H. Maeda

Japan Atomic Energy Agency, Muramatsu, Niigata, Japan

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Publications (215)347.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A new mode of improved energy confinement, for which the confinement time is not worse or is sometimes even better than that for the well known H-mode and the density of which is in a quasi-stationary state, has been obtained in neutral beam heating experiments on JFT-2M. The new mode is different from the H-mode in many respects. The central electron temperature is higher in the new mode than in the H-mode. Radiation loss and density are reduced in the peripheral region but not in the central region. Therefore, the density and radiation profiles are highly peaked in the new mode, in contrast to the broad profiles in the H-mode. Particle confinement in the peripheral region seems to be worse in the new mode than in the H-mode. The new mode can be obtained in both divertor configurations and limiter discharges in JFT-2M.
    Nuclear Fusion 01/2011; 28(10):1892. · 2.73 Impact Factor
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    ABSTRACT: The characteristics of the edge localized model (ELM), also referred to as edge relaxation phenomenon, were investigated in H-mode plasmas of JFT-2M. It was found that the ELM is mainly a density fluctuation phenomenon at the edge and that the electron temperature at the edge, except near the separatrix, is not very strongly perturbed. Some experimental conditions controlling the ELM intensity are plasma density, plasma ion species, heating power and plasma current ramping. ELMs found in low density deuterium discharges are suppressed by raising the density. ELMs are more strongly pronounced in hydrogen plasmas than in deuterium plasmas. ELMs seen in hydrogen plasmas or in near marginal H-mode conditions are suppressed by increasing the heating power. ELMs are found to be suppressed by plasma current ramp-down whereas they are enhanced by current ramp-up. The MHD aspects of the ELM were investigated. Although no consistent MHD features of the ELMs were found, they sometimes trigger bursts of the m = 2 mode. The enhanced broadband magnetic fluctuation suggests a temporary return to the L-mode during the ELM event.
    Nuclear Fusion 01/2011; 31(1):93. · 2.73 Impact Factor
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    ABSTRACT: The coupling characteristics of the fast wave excited by a phased four-loop antenna array are described. Fast waves in the lower hybrid frequency range have the potential of generating plasma current in hot and dense plasmas. Fast waves are excited at the plasma edge by an oscillating magnetic field parallel to the toroidal direction. The parallel wavenumber of the excited fast waves is determined by the relative phase of the RF current on each antenna. The loading resistance of the antenna increases with density, but at densities above 2 × 1019 m−3 it starts to decrease with density, because of the steepening of the density gradient. The loading resistance is strongly dependent on the antenna phasing. The maximum loading due to surface wave excitation is obtained at Δ = 0 and the minimum at Δ = π. Substantial absorption of the excited fast waves is observed at the plasma centre when the antenna phasing is Δ = π. The absorption efficiency rises with decreasing phase velocity of the excited waves. The experimental results obtained in the coupling experiment are consistent with theoretical predictions.
    Nuclear Fusion 01/2011; 30(2):297. · 2.73 Impact Factor
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    ABSTRACT: The radiation loss in DIVA is measured quantitatively by a pyroelectric detector calibrated by heavily contaminated resistive discharges. The results are: (1) radiation loss power due to low-Z impurities does not play an important role; (2) radiation loss power due to the pseudo-continuum from high-Z impurities affects the power balance of the confined plasma; (3) the time dependence of energy loss channels is clarified in the gross power balance, a fact which attests to the reliability of the radiation measurements carried out by the pyroelectric detector.
