A theoretically predicted instability of the ion-focus–regime guiding of intense relativistic electron beams, known as the ion hose instability, has been observed in an experiment. A 1.5-μs, 1.2-kA, 2.2-MeV electron beam has been successfully injected into and propagated down a 21-m preionized channel. Off-axis beam oscillations have been found to grow and then saturate at large amplitude. Observed growth times, real frequencies, and saturated amplitudes are in agreement with theoretical scaling predictions and with numerical simulation of the ion hose instability.

We report on molecular-dynamics simulations of the low–Reynolds-number flow of Lennard-Jones fluids through a channel. Application of a pressure gradient to a single fluid produces Poiseuille flow with a no-slip boundary condition and Taylor-Aris hydrodynamic dispersion. For an immiscible two-fluid system we find a (predictable) static contact angle and, when accelerated, velocity-dependent advancing and receding contact angles. The approximate local velocity field is obtained, in which the no-slip condition appears to break down near the contact line.

We find experimentally that in the forward direction the length scale of quasiballistic photons dominates the dynamic intensity-intensity correlation function of light multiply scattered from randomly diffusing spheres. This implies a breakdown of the photon-diffusion approximation, which is characterized by a single length, the transport mean free path. We suggest why this breakdown occurs, introduce phenomenologically a dynamic correlation length as an additional length scale, and with this extension reconcile the theory with experiment.

We obtain useful information from the intensity autocorrelations of light scattered from systems which exhibit strong multiple scattering. A phenomenological model, which exploits the diffusive nature of the transport of light, is shown to be in excellent agreement with experimental data for several different scattering geometries. The dependence on geometry provides an important experimental control over the time scale probed. We call this technique diffusing wave spectroscopy, and illustrate its utility by studying diffusion in a strongly interacting colloidal glass.

The wake of an oscillating cylinder at low Reynolds numbers is a nonlinear system in which a limit cycle due to natural vortex shedding is modulated, generating in phase space a flow on a torus. We experimentally show that the system displays Arnol’d tongues for rational frequency ratios, and approximates the devil’s staircase along the critical line. The ‘‘singularity spectrum’’ as well as spectral peaks at various Fibonacci sequences accompanying quasiperiodic transition to chaos shows that the system belongs to the same universality class as the sine circle map.

The stability of Taylor-Couette flow is studied as a function of the angular velocities of the inner and outer cylinders, ${\mathrm{\Omega }}_1$ and ${\mathrm{\Omega }}_2$=ε${\mathrm{\Omega }}_1$ cos(ωt), respectively. Contrary to the theoretical prediction, outer cylinder modulation stabilizes the flow. The critical Reynolds number, ${\mathrm{Re}}_{1\mathrm{c}}$, is a function of the dimensionless frequency, γ=(ω${\mathrm{d}}^2$/2ν${)}^{1/2}$, where d is the gap size and ν is the kinematic viscosity, and a monotonically increasing function of ε over the range examined: 0.3≤ε≤2.0. At larger ε stabilization is spectacular, ${\mathrm{Re}}_{1\mathrm{c}}$ reaching a value nearly double the value for stationary outer cylinder.

By use of the recently proposed model of the dynamics of a macromolecule in transient extensional flows, the effective viscosity increase in turbulence due to unraveling macromolecules is calculated, and a relation between this viscosity increase and a drag reduction parameter (the slope increment) is derived. The predictions of the theory are in good agreement with experiment.

We present a complete first-order quantum electrodynamics calculation of the spin polarization in a relativistic electron storage ring. The result differs from previous semiclassical calculations of other authors in several respects. Under ideal conditions a maximum polarization of 99.2% may be obtained.

The Karman vortex street in the wake of a circular cylinder is shown to be due to an absolute instability of the flow in the near wake. A new means of instability analysis is used, involving mappings from the complex k plane to the complex ω plane.

We present an analytical solution to the problem of pulse backscattering from a randomly stratified half-space. It is shown that the power spectrum μ for the backscattered wave is characterized by a function of χ, where χ=(distance traveled by the pulse at time τ)/(the frequency-dependent localization length). For the matched-impedance and the total-reflection boundary conditions, μ is given by χ/(1+χ${)}^2$ and 4χ, respectively. Implications for the time-domain measurement of the localization length are discussed.

