The intermittent fine structure of fully developed turbulence is studied by means of a visualization experiment. Characteristic locally ordered flow patterns, tentatively identified as layered vortex sheets, are observed.

We observed a shear-induced alignment of colloidal particles in a Brownian dynamics simulation carried out in the presence of an oscillating shear flow. As the shear rate γ is increased for temperatures near the melting point, we first observe shear thinning followed by a transition to a state with two-dimensional order in the plane perpendicular to the flow. In contrast to earlier nonequilibrium molecular-dynamics studies which also found a similar result, no thermostat is needed to keep the system stable.

We show that stable localized waves can be generated in the vicinity of an inverted Hopf bifurcation. We compute the size of the localized wave envelope perturbatively in the case of slightly dissipative systems. The size selection traces back to the broken scale invariance by the dissipative terms. This mechanism is a possible explanation for the localized structures, widely observed in various hydrodynamic flows in dissipative systems driven far from equilibrium.

We compare the radiation output from simulations of a finite-pulse high-power laser for several tapering rates. A fast taper leads to a tenfold increase in efficiency as compared to a slow taper, with little change in peak radiation intensity. The enhanced power for fast tapering rates is due to an increase in the optical pulse cross section, brought about by a reduction in refractive guiding. This is analyzed by an envelope equation for the radiation beam. For the tapering rates leading to the highest powers, the optical pulse is virtually free of sideband modulation.

In the opening gap between two moving surfaces, the interface between air and a viscous fluid can form a linear pattern of regular cells analogous to those of directional solidification. Depending on the values of the surface velocities, this pattern destabilizes in different ways. One regime presents the main characteristics of spatiotemporal intermittency of the type observed in the Kuramoto-Sivashinsky model. In the other, the cells become a propagative structure. Near one of the thresholds they show up as solitary waves; above, they form traveling domains of increasing size.

We discuss the interaction of localized solutions as they arise for the subcritical bifurcation to traveling waves. We find that for a large parameter range the localized solutions can interact so that they emerge after the collision with a size and shape unchanged compared to that well before the collision. The mechanism for this behavior, which is unusual for a strongly dissipative system, is qualitatively different from that associated with solitons for completely integrable systems. In accord with this we find that for other parameter values counterpropagating localized solutions can annihilate.

We have observed viscous fingering flow in a Hele-Shaw cell, starting from a carefully prepared initial state of fourteen almost identical fingers. As the flow progresses, killing off thirteen of the fingers on its way to the single-finger steady state, several systematic effects stand out, including (1) a characteristic trajectory for a finger which depends not on its position in the ‘‘starting gate’’ but rather on its final position in the ‘‘race’’ and, most importantly, (2) a cascade of reverse bifurcations, stages in which the number of competing fingers is reduced, typically by a factor of 2.

A systematic method is outlined for constructing workable approximations to the joint probability distribution scrP(ψ,ξ) of amplitude ψ and spatial gradient ξ of an active or passive scalar field that is advected by a prescribed isotropically distributed stochastic velocity field and subjected to molecular diffusion. scrP(ψ,ξ) is sampled along fluid-element paths, and closure is obtained by taking a multivariate-Gaussian reference field ${\mathrm{\psi }}_0$(x) and distorting it locally in x space so that it exhibits the current scrP(pxi,ξ).

We consider the kinetics of a steady-state aggregation process in which the basic dynamical variable is a ‘‘charge’’ which can assume both positive and negative values. We rigorously show that the charge distribution follows a power law in the steady state which is sustained by random injection. The exponent of the power law depends both on the type of injection and on the spatial dimension.

This paper addresses the development of a new high-voltage-gradient linear accelerator. This accelerator was energized by a modulated intense relativistic electron beam with power in the multigigawatt range. A 0.2-cm-diam electron beam emerged from the accelerator with a peak current of ∼200 A and peak kinetic energy ≥60 MeV. The length of the accelerating structure was 1 m. Simple scaling laws suggest that similar accelerators with much higher average electric fields can be built.

