The structure of the modulated phases that can occur in systems like chiral tilted smectic films and monolayers of tilted amphiphiles is studied numerically within the mean-field approximation. Two types of modulated phases, uniaxial and hexagonal, are considered. The uniaxial phase is composed of an array of nontopological line defects and is therefore different from modulated structures occurring near ordinary commensurate-incommensurate phase transitions. The hexagonal phase displays point defects and topological π line defects. We discuss the energetics of these structures and the nature of the modulated-smectic-C phase transition.

We have investigated, employing the techniques of molecular-dynamics computer simulation, the onset of the freezing region of a two-dimensional fluid interacting with a Lennard-Jones potential. We have studied the pair-distribution function g(r) and its Fourier transform S(q) for unique, qualitative, and quantitative structural characteristics associated with the onset of freezing and for the validity of a universal freezing criterion. We conclude that such a criterion exists, but its value is sensitive to even slight changes in density and temperature.

We have carried out a molecular-dynamics simulation of a two-dimensonal Lennard-Jones liquid near the triple point. We have compared some of the static- and dynamic-correlation functions with those of hard disks to study the effect of potential and with those of a three-dimensional triple-point Lennard-Jones liquid to study the effect of dimensionality. It is noted that while the behavior is qualitatively similar, there are distinct potential and dimensionality differences.

The harmonic measure on off-lattice diffusion-limited (DL) aggregates is defined in detail. The formulation requires the continuum Laplacian and the regularization of divergences in the density of the measure due to folds between atoms. An ultraviolet cutoff is needed for numerical simulations, but in this off-off-lattice formulation, the cutoff can be varied at will. The cutoff’s effects on the distribution of the Hölders α is studied. We find that the right-hand tail of the distribution of α is very dependent on the lower lattice cutoff. However, the cutoff does not effect the transformations that collapse the distributions corresponding to clusters of different sizes. The shapes of off-lattice DL aggregates giving rise to extremely small minimal harmonic measures are illustrated. We discuss the notion of the smallest harmonic measure in an ensemble of clusters, and also the scaling properties of the distribution of α on small clusters.

Irreversible stochastic models for the growth of clusters on a square lattice are formulated and studied by Monte Carlo simulation in which diffusing particles are biased globally and locally. In the case of global bias, the preferred directions of incoming particles are diagonal. Contrary to the biased walks on continuum space, the lattice anisotropy is enhanced and results in diagonally elongated clusters. With the local biased walk controlled by a parametric ζ, it seems that there is a crossover length L(ζ). For the region r<L(ζ), the cluster is more or less compact while for the region r>L(ζ) the shape of the clusters becomes similar to that of the ordinary diffusion-limited aggregation. The comparison of clusters of our model is made with those of the model considered recently by Meakin, Feder, and Jo/ssang [Phys. Rev. A 43, 1952 (1991)].

Refined Monte Carlo simulations have been performed on the diffusion-controlled A+B→0 and A+A→0 reaction on two- and three-dimensional critical percolation clusters and in one dimension. For comparison, we also tested the single random walker ${\mathit{S}}_{\mathit{N}}$ (number of distinct sites visited in N steps). Effects of excluded volume, time counting, random number generators, averaging procedures, and boundary conditions have been tested and found to be within our simulation uncertainties. However, finite-size and finite-time effects are important, as well as the initial particle density and the number of runs. All the A+A and ${\mathit{S}}_{\mathit{N}}$ data agree with the well-known scaling predictions, i.e., the time exponent is ${\mathit{d}}_{\mathit{s}}$/2, where ${\mathit{d}}_{\mathit{s}}$ is the spectral (fracton) dimension. The same is true for A+B reactions in one dimension (the exponent being 1/4). Finally, for the A+B reactions on two- and three-dimensional critical percolation clusters, our refined data are consistent with the classical Kang and Redner [Phys. Rev. Lett. 52, 944 (1984)] scaling result as well as the bounds (given in more recent work) involving other fractal dimensions.

High-energy modes of oscillation in a zero-temperature relativistic electron gas in a strong background magnetic field are reported. The modes propagate parallel to the magnetic field and appear both in a longitudinal and in two transverse polarizations. The underlying mechanism is the binding between electrons near the Fermi surface and virtual positrons, which is enhanced by the presence of the filled Fermi distribution, in a Cooper-pair-like phenomenon. The energy of the mode is of the order of the pair energy (over 1.02 MeV), and the mode exists only for wave numbers k above a critical value, such that the mode group velocity exceeds the velocity of an electron on the Fermi surface. Damping of the mode is insignificant at the critical wave number and increases with k to a relatively small maximum value.

