S. Kobayashi

University of Leicester, Leiscester, England, United Kingdom

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Publications (932)

  • S. Kobayashi · S. Ohshima · H. Matsuda · [...] · T. Mizuuchi
    [Show abstract] [Hide abstract] ABSTRACT: This paper describes the development study of the beam emission spectroscopy (BES) for the turbulent transport study in Heliotron J. Modification of the sightlines (10 × 4 for edge and 10 × 2 for edge) enables us to obtain 2-dimensional BES imaging. The cooling effect on the reduction in the electrical noise of avalanche photodiode (APD) assembly has been investigated using a refrigerant cooling system. When the temperature of the APD element has set to be −20 °C, the electrical noise can be reduced more than 50%. The measurement error of the phase difference in the case of low signal level has been tested by two light-emitting diode lamps. The APD cooling has an effect to improve the measurement error at the low signal level of APD.
    Article · Nov 2016 · Review of Scientific Instruments
  • [Show abstract] [Hide abstract] ABSTRACT: A Faraday-cup type lost-fast ion probe (FLIP) has been designed and installed in Heliotron J for the purpose of the studies of interaction between fast ions and MHD instabilities. The FLIP can measure the co-going fast ions whose energy is in the range of 1.7–42.5 keV (proton) and pitch angle of 90∘–140∘, especially for fast ions having the injection energy of neutral beam injection (NBI). The FLIP successfully measured the re-entering passing ions and trapped lost-fast ions caused by fast-ion-driven energetic particle modes in NBI heated plasmas.
    Article · Nov 2016 · Review of Scientific Instruments
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: A gravitational-wave transient was identified in data recorded by the Advanced LIGO detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the gravitational wave data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network Circulars, giving an overview of the participating facilities, the gravitational wave sky localization coverage, the timeline and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the electromagnetic data and results of the electromagnetic follow-up campaign are being disseminated in papers by the individual teams.
    Full-text Article · Jul 2016 · The Astrophysical Journal Letters
  • [Show abstract] [Hide abstract] ABSTRACT: The first direct detection of gravitational waves was made in late 2015 with the Advanced LIGO detectors. By prior arrangement, a worldwide collaboration of electromagnetic follow-up observers were notified of candidate gravitational wave events during the first science run, and many facilities were engaged in the search for counterparts. No counterparts were identified, which is in line with expectations given that the events were classified as black hole - black hole mergers. However these searches laid the foundation for similar follow-up campaigns in future gravitational wave detector science runs, in which the detection of neutron star merger events with observable electromagnetic counterparts is much more likely. Three alerts were issued to the electromagnetic collaboration over the course of the first science run, which lasted from September 2015 to January 2016. Two of these alerts were associated with the gravitational wave events since named GW150914 and GW151226. In this paper we provide an overview of the Liverpool Telescope contribution to the follow-up campaign over this period. Given the hundreds of square degree uncertainty in the sky position of any gravitational wave event, efficient searching for candidate counterparts required survey telescopes with large (~degrees) fields-of-view. The role of the Liverpool Telescope was to provide follow-up classification spectroscopy of any candidates. We followed candidates associated with all three alerts, observing 1, 9 and 17 candidates respectively. We classify the majority of the transients we observed as supernovae.
    Article · Jun 2016 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract] ABSTRACT: Using ion cyclotron heating and electron cyclotron heating (ECH), or solo ECH, trials of steady state plasma sustainment have been conducted in the superconducting helical/stellarator, large helical device (LHD) (Ida K et al 2015 Nucl. Fusion 55 104018). In recent years, the ECH system has been upgraded by applying newly developed 77 and 154 GHz gyrotrons. A new gas fueling system applied to the steady state operations in the LHD realized precise feedback control of the line average electron density even when the wall condition varied during long pulse discharges. Owing to these improvements in the ECH and the gas fueling systems, a stable 39 min discharge with a line average electron density ne-ave of 1.1 × 1019 m-3, a central electron temperature Te0 of over 2.5 keV, and a central ion temperature Ti0 of 1.0 keV was successfully performed with ∼350 kW EC-waves. The parameters are much improved from the previous 65 min discharge with ne-ave of 0.15 × 1019 m-3 and Te0 of 1.7 keV, and the 30 min discharge with ne-ave of 0.7 × 1019 m-3 and Te0 of 1.7 keV.
