S Okamura

National Institute for Fusion Science, Tokitsu-chō, Gifu, Japan

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Publications (637)1447.64 Total impact

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    ABSTRACT: A scenario for non-inductive current ramp-up has been demonstrated using electron cyclotron waves in the spherical tokamak QUEST. The configuration was characterized by a high toroidal magnetic mirror ratio of 2 and a steady vertical magnetic field of more than 10% of the toroidal magnetic field. The generation and confinement of energetic electrons having energy greater than 10 keV were studied using hard x-rays. Because of the energetic electron pressure, a natural divertor formed with an inboard poloidal field null at the high poloidal beta (approximately 3–4).
    Nuclear Fusion 02/2014; · 2.73 Impact Factor
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    The Astrophysical Journal 09/2013; 775(2):140. · 6.73 Impact Factor
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    ABSTRACT: We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M87 in the Virgo cluster, using planetary nebulas (PNs) as tracers. The surveyed areas (0.43 squared degrees) cover M87 up to a radial distance of 150 kpc, in the ransition region between galaxy halo and intracluster light (ICL). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour-magnitude diagram and the properties of their point-spread function. We extract a catalogue of 688 objects down to m_5007=28.4, with an estimated residual contamination from foreground stars and background Lyalpha galaxies, which amounts to ~35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M87. We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with the superposition of two components associated with the halo of M87 and with the ICL, which have different luminosity specific PN numbers, the ICL contributing three times more PNs per unit light. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Comparing the PNLF of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes.
    Astronomy and Astrophysics 08/2013; · 5.08 Impact Factor
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    ABSTRACT: We present the results of Magellan/MMIRS and Keck/NIRSPEC spectroscopy for five Lyα emitters (LAEs) at z 2.2 for which high-resolution FUV spectra from Magellan/MagE are available. We detect nebular emission lines including Hα on the individual basis and low-ionization interstellar (LIS) absorption lines in a stacked FUV spectrum, and measure average offset velocities of the Lyα line, Δv Lyα, and LIS absorption lines, Δv abs, with respect to the systemic velocity defined by the nebular lines. For a sample of eight z ~ 2-3 LAEs without active galactic nucleus from our study and the literature, we obtain Δv Lyα = 175 ± 35 km s–1, which is significantly smaller than that of Lyman-break Galaxies (LBGs), Δv Lyα 400 km s–1. The stacked FUV spectrum gives Δv abs = –179 ± 73 km s–1, comparable to that of LBGs. These positive Δv Lyα and negative Δv abs suggest that LAEs also have outflows. In contrast to LBGs, however, the LAEs' Δv Lyα is as small as |Δv abs|, suggesting low neutral hydrogen column densities. Such a low column density with a small number of resonant scattering may cause the observed strong Lyα emission of LAEs. We find an anti-correlation between Lyα equivalent width (EW) and Δv Lyα in a compilation of LAE and LBG samples. Although its physical origin is not clear, this anti-correlation result appears to challenge the hypothesis that a strong outflow, by means of a reduced number of resonant scattering, produces a large EW. If LAEs at z > 6 have similarly small Δv Lyα values, constraints on the reionization history derived from the Lyα transmissivity may need to be revised.
    The Astrophysical Journal 02/2013; 765(1):70. · 6.73 Impact Factor
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    ABSTRACT: The luminosity function of planetary nebulae populations in galaxies within 10-15 Mpc distance has a cut-off at bright magnitudes and a functional form that is observed to be invariant in different galaxy morphological types. Thus it is used as a secondary distance indicator in both early and late-type galaxies. Recent deep surveys of planetary nebulae populations in brightest cluster galaxies (BCGs) seem to indicate that their luminosity functions deviate from those observed in the nearby galaxies. We discuss the evidence for such deviations in Virgo, and indicate which physical mechanisms may alter the evolution of a planetary nebula envelope and its central star in the halo of BCGs. We then discuss preliminary results for distances for the Virgo, Hydra I and Coma clusters based on the observed planetary nebulae luminosity functions.
    Proceedings of the International Astronomical Union 12/2012;
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    ABSTRACT: Non-inductive current start-up via relativistic electron cyclotron resonance interaction is investigated for the high ratio (~10 %) of vertical Bv to toroidal Bt fields and the concave field lines in the QUEST spherical tokamak. In the start-up scenario with an internal poloidal field null (IPN), the fast current start-up rate of 0.3-0.5 MA/sec and correlation with mildly relativistic electrons accelerated due to multiple ECR interaction are observed. In steady state high p equilibrium characterized by the inboard null (Rs ~ 0.7×R0) and p of 1.5 is achieved, where p are the inverse aspect ratio and poloidal beta, respectively. Relaxation oscillations in this equilibrium and confinement of the energetic electrons are discussed.
