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REBCO coated conductors (CCs) are suitable for high field magnets due to their high in-field critical current density ( J c) and high tensile strength. However, in order to apply the REBCO CCs to a high field magnet, it is important not only to evaluate the characteristics of short samples but also to clarify the behavior of the REBCO coil under strong electro-magnetic forces in high magnetic fields. In this study, we investigated the tolerance to the electro-magnetic force by applying hoop stresses to a REBCO double pancake coil in high magnetic fields. The REBCO coil was wound with BHO-EuBCO CCs and impregnated with epoxy resin. In order to prevent a degradation due to thermal delamination stress, fluorine-coated polyimide tapes were wound on the REBCO CCs. When a maximum hoop stress of 608 MPa was applied to the coil, normal voltage was generated and I c degradation was confirmed in one side of double pancake. As a result of the investigation, the degradation occurred only at lap joint part between CCs in the windings, and the cause of the degradation was thought to be stress concentration at the both edges of lap joint. Since there was no I c degradation except for the lap joint part, it is considered that the REBCO coil can withstand larger hoop stress if we can reduce the stress concentration at the joint part. We therefore proposed a new joint structure and confirmed that it has higher tensile strength than a conventional lap joint structure.

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 J c 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.

The influence of air temperature on adhesive small bowel obstruction (ASBO) is unknown. This study aimed to investigate the relationship between air temperature and postoperative ASBO.Overall, 312 patients with postoperative ASBO were included. They were categorized into two groups: the surgery group (n = 83) comprising patients who needed surgery, and the non-surgery group (n = 229) comprising patients who responded to conservative treatment. The associations between patients' characteristics and weather variables on days of symptom onset with the need for surgical management were investigated. Weather variables included the daily mean barometric pressure, daily mean air temperature, day-to-day differences, daily variances, and diurnal variation in the air temperature. Day-to-day differences in weather variables were calculated as the daily mean variables on the day of symptom onset minus those on the previous day. The daily variances in weather variables were defined as the absolute value of day-to-day differences.Compared to the non-surgery group, the surgery group had older patients (75 vs. 70 years, p = 0.009), a higher proportion of female patients (44.6% vs. 32.3%, p = 0.046), increased incidence of closed-loop sign (50.6% vs. 0.9%, p0.001), a lower proportion of feces sign (18.1% vs. 31.3%, p = 0.024), and a prolonged hospital stay (11 vs. 22 days, p0.001). The number distribution of patients in the surgery group in day-to-day differences in air temperature was different from that of the non-surgery group; the former has several peaks whereas the latter has almost one peak. Daily variance in mean air temperature on the day of symptom onset was higher in the surgery group than in the non-surgery group (2.3 vs. 1.3℃, p0.001). Multiple logistic regression analysis revealed that increased daily variance in air temperature on the onset day was associated with the need for surgical management (odds ratio 1.254, p = 0.002) and closed-loop obstruction (odds ratio 1.235, p = 0.017). Regarding seasonal variations, the risk of the need for surgery and closed-loop obstruction in each ASBO patient was the highest in spring, followed by that in summer, autumn, and winter. Consistently, the daily variance in mean air temperature in spring was higher than that in summer, autumn, and winter (p0.0001, p0.0001, and p = 0.0047, respectively). The risk of the need for surgery and closed-loop obstruction in each ASBO patient was the highest in spring, followed by that in summer, autumn, and winter. Consistently, daily variance in mean air temperature was higher in spring than that in summer, autumn, and winter (p0.0001, p0.0001, and p = 0.0047, respectively).Increased daily variance in mean air temperature on the day of onset is associated with the need for surgical management and closed-loop obstruction. Spring is characterized by the highest daily variance in mean air temperature among the four seasons, and is associated with high proportions of the need for surgery and closed-loop obstruction. These results can be clinically useful in terms of hospital resource reallocation and staffing, and can help clarify the pathogenesis of ASBO.

