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Biomedical subjects

Z Yoshida

Publications and source records attributed to Z Yoshida.

At least 19 recordsLinked to original sources
PubMed2004

Statistical characterization of the interchange-instability spectrum of a separable ideal-magnetohydrodynamic model system.

A Suydam-unstable circular cylinder of plasma with periodic boundary conditions in the axial direction is studied within the approximation of linearized ideal magnetohydrodynamics (MHD). The normal mode equations are completely separable, so both the toroidal Fourier harmonic index n and the poloidal index m are good quantum numbers. The full spectrum of eigenvalues in the range 1< or = m < or = m(max) is analyzed quantitatively, using asymptotics for large m, numerics for all m, and graphics for qualitative understanding. The density of eigenvalues scales like m(2)(max) as m(max) -->infinity . Because finite-m corrections scale as 1/ m(2)(max) , their inclusion is essential in order to obtain the correct statistics for the distribution of eigenvalues. Near the largest growth rate, only a single radial eigenmode contributes to the spectrum, so the eigenvalues there depend only on m and n as in a two-dimensional system. However, unlike the generic separable two-dimensional system, the statistics of the ideal-MHD spectrum departs somewhat from the Poisson distribution, even for arbitrarily large m(max) . This departure from Poissonian statistics may be understood qualitatively from the nature of the distribution of rational numbers in the rotational transform profile.

Journal Article↗
PubMed2002

Empirical method for prediction of the coordination environment of Eu(III) by time-resolved laser-induced fluorescence spectroscopy.

The number of water molecules in the inner-sphere (N(H2O)) was determined for Eu(III) and the strength of ligand field (R(E/M)) was evaluated for a variety of coordination environments from the luminescence lifetime and the relative intensity at 615 nm and at 592 nm, by time-resolved laser-induced fluorescence spectroscopy. When R(E/M) and deltaN(H2O) for Eu(III) with a known coordination environment were plotted clear regularity was apparent between the location of the R(E/M)-deltaN(H2O) plot and the coordination environment of Eu(III). Here, deltaN(H2O) was calculated by use of the equation, deltaN(H2O)=9-N(H2O). Unknown coordination environments of Eu(III) can, in turn, be characterized, including both the inner- and the outer-sphere, simply by plotting R(E/M) and deltaN(H2O) for Eu(III) on the diagram. This empirical method is effective for prediction of the coordination environment of hydrated and complexed Eu(III) in solutions and that of the adsorbed Eu(III) on ion-exchange resins and by microorganisms.

Journal Article↗
PubMed2002

Dynamics of self-trapped singular beams in an underdense plasma.

Dynamics of an intense short laser pulse with a phase singularity, propagating in an underdense cold plasma, is investigated. Such a pulse can propagate as a vortex soliton in a self-created channel. It is shown that vortices with the topological charge m=1,2 (and a corresponding angular momentum) are unstable against symmetry-breaking perturbations; the breakup of the original vortex leads to the formation of stable spatial solitons that steadily fly away tangentially from the initial ring of vortex distribution.

Journal Article↗
PubMed2002

Self-trapping of strong electromagnetic beams in relativistic plasmas.

Interaction of an intense electromagnetic (em) beam with a relativistic electron-positron (e-p) plasma is investigated. It is shown that the thermal pressure brings about a fundamental change in the dynamics-localized, high amplitude, em field structures, not accessible to a cold (but relativistic) plasma, can now be formed under well-defined conditions. The possibilities of trapping em beams in self-guiding regimes to form stable two-dimensional solitonic structures in a pure e-p plasma are worked out.

Journal Article↗
PubMed2002

Injection of electron beam into a toroidal trap using chaotic orbits near magnetic null.

Injection of charged particle beam into a toroidal magnetic trap enables a variety of interesting experiments on non-neutral plasmas. Stationary radial electric field has been produced in a toroidal geometry by injecting electrons continuously. When an electron gun is placed near an X point of magnetic separatrix, the electron beam spreads efficiently through chaotic orbits, and electrons distribute densely in the torus. The current returning back to the gun can be minimized less than 1% of the total emission.