A simple derivation in the Coulomb gauge of the nonlinear Schrödinger equation describing propagation of powerful ultrashort circularly polarized laser pulses in underdense cold inhomogeneous plasmas is presented. Numerical solutions are given for the two-dimensional axisymmetric case for both initially homogeneous plasmas and static preformed plasma columns. These solutions account for (i) diffraction, (ii) refraction arising from variations in the refractive index due to the spatial profile of the electron density distribution, (iii) the relativistic electronic mass shift, and (iv) the charge displacement resulting from the transverse ponderomotive force. The most important spatial modes of propagation corresponding to (1) purely relativistic focusing and (2) the combined action of both the relativistic and charge-displacement mechanisms are described. The latter leads to the formation of stable confined modes of propagation having paraxially localized regions of high intensity and corresponding paraxially situated cavitated channels in the electron density. It is further demonstrated that the dynamical solutions of the propagation tend asymptotically to the lowest eigenmodes of the governing nonlinear Schrödinger equation. Finally, the calculations illustrate the dynamics of the propagation and show that the relativistic mechanism promotes the initial concentration of the radiative energy and that the subsequent charge displacement stabilizes this confinement and produces waveguidelike channels.

Heavy atoms Au (Z=79), Pb (Z=82), Bi (Z=83), and U (Z=92) have been introduced in the low-density (${\mathit{n}}_{\mathit{e}}$∼${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$) high-temperature (${\mathit{T}}_{\mathit{e}}$≥1 keV) TEXT tokamak (Fusion Research Center, University of Texas at Austin) plasma. The emission has been measured in the 50–200-Å range using a photometrically calibrated, time-resolving grazing-incidence spectrometer. The O-shell ion emission has been identified by comparison with ab initio energy-level calculations and line-intensity predictions of collisional radiative models for various charge states with 5${\mathit{p}}^6$5${\mathit{d}}^{\mathit{k}}$ ground-state configurations.

Most theories of electron flow have involved laminar flows or special flows of a single-orbital type. A general theory of electron flow has been published [C. W. Mendel, Jr., D. B. Seidel, and S. A. Slutz, Phys. Fluids 26, 3628 (1983)]. This theory can be used to make some general statements about these flows, but solutions to particular problems are complex and difficult. By constraining the general theory to flows where the electron pressure normal to the flow direction is small compared to the electromagnetic pressure, finite electron orbits can be taken into account in a general manner. An example of the utility of the theory under this assumption is given.

A possible explanation of high-power microwave radiation from a plasma-filled backward-wave oscillator in the presence of an axial magnetic field has been provided. The increase in power output with magnetic field can be explained by taking into account the simultaneous existence of Cherenkov and cyclotron resonances.

A comprehensive analysis is presented that describes the generation of harmonic radiation by the stimulated backscattering of intense laser fields from electron beams and from plasmas. The dynamics of the intense laser-electron interaction are analyzed using a fully nonlinear, relativistic, cold fluid model valid to all orders in the amplitude of the pump laser. In general, the backscattered radiation, from an electron beam or stationary plasma, occurs at odd harmonics of the Doppler-shifted incident laser frequency. The strength of the harmonics is strongly dependent on the incident laser intensity. The growth rate and saturation level of the backscattered harmonics are calculated, and the limitations due to thermal, space-charge, and collisional effects are discussed. Significant radiation generation at high harmonics requires sufficiently intense pump laser fields and sufficiently cold axial electron distributions. This mechanism may provide a practical method for producing coherent radiation in the xuv regime.

The many-body problem in a hot dense plasma is treated using the ‘‘effective one-particle’’ description of density-functional theory. Three key ideas emerge, viz., (i) the neutral-pseudoatom (NPA) concept to describe a nucleus plus its electron distribution in the plasma, (ii) the concept of hopping electrons describing the effect of the ion-distribution on the electrons, which are shown to exist as bound, hopping, and free electrons, and (iii) the concept of the mobility edge replacing that of the continuum edge as we go to denser plasmas. The NPA concept shows that the electron-density profile around an ion in a plasma is very similar to that of the isolated neutral atom in a relevant configuration, and explains the absence of large polarization shifts predicted by simple plasma-screening theories. Several models of the ion distribution are used to study level shifts as a function of temperature and density. The change of continuum phase shifts with density is shown to give information about level formation. The appearance of hopping electrons signals the breakdown of Saha theory and the need to evaluate the effective charge ${\mathit{Z}}^{\mathrm{*}}$ of an ion in a dense plasma as a function of the ion distribution. The ion-correlation sphere is shown to be the ‘‘optimal volume’’ that maximizes the number of hopping states in the plasma. The mobility edge for the plasma percolation cluster is calculated and shown to depend on exchange-plus-correlation effects of electrons and ions, a Friedel-sum contribution, and a percolation contribution.

Experiments on the excitation of cylindrical and spherical solitons in a positive-ion–negative-ion–electron plasma are described. The results are in substantial agreement with the theoretical model proposed by Das and Singh [Aust. J. Phys. 44, 523 (1991)]. In addition, experiments on the oblique collision of solitons in a cylindrical geometry are described.