Introduction. Acute mesenteric ischemia is a life-threatening complication after cardiovascular surgery with a mortality rate of 52.9–81.3%. However, few studies have evaluated the predictors of clinical outcome after treatment for acute mesenteric ischemia following cardiovascular surgery. Therefore, this study aimed to elucidate prognostic factors in patients who underwent laparotomy for acute mesenteric ischemia after cardiovascular surgery. Methods. We retrospectively analyzed 29 patients (20 men and 9 women; median age, 71.0 years) who underwent laparotomy for acute mesenteric ischemia after cardiovascular surgery between January 2010 and August 2020. These patients were classified into the survivor group (comprising patients who were discharged or referred to another hospital, n = 16) and the nonsurvivor group (comprising those who experienced in-hospital mortality, n = 13). We compared clinical parameters between the groups to identify the predictors of outcomes. Results. More patients in the nonsurvivor group underwent emergency cardiovascular surgery (62.5% vs. 100%, p = 0.017) and received hemodialysis (12.5% vs. 61.5%, p = 0.008) at the onset of acute mesenteric ischemia than those in the survivor group. The prelaparotomy serum creatinine level was higher in the nonsurvivor group than in the survivor group (1.27 vs. 2.33 mg/dL, p = 0.004). Logistic regression analysis revealed an association between preoperative serum creatinine level and in-hospital mortality (odds ratio 5.047, p = 0.046), and Cox regression analysis demonstrated a relationship between serum creatinine level and in-hospital mortality (hazard ratio 1.610, p = 0.009). The area under the curve (receiver operating characteristic analysis) for the serum creatinine level was 0.813. Furthermore, the optimal cutoff value of the serum creatinine level was 1.59 mg/dL with a sensitivity and specificity of 0.846 and 0.687, respectively, in predicting in-hospital mortality. Conclusions. The elevated serum creatinine level was associated with a poor clinical outcome after surgery for acute mesenteric ischemia following cardiovascular surgery.

A prototype high-temperature superconductor (HTS) sextupole magnet for the SuperKEKB Interaction Region was fabricated and tested in order to gain experience in the fabrication of HTS magnets as well as to understand their characteristics. The designed field strength of the sextupole magnet is 200 T/m2 at 30 K. The coil of the magnet has an inner radius of 75 mm and a length of 200 mm. Furthermore, the magnet consists of six rectangular coils wound with a REBa2Cu3O7-x (REBCO) coated conductor, which is 4 mm wide and was impregnated with epoxy resin. After fabrication, the magnet was tested in LN2 and it was confirmed that the magnet had been fabricated with no degradation of the conductor properties. In this paper, we briefly present the design and fabrication of the magnet, and the test results at 77 K including the properties of the magnetic field.

REBa2Cu3Ox (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 (Jc) performance and high tensile tolerance. Fujikura, Ltd., has been developing BaHfO3 (BHO)-doped EuBa2Cu3Ox (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 (Fp) 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 Jc, 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.

Background Neurofibromatosis type 1 is an autosomal dominant inherited disease associated with multiple skin neurofibromas or other neurogenic tumors, such as nodular plexiform neurinoma or cerebrospinal tumor. Gastrointestinal stromal tumors are often complicated in patients with neurofibromatosis type 1, although involvement of the appendix is rare, and there have been few reports of appendiceal ganglioneuroma. Case presentation The patient was a 29-year-old man diagnosed with neurofibromatosis type 1 based on physical findings and his family history. During the follow-up of neurofibromatosis, computed tomography was performed to detect neurological tumors, such as neurofibromas in the brain, spinal cord, and gastrointestinal tract. Computed tomography showed a markedly thickened appendix wall, and an appendiceal tumor was suspected. Laparoscopic appendectomy was performed, and a 50 × 35 mm appendiceal submucosal tumor was resected with a negative resection margin. At histopathological examination, the tumor was diagnosed as ganglioneuroma; it showed short spindle-shaped cells and ganglion cells diffusely infiltrated into the proper muscle layer and fibrous tissue that grew around nerve cells. The patient was discharged on the 5th postoperative day without postoperative complications and was doing well at 13 months following the operation. Conclusions Gastrointestinal stromal tumor and neurofibroma are the most common gastrointestinal tumors associated with neurofibromatosis type 1, but ganglioneuroma of the appendix is rare. Appendiceal neurogenic tumors should be considered in patients with neurofibromatosis type 1, and surgical resection is necessary because of the risk of malignancy.