    Article · Apr 2016 · Nuclear Fusion
  • [Show abstract] [Hide abstract] ABSTRACT: A moment approach to calculate neoclassical transport in non-axisymmetric torus plasmas composed of multiple ion species is extended to include the external parallel momentum sources due to unbalanced tangential neutral beam injections (NBIs). The momentum sources that are included in the parallel momentum balance are calculated from the collision operators of background particles with fast ions. This method is applied for the clarification of the physical mechanism of the neoclassical parallel ion flows and the multi-ion species effect on them in Heliotron J NBI plasmas. It is found that parallel ion flow can be determined by the balance between the parallelviscosity and the external momentum source in the region where the external source is much larger than the thermodynamic force driven source in the collisional plasmas. This is because the friction between C6+ and D+ prevents a large difference between C6+ and D+flowvelocities in such plasmas. The C6+flowvelocities, which are measured by the charge exchange recombination spectroscopy system, are numerically evaluated with this method. It is shown that the experimentally measured C6+ impurity flowvelocities do not contradict clearly with the neoclassical estimations, and the dependence of parallelflowvelocities on the magnetic field ripples is consistent in both results.
    Article · Mar 2016 · Physics of Plasmas
  • [Show abstract] [Hide abstract] ABSTRACT: An HCN laser (λ = 337 μm) interferometer with a high time resolution has been developed in a helical device, Heliotron J, for the study of plasma confinement and transport. Using the new interferometer in combination with a microwave interferometer, a gas puff modulation experiment has been performed to clarify the particle transport in ECH and ECH + NBI heated plasmas. Based on the particle balance equation, the diffusion coefficient D and the convection velocity V are evaluated on the assumption of profile shapes for D, V and particle source. The result indicates that ECH plasma has better particle transport characteristics, smaller value on D and V, than the case of NBI heated plasmas. The influence of the source profile shape on this analysis is considered, because there is ambiguity on the edge plasma parameters around LCFS, which determines the source profile shape. Although evaluated values of D and V can depend on the source profiles, the difference still remains within the error bars at the present accuracy in this experimental condition, suggesting that more careful treatment of the assumption on particle source is required for the particle transport study with higher accuracy.
    Article · Feb 2016 · Journal of Instrumentation
  • S. Ohshima · S. Kobayashi · S. Yamamoto · [...] · F. Sano
    [Show abstract] [Hide abstract] ABSTRACT: Two different responses to an energetic-particle-driven magnetohydrodynamic (MHD) instability, modulation of the turbulence amplitude associated with the MHD instability and dynamical changes in the radial electric field (Er) synchronized with bursting MHD activities, are found around the edge plasma in neutral beam injection (NBI) heated plasmas of the Heliotron J device using multiple Langmuir probes. The nonlinear phase relationship between the MHD activity and broadband fluctuation is found from bicoherence and envelope analysis applied to the probe signals. The structural changes of the Er profile appear in perfect synchronization with the periodic MHD activities, and radial transport of fast ions are observed around the last closed flux surface as a radial delay of the ion saturation current signals. Moreover, distortion of the MHD mode structure is clarified in each cycle of the MHD activities using beam emission spectroscopy diagnostics, suggesting that the fast ion distribution in real and/or velocity spaces is distorted in the core plasma, which can modify the radial electric field structure through a redistribution process of the fast ions. These observations suggest that such effects as a nonlinear coupling with turbulence and/or the modification of radial electric field profiles are important and should be incorporated into the study of energetic particle driven instabilities in burning plasma physics.
    Article · Jan 2016 · Nuclear Fusion
  • G. Greco · D. Kondrashov · S. Kobayashi · [...] · A. Ortolan
    [Show abstract] [Hide abstract] ABSTRACT: We present a data-adaptive spectral method – Monte Carlo Singular Spectrum Analysis (MC-SSA) – and its modification to tackle astrophysical problems. Through numerical simulations we show the ability of the MC-SSA in dealing with 1∕f β power-law noise affected by photon counting statistics. Such noise process is simulated by a first-order autoregressive, AR(1) process corrupted by intrinsic Poisson noise. In doing so, we statistically estimate a basic stochastic variation of the source and the corresponding fluctuations due to the quantum nature of light. In addition, MC-SSA test retains its effectiveness even when a significant percentage of the signal falls below a certain level of detection, e.g., caused by the instrument sensitivity. The parsimonious approach presented here may be broadly applied, from the search for extrasolar planets to the extraction of low-intensity coherent phenomena probably hidden in high energy transients.