A number of well-known features of a constricted discharge in plasma near-electrode layers (e.g., the normal current-density effect) proceeds from the fact that the layer thickness is much smaller than longitudinal dimensions. A better understanding of these features may be achieved by means of the asymptotic approach, which treats the ratio of the above-mentioned lengths as a small parameter. In the vicinity of extreme points of the current-voltage characteristic of the distributed discharge regime (regime with a uniform distribution of the current density over the electrode surface), this approach is similar to the perturbation method, reducing the reaction-diffusion equations in the vicinity of an instability point to the Ginzburg-Landau equation, and results in a Fisher-type equation for perturbations of current-density distribution. Using this equation, stationary perturbations are found and their stability is analyzed. In addition, the above-noted asymptotic approach is applied to the analysis of normal current-density regimes. In particular, it is shown that interaction of a current spot (covered area) with lateral boundaries and/or other spots is transmitted by means of the exponentially small perturbations introduced by spots into regions occupied by the cold and hot phases. Application of the results obtained to the transition between the normal and abnormal regimes of the glow-discharge near-cathode region is discussed.

An idealized model for ion transport across an oscillating plasma sheath is analyzed to obtain insight into qualitative features of the ion energy distributions observed in low-pressure rf discharges. The sheath is characterized by a constant electric field over an extent that varies sinusoidally with time, and ions incident on it correspond to a monoenergetic flux independent of phase φ in the rf cycle. The dimensionless parameters α=${\mathit{qV}}_{\mathit{s}}$/m${\mathrm{\omega }}^2$${\mathit{d}}^2$ and β=${\mathit{v}}_0$/ωd (where d and ${\mathit{V}}_{\mathit{s}}$ are the mean sheath thickness and potential drop, ω is the excitation frequency, and ${\mathit{v}}_0$ and q/m are the incoming ion speed and charge-to-mass ratio) govern the ion trajectories, which are found to divide into groups, delimited by two ‘‘critical’’ values of φ, that undergo N and N+1 encounters with the field. The first critical phase depends only weakly on β, whereas the second is sensitive to both α and β and cycles continuously as these parameters diminish. Correspondingly, within the ‘‘transition regime’’ where α and β are neither very small nor greater than (or comparable to) unity, the precise form of the incident-ion energy spectrum exhibits rapid variations, superposed on a systematic narrowing, as the frequency ω is increased.

The dispersion properties of low-frequency electrostatic modes in a dusty plasma in the presence of a static homogeneous magnetic field are examined. It is found that the presence of the dust particles and the static magnetic field have significant effects on the dispersion relations. For the parallel propagation, the electrostatic mode is slightly modified by the dust for the ion-acoustic branch. An alternative longitudinal mode arises at the extreme low-frequency limit, which is unaffected by the magnetic field for the parallel propagation. For the transverse propagation, the ion-acoustic mode is not affected by the magnetic field. However, the undamped extreme low-frequency mode is significantly modified by the presence of the magnetic field for the propagation transverse to the direction of the magnetic field.

We calculate the rigorous perturbation-theoretic result to first order in the plasma parameter γ (=${\mathrm{\kappa }}^3$/4πn, κ being the Debye wave number and n the density) for the partial density response functions and for the dielectric functions of a multicomponent plasma. The model used allows for the modification of the bare Coulomb potential due to short-range quantum effects into a general set of pseudopotentials. The derivation is based on applying the Vlasov-Dupree operator formalism in the first two equations of Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy. The result is given in terms of a compact expression which is valid for arbitrary k and ω values.

We establish the multispecies generalization of the velocity-average formalism for the calculation of plasma response functions at arbitrary values of the coupling. We use a set of pseudopotentials rather than the bare Coulomb potential to represent the interaction between the particles in order to describe quantum effects due to the formation of bound states, diffraction, inner-shell-electron exchange, etc. The result is a self-consistent integral equation for the partial response functions. In the weak-coupling limit the calculation can be carried out and the outcome compared with the result of the exact perturbation-theoretical calculations: in the long-wavelength k=0 limit they are in total agreement.

The classical trajectory Monte Carlo model has been used to computationally study the charge-state distributions that result from interactions between a high-energy, multielectron projectile and neutral and fully ionized targets. These studies are designed to determine the properties of a plasma for producing highly stripped ions as a possible alternative to gas and foil strippers that are commonly used to enhance the charge states of energetic ion beams. The results of these studies clearly show that a low-atomic-number, highly ionized plasma can yield higher charge states than a neutral target of the same density. The effect is principally attributable to the reduction in the number of available electron-capture channels. In this article, we compare the charge-state distributions that result during passage of a 20-MeV Pb projectile through neutral gas and fully ionized (singly charged) plasma strippers and estimate the effects of multiple scattering on the quality of the beam.