We fabricated a compact (13$\times$12$\times$12.5 mm${^3}$) trapped-field magnet by stacking 200 pieces of EuBa${_2}$Cu${_3}$O${_7}$ coated conductors with BaHfO${_3}$ nanorods as artificial pinning centers. It was magnetized by field-cooling method using a 18 T superconducting magnet, and the maximum field of 14.8 T was trapped at 10 K at a field-ramp rate of 0.1 T/min. At higher temperatures, although the trapped field was decreased, we could trap the field with much faster ramp rate. In order to understand these results, we also performed calculations of the trapped field based on $J_\mathrm{c}-H$ characteristics of the coated conductor. The calculated trapped field value was larger than the experimental value at 10 K. This discrepancy can be understood by considering the reduction of the effective size of the stacked coated conductors due to the existence of extended regions of irregular flux penetration called "flux jets"., 19 pages, 9 figures, to be published in Superconductor Science and Technology

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 BaMO 3 (BMO, M: Zr or Hf) to REBCO layer in order to enhance in-field critical current density (J c ). In the course of optimizing deposition conditions of PLD for BaHfO 3 doped EuBCO films on IBAD templates, “high J c condition” and “high growth rate condition” were found. For samples prepared under these conditions, the magnetic field angle dependence of I c and REBCO thickness dependence were investigated. Finally, we succeeded in fabricating a 300-m-long CC with uniform I c distribution at the high-growth-rate condition. The longitudinal I c 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 I c for the both end points were 1756 and 1786 A/cm at 30 K, 2 T, respectively.

Fujikura has developed production techniques of REBa 2 Cu 3 O y (REBCO, RE = rare earth) coated conductors (CCs) and provided high-performance REBCO CCs with long-length and high homogeneity of the critical current. REBCO CCs have a low c-axis strength (delamination strength) which originates from the layered structure. When an epoxy-impregnated REBCO coil is cooled, the radial thermal stress is generated. The stress could cause the delamination of REBCO CCs and lead to degradation of the coil. Therefore, it is important to evaluate the exact delamination strength of the CCs. In this paper, we evaluated the failure probability of the epoxy-impregnated coils at 77 K. We introduced Weibull analysis considering the stress distribution and the sample size effect, and estimated the actual delamination strength of the REBCO CCs. By using the evaluation method, we confirmed the strength enhancement by process improvement.

We investigated the relationship between current capacity and local critical current distribution of a Cu-sheathed multifilamentary RE-123 coated conductor (CC). Patterning multifilamentary structure on CC will be a promising solution for reducing magnetization to assure spatial homogeneity and its temporal stability of magnet applications such as MRI and NMR. On the other hand, it will become more difficult to maintain the current capacity because a smaller defect can block the current flow in a narrower filament. Permitting electrical coupling among the filaments will work for maintaining current capacity because current can avoid such a defect by flowing into the adjacent filament. However, too small interfilamentary resistance will result in long time constant of filament coupling, which will affect spatial homogeneity and its temporal stability of magnet applications. Therefore, to design a multifilamentary CC satisfying the requirement from magnet applications, it is necessary to understand the quantitative impact of interfilamentary resistance of the multifilamentary CC on its current capacity under the influence of spatial variation of local critical currents. In this study, we estimated global critical current of a Cu-sheathed multifilamentary CC as a function of interfilamentary resistance by considering its local critical current distribution in each filament. As a result, it was confirmed that the electrical coupling among the filaments was very effective to improve the current capacity of such a multifilamentary CC especially for a section with spatially inhomogeneous local critical currents. Furthermore, it was also found that local heat generation could be significantly suppressed even for a section with relatively homogeneous local critical currents.

We have successfully trapped a field of 17.89 T at 6.5 K at the center of a compact coated-conductor (CC) stacks (13 × 12 × 11.7 mm3) within 75 min by suppressing flux jumps. The CC stack consists of 200 sheets of EuBa2Cu3O7 CCs with BaHfO3 nanorods to increase the critical current density at high fields and low temperatures. To enhance thermomagnetic stability, the central 50 CCs are coated with 1 µm thick Pb with large specific heat at low temperatures. Numerical calculations based on the actual J c–H characteristics reproduces the trapped field quantitatively. New directions for achieving even higher trapped field at higher temperatures and making use of the trapped field are discussed.

