A Okamoto

The Jikei University School of Medicine, Edo, Tokyo, Japan

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Publications (199)315.16 Total impact

  • H. Takahashi · A. Okamoto · T. Takahashi · S. Kitajima ·
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    ABSTRACT: This technical note reports the first experimental investigation aiming for divertor plasma dynamics study using helium recombining plasma and energetic helium ion beam. A divertor plasma relevant experiment was performed using a linear plasma device, DT-ALPHA, and a multicusp ion source. The radial profiles of the line emission intensities from helium Rydberg atoms were investigated, and the reaction rate of volumetric recombination was confirmed to increase in the central region of the cylindrical plasma. Then an energetic helium ion beam was superimposed onto the helium recombining plasma. The optical emission intensities from the helium recombining plasma with and without energetic ions were obtained and compared. The temporal evolution of the optical emission indicated that charge-exchange momentum transfer between energetic helium ions and ground state helium atoms can affect population density even in recombining plasma where population density is dominated by recombining processes rather than ionizing processes.
    Fusion Science and Technology 07/2015; 68(1):190-195. · 0.49 Impact Factor
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    ABSTRACT: In the biased plasma in TU-Heliac, a steep density profile around the core plasma region suddenly collapsed, accompanied with a high-frequency bursting turbulent fluctuation (100 kHz < f < 500 kHz). Envelope analysis was applied and the moment of the bursting event occurrence was detected in the biased plasma. The profile of the density gradient and bursting fluctuation were obtained using two Langmuir probes, one as a monitor probe for the bursting fluctuation and the other as a profile scan probe with a conditional average method. The fluctuation increased and showed a peak at the low field side of the steep gradient, from which the fluctuation propagated to the inner and outer directions at a velocity of 1 km s−1. The density gradient and the fluctuation had a non-linear relation and indicated a hysteresis loop.
    Nuclear Fusion 11/2014; 54(11):114013. DOI:10.1088/0029-5515/54/11/114013 · 3.06 Impact Factor
  • M Inoue · N Yanaihara · A Okamoto ·
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    ABSTRACT: Salmonella ovarian abscess in a patient with rheumatoid arthritis (RA) is reported here. A 33-year-old nulliparous woman with a 16-year history of RA who had been treated with corticosteroid and immunosuppressive drugs was diagnosed as having a non-typhoidal Salmonella ovarian abscess which might have been preceded by an occurrence of endometriotic cyst. Multidisciplinary therapy including surgical intervention was required to complete the eradication of infection. Although Salmonella ovarian abscess is rare, it may cause a serious complication in the ovary harboring endometriotic cyst through sustained presence of Salmonella bacteraemia.
    Clinical and experimental obstetrics & gynecology 08/2014; 41(4):465-7. DOI:10.12891/ceog16952014 · 0.42 Impact Factor
  • T Seki · N Yanaihara · Y Hirata · M Fukunaga · T Tanaka · A Okamoto ·
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    ABSTRACT: Choriocarcinoma is categorized as either gestational or nongestational depending on its origin. Nongestational choriocarcinoma originated in the trophoblastic differentiation is a rare but an aggressive tumor. This article reports a nongestational case of a uterine endometrial carcinoma with trophoblastic differentiation. A 54-year-old woman with a history of atypical genital bleeding that underwent semi-radical hysterectomy, bilateral salpingo-oophrectomy, and pelvic lymph nodes dissection. Pathological investigation showed that the tumor had endometrioid adenocarcinoma and choriocarcinomatous components. Although a series of multimodality treatments including craniotomy were performed, she died of aggressive lung and brain metastases one year after the primary surgery.
    European journal of gynaecological oncology 08/2014; 35(4):461-4. DOI:10.12892/ejgo24642014 · 0.61 Impact Factor
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    ABSTRACT: HYPER-I (High Density Plasma Experiment-I) is a linear device that combines a wide operation range of plasma production with flexible diagnostics. The plasmas are produced by the electron cyclotron resonance (ECR) heating with parallel injection of right-handed circularly polarized microwaves of 2.45 GHz from the high-field side. The maximum attainable electron density is more than two orders of magnitude higher than the cutoff density of ordinary waves. Spontaneous formation of a variety of large-scale flow structures, or vortices, has been observed in the HYPER-I plasmas. Flow-velocity field measurements using directional Langmuir probes (DLPs) and laser-induced fluorescence (LIF) method have clarified the physical processes behind such vortex formations. Recently, a new intermittent behavior of local electron temperature has also been observed. Statistical analysis of the floating potential changes has revealed that the phenomenon is characterized by a stationary Poisson process.
    Journal of Plasma Physics 04/2014; 81(02). DOI:10.1017/S0022377814001147 · 0.86 Impact Factor
