Table of contents

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.

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 quench of a superconducting magnet in a high magnetic field environment causes irreversible damage to the magnet. A 32.4 T completely superconducting magnet consisting of a low-temperature superconducting (LTS) external magnet and a high-temperature superconducting (HTS) inserting magnet was developed and tested. The LTS external magnet quickly discharged after quenching when the system's peak center magnetic field reached 32.4 T. The inserting magnet, particularly the Coil 1 magnet on the magnet's innermost side, was damaged due to the quick current change in the LTS external magnet. This created a significant current inside the HTS magnet due to the mutual inductance effect. The overcurrent caused the degradation of Coil 1 magnet. The continuous critical current of DP69, DP80, DP85, and DP94 in the magnet was measured to investigate the degradation. Numerous circular burn sites in areas of degraded critical current were observed by optical and scanning electron microscopy (SEM). To analyze the causes of the degradation, a dynamic coupling model of LTS and HTS magnet during quench was developed. The simulation outcomes indicated that the locations exhibiting high hoop stress correspond to those where the normalized critical current density is observed to be notably low, proving that excessive hoop stress is the main reason for the critical current degradation in the coils. Furthermore, the model quantified the turn-to-turn loss in the Coil 1 magnet during the quenching process of the LTS magnet. It offered potential evidence for the circular burn-through points in the superconducting layer.

High-T_c superconducting REBa₂Cu₃O${\,}_{7-x}$ (REBCO) bulks are actively researched for use in superconducting magnetic bearings (SMBs). SMBs are attractive in applications which require zero friction, ultra high speeds, or integration within cryogenic environments. However, SMBs are limited by low stiffness values compared to mechanical bearings, prohibiting their use in turbulent environments such as rocketry or aviation. To improve the performance of SMBs, a comprehensive understanding of the precise distribution of currents and magnetic fields within the material during magnetic suspension is required. Hence, the results of vertical and lateral levitation force measurements between a single directional melt growth REBCO bulk and an NdFeB magnet are reported. The measurements are carried out between 77 K and 90 K and for bulk thickness between 0.5 mm to 4.5 mm. By reducing the bulk thickness, the depth to which the induced currents penetrate the bulk is identified in zero-field cooling conditions. At 77 K, the current and magnetic field occupy the top 1 mm, closest to the magnet. By reducing the bulk thickness further, the levitation force profiles vary significantly. Field-cooling results are similarly impacted; reducing the bulk thickness below 1 mm dramatically reduces the lateral stiffness. The above results are interpreted using finite element simulations employing experimentally measured J_c(B,θ) data from the bulk, following the completion of the levitation force measurement routine. These results are relevant for future bearing designs, helping to inform the optimal distribution of superconducting material for a given application.

The advent of no-insulation (NI) winding technique in high temperature superconducting magnets has marked a milestone, demonstrating enhanced thermal stability and mechanical integrity in various high-field applications. There have been reports on numerical and experimental results of the application of NI-class winding techniques to low temperature superconducting (LTS) magnets, showing improved thermal stability such as significant reduction of training, stable behavior during over-current operations, and even recovery after heater-fired quench as a result of current bypass near a hot spot. Thus, the NI approach might offer potential advantage to large scale LTS fusion magnet such as reduction of peak coil terminal voltage or decrease of the copper fraction within the conductor to achieve a more compact design. However, it is essential to firstly assess the potential risks associated with NI approach to large scale LTS fusion magnet to identify any critical barriers. This paper explores the feasibility of partially insulated Nb₃Sn EU DEMO toroidal field (TF) coil over a wide range of contact resistivity through two distinct partial insulation winding schemes: (1) inter-turn NI and (2) inter-layer NI scheme. Our investigation encompasses electromagnetic, structural, and thermal-hydraulic analyses to evaluate current distribution, magnet charging time, stress distribution, and temperature rise within the winding pack during magnet charging. The analysis numerically confirms the existence of a feasible design space for a partially insulated TF coil, which enables stable charging and normal operation of the magnet.

Magnetic resonance imaging (MRI) systems with magnetic field strengths higher than 7 Tesla (T), offer exceptional image resolution and signal-to-noise ratio, enabling more detailed visualization of fine anatomical structures, especially for brain, musculoskeletal, and cardiovascular studies. Only a few of them are operating in the whole world. An 11.74 T magnet for human MRI has been commissioned at Gachon University Gil Medical Center (GUGMC) in South Korea in 2022 and has now exceeded two-year operational life at full field. An overview of its features, the commissioning test campaign, and the experience gained after operations is presented.

