Mechanisms of parasitic wave excitation in a high-current Raman-regime free-electron laser (FEL) are investigated by use of a one-dimensional multimode FEL amplification code. It is found that multimode coupling among sideband waves and the primary wave plays an important role in the parasitic wave excitation in addition to the sideband instability. Such a multimode coupling process is observed in both the linear amplification stage and the nonlinear trapping stage, and modulates the output signal to a level comparable with that solely due to the sideband instability.

The macrotemporal structure of a storage-ring free-electron laser is studied in relation to deterministic chaos laws. Comparison between experimental and theoretical results shows, for the first time, that the laser is determined by these laws. This can have important consequences for the future operation of free-electron lasers.

We examine optical phase conjugation for a shaped-microparticle suspension maintained in a uniform electric field. Theory asserts that the presence of a Stark field decreases the grating response time and enhances the composite’s four-wave-mixing coefficient. These changes arise from and reflect the narrow angular range in which the particles are constrained to point, giving rise to reduced rotational times and the appearance of a new coherent scattering mechanism.

A heurisitic model for evolution of the probability distribution (PDF) of transverse velocity gradient s in incompressible Navier-Stokes turbulence is distilled from an analytical closure for Burgers turbulence. At all Reynolds number scrR, the evolved PDF is ∝‖s${\mathrm{\Vert }}^{\mathrm{-}1/2}$ exp(-const×‖s‖/〈${\mathit{s}}^2$${\mathrm{〉}}^{1/2}$) for large ‖s‖. The model suggests that skewness and flatnesses are asymptotically independent of scrR, and that cascade to smaller scales is not a fractal process. For Burgers dynamics, both simulations and the analytical closure give a PDF ∝‖ξ${\mathrm{\Vert }}^{\mathrm{-}1}$ exp(-const×‖ξ‖/〈${\xi }^2$${\mathrm{〉}}^{1/2}$) for large negative velocity gradient ξ.

Microwave phase conjugation was observed in a liquid suspension of elongated microparticles via degenerate four-wave mixing in a collinear waveguide geometry. Longitudinal phase corrections of a double-passed phase shifter were demonstrated by interferometry. Nondegenerate four-wave mixing was demonstrated with a FWHM bandwidth of 70 MHz. The 10% scattering from the induced grating indicates that high conversion efficiency and gain can be achieved with higher pump power.

The Ni-like 4d-4p laser scheme has been extended to wavelengths near the K absorption edge of carbon. A gain of 2.3 ${\mathrm{cm}}^{\mathrm{-}1}$ with a duration of 250 psec was observed in Ni-like Ta at 44.83 Å (a wavelength close to optimal for holographic imaging of live cells). Ni-like W produced a gain of 2.6 ${\mathrm{cm}}^{\mathrm{-}1}$ with a total of seven gainlengths of amplification at 43.18 Å. This is the first demonstration of an x-ray amplifier on the short-wavelength side of the carbon K edge, within the ‘‘water window.’’ Both lasers should be scalable to coherent power sufficient for holographic imaging and other applications.

The task of electrical-impedance tomography is to invert boundary measurements for the conductivity distribution of a body. This inverse problem can be formulated so the primary data are the measured powers dissipated across injection electrodes. Then, since these powers are minima of the pertinent (dual) variational principles, feasibility constraints can be found for the nonlinear inversion problem. When power may be measured accurately, the existence of these dual variational principles implies that any exact solution must lie at a point of intersection of the two feasibility boundaries.

Spatiotemporal dynamics of multiple coupled laser systems are predicted as an example of distinct phenomena in high-dimensional nonlinear systems far from thermal equilibrium. Generic dynamics, including a self-induced phase turbulence which results in emitter partition noise, chaotic itinerancy between ruins of local attractors (supermodes) resulting from phase hopping, as well as superslow relaxation into equilibria, are found. It is shown theoretically that such inherent dynamics result from the destruction of phase locking of multiple emitters due to population dynamics.

We show that the Marangoni effect drives the fingering instability observed at the edge of an aqueous surfactant drop spreading on a thin film of water. A calculation of the unperturbed flow profile demonstrates that the spreading of the drop is controlled by the dynamics of a thin layer which develops in front of the drop. The surface-tension gradient in this region leads to the fingering instability via a mechanism mathematically similar to that in Hele-Shaw flow despite the very different underlying physics.

