N. Yanagi

National Institutes Of Natural Sciences, Edo, Tōkyō, Japan

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Publications (324)269.48 Total impact


  • No preview · Article · Jan 2016 · IEEE Transactions on Applied Superconductivity

  • No preview · Article · Jan 2016 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: The Large Helical Device (LHD) is a heliotron-type fusion plasma experimental machine which consists of a fully superconducting magnet system cooled by a helium refrigerator having a total equivalent cooling capacity of 9.2 kW@4.4 K. Seventeenplasma experimental campaigns have been performed successfully since1997 with high reliability of 99%. However, sixteen years have passed from the beginning of the system operation. Improvements are being implementedto prevent serious failures and to pursue further reliability.The LHD cryogenic control system was designed and developed as an open system utilizing latest control equipment of VME controllers and UNIX workstations at the construction time. Howeverthe generation change of control equipment has been advanced. Down-sizing of control deviceshas beenplanned from VME controllers to compact PCI controllers in order to simplify the system configuration and to improve the system reliability. The new system is composed of compact PCI controller and remote I/O connected with EtherNet/IP. Making the system redundant becomes possible by doubling CPU, LAN, and remote I/O respectively. The smooth renewal of the LHD cryogenic controlsystem and the further improvement of the cryogenic system reliability are reported.
    Preview · Article · Dec 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: Based on the successful plasma experiments in the Large Helical Device (LHD), design activities of the LHD-type helical fusion reactor FFHR-d1 are progressing at National Institute for Fusion Science (NIFS). A 100 kA current capacity is required for the winding conductor under the maximum magnetic field of similar to 12 T. The high-temperature superconductor (HTS) is a promising option for the helical coil conductor. For the development of such a HTS conductor suitable for the helical fusion reactor, we fabricated 30 kA-class HTS conductor samples, and the excitation tests were successfully carried out. We then fabricated and tested a 100-kA class HTS conductor. The conductor sample is a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, then the critical current of the sample was measured. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the simply-stacked HTS tapes to verify the measured critical current of the samples. The critical current characteristics of a single HTS tape is evaluated by the percolation model in the precise analysis.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: This paper discusses tensile characteristics of a mechanical lap joint, which has been proposed and tested for joint-winding of a high-temperature superconducting (HTS) helical coil in the Large Helical Device type helical fusion reactor, FFHR-d1. Structural analysis for the helical coils in FFHR-d1 indicates that the maximum tensile strain along the HTS conductors and the maximum shear stress in HTS tapes region are 0.145% and 32 MPa, respectively under electromagnetic forces. Tensile test of the mechanical lap joint using Gadolinium Barium Copper Oxide (GdBCO) tapes with an inserted indium foil showed that the joint having 5-mm length and 5-mm width with a joint pressure of > 11 MPa had tensile strength exceeding that of the GdBCO tape itself, which also allows the tensile strain of 0.145% when used in the helical coil. To tolerate the shear stress of 32 MPa, the joint pressure of > 50 MPa was found to be needed according to the experimental results.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: Propagation of a short normal zone was observed in a helical coil of the Large Helical Device, when the coil was cooled with subcooled helium, of which the inlet and outlet temperatures are 3.2 K and below 4.0 K, respectively. The normal zone was induced at the bottom position of the coil. It propagated to only the downstream side of the current with recovery from the opposite side, and stopped after passing the outer equator of the torus. The induced balance voltage is obviously lower and the propagating time is shorter than those of propagating normal zones observed in the helical coil cooled with saturated helium at 4.4 K. According to the simulation of the propagation of a normal zone, it is considered that such a short normal zone at the current close to the minimum propagating current propagates without full transition to film boiling.
    No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: An innovative winding method is developed by connecting high-temperature superconducting (HTS) conductors to enable efficient construction of a magnet system for the helical fusion reactor FFHR-d1. A large-current capacity HTS conductor, referred to as STARS, is being developed by the incorporation of several innovative ideas, such as the simple stacking of state-of-the-art yttrium barium copper oxide tapes embedded in a copper jacket, surrounded by electrical insulation inside a conductor, and an outer stainless-steel jacket cooled by helium gas. A prototype conductor sample was fabricated and reached a current of 100 kA at a bias magnetic field of 5.3 T with the temperature at 20 K. At 4.2 K, the maximum current reached was 120 kA, and a current of 100 kA was successfully sustained for 1 h. A low-resistance bridge-type mechanical lap joint was developed and a joint resistance of 2 nΩ was experimentally confirmed for the conductor sample.