Transition edge sensors (TESs) are highly sensitive thermal detectors that operate across a wide frequency range, from millimeter waves to gamma rays. For optical-single photon TES detectors, a high energy resolution and large saturation energy are required, primarily limited by factors such as heat capacitance and temperature sensitivity, α_I. In this study, we evaluated TES devices of varying sizes by conducting complex impedance measurements using transfer function calibration, revealing that our titanium-based TES exhibits higher heat capacities compared to theoretical values. We obtained the measured and theoretical time constants, energy resolutions, and saturation energies for the different-sized TESs, notably constructing histograms from pulse area integral analysis. Our devices achieved an optimal energy resolution of 0.2 eV, with the largest TES demonstrating an energy resolution of 0.34 eV, a saturation energy of 35 eV and the ability to discriminate up to 20 photons at 438 nm. These results provide valuable insights for optimizing TES performance in precise photon detection.

The effect of thermal fluctuations in yttrium barium copper oxide SQUID filters (SQIFs), containing arrays of 258, 2304, and 4080 superconducting quantum interference devices (SQUIDs), were investigated using the electrical resistivity and applied magnetic field up to 9 T. Our study reveals that the SQIFs experience a two-step superconductor transition. The transition at lower temperature is only present in the SQIF samples, not in the control sample without the SQIFs, and it is investigated here in terms of two primary dissipation mechanisms. The first dominates near the transition temperature and has the characteristics of thermally activated phase slippage. At lower temperatures, the dominant characteristics are those of thermally-activated magnetic flux flow. The mechanisms producing such behaviors contribute significantly to dissipation effects, resulting in an additional DC voltage that overlays the Josephson current, regardless of the quantity of SQUIDs in the SQIFs. These dissipation effects can have significant implications for technologies utilizing high-temperature superconductor SQIFs unless the operating temperature is lowered significantly below the superconducting transition temperature.

Increasing the normal zone propagation velocity (NZPV) in superconducting cables based on rare earth barium copper oxide (REBCO) tapes is expected to cause a paradigm shift by enabling the development of faster and more reliable quench detection systems to mitigate the development of destructive hot spots. Furthermore, a higher NZPV is beneficial in terms of fault current limitation capabilities by accelerating the homogenization of the quench in superconducting power devices. One way envisioned to increase the NZPV of cables based on REBCO tapes is the current flow diverter (CFD) concept. A Conductor on Round Core (CORC^®) cable made with CFD REBCO tapes, called CFD CORC^® cable, and a CORC^® cable made with regular REBCO tapes, were fabricated and tested. The critical current of the CFD and regular CORC^® cables were obtained at temperatures ranging from 67 to 77 K in self-field. Measurements showed that the NZPV was increased by a factor of 4–4.5 in the case of the CFD CORC^® cable. Furthermore, the results suggest that the NZPV depends only on the applied current, similar to what has been observed previously on single REBCO tapes. These results demonstrate the successful integration of CFD REBCO tapes in CORC^® cables, without compromising their superconducting properties or their enhanced NZPV. The possibility of enhancing the NZPV of REBCO-based cables could facilitate quench detection, which remains an important issue in HTS magnet applications. This advancement also holds promise for the fault current limitation functionality of CORC^® cables.

This paper presents the design of a non-insulated (NI) high-temperature superconductor (HTS) 15 T solenoid with a 72 mm diameter warm bore, intended for use in the Paul Scherrer Institute positron production (P³) experiment. The P³ experiment, scheduled to start in Q3 2026, aims to demonstrate a high-yield positron source that is relevant in the context of FCC-ee. The coils will be solder-impregnated using techniques developed with a small-bore HTS NI coil stack. This magnet produced a magnetic field of 18 T at 12 K and 2 kA, in a cryogen-free, conduction-cooled setup. Similarly, the P³ magnet will be conduction-cooled by two cryocoolers. This larger-bore magnet is designed to operate at 15 K with a nominal operating current of 1.2 kA. To ensure the protection of the NI magnet, quench prevention is the preferred strategy. Several potential failure modes are analyzed, including thermal runaway in the event of failures in the current leads, power supply, or cryocoolers. By enhancing the cold mass' heat capacity through the addition of a large lead mass, the stored magnetic energy can be safely dissipated in the cold mass through the electrical path formed by the superconductor and the solder. Mechanical analysis indicates that the hoop, radial and axial stresses are kept below allowable limits.

