The technique of point-projection spectroscopy has been shown to be applicable to the study of expanding aluminum plasmas generated by ∼80 ps laser pulses incident on massive, aluminum stripe targets of ∼125 μm width. Targets were irradiated at an intensity of 2.5±0.5×${10}^{13}$ W/${\mathrm{cm}}^2$ in a line focus geometry and under conditions similar to those of interest in x-ray laser schemes. Hydrogenic and heliumlike aluminum resonance lines were observed in absorption using a quasicontinuous uranium backlighter plasma. Using a pentaerythrital Bragg crystal as the dispersive element, a resolving power of ∼3500 was achieved with spatial resolution at the 5-μm level in frame times of the order of 100 ps. Reduction of the data for times up to 150 ps after the peak of the incident laser pulse produced estimates of the temperature and ion densities present, as a function of space and time. The one-dimensional Lagrangian hydrodynamic code medusa coupled to the atomic physics non-local-thermodynamic-equilibrium ionized material package was used to simulate the experiment in planar geometry and has been shown to be consistent with the measurements.

Layered-target experiments at 1.06 μm have been performed in order to measure the mass-ablation rate ṁ and ablation pressure ${\mathit{P}}_{\mathit{a}}$ as a function of absorbed laser intensity ${\mathit{I}}_{\mathit{a}}$ and wavelength ${\lambda }_{\mathit{L}}$ at irradiances of ${10}^{13}$–${10}^{15}$ W/${\mathrm{cm}}^2$. The result can be put in the form ṁ(kg/sec ${\mathrm{cm}}^2$)≃143[${\mathit{I}}_{\mathit{a}}$(W/ ${\mathrm{cm}}^2$)/${10}^{14}$${]}^{1/3}$[${\lambda }_{\mathit{L}}$(μm)${]}^{\mathrm{-}4/3}$ and ${\mathit{P}}_{\mathit{a}}$(Mbar)≃11.9[${\mathit{I}}_{\mathit{a}}$(W/ ${\mathrm{cm}}^2$)/${10}^{14}$${]}^{2/3}$[${\lambda }_{\mathit{L}}$(μm)${]}^{\mathrm{-}2/3}$. Hydrodynamical simulations using the code medusa show heat-flux inhibition. This flux limitation is discussed in terms of a small lateral transport and the eventual presence of an intense magnetic field of order 0.76 MG.

When a laser is focused it develops a longitudinal component. This could be used to develop a laser particle accelerator. A lens waveguide array is discussed, and it is shown that such a system could generate high-energy particle beams. The possibility of using a ‘‘diffraction-free’’ Bessel beam is discussed, and a possible configuration is suggested. To accelerate electrons from one to a few MeV seems possible using a well-focused, 1-J, 1-ps laser pulse. This would provide a simple proof-of-principle experiment. In order to accelerate heavier particles, such as protons, the injected particle beam would have to be ultrarelativistic such as that produced by the superconducting supercollider.

Iterative Monte Carlo simulations are employed to study ion transport across the self-consistent electric field of a plasma sheath, under the influence of elastic collisions and charge-transfer reactions with ambient neutral species. Invoking arguments from kinetic theory, we express the retarding effect of encounters with neutral species as a ‘‘dynamical friction’’ force, proportional to the square of the mean ion velocity, whose coefficient κ(z) is a function of position in the sheath through its dependence on the ion distribution f(v,z). We show that κ(z) is determined by basic sheath parameters, such as the ion–to–neutral-species-mass ratio m/M, the mean free paths ${\lambda }_{\mathit{e}}$ and ${\lambda }_{\mathit{c}}$ for elastic scattering and charge transfer, and the ion distribution ${\mathit{f}}_0$(v)=f(v,0) at the presheath-sheath boundary. When m/M≥1 or ${\lambda }_{\mathit{c}}$≪${\lambda }_{\mathit{e}}$, the Monte Carlo models indicate that κ(z) is a relatively weak function, amenable to approximation by simple functional forms using parameters estimated from the simulations. By substituting such approximations into a simple continuum description—consisting of coupled ordinary differential equations for the electric field and mean ion velocity—substantively accurate models of the sheath structure are obtained at nominal computational expense, for regimes ranging from the nearly collisionless to the collision-dominated extremes.

It is shown that so-called ‘‘relativistic’’ electric fields [T. Ivezić, Phys. Lett. A 144, 427 (1990)] from stationary current-carrying conductors are responsible for the existence of longitudinal forces that subject the conductor to tension. The exploding-wire phenomenon [P. Graneau, IEEE Trans. Magn. MAG-20, 444 (1984)] is explained in terms of these longitudinal electric forces. Other unexplained electromagnetic experiments are also discussed.