    No preview · Article · May 2015 · Nuclear Fusion
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    ABSTRACT: The heat flux at the divertor in a fusion reactor is considered to have a peak of >10 MW/m2. In a design study of the helical reactor FFHR-d1, the feasibility of employing a copper alloy for divertor cooling pipes was investigated; however, radiation in the divertor area would quickly damage the copper alloy. The neutron load on the divertor can be reduced by a blanket arrangement; nevertheless, in the present divertor structure, irradiation damage of materials on the inboard side of the torus remains relatively high. If the divertor could be moved to an area receiving much less radiation, then the lifetimes of divertor materials should increase. In this paper, a novel divertor structure is introduced in which the coil-support structure is modified to create a region receiving relatively low amounts of radiation without changing the geometry of the helical or vertical field coils. Using this proposed design would increase the lifetime of the copper alloy in divertor components to more than an estimated six years. In addition, the divertor could be accessed from either the upper or lower sides of the device, simplifying maintenance.
    No preview · Article · Feb 2015 · Fusion Engineering and Design
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    ABSTRACT: Since the discovery of high temperature superconductors (HTS) more than 2 decades ago, there has been interest in their use for future fusion machines. Lack of performance of commercially available materials, however, dampened the initial optimism. However, recent advances in HTS materials, mostly second-generation tapes, open attractive topologies. In addition to reduced cryogenic loads and increased superconducting stability, the HTS tapes may allow demountable magnets that could be very helpful in the long term (for reactor maintenance) and in the intermediate term, for component-testing machines which require large access. Tests on joints have demonstrated that the thermal load due to the Joule dissipation in these joints is small, allowing operation with very long pulses without restrictions on cost of electricity or power availability. There are challenges in the use of HTS in magnets in general, and fusion specifically. The excellent properties of HTS materials, e.g., YBCO (YBa 2Cu 3O 7-δ) superconductors operating at elevated temperatures (> 30K) also offer operational advantages for fusion machines, but there are challenges, such as the manufacturing of high current cables and methods of quench protection. In addition to tapes, HTS can be fabricated as monoliths. These monoliths offer the possibility of field control for complex geometries, such as generating stellarator-like fields from simple toroidal fields. This paper summarizes work at MIT and in Japan on concept development and testing, as well as challenges ahead.
    No preview · Article · Feb 2015 · Fusion Science and Technology
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    ABSTRACT: Fusion power plants need larger scale and higher field superconducting magnets than the ITER magnets. Therefore, higher current superconductors with high strength against strong electromagnetic forces are needed. In order to examine the superconducting properties of such large conductors in real conditions, we are preparing a new test facility that is equipped with a solenoid coil of the highest field of 13 T with the bore of 0.7 m, a pair of temperature-variable current leads, and a vacuum chamber for conductor samples. The highest field can be increased to 15 T by installing an additional coil with the cold bore of 0.6 m. Since the inlet temperature of the samples can be varied from 4.4K to 50 K, it is possible to examine properties of advanced conductors at actual operating temperatures including high-temperature superconductors. We propose coil-shaped conductor samples instead of straight samples in order to apply electromagnetic hoop forces on the conductors to realize the real condition. A reference design of a sample for a cable-in-conduit conductor is provided. © 2015 The Japan Society of Plasma Science and Nuclear Fusion Research.
    No preview · Article · Jan 2015
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    ABSTRACT: NIFS launched the Fusion Engineering Research Project (FERP) in preparation for DEMO by starting the redesign of the LHD-type helical reactor FFHR-d1. In the first round, the main parameters were selected. The second round is preparing detailed three-dimensional (3D) design of the superconducting magnet support structures, and 3D neutronics analyses, where the diverter targets can be efficiently shielded from fast neutrons. A new Flinabe blanket mixed with metal powder was proposed. Fabrication of helical coils by connecting half-helical-pitch segments of 100 kA-class YBCO high-temperature superconductors is proposed as a promising method. Also in progress is improvement of the first round of the core plasma design, ignition start-up analyses, and fueling scenario. As a consequence, a multi-path strategy on FFHR-d1 has been introduced with versions of -d1A, -d1B, and -d1C, where design flexibility is expanded to include subignition with options FFHR-c1 for “before demo, compact, and component-test.”
