arXiv subjects

K. Sato

Publications and source records attributed to K. Sato.

At least 145 records · Page 8Linked to original sources

arXiv2004

Ion acceleration during internal magnetic reconnection events in TST-2

Characteristics of ion acceleration in the internal magnetic reconnection events (IRE) have been studied by means of a neutral particle energy analyzer (NPA) in Tokyo Spherical Tokamak (TST-2). The major and minor radii are 0.38 m and 0.25m, respectively. The magnetic field strength is 0.3T and the maximum plasma current is up to 140 kA. The electron and ion temperatures are 0.4-0.5 keV and 0.1 keV, respectively and the electron density is ~1x1019 m-3. The NPA can be scanned toroidally from q = 74° (cw) to q = 114° (ccw), where q = 90° corresponds to the perpendicular sightline. The direction of the plasma current is cw. The NPA signals are digitized at every 50 ms. The NPA is calibrated in the energy range of 0.1 keV < E < 8.4 keV. When the IRE occurs, it is observed that the plasma current increases by ~ 20% and the loop voltage drops from 0.6 V to-5 V for ~ 0.1 ms. The enhanced charge exchange flux is observed by more than one order of magnitude at ~ 1 keV for this reconnection phase. The ion temperature increases by 80 eV at IREs. The angle q dependence of increment of Ti shows that DTi (q = 74°) is higher than that for q = 114°. This observation suggests that an ion is accelerated initially in the direction of magnetic field lines. The time evolution of the ion distribution function is simulated with a Fokker-Planck code taking into account the electric field effects.

physics.plasm-ph↗

arXiv2004

Estimation of Power Balance in Steady State LHCD Discharges on TRIAM-1M

On TRIAM-1M, a long duration discharge for more than 5 h was achieved successfully by fully non-inductive lower hybrid current drive. Heat load distribution to the plasma facing components (PFCs) in the 5 h discharge was investigated by using calorimetric measurements. The injected RF power was coincident with the total amount of heat load to PFCs estimated by calorimetric measurement. The power balance including the portion of direct loss power of fast electron, heat flux due to the charge exchange process was able to be estimated in this long duration discharge.

physics.plasm-ph↗

arXiv2004

Low temperature ferromagnetism in (Ga, Mn)N

The magnetic properties of dilute magnetic semiconductors (DMS) are calculated from first-principles by mapping the ab initio results on a classical Heisenberg model. It is found that the range of the exchange interaction in (Ga, Mn)N is very short ranged due to the exponential decay of the impurity wave function in the gap. Curie temperatures (Tc) of DMS are calculated by using the Monte Carlo method. It is found that the Tc values of (Ga, Mn)N are very low since, due to the short ranged interaction, percolation of the ferromagnetic coupling is difficult to achieve for small concentrations.

cond-mat.mtrl-sci↗

arXiv2004

VERA Observation of the W49N H2O Maser Outburst in 2003 October

We report on a strong outburst of the W49N H2O maser observed with VERA. Single-dish monitoring with VERA 20 m telescopes detected a strong outburst of the maser feature at V_LSR = -30.7 km/s in 2003 October. The outburst had a duration of ~100 days and a peak intensity of 7.9 x 10^4 Jy, being one of the strongest outbursts in W49N observed so far. VLBI observations with the VERA array were also carried out near to the maximum phase of the outburst, and the outburst spot was identified in the VLBI map. While the map was in good agreement with previous studies, showing three major concentrations of maser spots, we found a newly formed arc-like structure in the central maser concentration, which may be a shock front powered by a forming star or a star cluster. The outburst spot was found to be located on the arc-like structure, indicating a possible connection of the present outburst to a shock phenomenon.

astro-ph↗

arXiv2003

T_c-Enhanced Codoping Method for GaAs-based Dilute Magnetic Semiconductors

Based on ab initio calculations of Ga_{1-x}Mn_xN_yAs_{1-y} and Ga_{1-x}Mn_xC_yAs_{1-y}, we propose a new codoping method to enhance the Curie temperature T_c of diluted magnetic semiconductors. The solubility of Mn can be increased up to high concentration by the codoping of N or C to reduce the lattice and volume expansion caused by Mn doping. It is found that the impurity band of the majority spin is strongly broadened and pushed up into the higher energy region due to the strong p-d hybridization caused by the codoping, and the T_c becomes higher than the room temperature at x>0.06.

