Y. Kojima

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (126)203.19 Total impact

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    Dataset: mopro110(1)
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    ABSTRACT: Circular motion of particles, dust grains and fluids in the vicinity of compact objects has been investigated as a model for accretion of gaseous and dusty environment. Here we further discuss, within the framework of general relativity, figures of equilibrium of matter under the influence of combined gravitational and large-scale magnetic fields, assuming that the accreted material acquires a small electric charge due to interplay of plasma processes and photoionization. In particular, we employ an exact solution describing the massive magnetic dipole and we identify the regions of stable motion. We also investigate situations when the particle dynamics exhibits the onset of chaos. In order to characterize the measure of chaoticness we employ techniques of Poincar\'e surfaces of section and of recurrence plots.
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    ABSTRACT: The recirculation loop of the Compact ERL (cERL) was constructed from July to November in 2013 after commissioning of the injector. Then commissioning of the entire cERL was started in December 2013 and the beam could be accelerated up to 20 MeV in a short time. The beam recirculation and energy recovery were also achieved without significant beam loss in February 2014. Generation of laser-Compton scattering X-rays is scheduled for the end of FY2014 and generation of THz coherent radiation is planned for FY2015.
    IPAC2014; 06/2014
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    Yasufumi Kojima · Yugo E. Kato
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    ABSTRACT: We present a model of the magnetosphere around an oscillating neutron star. The electromagnetic fields are numerically solved by modeling electric charge and current induced by the stellar torsional mode, with particular emphasis on outgoing radiation passing through the magnetosphere. The current is modeled using Ohm's law, whereby an increase in conductivity results in an increase in the induced current. As a result, the fields are drastically modified, and energy flux is thereby enhanced. This behavior is however localized in the vicinity of the surface since the induced current disappears outwardly in our model, in which the exterior is assumed to gradually approach a vacuum.
    01/2014; 2014(2). DOI:10.1093/ptep/ptu014
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    27th Linear Accelerator Conference; 01/2014
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    ABSTRACT: A capture cavity cryomodule was fabricated and used in a beam line for quantum beam experiments at the Superconducting RF Test Facility (STF) of the High Energy Accelerator Research Organization in Japan. The cryomodule is about 4 m long and contains two nine-cell cavities. The cross section is almost the same as that of the STF cryomodules that were fabricated to develop superconducting RF cavities for the International Linear Collider. An attempt was made to reduce the large deflection of the helium gas return pipe (GRP) that was observed in the STF cryomodules during cool-down and warm-up. This paper briefly describes the structure and cryogenic performance of the captures cavity cryomodule, and also reports the measured displacement of the GRP and the cavity-containing helium vessels during regular operation.
    12/2013; 1573(1). DOI:10.1063/1.4860786
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    ABSTRACT: Recent accelerator projects at KEK, such as the Superconducting RF Test Facility (STF) for R&D; of the International Linear Collider (ILC) project and the compact Energy Recovery Linac (cERL), employ superconducting RF cavities made of pure niobium, which can generate high gradient acceleration field. Since the operation temperature of these cavities is selected to be 2 K, we have developed two 2 K superfluid helium cryogenic systems for stable operation of superconducting RF cavities for each of STF and cERL. These two 2 K superfluid helium cryogenic systems are identical in principle. Since the operation mode of the cavities is different for STF and cERL, i.e. the pulse mode for STF and the continuous wave mode for cERL, the heat loads from the cavities are quite different. The 2 K superfluid helium cryogenic systems mainly consists of ordinary helium liquefiers/refrigerators, 2 K refrigerator cold boxes, helium gas pumping systems and high-performance transfer lines. The 2 K refrigerators and the high-performance transfer lines are designed by KEK. Some superconducting RF cavity cryomodules have been already connected to the 2 K superfluid helium cryogenic systems for STF and cERL respectively, and cooled down to 2 K successfully.
    12/2013; 1573(1). DOI:10.1063/1.4860863
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    ABSTRACT: The 35-MeV Compact Energy Recovery Linac (cERL) is a superconducting test accelerator for the future 3-GeV ERL project (PEARL) at KEK. During the past year, we have finished key devices such as a 500-kV DC photocathode electron gun and 1.3-GHz superconducting-cavity (SCC) cryomodules for both an injector and a main linac. We installed these devices into a shielding room of the cERL, and carried out high-voltage or high-power tests successfully. A 5-MeV injector of the cERL has been completed and commissioned.
    IPAC2013; 05/2013
  • Yasufumi Kojima
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    ABSTRACT: Dynamics of magnetic field decay is numerically studied. For neutron stars with strong magnetic fields, the Hall drift timescale in their crust is very short, and therefore the evolution is significantly affected. The nonlinear coupling between poloidal and toroidal components of the magnetic field is studied. It is also found that the polar field at the surface is highly distorted during the Hall drift timescale. For example, polar dipole field-strength temporarily decreases not by dissipation but by advection. This fact suggests that the dipole field-strength is not sufficient to determine the border between pulsars and magnetars.
