Search Results (6 total)

Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. We model this interaction using a coordinate system that expands with the rapidly diffracting optical mode from the ends of the undulator to the mirrors. Simulations show that the interaction of the strongly focused optical mode with a narrow electron beam inside the undulator distorts the optical wave front so it is no longer in the fundamental Gaussian mode. The simulations are used to study how mode distortion affects the single-pass gain in weak fields, and the steady-state extraction in strong fields.

The vortex structure of immiscible liquid-liquid Taylor-Couette-Poiseuille flow was studied using photographic techniques. Several parameters were considered, including the feed composition and the inner cylinder rotation rate. For certain feed compositions and sufficiently large rotation rates a translating banded structure, which consisted of alternating aqueous and organic-rich vortices, persisted indefinitely. At lower rotation rates, either a spatially homogeneous emulsion evolved or sustained oscillations between the banded and homogeneous structures developed.

Hierarchical mean-field rate equations and lattice-gas simulations were developed to elucidate the effects of the breakdown of the mean-field approximation for a model heterogeneous chemical oscillator that represents a simple extension of the well-known monomer-dimer surface reaction model. The bifurcation structure of the reaction kinetics depends sensitively on the details of surface transport processes, and the oscillatory behavior exhibited by the site approximation rate equations is not generally robust with respect to spatial correlations. © 1996 The American Physical Society.

The random sequential adsorption of parallel (aligned) squares is studied by computer simulation. A new precise value of the maximum (jamming) coverage is found: θ_J=0.562 009±0.000 004. The dynamics for times t≳6000 agree with Swendsen’s prediction θ_J-θ(t)∼c(lnt)/t. Various two-point correlation functions are measured, and the effects of the finite size and the discretization of the adsorbing surface are also investigated.

We introduce a model of coagulation with single-particle breakoff, described by the kernels K_ij=ij and F_ij=α((j+1)δ_i1+(i+1)δ_j1). For α above a critical value α_c, the system either gels or reaches a steady-state size distribution, depending upon initial conditions. Below α_c, gelation always occurs. At α=α_c, the scaling exponent τ, which describes the large-size behavior of the steady-state size distribution, is frac72; rather than the usual value frac52;, indicating that this process belongs to a new universality class of gelation.