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    ABSTRACT: Spatial distribution of electron density is considered. By using a one-dimensional recombining plasma model, effects of transient energetic ion flux are investigated. The time response of the system against the transient flux is dominated by the recombination frequency. The magnitude of modification of the spatial distribution is determined by the ratio between the ionization due to the energetic ion and the recombination of the bulk plasma.
    Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • A Daibo · A Okamoto · H Takahashi · T Kumagai · T Takahashi · S Tsubota · S Kitajima ·
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    ABSTRACT: A helium ion beam is injected into a linear plasma device for the development of an ion beam source simulating high energy particle flux in divertor plasma. Beam current density more than 10 mA/cm(2) is extracted. Measurement of beam currents indicates that the beam is transported along the linear device and reaches to the downstream end plate.
    The Review of scientific instruments 02/2014; 85(2):02B307. DOI:10.1063/1.4826099 · 1.61 Impact Factor
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    ABSTRACT: Angle-resolved energy distribution functions of positive and negative hydrogen ions produced from a rough-finished Si surface under 1 keV proton irradiation have been measured. The corresponding distribution from a crystalline surface and a carbon surface are also measured for comparison. Intensities of positive and negative ions from the rough-finished Si are substantially smaller than those from crystalline Si. The angular distributions of these species are broader for rough surface than the crystalline surface. No significant temperature dependence for positive and negative ion intensities is observed for all samples in the temperature range from 300 to 400 K.
    The Review of scientific instruments 02/2014; 85(2):02C311. DOI:10.1063/1.4855455 · 1.61 Impact Factor
  • A Okamoto · T Oku · S Koike · Y Sato · K Ishii · S Kitajima ·
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    ABSTRACT: To develop a compact ion injector made of hydrogen storage material, we investigate a plasma response for a positively biased hydrogen storage electrode in a small standard heliac device. When the bias voltage of the electrode increases, a positive potential between the electrode and the space potential on the magnetic surface emerges. The emission of hydrogen atom line also increases. To clarify the origin of the increase in emission, we designed an imaging system with sufficient temporal resolution.
    The Review of scientific instruments 02/2014; 85(2):02B302. DOI:10.1063/1.4825162 · 1.61 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
  • A. Okamoto · M. Isobe · S. Kitajima · M. Sasao ·
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    ABSTRACT: Full orbit-following calculations for the final orbit of lost alpha particles in an ITER equilibrium are performed using a Lorentz orbit code LORBIT [M. Isobe, et al. Rev. Sci. Instrum. 70 (1999) 827] to obtain parameter dependence of the footprint of alpha particle bombardment on the first wall. Determination of the poloidal location for the footprint is dominated by the pitch angle at the starting point. Toroidal distribution of the footprint location is inhomogeneous even for equipartitioned starting points because of the shadow of the magnetic field line on the first wall surface. Due to a hopping motion of an alpha particle on the surface of the first wall, a lined footprint is generated. Energy deposition is found to be dispersed along the line.
    Journal of Nuclear Materials 07/2013; 438. DOI:10.1016/j.jnucmat.2013.01.191 · 1.87 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
  • Seiji Ishii · Takato Yano · Akihiro Okamoto · Takeshi Murakawa · Hideyuki Hayashi ·
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    ABSTRACT: The ATP-binding cassette (ABC) transporter ComA is a key molecule essential for the first step of the quorum-sensing system of Streptococcus. The nucleotide binding domains (NBD) of S. mutans ComA with different N termini, NBD1 (amino acid residues 495-760), NBD2 (517-760), and NBD3 (528-760), were expressed, purified, and characterized. The shortest NBD3 corresponds to the region commonly defined as NBD in the database searches of ABC transporters. A kinetic analysis showed that the extra N-terminal region conferred a significantly higher ATP hydrolytic activity on the NBD at a neutral pH. Gel-filtration, X-ray crystallographic, and mutational analyses suggest that at least four to five residues beyond the N-terminal boundary of NBD3 indeed participate in stabilizing the protein scaffold of the domain structure, thereby facilitating the ATP-dependent dimerization of NBD which is a prerequisite to the catalysis. These findings, together with the presence of a highly conserved glycine residue in this region, support the redefinition of the N-terminal boundary of the NBD of these types of ABC exporters.
    Biochemistry 03/2013; 52(15). DOI:10.1021/bi3017069 · 3.02 Impact Factor
  • N Tanaka · M Sasao · K Terai · A Okamoto · S Kitajima · H Yamaoka · M Wada ·
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    ABSTRACT: Proof of principle experiments of neutral helium beam production for alpha particle diagnostics was carried out on a test stand. Negative helium ions were produced in the Li charge exchange cell, in which stable and long time operation was possible. He(-) beam was accelerated to 157 keV. Finally, He(0) beam was successfully produced after the flight in the drift-tube through the auto-electron-detachment process from He(-) to He(0). A neutral beam detector using a pyroelectric device was also developed to measure He(0) beam intensity. The metastable component in the neutral helium beam was found to be less than 2%.