The decimated-amplitude scheme (DAS) devised by Kraichnan is applied to a random-coupling model of turbulence originally introduced by Betchov to test the direct-interaction approximation (DIA). Using a system of 32 variables, forced stochastically under appropriate statistical constraints, it is shown that the DAS can accurately represent the autocorrelation function of a full Betchov system of 96 variables. A comparison is also made between the DAS and the DIA.

We have observed subpicosecond electrical pulses to propagate on 5-μm coplanar transmission lines at velocities faster than the phase velocity in the underlying dielectric. This situation produces an electromagnetic shock wave in a manner similar to Cherenkov radiation and electro-optic Cherenkov radiation. Using time-domain spectroscopy we have measured the strong frequency-dependent loss of energy in the propagating electrical pulse due to this radiation.

A new approximation to the short-wavelength scattering amplitude from an impenetrable sphere is presented. It is uniform in the scattering angle and it is more accurate than previously known approximations (including Fock’s theory of diffraction) by up to several orders of magnitude. It remains valid in the transition to long-wavelength scattering. It leads to a new physical picture of diffraction, as tunneling through an inertial barrier.

Experiments with refractive-index–matched materials show that evaporation of a liquid from a porous medium produces a modified form of immiscible displacement. In this process the displaced fluid is removed from the pore structure by counterflow in the same pore channels as the displacing fluid. Pressure gradients that arise from fluid flow result in displacement fronts that, unlike conventional displacement fronts, are inherently stable irrespective of the relative viscosities of the fluids.

Unsteady fast dynamo action is obtained in a family of stretch-fold-shear maps applied to a spatially periodic magnetic field in three dimensions. Exponential growth of a mean field in the limit of vanishing diffusivity is demonstrated by a numerical method which alternates instantaneous deformations with molecular diffusion over a finite time interval. Analysis indicates that the dynamo is a coherent feature of the large scales, essentially independent of the cascade of structure to small scales.

Flow-visualization studies and heat-flux measurements during the early evolution of convection in a cylindrical container of aspect ratio 10 (radius/height) are reported. The heat current was ramped linearly, or modulated time periodically, from below to above the onset value. A novel sidewall design allowed pattern formation to occur independently of container geometry. In the earliest stages, the patterns were composed of irregularly arranged cells and varied randomly between experimental runs. The results demonstrate the importance of stochastic effects during the pattern evolution.

We compare the effects of finite thickness and absorption on the line shape of the coherent backscattering peak from a disordered medium. By termination of the coherent random walk of photons through confined goemetry and/or absorption, contributions of different time-reversed paths to the line shape of the backscattering peak have been determined. Using circularly polarized light, we find good agreement between the experimental and calculated line shape of the helicity-preserving channel whose coherent backscattering peak closely resembles that of a scalar field.

A simple model is presented for the energy-cascading process in the inertial range that fits remarkably well the entire spectrum of scaling exponents for the dissipation field in fully developed turbulence. The scheme is a special case of weighted curdling and its one-dimensional version is a simple generalized two-scale Cantor set with equal scales but unequal weights (with ratio∼(7/3). This set displays all the measured multifractal properties of one-dimensional sections of the dissipation field.

The Lyapunov dimension of an incompressible, homogeneous, two-dimensional periodic shear flow with moderate resolution (from ${16}^{\mathrm{*}}$16 up to ${128}^{\mathrm{*}}$128 points) is calculated. The results show that the dimension of the chaotic attractor is much lower than the total number of degrees of freedom represented in the numerical system, and depends essentially on the scale at which energy is injected in the system, and not (or very weakly) on the Reynolds number, suggesting that only a fraction of the modes at a scale larger than that of the forced mode is turbulent.

Modulation instability that leads to breakup of intense cw radiation into a train of ultrashort pulses during propagation in optical fibers occurs only in the presence of anomalous group-velocity dispersion. It is shown that a new kind of modulation instability can occur even in the normal-dispersion regime when two copropagating optical fields interact with each other through cross-phase modulation initiated by the nonlinearity. The quantitative aspects of this cross-phase–modulation–induced modulation instability are discussed and illustrated by use of a realistic experimental example.