A Shimizu

National Institutes Of Natural Sciences, Edo, Tokyo, Japan

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Publications (375)741.62 Total impact

  • A Fujisawa · Y Nagashima · S Inagaki · T Onchi · S Ohshima · A Shimizu
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    ABSTRACT: A system for plasma turbulence tomography has been developed in a linear cylindrical plasma as a prototype with aiming at future application on toroidal plasma of higher temperature. This paper describes the diagnostic system in both aspects of the soft- and hardware, and reports the first results of tomographic reconstruction that can successfully produce local emission and its fluctuations. In the reconstruction process, two dimensional view of plasma is obtained for approximately 0.6 ms in every sampling time of 1 μs using parallel processing of 120 cores with 10 personal computers. The results include the steady state analysis of local fluctuation power spectra using fast Fourier transform, analysis of temporal behavior of fluctuation power spectra with wavelet transform, and analyses of the structural deformation or pattern of local plasma emission, demonstrating that the success of tomography as a promising diagnostic tool for plasma turbulence.
    No preview · Article · Feb 2016 · Plasma Physics and Controlled Fusion
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    ABSTRACT: In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities.
    Full-text · Article · Jan 2016 · Physical Review Letters
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    ABSTRACT: Abrupt and strong excitation of a mode has been observed when the frequency of a chirping energetic-particle driven geodesic acoustic mode (EGAM) reaches twice the geodesic acoustic mode (GAM) frequency. The frequency of the secondary mode is the GAM frequency, which is a half-frequency of the primary EGAM. Based on the analysis of spatial structures, the secondary mode is identified as a GAM. The phase relation between the secondary mode and the primary EGAM is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. The results suggest that the primary mode (EGAM) contributes to nonlinear destabilization of a subcritical mode.
    No preview · Article · Jan 2016 · Physical Review Letters
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    ABSTRACT: A resistive interchange mode of the structure (, : poloidal and toroidal mode numbers, respectively) with a bursting character and rapid frequency chirping in the range less than 10 kHz is observed for the first time in the edge region of the net current-free, low beta LHD (Large Helical Device) plasmas during high power injection of perpendicular neutral beams. The mode resonates with the precession motion of helically trapped energetic ions (EPs), following the resonant condition. The radial mode structure is recognized to be similar to that of the pressure-driven resistive interchange mode, of which radial displacement eigenfunction quite localizes around the mode rational surface, and evolves into an odd-type (or island-type) during the late of frequency chirping phase. This beam driven mode is excited when the beta value of helically trapped EPs exceeds a certain threshold. This instability is thought to be a new branch of resistive interchange mode destabilized by the trapped energetic ions. The radial transport, i.e. redistribution and losses, of helically trapped energetic ions induced by the mode transiently generates significant radial electric field near the plasma peripheral region. The large shear of thus generated radial electric field is thought to contribute to the observed suppression of micro-turbulence and transient increases of the temperature of fully ionized carbon impurity ions and electron density, suggesting improvement of bulk plasma confinement.
    No preview · Article · Jan 2016 · Nuclear Fusion
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    Full-text · Dataset · Sep 2015
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    ABSTRACT: The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti).
    No preview · Article · Sep 2015 · Nuclear Fusion
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    ABSTRACT: n = 0 modes with frequency chirping have been observed by a heavy ion beam probe and Mirnov coils in the large helical device plasmas, where n is the toroidal mode number. The spatial structures of the electrostatic potential fluctuation and the density fluctuation correspond to those of the geodesic acoustic mode (GAM). The modes are observed only during the tangential neutral beam injection with the energy of 175 keV. The energy spectra of fast ions measured by a neutral particle analyzer implies that the modes are excited by the fast ions through the inverse Landau damping. The absolute values and the temperature dependence of the frequency of the mode can be interpreted by the dispersion relation taking into account the measured energy spectra of the fast ions. Therefore, the observed n = 0 modes are identified as the energetic-particle driven GAM.
    No preview · Article · Aug 2015 · Nuclear Fusion
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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.
    No preview · Article · Jul 2015 · Nuclear Fusion
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    ABSTRACT: A new bursting m=1/n=1 instability (m,n: poloidal and toroidal mode numbers) with rapid frequency chirping down has been observed for the first time in a helical plasma with intense perpendicular neutral beam injection. This is destabilized in the plasma peripheral region by resonant interaction between helically trapped energetic ions and the resistive interchange mode. A large radial electric field is induced near the edge due to enhanced radial transport of the trapped energetic ions by the mode, and leads to clear change in toroidal plasma flow, suppression of microturbulence, and triggering an improvement of bulk plasma confinement.
    No preview · Article · Apr 2015 · Physical Review Letters
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    ABSTRACT: The driving and damping mechanism of plasma flow is an important issue because flow shear has a significant impact on turbulence in a plasma, which determines the transport in the magnetized plasma. Here we report clear evidence of the flow damping due to stochastization of the magnetic field. Abrupt damping of the toroidal flow associated with a transition from a nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 in the large helical device. This flow damping and resulting profile flattening are much stronger than expected from the Rechester-Rosenbluth model. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that the flow damping is due to the change in the non-diffusive term of momentum transport.
    Preview · Article · Mar 2015 · Nature Communications
  • A Shimizu · T Ido · M Kurachi · R Makino · M Nishiura · S Kato · A Nishizawa · Y Hamada
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    ABSTRACT: Two-dimensional potential profiles in the Large Helical Device (LHD) were measured with heavy ion beam probe (HIBP). To measure the two-dimensional profile, the probe beam energy has to be changed. However, this task is not easy, because the beam transport line of LHD-HIBP system is very long (∼20 m), and the required beam adjustment consumes much time. To reduce the probe beam energy adjustment time, an automatic beam adjustment system has been developed. Using this system, required time to change the probe beam energy is dramatically reduced, such that two-dimensional potential profiles were able to be successfully measured with HIBP by changing the probe beam energy shot to shot.