We report first evidence in support of chaotic stimulated Brillouin scattering under cw pump conditions involving a single Stoke and pump signal. A single-mode optical fiber is used as the nonlinear medium. The inclusion of external optical feedback modifies the form of the chaotic dynamics and results in a rich variety of classifiable precursor dynamical features.

The large-scale, long-time properties of a compressible fluid stirred by a Gaussian random force with correlation 〈${\mathit{f}}_{\mathit{i}}$${\mathit{f}}_{\mathit{j}}$〉∝${\mathit{k}}^{\mathrm{-}\mathit{y}}$ are investigated. It is shown that when ε=4+y-d>3 (d=space dimension) the effective sound velocity becomes scale dependent in the limit k→0 and the fluid obeys a universal equation of state. The effective Mach number is also scale dependent, reaching a fixed-point value ${\mathit{Ma}}^{\mathrm{*}}$(k)<1 when k→0. The predictions of the theory are compared with the results of direct numerical simulations.

We present experimental evidence for localization of acoustic waves in a novel three-dimensional system. The system consists of a polymer melt solidifying by the growth of spherical semicrystalline nuclei. For sufficiently high excitation frequencies, we find renormalization of sound speed and intense absorption peaks over very narrow ranges of wave number. We generalize theory to include shear modes in the scatterers, and find that localization is consistent with our observations.

We show that a semiconductor laser switched from below to above threshold by a gradual increase of the optical gain, which follows a stepwise change of the injected current (gain switching), exhibits, besides a random distribution of the turn-on times, peculiar features in the time-intensity evolution. In particular, we find that the switching trajectories in the time-intensity plane cluster under a line at a nonzero angle off the time axis. This new effect can be used to discriminate transient fluctuations due to spontaneous emission against those arising from other effects.

A new method of measuring a coherence length in weakly turbulent systems at the onset of spatiotemporal intermittency is suggested. Our method is based on direct measurement of the distributions of the velocity and its spatial derivatives. We consider a coupled circle-map lattice, identify a coherence length, and determine its scaling properties. We also connect the coherence length to the Lyapunov exponent. The second-order phase transition observed when a cutoff is introduced is described in terms of percolation theory.

We present a variational calculation of free-electron-laser gain the exponential regime before saturation using a dispersion relation incorporating the energy spread, emittance, and focusing of the electron beam, and the diffraction and guiding of the radiation. Rapid computation facilitates free-electron-laser design optimization. Results are expressed in a universal scaled form.

Axial and circular birefringence effects are introduced which conserve parity and time-reversal invariance, and which are induced by time-varying electric fields and electromagnetic fields. In each case the symmetry of the complete experiment is considered, and equations for the birefringence are given in terms of molecular-property tensors of the ensemble.

We report experimental evidence that, beyond the transition for spatiotemporal chaos, Rayleigh-Bénard convection displays thermodynamic properties in Fourier space. It is shown that a generalized temperature may be defined using the natural fluctuations of the system, and that a suitable free energy accounts for the experimental results.

We overcame the strong Gγ=-3+${\nu }_{\mathit{y}}$ intrinsic depolarizing resonance using a Siberian snake acting on a stored beam of 177-MeV polarized protons at the Indiana University Cooler Ring. We also saw the first evidence for a synchrotron depolarizing resonance in a proton ring. Two synchrotron resonances were studied by varying the rf accelerating voltage and the imperfection field at 104 MeV. The Siberian snake also overcame these synchrotron depolarizing resonances.

We describe the first investigation of the nonclassical radiation field of a micromaser. The maser cavity is cooled to 0.5 K to exclude thermal photons and has a quality factor 3×${10}^{10}$. Velocity-selected Rydberg atoms pump the maser. The photon statistics in the cavity are measured via the statistics of the atoms leaving the cavity in the lower maser level. Taking into account a detection efficiency of 10%, their fluctuations are reduced up to 40% below the Poisson level. This corresponds to photon-number fluctuations in the cavity of up to 70% below the vacuum-state limit.

We have measured the correlation in frequency-dependent intensity fluctuations in the total transmission through random dielectric samples, using visible light in a (essentially) plane-wave geometry. The correlation function, of which the width at half maximum is proportional to ${\mathit{L}}^{\mathrm{-}2}$ (L is the thickness of the sample), decays as (Δω${)}^{\mathrm{-}1/2}$. This constitutes the experimental proof for the buildup of long-range intensity correlations in the propagation of classical waves by multiple scattering.