumps (TRFPs) via a simulation previously validated against an experimental system. Upgrades to the model are introduced, namely the transition to a fully hysteretic transformer reluctance model and the introduction of a conduction cooling thermal model. The conduction cooling model allows direct comparison between liquid cryogen and conduction cooled systems, highlighting the distinct variations in response. This study focuses on parameter sweeps over some of the most important aspects of a TRFP such as; input power, cooling paths and switch length. The findings from these variable sweeps shed light on how to best design the different aspects of a TRFP for most applications and the conclusions drawn throughout this work can be applied to most TRFP types. Some of the major findings from this work are; i. the need for dedicated cooling busses in TRFP switches, ii. that increased switch lengths increase voltage generation but negatively impact cooling effectiveness iii. that the reduction of the metallic coatings surrounding commercial coated conductors results in larger switch voltages.

A fully three-dimensional multi-physics model to simulate quench propagation in HTS conductors for fusion applications is presented. It accounts for thermal, electric and fluid dynamics throughout the entire transient. The need for high-fidelity models for quench simulations comes from the bulky layouts of many HTS conductors that are being proposed. The model is then validated against experimental data, showing a good agreement on all the relevant quentities (local voltage and temperatures). It is shown that the detailed model improves the quality of the agreement with the measured data with respect to more simplified models. It also allows an insight on the temperature distributions in the conductor cross-section, which can be relevant for the interpretation of experimental data as well as to support the design of quench detection strategies which rely on local temperature variations.

The process to obtain a superconducting joint between Coated Conductors (CCs) involves the simultaneous application of temperature and transverse compressive pressure to promote the joining of the two adjacent REBCO layers. We performed experiments to simulate this procedure by subjecting samples from commercial CCs to different combinations of pressure, and temperature. The objective was to understand the effects of the thermomechanical cycles on the critical current (Ic) and, therefore, determine the upper limit for the achievable current in a superconducting joint between CCs. We observed a reduction of the Ic across the investigated parameter range that is accelerated at higher temperatures and pressures. For instance, the average reduction of Ic measured at 77 K in self-field varied from 45% to 90% when increasing the pressure for samples heated at 820 °C, as compared to samples heated at the same temperature without applied pressure. As a complement to electrical transport measurements, we carried out TEM and EDX investigations to study the relation between the degradation of Ic and alterations in the microstructure of REBCO. These analyses indicate that the concurrent application of temperature and pressure accelerates the decomposition of the REBCO phase. The EDX data suggests that the decomposition products could correspond with a peritectic reaction of the REBCO occurring at lower temperatures, accelerated by the presence of an external pressure. This early decomposition occurs in localized areas of the REBCO layer, constricting the available cross-section for current flow and thereby diminishing the critical current.

The threshold magnetic field is a key parameter for evaluating the current decay caused by dynamic resistance in superconducting windings and magnets. For a DC-carrying superconducting slab under an AC parallel magnetic field, the analytical theory clearly shows that there is only one electric central line (ECL) across the slab width at the onset of dynamic-resistance. However, threshold magnetic fields in superconducting strips and coils have not been fully investigated. Based onthe one-ECL criterion, this paper first presents a method for numerically determining the threshold magnetic field via the evolving internal magnetic field in superconducting strips and coils. By probing transient electromagnetic behaviours, interestingly, we found a distinctive feature of superconducting strips in which a wide region of zero electrical field is observed when dynamic resistance/loss initially occurs. With increasing magnetic fields, this region gradually shrinks and eventually become the ECL. More importantly, this numerical method can analyse the local threshold magnetic field in a targeted coil turn. The ability to quantify threshold magnetic field provides a clear guidance on the acceptable level of ripple and harmonic magnetic fields for coil windings in superconducting maglev trains and field windings of superconducting machines operating at persistent current mode.&#xD;

A low normal zone propagation velocity (NZPV) combined with critical current inhomogeneities favor the nucleation of destructive hot spots in rare-earth barium copper oxide (REBCO) tapes. Increasing the NZPV using the current flow diverter (CFD) concept is a promising solution to mitigate the risk of developing hot spots. The fabrication method of CFD REBCO tapes implies several steps consisting in masking, silver etching, mask removal, and silver deposition, which takes time and remains a barrier to the implementation of a low-cost industrial production of long-length CFD REBCO tapes. This work presents a cost-effective and maskless CFD fabrication approach that relies on inkjet printing (IJP) of silver patterns directly on top of the REBCO layer to create a non-uniform interfacial resistance between the silver and the REBCO surface, along the width of the tape. The parameters of IJP and oxygen annealing were optimized to obtain highly conductive silver patterns deposited on the surface of the REBCO layer. CFD REBCO tapes were successfully fabricated using commercial REBCO tapes and the proposed method without degrading the superconducting properties. Experimental measurements revealed an increase of the NZPV by a factor of 6-7 compared to commercial REBCO tapes.