Quench protection of full-size, high-field accelerator magnets poses significant challenges. Maintaining the hot-spot temperature and peak voltage-to-ground within acceptable limits requires a protection system that quickly transitions most of the coil turns to the normal state. Existing magnet protection technologies, such as quench protection heaters or the coupling loss induced quench system (CLIQ), have been successfully applied. However, they both present shortcomings since they require either thin insulation between the heaters and the magnet conductor—in the case of heaters, or direct electrical connections to the magnet coil—in the case of CLIQ. A novel quench protection method, energy shift with coupling (ESC), is presented, which can achieve excellent quench protection performance without requiring any electrical connection or close physical contact with the coils to protect. ESC relies on normal-conducting auxiliary coils strongly magnetically coupled with the magnet coils to protect. Upon quench detection, capacitive units connected across the auxiliary coils introduce a high current change in the auxiliary coils, causing a rapid shift of magnet stored energy from the magnet coils to the auxiliary coils. This has three beneficial effects: sudden reduction of ohmic loss in the normal zone of the magnet conductor, introduction of high transient losses in the magnet conductor, thus causing a quick transition to the normal state, and extraction of a part of the magnet's stored energy to the auxiliary coils. The applicability of the ESC concept on an existing magnet design is analyzed with electro-magnetic and thermal transient simulations performed with the STEAM-LEDET program. The advantages and disadvantages of ESC are discussed and compared to other conventional quench protection methods. Simulation results show that ESC can be applied to protect full-scale magnets with reasonable requirements in terms of size and location of the auxiliary coils, and of capacitive unit parameters.

Tantalum (Ta) films with a body-centered cubic α-phase play a crucial role in high-performance Ta-based superconducting quantum processors. The growth of pure α-Ta at low temperatures has been a long-term goal. Existing experience with Ta growth on silicon substrates has led to the belief that α-Ta can only be stabilized at high temperatures. In this study, we systematically analyzed the growth mechanism of Ta films on sapphire substrates using first-principles calculations, revealing a narrow growth energy window for pure α-Ta at room temperature (RT). This finding indicates the feasibility of obtaining pure α-Ta at significantly lower temperatures. Experimentally, we successfully achieved pure α-Ta at RT via magnetron sputtering by meticulously adjusting the growth parameters. The resulting superconducting resonators based on the developed RT α-Ta exhibit an internal quality factor ($Q_\textrm{i}$) of approximately $2.1 \times 10^6$ at single-photon levels, comparable to those fabricated at high temperatures. Additionally, fixed-frequency transmon qubits fabricated using the RT α-Ta film demonstrate a coherence time exceeding 280 µs, underscoring the film's high quality. The RT α-Ta without a seed layer simplifies the fabrication process, offering a more efficient and flexible approach for producing Ta-based superconducting quantum processors.

This work investigates the distribution of flux states of hysteretic dc superconducting quantum interference devices (SQUIDs) obtained from standard current ramp measurement at 2.50 and 0.35 K. Each flux state corresponds to a specific number of flux quanta trapped in the superconducting loop, associated with a metastable state in the two-dimensional potential energy of the hysteretic SQUID. The flux states can be distinguished by the multiple switching currents. The observed distribution is different from the distribution caused only by thermal excitation at a specific temperature. There are notable systematic deviations between the measurement results of three samples and the standard Boltzmann distribution, which can be clearly identified from the ratios between the proportions of different flux states. A formula resembling the Boltzmann distribution, involving multiple effective temperatures, is proposed to provide a better fit to the measured results. The finding suggests that the observed distribution of flux states is established during the SQUID's transition from the dissipative state to the zero-voltage state, and multiple equivalent temperatures may be involved in the process.

High-temperature superconducting (HTS) magnets are widely used in high field applications due to their exceptional current-carrying capabilities. Traditionally, these magnets are powered by high current power supplies via current leads, which complicates insulation between cryogenic and room temperature environments and introduces significant heat leaks. Recent advancements in flux pumps for HTS magnets now allow charging currents in the kA range without the need for traditional power supplies. However, while some research has realized conduction-cooled traveling wave HTS flux pumps, much of the focus for transformer rectifier flux pumps remains on operating them to charge magnets at liquid cryogen temperatures, with limited attention given to the heat load and cooling capabilities of modern dry cryocoolers. This study presents a compact, modular flux pump designed for operation in conduction-cooled environments. It examines the impact of cooling power on the flux pump's performance in such settings. A lumped element model implemented in PLECS is validated through experiments conducted at 70 K and 30 K, demonstrating consistent results. Based on this validation, we demonstrate the capability to charge an HTS magnet to 300 mT (${\approx}$800 A) in a conduction-cooled environment using a compact, self-regulating HTS flux pump.