    Proceedings of the International Astronomical Union 03/2013; 8(S291):425-427. DOI:10.1017/S1743921312024374
  • Yasufumi Kojima
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    ABSTRACT: Recoil velocity is examined as a back reaction to the magnetic dipole and quadrupole radiations from a pulsar/magnetar born with rapid rotation. The model is extended from notable Harrison-Tademaru one by including arbitrary field-strength of the magnetic quadrupole moment. The process is slow one operating on a spindown timescale. Resultant velocity depends on not the magnitude, but rather the ratio of the two moments and their geometrical configuration. The model does not necessarily lead to high spatial velocity for a magnetar with a strong magnetic field. This fact is consistent with the recent observational upper bound. The maximum velocity predicted with this model is slightly smaller than that of observed fast-moving pulsars.
    Proceedings of the International Astronomical Union 03/2013; 8(S291):428-430. DOI:10.1017/S1743921312024386
  • Yasufumi Kojima
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    ABSTRACT: The evolution of magnetic field is numerically studied for an isolated magnetar, assuming vacuum exterior. Nonlinear coupling between poloidal and toroidal components of the magnetic field can be seen in the initial Hall-drift timescale. Consequently, the polar field at the surface is highly distorted during the phase. This result is suggestive. Fixed dipole magnetic field has been used so far in the theoretical study of the interaction between magnetosphere and accreting matter. In the accretion to magnetar, time-dependent polar magnetic field should be taken into account.
    Proceedings of the International Astronomical Union 02/2013; 8(S290):235-236. DOI:10.1017/S1743921312019783
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    ABSTRACT: Within the framework of Bonnor's exact solution describing a massive magnetic dipole, we study the motion of neutral and electrically charged test particles. In dependence on the Bonnor spacetime parameters, we determine regions enabling the existence of stable circular orbits confined to the equatorial plane and of those levitating above the equatorial plane. Constructing Poincar\'e surfaces of section and recurrence plots, we also investigate the dynamics of particles moving along general off-equatorial trajectories bound in effective potential wells forming around the stable circular orbits. We demonstrate that the motion in the Bonnor spacetime is not integrable. This extends previous investigations of generalized St\"ormer's problem into the realm of exact solutions of Einstein-Maxwell equations, where the gravitational and electromagnetic effects play a comparable role on the particle motion.
    Classical and Quantum Gravity 01/2013; 30(2). DOI:10.1088/0264-9381/30/2/025010 · 3.10 Impact Factor
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    ABSTRACT: Near a rotating black hole, circular motion of particles, dust grains and complex fluids have been investigated as a model for accretion of gaseous and dusty environment in the toroidal geometry. Here we further discuss, within the framework of general relativity, figures of equilibrium of matter under the influence of combined gravitational and large-scale magnetic fields, assuming that the accreted material acquires a small (but non-vanishing) electric charge due to the interplay of plasma processes and photoionization. We employ different solutions for the central body (magnetized Kerr metric, or a massive magnetic dipole) and we identify the corresponding regions of stability. The action of gravitational and electromagnetic forces jointly determine the regions of stable motion, in particular, whether the halo lobes develop where particles can be captured in permanent circulation around the central body. Therefore, our set-up is relevant in the context of accreting compact objects where the halo motion can describe the overall global motion through corona of an accretion disc or a geometrically thick torus. We also investigate situations when the motion exhibits the onset of chaos. In order to characterize the measure of chaoticness we employ techniques of Poincare surfaces of section and Recurrence plots. Acknowledgments: Czech-US collaboration project (ref. ME09036) and the Czech Science Foundation program (ref. P209/10/P190) are gratefully acknowledged for their continued support.
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    Yasufumi Kojima · Shota Kisaka
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    ABSTRACT: The dynamics of magnetic field decay with Hall drift is investigated. Assuming that axisymmetric magnetic fields are located in a spherical crust with uniform conductivity and electron number density, long-term evolution is calculated up to Ohmic dissipation. The nonlinear coupling between poloidal and toroidal components is explored in terms of their energies and helicity. Nonlinear oscillation by the drift in strongly magnetized regimes is clear only around the equipartition between two components. Significant energy is transferred to the poloidal component when the toroidal component initially dominates. However, the reverse is not true. Once the toroidal field is less dominant, it quickly decouples due to a larger damping rate. The polar field at the surface is highly distorted from the initial dipole during the Hall drift timescale, but returns to the initial dipole in a longer dissipation timescale, since it is the least damped one.
    Monthly Notices of the Royal Astronomical Society 01/2012; 421(3). DOI:10.1111/j.1365-2966.2012.20509.x · 5.23 Impact Factor
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    Shota Kisaka · Yasufumi Kojima
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    ABSTRACT: Recent obserbations by Fermi Gamma-Ray Space Telescope of gamma-ray pulsars have revealed further details of the structure of the emission region. We investigate the emission region for the multi-wavelength light curve using outer gap model. We assume that gamma-ray and non-thermal X-ray photons are emitted from a particle acceleration region in the outer magnetosphere, and UV/optical photons originate above that region. We also assume that gamma-rays are radiated only by outwardly moving particles, whereas the other photons are produced by particles moving inward and outward. We parametrize the altitude of the emission region. We find that the outer gap model can explain the multi-wavelength pulse behavior. From observational fitting, we also find a general tendency for the altitude of the gamma-ray emission region to depend on the inclination angle. In particular, the emission region for low inclination angle is required to be located in very low altitude, which corresponds to the inner region within the last-open field line of rotating dipole in vacuum. This model suggests a modification of statistics about observed gamma-ray pulsars.