The SuperKEKB project underway at KEK will increase the KEKB luminosity by 40 times using nanobeam interaction-region focusing optics. The chromaticity correction of the collision beams is crucial to achieve the high luminosity. Sixteen special sextupole magnets, which can simultaneously produce normal and skew sextupole fields, are required in the 100-m-long straight sections around the interaction point for the chromaticity correction. As a candidate for the sextupoles, a cryocooled high-temperature superconducting (HTS) magnet was proposed and the development has started. In HTS magnets, quench protection is a major concern. Especially for accelerator magnets, reliable quench detection and protection systems are essential. In this study, we have performed heat-induced quench tests and numerical simulations on the ReBCO-impregnated double pancake coils at operating temperatures of 30, 40, and 50 K. Two test coils with different thickness of the copper stabilizer were prepared for the tests to compare the quench characteristics. The hot spot temperature was discussed with the operating conditions and quench detection criteria. The copper stabilizer thickness of the sextupole ReBCO coils was designed by using the validated numerical model.

BaMO 3 (BMO, M: Zr or Hf)-doped REBCO-coated conductors were prepared by Hot-Wall PLD method on IBAD template. The highest J c was obtained for BaHfO 3 -doped EuBCO. Quite strong pinning force density (F p ) of 1.6 TN/m 3 was observed at 4.2 K, 15 T, for a sample deposited with growth rate of ~5 nm/s. It had strong c-axis correlated pinning at the temperature above 20 K with the matching field of ~5 T. A 45-m-long sample was formed by productive high growth rate over 20 nm/s, being consistent with commercial nondoped conductors. It had also large I c of 1687 A/cm and F p of 0.96 7 TN/m 3 was obtained at 4.2 K, 15 T. The end-to-end longitudinal Ic homogeneity (σ) at 77 K in low field was evaluated to be 2.2% by magnetization. Transport in-fields I c were also measured for the both end points at 30 K, 2 T, resulting in 1863 and 1883A/cm, respectively.

We have carried out the characterization of local critical current distribution in a 113-m-long multifilamentary coated conductor (CC) based on reel-to-reel scanning Hall-probe microscopy. Patterning multifilamentary structure on CCs is a key technology for the reduction of ac losses due to magnetization. Recently, this advantage has also been expected for the reduction of the magnetization in a coil winding which has been recognized as a critical problem for HTS magnet applications such as accelerators, MRI, and NMR from the viewpoint of field homogeneity and temporal stability. However, it has been difficult for the conventional techniques such as four-probe method, TAPESTAR, etc., to make diagnostics for multifilamentary CCs due to the limitation of spatial resolution. On the other hand, our measurement has an advantage in taking two-dimensional field image. This enabled us to confirm that multifilamentary structure as well as the corresponding magnetization reduction was successfully achieved in a long-length CC. At the same time, by the estimation of local critical current distribution for each filament, it was also found that there were still some local defects, which affected the global performance of the multifilamentary CC, even if the probability density of such defects was only in the order of 10 -5 . These findings will become crucial information for the optimization of fabrication processes of multifilamentary CCs and for their nondestructive quality assurance.

REBa2Cu3Ox (REBCO, RE = rare earth) coated conductors (CCs) are expected to show high performance for superconducting coil applications due to their high tensile strength and high critical current density under magnetic fields. One of the most serious problems on the coil application is the significant screening current, which is caused by the configuration of CC. Since screening current is dependent on the width of the REBCO layer, filamentization of the REBCO layer is strongly desired. We have succeeded in fabricating long length four-divided multifilamentary CCs with 4 mm wide over 100 m long by using the process of scratching the IBAD-MgO layer (scratching process). In this work, we fabricated two epoxy-impregnated coils and investigated the effects of the screening current. One of the coils is wound with multifilamentary CCs fabricated by scratching process, another is wound with conventional mono-filamentary CC. As the results, it is confirmed that the screening current is reduced and the coupling current between the filaments might be in the acceptable range on the DC magnet application.