cond-mat.mtrl-sci↗

arXiv2003

The effect of the spin-orbit interaction on the band gap of half-metals

The spin-orbit interaction can cause a nonvanishing density of states (DOS) within the minority-spin band gap of half-metals around the Fermi level. We examine the magnitude of the effect in Heusler alloys, zinc-blende half metals and diluted magnetic semiconductors, using first-principles calculations. We find that the ratio of spin-down to spin-up DOS at the Fermi level can range from below 1% (e.g. 0.5% for NiMnSb) over several percents (4.2% for (Ga,Mn)As) to 13% for MnBi.

cond-mat.mtrl-sci↗

arXiv2003

Small Scale Clustering in the Isotropic Arrival Distribution of Ultra-High Energy Cosmic Rays and Implications for Their Source Candidates

We present numerical simulations on the propagation of UHE protons with energies of $(10^{19.5}-10^{22})$ eV in extragalactic magnetic fields over 1 Gpc. We use the ORS galaxy sample, which allow us to accurately quantify the contribution of nearby sources to the energy spectrum and the arrival distribution, as a source model. We calculate three observable quantities, cosmic ray spectrum, harmonic amplitude, and two point correlation function from our data of numerical simulations. With these quantities, we compare the results of our numerical calculations with the observation. We show that the three observable quantities including the GZK cutoff of the energy spectrum can be reproduced in the case that the number fraction $\sim 10^{-1.7}$ of the ORS galaxies more luminous than -20.5 mag is selected as UHECR sources. In terms of the source number density, this constraint corresponds to $10^{-6}$ Mpc$^{-3}$. However, since mean number of sources within the GZK sphere is only $\sim 0.5$ in this case, the AGASA 8 events above $10^{20.0}$ eV, which do not constitute the clustered events with each other, can not be reproduced. On the other hand, if the cosmic ray flux measured by the HiRes, which is consistent with the GZK cutoff, is correct and observational features about the arrival distribution of UHECRs are same as the AGASA, our source model can explain both the arrival distribution and the flux at the same time. Thus, we conclude that large fraction of the AGASA 8 events above $10^{20}$ eV might originate in the topdown scenarios, or that the cosmic ray flux measured by the HiRes experiment might be better. We also discuss the origin of UHECRs below $10^{20.0}$ eV through comparisons between the number density of astrophysical source candidates and our result ($\sim 10^{-6}$ Mpc$^{-3}$).

astro-ph↗

arXiv2003

A New Method for Calculating Arrival Distribution of Ultra-High Energy Cosmic Rays above 10^19 eV with Modifications by the Galactic Magnetic Field

We present a new method for calculating arrival distribution of UHECRs including modifications by the galactic magnetic field. We perform numerical simulations of UHE anti-protons, which are injected isotropically at the earth, in the Galaxy and record the directions of velocities at the earth and outside the Galaxy for all of the trajectories. We then select some of them so that the resultant mapping of the velocity directions outside the Galaxy of the selected trajectories corresponds to a given source location scenario, applying Liouville's theorem. We also consider energy loss processes of UHE protons in the intergalactic space. Applying this method to our source location scenario which is adopted in our recent study and can explain the AGASA observation above 4 \times 10^{19} eV, we calculate the arrival distribution of UHECRs including lower energy (E>10^19 eV) ones. We find that our source model can reproduce the large-scale isotropy and the small-scale anisotropy on UHECR arrival distribution above 10^19 eV observed by the AGASA. We also demonstrate the UHECR arrival distribution above 10^19 eV with the event number expected by future experiments in the next few years. The interesting feature of the resultant arrival distribution is the arrangement of the clustered events in the order of their energies, reflecting the directions of the galactic magnetic field. This is also pointed out by Alvarez-Muniz et al.(2002). This feature will allow us to obtain some kind of information about the composition of UHECRs and the magnetic field with increasing amount of data.

astro-ph↗

arXiv2003

Explosive Nucleosynthesis Associated with Formation of Jet-induced GRBs in Massive Stars