    The Review of scientific instruments 02/2012; 83(2):02B115. DOI:10.1063/1.3672113 · 1.61 Impact Factor
  • T Kobuchi · M Sasao · M Kisaki · K Tsumori · N Tanaka · A Okamoto · S Kitajima · O Kaneko · K Shinto · M Wada ·
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    ABSTRACT: A strongly focusing He(+) ion beam source equipped with concave multi-aperture electrodes was developed for production of He(-) through a charge exchange cell. The beam was extracted at a voltage less than 20 kV from 301 apertures distributed in an area of 100 mm φ, and focused at 750 mm distance. The beam current and the beam size of 2 A and 20 mm in diameter, respectively, were achieved with an arc power less than 10 kW. The optimum perveance was obtained at 0.02 A∕kV(1.5) at the beam energy less than 20 keV which is suitable for the conversion to He(-) in an alkali vapor cell.
    The Review of scientific instruments 02/2012; 83(2):02B120. DOI:10.1063/1.3673630 · 1.61 Impact Factor
  • M Sasao · M Kisaki · T Kobuchi · K Tsumori · N Tanaka · K Terai · A Okamoto · S Kitajima · O Kaneko · K Shinto · M Wada ·
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    ABSTRACT: An energetic helium neutral beam is involved in the beam neutralization measurement system of alpha particles confined in a DT fusion plasma. A full size strong-focusing He(+) ion source (2 A, the beam radius of 11.3 mm, the beam energy less than 20 keV). Present strong-focusing He(+) ion source shows an emittance diagram separated for each beamlet of multiple apertures without phase space mixing, despite the space charge of a beamlet is asymmetric and the beam flow is non-laminar. The emittance of beamlets in the peripheral region was larger than that of center. The heat load to the plasma electrode was studied to estimate the duty factor for the ITER application.
    The Review of scientific instruments 02/2012; 83(2):02B718. DOI:10.1063/1.3673493 · 1.61 Impact Factor
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    ABSTRACT: The next generation of compact tandem‐type DD or DT neutron generators requires a robust electron stripper with high charge exchange efficiency. In this study, stripping foils of various types were tested, and the H− to H+ conversion efficiency, endurance to the heat load, and durability were investigated in terms of suitability in the tandem‐type neutron generator. In the experiments, a H− beam was accelerated to about 180 keV, passes through a stripping foil, and produces a mixed beam of H−, H0, and H+. These ions were separated by an electric field, and detected by a movable Faraday cup to determine the conversion efficiency. The experimental results using thin foils of diamond‐like carbon, gold, and carbon nano‐tubes revealed issues on the robustness. As a new concept, a H− beam was injected onto a metal surface with an oblique angle, and reflected H+ ions are detected. It was found that the conversion efficiency, H+ fraction in the reflected particles, depends on the surface condition, with the maximum value of about 90%.
    09/2011; 1390(1):684-691. DOI:10.1063/1.3637441
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    ABSTRACT: The transition to the improved confinement mode by electrode biasing was observed for the first time in the Large Helical Device (LHD). A negative resistance was observed in the confinement mode sustained by the cold electrode biasing. The electrode current showed a clear decrease against an increase in the electrode voltage and was characterized by a hysteresis in the transition phenomena. The decrease in the electrode current suggested an improvement in the radial particle transport. An increase in the energy confinement time and remarkable suppression of density fluctuations, which correspond to the transition, were also observed. These results indicated that the electrode biased plasma in LHD showed similar improvements in confinement to those in the H-mode plasmas in tokamaks and stellarators.
    Nuclear Fusion 07/2011; 51(8):083029. DOI:10.1088/0029-5515/51/8/083029 · 3.06 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 11/2010; 22(44):243-243. DOI:10.1002/chin.199144243
  • Source
    A Okamoto · M Isobe · S Kitajima · M Sasao ·
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    ABSTRACT: Full orbit-following calculation is performed for the final orbit of the lost alpha particles, showing some orbits escaping from the last closed flux surface could be detected by a concealed lost ion probe (CLIP) installed under the shadow of the original first wall surface. While both passing and trapped orbits hit the same wall panel, detecting a trapped orbit by the CLIP is easier than detecting passing orbits. Whether the final orbit is detected or not is determined by the position of the reflection point. The CLIP successfully detects the trapped orbits, which are reflected before they hit to a first wall. Then the pitch angles of the orbits at the CLIP are close to and smaller than 90°. Optimization of the position of the CLIP in terms of broader detection window is investigated.
    The Review of scientific instruments 10/2010; 81(10):10D312. DOI:10.1063/1.3489968 · 1.61 Impact Factor