    No preview · Article · Nov 2014 · Review of Scientific Instruments
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    ABSTRACT: Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density n e_fir = 1 × 1019 m−3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of n e_fir. The plasma stored energy of 530 kJ with n e_fir = 3.2 × 1019 m−3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.
    No preview · Article · Jun 2014 · Physics of Plasmas
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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.
    No preview · Article · May 2014 · Physics of Plasmas
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    ABSTRACT: Chronic active Epstein-Barr virus infection (CAEBV) is characterized by chronic infectious mononucleosis-like symptoms. We report a very rare case with autoimmune hepatitis (AIH) complicated by CAEBV. A 50-year-old woman with systemic lupus erythematosus (SLE) complicated by AIH began to suffer from acute respiratory failure and her clinical symptoms improved rapidly in response to steroid treatment. However, during the gradual tapering of the steroid dose, a steady increase of the serum hepatobiliary enzyme levels subsequently was observed and the patient began to have continuous fever. Moreover, upper gastrointestinal endoscopy revealed multiple intractable gastric ulcers. When EBER-ISH was performed on liver biopsy and gastric mucosal biopsy specimens, EBER-positive lymphocytes were observed. When peripheral blood was examined, 2.1 × 10(6) copies/µg of EBV-DNA were observed in the CD4-positive T cells, confirming the diagnosis of CAEBV. A cooling therapy was started by steroid and cyclosporine. Thereafter, despite the start of CHOP therapy, she developed a malignant lymphoma (PTCL-NOS) and died of hepatic failure. When treatment-resistant AIH patients are encountered, not only AIH exacerbation but also CAEBV should be considered in the differential diagnosis.
    No preview · Article · Mar 2014 · Lupus
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    ABSTRACT: Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1 MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5 MW/CW) was newly installed in 2012 and the total injection power of ECRH reached 4.6 MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature T-e0 = 13.5 keV with n(e) = 1x10(19) m(-3). In the LHD, an electron-internal-transport barrier (e-ITB) related to the production of high T-e plasmas has been realized by strongly centre-focused ECRH. The electron thermal confinement clearly improved inside the e-ITB. The radial electric field was measured using the heavy ion beam probe. The formation of the positive E-r was observed in the core region, which well agreed with the prediction of the neoclassical transport theory. The energy confinement characteristics have been investigated in the ECRH plasmas. It was found that higher plasma stored energy and lower radiation power was realized in the outward configuration. The plasma stored energy of 530 kJ with n(e) = 3.2x10(19) m(-3), which is the 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved.
    No preview · Conference Paper · Feb 2014
  • K. Ogawa · M. Isobe · K. Toi · A. Shimizu · M. Osakabe
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    ABSTRACT: In this study, using a scintillator-based lost fast-ion probe, we have measured fast-ion losses in the Large Helical Device, in which the losses were caused by externally applied static magnetic perturbations (MPs) having small amplitudes. Energy and pitch-angle resolved measurements reveal that the effects of MPs on fast-ion losses occur in both high and low pitch-angle regions. A Lorentz orbit-following calculation indicates that static MPs can change the orbits of fast ions, which can enhance the transport/loss of fast ions. © 2014 The Japan Society of Plasma Science and Nuclear Fusion Research.
    No preview · Article · Jan 2014
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    ABSTRACT: We report observations of the dynamic response of micro-fluctuations and turbulent flux to a low-frequency heating power modulation in the Large Helical Device. The responses of heat flux and micro-fluctuation intensity differ from that of the change in temperature gradient. This result violates the local transport model, where turbulence is determined by the local temperature gradient. A new relationship between flux, gradient and turbulence is found. In addition to the temperature gradient, the heating rate is proposed as a new, direct controlling parameter of turbulence to explain the fast response of turbulence against periodic modulation of heating power.
    No preview · Article · Nov 2013 · Nuclear Fusion
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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.
    No preview · Article · Oct 2013 · Nuclear Fusion
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    ABSTRACT: Objective: Silver Diamine Fluoride (SDF) is focused on its inhibitory efficacy of tooth demineralization. A portable chair-side hardness tester ‘CariotesterTM‘ (CT) is reportedly capable of investigating the inhibitory efficacy in vivo. As a preliminary study prior to in vivo evaluation, the present study was performed with the purpose of evaluating the acid resistance of dentin treated by SDF using CT. Method: A hundred bovine root dentin were divided into ten groups. Five groups received SDF application onto dentin (SDF groups) and the remaining groups did not (control groups). The ten groups were subjected to immersion in acidic buffer for 1, 3, 7, 14 or 21 days for demineralization (n = 10). Surfaces of the demineralized specimens were indented using CT and the indentation depths (CT depth) were measured. Following the depth measurement, the specimens were sliced and analyzed using TMR to obtain the lesion depth (LD). Data were statistically analyzed using Bonferroni multiple comparison at the 0.05 level of significance. Result: SDF groups demonstrated significantly smaller CT depth than the control groups at every immersion periods, that indicates the efficacy of SDF lasted for 21 days. Strong linear correlations were exhibited between the square root of immersion period and CT depth or LD. Strong correlation was also revealed between CT depth and LD. CT depth and LD was previously reported to correlate in dentin treated with NaF or APF. The results with CT are consistent with those with TMR. Accordingly these findings might suggest that CT depth is a meaningful parameter to evaluate the acid resistance of fluoride-treated dentin. Conclusion: Application of SDF was capable of modifying the dentin into acid-resistant one indicating an inhibitory efficacy on root caries development. CT must be a promising device for follow-up assessment of root caries.
    No preview · Conference Paper · Aug 2013
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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.
    Preview · Article · Jul 2013 · Nuclear Fusion