In the commercial coated conductor tapes, the twinned structure of REBCO (REBa2Cu3O7-d, RE = Y and Gd) is characterized as either or orientation based on the tape length direction. In this study, we investigate the effects of the two different twinned structures on the critical current Ic of the REBCO tapes by combining; transport critical current and synchrotron radiation diffraction measurements. For the tapes with oriented twins, we observed the inverted parabolic strain behavior on the uniaxial strain dependence of Ic. In contrast, the ones with oriented twins showed a weak strain behavior without any maximum appeared in the strain dependence. Such a different uniaxial strain dependence was analyzed by using the one-dimensional twin model with different fractional lengths of A-domains and B-domains of REBCO twins. This model explains the essential features of the different uniaxial strain dependence we observed.

Superconductors can trap large magnetic fields. However, magnetic fields perpendicular to the trapped field reduces the trapped field effective, which is known as crossed-field demagnetization. Crossed-field demagnetization becomes a problem for the application of stacked magnet. We investigated the crossed-field demagnetization of a EuBa2Cu3O7 coated conductor with BaHfO3 nonorods which has large critical current density. When an AC transverse field is applied after the magnetization of the coated conductor, the decay of trapped field was significantly accelerated. However, for the AC field smaller than the full penetration field parallel to the ab-plane, the decay becomes slow enough. All these results suggests that we can apply the stacked magnet for rotating machines if the alternating transverse magnetic field is reduced below the penetration field.

Fujikura Ltd has developed production techniques of REBa2Cu3O X (REBCO, RE = rare earth) coated conductors (CCs) using large-area ion-beam-assisted-deposition and hot-wall type pulsed-laser-deposition. We have provided high-performance REBCO CCs with long length and high homogeneity of critical current. The CCs are required to maintain their superconducting properties under mechanical stresses for a long-term. Therefore, it is important to evaluate the fatigue properties of the CCs. Although there are many investigations on cyclic tensile fatigue in the longitudinal direction of the CCs, few studies on fatigue properties in the through-thickness direction have been reported. In the case of the through-thickness stress, i.e. delamination stress, the stress is applied to the REBCO ceramic layer without substrate reinforcement, static fatigue fracture may occur by subcritical crack growth peculiar to ceramics. In this work, we carried out dynamic fatigue tests under delamination stresses at room temperature (RT) and liquid nitrogen (LN2) temperature 77 K to determine the fatigue coefficient N characterizing the static fatigue. As a result, we found that the N-value was relatively low (N ∼ 20) at RT, while the N-value was extremely high (N ∼ 150) at LN2 temperature. This result shows that the crack growth of the REBCO layer under the through-thickness tensile stress is extremely slow at low temperatures as well as that reported for bulk ceramics such as SiO2 glass. Therefore, it is expected that the static fatigue in the through-thickness direction of REBCO CC hardly occurs at low temperatures.

A method for making multi-filamentary REBCO tapes by only scratching buffer layer was developed for coil application which requires accurate magnetic fields. By continuous I c measurement, we found that our new multi-filamentary tape could provide almost equal I c compared to conventional tapes. Then, using EBSD and RTR-SHPM methods, a divided structure of REBCO layer was surely confirmed. AC loss was also decreased. Furthermore, the result of delamination test of our new multi-filamentary tape showed enough mechanical property. As a result, we have succeeded in developing 100 m class multi-filamentary tape for superconducting coil.

In this paper, normal zone in a REBCO single-pancake coil induced by quenches in a low-temperature superconducting outsert coil is investigated in both experimental and numerical methods to discuss the quench protection of REBCO coils. The normal zone is experimentally clarified by measuring a partial voltage in the REBCO coil at temperatures from 30 K to 50 K during quenches of the outsert coil charging 5 T. Using practical fittings for the physical properties of the components of the REBCO coil in ranges of 0-5 T and 10-77 K, the numerical results have good agreement with the experimental results. The simulation reveals the heating distribution with initial temperatures from 10 K to 65 K, which indicates that the maximum temperature in a hot spot becomes higher for the lower initial temperature. Unexpectedly, the temperature distribution becomes rather homogeneous with an initial temperature of 65 K. It suggests that the hot spot at higher temperature has a lower risk of burning out. Finally, possible coil protections are discussed as the operation at the high temperature or the quench heater method to keep the REBCO coil at high temperature when a hot spot appears.