Thanks to their compact size, reduced energy losses, and enhanced power density, superconducting motors utilising high-temperature superconducting (HTS) materials are particularly appealing for propulsion systems such as those used in electric aircraft. Upon magnetisation, stacked HTS coated conductors are transformed into trapped field stacks (TFSs), which represent a key variant of trapped field magnets. TFSs offer a promising alternative to traditional permanent magnets, providing rotor fields for motor operation, particularly in synchronous machines. To date, there has been a lack of comprehensive investigation into the interaction between HTS TFSs and cryocooled iron cores, as well as the losses generated in iron cores during pulsed field magnetisation (PFM). Furthermore, the optimal magnetisation strategy for achieving the maximum trapped field in TFSs through PFM for motor applications remains an unresolved issue. To address these challenges, a 2D electromagnetic-thermal coupled numerical model was built using the finite element method and validated against experimental data. This model was employed to analyse a superconducting machine featuring TFSs as rotor magnets and cryocooled iron cores. This study introduces two key novelties. The primary novelty lies in identifying the optimal combination of pulsed current amplitudes and TFS layer numbers required to achieve maximum magnetisation fields. The secondary novelty involves evaluating the overall losses in both superconducting and non-superconducting components of the machine during PFM. This includes heat generation in various layers of the TFS and losses in the motor's active materials, such as copper windings and iron cores. Additionally, two practical motor configurations were proposed, and two calculation methods using linear interpolation of iron losses and steel grades were introduced to estimate the iron losses for the studied iron material, M270-35 A. This pioneering study is expected to serve as a valuable reference for loss analyses and structural design considerations in developing superconducting machines.

The mechanism of critical current density (J_c) enhancement in rapid heating, quenching and transformation (RHQT) Nb₃Al wires with nanoparticles was investigated from microscopic defects. As shown in transmission electron microscopy results, the stacking faults (SF) density in RHQT Nb₃Al could be improved by incorporating nano-oxide particles, because the nanoparticles distort the Nb₃Al crystal lattice and expand the distortion generated SFs. The macroscopic measurements based on electron backscatter diffraction showed that many SFs were parallel to the Lorentz force (F_L//SF) in the pure sample, but that the orientation of SFs are distributed randomly in the Nb₃Al wires with nano-oxide. SFs that are normal to the Lorentz force (F_L⊥SF) could hinder the motion of flux lines effectively and thus enhance the total flux pinning force (F_p). As a result, J_c of the incorporated nano-oxide Nb₃Al wires were significantly elevated, where the 1 at.% nano-ZrO₂ and 1 at.% nano-Dy₂O₃ samples exhibited J_c values of 1.86 × 10⁵ A cm⁻² and 1.95 × 10⁵ A cm⁻²@4.2 K,12 T, which improved by about 60% compared to the pure sample. The density and orientation regulation of SFs in RHQT Nb₃Al revealed a new mechanism of J_c enhancement, which provides a new route to further improve the high-field J_c of Nb₃A in future research.

The applications of superconducting cable or magnet require that the superconductors are made into wires or tapes. For cuprate superconductors, this is a big challenge because of the friability and short coherence length of the materials; weak links are easy to be formed at the grain boundaries hindering the flowing of the supercurrent. One of the ways is to fabricate superconducting films on flexible metallic tapes with oxide buffer layers. The successful one so far is the REBa₂Cu₃O₇ (RE = rare earth elements) films in tape form, as called the coated conductors. While the superconducting transition temperature of REBCO system is limited to around 90 K. Here we report the successful fabrication of another new non-toxic superconducting film, namely (Cu,C)Ba₂Ca₂Cu₃O$_{9\pm\delta}$ on these flexible metallic tapes with LaMnO₃ and CeO₂ as the top buffer layers. The onset superconducting transition occurs at 112 and 110 K, and the zero-resistance transition temperatures are about 96 and 98 K, respectively. The temperature dependent resistivity under magnetic fields in different directions reveals a relatively small anisotropy. Further optimization of the films will improve the zero resistance transition temperature, thus can also improve the characteristic properties for applications. Our results show that the (Cu,C)Ba₂Ca₂Cu₃O$_{9\pm\delta}$ is a promising candidate material for the high power applications in liquid nitrogen temperature region.