    24th IAEA Fusion Energy Conference; 10/2012
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    ABSTRACT: We present the results of Magellan/MMIRS and Keck/NIRSPEC spectroscopy for five Lya emitters (LAEs) at z=2.2 for which high-resolution FUV spectra from Magellan/MagE are available. We detect nebular emission lines including Ha on the individual basis and low-ionization interstellar (LIS) absorption lines in a stacked FUV spectrum, and measure average offset velocities of the Lya line, Delta_v_Lya, and LIS absorption lines, Delta_v_abs, with respect to the systemic velocity defined by the nebular lines. For a sample of eight z~2-3 LAEs without AGN from our study and the literature, we obtain Delta_v_Lya = 175+/-35 km s^{-1}, which is significantly smaller than that of Lyman-break Galaxies (LBGs), Delta_v_Lya=400 km s^{-1}. The stacked FUV spectrum gives Delta_v_abs = -179 +/- 73 km s^{-1}, comparable to that of LBGs. These positive Delta_v_Lya and negative Delta_v_abs suggest that LAEs also have outflows. In contrast to LBGs, however, the LAEs' Delta_v_Lya is as small as |Delta_v_abs|, suggesting low neutral hydrogen column densities. Such a low column density with a small number of resonant scattering may cause the observed strong Lya emission of LAEs. We find an anti-correlation between Lya equivalent width (EW) and Delta_v_Lya in a compilation of LAE and LBG samples. Although its physical origin is not clear, this anti-correlation result appears to challenge the hypothesis that a strong outflow, by means of a reduced number of resonant scattering, produces a large EW. If LAEs at z>6 have similarly small Delta_v_Lya values, constraints on the reionization history derived from the Lya transmissivity may need to be revised.
    06/2012;
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    ABSTRACT: We derived the luminosity function (LF) of dwarf galaxies in the Coma Cluster down to M_R=-10 at three fields located at the center, intermediate, and outskirt. The LF (-19<M_R<-10) shows no significant differences among the three fields. It shows a clear dip at M_R\sim-13, and is composed of two distinct components of different slopes; the bright component with -19<M_R<-13 has a flatter slope than the faint component with -13<M_R<-10 which has a steep slope. The bright component (-19<M_R<-13) consists of mostly red extended galaxies including few blue galaxies whose colors are typical of late-type galaxies. On the other hand, the faint component (-13<M_R<-10) consists of largely PSF-like compact galaxies. We found that both these compact galaxies and some extended galaxies are present in the center while only compact galaxies are seen in the outskirt. In the faint component, the fraction of blue galaxies is larger in the outskirt than in the center. We suggest that the dwarf galaxies in the Coma Cluster, which make up the two components in the LF, are heterogeneous with some different origins.
    The Astronomical Journal 05/2012; 144(2). · 4.97 Impact Factor
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    ABSTRACT: We present the results of deep imaging and spectroscopic observations of very extended ionized gas (EIG) around four member galaxies of the Coma cluster of galaxies: RB199, IC4040, GMP2923 and GMP3071. The EIGs were serendipitously found in an H-alpha narrow band imaging survey of the central region of the Coma cluster. The relative radial velocities of the EIGs with respect to the systemic velocities of the parent galaxies from which they emanate increase almost monotonically with the distance from the nucleus of the respective galaxies, reaching -400 - -800 km/s at around 40 - 80 kpc from the galaxies. The one-sided morphologies and the velocity fields of the EIGs are consistent with the predictions of numerical simulations of ram pressure stripping. We found a very low-velocity filament (v_rel = -1300 km/s) at the southeastern edge of the disk of IC4040. Some bright compact knots in the EIGs of RB199 and IC4040 exhibit blue continuum and strong H-alpha emission. The equivalent widths of the H-alpha emission exceed 200 A, and are greater than 1000 A for some knots. The emission line intensity ratios of the knots are basically consistent with those of sub-solar abundance HII regions. These facts indicate that intensive star formation occurs in the knots. Some filaments, including the low velocity filament of the IC4040 EIG, exhibit shock-like emission line spectra, suggesting that shock heating plays an important role in ionization and excitation of the EIGs.
    The Astrophysical Journal 02/2012; 749(1). · 6.73 Impact Factor
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    ABSTRACT: Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.