The SuperKEKB project underway at KEK will increase the KEKB luminosity by 40 times using nanobeam-interaction-region focusing optics. To achieve this high luminosity, the chromaticity correction of the collision beams is crucial; thus, special sextupole magnets, which can simultaneously produce normal and skew sextupole fields, are required in the straight sections around the interaction point. As a candidate for the sextupole, a cryocooled high-temperature superconducting magnet was proposed; development has begun. Here, the basic magnet design and the results of a small coil test are presented. The test studied the quench characteristics of the REBCO-impregnated coil.

VAMAS TWA 16 promoted an international round robin test (RRT) on critical current (I c ) measurement of RE-BaCu-O (REBCO; RE = rare earth) superconductors to establish the transport I c measurement method in liquid nitrogen bath without an external magnetic field. Ten laboratories from five countries participated in the RRT and tested commercially available REBCO superconductors. I c and measurement conditions were reported. In parallel, intrinsic I c nonuniformity of the superconductors were evaluated by reel-to-reel scanning Hall-probe microscopy for 10-m conductors. Statistical analyses of I c provided uncertainty of the measurement method. Combined standard uncertainty was analyzed considering the intrinsic nonuniformity of I c . The analysis indicated that the intrinsic nonuniformity has the largest contribution to the uncertainty of I c .

Recent development of production technique for RE123 coated conductors was reviewed, using Hot-wall PLD/ IBAD process. Lot-to-lot distributions for both average value and longitudinal uniformity of Ic at 77 K, 0 T were summarized for 127 run of 300-500-m-long 10-mm-wide CC. By optimizing hotwall PLD process parameters we found a reproducible high Jc production condition (20% enhancement in field at 20 K), less angular dependent to applied magnetic field. Furthermore, new hermetic type Cu stabilization foil configuration was developed and general mechanical properties against tensile strain, etc. were surveyed.

A 25-T cryogen-free superconducting magnet (25T-CSM) is being developed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In the case of a cryogen-free superconducting magnet, the coil temperature rises during a sweep of an operating current due to ac losses. Hence, the ac loss estimation is very important for the cooling design. The critical current density and the magnetization of Gd123 tapes at 4.2 K were measured in order to estimate the ac loss of a Gd123 insert of the 25T-CSM. The ac loss is actually the hysteresis loss, which are calculated from the J c properties taking the magnetic field distribution in the Gd123 coil into account. Regarding the effect of tape stacking (or winding) in a pancake coil, the slab approximation can be used for hysteresis loss calculation. In the case of slab model, the full penetration field at the center of the tape becomes higher than the maximum applied magnetic field in the most part of the coil. As a result, the hysteresis loss increases with increasing a magnetic field when the magnet energizes. The hysteresis losses assuming the slab model, however, show an opposite field dependence to those calculated from the strip model without the stacking effect. Hence, the ac loss of 5 W is estimated when the magnet energizes to 25.5 T within 60 min.

High temperature superconducting (HTS) cables are able to achieve large power capacity and low-loss power transmission. Fujikura Ltd. worked on developing a 66 kV-5 kArms HTS power cable using high critical current (I c ) REBCO tapes in the Japanese national project. One of the technical targets in this project was to reduce AC loss to less than 2 W/m at 5 kArms. REBCO tapes with 240 A/4 mm-width of I c at 77 K, self field, which were fabricated by ion-beam-assisted-deposition (IBAD) and pulsed laser deposition (PLD) methods, were applied to the HTS power cable. As a result, we succeeded in developing the 20 m-long 66 kV-5 kArms HTS power cable. The measured AC loss was achieved 1.4 W/m at 77 K and 1.0 W/m at 67 K at 5 kArms. On the other hand, it is considered that the eddy current loss of the stainless-steel cryostat pipe of HTS cable become obvious in the case of the single-core cable with the large current capacity. It is suggested that the eddy current loss of the stainless-steel cryostat pipe is due to the internal magnetic flux. Therefore, the eddy current loss of the stainless-steel cryostat pipe as well as uniform current distribution of HTS conductor and shield layer should be taken into account for designing the winding direction and pitch of HTS conductor and shield layer of HTS cable. In this work, the measured eddy current loss of the stainless-steel cryostat pipe is also compared with the calculated one.

