[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
Progress of Theoretical and Experimental Physics 01/2014; 2014(2). DOI:10.1093/ptep/ptu014 · 2.49 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Construction of capture cryomodule for Quantum Beam Project  has started since September, and finished by the end of December in 2011 at KEK-STF. Two cavities (MHI#12, #13), which reached ILC specification (0.8x10 10 at 35MV/m) at the vertical test , were installed into a short cryomodule with improved input couplers. Slide-Jack tuner was attached at different position (center or end of helium jacket) for each cavity same as S1-Global . From February 2012, this cryomodule was cooled down to 2K, and the high power test started including check of the cavity/coupler/tuner performance, LFD measurement, LFD compensation by Piezo, dynamic loss measurement and so on. From April, the beam commissioning started with a small beam current and the maximum beam energy of 40MeV. During the beam commissioning, two cavities are being operated stably without any trouble at the accelerating gradient of 15-20MV/m. In this report, the test results of various performances at the Quantum Beam Project will be presented in detail.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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; · 5.99 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 4.38 Impact Factor