    The Review of scientific instruments 01/2012; 83(1):013501. · 1.52 Impact Factor
  • 07/2011;
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    ABSTRACT: We carried out extended spectroscopic confirmations of Lyα emitters (LAEs) at z = 6.5 and 5.7 in the Subaru Deep Field. Now, the total number of spectroscopically confirmed LAEs is 45 and 54 at z = 6.5 and 5.7, respectively, and at least 81% (70%) of our photometric candidates at z = 6.5 (5.7) have been spectroscopically identified as real LAEs. We made careful measurements of the Lyα luminosity, both photometrically and spectroscopically, to accurately determine the Lyα and rest-UV luminosity functions (LFs). The substantially improved evaluation of the Lyα LF at z = 6.5 shows an apparent deficit from z = 5.7 at least at the bright end, and a possible decline even at the faint end, though small uncertainties remain. The rest-UV LFs at z = 6.5 and 5.7 are in good agreement, at least at the bright end, in clear contrast to the differences seen in the Lyα LF. These results imply an increase in the neutral fraction of the intergalactic medium from z = 5.7 to 6.5. The rest-frame equivalent width (EW0) distribution at z = 6.5 seems to be systematically smaller than z = 5.7, and it shows an extended tail toward larger EW0. The bright end of the rest-UV LF can be reproduced from the observed Lyα LF and a reasonable EW0-UV luminosity relation. Integrating this rest-UV LF provides the first measurement of the contribution of LAEs to the photon budget required for reionization. The derived UV LF suggests that the fractional contribution of LAEs to the photon budget among Lyman break galaxies significantly increases toward faint magnitudes. Low-luminosity LAEs could dominate the ionizing photon budget, though this inference depends strongly on the uncertain faint-end slope of the Lyα LF.
    The Astrophysical Journal 06/2011; 734(2):119. · 6.73 Impact Factor
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    ABSTRACT: We present the average metallicity and star-formation rate of Lya emitters (LAEs) measured from our large-area survey with three narrow-band (NB) filters covering the Lya, [OII]3727, and Ha+[NII] lines of LAEs at z=2.2. We select 919 z=2.2 LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted [OII] and Ha nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by narrow-band imaging or deep spectroscopy. The stacked object has an Ha luminosity of 1.7x10^{42} erg s^{-1} corresponding to a star formation rate (SFR) of 14 M_{sun} yr^{-1}. We place, for the first time, a firm lower limit to the average metallicity of LAEs of Z>~0.09 Z_{sun} (2sigma) based on the [OII]/(Ha+[NII]) index together with photo-ionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at z~2, are extremely metal poor (Z<2x10^{-2} Z_{sun}) young galaxies at the 4sigma level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of z~2 UV-selected galaxies toward lower masses (5x10^{8} M_{sun}), but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The Ha and Lya luminosities of our NB-selected LAEs indicate that the escape fraction of Lya photons is ~12-30 %, much higher than the values derived for other galaxy populations at z~2.
    05/2011;
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    ABSTRACT: We carried out extended spectroscopic confirmations of Ly-alpha emitters (LAEs) at z=6.5 and 5.7 in the Subaru Deep Field. Now, the total number of spectroscopically confirmed LAEs is 45 and 54 at z=6.5 and 5.7, respectively, and at least 81% (70%) of our photometric candidates at z=6.5 (5.7) have been spectroscopically identified as real LAEs. We made careful measurements of the Ly-alpha luminosity, both photometrically and spectroscopically, to accurately determine the Ly-alpha and rest-UV luminosity functions (LFs). The substantially improved evaluation of the Ly-alpha LF at z=6.5 shows an apparent deficit from z=5.7 at least at the bright end, and a possible decline even at the faint end, though small uncertainties remain. The rest-UV LFs at z=6.5 and 5.7 are in good agreement, at least at the bright end, in clear contrast to the differences seen in the Ly-alpha LF. These results imply an increase in the neutral fraction of the intergalactic medium from z=5.7 to 6.5. The rest-frame equivalent width (EW_0) distribution at z=6.5 seems to be systematically smaller than z=5.7, and it shows an extended tail toward larger EW_0. The bright end of the rest-UV LF can be reproduced from the observed Ly-alpha LF and a reasonable EW_0-UV luminosity relation. Integrating this rest-UV LF provides the first measurement of the contribution of LAEs to the photon budget required for reionization. The derived UV LF suggests that the fractional contribution of LAEs to the photon budget among Lyman break galaxies significantly increases towards faint magnitudes. Low-luminosity LAEs could dominate the ionizing photon budget, though this inference depends strongly on the uncertain faint-end slope of the Ly-alpha LF.