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    Shota Kisaka · Yasufumi Kojima
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    ABSTRACT: Using the outer gap model, we investigate the emission region for the multi-wavelength light curve from energetic pulsars. We assume that γ-ray and non-thermal X-ray photons are emitted from a particle acceleration region in the outer magneto-sphere, and UV/optical photons originate above that region. We assume that γ-rays are radiated only by outwardly moving particles, whereas the other photons are produced by particles moving inward and outward. We parameterize the altitude of the emission region as the deviation from the rotating dipole in vacuum and determine it from the observed multi-wavelength pulse profile using the observationally constrained magnetic dipole inclination angle and viewing angle of the pulsars. We find that the outer gap model can explain the multi-wavelength pulse behavior by a simple distribution of emis-sivity, and discuss the possibility of further improvement. From observational fitting, we also find a general tendency for the altitude of the γ-ray emission region to depend on the inclination angle. In particular, the emission region for low inclination angle is required to be located in very low altitude, which corresponds to the inner region within the last-open field line of rotating dipole in vacuum. This model suggests a modification of statistics about observed γ-ray pulsars. Number of the sources with low inclination and viewing angles increases compared with previous estimate.
    The Astrophysical Journal 07/2011; 739(1). DOI:10.1088/0004-637X/739/1/14; · 6.28 Impact Factor
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    ABSTRACT: DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the planned Japanese space gravitational wave antenna, aiming to detect gravitational waves from astrophysically and cosmologically significant sources mainly between 0.1 Hz and 10 Hz and thus to open a new window for gravitational wave astronomy and for the universe. DECIGO will consist of three drag-free spacecraft, 1000 km apart from each other, whose relative displacements are measured by a differential Fabry-Perot interferometer. We plan to launch DECIGO in middle of 2020s, after sequence of two precursor satellite missions, DECIGO pathfinder and Pre-DECIGO, for technology demonstration required to realize DECIGO and hopefully for detection of gravitational waves from our galaxy or nearby galaxies.
    Classical and Quantum Gravity 05/2011; 154(9). DOI:10.1088/0264-9381/28/9/094011 · 3.10 Impact Factor
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    Yasufumi Kojima · Junpei Oogi · Yugo E. Kato
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    ABSTRACT: Relativistic effects in the oscillatory damping of magnetic disturbances near two-dimensional X-points are investigated. By taking into account displacement current, we study new features of extremely magnetized systems, in which the Alfv\'en velocity is almost the speed of light. The frequencies of the least-damped mode are calculated using linearized relativistic MHD equations for wide ranges of the Lundquist number S and the magnetization parameter $\sigma$. These timescales approach constant values in the large resistive limit: the oscillation time becomes a few times the light crossing time, irrespective of $\sigma$, and the decay time is proportional to $\sigma$ and therefore is longer for a highly magnetized system.
    Astronomy and Astrophysics 04/2011; 531. DOI:10.1051/0004-6361/201116562 · 4.48 Impact Factor
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    Yasufumi Kojima · Yugo E. Kato
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    ABSTRACT: We examine the recoil velocity induced by the superposition of the magnetic dipole and quadrupole radiation from a pulsar/magnetar born with rapid rotation. The resultant velocity depends on not the magnitude, but rather the ratio of the two moments and their geometrical configuration. The model does not necessarily lead to high spatial velocity for a magnetar with a strong magnetic field, which is consistent with the recent observational upper bound. The maximum velocity predicted with this model is slightly smaller than that of observed fast-moving pulsars.
    The Astrophysical Journal 12/2010; 728(2). DOI:10.1088/0004-637X/728/2/75 · 6.28 Impact Factor
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    ABSTRACT: A space gravitational-wave antenna, DECIGO (DECI-hertz interferometer Gravitational wave Observatory), will provide fruitful insights into the universe, particularly on the formation mechanism of supermassive black holes, dark energy and the inflation of the universe. In the current pre-conceptual design, DECIGO will be comprising four interferometer units; each interferometer unit will be formed by three drag-free spacecraft with 1000 km separation. Since DECIGO will be an extremely challenging mission with high-precision formation flight with long baseline, it is important to increase the technical feasibility before its planned launch in 2027. Thus, we are planning to launch two milestone missions. DECIGO pathfinder (DPF) is the first milestone mission, and key components for DPF are being tested on ground and in orbit. In this paper, we review the conceptual design and current status of DECIGO and DPF.
    Classical and Quantum Gravity 04/2010; 27(8):084010. DOI:10.1088/0264-9381/27/8/084010 · 3.10 Impact Factor