記述言語：英語   掲載種別：研究論文（学術雑誌）

In order to improve the electrical transport properties of REBa2Cu3O7-δ nanocomposite films, SmBa2Cu3O7-δ films with and without BaHfO3 nanoparticles were grown by chemical solution deposition, and their microstructural and transport properties were investigated in a detailed study using transmission electron microscopy and transport measurements in magnetic fields up to 24 T. The optimization process of the crystallization step (temperature and oxygen partial pressure) as well as an aging effect, which is due to the release of trapped fluorine, are described. Critical temperature and critical current densities surprisingly improve initially during the aging. Due to the complex microstructure, the additional BaHfO3 nanoparticles have only a positive effect at low magnetic fields for our samples.

DOI： 10.1088/1361-6668/ac7b4d

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

To understand the temperature, magnetic field, and angular dependence of the critical current density, Jc(T, B, θ), of (Bi, Pb)2Sr2Ca2Cu3Oy (Bi2223) core composed of commercial Bi2223 tapes, we investigated Jc(T, B, θ) of a bundle of Bi2223 filaments extracted from Ag-sheathed Bi2223 tapes under broad (T, B, θ) conditions. We found that flux pinning curves for B||c (θ = 0°) exhibit a temperature-scaling relation at high T (50-77.3 K) but not at low T (30-40 K). We also found that Jc(θ) exhibits a broad peak in the B||ab (θ = 90°) direction and attributed it to the broadening of a sharp peak due to the intrinsic pin caused by a c-axis misalignment. We plotted Jc(T, B, θ) as a function of the tape normal component of the magnetic field considering the c-axis misalignment, and it agrees well with the measured Jc(T, B, 0°). This indicates that the intrinsic pinning is strong in Bi2223 and Jc(θ) of Bi2223 is dominated by the flux pinning for the B||c component. Based on these findings, we succeeded in reproducing Jc(T, B, θ) at 50-77.3 K by a model taking account of the T-scaling law, c-axis misalignment, and intrinsic pinning.

DOI： 10.1109/TASC.2021.3129727

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core ω0τcore is 1 ± 0.5, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small ω0τcore value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion.

DOI： 10.7566/JPSJ.90.094704

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We attempted to control the volume fraction of the in-plane domains of REBa$_{2}$Cu$_{\bf 3}$O$_{\bf 7-\delta }$coated conductors (REBCO-CCs) by annealing under a bending strain applied at high temperature. We succeeded in changing the volume fraction of the region where the $a$axis of REBCO aligns along the longitudinal direction of REBCO-CC, $f_{\rm A}$, up to $\simeq\! 90\%$when a compressive bending strain of $\simeq\! -1.35\%$is applied along the longitudinal direction. We found that $f_{\rm A}$evaluated by repeated X-ray diffraction measurements decreases over time if the domain-controlled REBCO-CCs with a high $f_{\rm A}$value is kept flat, whereas $f_{\rm A}$remains high if the sample remains bent. In addition, we also observed that $f_{\rm A}$in the as-received sample increases over time if the sample is exposed to a compressive bending strain at room temperature. These results suggest that oxygen atoms can migrate in REBCO even at room temperature when a uniaxial strain is applied to the REBCO-CC. Such a temporal variation of $f_{\rm A}$probably leads to changes in the superconducting properties, and may become important for the design of REBCO pancake coils.

DOI： 10.1109/TASC.2021.3063995

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We investigated the mechanical properties and superconducting characteristics of high-strength (Bi, Pb)2Sr2Ca2Cu3O10+δ tapes reinforced by Ni-based alloy lamination over a wide range of temperature (30 K≤T ≤ 300 K), magnetic field (B ∥ c ≤ 6 T), and uniaxial tensile strain (ε ≤ 0.6%) conditions. We found that the Young’s modulus evaluated from the stress–strain curves increases gradually with decreasing temperature (120 GPa at 300 K and 142 GPa at 40 K), indicating the importance of low-T data for designing high-field magnets. The enhancement of the Young’s modulus of the tape at such low temperatures is probably attributable to the use of a Ni-based alloy for reinforcement and is also reflected in the irreversible stress. The critical current, Ic, measured under various (T, B, ε) conditions decreases almost linearly with uniaxial tensile strain in the reversible region. We constructed a phenomenological model of Ic(T, B, ε) that can roughly reproduce the observed Ic under all the (T, B, ε) conditions we investigated. We expect that fine adjustment of the model parameters based on more detailed measurements will make it possible to estimate Ic(T, B, ε) under arduous measurement conditions from data obtained under conditions where measurement is relatively easy.