    04/2011;
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    ABSTRACT: The loss flux to an azimuthally segmented limiter in the central cell of the cusp end cell stabilized, axisymmetric mirror machine, RFC-XX, shows that non-axisymmetric illumination by either ICH or ECH can induce azimuthal non-uniformities in the plasma. Measurements of the floating potential in the vicinity of the limiter and by the limiter itself suggest that a large stationary azimuthal electric field is induced which can convectively transport plasma across magnetic field lines. The local radial component of the electric drift velocity is as large as 7 × 104 cms−1. Examinatio n of the plasma end loss versus azimuth and probe measurements deep in the plasma show that the non-uniformities extend into the plasma and are not localized near the outer boundary. The data suggest that the RF fields are driving an anomalous radial loss.
    Nuclear Fusion 01/2011; 26(5):611. · 2.73 Impact Factor
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    ABSTRACT: Experimental data for ion cyclotron resonance heating in the RFC-XX machine in IPP-Nagoya are presented. The achieved ion temperature is as high as 100 eV at n = 1013 cm−3 and 1 keV at n = 1012 cm−3. The ion energy confinement becomes worse by the application of a longer pulse, which is found to be due to the enhanced charge-exchange loss and/or electron drag. Axially and azimuthally arrayed antennas are used in the heating, and the importance of the phasing is demonstrated. A simple model of the multiple-antenna problem is also given and used to interpret the experimental data.
    Nuclear Fusion 01/2011; 22(10):1359. · 2.73 Impact Factor
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    ABSTRACT: A method of plasma production by ion cyclotron wave heating has been developed in RFC-XX. Nagoya Type-III antennas were used for wave excitation, and gas was supplied through a gas box. The effect of the rotating field excitation on plasma production and heating was investigated. In the m = − 1 rotational mode (rotation in the direction of ion cyclotron motion), the plasma density profile is flat within the gas box bore with a line integrated density nℓ = 3 × 1013cm−2, and the ion temperature is Ti ≈ 150 eV. For the m = +1 mode, a high density plasma was obtained with a different profile having a peak at the centre. In this mode, the line integrated density is nℓ = 3 × 1014cm−2, with the peak density n(0) = 7 × 1013cm−3.
    Nuclear Fusion 01/2011; 26(11):1491. · 2.73 Impact Factor
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    ABSTRACT: We present a wide-field Halpha imaging survey of the rich cluster CL0939+4713 (A851) at z = 0.41 with Suprime-Cam on the Subaru Telescope, using the narrow-band filter NB921. The survey is sensitive to active galaxies with star formation rates (SFRs) down to ~0.3 M sun yr-1 throughout the 27' × 27' field. We identified 445 Halpha emitters along the large-scale structures around the cluster. Using this sample, we find that (1) the fraction of Halpha emitters is a strong function of environment and shows a clear decline toward the cluster central region, and (2) the color of Halpha emitters is clearly dependent on environment. The majority of the Halpha emitters have blue colors with B - I < 2, but we find Halpha emitters with red colors as well. Such red emitters are very rare in the cluster center or its immediate surrounding regions, while they are most frequently found in groups located far away from the cluster center. These groups coincide with the environment where a sharp transition in galaxy color distribution is seen. This may suggest that dusty star formation activity tends to be involved in galaxy truncation processes that are effective in groups, and that it is probably related to the "pre-processing" that generates present-day cluster S0 galaxies. Finally, we confirm that (3) the mass-normalized integrated SFR in clusters (i.e., the total SFR within 0.5 × R 200 from the cluster center divided by the cluster dynamical mass) rapidly increases with look-back time following approximately vprop(1 + z)6 and is also correlated with the cluster mass.
    The Astrophysical Journal 01/2011; 734. · 6.73 Impact Factor
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    ABSTRACT: We investigate the stellar populations of Lyalpha emitters (LAEs) at z = 5.7 and 6.6 in a 0.65 deg2 sky of the Subaru/XMM-Newton Deep Survey (SXDS) Field, using deep images taken with the Subaru/Suprime-Cam, United Kingdom Infrared Telescope/Wide Field Infrared Camera, and Spitzer/Infrared Array Camera (IRAC). We produce stacked multiband images at each redshift from 165 (z = 5.7) and 91 (z = 6.6) IRAC-undetected objects to derive typical spectral energy distributions (SEDs) of z ~ 6-7 LAEs for the first time. The stacked LAEs have as blue UV continua as the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) z-dropout galaxies of similar M UV, with a spectral slope beta ~ -3, but at the same time they have red UV-to-optical colors with detection in the 3.6 mum band. Using SED fitting we find that the stacked LAEs have low stellar masses of ~(3-10) × 107 M sun, very young ages of ~1-3 Myr, negligible dust extinction, and strong nebular emission from the ionized interstellar medium, although the z = 6.6 object is fitted similarly well with high-mass models without nebular emission; inclusion of nebular emission reproduces the red UV-to-optical colors while keeping the UV colors sufficiently blue. We infer that typical LAEs at z ~ 6-7 are building blocks of galaxies seen at lower redshifts. We find a tentative decrease in the Lyalpha escape fraction from z = 5.7 to 6.6, which may imply an increase in the intergalactic medium neutral fraction. From the minimum contribution of nebular emission required to fit the observed SEDs, we place an upper limit on the escape fraction of ionizing photons of f ion esc ~ 0.6 at z = 5.7 and ~0.9 at z = 6.6. We also compare the stellar populations of our LAEs with those of stacked HST/WFC3 z-dropout galaxies. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.