    No preview · Article · Oct 2014 · Fusion Engineering and Design
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    ABSTRACT: FFHR-d1 is a conceptual design of the helical reactor being developed at the National Institute for Fusion Science. The maintenance of in-vessel components is very important for the fusion demo reactor. In addition, sufficient pathways are needed for the divertor exhaust. To implement these, the vacuum vessel, coil support structure, and cryostat require large apertures. However, the coil support structure has to be sufficiently rigid to remain within soundness and deformation limits. A design combining the structural components in the FFHR-d1A was developed from mechanical and thermal viewpoints. Consequently, components having a sufficiently large port area were provided. An investigation of the maintenance and exhaust schemes has been planned on the basis of this fundamental design.
    No preview · Article · Oct 2014 · Fusion Engineering and Design
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    ABSTRACT: Objectives: This study examines the relationship between toner exposure and its health effects in terms of biomarkers which are known to assess the damages to humans caused by toxic material exposure. Methods: The subjects were 1504 male workers aged below 50 in 2003 in a Japanese toner and photocopier manufacturing company. Personal exposure measurements, pulmonary function tests, chest X-ray examinations, biomarker measurement, and a questionnaire about respiratory symptoms were conducted. We will report about biomarker measurement in this study. Cross-sectional survey studies and a longitudinal study from 2003 to 2008 were conducted. Results: Few significant findings were associated with the toner exposure in both the cross-sectional and the longitudinal studies. The higher toner exposure concentrations did not induce effects on increasing biomarkers. Conclusion: There was no evidence of excessive inflammatory, allergic, or oxidative stress reaction in toner-handling workers as compared to non-handling workers, despite some sporadically significant findings. There are no other reports of a longitudinal epidemiological study with regard to toner exposure; this report significantly contributes to toner exposure literature. Although in the current well-controlled working environment, the toner exposure concentrations are quite low; further studies are needed to completely understand the health effects toner may have, however small they may be.
    No preview · Article · Jul 2014 · Human & Experimental Toxicology
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    ABSTRACT: Objectives: This study uses pulmonary function tests and chest x-ray examinations to examine the relationship between toner-handling work and its health effects. Methods: The subjects were 1504 male workers in a Japanese toner and photocopier manufacturing company, in the age range from 19 to 50 years in 2003. Personal exposure measurements, pulmonary function tests, chest x-ray examinations, biomarker measurements, and a questionnaire about respiratory symptoms were conducted. The present study reports the results of pulmonary function tests and chest x-ray examinations conducted in the subjects, which includes a cross-sectional study on the toner handling and non-handling workers and a longitudinal study from 2003 to 2008. Results: Few significant findings were suspected to be caused by toner exposure found in pulmonary function indices in both the cross-sectional and longitudinal studies. Any obvious fibrotic findings in chest x-ray findings related to the toner exposure could not be found out. Conclusion: No evidence of adverse effects on pulmonary function indices and chest x-rays was present in the toner-handling workers as compared to the nonspecifically exposed workers. Although the toner exposure concentration is quite low in the current well-controlled working environment, even among the toner-handling workers, we would like to continue this study in the future to verify the toner exposure health effects.
    No preview · Article · Jul 2014 · Human & Experimental Toxicology
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    ABSTRACT: In this report, we propose segment-fabricated high-temperature superconducting (HTS) magnets as candidates for the FFHR-d1 heliotron-type fusion reactor. The FFHR-d1 requires 100-kA-class superconducting conductors used at 12 T for a pair of helical coils. We fabricated and tested two 30-kA-class GdBCO conductors with bridge-type mechanical lap joints (mechanical bridge joints). This report details the design of the joint section and the experimental results of those samples, especially, those of their joints. We improved the geometry of the joint region in a second sample, based on our results from the first. The second sample has sufficiently low joint resistance (less than 5 nΩ), and we could apply 70 kA to it without causing quenching at the joint. Its joint resistance was also acceptable for providing the electric power required to run the cryoplant for the segmented HTS helical coils.