DOI： 10.1088/1361-6668/abced2

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We developed a new microwave Hall effect measurement method for materials in the skin depth region at low temperatures using a cross-shaped bimodal cavity. We analytically calculated electromagnetic fields in the cross-shaped cavity and the response of the cavity including the sample, whose property is represented by the surface impedance tensor; furthermore, we constructed the method to obtain the Hall component of the surface impedance tensor in terms of the change in resonance characteristics. To confirm the validity of the new method, we applied our method to measure the Hall effect in metallic Bi single crystals at low temperatures, and we confirmed that the microwave Hall angles coincide with the DC Hall angle. Thus, it becomes clear that the Hall angle measurement under cryogenic conditions becomes possible without any complicated tuning mechanisms, and our bimodal cavity method can be used to measure the microwave Hall effect on materials in the skin depth region. The result opens a new approach to discuss the Hall effect in condensed matter physics such as the microwave flux-flow Hall effect in superconductors.

DOI： 10.1063/5.0033777

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The electric transport properties of a GdBa2Cu3O7-δ thin film containing 12 mol% nano-sized BaHfO3 (BHO) particles grown by chemical solution deposition were investigated in a wide range of temperatures (4.2 ≤T ≤ 77 K) and magnetic fields up to μ0H = 19 T. The exponent n of the electric field-current density characteristics (E ∞Jn) depends on critical current density Jc as (n-1)∞Jαc (α∼0.45) irrespective of measurement temperature for H ≈ c. On the other hand, this relation does not hold for H ≈ ab. The angular dependence of Jc is almost similar to that of n except for the angle close to the ab-plane. A dip of n around this angle regime was observed below 77 K, whereas Jc exhibited a maximum. At T ≤50 K a tiny peak in the dip was observed that increases with decreasing temperatures. These results suggest that the pinning mechanism changes with temperature for H ≈ ab.

DOI： 10.1088/1361-6668/abb205

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

One of the most significant issues for superconductivity is clarifying the momentum-dependent superconducting gap Δ(k), which is closely related to the pairing mechanism. To elucidate the gap structure, it is essential to investigate Δ(k) in as many different physical quantities as possible and to crosscheck the results obtained in different methods with each other. In this paper, we report a combinatorial investigation of the superfluid density and the flux-flow resistivity of iron-pnictide superconductors; LiFeAs and BaFe2(As1−xPx)2 (x = 0.3, 0.45). We evaluated Δ(k) by fitting these two-independent quantities with a two-band model simultaneously. The obtained Δ(k) are consistent with the results observed in angle-resolved photoemission spectroscopy (ARPES) and scanning-tunneling spectroscopy (STS) studies. We believe our approach is a powerful method for investigating Δ(k) because it does not require a sample with clean surface unlike ARPES and STS experiments, or a rotational magnetic-field system for direct measurements of the angular dependence of thermodynamic quantities.