    Nuclear Fusion 01/2011; 18(10):1337. · 2.73 Impact Factor
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    ABSTRACT: The absorption characteristics of fast waves excited by a phased four-loop antenna array have been investigated experimentally in JFT-2M. The frequency of the fast waves is 200 MHz, which corresponds to approximately the tenth harmonic of the ion cyclotron frequency of hydrogen. The fast wave power is absorbed mainly by the bulk thermal electrons. It is shown that the absorption efficiency of the excited fast waves is improved with increasing density and temperature, and with decreasing phase velocity of the fast wave. The results are consistent with the theoretical predictions obtained from ray-tracing calculations. The power deposition profile is obtained through synchronous detection of the electron cyclotron emission modulated by a periodic heat source. In this modulation experiment with a limiter plasma on JFT-2M the electron thermal diffusivity is 2-3 m2s−1 and the convection velocity is 20-40 ms−1 at e = 2 × 1019 m−3 and Ip = 230 kA. The resultant power deposition profile has a peak at the plasma centre and agrees well with that calculated with the ray-tracing code. The absorption efficiency calculated from the power deposition profile is 0.3-0.4 at e = 2 × 1019 m−3 and Te0 = 0.7 keV, which agrees roughly with that estimated from the initial rise of the plasma stored energy. The electron heating efficiency estimated from the absorption efficiency is (4–5) × 1019 eVm−3kW−1 and the incremental confinement time is 8-10 ms, which is slightly longer than that in L-mode plasmas heated by neutral beam injection and/or ion cyclotron heating in JFT-2M.
    Nuclear Fusion 01/2011; 30(5):831. · 2.73 Impact Factor
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    ABSTRACT: ICRF heating of impurity-free tokamak plasmas is studied in DIVA. The frequency is fixed at 25 MHz, which is a second-harmonic cyclotron frequency of deuteron with a toroidal magnetic field of 16.4 kG. The toroidal field BT and the ratio of proton-to-deuteron density p are varied over ranges of 12 ≤ BT ≤ 19 kG and 2 p 40%, respectively. The most favourable ion heating is obtained at BT = 18 kG and p = 5 ~ 10%. Ion temperature increases from 200 to 300 eV were measured by charge-exchange neutral analysis and also by spectroscopy, where 100 kW RF net power were applied. The results are consistent with mode-conversion theory.
    Nuclear Fusion 01/2011; 19(11):1499. · 2.73 Impact Factor
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    ABSTRACT: Experimental studies on the origin of metal impurities in DIVA are described. Three processes of metal impurity release, i.e. ion sputtering, evaporation and arcing, have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, which is characterized by the lack of spikes in the loop voltage as well as by the fact that no heat flux concentration towards a specific part of the first wall takes place. Moreover, it is demonstrated that a honeycomb structure can decrease the metal impurity release.
    Nuclear Fusion 01/2011; 18(6):872. · 2.73 Impact Factor
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    ABSTRACT: A pulse of methane gas and a short burst of aluminium have been injected into DIVA plasma at the point farthest from the divertor. For the same amount of injected impurity, the accumulation of injected impurity in the main plasma is reduced by a factor of 2 to 4 by the action of the divertor. The impurity shielded by the scrape-off plasma is rapidly guided into the burial chamber. The time needed for the poloidal excursion of the injected impurity ions in the scrape-off layer is roughly equal to that of the scrape-off plasma. Sweeping-out of the impurity ions diffusing from the main plasma into the burial chamber is also observed.
    Nuclear Fusion 01/2011; 18(10):1347. · 2.73 Impact Factor
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    ABSTRACT: Heat flux to the material surfaces in the scrape-off layer of a tokamak plasma is investigated experimentally. The time response of heat flux measurement has been improved to 0.2 ms by the recently developed thin-film thermometer. The result shows that the heat flux is calculated from a simple sheath model including secondary electron emission effects even if epithermal electrons are present.
    Nuclear Fusion 01/2011; 18(9):1195. · 2.73 Impact Factor
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    ABSTRACT: Superthermal electrons are observed in the scrape-off layer of DIVA. The superthermal electrons discussed in this paper are divided into two groups, high-energy electrons (10 keV – 100 keV) and epithermal electrons (150 eV – 500 eV). The epithermal electrons are studied quantitatively and their loss is shown to be explained by the destruction of magnetic surfaces near the separatrix owing to non-axisymmetric perturbations. The two-dimensional path of the high-energy electrons is studied, and the effects of nonaxisymmetric perturbations on the drift surface are discussed.