    No preview · Article · Jun 2014 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: The high-temperature superconducting magnet option is being explored in the conceptual design studies of the LHD-type helical fusion reactor FFHR-d1. A 100 kA-class conductor is being developed by simply stacking REBCO tapes in a copper and stainless-steel jacket. One of the design options of the HTS conductor includes internal insulation so that the windings do not require vacuum pressure impregnation process. Innovative winding method of the huge helical coils is being investigated based on the segment fabrication of half-helical-pitch conductors by developing a bridge-type mechanical lap joint. A “30 kA-class” prototype conductor sample was fabricated using GdBCO tapes and successfully tested. The critical current was measured at various temperatures at 4.2-40 K and magnetic field <; 8 T. The joint resistance was evaluated by changing the applied stress. These experimental results are boosting the HTS magnet design of FFHR-d1.
    No preview · Article · Jun 2014 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: The upgrading plan of the Mini-RT which is an experimental device for plasma physics was initiated to replace the magnetically-levitated high temperature superconducting (HTS) coil by the new one wound with the latest REBCO tapes. The performance of the new HTS coil has been examined at the National Institute for Fusion Science. The coil was indirectly cooled to 35 K by forced flow of cold helium gas. The coil was successfully excited up to 100 A with a proper persistent current switch (PCS) operation. The central field given by the Hall probe was 0.21 T, which was the same with that expected by a numerical calculation. The time constant of the current decay during the persistent current mode was evaluated at the coil temperature of 36 and 41 K to be 306 and 228 h, respectively. These values are consistent with the estimation based on the measured joint resistances and they are much longer than that of the previous floating coil of Mini-RT wound 10 years ago (41 h before aged deterioration). By the test results, it has been confirmed that the promising performance of the manufactured REBCO floating coil has been achieved to further promote the Mini-RT project.
    No preview · Article · Jun 2014 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: Design activities on the helical-type fusion DEMO reactor, FFHR-d1, are progressing at NIFS. A 100 kA current-capacity is required for the helical coil conductors under the maximum magnetic field of ~ 13 T. High-temperature superconducting conductor has been proposed as one of the conductor options for the FFHR-d1 magnet. In this study, a 30 kA class HTS conductor sample has been fabricated and tested. The sample had no current feeders and the current was induced by changing the background magnetic field generated by the 9 T split coils in the cryostat. Rogowski coils and Hall probes were used for the measurement of the transport current of the sample. The critical current of the sample was measured at various temperatures and bias magnetic fields. To verify the self-field effect of the sample, a numerical analysis was performed by considering the current and magnetic field distribution among the tapes self-consistently. The analysis result was compared with the experimental observation.
    No preview · Article · Jun 2014 · IEEE Transactions on Applied Superconductivity
  • T. Watanabe · N. Yanagi · A. Sagara
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    ABSTRACT: We studied the optimization of the magnetic configuration for the LHD-type fusion energy reactor (FFHR). We propose thin and flat helical coil systems, which are partitioned to three blocks with independent current value control, to satisfy the following requirements: (1) sufficient blanket space and large plasma volume under the helical coils with appropriate major radius and (2) divertor legs with little disorder that turn to the back of the helical coils. The cross-section of the plasma boundary changes from elliptical to racetrack-type. By extending the coil height, the coil current density of the central block for helical coil is observed to decrease gradually. © 2014 The Japan Society of Plasma Science and Nuclear Fusion Research.
    No preview · Article · Jan 2014

Publication Stats

2k Citations
269.48 Total Impact Points

Institutions

  • 2009-2015
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
  • 1991-2015
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 2014
    • University of Occupational and Environmental Health
      • Institute of Industrial Ecological Sciences
      Kitakyūshū, Fukuoka, Japan
  • 1999-2014
    • The Graduate University for Advanced Studies
      • Department of Fusion Science
      Миура, Kanagawa, Japan
  • 1986-2011
    • Kyoto University
      • Graduate School of Energy Science
      Kyoto, Kyoto-fu, Japan
  • 2006
    • University of Tsukuba
      • Applied Physics
      Tsukuba, Ibaraki, Japan
  • 2000
    • Kyushu University
      Hukuoka, Fukuoka, Japan
  • 1995
    • Slovak Academy of Sciences
      Presburg, Bratislavský, Slovakia
    • Kagoshima University
      Kagosima, Kagoshima, Japan