DOI： 10.1038/s41598-020-63304-0

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

REBa2Cu3O coated conductors (REBCO-CCs; RE: rare-earth element or Y) have garnered considerable attention in high-field magnet applications. However, the twin structure existing in the ab plane of REBCO complicates the dependence of critical current density on temperature, magnetic field, and uniaxial strain:. In particular, the mechanism for the appearance of anomalous double-peak structure in of REBCO-CCs with c-axis-correlated pins or with random pins as a function of in-plane uniaxial strain and magnetic field applied parallel to the c axis. In the case of (Y,Gd)BCO-CC with BaZrO3 nanorods, we only observed a mountain-shaped response of under the entire magnetic field. However, GdBCO-CC without artificial pinning centers, where random pinning is dominant, exhibited a valley-shaped response at low fields and then a mountain-shaped response at high fields. These results suggest that the variation in the pinning mechanism (correlated- or random pins) plays a crucial role in the absence/presence of double-peak structure in REBCO-CCs. We developed a phenomenological model for in which (i) strain dependence of superconducting properties in two domains of REBCO and (ii) ideal behavior of correlated- or random pinning are considered. Consequently, we observed that the valley-shaped response of can be realized in the case of ideal random pinning with limited parameters, while ideal correlated pinning always exhibits a mountain-shaped response of. Furthermore, a crossover from a valley-shaped response under low fields to a mountain-shaped response under high fields in the strain dependence of is qualitatively consistent with our observations. This qualitative agreement between our model and experimental data validates the efficacy of our model to provide a clear understanding of the strain dependence of in REBCO-CCs, including the anomalous double-peak structure.

DOI： 10.1088/1361-6668/aba352

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We have measured the temperature and angular dependence of the upper critical field (BC2) for three state of the art (RE)BCO HTS tapes using variable-temperature ac susceptibility measurements in applied fields up to 35 T. The three tapes measured were a Fujikura tape without artificial pinning centers and Superpower tapes with and without artificial pinning centers. We have obtained fits to our BC2(T ,θ) data using both the anisotropic Ginzburg-Landau (G-L) and Klemm's models for layered superconductors. Our calculations suggest that these tapes are three-dimensional (3-D) at all temperatures in zero field but become 2-D in magnetic fields above a crossover field of 457/γ T, where γ = √mc/mab is the G-L anisotropy parameter. The values of γ were measured and show that the 3-D-2-D crossover fields in these tapes are at least 130 T.

DOI： 10.1109/TASC.2019.2895218

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

To clarify the effects of longitudinal magnetic field (LMF) on Y0.77Gd0.23Ba2Cu3O7-δ coated conductors fabricated by the trifluoroacetic-acid based metal organic decomposition method with the ultra-thin-once-coating process (UTOC-MOD (Y,Gd)BCO-CCs), and to understand the contributions of BaHfO3 nanoparticles (BHO-NPs) to this, we investigated the critical current density Jc under a wide range of temperature and magnetic field conditions. We found that Jc in the LMF configuration increased compared to those in the in-plane transverse magnetic field configuration under whole conditions in samples both with (w/) and without (w/o) BHO-NPs. Jc enhancement due to the LMF configuration was remarkable in the w/BHO-NPs sample at higher temperatures, indicating that introduction of BHO-NPs to UTOC-MOD (Y,Gd)BCO-CC would be useful for LMF DC cable applications.

DOI： 10.1109/TASC.2019.2899745

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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 ϵθ 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.

DOI： 10.1109/tasc.2024.3524490

DOI： 10.1109/tasc.2024.3524490

記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/tasc.2025.3525608

DOI： 10.1109/tasc.2025.3525608

記述言語：英語   掲載種別：研究論文（学術雑誌）

The High Field Laboratory for Superconducting Materials (HFLSM) is developing a 33 T Cryogen-free Superconducting Magnet (CSM). The major part of the magnetic field will be achieved thanks to a High Temperature Superconductor (HTS) insulated insert based on the Robust Coil concept. This design features, for each turn, two tapes wound together, in direct contact to enhance the coil's protection. However, a drawback of this architecture is the development of coupling currents caused by variations in the axial component of the magnetic field. These currents lead to additional losses and affect the quality of the magnetic field. This work focuses specifically on the impact of these coupling currents on the magnetic field quality at the location of the future samples in the context of developing a user magnet. The case study of this work is a large-scale prototype of the future HTS insert. To achieve this objective, numerical simulations were conducted to differentiate what are the qualitative effects of coupling currents and screening currents. The results showed that these currents have opposite effects on the magnetic field at the sample position. Additionally, the large-scale prototype experienced long current plateaus, during which the magnetic field was measured. The variations of the latter were driven by the relaxation of these opposing effects, each exhibiting different rates and relaxation times.