Superconducting (SC) magnets can generate exceptionally high magnetic fields and can be employed in various applications to enhance system power density. In contrast to conventional coil-based SC magnets, high-temperature superconducting (HTS) trapped field magnets (TFMs), namely HTS trapped field bulks (TFBs) and trapped field stacks (TFSs), can eliminate the need for continuous power supply or current leads during operation and thus can function as super permanent magnets. TFMs can potentially trap very high magnetic fields, with the highest recorded trapped field reaching 17.89 T, achieved by TFSs. TFMs find application across diverse fields, including rotating machinery, magnetic bearings, energy storage flywheels, and magnetic resonance imaging (MRI). However, a systematic review of the advancement of TFMs over the last decade remains lacking, which is urgently needed by industry, especially in response to the global net zero target. This paper provides a comprehensive overview of various aspects of TFMs, including simulation methods, experimental studies, fabrication techniques, magnetisation processes, applications, and demagnetisation issues. Several respects have been elucidated in detail to enhance the understanding of TFMs, encompassing the formation of HTS bulk composites and TFSs, trapped field patterns, enhancement of trapped field strength through pulsed field magnetisation (PFM), as well as their applications such as SC rotating machines, levitation, and Halbach arrays. Challenges such as demagnetisation, mechanical failure, and thermal instability have been illuminated, along with proposed mitigation measures. The different roles of ferromagnetic materials in improving the trapped field during magnetisation and in reducing demagnetisation have also been summarised. It is believed that this review article can provide a useful reference for the theoretical analysis, manufacturing, and applications of TFMs within various domains such as materials science, power engineering, and clean energy conversion.

$\textrm{Nb}_x\textrm{Re}_{1-x}$ is a non-centrosymmetric superconductor recently realized in thin film form. The access to the basic physics that governs this material is hindered by the polycrystalline structure of the films which consist of grains of small dimensions. In these conditions, films are not decisive in revealing the nature of the superconducting order parameter, and, in particular, the possible presence of a spin-triplet component. In this work, post-deposition annealing procedures were performed to improve the crystalline quality of the as-grown films. As determined by x-ray diffraction measurements, by tuning the annealing process, it was possible to increase the crystalline size from 1–2 nm up to several nanometers, beyond the values of the superconducting coherence length. Pair distribution function analyses revealed the non-centrosymmetric crystal structure of the treated NbRe films. Finally, electrical transport measurements, performed to study the relationship between structural and transport properties of the samples, showed an improved metallicity and a reduction of the superconducting critical temperature after the thermal treatment. This last result can be reasonably ascribed, for instance, to oxygen contamination, modification of the electronic density of states at the Fermi level, or change in the electron–phonon coupling. The results of this work are useful to realize films for future experiments meant to access nature of the superconducting order parameter.

Brain imaging MRI comprises a significant proportion of MRI scans, but the requirement for including the shoulders in the magnet bore means there is not a significant size reduction in the magnet compared to whole-body magnets. Here we present a new design approach for brain imaging MRI magnets targeting ±20 kHz B₀ variation over the imaging volume rather than the more usual ±200 Hz making use of novel high-bandwidth MRI pulse sequences and distortion correction. Using this design approach, we designed and manufactured a 1.5 T class ReBCO cryogen-free magnet. The magnet is dome-like in form, completely excludes the shoulders and is <400 mm long. The magnet was wound using no-insulation style coils with a conductive epoxy encapsulant where the contact resistance of the coils was controlled so the emergency shut-down time of the magnet was less than 30 s. Despite acceptable coil testing results ahead of manufacture, during testing of the magnet, several of the epoxy coils showed signs of damage limiting stable performance to <55 A compared to the designed 160 A. These coils were replaced with insulated paraffin encapsulated coils. Subsequently the magnet was re-ramped and was stable at 81 A, generating 0.71 T as several other coils had sustained damage not visible in the first magnet iteration. The magnet has been passive shimmed to ±20 kHz B₀ variation over the imaging volume and integrated into an MRI scanner. The stability of the magnet has been evaluated and found to be acceptable for MRI.