This work presents the construction, testing, and analyses work of the 835 MHz high-temperature superconducting REBCO insert magnet (H835), a critical component of the ongoing MIT 1.3 GHz HTS/LTS NMR Magnet project (1.3 G). H835 consists of 40 double-pancake coils operating in a solid nitrogen environment at temperatures ranging from 4 to 17 K. It is designed to generate a central magnetic field of 19.6 T within a 79.4 mm clear bore that will house a 54 mm standard warm bore in the future 1.3 GHz NMR system. Building on lessons learned from our previous 18.8 T REBCO insert magnet (H800), which experienced quenching in 2018 that resulted in permanent damage, several improvements have been implemented in this new H835 design and construction. We have charged H835 to the rated current of 230 A and maintained its central field of 19.43 T for over 15 h without any issues before ramping down the current. The 0.17 T (0.87%) error field between measurement and design was due to screening current effect. H835 will be combined with the low-temperature superconducting 500 MHz background magnet (L500) to complete 1.3 G. We believe that this solid-nitrogen-cooled H835 can provide valuable insights for developing high-field liquid-helium-free REBCO magnets.

In this paper, we propose the Reduced Order Hysteretic Magnetization (ROHM) model to describe the magnetization and instantaneous power loss of composite superconductors subject to time-varying magnetic fields. Once the parameters of the ROHM model are fixed based on reference simulations, it allows to directly compute the macroscopic response of composite superconductors without the need to solve the detailed current density distribution. It can then be used as part of a homogenization method in large-scale superconducting models to significantly reduce the computational effort compared to detailed simulations. In this contribution, we focus on the case of a strand with twisted superconducting filaments subject to a time-varying transverse magnetic field. We propose two variations of the ROHM model: (i) a rate-independent model that reproduces hysteresis in the filaments, and (ii) a rate-dependent model that generalizes the first level by also reproducing dynamic effects due to coupling and eddy currents. We then describe the implementation and inclusion of the ROHM model in a finite element framework, discuss how to deduce the model parameters, and finally demonstrate the capabilities of the approach in terms of accuracy and efficiency over a wide range of excitation frequencies and amplitudes.

Second-generation high-temperature superconducting coated conductors (CCs) have excellent properties such as high irreversibility magnetic field, high critical current, and high critical temperature, which offer an extensive potential application. These conductors are typically employed in extremely complex environments characterized by low temperatures, high magnetic fields, and large current-carrying capacities, which lead to a pronounced coupling of mechanical, thermal, electrical, and magnetic fields. Currently, quenching represents a critical challenge limiting their application. To investigate the quenching mechanism and failure modes of superconducting CCs, this study has developed a method called digital image correlation-aided fluorescent thermometry (FT). Based on the different channels of a single color camera, this method facilitates in-situ, simultaneous measurement of deformation and temperature fields, while significantly minimizing the errors associated with positional changes that plague conventional FT. Through non-contact, comprehensive field measurements of the quenching process, triggered by uniaxial stretching of a superconducting CC at 77 K, it unveils the patterns of temperature and deformation during the propagation of quenching in superconducting CCs. This investigation lays the groundwork for future efforts in quench protection and design optimization.

We investigate the correlation between chemical composition and vortex dynamics in Ni-doped RbCa₂(Fe_1−xNi_x)₄As₄F₂ (x = 0, 0.03, 0.05, and 0.07) single crystals using electrical transport and magnetization measurements. In the mixed state, the vortex slush phase, typically observed in a system with moderate disorder, gradually disappears as the Ni doping concentration increases. The activation energy U₀, derived from Arrhenius plots of resistivity, exhibits an H^−α dependence and decreases progressively. Magnetic hysteresis loops measurements reveal the second magnetization peaks and its evolution from non-monotonic to monotonic behavior with temperature. A peak in critical current density J_c with Ni doping is observed, and the maximum J_c occurs at x = 0.03, with an estimated value of 1.7 × 10⁶ A cm⁻². The monotonic decline in T_c and non-monotonic variation in J_c suggest that the partial substitution of Fe by Ni increases the lattice disorder and improves the pinning performance. Referring to the Dew-Hughes model, the fitting results of vortex pinning force density against the reduced magnetic fields (h= H/H_irr and H/H_max) manifest that both normal point pinning and surface pinning contribute to the pinning effect on the critical current, with normal point pinning gaining prominence at higher Ni doping concentrations. The findings offer valuable insights into the role of chemical doping in vortex pinning and critical current density enhancement, deepening our understanding of the pinning mechanisms in iron-based superconductors with intergrowth structures.