DOI： 10.1109/TASC.2024.3514994

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We investigated the temperature (T) dependence of the irreversibility field Hirr(T) in high-critical-temperature cuprate Bi2+xSr2−xCa1−yYyCu2O8+δ (Bi-2212) single crystals over a wide range of hole doping level (p). Hirr(T) was evaluated by measuring the magnetization hysteresis loop. The value of Hirr(T) extrapolated to T = 0 K [Hirr(0)], is either equal to or sets the lower boundary for the upper critical field at T = 0 K [Hc2(0)]. Tc shows a parabolic p-dependence (peak at p = 0.16), whereas μ0 Hirr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, Tc = 24 K) to 209 T for the most overdoped sample (p = 0.200, Tc = 75 K). The present results qualitatively agree with Hc2(0) values evaluated from the specific heat measurements. The observed p-dependence of Hirr(0) in Bi-2212 is distinct from those in YBa2Cu3O7−δ and HgBa2CuO6+δ, in which a pronounced dip structure appears in the underdoped region. Considering that the dip structures observed in these two systems are likely associated with the formation of competing orders (most likely field-induced charge orders), the present results indicate that the influence of the competing order in Bi-2212 is less prominent than that in the other two systems.

DOI： 10.7566/JPSJ.93.104705

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities Jc. The typical approach to increasing Jc is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum Jc of only ∼30% of the depairing current density Jd, which depends on the coherence length and the penetration depth. Here we dramatically boost Jc in SmFeAsO1–xHx films using a thermodynamic approach aimed at increasing Jd and incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupled Jd reaches 415 MA cm–2, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high Jc values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.

DOI： 10.1038/s41563-024-01952-7

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The field and temperature dependence of the critical current Ic is a vital factor in designing the superconducting magnets. The standard method of measuring Ic involves applying a dc current and measuring voltage. However, when assessing the field dependence of Ic, the high magnetic field's space constraints and cooling power limit the injection of dc current into the sample. To address this, we have developed a method using pulsed current to significantly reduce heat generation. Our previous research reported an Ic measurement system capable of handling up to 19 T and variable temperatures with a pulse current up to 500 A. In this study, we have enhanced the system with electric double layer capacitors, known as supercapacitors, allowing currents up to 2 kA. This upgrade enables the measurement of Ic in the high-temperature superconducting REBCO tapes, which are 4 mm wide, at cryogenic temperatures as low as 4.2 K in magnetic fields or 20 K even under a self-field. We have verified the system's reliability by comparing measurements of dc and pulsed currents. Additionally, we contrasted the Ic values of full width samples with those of samples containing micro-bridges.

DOI： 10.1109/TASC.2023.3343670

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2024.3367620

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Commonwealth Fusion Systems (CFS) is pursuing the high-field approach to fusion energy, enabled by magnets composed of 4 mm 2G REBCO wire. Most of these magnets will operate in the high field (20 T) and low temperature (∼20 K) regime and therefore require characterization of the critical current at those parameters. The industry standard is to use (77 K, self-field) data for rapid characterization of critical current. However, Ic (77 K, SF) is a poor predictor of performance at high (>6 T) field, low temperature applications, and so more predictive correlations are extremely desirable. This study examines critical current correlations between high temperature (44-77 K) with low field (0-5 T) and low temperatures (20 K) with high field (6-20 T) from multiple manufacturers. Employing these correlations will allow for the use of cheaper and simpler critical current test systems that can still accurately predict high field, low temperature performance. Moreover, they will provide higher confidence in the predicted performance of magnets. This paper presents the predictive ability of these correlations and the associated error analysis.