    Nuclear Fusion 01/2011; 18(2):205. · 2.73 Impact Factor
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    ABSTRACT: A pulse of methane gas is injected into the DIVA tokamak. The major portion of the injected impurity is shielded by the outer edge plasma, i.e. only 20% of the injected carbon are introduced into the discharge. After radial penetration of the injected carbon has been accomplished, accumulation doubles and a new steady state is sustained over the period of proton confinement. The transient radial diffusion process of the carbon is compared with calculations using an impurity transport code. The behaviour of the carbon impurity in the whole minor radius is well described by assuming perfect re-cycling at the gold-plated shell surface and using neoclassical diffusion superposed by anomalous diffusion of protons. It is shown that spreading caused by anomalous diffusion is important. The calculated result indicates that the carbon ions are lost dominantly in a He-like state.
    Nuclear Fusion 01/2011; 18(9):1217. · 2.73 Impact Factor
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    ABSTRACT: Re-cycling of self-sputtered impurities from a limiter or a divertor neutralizer plate is investigated numerically. An upper limit for the boundary plasma temperature is determined from the condition that wall material impurity ions do not increase. This temperature is shown to be very low in a conventional tokamak but not limited in a divertor tokamak with a molybdenum neutralizer plate.
    Nuclear Fusion 01/2011; 19(10):1327. · 2.73 Impact Factor
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    ABSTRACT: Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.
    Nuclear Fusion 01/2011; 28(2):301. · 2.73 Impact Factor
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    ABSTRACT: An unusual systemic arteriovenous malformation arising from the left 9th intercostal artery, taking a caudal course through the left paravertebral veins, eventually drained into the left ascending lumbar vein, as illustrated at computed tomography and angiography. An embryologic maldevelopment of the azygos venous system is not the only possible explanation for venous drainage in the caudal direction. We believe that predominant venous drainage can take place in either direction because of the potentially bidirectional flow in the vertebral venous systems.
    Acta Radiologica 01/2010; 30(5):487-9. · 1.33 Impact Factor
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    ABSTRACT: Ag-sheathed Bi-2212 tapes with varying core thickness were heat treated in flowing O2 in magnetic fields up to 15 T. A uniform, high-degree of texture is achieved throughout the thickness of the tapes when heat treated in a magnetic field, whereas a large portion of non-textured region exists without magnetic field. The critical current density Jc of the tapes increases with increasing field strength due to enhancement in texture. The self field critical current Ic > 1000 A is achieved for tapes with a core thickness of 180 mum. For tapes with larger core thicknesses, however, Ic decreases due to inhomogeneous melting. The magnetic field is more effective in enhancing texture in the early stages of crystal growth.
    01/2005;
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    ABSTRACT: Materials synthesis processes in high magnetic fields are investigated for high-Tc superconductors at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. On a melt growth process of YBa2Cu3O7 bulk in magnetic fields, it was found that the crystallinity is improved. These results are due to not only the magnetic field orientation but also the decrease of the crystal growth rate by the magnetic field, which is suggested from a differential temperature analysis measurement in magnetic fields. On a chemical vapor deposition process of YBa2Cu3O7 films deposited on the polycrystalline Ag substrates in high magnetic fields, the microstructure changes drastically from the rectangular large grains with 10 μm in size to the small complex shaped grains. The spiral growth mode is observed for the samples prepared at 0 T but it changes into the island growth mode for that prepared in the presence of a magnetic filed. On a partial-melt process of Bi2Sr2CaCu2O8 tapes in magnetic fields, the homogeneous texture is obtained and it results in the increase of the critical current density.
