Edge impregnation is a key technology for fabricating robust, high-field REBCO insert magnets. However, the effect of screening current-induced stress (SCIS), a critical challenge in ultra-high field magnet design, has not been fully clarified. This study aims to investigate the influence of SCIS on edge-impregnated REBCO coils through an electromagnetic-mechanical analysis. The investigation focuses on the 33 T cryogen-free superconducting magnet (33T-CSM) under development at HFLSM, Tohoku University. The analysis revealed that SCIS significantly amplified the hoop strain in edge-impregnated coils, predicting a maximum strain exceeding the conductor's irreversible limit. Compared with the full impregnation, the edge impregnation was insufficient to suppress SCIS to the acceptable level even with mechanical reinforcement although it made the delamination stress negligible. Notably, the analysis overestimated the hoop strain compared with experiments because the large-scale prototype coil (33T-LPC) has been successfully validated without degradation. Possible origins for the overestimation are discussed, including interfacial friction between the stacked pancake coils.

High temperature superconductors, especially YBa2Cu3O7-delta (YBCO), are considered a key enabling technology toward a clean energy future. Hole doping in YBCO is a prerequisite for the emergence of its unchallenged superconducting properties. Up to now, research was focused on the under- and optimally doped region, due to practical limitations in reaching the overdoped state, despite being highly interesting from fundamental and applied aspects as competing orders vanish and critical current densities are expected to peak. Here, we deploy for the first time an electrochemical method to access the mostly uncharted overdoped region. We demonstrate precise control over the bulk oxygen concentration in YBCO thin films across the full off-stoichiometry window () using electrochemical oxidation combined with in situ X-ray diffraction (XRD) and electrical measurements. Resulting high doping states and critical current densities are confirmed using a multi modal approach, including XRD, electrical, Hall and magnetic characterization. Thus, this work opens a promising pathway based on electrochemical oxidation toward electronically clean, oxygen overdoped cuprate superconductors and therefore will assist to further push the critical current density to its intrinsic limit.

REBCO HTS material makes it possible to consider very high engineering current densities and energy densities in coil windings Their use for very high field insert is already extensively studied, but they are also attractive for high field outserts or other meter-scale standalone magnets. In this work present preliminary studies for an insulated 500 mm split REBCO magnet concept. The conductor consist of co-wound REBCO and Hastelloy reinforcement tapes, In order to use efficiently the conductor, full impregnation is considered to enable effective stress averaging in the winding. Such impregnation is made possible by the use of thin film insulation on the co-wound metal reinforcement tape limiting the risk of delamination when cooling down. This concept was validated on a small-scale pancake tested under high stress in background field. Mechanical modeling compared to experimental strain measurement demonstrate the stress redistribution.

A 33T cryogen-free superconducting magnet (33TCSM) project is now in progress at HFLSM, IMR, Tohoku University. The 33T-CSM consists of a f 68mm- 19 T REBCO (HTS) insert and a f 320mm-14 T CuNb/Nb3Sn and NbTi Rutherford (LTS) magnets. The 33T-CSM system has been installed and tested without the HTS insert in March 2024. The LTS outsert magnet consists of three CuNb/Nb3Sn Rutherford cable coils and two NbTi Rutherford cable coils with an epoxy impregnation. It can generate 14 T in a 320 mm bore with 879 A. This winding makes use of advanced high strength CuNb/ Nb3Sn strands specifically developed for the 33T-CSM project, enabling high stress design with about 275 MPa in the Nb3Sn coil at 14 T. The 19 T-HTS insert is designed based on the robust REBCO coil technology we proposed in previous works. For the cooling system, a 9W GM/JT cryocooler is used for the LTS coils, four 4K-GM cryocoolers cool the REBCO coils (1.5 W each at 4.2 K) and two single-stage cryocoolers are used for the radiation shield and current leads. Helium circulation with compressors ensure the thermal connection between the coils and the cryocoolers. The initial cooling is about 7.3 days from room temperature. The test of the 14 T LTS magnet was successfully completed up to 839 A with the nominal maximum electromagnetic stress of 275 MPa after one training quench.