DOI： 10.1109/TASC.2024.3360937

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The existence of screening currents is one large drawback coming from the use of REBa2Cu3O7-x with RE standing for Rare Earth (REBCO) tapes to develop superconducting coils. It not only alters the quality of the magnetic field but also implies losses and additional mechanical stresses on the tapes. The High Field Laboratory for Superconducting Materials (HFLSM) from Tohoku University, Japan, together with Toshiba are now developing a 33 T all-superconducting magnet, cooled by conduction, with a 19T Robust REBCO Insert. One important element of the design is a turn made of two-tape bundle and a turn-to-turn insulation. In addition to the screening currents, magnetic field variations will induce coupling currents developing between these two REBCO tapes in direct contact. These coupling currents also affect the magnetic field quality. Recently, a large-scale prototype of this insert was manufactured to validate the mechanical design and the protection system in the event of local dissipation coming from defects in the tapes. This article presents detailed behaviors of the screening currents and coupling currents induced fields obtained over this prototype focusing on the impact of some operating conditions such as the current ramping rate, applied-field history, and temperature of operation. The magnetic field error, i.e., the difference between the measured magnetic field and the one obtained with a homogeneous current distribution, is the main tool used to carry out this study. It reached about 0.5 T and was significantly changed by the temperature of operation and the existence of previous magnetizations, as predicted by simulations results carried out by several groups.

DOI： 10.1109/TASC.2023.3337060

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A 25 T cryogen-free superconducting magnet (25 T-CSM) was installed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University in 2015. The 25T-CSM consists of low-Temperature superconducting (LTS) outsert coils and a high-Temperature superconducting (HTS) coil; high strength CuNb/Nb3Sn and NbTi Rutherford cables, and Bi2Sr2Ca2Cu3Oy (Bi2223) tape were adopted for the LTS and HTS coil, respectively. The Bi2223 coil was a stack of impregnated double pancake (DP) coils. The first Bi2223 coil generated 10.6 T in a background field of 14.0 T by the LTS coils in 2015. Because of the coil stiffness insufficiency, it had an electrical instability. The new Bi2223 coil, made by improving the stiffness of DPs, successfully generated 11.1 T (25.1 T in total) in 2020. Both Bi2223 coils experienced accidental quenches during the operations as a user magnet. However, the magnet's quench detection and protection system protected the coils from the damage including the LTS coils. The 25T-CSM has been operated as a multipurpose user magnet for more than 1100 days in seven years, meaning that almost 100% of the available days were utilized for user experiments.

DOI： 10.1109/TASC.2024.3350566

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Given their excellent superconducting properties, REBa2Cu3Oy (REBCO)-coated conductors (CCs) are anticipated to be utilized in a variety of magnet applications. To further increase the critical current density J c of these materials to levels needed for commercial applications, this study employs reel-to-reel (RTR) pulsed laser deposition (PLD) to fabricate REBCO+BaHfO3 (BHO) CCs. PLD creates BHO nanorods, which serve as flux-pinning defects. The material is subjected to O2+ irradiation to introduce more defects. The irradiation-induced defects serve as flux-pinning centers to the REBCO+BHO-nanorod CCs, increasing J c along the c axis and over a wide range of magnetic-field angles compared with conventional REBCO+BHO-nanorod CCs. Both nanorods and irradiation-induced defects are demonstrated to be effective pinning centers in this material.

DOI： 10.35848/1347-4065/ad45d0

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Nanorods are a very effective pinning center in YBa2Cu3O7 films, and the vortex pinning mechanism should be understood to further improve the critical current density (J c) in the nanocomposite films. Matrix defects are naturally formed during nanocomposite growth, and in this study the effect of matrix defects on the J c in the YBa2Cu3O7 films containing nanorods is discussed. YBa2Cu3O7 + BaHfO3 and YBa2Cu3O7 + BaSnO3 films were prepared by varying the laser repetition frequency to control the matrix defects. All the films contained the nanorods. The YBa2Cu3O7 + BaHfO3 film of 2 Hz exhibits high J c for magnetic field parallel to the c-axis (B//c) due to the nanorods and the isolated random point defects. In the YBa2Cu3O7 + BaHfO3 film of 10 Hz, while the short stacking faults and networked random point defects decreased the J c for B//c, the short stacking faults improved the J c for tilted magnetic field. In the YBa2Cu3O7 + BaSnO3 film of 10 Hz, the isolated random point defects improved J c regardless of the magnetic field direction. While the ab-plane correlated defects are analyzed by the structural observation, the network or isolated nature of the random point defects can be discussed by the normal state resistivity. The matrix defects of YBa2Cu3O7 nanocomposite films should be designed considering the correlated/network/isolated nature which is determined by the morphology and defect concentration.