Computer experiments have been performed to study the formation and propagation processes of solitons in a nonlinear transmission line composed of 21 LC-circuit sections. A solitonlike wave form already appears at the first section (or stage) and a stable soliton, which propagates without changing line shape and intensity, is formed, depending on the area of the input pulse. One soliton and subsequently two, three, and multiple solitons are generated as the area is increased. The minimum area to generate a stable soliton is found to be 0.50 μsec V, a value consistent with the thresholds for one-, two-, three-, and multiple-soliton generation. Discussion is given of the low-amplitude oscillation appearing behind the soliton wave form and also of the relative strengths of the nonlinear effect and the dispersion effect.

Computer experiments have been performed to study the interaction of solitons with an impurity capacitor in a nonlinear transmission line composed of 21 LC-circuit sections. It is found that the wave form of solitons is changed at the impurity capacitor position, with the amount of change depending on the strength of the nonlinearity of the capacitance. The soliton amplitude is decreased by a light-mass impurity of low nonlinearity, while it is increased by a heavy-mass impurity of higher nonlinearity.

Within the simple displaced oscillator state ansatz of Davydov [Phys. Scr. 20, 387 (1979)], called the ${\mathit{D}}_2$ ansatz state, the soliton remains stable against strong disorder in the sequences of masses, spring constants, and coupling constants. However, weak diagonal disorder or disorder in the dipole coupling constants destroys the solitons. Within the ${\mathit{D}}_1$ ansatz, in which the quantum nature of the lattice plays a greater role than in the classical ${\mathit{D}}_2$ state, the soliton appears only from nonlinearities roughly 3 to 4 times larger than those in ${\mathit{D}}_2$ models. The sensitivity of such solitons to disorder is practically opposite that for the ${\mathit{D}}_2$ state. Within the partial dressing model we find only dispersing solitary waves, no real traveling solitons. The sensitivity of such waves to disorder is similar to the ${\mathit{D}}_1$ case.

We have measured the low-frequency (<200 ${\mathrm{cm}}^{\mathrm{-}1}$) Raman activity of dehydrated fibers and films of polynucleotides and random-sequenced nucleic acids. The spectra exhibit a pronounced, unresolved band in the range 10–150 ${\mathrm{cm}}^{\mathrm{-}1}$. A nonlinear least-squares algorithm optimally fits the spectra from all samples with three low-frequency modes in the range 25–100 ${\mathrm{cm}}^{\mathrm{-}1}$ and a fourth mode near 200 ${\mathrm{cm}}^{\mathrm{-}1}$. A mode near 25 ${\mathrm{cm}}^{\mathrm{-}1}$ is sequence independent, but does shift to marginally higher frequency in RNA. The mode frequencies and oscillator strengths of two modes in the range 35–100 ${\mathrm{cm}}^{\mathrm{-}1}$ exhibit sequence dependence.

We study the learning dynamics of neural networks from a general point of view. The environment from which the network learns is defined as a set of input stimuli. At discrete points in time, one of these stimuli is presented and an incremental learning step takes place. If the time between learning steps is drawn from a Poisson distribution, the dynamics of an ensemble of learning processes is described by a continuous-time master equation. A learning algorithm that enables a neural network to adapt to a changing environment must have a nonzero learning parameter. This constant adaptability, however, goes at cost of fluctuations in the plasticities, such as synapses and thresholds. The ensemble description allows us to study the asymptotic behavior of the plasticities for a large class of neural networks. For small learning parameters, we derive an expression for the size of the fluctuations in an unchanging environment. In a changing environment, there is a trade-off between adaptability and accuracy (i.e., size of the fluctuations). We use the networks of Grossberg [J. Stat. Phys. 48, 105 (1969)] and Oja [J. Math. Biol. 15, 267 (1982)] as simple examples to analyze and simulate the performance of neural networks in a changing environment. In some cases an optimal learning parameter can be calculated.

The one-dimensional kinetic Ising models with order-parameter conservation are studied for different transition rates. The effect on critical dynamics of global and local conservation laws is considered. Rigorous lower bounds for the dynamical exponent (z) are obtained from the initial response rate and the scaling hypothesis for the relaxation time. It is shown for several dynamical models that z≥5 even if the order-parameter conservation occurs only globally. For models in which the maximal separation between two exchanged spins is proportional to the equilibrium correlation length, the inequality z≥3 is obtained. The d-dimensional case is discussed briefly.