A 33 T cryogen-free superconducting magnet (33 T-CSM) is under development. The 33 T-CSM consists of a REBCO insert coil and Nb3Sn/NbTi outsert coils. The REBCO insert coil is designed to generate 19 T in the external field of 14 T. The REBCO insert coil is composed of stacked 64 single pancake coils wound with two bundled REBCO tapes. The inner and outer diameters of the REBCO insert coil are 68 mm and 295 mm, respectively. The REBCO coil is impregnated with epoxy resin for conduction cooling. To prevent delamination of the superconducting layer by thermal stress, the fluorine-coated polyimide tape is co-wound with REBCO tapes and to prevent degradation of superconductivity by electromagnetic stress, reinforcing tape is also co-wound. According to 2D-FEM, it is shown that the circumferential strain & varepsilon;(theta) under applying electromagnetic force is 0.29% . The results of 2D-FEM also suggest that stress concentration occurs at the connection between the coil and the bus bar, and at the widthwise end of the REBCO tape. In this paper, the basic design of the insert coil and the results of FEM analysis will be described.

The development of a 33 T cryogen-free magnet is taking place at the High Field Lab. for Superconducting Materials, Tohoku University. It consists of a 14 T large bore LTS background magnet with a 19 T REBCO insert, which is the focus of this work. As it is conduction-cooled and must be ramped-up rapidly, a conventional isolated pancake-based design is preferred for the insert coil. It will follow the robust REBCO coil concept already introduced in previous works, with the so-called edge-impregnation technique enabling stress redistribution as well as conduction-cooling and a conductor made of two tapes co-wound for reliability. Still, the exact conductor structure is not yet fixed, in particular in terms of mechanical reinforcement. In this work, the REBCO coil design is discussed from two point of views: coil mechanical behavior and protection against local thermal runaway when operated in combination with the LTS outsert, using modelling approaches. The necessary tradeoff between mechanical and electrical margins is highlighted, guiding the choice for the future 33 T insert conductor structure.

REBCO Coated Conductors have high tensile strength along their length. They are however sensitive to delamination when traction force is applied on the tape surface, which in a pancake winding corresponds to radial tensile stress. For this reason, most REBCO magnets are dry-wound (not impregnated). One of the drawbacks of dry winding is that conduction cooling to low temperature (below 20 K) would be inefficient. Edge impregnation structures were developed to mitigate this problem but retaining a soft non-stick insulating layer in between the turn. In this paper, we propose an improved pancake structure with the benefits of edge-impregnation but using Hastelloy tape with thin insulation coating to reinforce the winding. The mechanical behavior of such structure is first estimated then validated with high current tests under large background magnetic field. The insulation performance is then tested by rapid ramping and fast discharge.

For next generation of high field cryogen-free superconducting user magnets, we adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration, a fluorine-coated polyimide tape as the inter-bundle insulation and thin FRP plates glued on the coil edges. To validate our coil design, we investigated electromagnetic and current-voltage properties of 20-stacked REBCO pancake coils wound with Fujikura EuBCO tapes with the robust coil concept at 4.2 K under a background field of 14 T. The inner and outer diameters and total height of the coils are 68 mm, 266 mm, and 101 mm, respectively. The average of measured hoop and axial strain on the outermost winding at the mid plane of the 20-stacked coils were 0.23% and −0.15% respectively at a hoop stress of 370 MPa estimated by the BJR relation. The I - V properties of the coils showed no anomalous behavior up to 25 T totally generated by the coils and background field, suggesting that our coil structure has an advantage against large electromagnetic stress.