    Chapter · Jan 2016
  • Article · Jan 2016 · Reproduction Fertility and Development
  • [Show abstract] [Hide abstract] ABSTRACT: Real-time polarization scan experiments were performed on the Large Helical Device (LHD) to search an optimal incident wave polarization for electron cyclotron resonance heating. The obtained optimal polarization state to maximize the power absorption to the LHD plasma is compared with the ray-tracing code that includes mode content analyses, which indicates that the calculated results are generally in good agreement with the experimental results. The analyses show that optimal coupling to plasma waves requires a fine adjustment for an incident wave polarization even for perpendicular injection due to the finite density profile and the magnetic shear at the peripheral region. © 2016 The Japan Society of Plasma Science and Nuclear Fusion Research.
    Article · Jan 2016
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    [Show abstract] [Hide abstract] ABSTRACT: This Supplement provides supporting material for arXiv:1602.08492 . We briefly summarize past electromagnetic follow-up efforts as well as the organization and policy of the current electromagnetic follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the electromagnetic follow-up observations that were performed in the different bands.
    Full-text Article · Jan 2016 · The Astrophysical Journal Supplement Series
  • [Show abstract] [Hide abstract] ABSTRACT: The power injection system for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) was modified and upgraded. An outside horizontal port 2-O on the Large Helical Device (LHD) was furnished with two antenna systems for the EC-waves of the frequencies of 77 and 154 GHz, respectively. In addition to them, two new antenna systems for 77 and 154 GHz waves were installed in the 2-O port. Each antenna in the 2-O port has wide range of EC-wave beam direction control so that these are suitable for ECCD which requires toroidally oblique EC-wave beam injection. In the LHD 18th experimental campaign in 2014-2015, an ECCD experiment with second harmonic resonance condition, on-axis magnetic field of 1.375 T for 77 GHz waves, was performed in which some combination patterns of two 77 GHz ECCDs were applied. The discharges of dual co- and dual counter-ECCDs showed remarkable plasma currents of ~±26 kA in both of the co- and counter-directions, by 6 s pulse duration and injection powers of 366 and 365kW. The new antenna has nearly the same capability for ECCD with that of the existing antenna. The improvement in the flexibility of the ways of applying plural ECCDs will offer a highly useful tool for investigations on the phenomena concerning with the plasma current such as magnetohydro-dynamics. © 2016 The Japan Society of Plasma Science and Nuclear Fusion Research.
    Article · Jan 2016
  • S. Kobayashi · K. Nakazawa · K. Makishima
    [Show abstract] [Hide abstract] ABSTRACT: Attempts were made to construct a unified description of the spectra of ULX (Ultra Luminous X-ray source) objects, including their Power-Law (PL) state and Disk-like state. Among spectral models proposed to explain either state, the present work adopts the one which combines multi-color disk (MCD) emission and its thermal Comptonization (THC). This model was applied to several datasets of ULXs obtained by Suzaku, XMM-Newton, and Nustar. The model well explains all the spectra, regardless of the spectral states, in terms of a cool disk (inner radius temperature of 0.2-0.5 keV) and a cool thick (electron temperature of 1-3 keV, and optical thickness ~10) corona. The fit results can be characterized by two new parameters. One is Q (defined as the electron temerature divided by the inner radius temperature) which describes balance between the Compton cooling and gravitational heating of the coronal electrons, while the other is F, namely, the covering fraction of the MCD by the corona. Here, F is calculated from the percentage of the directly-visible disk luminosity in the total radiation. Then, the PL-state spectra have been found to show Q~10 and F~0.5, while those of the Disk-like state Q~3 and F~1. Thus, the two states are clearly separated in terms of Q and F. The obtained results are employed to argue for their interpretation in terms of high-mass (several tens to several hundreds solar masses) black holes.
    Article · Nov 2015 · Astronomische Nachrichten
  • [Show abstract] [Hide abstract] ABSTRACT: In the medium-sized heliotron device Heliotron J, edge density fluctuation has been measured simultaneously with a high-speed video camera and a Langmuir probe. Poloidally propagating, parallel elongating filamentary structures with 20- to 30-kHz frequency and similar to 14-cm poloidal wave length were observed by a camera. However, the radial position of this density mode is not well known with only camera data because the camera lens axis is perpendicular to the torus plane. To identify the span of this density mode, plasma-surface interaction (PSI) between the probe and the plasma has been analyzed. As the probe scanned into the plasma, enhanced brightness due to PSI was clearly observed in camera images. By comparing this enhanced brightness among different probe positions, the outmost margin of the 20- to 30-kHz mode observed by the camera has been identified to be within 10 mm outside from the last closed flux surface. This conclusion is supported by the spectrum of the probe data.