Recently, high performance ReBa2Cu3Ox (ReBCO) coated conductor, such as those with over 500 A per 1 cm in width at 77 K, have become commercially available. The quench detection of ReBCO pancake coils under conduction-cooled conditions is an important issue for the safe operation of superconducting applications. Although experimental data on the quench characteristics of ReBCO coated conductors have been reported, there is no sufficient database available regarding ReBCO impregnated pancake coils. In this work, a ReBCO impregnated pancake coil was fabricated using ReBCO coated conductors laminated with a 75-μm-thick copper stabilizer. The inner diameter of the ReBCO coil was 50 mm, and the outer diameter was approximately 73 mm. The measured longitudinal normal-zone propagation velocities (NZPV) in the ReBCO coil were 3–27 mm/s at 50–20 K. It was also confirmed that the ReBCO coil was shut down from 100 A to 240 A by a balance voltage detection at 40–100 mV under conduction-cooled conditions without degradation.

REBa2Cu3Ox (REBCO, RE = rare earth) coated conductors are expected to show high performance for superconducting applications because of their high current density and high mechanical strength. Fujikura has developed REBCO coated conductor over 570 A/cm width at 77 K, self field with over 800 m length. In addition, we have succeeded in developing a 5 T cryocooled REBCO magnet with a stored energy of 426 kJ. The magnet is composed of 24 pancake coils with an inner diameter of 260 mm, and the total length of the REBCO coated conductors is approximately 7.2 km. We confirmed that the REBCO magnet could be excited up to 5 T for 720 min 9 months after the fabrication of the magnet. It is important to validate the long-term operation test results of the REBCO magnets with larger diameters in order to use these conductors in practical superconducting applications. On the other hand, we have fabricated and evaluated a model magnet before we fabricated a 426 kJ cryocooled magnet. The model magnet is composed of 6 REBCO pancake coils and 18 “dummy coils” with an inner diameter of 260 mm and an outer diameter of 515 mm. The model magnet is about the same size and shape as the 426 kJ cryocooled magnet. We have confirmed that the calculated critical current (Ic) of the top pancake coil of the model magnet was in good agreement with the measured coil Ic from 30 to 77 K under the conduction-cooled conditions. In addition, we have evaluated the thermal runaway behavior of the model magnet under the conduction-cooled conditions. As a result, we have confirmed that the model magnet with a 0.3-mm-thick copper stabilizer had sufficient time to ramp down without degradation at 361 A transport current after the detected normal transition.

A YBCO insert coil has been developed for upgrading a cryogen-free 18 T superconducting magnet installed in the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University to a 22 T superconducting magnet. The YBCO insert coil is designed to generate 6.5 T at 200 A in 15.5 T outer LTS coils. The YBCO insert coil is composed of a stack of 50 single pancake coils wound with YBCO-coated conductors (0.23 mm × 4 mm). The inner and outer diameters of the YBCO insert coil are 96 mm and 178 mm, respectively, and the total conductor length is about 3 km. The maximum hoop stress of the YBCO insert coil was estimated to be 310 MPa when the central magnetic field was 22 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. Thermal runaway may cause burnout of the YBCO insert coil, and therefore, it is important to calculate the voltage-current characteristics of the coil from the superconducting properties of the YBCO-coated conductors. The coil should have no damaged area in the windings because a damaged area would generate heat locally, eventually resulting in thermal runaway. Therefore, a demonstration coil with almost the same size as the pancake coils used for the YBCO insert coil was fabricated and tested in conduction cooling conditions in order to evaluate whether the coil could withstand the thermal stresses and electromagnetic force.

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

A bulk magnet was fabricated by stacking coated conductors with enhanced J c by introducing nanorods. The fabricated CC stack consists of 100 pieces of CCs, and it was magnetized by field-cooling method in an applied field of 14.5 T down to 4.2 K. By reducing the magnetic field down to zero at a ramp of rate 1 T/min, we have succeeded in trapping 12.6 T at the center of the CC stack. We also confirmed that the CC stack can sustain the high trapped field for more than 20 min, which demonstrates the practical usefulness of such a staked CC magnet.

