The possibility of the existence of an essentially single-species plasma state represented by a stable packet of charged particles moving collectively through space-time is examined. The collective plasma state is catalyzed by a localized electromagnetic wave. Condensation to this state is shown to occur on a very short time scale. The model treats the particle packet as a warm electron plasma (fluid) and self-consistently incorporates the resulting electromagnetic field. Predicted characteristics of the localized particle packet and its associated electromagnetic fields compare favorably with recent experimental data.

Emission and transport characteristics of sub-keV x rays in laser-produced gold plasmas are investigated. Absolute x-ray spectra under three different experimental conditions have been measured with high spectral resolution to study dependences of x-ray emission on laser incidence angles, irradiation intensity, and thickness of a foil target. Simulations by a one-dimensional hydrodynamic code, which incorporates a detailed atomic physics, replicate quite well the general x-ray emission features obtained in the experiments. There remain, however, discrepancies in some details of spectral shapes, notably in the range between 0.5 and 0.9 keV. It is also shown that the transport of radiation heat wave deduced from x-ray emissions at the front and the rear sides of a thin gold foil may be interpreted by the self-similar solutions presented by Pakula and Sigel [Phys. Fluids. 28, 232 (1985); 29, 1340 (E) (1986)] with an approximated gold opacity under local thermodynamic equilibrium.

Accurate measurement of transport and rate coefficients is complicated by the presence of non- equilibrium regions near electrodes. The purpose of this work is to estimate the spatial extent of the nonequilibrium regions. Two different theoretical methods for accomplishing this purpose are discussed: (a) the five-moment (FM) method and (b) the density-gradient expansion (DGE) method. The FM method leads to three coupled differential equations for conservation of electron density, momentum, and energy. These equations are solved numerically in one dimension for the steady state to give the motion of electrons subjected to a steady, uniform electric field, similar to conditions found in drift tube measurements of electron average velocity and characteristic energy. FM solutions predict regions near each electrode where electrons are not in equilibrium with the electric field. The equilibration length is predicted to be the same for both drift velocity and average energy; it varies linearly with electric field and inversely as the square of the gas pressure. The DGE method also leads to three coupled differential equations for conservation of electron density, momentum, and energy. DGE solutions are examined to fourth order in the expansion. Convergence is not found for any order studied. It is concluded that the density-gradient expansion method is inadequate to deal with problems of the sort discussed in this paper because proper convergence cannot be established.

Ambipolar diffusion theory of the negative glow (NG) and Faraday dark space (FDS) of a cylindrical gas discharge maintained by thermionic emission from a hot cathode is discussed. Electrons are divided into two groups: thermionic beam electrons and plasma electrons. Radial diffusion losses are handled by introducing the usual diffusion length. Inputs to the model include probabilities of excitation and ionization by thermionic beam electrons and by plasma electrons, electron and ion temperatures and mobilities, and discharge tube radius. The model predicts the following measured quantities with good qualitative agreement: (a) combined length of NG plus FDS, (b) axial distribution of plasma electron density, and (c) ion current to cathode. The model also predicts two reversals in axial electric field: one in the NG near the cathode where axial current is supported by ambipolar diffusion, and one near the end of the FDS at the beginning of the positive column where axial current can no longer be supported by ambipolar diffusion.

This paper presents parameter studies to find the optimum conditions for the production of a cold highly ionized plasma suitable for the generation of recombination x-ray lasers. The effect of space-charge fields on the residual electron energy of optical-field-ionized plasmas is studied using a classical theory of above-threshold ionization and particle-in-cell simulations. The dependence of the residual energy on the pump-laser wavelength and peak intensity is strongly influenced by the dependence of the quiver energy on these parameters. The effect of space-charge fields indicates that the residual energy can be minimized by a suitable matching of the laser pulse length to the plasma density. The effect of collisional heating on the residual energy is also discussed.

Radiative losses in a collision of two solitons in the nonlinear Schrödinger model of an optical fiber including the third-order linear dispersion, and the corresponding changes of amplitudes and velocities of the solitons, are found analytically for the case when the solitons’ relative velocities are much larger than their amplitudes.

A one-dimensional model for hydrogen-bonded chains is investigated by including inhomogeneities (impurities) produced by changing parameters of the hydrogen as well as oxygen subsystems. The soliton scattering by a local impurity is studied analytically in the framework of the collective-coordinate approach, and the effective potential describing such an interaction is derived. The threshold velocity, which corresponds to the soliton repulsion by the impurity, is calculated. It is demonstrated that the soliton scattering produces an emission, mostly in the oxygen subsystem. Due to the emission the velocity of the soliton decreases, and it may be trapped by an attractive impurity. We also calculate the threshold velocity for such an effect. The decay of the soliton transmission coefficient in a disordered hydrogen-bonded system is analyzed using the independent-scattering approach.

