A new foilless diode has been developed that does not require an external magnetic field to control beam expansion of an intense relativistic electron beam (IREB). An ionized channel, formed with use of a laser, extends through the transport region and terminates at the cathode of an IREB accelerator. When a negative potential is applied to the cathode, channel electrons are expelled leaving a positive core that electrostatically attracts and focuses the emitted electrons. A 3.6-MeV, 25-kA electron beam has been generated and transported 1.3 m by use of this technique.

Chemical dissolution of a two-dimensional porous medium by a reactive fluid which produces fractal patterns is studied. A new model is proposed that introduces a cumulative erosion process which broadens the branches of the cluster and gives structures very similar to the experimental ones. Diffusion-limited aggregation is a limiting case of this model. An interpretation of the evolution of the injection pressure with time which yields the fractal dimension is also presented, and agrees with the experimental results.

We report the observation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating, intense, pulsed ${\mathrm{CO}}_2$ laser beams, operating at different frequencies in a stimulated Compton scattering scheme. The interaction takes place in the nonlinear (trapping) regime, and its physics is the same as in laser accelerators and efficiency-enhanced free-electron lasers with long wigglers. Two different mechanisms of enhanced energy transfer were observed—electron trapping and phase-area displacement.

We report the first direct observation of electron inelastic scattering from the pondermotive potential of an intense laser pulse in vacuum. Electrons gained up to 0.2 eV when scattered from the temporal leading edge of a 1064-nm (1.165 eV), 140-psec laser pulse, and lost comparable amounts of energy when scattered from the trailing edge. When directed to the most intense part of the pulse, they were deflected out of the beam.

The dynamics of the interaction of a single Rydberg atom with a single mode of an electromagnetic field in a superconducting cavity was investigated. Velocity-selected atoms were used and the evolution of the atomic inversion as atom and field exchange energy was observed. The quantum collapse and revival predicted by the Jaynes-Cummings model were demonstrated experimentally for the first time. The evaluation of the dynamic behavior of the atoms allows us to determine the statistics of the few photons in the cavity.

The four-wave-mixing interactions produced by pumping at two well-separated frequencies can couple four different radiation modes to generate a new kind of squeezed state of light. A dual-frequency homodyne detection scheme is described and is used to observe this type of nonclassical correlation. Initial experimental results in an optical fiber show a noise level 20% below the vacuum noise level for the output of the dual-frequency detector, even though the noise at each individual detector remains above the vacuum level.

We report on the dependence of enhanced backscattering of light in a random medium on sample thickness, mean free path, and size of scattering centers. Introduction of a new difference technique allows for independent observation of contributions from long and short light paths in a suspension of polystyrene spheres. A new anisotropy effect, which has been found to be most prominent for the smallest particles studied, could be classified as a lower-order multiple-scattering phenomenon.

The competition between the bunching force of the ponderomotive potential and the repulsive force of the self electrostatic potential in a tapered free electron laser limits the beam density, deceleration rate due to tapering, and the local electric field spatial exponentiation rate. The limit on the spatial exponentiation rate is below that predicted by linear theory at the transition from the Compton to Raman regimes. Furthermore, this limit is independent of the form of the particle energy distribution function.

The formation of laser-driven shocks in fused silica was observed from shock trajectory measurements. The result reveals an anomalously slow buildup of the strong shock which suggests a sig- nificant volume change at pressures ≳ 1 Mbar. Nonsteady propagation of the strong shock was also observed for a transient period immediately following its formation.

The general form of the field correlations within a fluctuating homogeneous medium is determined in terms of the corresponding source correlations. Under very general conditions, the degree of spectral coherence has a universal form, independent of the source, if the fluctuations are isotropic. It they are anisotropic, the degree of coherence at separations that are large compared with the wavelength directly yields the generalized dynamical structure factor of the medium.

The transition to turbulence via spatiotemporal intermittency observed in a partial differential equation displays statistical features typical of critical phenomena. Analogy with directed percolation is drawn.

We discuss the onset of convection in viscoelastic liquids. We find for the first time that for some value of the parameters the tricritical point of one of the instability branches coincides with the codimension two point. Such a phenomenon seems to have no analog in systems close to thermodynamic equilibrium. In addition we predict that the oscillatory instability sets in as finite-amplitude traveling waves for most Prandtl numbers.

Considerable progress has been made in the experimental observation of Anderson localization and quantum eigenstate coherence for electrons in disordered solids. The coherence effects deduced from conductivity measurements require some corrections for inelastic scattering, and often the necessary information is imprecisely known. In this letter we report detailed observations of inelastic effects in an acoustic localization experiment where parameters may be precisely controlled and measured; some phase correlation effects are evident.

A semiclassical approach is used to derive, and extend to first order in g-2, the equilibrium degree of polarization in high-energy electron storage rings (the Derbenev-Kondratenko formula). Statistical concepts are shown to be essential for an understanding of this phenomenon. In so doing, some aspects of the polarization mechanism not previously recognized are uncovered.

An induction accelerator system has been successfully operated to postaccelerate an intense pulsed ion beam that is highly space-charge and current neutralized. An annular ion beam (energy 90 keV, current 5.7 kA, pulse width 750 nsec) extracted from an applied-${\mathrm{B}}_{\mathrm{r}}$ magnetically insulated diode is injected into the induction accelerator, where a short pulse (210 kV, 50 nsec) is applied. Measurements of beam energy by a Thomson-parabola spectrometer before and after the postacceleration have confirmed the increase in beam energy of ${\mathrm{H}}^{+}$ up to ∼240 keV after being postaccelerated.

We consider the effect of a spatially periodic forcing at the onset of one-dimensional periodic patterns in nonequilibrium systems. It is shown that the competition between periodicities can lead to a commensurate-incommensurate transition mediated by solitons as in equilibrium systems. This result is in agreement with a recent experiment.

A new phenomenon is reported in which light satisfying a Bragg-type resonance condition gets strongly reflected by a weak coarse-period (tens of microns) superlattice that is superimposed on a fine-period (submicron) stratified medium. A couple-wave theory is developed to describe this interaction.

By means of the Poincaré-sphere representation, new exact solutions for the evolution of the polarization in anisotropic crystals exhibiting a third-order nonlinearity and subjected to a dc electric field are found. This allows us to discover the existence of bifurcations of the nonlinear eigenpolarizations. A stability analysis of the eigenpolarizations shows how the intrinsic instabilities evolve in the interaction between self-induced and dc-field–induced birefringence.

The experimental observation of the behavior of acoustically rotated free drops in the microgravity environment of low earth orbit has yielded the first data which meet the restrictions of existing theoretical treatments. Rotating drops displayed both ‘‘axisymmetric’’ and ‘‘two-lobed’’ shapes in the presence and absence of applied torques. Their dimensions and general development confirm the existence of the distinct shape families and the bifurcation point between them. The data also indicate that the secular instability in the axisymmetric family occurs at a lower rotation velocity than predicted.

Because of relativistic nonlinear effects large amplitude cyclotron motion of a free electron can be excited by two laser beams with frequencies much higher than the cyclotron frequency Ω. The laser frequencies can differ either by Ω (three-photon resonance) or 2Ω (four-photon resonance). The excited cyclotron motion displays a hysteretic resonance based solely on the relativistic mass effect. To observe three-photon hysteretic excitation, a He-Ne laser with a few microwatts of power is sufficient.