Y. Nagayama

National Institutes Of Natural Sciences, Edo, Tōkyō, Japan

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Publications (441)515.4 Total impact

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    K Ida · K Nagaoka · S Inagaki · H Kasahara · T Evans · M Yoshinuma · K Kamiya · S Ohdach · M Osakabe · M Kobayashi · [...] · N Tamura · H Tanaka · K Toi · B Wieland · I Yamada · R Yasuhara · H Zhang · O Kaneko · A Komori · Collaborators
  • K Ida · K Nagaoka · S Inagaki · H Kasahara · T Evans · M Yoshinuma · K Kamiya · S Ohdach · M Osakabe · M Kobayashi · [...] · N Tamura · H Tanaka · K Toi · B Wieland · I Yamada · R Yasuhara · H Zhang · O Kaneko · A Komori · Collaborators
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    ABSTRACT: The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion 53 095024), is reviewed. Plasma with high ion and electron temperatures ( T i(0) ∼ T e (0) ∼ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 10 19 m −3 and with high electron and ion temperatures ( T i(0) ∼ T e (0) ∼ 2 keV), resulting in 3.36 GJ of input energy, is achieved.
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    The European Physical Journal Conferences 01/2015; 87:03010. DOI:10.1051/epjconf/20158703010
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    ABSTRACT: Non-linear coupling between fluctuations at different radial locations is observed on the Large Helical Device (LHD) plasma. The summed bi-coherence between the low frequency macro-scale fluctuation (frequency is f = 2.75 kHz) at ρ = reff/a99 = 0.63 (where a99 is averaged plasma radius) and the high-frequency micro-scale fluctuations (frequency range of f > 150 kHz) at ρ = 0.88 is found to converge to a non-zero and finite value. The bi-phase analysis provides another basis for the non-linear interaction across the long distance. These results are associated with the violation local closure for transport relation.
    Nuclear Fusion 11/2014; 54(11):114014. DOI:10.1088/0029-5515/54/11/114014 · 3.06 Impact Factor
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    ABSTRACT: We propose a new interferometer system for density profile measurements. This system produces multiple measurement chords by a leaky-wave antenna driven by multiple frequency inputs. The proposed system was validated in laboratory evaluation experiments. We confirmed that the interferometer generates a clear image of a Teflon plate as well as the phase shift corresponding to the plate thickness. In another experiment, we confirmed that quasi-optical mirrors can produce multiple measurement chords; however, the finite spot size of the probe beam degrades the sharpness of the resulting image.
    Review of Scientific Instruments 11/2014; 85(11):11D411. DOI:10.1063/1.4893430 · 1.61 Impact Factor
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    ABSTRACT: A new antenna array is proposed in order to improve the sensitivity and complexity of microwave imaging diagnostics systems such as a microwave imaging reflectometry, a microwave imaging interferometer, and an electron cyclotron emission imaging. The antenna array consists of five elements: a horn antenna, a waveguide-to-microstrip line transition, a mixer, a local oscillation (LO) module, and an intermediate frequency amplifier. By using an LO module, the LO optics can be removed, and the supplied LO power to each element can be equalized. We report details of the antenna array and characteristics of a prototype antenna array.
    Review of Scientific Instruments 10/2014; 85(11). DOI:10.1063/1.4885471 · 1.61 Impact Factor
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    ABSTRACT: We have studied two types of spatio-temporal turbulence dynamics in plasmas in the Large Helical Device, based on turbulence measurements with high spatial and temporal resolution. Applying conditional ensemble-averaging to a plasma with Edge-Localized Modes (ELMs), fast radial inward propagation of a micro-scale turbulence front is observed just after ELM event, and the propagation speed is evaluated as ∼100 m/s. A self-organized radial electric field structure is observed in an electrode biasing experiment, and it is found to realize a multi-valued state. The curvature of the radial electric field is found to play an important role for turbulence reduction.
    Physics of Plasmas 05/2014; 21(5):055904. DOI:10.1063/1.4876619 · 2.14 Impact Factor
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    ABSTRACT: We present a new TCP/IP-based file transfer protocol which enables high-speed daisy-chain transfer. By using this protocol, we can send a file to a series of destination hosts simultaneously because intermediate hosts relay received file fragments to the next host. We achieved daisy-chain file transfer from Japan to Europe via USA at about 800 Mbps by using a prototype. The experimental result is also reported. A total link length of a data delivery network can be reduced by daisy chaining, so it enables cost-effective international data sharing.
    Fusion Engineering and Design 05/2014; 89(5). DOI:10.1016/j.fusengdes.2014.02.028 · 1.15 Impact Factor
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    ABSTRACT: The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.
    Nuclear Fusion 10/2013; 53(10):104015. DOI:10.1088/0029-5515/53/10/104015 · 3.06 Impact Factor
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    ABSTRACT: Electrode biasing experiments were tried in various magnetic configurations on the Large Helical Device (LHD). The transitions of poloidal viscosity, which were accompanied with bifurcation phenomena characterized by a negative resistance, were clearly observed on LHD by the electrode biasing. The critical external driving force required for transition were compared with the local maximum in ion viscosity, and the radial resistivity before the transition also compared with the expected value from a neoclassical theory. The critical driving force increased and the radial resistivity decreased with the major radius of the magnetic axis R ax going outward. The configuration dependence of the transition condition and the radial resistivity qualitatively agreed with neoclassical theories. The radial electric field and the viscosity were also evaluated by the neoclassical transport code for a non-axisymmetric system, and estimated electrode voltage required for the transition, which was consistent with the experimental results.