    Current Applied Physics 10/2003; 3(5):391-395. · 2.03 Impact Factor
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    ABSTRACT: Ag-sheathed Bi2212 monocore tapes with core thickness between 80 and 700 μm were heat-treated in flowing O<sub>2</sub> in magnetic fields up to 15 T. A uniform high-degree of texture is achieved throughout the thickness of the tapes when heat-treated in a magnetic field (H<sub>a</sub>) whereas a large portion of nontextured region exists without magnetic field. The critical current density J<sub>c</sub> of the tapes increases with increasing H<sub>a</sub> due to enhancement in texture. The self field critical I<sub>c</sub>>1000 A was achieved for tapes with a core thickness of 180 μm. However, for tapes with larger core thicknesses, I<sub>c</sub> decreases due to inhomogeneous melting. The magnetic field is more effective in enhancing texture in the early stages of crystal growth.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: Vertical magnetic fields up to 15 T were applied to the magnetic melt-processing (MMP) of Bi2212 bulks and Ag-sheathed tapes with a core thickness above 80 μm, which were set horizontally. Texture with the c-axis along the direction of magnetic field applied during MMP is developed due to the anisotropy in magnetic susceptibility of Bi2212. The degree of texture and the anisotropy factor in magnetization increase almost linearly as the magnetic field strength Ha during MMP is increased. The anisotropy factor in magnetization reaches 6.5 at a Ha of 13 T for Ag-doped Bi2212 bulks, whereas for Bi(Pb)2212 in which Pb partially substitutes for Bi the factor is suppressed to a half of that of Ag-doped Bi2212. The transport critical current Ic and the critical current density Jc of Bi2212 tapes also increase with increasing Ha due to the texture development and Ic reaches above 1000 A in self-filed for the tapes with a core thickness of 180 μm. However, for further thicker tapes, Ic decreases. The suppressions in the Ic for the tapes and in the anisotropy factor in magnetization for Bi(Pb)2212 bulks are due to inhomogeneous melting, and inhomogeneous nucleation and growth of crystals during MMP. When crystal growth is restricted, crystal alignment is suppressed.
    Physica C Superconductivity 01/2003; · 0.72 Impact Factor
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    ABSTRACT: Measurements obtained by the novel technique of an assembly of planar Langmuir probes rotating in the scrape-off layer of JFT-2M are described. The probes scan the radial direction and rotate 360 degrees so measuring the direction of the magnetic field and the density of current in the magnetic surfaces. Experiments carried out with and without divertor biasing lead to the following main results: (1) The direction of the ion flow has been measured. (b) It has been found that j*B not=0 with the resulting force always directed towards the magnetic axis. (c) By applying a negative divertor bias a narrow layer appears where the current flows in a direction opposite to the main plasma current
    Nuclear Fusion 10/2002; 34(11):1453. · 2.73 Impact Factor
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    ABSTRACT: A Langmuir probe is used to study the plasma behaviour in the scrape-off layer of JFT-2a, (a Tokamak with an axisymmetric divertor). The measurements reveal particle fluxes to the divertor and the wall as well as cross-field profiles of density and potential in the scrape-off layer. On the basis of these measurements, the cross-field diffusion coefficient in the scrape-off layer and the divertor efficiency are estimated. The existence of the magnetic limiter is also verified by the method.
    Plasma Physics 10/2002; 20(8):749.

Publication Stats

940 Citations
347.29 Total Impact Points

Institutions

  • 2011
    • Japan Atomic Energy Agency
      Muramatsu, Niigata, Japan
  • 1987–2010
    • Nagasaki University Hospital
      Nagasaki, Nagasaki, Japan
  • 2003
    • Florida State University
      • Center for Advanced Power Systems (CAPS)
      Tallahassee, FL, United States
  • 2002
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2000–2001
    • Kitami Institute of Technology
      Notsukeushi, Hokkaidō, Japan
  • 1997–2001
    • Tohoku University
      • Institute for Materials Research
      Sendai-shi, Miyagi-ken, Japan
    • Tokyo Institute of Technology
      • Department of Energy Sciences
      Tokyo, Tokyo-to, Japan
  • 1998–1999
    • Universiteit Twente
      • Faculty of Science and Technology (TNW)
      Enschede, Provincie Overijssel, Netherlands
    • The University of Tokyo
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1990–1997
    • Tsukuba Research Institute
      Edo, Tōkyō, Japan
  • 1985–1996
    • Toshiba Corporation
      Edo, Tōkyō, Japan
  • 1994
    • Okano Cable Co., Ltd.
      Зама, Kanagawa, Japan
  • 1991
    • Tokyo University of Agriculture and Technology
      • Department of Electronic and Information Engineering
      Edo, Tōkyō, Japan
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States