Exploration of superconducting metal diborides with a high critical temperature (T_c) has attracted much attention since the discovery of MgB₂. Recently reported high T_c in compressed MoB₂ has motivated us to investigate the electrical transport properties under high pressure in other metal diborides. We focus on the relationship between T_c and a stretched c-lattice parameter (c/a), known as a crucial parameter for superconductivity in MgB_2, which shows the primary contribution of p electrons to the electronic structure. In this study, the correlation between T_c and c/a is investigated in NbB₂- and MoB₂-based superconducting diborides, which have totally different electronic structures mainly comprising d electrons from transition metals. Our high-pressure electrical transport measurements and structural analysis revealed that a common positive correlation between T_c and c/a is observed in NbB₂, Zr-stabilized MoB_2, and these defected systems. Our insights open further avenues for the development of superconducting metal diborides.

The U.S. Magnet Development Program (US-MDP) explores high-field accelerator magnets compatible with operational conditions beyond the limits of Nb₃Sn technology. The ongoing R&D High-Temperature Superconductors (HTS) suggests using Bi₂Sr₂CaCu₂O$_{8-x}$ (Bi-2212) as superconducting element. Bi-2212 Rutherford cables maintain a high critical current ($I_{\mathrm{C}}$) when exposed to a large external magnetic field. However, Bi-2212 exhibits an oversensitive stress–strain response when subject to large Lorentz forces. This paper reports on the magnetic and mechanical analysis of the Bi-2212 cosine-theta insert being developed at Fermilab for a hybrid magnet composed of two external layers of Nb₃Sn and two internal layers of Bi-2212. We performed a FEM analysis of the insert to estimate the HTS stress state in the coil's strands under magnetic and mechanical loads.

High-temperature superconducting (HTS) maglev technology has gained significant attention for its self-stabilizing levitation and guidance properties. However, the development of electromagnetic turnouts for HTS maglev systems remains underexplored, limiting their application in complex rail networks. This study processed an electromagnetic turnout with trapezoidal switch pole based on the evaluation that incorporates magnetic field and gradients. The levitation force characteristics of HTS maglev during multiple turnout crossings are analyzed with released both lateral and yaw (BLY) degrees of freedom. Furthermore, three constraint states of the levitation system are examined and found that the BLY constraint significantly reduces levitation force fluctuations and ensures smoother turnout transitions. The findings provide critical insights for designing electromagnetic turnouts and optimizing constraint conditions for HTS maglev systems, offering potential for improved performance in turnout and complex curve crossings.

A bulk high-temperature superconductor (HTS) employed in the high-temperature superconducting levitation device is susceptible to cracks or defects during its fabrication and utilization. This may cause changes in the electromagnetic (EM) and vibrational behaviors of the HTS, even affecting the normal operation of the device. To this end, based on the magnetic, thermal and motion field equations for the transversely isotropic superconducting material, we establish a 3D coupled model of a levitation system containing an HTS with a slot defect and permanent magnet (PM), to investigate the vibrational and EM behaviors of the HTS with the defect above the PM, and discuss the effects of the depth of the defect, frequency of the external excitation, and combined defect on the behaviors in the time or frequency domains. The results show that the intrinsic frequencies of the HTS with defects of different depths are in the ranges of 6–8 Hz, 13–16 Hz and 19–20 Hz. With the increase of the defect depth, the intrinsic frequencies and EM losses increase but the vibration amplitude decreases. Under the action of the simple harmonic excitation with the same amplitude, the larger the defect depth, the lower the vibration amplitude and the better the stability of the phase trajectory. With the increase of the excitation frequency, the vibration amplitude decreases and the losses increase. However, when the excitation frequency is close to the intrinsic frequency, the HTS resonates and its internal fluxes undergo the violent motion. When the HTS contains a composite defect consisting of two single defects, its intrinsic frequencies are higher than those for the HTS with either defect included. And its EM losses are greater than the sum of their losses. The modeling method presented here can provide an idea and a reference for the research of EM-vibration behaviors of superconducting levitation systems containing defects.