We have used relativistic klystron technology to extract 290 MW of peak power at 11.4 GHz from an induction linac beam, and to power a short 11.4-GHz high-gradient accelerator. We have measured rf phase stability, field emission, and the momentum spectrum of an accelerated electron beam. An average accelerating gradient of 84 MV/m has been achieved with 80 MW of relativistic klystron power.

We study the frequency lockings of two intrinsic hydrodynamic modes of a convecting ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixture by independently varying the Rayleigh and Prandtl numbers. We establish points on the critical line in this parameter space using a transient technique to locate the spiral-node transition in the interior of three resonance horns. Universal scaling is demonstrated at winding numbers with golden mean tails by computing the f(α) singularity spectrum.

We report the first experimental observation of type-II intermittency in an optical system. The type-II intermittency is observed in the light intensity of a GaAs/GaAlAs semiconductor laser with external feedback in the regime of the coherence collapse. We conclude the occurrence of a time-inverted type-II intermittency from the time distribution of the observed intensity breakdowns, with the injection current as control parameter. This interpretation is confirmed by a reconstructed Poincaré plot exhibiting the spiraling behavior of the type-II intermittency.

Wave multiple scattering is responsible for making a random medium cloudy in appearance and opaque in the sense of structure delineation. For a randomly layered medium such as the earth’s subsurface, however, knowledge about the generic behavior of multiple scattering enables us to construct a theory of statistical inversion which can recover from a single data set the slowly varying mean character of a medium with signal amplitude only ${10}^{\mathrm{-}3}$ that of the multiple-scattering noise. Inversion accuracy improves systematically with the availability of statistically redundant data.

The bifurcation diagram corresponding to the Eckhaus stability curve has been constructed for the one-dimensional Swift-Hohenberg equation in a finite domain. Finite amplitude solutions with particular spatial wavelength recover linear stability, as predicted by the Eckhaus curve, after a sequence of secondary bifurcations from the branch of solutions with this wavelength. No connectivity between the primary solution branches is admissible if the stability predicted by this bifurcation diagram is to correspond to the prediction of the Eckhaus analysis. The Eckhaus curve does not exist if nonlinear couplings destroy this bifurcation pattern. This is demonstrated by analysis of a coupled pair of swift-Hohenberg equations which allow new types of secondary bifurcation from each primary family.

A model for subcritical parity-breaking bifurcation of a periodic pattern is introduced. It is found that nucleated regions of the new asymmetric state propagate with a well-defined velocity in a direction determined by their parity, and leave in their wake a pattern with an altered wavelength. Successive passage of these ‘‘parity bubbles’’ enables the system to relax to a selected wavelength. The possible relevance of these findings to recent observations of ‘‘solitary modes’’ in directional solidification and hydrodynamic experiments is discussed.

We report a study of a fingering instability which occurs during the spreading of a spinning drop. We measure the time evolution of the drop profile, the critical radius for instability onset, and the fastest growing instability wavelength. The experimental results are compatible with the predictions of lubrication theory when the centrifugal effect is not too large compared to the capillary one.

Diffusion of a passive scalar in a random velocity field v(x,t) is considered. An exact, closed form of the limiting (t→∞) probability distribution of the normalized scalar is derived. The predictions of the theory are compared with the results of numerical simulations.

A theory of probability distributions of temperature in high-Rayleigh-number turbulent Bénard convection is presented.

Coherent pair creation during the collision of ${\mathrm{e}}^{+}$${\mathrm{e}}^{\mathrm{-}}$ beams in linear colliders is examined. This includes contributions from both real and virtual photons, where the real photons are predominantly from beamstrahlung. The pair creation probability is shown to be orders of magnitude larger than the corresponding incoherent processes. The energy spectrum is also calculated where the effective threshold energy is shown to be inversely proportional to the beamstrahlung parameter Υ. Implications of this effect on future ${\mathrm{e}}^{+}$${\mathrm{e}}^{\mathrm{-}}$, eγ, and γγ linear colliders are discussed.