Journal Article↗
PubMed2002

Variational principles and self-organization in two-fluid plasmas.

Self-organization of an ordered structure occurs in a plasma under rather restrictive conditions. A new framework for a variational principle invokes a coercive form that results in a criterion for self-organizing relaxation of a two-fluid plasma. The constraints (constants of motion of the ideal model) are adjusted, through a weakly dissipative process, so that the relaxed state, under well-defined conditions, is a stable equilibrium independent of the direct effects of dissipation.

Journal Article↗
PubMed2001

Asymptotic analysis and renormalized perturbation theory of the non-Hermitian dynamics of an inviscid vortex.

An analysis of the non-Hermitian fluid systems described by the Rayleigh equation in an unbounded domain has been carried out in the regime of large wave numbers. The evolution of a special class of localized vorticities is also discussed. Asymptotic and perturbative approaches lead to the same final result. In the limit considered, the system is stable. The perturbation analysis reveals interesting pathologies of the non-Hermitian systems. Under specific conditions, the expansion is found to show secular growth. A discussion about the mechanism of insurgence of such singular behavior is presented. It is also shown that the divergent expansion is renormalizable by means of the renormalization group method-the renormalized results are in complete conformity with the asymptotic solutions.

Journal Article↗
PubMed2001

Resonant production of secondary electrons generating a discrete energy structure in a magnetized electron beam system.

In a system of magnetized electron beams, multiplication of electrons can occur if the incident and return beams (the latter one is produced by secondary electrons emitted at the end of the incident beam) satisfy a resonance condition on the gyration phase. The beams propagate between two boundaries; one is an electron gun and the other is a Faraday cup detector. The resonance condition demands discrete energies (eigenvalues) of the Hamiltonian that generates a propagator of the beam. This model has been compared with Varma's formulation of discrete energies [R. K. Varma and A. M. Punithavelu, Mod. Phys. Lett. A 8, 167 (1993)].

Journal Article↗
PubMed2000

Nonlinear density wave theory for the spiral structure of galaxies.

The theory of nonlinear waves for plasmas has been applied to the analysis of the density wave theory of galaxies which are many-body systems of gravity. A nonlinear Schrödinger equation has been derived by applying the reductive perturbation method on the fluid equations that describe the behavior of infinitesimally thin disk galaxies. Their spiral arms are characterized by a soliton and explained as a pattern of a propagating nonlinear density wave.

Astronomical Phenomena↗
PubMed1986

Crystal structures of modified myoglobins. II. Relation between oxygen affinity properties and structural changes around heme in myoglobins reconstituted with 2,4-diisopropyldeuteroheme, 2-isopropyl-4-vinyldeuteroheme, and 2-vinyl-4-isopropyldeuteroheme.

The crystal structures of sperm whale metmyoglobins reconstituted with three kinds of modified hemes, 2,4-diisopropyldeuteroheme, 2-isopropyl-4-vinyldeuteroheme, and 2-vinyl-4-isopropyldeuteroheme, have been determined and refined at 2.2 A resolution to R = 0.216, 0.219, and 0.195, respectively. All the crystals of these myoglobins are isomorphous with that of native metmyoglobin. The 2-vinyl-4-isopropyldeuteroheme was found to be in a reverse orientation, in which the heme plane is rotated by 180 degrees about an axis through the alpha-gamma-meso carbons, whereas the orientations of the other two hemes were the same as that of protoheme in native myoglobin. In the myoglobins with 2,4-diisopropyldeuteroheme and 2-vinyl-4-isopropyldeuteroheme, both of which have lower oxygen affinities than native myoglobin, the bulky isopropyl side chain pushes Phe 43 0.7 A toward His 64 (the distal histidine) in the former, and the whole E helix at most 1.5 A, including a 0.7 A shift of the His 64 imidazole ring, in the latter. The changes of the structures prevent His 64 from forming a hydrogen bond with the liganded oxygen molecule, so that these two modified myoglobins show low oxygen affinities. On the other hand, there is no such drastic displacement in myoglobin with 2-isopropyl-4-vinyldeuteroheme, which has a slightly higher oxygen affinity than native myoglobin.

Animals↗