    Nuclear Fusion 07/2013; 53(7). DOI:10.1088/0029-5515/53/7/073014 · 3.06 Impact Factor
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    ABSTRACT: A correlation between drift-type fluctuations and anomalous radial transport was observed in GAMMA 10, and these fluctuations were suppressed by the electron-cyclotron-heating-driven radial electric field. We developed new diagnostics to conduct these studies: a simultaneous two-point-measurement gold neutral beam probe (GNBP) for the radial electric field and potential fluctuations, and a high-speed end-plate potential fluctuation measurement system. The electric field and its fluctuations were successfully obtained in a single plasma shot for the first time. Coherence between the drift-type potential fluctuations measured in the core plasma by the GNBP and those obtained by end-plate measurements was clearly observed. These potential and electric field fluctuations were clearly suppressed by the positive electric fields measured by two-point potential measurements using the GNBP and the end-plate system. We can obtain a powerful tool for probing the radial electric field and potential fluctuations in the core plasma because the end-plate potential fluctuation is the same as that in the core plasma measured using the new GNBP.
    Nuclear Fusion 06/2013; 53(7):073031. DOI:10.1088/0029-5515/53/7/073031 · 3.06 Impact Factor
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    ABSTRACT: Progress in micro- to millimeter-wave technologies have made possible advanced diagnostics (imaging) for various applications, such as, plasma diagnostics, radio astronomy, alien substance detection, airborne and spaceborne imaging radars called as synthetic aperture radars, living body measurements. In this report we focus on the reflectometric (radar) techniques applied to plasma diagnostics and biomedical measurement.
    Electromagnetic Theory (EMTS), Proceedings of 2013 URSI International Symposium on; 01/2013
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    ABSTRACT: This paper presents a system called “TSMAP” that maps electron temperature profiles to flux coordinates for the Large Helical Device (LHD). Considering the flux surface is isothermal, TSMAP searches an equilibrium database for the LHD equilibrium that fits the electron temperature profile. The equilibrium database is built through many VMEC computations of the helical equilibria. Because the number of equilibria is large, the most important technical issue for realizing the TSMAP system is computational performance. Therefore, we use multiple personal computers to enhance performance when building the database for TSMAP. We use virtual machines on multiple Linux computers to run the TSMAP program. Virtual machine technology is flexible, allowing the number of computers to be easily increased. This paper discusses how the use of virtual machine technology enhances the performance of TSMAP calculations when multiple CPU cores are used.
    Fusion Engineering and Design 12/2012; 87(12):2076–2080. DOI:10.1016/j.fusengdes.2012.02.127 · 1.15 Impact Factor
  • Yoshio Nagayama · Masahiko Emoto · Masanobu Yoshida · Chie Iwata
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    ABSTRACT: The large helical device (LHD), which is the world largest helical confinement system, is a national project serving Japanese fusion community. In LHD, experiments of 7000 shots are carried out for 250 proposals every year. Efficient experiment arrangement is required in order to carry out many collaborators’ proposal. Sometimes collaborators who are not familiar to LHD stay at NIFS only a few day to join the experiment. Issues are as follows: how to reduce collaborator's effort, how to reduce manager's effort to optimize the schedule; how to publicize plan and results of the experiment. We have developed web systems of virtual printer, experimental proposal and scheduling by using Ruby on Rails (RoR), which encapsulates relational data base (RDB) and AJAX. RDB enables to make tables by searching and sorting data with key words. Web servers are equipped in the virtual computer system in order to minimize efforts and cost of maintenance. The LHD web portal has been also developed in order to provide collaborators an efficient and intuitive interface to access the above systems, to take LHD information, and to use tools for LHD data analysis. The web systems have reduced collaborators’ and managers’ efforts significantly.
    Fusion Engineering and Design 12/2012; 87(12):2218–2222. DOI:10.1016/j.fusengdes.2012.09.012 · 1.15 Impact Factor
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    ABSTRACT: “Fusion virtual laboratory (FVL)” is the experiments’ collaboration platform covering multiple fusion projects in Japan. Major Japanese fusion laboratories and universities are mutually connected through the dedicated virtual private network, named SNET, on SINET4. It has 3 different categories; (i) LHD remote participation, (ii) bilateral experiments’ collaboration, and (iii) remote use of supercomputer. By extending the LABCOM data system developed at LHD, FVL supports (i) and (ii) so that it can deal with not only LHD data but also the data of two remote experiments: QUEST at Kyushu University and GAMMA10 at University of Tsukuba. FVL has applied the latest “cloud” technology for both data acquisition and storage architecture. It can provide us high availability and performance scalability of the whole system. With a well optimized TCP data transferring method, the unified data access platform for both experimental data and numerical computation results could become realistic on FVL. The FVL project will continue demonstrating the ITER-era international collaboration schemes and the necessary technology.