We perform 2-dimensional relativistic hydrodynamical simulations in the context of collapsar model. Calculations of explosive nucleosynthesis are also accomplished. We investigate the influence of the structure of the progenitor and energy deposition rate on the resulting explosive nucleosynthesis, assuming that 56Ni is mainly synthesized in the jet launched by the neutrino heating. We show the amount of 56Ni is very sensitive to the energy deposition rate. Thus we conclude that it is quite natural not to detect an underlying supernova in some X-ray afterglows as in GRB 010921. We also point out the possibility that the relative abundance of the elements with intermediate mass number such as Si and S in the X-ray afterglow of GRB 011211 may be naturally explained if the energy deposition rate at the central engine is relatively long because little amount of 56Ni should be synthesized under such an environment. If this discussion is true, there should be correlation between the line features in the X-ray afterglow and duration of the GRB. It should be noted that the duration of GRB 011211 is 270 seconds, making it the longest burst ever observed by Beppo-SAX although it suffers from the effect of red-shift (z_host=2.14), which supports our conclusion. Our results also suggest that the type I collapsar model in which the energy deposition rate is relatively low (\dot{E} \sim 10^{51} erg/s) might have difficulty in reproducing the observed amount of 56Ni in a hypernova such as SN 1998bw. This means that the mechanism of the central engine of a hypernova accompanying GRB may be constrained by the discussion of explosive nucleosynthesis.

astro-ph↗

arXiv2003

First Fringe Detection with VERA's Dual-Beam System and its Phase Referencing Capability

We present the results of first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The first dual-beam observations of a pair of H2O maser sources W49N and OH43.8-0.1 have been carried out on 2002 May 29 and July 23, and fringes of H2O maser lines at 22 GHz have been successfully detected. While the residual fringe phases of both sources show rapid variations over 360 degree due to the atmospheric fluctuation, the differential phase between the two sources remains constant for 1 hour with r.m.s. of 8 degree, demonstrating that the atmospheric phase fluctuation is removed effectively by the dual-beam phase referencing. The analysis based on Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for arbitrarily long time. These results demonstrate VERA's high capability of phase referencing, indicating that VERA is a promising tool for phase referencing VLBI astrometry at 10 microarcsec level accuracy.

astro-ph↗

arXiv2003

Effects of neutrino oscillation on supernova neutrino: inverted mass hierarchy

We study the effects of neutrino oscillation on supernova neutrino in the case of the inverted mass hierarchy ($m_{3} \ll m_{1} < m_{2}$) as well as the normal mass hierarchy ($m_{1} < m_{2} \ll m_{3}$). Numerical analysis using a realistic supernova and presupernova model allows us to discuss quantitatively a possibility to probe neutrino oscillation parameters. We show that information about the mass hierarchy can be obtained if $θ_{13}$ is rather large ($\sin^{2}{2 θ_{13}} > 10^{-3}$) and that $θ_{13}$ can be well probed by SuperKamiokande if the neutrino mass hierarchy is the inverted case. Errors due to the uncertainty of the original neutrino spectra and the Earth effect are also discussed.

hep-ph↗

arXiv2003

The CDFII Time-Of-Flight Detector and Impact on Beauty Flavor Tagging

The new CDFII detector incorporates a Time-of-Flight detector (TOF), employing plastic scintillator bars and fine-mesh photomultipliers. Since August 2001 the TOF system has been fully instrumented and integrated into the CDFII data acquisition system. With a design goal of $100 ps$ resolution the TOF system will provide at least two standard deviations separation between $K^{\pm}$ and $π^{\pm}$ for momenta $p<1.6 GeV/c$, complementing low momentum particle identification by means of the specific ionization energy loss measured in the drift chamber. We describe the design of the TOF detector and discuss the current status of its calibration and initial performances. Finally we review the expected impact of the TOF detector in the flavor tagging of neutral $B_S$ meson.

hep-ex↗

arXiv2003

In situ photoemission study of the room-temperature ferromagnet ZnGeP_2:Mn

The chemical states of the ZnGeP$_{2}$:Mn interface, which shows ferromagnetism above room-temperature, has been studied by photoemission spectroscopy. Mn deposition on the ZnGeP$_2$ substrate heated to 400$^{\circ}$C induced Mn substitution for Zn and then the formation of metallic Mn-Ge-P compounds. Depth profile studies have shown that Mn 3$d$ electrons changed their character from itinerant to localized along the depth, and in the deep region, dilute divalent Mn species ($\textless$ 5 % Mn) was observed with a coexisting metallic Fermi edge of non-Mn 3$d$ character. The possibility of hole doping through Mn substitution for Ge and/or Zn vacancy is discussed.

cond-mat.str-el↗

arXiv2003

Shock propagation and neutrino oscillation in supernova

The effect of the shock propagation on neutrino oscillation in supernova is studied paying attention to evolution of average energy of $ν_{e}$ and $\barν_{e}$. We show that the effect appears as a decrease in average $ν_{e}$ (in case of inverted mass hierarchy, $\barν_{e}$) energy at stellar surface as the shock propagates. It is found that the effect is significant 2 seconds after bounce if $3 \times 10^{-5} < \sin^{2}{θ_{13}} < 10^{-2}$.