Publication Stats

4k Citations
741.62 Total Impact Points

Institutions

  • 2010-2016
    • National Institutes Of Natural Sciences
      Edo, Tokyo, Japan
  • 2015
    • Japan Atomic Energy Agency
      • Nuclear Science and Engineering Directorate
      Muramatsu, Niigata, Japan
  • 1998-2015
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 2014
    • National Center for Global Health and Medicine in Japan
      Edo, Tōkyō, Japan
  • 2013
    • Tsurumi University
      • Department of Operative Dentistry
      Yokohama, Kanagawa, Japan
    • Hyogo College of Medicine
      • Department of Dentistry and Oral Surgery
      Nishinomiya, Hyōgo, Japan
  • 2009-2013
    • The Graduate University for Advanced Studies
      Миура, Kanagawa, Japan
  • 2012
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, Florida, United States
  • 1995-2010
    • Kyushu University
      • • Interdisciplinary Graduate School of Engineering Sciences
      • • Research Institute for Applied Mechanics
      • • Department of Surgery and Science
      Hukuoka, Fukuoka, Japan
  • 1994-2010
    • Gifu University
      • Faculty of Engineering
      Gihu, Gifu, Japan
  • 1979-2010
    • Osaka University
      • Research Center for Nuclear Physics
      Suika, Ōsaka, Japan
  • 1993-2009
    • Osaka Medical College
      • First Department of Internal Medicine
      Takatuki, Ōsaka, Japan
  • 2003-2006
    • Tohoku University
      • • Division of Life Sciences
      • • Graduate School of Engineering
      Sendai, Kagoshima-ken, Japan
  • 2000-2002
    • Nagoya University
      • • Graduate School of Engineering
      • • Department of Energy Engineering and Science
      Nagoya, Aichi, Japan
  • 1985-1994
    • Kyoto University
      • • Department of Biophysics
      • • Department of Medical Chemistry
      • • Department of Molecular Genetics
      • • Department of Molecular and Cellular Biology
      • • Institute for Virus Research
      Kyoto, Kyoto-fu, Japan
    • Ibaraki University
      Mito-shi, Ibaraki, Japan
  • 1989
    • Howard Hughes Medical Institute
      Ashburn, Virginia, United States
  • 1983-1986
    • Tokyo Institute of Technology
      • Department of Biological Sciences
      Edo, Tōkyō, Japan