共查询到10条相似文献,搜索用时 140 毫秒
1.
Algorithms for generating new exact solutions of the Einstein-Klein-Gordon field equations, which describe inhomogeneous universes with S3 topology of spatial sections, are developed. The known exact vacuum and stiff-fluid solutions with S3 topology are used as an input. The methods developed are further applied to derive inhomogenous generalizations of Bianchi type IX solutions and inhomogeneous S3 Gowdy models with gravitational and scalar waves. It is shown that the new solutions, which are generalizations of the Bianchi type IX models, permit identification of the scalar field with the velocity potential of the stiff irrotational fluid. The latter result is further used to study the growth rate of density perturbations of the isotropic and anisotropic Bianchi type IX universes in a fully nonlinear relativistic regime. The role of anisotropy of the rate of growth of density perturbations is studied in detail. 相似文献
2.
Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B1) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B1 field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B1 field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI. 相似文献
3.
Amorphous FeSiB wires with positive magnetostriction are very perspective soft magnetic materials for many applications, e.g. torque, field or current sensors, pulse generators and highly sensitive magnetometers. The appearance of the Large Barkhausen Effect (LBE) during slow magnetization of FeSiB wires is described by means of the core-shell model assuming a residual radial tensile stresses in the as-cast state. In this work, the LBE during magnetization reversal of Fe77.5Si7.5B15 amorphous wire in the as-cast state was analysed. We have studied the kinetics of the reverse domain in the core region of the wire by means of Sixtus-Tonks method of two small pick-up coils placed in an asymmetric way with respect to the ends of the wire. We estimated the velocity of the reverse domain wall and the core region volume of the wire. It was found that the residual radial tensile stress distribution of the shell region strongly influences the magnetization reversal in the FeSiB wire. 相似文献
4.
A molecular dynamics simulation of neutral clusters (H2O)
n ≤ 21 has been performed in the framework of the flexible polarized model. The formation and evolution of the dipole moment of
the cluster have been investigated with a change in the size and temperature of the cluster and an external electric field.
It has been shown that at low electric fields corresponding to the experiments on the deflection of clusters in the transverse
inhomogeneous electrostatic field (Moro et al., 2006), the induced polarization of the cluster is determined by the orientational
polarizability of the “rigid” cluster, rather than by the intracluster reorientation of the molecules. The calculated dependence
of the effective polarizability of the cluster in the low field on n qualitatively reproduces the experimental results, but the calculated polarizability is numerically much higher than the
experimental value by, e.g., a factor of 4 for n ≈ 20. 相似文献
5.
The Bethe-Salpeter equation in the form due to Cung et al. (Ann. Phys. (N.Y.)98 (1976), 516) is investigated for the special case of instantaneous harmonic oseillator exchange, An exact reduction to a pair of coupled ordinary differential equations for the radial excitations of the 3(J ± 1)J modes is achieved. The equations in the mass zero case are brought to a form which is quite close to Whittaker's equation. This similarity to Whittaker's equation is exploited in a computer study of the level structure as a function of the quark mass. This study covers the region from a highly relativistic spectrum depending only upon J to the nonrelativistic regime where the spectrum depends only upon L. An expression for the leptonic width of a 3S1 state in terms of the Bethe-Salpeter wave function is derived and applied to the ψ-family. The effect of relativistic corrections is to reduce the predicted value of the leptonic width compared to the value calculated by assuming nonrelativistic kinematics. It is also shown that the relativistic treatment allows a 3D1 state to couple directly to a virtual photon. 相似文献
6.
T. Kikuchi T. Motobayashi N. Iwasa Y. Ando M. Kurokawa S. Moriya H. Murakami T. Nishio J. Ruan S. Shirato S. Shimoura T. Uchibori Y. Yanagisawa T. Kubo H. Sakurai T. Teranishi Y. Watanabe M. Ishihara M. Hirai T. Nakamura S. Kubono M. Gai R.H. France III K.I. Hahn T. Delbar P. Lipnik C. Michotte 《The European Physical Journal A - Hadrons and Nuclei》1998,3(3):213-215
We have measured the dissociation of 8B in the Coulomb field of 208Pb at Ein=51.9 MeV/nucleon and extracted the cross section of the 7Be(p,γ)8B reaction at 0.4 ≤ Erel≤ 3 MeV, which is of importance for the 8B solar-neutrino production rate. The extracted astrophysical S17 factors are consistent with our earlier Coulomb dissociation measurement, and agree with the values deduced from the direct
capture measurements by Filippone et al., Vaughn et al. and Hammache et al. The S factor at zero energy was extracted to be
18.9±1.8 eV-b with the help of theoretical energy-dependence.