    The Astrophysical Journal 12/2010; 724:1524-1535. · 6.73 Impact Factor
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    ABSTRACT: We present images of extended Hα clouds associated with 14 member galaxies in the Coma cluster obtained from deep narrowband imaging observations with the Suprime-Cam at the Subaru Telescope. The parent galaxies of the extended Hα clouds are distributed farther than 0.2 Mpc from the peak of the X-ray emission of the cluster. Most of the galaxies are bluer than g – r 0.5 and they account for 57% of the blue (g – r < 0.5) bright (r < 17.8 mag) galaxies in the central region of the Coma cluster. They reside near the red- and blueshifted edges of the radial velocity distribution of Coma cluster member galaxies. Our findings suggest that most of the parent galaxies were recently captured by the Coma cluster potential and are now infalling toward the cluster center with their disk gas being stripped off and producing the observed Hα clouds.
    The Astronomical Journal 11/2010; 140(6):1814. · 4.97 Impact Factor

Publication Stats

9k Citations
1,447.64 Total Impact Points

Institutions

  • 1993–2014
    • National Institute for Fusion Science
      Tokitsu-chō, Gifu, Japan
  • 1976–2013
    • The University of Tokyo
      • • Institute for the Physics and Mathematics of the Universe (IPMU)
      • • Department of Astronomy
      • • Department of Advanced Energy
      Edo, Tōkyō, Japan
  • 2012
    • Fukuoka Institute of Technology
      Hukuoka, Fukuoka, Japan
    • Hosei University
      • Faculty of Science and Engineering
      Edo, Tōkyō, Japan
  • 2011
    • Nagoya Institute of Technology
      Nagoya, Aichi, Japan
  • 1984–2011
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya-shi, Aichi-ken, Japan
  • 2009
    • Instituto de Astrofísica de Canarias
      San Cristóbal de La Laguna, Canary Islands, Spain
  • 2001–2008
    • The Graduate University for Advanced Studies
      • • Department of Astronomical Science
      • • Department of Fusion Science
      Миура, Kanagawa, Japan
    • Carnegie Mellon University
      • Department of Physics
      Pittsburgh, Pennsylvania, United States
    • Tohoku University
      • Astronomical Institute
      Sendai, Kagoshima, Japan
  • 1982–2008
    • National Astronomical Observatory of Japan
      • Division of Optical and Infrared Astronomy
      Edo, Tōkyō, Japan
  • 1992–2007
    • Kyoto University
      • • Department of Nuclear Engineering
      • • Institute of Advanced Energy
      • • Department of Astronomy
      Kioto, Kyōto, Japan
  • 2006
    • Fermi National Accelerator Laboratory (Fermilab)
      Batavia, Illinois, United States
    • California Institute of Technology
      • Spitzer Science Center
      Pasadena, California, United States
  • 2005
    • New Mexico State University
      • Department of Astronomy
      Las Cruces, New Mexico, United States
  • 2004
    • Durham University
      • Department of Physics
      Durham, ENG, United Kingdom
  • 2001–2004
    • University of Chicago
      • • Enrico Fermi Institute
      • • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
  • 2003
    • University of Maine
      • Department of Physics and Astronomy
      Orono, Minnesota, United States
  • 1999–2003
    • Princeton University
      • • Department of Astrophysical Sciences
      • • Princeton Plasma Physics Laboratory
      Princeton, New Jersey, United States
  • 2002
    • Kyushu University
      • Research Institute for Applied Mechanics
      Fukuoka-shi, Fukuoka-ken, Japan
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, Washington, United States
  • 2000
    • Australian National University
      • Plasma Research Laboratory (PRL)
      Canberra, Australian Capital Territory, Australia
  • 1997
    • Chubu University
      Касугай, Aichi, Japan
  • 1986
    • Gifu University
      Gihu, Gifu, Japan