    Article · Nov 2015 · Fusion Science and Technology
  • [Show abstract] [Hide abstract] ABSTRACT: The central electron temperature has successfully reached up to 7.5 keV in large helical device (LHD) plasmas with a central high-ion temperature of 5 keV and a central electron density of 1.3×1019 m-3. This result was obtained by heating with a newly-installed 154 GHz gyrotron and also the optimisation of injection geometry in electron cyclotron heating (ECH). The optimisation was carried out by using the ray-tracing code LHDGauss, which was upgraded to include the rapid post-processing three-dimensional (3D) equilibrium mapping obtained from experiments. For ray-tracing calculations, LHDGauss can automatically read the relevant data registered in the LHD database after a discharge, such as ECH injection settings (e.g. Gaussian beam parameters, target positions, polarisation and ECH power) and Thomson scattering diagnostic data along with the 3D equilibrium mapping data. The equilibrium map of the electron density and temperature profiles are then extrapolated into the region outside the last closed flux surface. Mode purity, or the ratio between the ordinary mode and the extraordinary mode, is obtained by calculating the 1D full-wave equation along the direction of the rays from the antenna to the absorption target point. Using the virtual magnetic flux surfaces, the effects of the modelled density profiles and the magnetic shear at the peripheral region with a given polarisation are taken into account. Power deposition profiles calculated for each Thomson scattering measurement timing are registered in the LHD database. The adjustment of the injection settings for the desired deposition profile from the feedback provided on a shot-by-shot basis resulted in an effective experimental procedure.
    Article · Nov 2015 · Nuclear Fusion
  • Source
    G. Greco · D. Kondrashov · S. Kobayashi · [...] · A. Ortolan
    [Show abstract] [Hide abstract] ABSTRACT: We present a data-adaptive spectral method - Monte Carlo Singular Spectrum Analysis (MC-SSA) - and its modification to tackle astrophysical problems. Through numerical simulations we show the ability of the MC-SSA in dealing with $1/f^{\beta}$ power-law noise affected by photon counting statistics. Such noise process is simulated by a first-order autoregressive, AR(1) process corrupted by intrinsic Poisson noise. In doing so, we statistically estimate a basic stochastic variation of the source and the corresponding fluctuations due to the quantum nature of light. In addition, MC-SSA test retains its effectiveness even when a significant percentage of the signal falls below a certain level of detection, e.g., caused by the instrument sensitivity. The parsimonious approach presented here may be broadly applied, from the search for extrasolar planets to the extraction of low-intensity coherent phenomena probably hidden in high energy transients.
    Full-text Article · Sep 2015
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Gamma-ray burst GRB 140430A was detected by the Swift satellite and observed promptly with the imaging polarimeter RINGO3 mounted on the Liverpool Telescope, with observations beginning while the prompt $\gamma$-ray emission was still ongoing. In this paper, we present densely sampled (10-second temporal resolution) early optical light curves in 3 optical bands and limits to the degree of optical polarization. We compare optical, X-ray and gamma-ray properties and present an analysis of the optical emission during a period of high-energy flaring. The complex optical light curve cannot be explained merely with a combination of forward and reverse shock emission from a standard external shock, implying additional contribution of emission from internal shock dissipation. We estimate an upper limit for time averaged optical polarization during the prompt phase to be as low as P < 12% (1$\sigma$). This suggests that the optical flares and early afterglow emission in this GRB are not highly polarized. Alternatively, time averaging could mask the presence of otherwise polarized components of distinct origin at different polarization position angles.
    Full-text Article · Sep 2015 · The Astrophysical Journal
  • [Show abstract] [Hide abstract] ABSTRACT: Structural materials used in radiation environments require radiation tolerance and sufficient mechanical properties in the controlled state. In order to offer SUS316L austenitic stainless steel with the assumed requirements, nanostructured SUS316L with TiC addition of 2% (SUS316L-2TiC) that is capable of exhibiting enhanced tensile ductility and flow strength sufficient for structural applications was fabricated by advanced powder metallurgical methods. The methods include MA (Mechanical Alloying), HIP (Hot Isostatic Pressing), GSMM (Grain boundary Sliding Microstructural Modification) for ductility enhancement, cold rolling at temperatures below Md (the temperature where the martensite phase occurs by plastic deformation) for phase transformation from austenite to martensite and heat treatment for reverse transformation from martensite to austenite. It is shown that the developed SUS316L-2TiC exhibits ultrafine grains with sizes of 90-270 nm, accompanied by TiC precipitates with 20-50 nm in grain interior and 70-110 nm at grain boundaries, yield strengths of 1850 to 900 MPa, tensile strengths of 1920 to 1100 MPa and uniform elongations of 0.6-21%, respectively, depending on the heat treatment temperature after rolling at -196 °C.