High temperature superconducting (HTS) cables are able to achieve large power capacity and low-loss power transmission. In the Japanese national project, Fujikura Ltd. worked on developing a 66 kV-5 kArms HTS power cable using high critical current ( I c ) REBa 2 Cu 3 O x (REBCO, RE = rare earth) tapes. One of the technical targets in this project is to reduce AC loss to less than 2 W/m at 5 kArms. The REBCO tapes with 240 A/4mm-width of I c at 77 K, self field, which were fabricated by Ion-beam-assisted-deposition (IBAD) and Pulsed Laser Deposition (PLD) method, were applied to a HTS power cable in order to achieve extremely low AC loss. As a result, we have succeeded in developing a 20 m-long 66 kV-5 kArms HTS power cable. The measured AC loss was achieved 1.4 W/m at 77 K and 1.0 W/m at 67 K at 5 kArms.

One of the most important factor for resistive-type fault current limiters is uniform voltage distribution under fault condition. Homogeneous distribution of Ic and resistance of nonsuperconducting state in the longitudinal direction are essential for uniform quenching. Recently, we have succeeded in manufacturing the highly homogeneous superconducting RE BCO coated conductor. In this paper, we report the voltage distribution in our coated conductor under fault condition, at 50 Hz for a half cycle. The experimental results show that applied voltage, which is necessary to quench over the entire length of sample, depends on Ic and normal state resistance on Tc.

REBCO coated conductors are expected to show high performance in superconducting applications, because of their high mechanical strength and high current density in the magnetic field. However, more characteristics data of REBCO magnet with larger diameter are required in order to apply these conductors in practical superconducting applications. In this work, we have succeeded in developing a 5 T cryo-cooled high-temperature superconducting (HTS) magnet, which was composed of 24 pancake coils with an inner diameter of 260 mm, using REBCO coated conductors. The stored energy of the magnet was 426 kJ. The total length of the REBCO coated conductors was approximately 7.2 km. These conductors were fabricated by Fujikura Ltd. using ion-beam-assisted deposition (IBAD) and pulsed laser deposition (PLD) methods. All pancake coils were confirmed the V-I characteristics without degradation in liquid nitrogen before fabricating the magnet. The magnet was cooled down to 24 K by a GM (Gifford-McMahon) cryocooler after the fabrication. The magnet could excite up to 5.0 T in 60 minutes, successfully. Furthermore, we confirmed that 1% current reversal of the coil current could stabilize the axial central magnetic field drift of this magnet.

We have developed an ion-beam-assisted-deposition (IBAD) and a pulsed-laser-deposition (PLD) technique to produce REBa 2 Cu 3 O x (RE123, RE: rare earth) coated conductors with high performances. Distributions of critical currents ( I c ) in the longitudinal direction were improved in most wires routinely fabricated under optimized process. A typical wire showed I c of 460 A/cm at 77 K and self-field, about 200 A/cm at 65 K and 3 T and about 300 A/cm at 50 K and 5 T. In addition, further high I c values over 800 A/cm were also obtained in thick films. Finally, we succeeded in fabricating an 816.4 m long wire with end-to-end I c of 572 A/cm, corresponding to the new world record I c × L value of 466,981 Am/cm.

RE1Ba2Cu3O7-≏(RE: Rare Earth, Y, Gd and so on, REBCO) superconducting tapes have high performance in critical current density, Jc , property even in high magnetic field and at liquid nitrogen temperature. We have proposed the ac loss reduction method of REBCO superconducting tape, which are the combination of a laser–scribing into a multifilamentary structure and a special winding process. Using the ac loss reduction technique, we could realize REBCO superconducting transformers with the advantage of light weight, compactness and high efficiency in comparison to conventional one. Further we aim to add a current limiting function to REBCO superconducting transformers. Recently we manufactured a 10kVA four-winding transformer with REBCO tapes. We carried out a sudden short circuit test and investigated the response against the fault excess current. We developed a numerical analysis program so as to quantitatively simulate the phenomena taking into account of a flux-flow resistance. As a result, it was shown that the whole of the REBCO winding did not shift to normal state even though the current exceeded the critical current all over the length. In this paper, we report the outlines.