Coherent Motion Of Particles Research Articles

Collective coherent emission of electrons in strong laser fields and perspective for hard x-ray lasers

Coherent motion of particles in a plasma can imprint itself on radiation. The recent advent of high-power lasers—allowing the nonlinear inverse Compton-scattering regime to be reached—has opened the possibility of looking at collective effects in laser–plasma interactions. Under certain conditions, the collective interaction of many electrons with a laser pulse can generate coherent radiation in the hard x-ray regime. This perspective paper explains the limitations under which such a regime might be attained.

Collective motion in repulsive self-propelled particles in confined geometries

We study the collective dynamics of a simple model of repulsive self-propelled particles in three different geometries bounded by reflecting walls. Order-disorder transition is observed in all three cases and the behavior of the phase diagram is similar to that of a system with periodic boundary conditions, which supports a view that the coherent motion of particles is induced by a microscopic mechanism. We also find that some interesting quasistable dynamic patterns may appear during the ordering process. The long lifetime of the transient structures indicates that the boundaries hinder the propagation of correlations.

Simulating cosmic-ray transport with adiabatic focusing

Context. Magnetized space plasmas such as the heliosphere contain a spatially variable guide magnetic field that is superposed by turbulence. Understanding the transport of energetic particles in such an environment is important for many applications.Aims. The modifications of the scattering mean free path caused by the effect of adiabatic focusing have so far been studied only analytically. Here, the exact analytical conditions are reproduced in a numerical simulation to allow for a detailed comparison.Methods. A Monte-Carlo simulation code was modified to include a guide magnetic field with a constant focusing length. The turbulence strength was kept constant either throughout the simulation space or relative to the variable background field strength.Results. In contrast to both quasi-linear and nonlinear analytical predictions, an increased parallel mean free path is found for almost all cases. Particle diffusion is found to be asymmetric depending on the initial particle velocity relative to the guide field. Conclusions. The increased parallel mean free path can be explained by a coherent particle motion toward lower magnetic field strengths. The possible reduction of the diffusion parameter is consequently obscured.

Dispersionless motion in a periodically rocked periodic potential

Recently, dispersionless (coherent) motion of (noninteracting) massive Brownian particles, at intermediate time scales, was reported in a sinusoidal potential with a constant tilt. The coherent motion persists for a finite length of time before the motion becomes diffusive. We show that such coherent motion can be obtained repeatedly by applying an external zero-mean square-wave drive of appropriate period and amplitude instead of a constant tilt. Thus, the cumulative duration of coherent motion of particles is prolonged. Moreover, by taking an appropriate combination of periods of the external field, one can postpone the beginning of the coherent motion and can even have coherent motion at a lower value of position dispersion than in the constant tilt case.

Dynamic control of the photonic smectic order of membranes

Self-assembled soft matter is often used in photonics because its characteristic length scale can be of the order of the wavelength of light. For example, a hyperswollen lamellar phase composed of bilayer membranes reflects visible light by the Bragg diffraction and acts as a photonic smectic crystal. The softness of such a structure allows us to dynamically control its photonic characteristics using an external field, as reported here. The smectic order of membranes is destabilized by doping charged colloidal particles into intermembrane water regions. However, we found that anisotropic coherent motion of particles along membranes induced by an alternating electric field enhances the degree of the photonic smectic order significantly. This demonstrates that entropic interactions can be controlled by modulating the membrane fluctuations through their dynamic coupling to an external field.

Crossover of the behavior of surface waves in a vibrated granular material

We study the formation of standing waves on the surface of two-dimensional vibrated granular system, using molecular dynamics simulations. Two distinct regimes are observed at low and high vibration frequencies. At low frequency, the standing waves are maintained by a sizable fraction of particles traveling long distances. Such movements are significantly reduced at high frequency due to frequent interparticle collisions, and more coherent motion of particles is observed. We also describe in detail the time evolution of the standing waves, especially on the collision process of the granular layer with the bottom plate, during which horizontal flux of particles is generated.

