For a model stationary flow with hexagonal symmetry, we study the recurrence-time statistics. This model flow has been shown elsewhere to provide a sharp transition from normal to anomalous transport. We show here that this transition from normal to anomalous transport is accompanied by a corresponding change of the recurrence-time statistics from ‘‘normal’’ to ‘‘anomalous.’’ In the anomalous case the distribution of recurrence times has a power tail. Recurrence-time statistics provide a local measurement to make evident the existence of anomalous transport.

We present experimental studies of the transitions between conduction, hexagons, and rolls in non-Boussinesq convection of gaseous ${\mathrm{CO}}_2$ in a cylindrical cell of radius-to-height ratio 86. Except for the transition from conduction to hexagons, transitions occur when the two states involved have nearly the same value of a generalized potential rather than at the stability limits. Conduction gives way to hexagons via the propagation of a front connecting the two states, while the transitions between hexagons and rolls are facilitated at the cell walls which appear to nucleate the minority state.

We report on a new regime of free-electron-laser operation with reversed axial guide magnetic field, in which the rotational motion of the electrons in the helical wiggler field is opposed by the presence of the uniform guide field. The 33.3-GHz free-electron-laser amplifier is driven by a mildly relativistic electron beam (750 kV, 300 A, 30 ns) and generates 61 MW of radiation with a 27% conversion efficiency. Measurements with the conventional orientation of the axial field show a considerable loss of power and efficiency.

We study the physics of a free-electron laser based on a ‘‘two-frequency’’ undulator (TFU) which induces large nonlinear effects, especially on the spectral dynamics. These effects are analyzed in an extended formalism where the spontaneous emission, the low-gain regime, and the strong-field saturation regime are studied. Numerical simulations show that an optimized TFU generates a laser field having both a large extraction efficiency and a narrow spectrum.

Stochastic motions of parametrically pumped electron oscillators synchronize abruptly when the pump power exceeds a threshold and the oscillators are radiatively cooled in a cylindrical Penning trap. Self-organized collective behavior far from thermal equilibrium is manifested by the coherent, phase-bistable motion of the electrons’ center of mass, which was observed. Synchronized electrons are an ideal probe of the radiation field of a trap cavity, opening the way to a new generation of electron magnetic-moment measurements and to sideband cooling of an electron motion to mK temperatures.

We model fluid-fluid displacement in d=2 by a diffusion-limited-aggregation (DLA) algorithm which takes random capillary forces into account. Interpore surface tension is neglected. The invading fluid is nonviscous. We find a crossover length ${\mathit{L}}_{\mathit{c}}$. On length scales much smaller (larger) than ${\mathit{L}}_{\mathit{c}}$, invasion percolation (DLA) patterns are obtained. We argue by scaling, and check by simulations, that ${\mathit{L}}_{\mathit{c}}$∼(Δp̃/Ca${)}_{\mathit{s}}^{2/(2+\mathit{D}}$), Δp̃ stands for a measure of spatial variations of the capillary pressure, Ca is the capillary number, and ${\mathit{D}}_{\mathit{s}}$ is the interface fractal dimension on small length scales (we find ${\mathit{D}}_{\mathit{s}}$=1.3).

Coherent transition radiation, generated by the passage of mm-long bunches of electrons through a thin metal foil, has been observed in the far infrared. The intensity was compared to the smaller amount of Cherenkov radiation produced when ${\mathrm{SF}}_6$ gas was introduced into the electron path. Wake-field radiation was ruled out by varying the geometry of the metal chamber near the electron beam. The intensity, the polarization, and the spectral and angular distribution of the transition radiation were measured and its coherent nature was identified.

It is shown that photons in a polar crystal with a high nonlinearity can feel an attractive effective interaction by exchange of virtual phonons. A coherent state is unstable with respect to such an interaction, and a superguiding state is formed through the association of photons in pairs with opposite wave vectors and spins. The photon pairs travel without scattering attenuations. If the photons propagating in a wave guide enter the superguiding state, the wave guide exhibits an ultralow energy loss.

The static and dynamic frictional forces of single-crystal muscovite mica are measured as a function of the lattice misfit angle between the two contacting cleavage lattices at a very light load under both dry and ambient atmospheres. The frictional forces are anisotropic with respect to the lattice misfit angle, i.e., they increase (decrease) when the contacting surfaces approach being commensurate (incommensurate). The underlying mechanisms of the anisotropy are discussed in terms of the commensurability between the contacting lattices.

We predict, on the basis of a model calculation, that Fano interference effects may be observed in type-II quantum-well structures resulting in pronounced nonreciprocal absorption and emission profiles and interference narrowing due to overlapping resonances.