DOI： 10.1088/1361-6668/acecad

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2023.3255178

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Using a low-cost fluorine-free metal organic decomposition (FF-MOD) method, REBCO films with BaMO3 (BMO, M = Zr, Hf) artificial pinning centers were fabricated. When M elements are conventionally added to the solution, the size of BMO formation depends on the growth conditions. But by adding preformed BMO nanoparticles, the size can be maintained on the nanometer order, meaning the nanoparticles act as effective pinning centers at low temperatures and magnetic fields. Adopting a polycrystallization sintering process enables the BaCO3 to decompose completely and polycrystalline REBCO films to form prior to REBCO orientation. After final sintering, REBCO orientation can be achieved over a wide temperature range even with the addition of BMO nanoparticles. By utilizing preformed BMO nanoparticles and a polycrystallization sintering process, BMO doped GdBCO over 100 m-long tapes which have a thickness of more than 3 μm can be achieved. This is the first report of 120-meter-long artificial pinned REBCO tapes with Ic exceeding 200 A/4 mm width at 77 K, self-field made by the FF-MOD method.

DOI： 10.1109/TASC.2022.3233807

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記述言語：英語   掲載種別：研究論文（学術雑誌）

GdBa2Cu3O7-d (GdBCO) coated conductors (CCs) are promising wire materials because of their high critical temperature and critical current density in magnetic fields. The CCs must contain superconducting joints to create long wires. Previously, we fabricated a GdBCO CC with a superconducting joint by face-to-face crystallization of two CC samples with a GdBCO precursor film. To obtain a high current and low resistance, the crystallinity of GdBCO and a wide joint area are key considerations. This study investigates the effect of the heat-treatment temperature and precursor film thickness on these two factors. GdBCO samples were prepared without or with a precursor film of ∼50-200 nm thickness via pulsed laser deposition. The samples were set face-to-face and crystallized at 700-820 °C under a mechanical pressure of 40 MPa in an electric furnace. X-ray diffraction (XRD) analysis and optical microscopy revealed that both the peak intensity ratio of crystallized GdBCO and the joint rate at 800 °C were respectively 1.5 and 1.6 times higher than that at 720 °C. As the precursor film thickness increased from 50- 200 nm, the joint rate increased from 15.7- 25.4%, indicating that the heat treatment at ∼800 °C with a thick precursor film, is suitable. The results of this article lead to that the proposed method used in GdBCO CCs to reduce the electrical resistance.

DOI： 10.1109/TASC.2022.3191636

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2022.3166469

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記述言語：英語   掲載種別：研究論文（学術雑誌）

REBa2Cu3O7-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 Jc(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 Ic 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.

DOI： 10.1109/TASC.2022.3163690

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2022.3171188

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2021.3135483

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate remarkable engineering current density (averaged over the full wire), JE, at very high magnetic fields, driving progress in fusion and other applications. The key challenge for HTS wires has been to offer an acceptable combination of high and consistent superconducting performance in high magnetic fields, high volume supply, and low price. Here we report a very high and reproducible JE in practical HTS wires based on a simple YBa2Cu3O7 (YBCO) superconductor formulation with Y2O3 nanoparticles, which have been delivered in just nine months to a commercial fusion customer in the largest-volume order the HTS industry has seen to date. We demonstrate a novel YBCO superconductor formulation without the c-axis correlated nano-columnar defects that are widely believed to be prerequisite for high in-field performance. The simplicity of this new formulation allows robust and scalable manufacturing, providing, for the first time, large volumes of consistently high performance wire, and the economies of scale necessary to lower HTS wire prices to a level acceptable for fusion and ultimately for the widespread commercial adoption of HTS.

