arXiv subjects

Y. Kato

Publications and source records attributed to Y. Kato.

At least 145 records · Page 8Linked to original sources

arXiv2009

Novel path towards compact laser ion accelerators for hadron therapy: Tenfold energy increase in laser-driven multi-MeV ion generation using a gas target mixed with submicron clusters

We demonstrate generation of 10-20 MeV/u ions with a compact 4 TW laser using a gas target mixed with submicron clusters, corresponding to tenfold increase in the ion energies compared to previous experiments with solid targets. It is inferred that the high energy ions are generated due to formation of a strong dipole vortex structure. The demonstrated method has a potential to construct compact and high repetition rate ion sources for hadron therapy and other applications.

physics.plasm-ph↗

arXiv2008

Evidence of the Θ^+ in the γd \to K^+K^-pn reaction

The γd \to K^+K^-pn reaction has been studied to search for the evidence of the Θ^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 σ, and the differential cross-section for the γn \to K^- Θ^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Θ^+ in the γn center-of-mass system. The obtained results support the existence of the Θ^+.

nucl-ex↗

arXiv2008

Bow wave from ultra-intense electromagnetic pulses in plasmas

We show a new effect of the bow wave excitation by an intense short laser pulse propagating in underdense plasma. Due to spreading of the laser pulse energy in transverse direction, the bow wave causes a large-scale transverse modulation of the electron density. This can significantly increase the electric potential of the wake wave since the wake wave is generated in the region much wider than the laser pulse waist.

physics.plasm-ph↗

arXiv2008

Photoproduction of Lambda(1405) and Sigma^{0}(1385) on the proton at E_γ= 1.5-2.4 GeV

Differential cross sections for $γp \to K^+Λ(1405)$ and $γp \to K^+Σ^0(1385)$ reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of $0.8<\cos(Θ)<1.0$ for the $K^+$ scattering angle in the center-of-mass system. This data is the first measurement of the $Λ(1405)$ photoproduction cross section. The lineshapes of \LamS measured in $Σ^+π^-$ and $Σ^-π^+$ decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the $Σπ$ scattering amplitudes. The ratios of \LamS production to \SigS production were measured in two photon energy ranges: near the production threshold ($1.5<E_γ<2.0$ GeV) and far from it ($2.0 <E_γ<2.4$ GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.

hep-ex↗

arXiv2008

Forward coherent $ϕ$-meson photoproduction from deuterons near threshold

Differential cross sections and decay asymmetries for coherent $ϕ$-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at $E_γ$= 1.5-2.4 GeV. This reaction offers a unique way to directly access natural-parity Pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity $π$-meson exchange is small. Nevertheless the deduced cross sections of $ϕ$-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional Pomeron model.

nucl-ex↗

arXiv2007

A GEM TPC End-Panel Pre-Prototype

A GEM TPC end panel pre-prototype was constructed for a large LC-TPC prototype to test its basic design philosophy and some of its engineering details. Its interim test results are presented.

physics.ins-det↗

arXiv2007

Relativistic Tennis with Photons: Demonstration of Frequency Upshifting by a Relativistic Flying Mirror through Two Colliding Laser Pulses

Since the advent of chirped pulse amplification1 the peak power of lasers has grown dramatically and opened the new branch of high field science, delivering the focused irradiance, electric fields of which drive electrons into the relativistic regime. In a plasma wake wave generated by such a laser, modulations of the electron density naturally and robustly take the shape of paraboloidal dense shells, separated by evacuated regions, moving almost at the speed of light. When we inject another counter-propagating laser pulse, it is partially reflected from the shells, acting as relativistic flying (semi-transparent) mirrors, producing an extremely time-compressed frequency-multiplied pulse which may be focused tightly to the diffraction limit. This is as if the counterstreaming laser pulse bounces off a relativistically swung tennis racket, turning the ball of the laser photons into another ball of coherent X-ray photons but with a form extremely relativistically compressed to attosecond and zeptosecond levels. Here we report the first demonstration of the frequency multiplication detected from the reflection of a weak laser pulse in the region of the wake wave generated by the driver pulse in helium plasma. This leads to the possibility of very strong pulse compression and extreme coherent light intensification. This Relativistic Tennis with photon beams is demonstrated leading to the possibility toward reaching enormous electromagnetic field intensification and finally approaching the Schwinger field, toward which the vacuum nonlinearly warps and eventually breaks, producing electron-positron pairs.