Resistive REBCO fault current limiter (FCL) is a promising solution to the fault current issue in electric grids, which is not fully satisfactorily solved. However, the cost of the superconducting (SC) REBCO conductor should be lowered to facilitate the deployment of this innovative device. What we refer to as REBCO conductor is usually a bare REBCO tape bonded to a stabilizer. The design of the stabilizer is fundamental to optimize the conductor and lower the superconducting FCL (SFCL) cost. The goal is to limit the temperature rise for any fault conditions, notably for any amplitude of the prospective fault current. The specificities of SC tapes, including critical current inhomogeneities along the length, must be considered in the design. Here, we study various stabilizer configurations. First, the case of “dielectric stabilizer,” electrically insulating is presented. The theoretical performances are interesting in terms of electric field under limitation, but the low thermal diffusivity remains an issue, as well as the implementation. Some experimental investigations are shown. The results are compared with classical metallic stabilizer designs, which are simpler to implement. To enhance the electric field under limitation, a variant of metallic stabilizer is then introduced, using a corrugated structure. A design showing an electric field under limitation of 200 V/m (clearing time of 50 ms) is presented. Further optimizations and higher electric fields seem achievable even if practical implementation remains very challenging.

We recently proposed a robust REBCO HTS (RareEarth BaCuO) insert coil concept in the framework of a project to upgrade the 25 T Cryogen-Free Superconducting Magnet at the High Field Laboratory For Superconducting Materials to reach 30 T. In this article, the conductor consists of a two-tape bundle to mitigate the risks posed by local tape degradation. So-called edge impregnation is used to enhance the mechanical stiffness while reducing delamination risk, providing conduction-cooling capability at the same time. This insert is to be protected against the risk of thermal runaway simply by using dump resistor, thanks to an early detection concept. The different ideas forming the robust REBCO insert coil concept were tested successfully on various small scale prototypes. We report here their first implementation on a large-size insert coil, consisting of 20 stacked pancakes from the 30 T CSM upgrade insert. The mechanical behavior of the coil is modelled and validated with experiments. The thermal runaway detection setup is introduced. Its sensitivity and selectivity is discussed based on previous works on thermal runaway scenarios.

There is an ongoing plan to upgrade the 25 T cryogen-free superconducting magnet (25T-CSM), at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, to a 30T-CSM by replacing the existing Bi2223 insert coils with REBCO coils. For the REBCO coils of the 30T-CSM, we will adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration with thin FRP plates glued on the edges. To confirm the effectiveness of this concept, we performed hoop stress tests on full-scale-size four stacked pancake coils fabricated with Fujikura EuBCO tapes at conduction-cooled 20 K under a background field of 11 T. The inner and outer diameters of the pancake coils were 68 and 268 mm, respectively. The averages of the measured strains on the outermost winding were 0.17–0.24% at a hoop stress of 400 MPa estimated by the BJR relation. The I-V properties of the coils indicated no anomalous voltage, implying that our coil structure was robust against large electromagnetic stress.

AC losses in a high temperature superconducting (HTS) coil are experimentally evaluated. Double-pancake (DP) coils with turn-to-turn electrical insulations are wound using bundle conductors formed with two pieces of rare-earth-based coated conductors located face-to-face without insulation to improve thermal stability. Five DP coils are stacked and subsequently four copper plates are soldered between the adjacent DP coils to form a small-size HTS coil for AC loss measurements. The fabricated HTS coil is immersed in liquid nitrogen. Before measuring the AC losses, the contact resistances between the DP coils are observed in DC operations of the HTS coil at first. After that, the total losses including the Joule losses in the joints between the DP coils are measured by integrating the products of almost resistive components extracted from the terminal voltages in the HTS coil and applied transport currents observed using a pickup coil over a cycle under AC operations. Net AC losses in the two-ply bundle conductor windings are obtained by subtracting the Joule losses from the measured ones. In order to understand the mechanism of AC losses in the two-ply bundle conductor windings, the influences of current amplitudes and frequencies are investigated experimentally and theoretically.