Viscosity in a Dense Planetary Ring with Self-Gravitating Particles

We have investigated the viscosity (the angular momentum flux) in dense, self-gravitating particle disks such as Saturn's main ring, by performing local N-body simulations. Viscosity could play important roles in evolution and structure formation of planetary rings. The ring's viscosity has been investigated with both theoretical and numerical approaches (e.g., Goldreich and Tremaine, 1978, Icarus 34, 227–239; Wisdom and Tremaine, 1988, Astron. J. 95, 925–940). However, these studies mainly considered systems including physical collisions of particles but not mutual gravitational interactions. Local N-body simulations by Salo (1995, Icarus 117, 287–312) and Daisaka and Ida (1999, Earth, Planet Space 51, 1195–1213) showed that a wake-like structure and clumps of particles are formed by the self-gravitational instability and that in such situations coherent motion of particles is dominant rather than random motion, which leads to an increase in radial velocity dispersion of particles. The wake structure and associated coherent motion are considered to affect the ring viscosity significantly. Our simulation in this paper shows that the viscosity is strongly enhanced by the wakes. When the wake structure strongly develops, the coherent motion considerably enhances the translational viscosity, which was usually referred to as a local component in previous studies, and the effective viscosity is dominated by both gravitational torque due to the wake structure and the enhanced translational viscosity. We also find that in the presence of the wakes, the viscosity ν is given as ν⋍ C G 2Σ 2/Ω 3 0, where G, Σ, and Ω 0 are the gravitational constant, the surface mass density of a ring, and the angular velocity, respectively. The non-dimensional correction factor C depends on the distance from the central planet. For example, C⋍6–20 for Saturn's B-ring. The effects of the enhanced viscosity on the structure and evolution of Saturn's main ring are also discussed.

Multiwavelet spectral and polarization analyses of seismic records

SUMMARY We present an algorithm, based on the wavelet transform and multiple taper spectral analysis, for providing a low-variance spectrum estimate of a non-stationary data process. The‘multiwavelet’algorithm uses, within each frequency band, a number of mutually orthogonal Slepian wavelets, optimally concentrated in frequency. The sum of squared wavelet transforms with the Slepian wavelets results in a spectrum estimate that is both low-variance and resistant to broad-band bias. The multiwavelet algorithm is used to estimate the time-varying spectral density matrix S(f, t) for two or more time series, in particular for three-component seismic data. Coherent three-component motion is described by motion along a single trajectory, with appropriate projections onto the three component axes. This trajectory is found by applying a singular value decomposition (SVD) to a matrix M(f, t) of wavelet transform values. The normalized first singular value of the SVD determines whether a correlation among the three components of the seismogram is statistically significant. Where significant, coherent particle motion is reconstructed by a linear combination of the wavelets with coefficients specified by the first left-singular vector. The polarization of this motion with respect to the coordinate axes is given by the first right-singular vector. Where the wavelets are real-valued, the usefulness of this method is limited to cases in which the three components of the seismic record oscillate in phase with each other, as is often the case for seismic body waves. Elliptical polarization is handled by pairing even and odd Slepian wavelets into complex-valued wavelets, capable of detecting phase shifts between components. We demonstrate the mutiwavelet spectrum and polarization estimators on seismic data from a large shallow earthquake in the Solomon Islands, and from the recent deep earthquakes beneath Fiji (1994 March 9) and Bolivia (1994 June 9).

"Confinement" in the one-dimensional Hubbard model: Irrelevance of single-particle hopping.

We demonstrate, by direct use of the asymptotic Green's functions of Korepin and Ren, that the one-dimensional repulsive U Hubbard model has the property of ``confinement,'' in that weak interchain coupling does not cause coherent particle motion in the transverse direction at absolute zero. Implications for real one-dimensional systems and for the two-dimensional Hubbard model are discussed.