It is proved that the expectation value of the kinetic energy becomes unbounded in time for a finite cord with dynamical disorder (perturbation random in time). This behavior is opposed to the other without disorder (periodic perturbation), where the energy remains bounded outside the resonance regime. The growth of the energy expectation value is given explicitly, and a diffusive behavior in the velocity was found.

A lattice-dynamical model has been developed of DNA in a crystalline lattice. This model is used to evaluate the harmonic nearest-neighbor force constants of highly oriented films of Na-, K-, Rb-, and Cs-DNA as a function of relative humidity from Raman data. These DNA-counterion systems were chosen since they all undergo an A to B transition. The analysis suggests that the A to B transition takes place in these different DNA’s when the effective force constant of the intermolecular bond falls below a ‘‘critical’’ value.

We show analytically that the expected number of fixed-point attractors in an analog associative memory neural network increases exponentially with network size with a scaling exponent that depends on the ratio of stored memories to neurons and on the maximum slope, or gain, of the neuron transfer function. The scaling exponent decreases with gain for a sigmoidal transfer function, indicating that gain reduction can improve computational performance by eliminating spurious fixed points. Numerical data based on fixed-point counts in small networks support the analytical results.

An exact cumulant sum rule is derived for the surface-height function that satisfies the Kardar-Parisi-Zhang [Phys. Rev. Lett. 56, 889 (1986)] equation. This identity may have bearing on dynamical scaling results previously arrived at by renormalization-group analysis and by numerical simulations. Confirmation of the sum rule by numerical simulation has already identified an algorithmic subtlety that impacts scaling-law numerics.

We consider a one-dimensional Lorenz-type mapping with slowly decaying correlation function, which produces anomalous diffusion with squared displacement growing faster then linearly. The statistical properties of the process are investigated by means of diffusion-index calculation. A simple approximation for probability distribution gives a good estimate for these indices.

We present a detailed analysis of the finite-size and boundary effects for one-dimensional irreversible monolayer adsorption of dimers on a lattice, within the rate-equation approach. Asymptotic expressions for short- and long-time coverages are derived for periodic and free boundary conditions.

A new mechanism for instability in compressible fluid systems in described. A rarefaction wave induces acceleration, which can destabilize the system at a contact surface. The instability grows until the wave has passed. Residual motion can persist for long times thereafter, leading to significant enhancing of the mixing. To illustrate this mechanism, the Rayleigh-Taylor instability excited by a rarefaction wave at a density discontinuity is discussed and investigated numerically.

Recent experiments on oil, water, and surfactant mixtures indicate that these systems can be brought close to a Lifshitz tricritical point. We investigate the interfacial properties of a system in the vicinity of such a point, based on a single-order-parameter Ginzburg-Landau model. We find that the microemulsion does not wet the oil-water interface all the way to the Lifshitz tricritical point. In addition, the scattering intensity shows a characteristic scaling form. The interfacial behavior near Lifshitz critical end points is also examined.

Vegard’s law is an approximate empirical rule which holds that a linear relation exists, at constant temperature, between the crystal lattice constant of an alloy and the concentrations of the constituent elements. Applications of a density-functional theory of nonuniform fluid mixtures to the fluid-solid transition of simple binary mixtures of hard spheres demonstrates the importance of relative atomic sizes in determining lattice constants and suggests that for sufficiently small disparities in atomic size Vegard’s law may also hold along the fluid-solid coexistence curve.

Diffusion-limited clusters formed under the constraints of a characteristic length are shown to possess a clearly defined transition from fractal to nonfractal topologies at the characteristic length, i.e., scaling exponents that vary from fractional to integer values at the specified length. The transition is useful in unifying commonly encountered fractal geometries or physical processes with the more traditional thermodynamic and material properties.

We show that two modes counterpropagating in a single-mode fiber in the presence of random coupling due to refractive-index fluctuations can interfere constructively and give rise to a coherent backscattering process analogous to that associated with backscattering from a disordered medium. Both strong- and weak-localization limits are studied and the interplay between localization and radiation losses is analyzed.

A recent paper by Banggu [Phys. Rev. A 42, 761 (1990)] claims that there are spherical solutions of the Boltzmann equation that violate particle-number conservation. The examples presented in this paper do not violate particle-number conservation, and the modification of the Boltzmann equation proposed by Banggu is incorrect.

The low value of the flux-limiter proposed in a recent article by Mirza and Murtaza [Phys. Rev. A 41, 4460 (1990)] is misleading. In order to justify their conclusions the authors made inappropriate comparisons with the results from certain experimental papers.