The resistive-type of superconducting fault current limiters (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) are well adapted to power grid protection. Almost electrically invisible in normal operation, it quenches and becomes highly resistive in the event of a fault. The tape operation at the transition from the superconducting state to the resistive state is the basis of a R-SFCL. In this scope, we studied the onset of a quench occurring in 2G HTS tapes using high-speed imaging to record the bubble generation in liquid nitrogen. In the first milliseconds of operation, the bubble generation appears highly inhomogeneous. Dissipation is initiated on multiple spots over the tape surface, which first expands in the direction of the width of the tape and then along its length. A detailed analysis of the REBCO layer is performed by means of scanning Hall probe microscopy (SHPM) after the optical study of the tapes is completed. Analysis of the local current density distribution demonstrates that the bubble generation occurs at positions of local inhomogeneities in the REBCO layer. These inhomogeneities, well recognized through SHPM, are not reliably described by the critical current as a function of the position derived magnetically (e.g., by traditional TapeStar measurements). Similar results were obtained on samples from Superpower and SuperOx.

Local degradation of critical currents in REBa2Cu3Oy (REBCO, RE: rare-earth and Y) coated conductor is one of the serious issues especially for high field superconducting magnets. It may give rise to a localized hotspot, followed by a burn-out in the magnet. To mitigate such phenomena, a co-winding of bundled two REBCO coated conductors is considered. The bundled REBCO double pancake coil with a local damaged area was made and tested in LN2 and conduction cooling conditions. The Ic of the damaged coil showed more than 90 % of that of the non-damaged coil. The comparison of the Ic distribution in the coil and the Ic value at the damaged area can explain the good Ic performance of the damaged coil. Even if the operation current exceeds the local Ic due to the local damage, the coil can be protected if we would detect the quench with a threshold of less than 18 mV for the practical 30 T cryogen-free superconducting magnet.

Inhomogeneous temperature elevations, also called hot spots, occurring in second generation high-temperature superconductor (2G HTS) tapes may lead to their destruction. A better understanding of inhomogeneous quenches would contribute to develop strategies to better protect superconducting devices based on 2G HTS tapes against hot spots. To do so, we investigated the current redistribution around a dissipative zone in a 2G HTS tape with a combination of experiments and numerical simulations based on the finite element method (FEM). Firstly, the inhomogeneous heat generation in a commercial 2G HTS bare tape (without copper cladding) was observed through the visualization of bubble generation. Secondly, the current redistribution around a dissipative zone in commercial 2G HTS bare tapes from two different manufacturers was investigated using voltage taps on both sides of the tape. The measured voltages showed that the current redistribution around the dissipative area in the top stabilizer layer of the tape is different from that in the bottom stabilizer layer. Using a 3D electro-thermal FEM model, we reproduced these behaviors, assuming a HTS tape architecture with an inhomogeneous local critical current density. Finally, using the same FEM model, we explored the impact of a lack of silver on one lateral side of a 2G HTS tape. Our results indicate that such a lack of silver does not critically affect the quench dynamics.

In the framework of the BOSSE project, a 12 T insulated REBCO solenoid is being manufactured. This magnet will be used as a SMES with the objective to reach 1 MJ and a specific energy of 20 kJ/kg for the winding. To manufacture this solenoid and reach such performances, 21 insulated REBCO Double Pancakes (DPs) will be assembled and cooled in a liquid helium bath at 4.2 K. To validate the solenoid's electromagnetic design, a prototype DP tested in self-field up to its critical current (972 A), thanks to a sensitive protection system, was presented in a previous work. Here we present the background magnetic field test of this prototype DP which allowed to validate the coil's mechanical design. In order to validate the electromagnetic performances of the 21 DPs, each of them is tested in liquid helium up to its rated current. The performances of the 17 DPs already tested and validated will also be presented. Finally, we will present the tests and results of 2 preliminary assemblies of 3 and 5 DPs. These assemblies were tested with two different protection philosophies. The results of these tests show that an individual protection of each DP is to be preferred in order to obtain a better sensitivity on the protection signal.