Publication Stats

3k Citations
315.16 Total Impact Points


  • 1992-2014
    • The Jikei University School of Medicine
      • Department of Obstetrics and Gynecology
      Edo, Tokyo, Japan
  • 1991-2014
    • Tohoku University
      • • Department of Quantum Science and Energy Engineering
      • • Graduate School of Engineering
      • • Department of Chemistry
    • NEC Corporation
      Edo, Tōkyō, Japan
  • 2005-2013
    • Tokai University
      • • Department of Biological Science and Technology
      • • School of High Technology for Human Welfare
      Hiratuka, Kanagawa, Japan
    • Chuo University
      • Faculty of Policy Studies
      Edo, Tōkyō, Japan
  • 2002-2012
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 1990-2010
    • Osaka City University
      • Faculty of Science
      Ōsaka, Ōsaka, Japan
  • 1991-2009
    • Osaka Medical College
      • • Department of Biochemistry
      • • Department of Medical Chemistry
      Takatuki, Ōsaka, Japan
  • 1987-2008
    • Tokyo Metropolitan Komagome Hospital
      Edo, Tōkyō, Japan
  • 2002-2006
    • Nagoya University
      • Graduate School of Science
      Nagoya-shi, Aichi-ken, Japan
  • 2000-2005
    • The University of Tokyo
      • Center for Human Genome
      Tōkyō, Japan
  • 2003
    • Seoul National University
      • Department of Nuclear Engineering
      Sŏul, Seoul, South Korea
  • 1998
    • University of Hamamatsu
      Hamamatu, Shizuoka, Japan

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