astro-ph↗

arXiv2002

Revision of the Selection Function of the Optical Redshift Survey using the Sloan Digital Sky Survey Early Data Release: Toward an Accurate Estimate of Source Number Density of Ultra-High Energy Cosmic Rays

If Ultra-High Energy Cosmic Rays (UHECRs) are originated from nearby galaxies, modeling of the distribution of nearby galaxies is important to an accurate estimate the source number density of UHECRs. We investigate uncertainty of the selection function of the Optical Redshift Survey (ORS), which we used to construct a source model of UHECRs. The investigation is based on a comparison of numbe counts of ORS galaxies with those of the spectroscopic sample of the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR). We carefully count galaxies in the same absolute magnitude bin from the two samples. We find a slight systematic overestimate of the ORS counts outside 5000 km s$^{-1}$ by about a factor of 2. We revise the selection function of the ORS assuming that the SDSS counts are correct. Our revision is based on the absorption given in the ORS catalog as well as that computed from Schlegel et al. (1998), which is systematically larger than the former by $A_B \sim 0.1$ mag in the region of low absorption. It is found that introduction of Schlegel et al.'s absorption changes one of the parameters of the ORS selection function by more than 10%. The revision should be taken into account in the future analysis of the source number density of UHECRs based on the ORS. Using the revised selection function, we determine the global structure of the Local Supercluster (LSC) with a source model of UHECRs, that is, a number-density model consisting of a uniform spherical halo and an exponential disk. We find that the revision is insignificant in terms of the structure of the LSC. However, the revised selection function will be useful to other studies such as peculiar velocity and correlation function.

astro-ph↗

arXiv2002

Irreversibility resulting from contact with a heat bath caused by the finiteness of the system

When a small dynamical system that is initially in contact with a heat bath is detached from this heat bath and then caused to undergo a quasi-static adiabatic processes, the statistical distribution of the system's energy differs from that of an equilibrium ensemble. Subsequent contact of the system with another heat bath is inevitably irreversible, hence the entire process cannot be reversed without a net energy transfer to the heat baths.

cond-mat.stat-mech↗

arXiv2002

GR@PPA_4b: A Four Bottom Quark Production Event Generator for PP/PPbar Collisions

We have developed an event generator, named GR@PPA_4b, for the four bottom quark (b bbar b bbar) production processes at PP and PPbar collisions. The program implements all of the possible processes at the tree level within the framework of the Standard Model. Users can generate events from the Higgs boson and photon/Z mediated processes, as well as those from pure QCD interactions. The integration and the event generation are performed within the newly developed GR@PPA framework, an extension of the GRACE automatic event-generator generation system to hadron collisions. This program is so designed that it can be embedded in a general-purpose event generator PYTHIA version 6.1. PYTHIA adds the initial- and final-state parton showers and simulates the hadronization and decays to make generated events realistic. It should be emphasized that a huge number of diagrams and complicated four-body kinematics are dealt with strictly in GR@PPA_4b. This program will be useful for studies of Higgs boson productions, especially those in extended models where the Yukawa coupling to b quarks is greatly enhanced. The source code is located in http://atlas.kek.jp/physics/nlo-wg/index.html .

hep-ph↗

arXiv2002

Detectability of the Supernova Relic Neutrinos and Neutrino Oscillation

We investigate the flux and the event rate of the supernova relic neutrino background (SRN) at the SuperKamiokande detector for various neutrino oscillation models with parameters inferred from recent experimental results. A realistic model of neutrino emission from supernova explosions and several models of the cosmic star formation history are adopted in the calculation. The number flux over entire energy range is found to be $11-15 \mathrm{cm^{-2}s^{-1}}$. We discuss the detection possibility of SRN at SuperKamiokande, comparing this SRN flux with other background neutrinos in more detail than previous studies. Even though there is no energy window in which SRN is dominant, we might detect it as the distortion of the other background event. We found in the energy range $17-25 \mathrm{MeV}$ the expected event rate at SuperKamiokande $0.4-0.8 ~\mathrm{yr^{-1}}$. In this range, ten-year observation might enable us to detect SRN signal (at one sigma level) in the case of LMA solar neutrino solution. We also investigate event rate at SNO and KamLAND. Although we can find energy window, the expected event rate is rather small (0.03 yr$^{-1}$ for SNO, 0.1 yr$^{-1}$ for KamLAND).

astro-ph↗