The concept of multifractality is extended to self-affine fractals in order to provide a more complete description of fractal surfaces. We show that for a class of iteratively constructed self-affine functions there exists an infinite hierarchy of exponents ${\mathit{H}}_{\mathit{q}}$ describing the scaling of the qth order height-height correlation function ${\mathit{c}}_{\mathit{q}}$(x)∼${\mathit{x}}_{\mathit{q}}^{\mathit{q}\mathit{H}}$. Possible applications to random walks and turbulent flows are discussed. It is demonstrated on the example of random walks along a chain that for stochastic lattice models leading to self-affine fractals ${\mathit{H}}_{\mathit{q}}$ exhibits phase-transition-like behavior.

A method for calculating quantum averages and static correlation functions of configurational observables is introduced, by extending a variational method, which permits the reduction of the problem to the computation of classical-like averages. This approach opens the possibility of obtaining novel results in many fields, and strongly shortens the computer time needed for numerical calculations. As an example, the well-known sine-Gordon model is considered, and its relevant quantum correlation functions are calculated on the whole temperature range.

Under certain conditions, the renormalization-group flow of models in statistical mechanics can change dramatically under just very small changes of given external parameters. This can typically occur close to bifurcations of fixed points, close to the complete disappearance of fixed points, or in regions where the renormalization-group flow becomes chaotic. We present some explicit examples of these phenomena for the case of a Lie group valued spin-model analyzed by means of a variational real-space renormalization group. By directly computing the free energy of these models around the parameter regions in which such nontrivial modifications of the renormalization-group flow occur, we can extract the physical consequences of these phenomena.

The clump lifetime of the correlated motion for two adjacent particles in a dissipative space-time-correlated stochastic field with a small initial separation is found to have a remarkable feature. Despite its monotonic long-time diffusive behavior, this work predicts that if the motion of these two particles undergoes dissipation, the formation of long-time density granulations or clumps presents a complex oscillatory behavior.

A quantum-mechanical theory of incoherent, inelastic neutron scattering from liquid water is presented. This theory is statistical and is similar to those describing the inelastic light scattering. The experimental spectra are reproduced theoretically. The role of vibrational relaxation processes as well as of those due to librational motions is emphasized.

We report measurements of the electric-field-induced molecular tilt in a homogeneously aligned, single-component chiral liquid crystal that undergoes a direct isotropic–to–smectic-A transition. Above the transition to the bulk isotropic phase, there exists an in-plane molecular tilt that is linearly proportional to the applied field. It is suggested that surface order plays an essential role in realizing this in-plane molecular tilt and that its temperature dependence is much stronger than that for materials possessing the nematic–to–smectic-A transition. The effects of the phase-transition sequence and the number of constituent molecular species on the nature of the surface ordering are also discussed.

For the cubic lattices of the liquid-crystalline blue phases I and II electrostriction has been observed recently, i.e., a distortion of the crystal by an electric field in quadratic order. Here we investigate the characterizing fourth-order electrostriction tensor. Because electrostriction results from a competition between electric and elastic forces, the tensor is the quotient of the elasto-optic and the elastic tensor. The latter are derived from the Landau–de Gennes theory of phase transitions in liquid crystals within the model of rigid helices and are expressed as functionals of the alignment tensor for the undistorted system. Comparison is made with experimental data. The limitations of the model of rigid helices are discussed.

Two- and three-dimensional behavior of the Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) instabilities is examined with a hydrodynamic code: cubic interpolated pseudoparticle. The mushroom structure owing to the KH instability in three dimensions is much smaller than that in two dimensions, and hence the nonlinear growth is faster in three dimensions. The simulation without gravity shows a similar behavior and hence this difference between two and three dimensions does not originate from the RT instability. This difference cannot be explained by linear analysis on the KH instability also.

We present a complete numerical implementation of an alternative approach for evaluating the electron radiative self-energy in high-Z hydrogenic ions. Using basis-set techniques, we demonstrate that the partial-wave expansion of the most numerically intensive term converges relatively rapidly as approximately 1/${\mathit{L}}^3$, making this an attractive calculational approach. Numerical results extrapolated to zero nuclear size are in agreement with Mohr [Ann. Phys. (N.Y.) 88, 26 (1974); 88, 52 (1974); Phys. Rev. Lett. 34, 1050 (1975); Phys. Rev. A 26, 2338 (1982)] for the K and L shells, and with the graphical results of Mohr and Kim (unpublished) for higher excited states. The algorithm is suitable for immediate generalization to non-Coulombic potentials.