    Article · Aug 2015
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    [Show abstract] [Hide abstract] ABSTRACT: We present a wide dataset of gamma-ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814A the best.
    Full-text Article · May 2015 · Monthly Notices of the Royal Astronomical Society

Publication Stats

9k Citations

Institutions

  • 2015
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom
  • 2009-2015
    • Liverpool John Moores University
      • Astrophysics Research Institute
      Liverpool, England, United Kingdom
  • 2006-2015
    • Ashikaga Institute of Technology
      Asikaga, Tochigi, Japan
    • Hiroshima University
      • Graduate School of Engineering
      Hirosima, Hiroshima, Japan
  • 2008-2014
    • National Livestock Breeding Center
      Hukusima, Fukushima, Japan
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, MD, United States
    • Pusan National University
      • Department of Physics
      Pusan, Busan, South Korea
  • 2004-2014
    • Ehime University
      • Department of Materials Science and Engineering
      Matuyama, Ehime, Japan
    • Kitasato University
      • Laboratory of Animal and Human Nutritional Physiology
      Edo, Tōkyō, Japan
    • Tokyo Metropolitan Institute
      Edo, Tokyo, Japan
  • 1967-2014
    • Kyoto University
      • • Institute of Advanced Energy
      • • Graduate School of Energy Science
      • • Department of Physics II
      Kioto, Kyōto, Japan
  • 2012
    • National Institutes Of Natural Sciences
      Edo, Tokyo, Japan
  • 2010-2012
    • Japan Aerospace Exploration Agency
      • Institute of Space and Astronautical Science (ISAS)
      Chōfu, Tōkyō, Japan
    • University of Liverpool
      Liverpool, England, United Kingdom
  • 2002-2011
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
    • Shimane University
      • Department of Internal Medicine
      Matsu, Shimane Prefecture, Japan
  • 1988-2011
    • Saitama University
      • • Graduate School of Science and Engineering
      • • Faculty of Engineering
      • • Department of Electrical and Electronic Systems
      Saitama, Saitama-ken, Japan
    • University of Oxford
      Oxford, England, United Kingdom
  • 2008-2010
    • Tohoku University
      • • Department of Life and Pharmaceutical Science
      • • Institute of Multidisciplinary Research for Advanced Materials (IMRAM)
      Sendai-shi, Miyagi, Japan
  • 2005-2009
    • Iwate University
      • Faculty of Engineering
      Morioka, Iwate, Japan
    • The Jikei University School of Medicine
      • Department of Surgery
      Edo, Tokyo, Japan
  • 2003-2009
    • National Taiwan University
      • Department of Physics
      Taipei, Taipei, Taiwan
  • 2004-2008
    • Waseda University
      • Advanced Research Institute for Science and Engineering
      Edo, Tokyo, Japan
  • 1999-2007
    • University of Tsukuba
      • Applied Physics
      Tsukuba, Ibaraki, Japan
    • Shinshu University
      • Department of Medicine
      Shonai, Nagano, Japan
  • 1973-2007
    • Saga University
      • Department of Physics
      Сага Япония, Saga Prefecture, Japan
  • 2002-2003
    • Budker Institute of Nuclear Physics
      Novo-Nikolaevsk, Novosibirsk, Russia
  • 1995-2002
    • Chuo University
      Edo, Tōkyō, Japan
    • Osaka Prefecture University
      • College of Engineering
      Sakai, Ōsaka, Japan
  • 2001
    • Aomori University
      Aomori, Aomori Prefecture, Japan
  • 2000
    • Teikyo University Hospital
      Edo, Tōkyō, Japan
  • 1996
    • Nara Hospital
      Ikuma, Nara, Japan
    • Gadjah Mada University
      Jogjakarta, Daerah Istimewa Yogyakarta, Indonesia
  • 1991
    • Toshiba Corporation
      Edo, Tōkyō, Japan