Bi₂Sr₂CaCu₂O_x (Bi-2212)/Ag round wires prepared by the powder-in-tube method is a promising high-temperature superconductor for ultra-high field applications. The current capacity of Bi-2212/Ag wires is negatively affected by the impurity phase of Bi₂Sr₂CuO_x (Bi-2201). In this work, a novel precursor powder fabrication technique was developed to eliminate the presence of large residual Bi-2201 grains and to enhance the critical current density (J_c) of Bi-2212/Ag wires. Bi-2212 incorporating (Sr, Ca)₁₄Cu₂₄O_y (14:24AEC) precursor powders was prepared using a combination of ultrasonic spray pyrolysis and calcination processes. The influence of the 14:24AEC phase on the phase, microstructure, melting behavior and superconducting properties of Bi-2212/Ag wires was systematically investigated. During the overpressure heat treatment, the addition of 14:24AEC into the wire promoted melting, effectively reducing the formation of large Bi-2201 grains. Moreover, the formation of narrow Bi-2201/Bi-2212 intergrowth and stacking faults were introduced into the Bi-2212 matrix. The Bi-2212/Ag wire with added 2 wt% 14:24AEC exhibited a maximum J_c of 4670 A mm⁻² at 4.2 K and 12 T. Therefore, precise tuning of the alkaline earth cuprate composition in precursor powders could be a valid approach for improving the current capacity of Bi-2212/Ag multi-filamentary wires.

Superconducting undulators (SCUs) are increasingly vital for synchrotron light sources and free-electron lasers in modern particle accelerators due to their ability to surpass the performance limitations of traditional permanent magnet undulators (PMUs). This paper delves into the theoretical framework and key technologies of SCUs, substantiating findings with experimental verifications. The theoretical analysis and numerical simulations of SCU magnetic fields are conducted based on Maxwell's equations, leading to optimized magnetic field distributions. Advanced manufacturing processes, such as high-precision coil winding and epoxy resin impregnation, are developed and refined. The paper also details the SCU's cryogenic, power supply, quench protection, vacuum, and magnetic field measurement systems, ensuring safe and stable operations in real-world conditions. Prototypes, including 1.5-meter-long SCUs, undergo rigorous tests at liquid helium temperatures to validate reliability, revealing that while the SCU meets the design goals in both the vertical and horizontal states, gravity effects in the horizontal state deteriorate the quality of the magnetic field and require iterative optimization. The improvement of the magnetic field quality is achieved by locally adjusting the tuning gap and the end correction coils. Despite the advances that have been made, it is essential to further enhance the performance of SCUs by optimizing the end design and adding means to regulate the magnetic field at low temperatures. This study lays a solid theoretical and technical foundation for the application of SCUs in advanced light sources, and is expected to provide a meaningful reference for the development of more SCUs in the future.

Cold-electron bolometers have shown suitability for use in modern fundamental physical experiments. In this study, the fabrication and measurements of the samples with cold-electron bolometers integrated into coplanar antennas are performed. The absorber layer was made using combined aluminum-hafnium technology to improve quality of aluminum oxide layer and decrease electron–phonon coupling. The samples of two types were measured in a dilution cryostat at various temperatures from 20 to 300 mK. The first sample with Ti/Au/Pd antenna shows response in the two frequency bands, at 7–9 GHz with bandwidth of about 20%, and also at 14 GHz with 10% bandwidth. The NEP below 10 aW$\sqrt{\mathrm{Hz}^{-1}}$ is reached at 300 mK for 7.7 GHz signal. The second sample with aluminum made antenna shows response in the frequency range 0.5–3 GHz due to the effect of kinetic inductance of superconducting aluminum.

Transition-metal-based carbides, renowned for their exceptional hardness, mechanical strength, and thermal properties, have recently emerged as promising candidates for topological superconductivity. In this study, we synthesized ReC in the NaCl structure at ambient pressure substituting Mo (x = 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 0.9) or W (x = 0.2, 0.5, 0.8) at the Re-site. We investigated the superconducting properties of Re_1-xT_xC (where T = Mo, W) for x = 0.5 using magnetization, resistivity and specific heat measurements. These compounds display type-II, fully gapped, weakly coupled superconductivity with high critical fields, establishing them as new members of superconducting ultra-hard materials at ambient pressure and paving the way for superconducting device applications under extreme conditions.