physics.plasm-ph↗

arXiv2005

Spatially Resolved NMR Relaxation Rate in a Noncentrosymmetric Superconductor

We numerically study the spatially-resolved NMR around a single vortex in a noncentrosymmetric superconductor such as CePt3Si. The nuclear spin-lattice relaxation rate 1/T1 under the influence of the vortex core states is calculated for an s+p-wave Cooper pairing state. The result is compared with that for an s-wave pairing state.

cond-mat.supr-con↗

arXiv2005

Basic Properties of a Vortex in a Noncentrosymmetric Superconductor

We numerically study the vortex core structure in a noncentrosymmetric superconductor such as CePt3Si without mirror symmetry about the xy plane. A single vortex along the z axis and a mixed singlet-triplet Cooper pairing model are considered. The spatial profiles of the pair potential, local density of states, supercurrent density, and radially-textured magnetic moment density around the vortex are obtained in the clean limit on the basis of the quasiclassical theory of superconductivity.

cond-mat.supr-con↗

arXiv2005

Spectral properties of Three-dimensional Magneto-hydrodynamical Accretion Flows

In spite of a large number of global three-dimensional (3D) magneto-hydrodynamical (MHD) simulations of accretion flows and jets being made recently, their astrophysical relevance for realistic situations is not well known. In order to examine to what extent the simulated MHD flows can account for the observed spectral energy distribution (SED) of Sagittarius A* (Sgr A*), for the first time we calculate the emergent spectra from 3D MHD flows in a wide range of wavelengths (from radio to X-ray) by solving the 3D radiative transfer equations. We use the simulation data by Kato, Mineshige, and Shibata (2004) and perform Monte Carlo radiative transfer simulations, in which synchrotron emission/absorption, free-free emission/absorption, and Compton/inverse Compton scattering are taken into account. We assume two temperature plasmas and calculate electron temperatures by solving the electron energy equation. Only thermal electrons are considered. It is found that the 3D MHD flow generally over-produces X-rays by means of bremsstrahlung radiation from the regions at large radii. A flatter density profile, r^{-a} with a<1, than that of the advection-dominated accretion flow (ADAF), r^{-3/2}, is the main reason for this. If we restrict the size of the emission region to be as small as 10r_s, where r_s is the Schwarzschild radius, the MHD model can reproduce the basic features of the observed SED of Sgr A* during its flaring state. Yet, the spectrum in the quiescent state remains to be understood. We also calculate the time-dependent spectral changes, finding that the fluxes fluctuate in a wide range of the frequency and the flux at each wavelength does not always vary coherently.

astro-ph↗

arXiv2004

Impurity Effect on Kramer-Pesch Core Shrinkage in s-Wave Vortex and Chiral p-Wave Vortex

The low-temperature shrinking of the vortex core (Kramer-Pesch effect) is studied for an isolated single vortex for chiral p-wave and s-wave superconducting phases. The effect of nonmagnetic impurities on the vortex core radius is numerically investigated in the Born limit by means of a quasiclassical approach. It is shown that in the chiral p-wave phase the Kramer-Pesch effect displays a certain robustness against impurities owing to a specific quantum effect, while the s-wave phase reacts more sensitively to impurity scattering. This suggests chiral p-wave superconductors as promising candidates for the experimental observation of the Kramer-Pesch effect.

cond-mat.supr-con↗

arXiv2004

Influence of electron capture and Coulomb explosion on electron screening in low energy nuclear reactions in laboratories

We discuss the effects of electron capture by the projectile and the Coulomb explosion of a molecular projectile on the electron screening in low energy nuclear reactions in laboratory. Using the idea of equilibrium charge, we show that the electron capture of projectile leads to a screening energy which significantly exceeds the adiabatic limit in the simple consideration for the D(d,p)T reaction and provides a possibility to explain the large screening energy claimed in the analysis of experimental data. We then show that the Coulomb explosion can result in a large apparent screening energy as large as that encountered in the analysis of ^3He(d,p)^4He reactions induced by the molecular D_2^+ and D_3^+ projectiles at very low energies.