Received: 15 August 1998 相似文献
7.
D. P. Bhattacharyya 《Annalen der Physik》1984,496(2):109-118
The all particle primary spectrum of GRIGOROV et al. surveyed by HILLAS has been fitted by a power law fit of the form Iall particle(>E) = 1.3 E?1.65 (cm2 s sr)?1 where E is the energy in GeV/nucleus. Using our recently determined conversion factor for protonnuclei flux ratio of equal energies the primary proton spectrum has been calculated and the result agrees with the Goddard Space Flight Centre primary proton spectrum data satisfactorily. The primary nucleon spectrum has also been calculated and follows the form Nnucleons(E) dE = 2.664 E?2.75 dE (cm2 s sr GeV/nucleon)?1. Using this primary nucleon spectrum as the source of hadrons and accelerator data for various inclusive reactions viz. used for the estimation of hadronic energy moments in the frame work of FEYNMAN- Scaling, the differential meson spectra have been estimated. The meson atmospheric diffusion equation after Bugaev et al. has been considered for the derivation of sea level muon spectrum. The magnetic spectrograph data of Allkofer et al., Ayre et al., Green et al., and MUTRON group are in accord with the calculated muon spectrum. 相似文献
8.
Anirudh Pradhan H. Amirhashchi H. Zainuddin 《International Journal of Theoretical Physics》2011,50(1):56-69
A new class of exact solutions of Einstein’s modified field equations in inhomogeneous space-time for perfect fluid distribution
with electromagnetic field is obtained in the context of normal gauge for Lyra’s manifold. We have obtained solutions by considering
the time dependent displacement field. The source of the magnetic field is due to an electric current produced along the z-axis. Only F
12 is a non-vanishing component of the electromagnetic field tensor. It has been found that the displacement vector β(t) behaves like the cosmological constant Λ in the normal gauge treatment and the solutions are consistent with the recent observations of Type Ia supernovae. Physical
and geometric aspects of the models are also discussed in the presence of magnetic field. 相似文献
9.
Small helical anisotropy was induced in amorphous ferromagnetic Co68.2Fe4.3Si12.5B15 wire by current annealing and simultaneous application of tensile stress and torsion. Presence of helical anisotropy was confirmed by measurement and analysis of the circular magnetic flux versus axial magnetic field hysteresis loops. These measurements also showed that a single domain wall between circular domains can be created by placing the wire in a sufficiently high inhomogeneous magnetic field generated by Helmholtz coils with opposite currents. The domain wall velocity versus axial driving field was measured. The results show that the basic dynamic properties (magnitude of the wall mobility, field interval in which linear dependencies between velocity and field are observed, accelerated increase of the velocity for higher fields) are very similar to those obtained for the domain wall between circular domains driven by a constant circular field. The Hall effect was detected in the eddy current loop generated by the moving domain wall. 相似文献
10.
The minimization of residual stresses in magnetic systems with quasi-force-free current distribution
The feasibility of a magnet experiencing very low mechanical stresses is studied. Megagauss fields can be generated without failure of the solenoid winding and excessively increasing the winding dimensions if the current distribution is force-free in its inner part and the axial current closes in its outer part. The approximation of the quasi-force-free current distribution by discrete conductive layers is considered. Analysis and computations are performed for two force-free winding approximations. In the former case, the conductors are divided into N paired balanced layers where the azimuth and poloidal currents, respectively, pass. The equal and oppositely directed forces arising in the layers of a pair are transferred to insulating spacers between the layers. Stresses in the spacers can be reduced down to values N times as low as the magnetic field pressure B 0 2 /(2μ0) at the solenoid axis. If the current direction in each of the layers meets the balance condition, the residual stresses can be N 2 times lower than B 0 2 /(2μ0), because the tensile and compressive forces in the layers partially cancel. 相似文献