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.

High-temperature superconductors, such as REBa₂Cu₃O_7−x (REBCO, RE = rare earth), are becoming pivotal for high-field magnet technology for future circular colliders and compact fusion reactors. The U.S. Magnet Development Program, in collaboration with industry, is developing REBCO magnet technology using round conductors consisting of multiple REBCO tapes. For these multi-tape cables, traditional instrumentation, such as voltage taps and resistive strain gauges, become insufficient to help measure and understand the performance-limiting factors in these model magnets. Distributed fiber-optic sensing (DFOS) is a potential solution to address this challenge. Although DFOS is well established for various applications, measuring temperature and strain in high-temperature superconducting magnets is in its infancy. Here we report the detailed implementation and test results of DFOS based on Rayleigh scattering in a subscale canted cosθ (CCT) dipole magnet using high-temperature superconducting CORC^® wires. We co-wound optical fibers in each layer of the CCT magnet and compared different types of commercial fibers and mold-release agents to reduce the power attenuation in the fibers. The DFOS allowed us to measure mechanical deformation and temperature along the conductor during tests at 77 and 4.2 K. The measured strain agreed quantitively with a finite-element mechanical model of the subscale magnet. Our results indicate that DFOS can effectively identify locations of strain and temperature changes, offering unique insight into magnet performance that can advance our understanding and development of the REBCO magnet technology for high-energy physics and fusion applications.

We measured the frequency-dependent magnetization losses of twisted multifilament MgB₂ wires subjected to ac transverse magnetic fields with a small amplitude, under which coupling loss dominates magnetization loss, and then, we determined their coupling time constants as well as their geometry factors of coupling losses at various temperatures. We also measured the hysteresis loops of their magnetizations using a vibrating sample magnetometer, and then, we calculated the hysteresis losses at various magnetic field amplitudes and at various temperatures. Note that such calculated values represent the hysteresis losses in completely decoupled filaments in a twisted multifilament MgB₂ wire. Using these values obtained experimentally, we calculated the magnetization losses of twisted multifilament MgB₂ wires under practical operating conditions, i.e. at various magnetic field amplitudes, at various magnetic field frequencies, and at various temperatures, considering the saturation of the filamentary region in the wire, which occurs at magnetic fields with high frequencies and large amplitudes.

Vanadium trioxide (V₂O₃) is an ideal candidate for electrical insulation materials in rare-earth barium copper oxide (REBCO) magnet applications owing to its metal-to-insulator transition (MIT) behaviour that is facilitated by its adjustable resistance, which shifts from high (at temperatures below 77 K) to low (above the transition temperature of ∼160 K). Optimising the turn-to-turn contact resistance (R_ct) in both normal and quenched states provides the benefits of both insulated and non-insulated (NI) winding methods. To implement V₂O₃ as a turn-to-turn insulator in REBCO magnets, ensuring responsive MIT activation between the turns is crucial. This study experimentally investigated the electrical behaviour of V₂O₃ insulation, which undergoes an electrical phase transition thermally triggered by transport current flow between the turns. Current–voltage (I–V) tests were performed on sandwiched REBCO tape samples with and without V₂O₃ insulation to analyse the electrical and thermal reactions. The I–V tests included stepped and pulsed current ramping to examine the repetitive behaviours under varying conditions. Furthermore, we verified the effect of mixing metal powder with V₂O₃ on the I–V characteristics. The results suggest that V₂O₃ insulation produces nonlinear I–V characteristics due to its negative temperature dependence, while the NI samples display linear I–V properties. The MIT characteristics of V₂O₃ insulation were reproducible across different current transport patterns and repeatable over thousands of cycles. Moreover, I–V measurements revealed that increasing the metal powder concentration reduced the R_ct values in the V₂O₃-insulated layer. Thus, maintaining high R_ct values (∼10⁷−∼10⁴μΩ cm²) in the normal operating range of below 77 K in REBCO magnets can effectively mitigate leakage current issues in the no-insulation winding technique. Conversely, transitioning to low R_ct values (∼10³μΩ cm²) in the transient operating range above 160 K is expected to minimise stability degradation caused by quenching or extended thermal runaway in the insulation winding technique. These findings provide valuable insights in designing V₂O₃-insulated REBCO magnets for diverse applications, such as rotating machines, accelerator and superconducting magnetic energy storage.