The transport of fluid particles on three-dimensional standing surface waves is studied by the inviscid irrotational theory. When two internally resonant modes are excited simultaneously, a second-order vortical drift velocity emerges on the surface proportional to the angular momentum. Particles can move periodically, quasiperiodically, or chaotically over a wide distance, depending on the size of the container. The theory presented is applied to the smallest system, having a transport effect that has been discovered in capillary-wave experiments.

A theory is presented for the time-dependent breakdown of a network of spring (fuse) elements where the probability of breaking an element under load σ is ${\mathrm{\sigma }}^{\mathrm{\eta }}$. For all η, it predicts the system-size scaling of the number of broken elements at breakdown found in simulations. The breakdown is shown to be percolationlike for η≤2 but is due to the dominance of one large growing crack, despite the absence of a failure threshold, for η>2. This transition in fracture behavior and in scaling at η>2 is found to be directly related to the dependence of crack tip stress enhancement on the square root of crack size.

We present the results of a new calculational technique that evaluates the beam-beam force due to an arbitrary charge distribution. We find coherent instabilities that dominate at certain operating points and depend strongly on the degree of damping in the system. We conclude that while these resonances may play a significant role for colliders with low damping, with a careful choice of operating points they are unlikely to present a danger to colliders with greater damping, such as the new generation of high-luminosity heavy-quark factories currently under design.

The ${\mathit{k}}^{\mathrm{-}3}$ spectrum, including the logarithmic correction, for the enstrophy range and the ${\mathit{k}}^{\mathrm{-}5/3}$ spectrum for the energy range of two-dimensional turbulence are derived using the renormalization-group method. The values of the Kolmogorov constants C’≃1.59 in the enstrophy range and C≃6.45 in the energy range are obtained without the need of adjustable parameters.

We have observed coherent patterns of the magnitude of the normalized vortex stretching vector ‖ω⋅∇u‖/‖ω‖ during the ‘‘bursting’’ reconnection phase of the evolution of identical initially orthogonally offset vortex tubes. We propose that the intensification and reconnection of initially tubelike vortex regions be viewed as a scattering process and that the topology of vortex lines during this phase of the process contributes to limiting vortex stretching (intensification). Furthermore, we advocate the use of these patterns (some rendered in color) as signatures of turbulent intermittency in exploring massive data sets.

We report heat transport measurements and optical shadowgraph visualization of rotating Rayleigh-Bénard convection. For dimensionless rotation rates 140<Ω<4300, the initial transition to convection, occurring at a Rayleigh number R much less than the linear-stability value for roll or vortex states, is a forward Hopf bifurcation to an azimuthally asymmetric state with mode number n. States with n=3, 4, 5, 6, and 7 exist at low to moderate R and precess with frequencies that depend on R and Ω. At higher R there is a continuous transition to a state with noisy, time-dependent heat transport, a distinct array of vortices in the central region, and a modulation of the precession speed of the outer structures.

Using the multifractal approach, we derive the probability distribution function (PDF) of the velocity gradients in fully developed turbulence. The PDF is given by a nontrivial superposition of stretched exponentials, corresponding to the various singularity exponents. The form of the distribution is explicitly dependent on the Reynolds number. The experimental data are in good agreement with the PDF predicted by the same random beta model used to fit the scaling of the velocity structure functions.

Using an acoustic technique profile analysis we have studied the growth of viscous fingers inside 3D porous media. The experimental data support the definition of an instability parameter which captures the essential features of the viscous fingering; its dependence on the viscosity ratio, on the flow rate, and on the nature of the porous medium is compared to available theory. Our data demonstrate the predicted crossover between diffusive and linear growth and the growth enhancement due to coupling between large viscosity ratio and velocity-dependent hydrodynamic dispersion.

Fluid-displacement experiments in Hele-Shaw cells filled with a viscoelastic fluid show a novel transition between a viscous fingering (VF) regime producing fractal patterns of ‘‘fingers’’ and a viscoelastic facturing (VEF) regime producing fractal patterns of ‘‘cracks.’’ VEF patterns are characterized by branching angles of 90° with respect to the main crack, behind the tip, and by a lower fractal dimension than VF. The transition is controlled by several parameters, including the Deborah number and the system deformability.

We have measured autocorrelation functions for light scattered from capillary waves at the interface in a near-critical binary liquid mixture both at two-phase equilibrium and after abruptly raising the temperature to drive the system toward one-phase equilibrium. Use of the capillary-wave dispersion relation allows the extraction of an effective ‘‘surface tension.’’ This surface tension represents a dynamical integrity of the nonequilibrium interface which may affect pattern formation when the interface is driven.