nucl-th↗

arXiv2004

Current induced electron spin polarization in strained semiconductors

The polarization of conduction electron spins due to an electrical current is observed in strained nonmagnetic semiconductors using static and time-resolved Faraday rotation. The density, lifetime, and orientation rate of the electrically-polarized spins are characterized by a combination of optical and electrical methods. In addition, the dynamics of the current-induced spins are investigated by utilizing electrical pulses generated from a photoconductive switch. These results demonstrate the possibility of a spin source for semiconductor spintronic devices without the use of magnetic materials.

cond-mat.mes-hall↗

arXiv2003

Local Manipulation of Nuclear Spin in a Semiconductor Quantum Well

The shaping of nuclear spin polarization profiles and the induction of nuclear resonances are demonstrated within a parabolic quantum well using an externally applied gate voltage. Voltage control of the electron and hole wave functions results in nanometer-scale sheets of polarized nuclei positioned along the growth direction of the well. RF voltages across the gates induce resonant spin transitions of selected isotopes. This depolarizing effect depends strongly on the separation of electrons and holes, suggesting that a highly localized mechanism accounts for the observed behavior.

cond-mat.mes-hall↗

arXiv2002

Generation of High Quality Laser Accelerated Ion Beams

In order to achieve a high quality, i. e. monoergetic, intense ion beam, we propose the use of a double layer target. The first layer, at the target front, consists of high-Z atoms, while the second (rear) layer is a thin coating of low-Z atoms. The high quality proton beams from the double layer target, irradiated by an ultra-intense laser pulse, are demonstrated with three dimensional Particle-in-Cell simulations.

physics.plasm-ph↗

arXiv2002

Effects of superconducting gap anisotropy on the flux flow resistivity in Y(Ni_{1-x}Pt_x)_2B_2C

The microwave complex surface impedance Z_s of Y(Ni_{1-x}Pt_x)_2B_2C was measured at 0.5 K under magnetic fields H up to 7T. In nominally pure YNi_2B_2C, which is a strongly anisotropic s-wave superconductor, the flux flow resistivity ρ_f calculated from Z_s was twice as large as that expected from the conventional normal-state vortex core model. In Pt-doped samples where the gap anisotropy is smeared out, the enhancement of ρ_f is reduced and ρ_f approaches to the conventional behavior. These results indicate that energy dissipation in the vortex core is strongly affected by the anisotropy of the superconducting gap.

cond-mat.supr-con↗

arXiv2002

Free flux flow resistivity in strongly overdoped high-T_c cuprate; purely viscous motion of the vortices in semiclassical d-wave superconductor

We report the free flux flow (FFF) resistivity associated with a purely viscous motion of the vortices in moderately clean d-wave superconductor Bi:2201 in the strongly overdoped regime (T_c=16K) for a wide range of the magnetic field in the vortex state. The FFF resistivity is obtained by measuring the microwave surface impedance at different microwave frequencies. It is found that the FFF resistivity is remarkably different from that of conventional s-wave superconductors. At low fields (H<0.2H_c2) the FFF resistivity increases linearly with H with a coefficient which is far larger than that found in conventional s-wave superconductors. At higher fields, the FFF resistivity increases in proportion to \sqrt H up to H_c2. Based on these results, the energy dissipation mechanism associated with the viscous vortex motion in "semiclassical" d-wave superconductors with gap nodes is discussed. Two possible scenarios are put forth for these field dependence; the enhancement of the quasiparticle relaxation rate and the reduction of the number of the quasiparticles participating the energy dissipation in d-wave vortex state.

cond-mat.supr-con↗

arXiv2001

Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud

One hundred and forty six long-period red variable stars in the Large Magellanic Cloud (LMC) from the three year MOA project database were analysed. A careful periodic analysis was performed on these stars and a catalogue of their magnitudes, colours, periods and amplitudes is presented. We convert our blue and red magnitudes to $K$ band values using 19 oxygen-rich stars. A group of red short-period stars separated from the Mira sequence has been found on a (log P, K) diagram. They are located at the short period side of the Mira sequence consistent with the work of Wood and Sebo (1996). There are two interpretations for such stars; a difference in pulsation mode or a difference in chemical composition. We investigated the properties of these stars together with their colour, amplitude and periodicity. We conclude that they have small amplitudes and less regular variability. They are likely to be higher mode pulsators. A large scatter has been also found on the long period side of the (log P, K) diagram. This is possibly a systematic spread given that the blue band of our photometric system covers both standard B and V bands and affects carbon-rich stars.

astro-ph↗