    Fusion Engineering and Design 12/2012; 87(12):2189–2193. DOI:10.1016/j.fusengdes.2012.04.027 · 1.15 Impact Factor
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    ABSTRACT: This paper proposes a clever usage of bispectral analysis for extracting or reconstructing a quasi-coherent and quasi-periodic structural evolution of plasma fluctuations. The method has been applied on electron temperature fluctuation signals measured with a multi-point electron cyclotron emission (ECE) radiometer in the Large Helical Device (LHD). The method successfully reconstructs the averaged spatiotemporal evolution of fluctuating coherent structure (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), while the structure is buried in (or comparable in power to) background fluctuations. The bicoherence analysis has found the faint fluctuating structure consisting of a fundamental mode and the harmonic modes from the existence of significant couplings between them. The reconstructed spatiotemporal structure with the proposed method is compared with that obtained with the lock-in (conditional) average. Three cases of spatiotemporal evolution of the non-sinusoidal waveform are presented.
    Plasma Physics and Controlled Fusion 11/2012; 54(11-11):115004. DOI:10.1088/0741-3335/54/11/115004 · 2.19 Impact Factor
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    ABSTRACT: Three-dimensional (3D) microwave imaging reflectometry has been developed in the large helical device to visualize fluctuating reflection surface which is caused by the density fluctuations. The plasma is illuminated by the probe wave with four frequencies, which correspond to four radial positions. The imaging optics makes the image of cut-off surface onto the 2D (7 × 7 channels) horn antenna mixer arrays. Multi-channel receivers have been also developed using micro-strip-line technology to handle many channels at reasonable cost. This system is first applied to observe the edge harmonic oscillation (EHO), which is an MHD mode with many harmonics that appears in the edge plasma. A narrow structure along field lines is observed during EHO.
    The Review of scientific instruments 10/2012; 83(10):10E305. DOI:10.1063/1.4729259 · 1.61 Impact Factor
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    ABSTRACT: We propose a new interferometer concept that can realize electron-density distribution measurement with high spatial and moderate temporal resolution. The image non-radiative dielectric guide antenna can probe a wide measurement area simultaneously. We fabricated the antenna with an electromagnetic simulator and confirmed that the simulated and measured radiation patterns are consistent with each other. In addition, we found that the antenna shows the required characteristics such as scanning characteristics, which depend on the input frequency.
    The Review of scientific instruments 10/2012; 83(10):10E347. DOI:10.1063/1.4734488 · 1.61 Impact Factor
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    ABSTRACT: The competing effects of the long-range fluctuation, which has been found on the Large Helical Device (Inagaki et al 2011 Phys. Rev. Lett.107 115001), on the net turbulent transport are discussed. The transport, which is driven directly by this mode, is evaluated. Then, the associated reduction of fluctuations in the range of drift wave turbulence is analyzed, by employing the predator–prey model. The transport driven by microscopic turbulence is reduced by the appearance of long-range fluctuations. Comparing these two competing processes, the condition is derived for the net transport to be reduced by the appearance of this long-range fluctuation. In addition, the experimental observations of microscopic fluctuations are discussed. The reduction of high-frequency fluctuations occurs in conjunction with the long-range fluctuations.
    Plasma Physics and Controlled Fusion 09/2012; 54(9). DOI:10.1088/0741-3335/54/9/095016 · 2.19 Impact Factor
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    ABSTRACT: Macro-scale temperature fluctuations, which have long radial correlation length (of the order of the plasma radius), have been discovered on Large Helical Device (LHD). In this paper, non-linear coupling between the long-range fluctuations and microscopic fluctuations is investigated using cross bi-coherence analysis. The significant cross bi-coherence between long-range fluctuations and micro-fluctuations is observed in the range of frequency f < 100 kHz, where f is the frequency of microscopic fluctuations. It is found that the squared bi-coherence and bi-phase for this three-wave coupling is weakly dependent on the frequency of microscopic fluctuations. These observations demonstrate that the microscopic fluctuations are coherently interacting with long-range fluctuations.
    Journal of the Physical Society of Japan 03/2012; 81(3):4501-. DOI:10.1143/JPSJ.81.034501 · 1.59 Impact Factor

Publication Stats

3k Citations
515.40 Total Impact Points


  • 2006–2014
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
  • 1996–2014
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 1999–2013
    • The Graduate University for Advanced Studies
      • Department of Fusion Science
      Миура, Kanagawa, Japan
  • 1986–2011
    • The University of Tokyo
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 2003
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya, Aichi, Japan
  • 1990–2001
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, New Jersey, United States
  • 1996–1998
    • University of Tsukuba
      • Applied Physics
      Tsukuba, Ibaraki, Japan