DOI： 10.1038/s41598-021-81559-z

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記述言語：英語   掲載種別：研究論文（学術雑誌）

An upgrading of the 25 T cryogen-free superconducting magnet (CSM) at the HFLSM to a 30T-class CSM is considered by replacing the present Bi2223 insert coil with a REBCO ones, since REBCO tapes have better mechanical and in-field critical current properties. In our strategy for the REBCO coil of the 30T-CSM, two tapes co-winding technique will be adopted in order to increase the space current density in the coil and to avoid a hotspot generation due to local degradation. Electromagnetic and mechanical properties of two bundle co-wound (two-ply) pancake coils are investigated as an R&D test coil for the 30T-CSM. The R&D coils were fabricated with Fujikura REBCO tapes that have 4.1 mm width and 0.11 or 0.15 mm thickness depending on the coil. The coil voltages and strains were measured at 4.2 K under a background field of 11 T. The maximum hoop stress up to 460-530 MPa at the outermost winding estimated by the BJR relation was applied, and no degradation by the hoop stress was found.

DOI： 10.1109/TASC.2021.3069685

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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 Ic 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 Ic margin if we accept a maximum hoop stress of 500 MPa.

DOI： 10.1109/TASC.2021.3061896

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

DOI： 10.1109/TASC.2021.3059604

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

AC losses and screening-current-induced fields (SCFs) in a high temperature superconducting (HTS) inserts for high field magnet are evaluated theoretically. The HTS inserts are composed of stacked pancake coils, which are wound using two-ply conductors, where the superconducting-layer sides of two rare-earth-based coated conductors are attached to each other without electrical insulation. The theoretical formulas of AC loss and SCF in the two-ply conductor are derived for the simultaneous applications of a transport current and an external magnetic field parallel to its broad face. By taking into account the magnetic-field profile in the HTS insert and the magnetic-field dependency of critical current density in the coated conductor, the AC losses and SCFs are estimated using the theoretical formulas for monotonical increase in a central field up to 25.5 T in 60 minutes in combination with low temperature superconducting outsert coils. It is guessed that the AC loss estimated for the HTS insert wound using the two-ply conductor could be cooled by using prepared cryocoolers. The magnitude of SCF in the HTS insert wound using the two-ply conductor becomes smaller than that for the single-Tape conductor estimated previously.

DOI： 10.1088/1742-6596/1559/1/012130

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We have developed BaHfO3 (BHO)-doped EuBa2Cu3Ox (EuBCO) coated conductors (CCs) using a hot-wall type pulsed-laser-deposition (PLD) with a high throughput condition on ion-beam-assisted-deposition (IBAD) template with a thickness of 50 μm including Hastelloy substrate. Although we have investigated the in-field Jc properties, mechanical properties are also one of the most important issues for high field magnet applications. In this article, we investigated the degradation characteristics of REBa2Cu3Ox (REBCO) CCs including the BHO-doped EuBCO CCs under uniaxial tensile strain and bending strain. As a result, dependences of irreversible strain on the REBCO film thickness were clarified. In addition, longitudinal fatigue properties of the CC were investigated by repeated tensile tests, and a fatigue behavior of metallic components of the CC was confirmed in more than 100 000 cycles.

DOI： 10.1109/TASC.2020.2965503

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Magnetic field stability and hysteresis can be serious problems in a high-field high-temperature superconducting magnet, because of the large shielding current effect. Accordingly, the field stability of a 25 T cryogen-free superconducting magnet (25T-CSM) was investigated. This particular system can generate a central field of 24.6 T in a 52-mm room-temperature bore. It was found that the magnetic field relaxation rate of the 25T-CSM with a Bi2Sr2Ca2Cu3Oy (Bi2223) insert was due to the flux creep effect, and was small enough to allow nuclear magnetic resonance (NMR) experiments. Based on these experiences in the development of the 25T-CSM, primitive designs for a 30 T cryogen-free superconducting magnet (30T-CSM) were suggested based on upgrading the present Bi2223 insert to a REBa2Cu3Oy (RE123, RE: rare earth and Yttrium) insert incorporating two co-wound RE123 tapes. It was confirmed that the prototype test coil could be operated at 4.2 K under large electromagnetic stresses of up to 540 MPa without experiencing degradation.