Dynamic speckle spatial coherence measurements utilizing a single-mode optical-fiber probe

The spatial coherence functions for light scattered by a suspension of particles undergoing Brownian motion are interrogated with a single-mode optical-fiber interferometer. The exposed core of a single-mode optical fiber is utilized as a mobile pointlike probe of the scattered optical field. Two such fiber probes, together with a three-coupler heterodyne interferometer, are used to measure the spatial coherence of fluctuations in the scattered light intensity, the heterodyne carrier amplitude, the intensity-weighted phase rate (IWPR), and the phase rate. Spatial coherence measurements for coherent particle motion are also presented for the IWPR and the phase rate. A theoretical analysis of the spatial coherence functions is developed and compared with the experimental results. The coherence function for fluctuations in the intensity and the IWPR that is due to Brownian particle motion exhibits a Gaussian functional form with a coherence lengths ξc of ~2 ± 1.2 μm at 1.0 kHz. This result is in good agreement with the theoretical result of 1.5 ± 0.9 μm based on the scattering volume dimensions and scattering geometry. Measurements of the phase-rate spatial coherence function for Brownian motion were inaccessible because of signal-to-noise-ratio limitations associated with the signal-processing electronics. The IWPR measurements for coherent particle motion exhibit a bandwidth-dependent coherence function, having a background level of 0.74 and a coherence length ξc of 4.5 μm at 1.8 kHz. The phase-rate coherence function for coherent particle motion was constant with a value of 0.5.

On the local power dissipation of h.f. waves in hot inhomogeneous plasmas

In a medium with spatial dispersion the power Pabs dissipated per unit volume by an electro-magnetic wave differs from (J.E) by the divergence of a vector which is interpreted as the correction delta Skin to the Poynting flux due to the coherent particle motion in the field. The authors obtain expressions for Pabs and delta Skin in hot, inhomogeneous, weakly collisional plasmas by evaluating the time-averaged rate of change of the total kinetic energy density of charged particles, generalizing a method suggested by McVey et al. (Phys. Rev. Lett. vol.55, p.507, 1985). The authors prove that these expressions are consistent with the time averaged Poynting theorem for Maxwell equations. By specializing them to uniform and plane-stratified geometries, they derive some interesting properties of wave-plasma energy exchanges, and they check that the well known expressions for Pabs and delta Skin in the case of a plane wave propagating in an infinite homogeneous plasma are recovered in the appropriate limit.

Two-Body Forces and Nuclear Saturation. I. Central Forces

The problem of nuclear saturation for the rapidly varying and nonmonotonic potentials of pseudoscalar meson theory has been investigated. In these potentials, variational methods using independent-particle trial functions are grossly inadequate. Although the problem can be approached using more general variational functions with interparticle correlation, the evaluation of the resulting expressions is very difficult since indirect correlations involving many more than two particles become important at high densities. An alternative procedure has been developed which allows a rather straightforward evaluation of the many-body energy even when the potentials are of great complexity. This method depends on a treatment of the coherent particle motion which is exact in the limit of very many scatterers, and treats the incoherent motion as a perturbation. In this case the many-body potential energy can be simply expressed in terms of the low-energy scattering amplitudes. This method has been applied to the two-body potentials given by pseudoscalar meson theory when the effects of nucleon pair formation are assumed to be small. In this approximation the many-body forces of the theory are negligible. These potentials given an excellent fit to the low-energy scattering parameters of the two-nucleon system and also an approximately correct description of scattering up to 90 Mev. They are characterized by repulsive cores of radii 0.3-0.4\ensuremath{\Elzxh}/\ensuremath{\mu}c and quite weak interactions in odd states. The many-body energy has been evaluated neglecting the tensor contributions which average to zero in first approximation. The result shows saturation at an energy per particle (neglecting Coulomb energy) of 12 Mev at a nuclear radius of $1.15\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{A}^{\frac{1}{3}}$ cm. The method has also been applied to potentials of the L\'evy type in which the odd-state potentials are rather strong and attractive. To give saturation with these near normal density, a 3-body force of the type given by the pair terms in the pseudoscalar coupling with a coupling constant $\frac{{g}^{2}}{4\ensuremath{\pi}}\ensuremath{\sim}3$ is required. Finally, the method can easily be extended to the determination of the elastic interaction of a slow neutron with a nucleus. The resulting "Weisskopf" potential has a depth of 35 Mev.