REBa 2 Cu 3 O 7-δ (RE: rare earth/Y) coated conductors have good critical current characteristics under high magnetic fields. Their application to superconducting magnets generating above 25T is under active study. However, performance inhomogeneity along their length may cause damaging hotspots. Adapted winding technologies are needed to mitigate this phenomenon. One of such winding techniques is the two-tape bundle co-winding method, where two REBCO tapes are co-wound along with an isolating tape to form the conductor. To test this solution, a double pancake coil was wound with one of the pancakes having an artificially degraded short section on one of the tapes, where the critical current is close to 0. The I-V characteristics of both damages and undamaged pancakes are compared and analyzed based on critical current density J c ( T,B ,θ) data, in terms of operation margins. At low voltage levels, similar I-V characteristics for damaged and undamaged pancakes are observed. The behavior is found similar to the expected behavior of a coil whose critical current I c is locally reduced by 50%, demonstrating the effectiveness of two-tape bundle for current redistribution in case of local defect, even under high current density.

AC losses in stacked bundle conductors exposed to external magnetic fields are numerically evaluated using a two-dimensional finite element method formulated with a self-magnetic field owing to currents induced in an analysis region. The bundle conductor is composed of two pieces of (Re)BCO coated conductors without electrical insulation to improve the thermal stability. In the analysis models, an idealized copper layer is sandwiched between a pair of superconducting layers in every bundle conductor. The external magnetic fields are increased monotonically from zero to simulate the electromagnetic responses in parts of a single pancake coil for a high field magnet. To understand only the geometrical effect on AC loss, the superconductors are assumed to be subject to the Bean model, in which the critical current density is independent of the local magnetic field. The influences of the number of bundle conductors, gap between bundle conductors, and applied angle of the magnetic field on AC loss are investigated.

The upgrade project of the 25T-CSM to 30 T is on-going at HFLSM, IMR, Tohoku University. The goal is to obtain 30 T in a 32 mm warm bore by replacing the present Bi2223 insert coil with one wound using REBCO. In order to develop a robust REBCO high field insert, we adopt a two-tape bundle conductor with a face-to-back configuration. We confirmed that this conductor configuration mitigates effectively the risk posed by local defects in an R&D coil: a stable operation was achieved with negligible degradation in coil I c even with a severe local defect on one of the two tapes. Quench protection from possible hotspot in the REBCO insert using an early detection method is made possible by the two-tape bundle configuration and the thick Cu stabilizer. Other R&D coils using such conductor were tested with large hoop stresses up to 460 MPa with no degradation. On the basis of these R&D studies, a robust REBCO insert is designed for the upgrade. The designed REBCO insert has a capability to achieve 35 T with enough I c margin if we accept a maximum hoop stress of 500 MPa.

The upgrade to 30 T of the existing 25 T cryogen-free magnet (25 T CSM) at the High Field Laboratory for Superconducting Materials, Tohoku Univ., will consist in replacing the existing High Temperature Superconductor insert made of Bi-2223 by one wound using REBCO tape. The new insert is designed to produce 16 T instead of 11 T. In order to have good dynamics and allow sweeping mode experiment, the insert will be conventionally isolated, as it was the case for the previous Bi-2223 version. In previous work we modelled thermal runaway phenomenon and showed that a sensitive dissipative voltage detection technique can give sufficient warning to protect an HTS coil using external dump resistor even in the case of sharp local critical current drop. For the 30 T insert, the conductor will consist of a two tape bundle so as to reduce the chance of such sharp Ic drop due to tape performances inhomogeneities or local damage. We simulate the quench dynamics in the LTS outsert coupled with the HTS insert to demonstrate that the existing detection/protection system of the 25 T CSM can be re-used and determine the current dumping dynamics. We then use our thermal runaway modelling tool to study the behavior of the future 16 T REBCO insert in case of local sharp defect and determine realistic detection threshold for its safe protection.