DOI： 10.1109/TASC.2019.2898699

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記述言語：英語   掲載種別：研究論文（学術雑誌）

REBa 2 Cu 3 O x (REBCO, RE = rare earth) coated conductors (CCs) are promising as superconducting wires for high-field magnets because of their high in-field critical current density (J c ) performance and high tensile tolerance. Fujikura, Ltd., has been developing BaHfO 3 (BHO)-doped EuBa 2 Cu 3 O x (EuBCO, Eu = Europium) CCs using a hot-wall-type pulsed-laser deposition (PLD) in order to further improve the in-field performance. Although a high deposition rate in the PLD process is necessary for mass production, it has been found that the in-field performance greatly differ depending on the deposition rate. In this study, flux pinning properties of BHO-doped EuBCO CCs fabricated with different deposition rates by the hot-wall PLD on IBAD substrates were investigated in detail. From the scaling characteristics of the flux-pinning force density (F p ) curve, the BHO precipitates in a fast deposition rate REBCO film appeared to behave like random pinning centers, which was almost consistent with a result of transmission electron microscope observation. On the other hand, from a decrease of anisotropy in the magnetic field angle dependence of J c , it was also confirmed that BHO precipitates were not completely random pinning centers, that is, it has some kind of anisotropy. Considering the critical current value per production time, it was also found that the fast deposition rate is advantageous since the REBCO layer could be thicker.

DOI： 10.1109/TASC.2019.2896535

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Superconducting joints have been demanded to lengthen REBa2Cu3O y -coated conductors for electrical power applications. In this study, we propose a new joint method for GdBa2Cu3O y -coated conductors via crystallizing precursor films additionally deposited on the conductors. Two GdBa2Cu3O y -coated conductors with precursor films were placed in a face-to-face manner in a furnace, then they were crystallized under mechanical pressure to joint them. A superconducting joint was achieved with a high critical temperature of 90.8 K. Microstructural observation by a transmission electron microscope revealed that there were only a few pores at the joint boundary without a secondary phase having c-axis-oriented GdBa2Cu3O y

DOI： 10.7567/1347-4065/ab0f23

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記述言語：英語   掲載種別：研究論文（学術雑誌）

There has been demand to lengthen superconducting joints of REBa2Cu3Oy coated conductors to realize magnet applications. In this work, jointed samples of GdBa2Cu3Oy coated conductors were prepared by crystallizing precursor films deposited by a pulsed laser deposition method under mechanical pressures, and influence of the pressures on superconducting and mechanical properties were investigated. While areal fraction and shearing stress of the joints were increased by increasing the pressure, superconducting properties were degraded. X-ray diffraction analysis indicated that the degradation was due to the formation of a secondary phase and decrease of oxygen contents in GdBa2Cu3Oy

DOI： 10.7567/1347-4065/ab0727

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Fujikura Ltd., Chiba, Japan, has developed stable and high-throughput production techniques for REBCO coated conductors (CC) by using large-area ion-beam-assisted-deposition (IBAD), and hot-wall type pulsed-laser-deposition (PLD). Recently, we have been attempting to dope BaMO3 (BMO, M: Zr or Hf) to REBCO layer in order to enhance in-field critical current density (Jc). In the course of optimizing deposition conditions of PLD for BaHfO3 doped EuBCO films on IBAD templates, "high Jc condition" and "high growth rate condition" were found. For samples prepared under these conditions, the magnetic field angle dependence of Ic and REBCO thickness dependence were investigated. Finally, we succeeded in fabricating a 300-m-long CC with uniform Ic distribution at the high-growth-rate condition. The longitudinal Ic homogeneity (defined as standard deviation divided by average) at 77 K evaluated by magnetization method was 2.6%, which is comparable to nondoped conductors. Transport in-field Ic for the both end points were 1756 and 1786 A/cm at 30 K, 2 T, respectively.

DOI： 10.1109/TASC.2018.2791805

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