L. A. Collins

Watching Dark Solitons Decay into Vortex Rings in a Bose-Einstein Condensate

We have created spatial dark solitons in two-component Bose-Einstein condensates in which the soliton exists in one of the condensate components and the soliton nodal plane is filled with the second component. The filled solitons are stable for hundreds of milliseconds. The filling can be selectively removed, making the soliton more susceptible to dynamical instabilities. For a condensate in a spherically symmetric potential, these instabilities cause the dark soliton to decay into stable vortex rings. We have imaged the resulting vortex rings.

Dark-soliton states of Bose-Einstein condensates in anisotropic traps

Dark soliton states of Bose-Einstein condensates in harmonic traps are studied both analytically and computationally by the direct solution of the Gross-Pitaevskii equation in three dimensions. The ground and self-consistent excited states are found numerically by relaxation in imaginary time. The energy of a stationary soliton in a harmonic trap is shown to be independent of density and geometry for large numbers of atoms. Large-amplitude field modulation at a frequency resonant with the energy of a dark soliton is found to give rise to a state with multiple vortices. The Bogoliubov excitation spectrum of the soliton state contains complex frequencies, which disappear for sufficiently small numbers of atoms or large transverse confinement. The relationship between these complex modes and the snake instability is investigated numerically by propagation in real time.

Evolution of ultracold neutral plasmas.

We present the first large-scale simulations of an ultracold neutral plasma, produced by photoionization of laser-cooled xenon atoms, from creation to initial expansion, using classical molecular-dynamics methods with open boundary conditions. We reproduce many of the experimental findings such as the trapping efficiency of electrons with increased ion number, a minimum electron temperature achieved on approach to the photoionization threshold, and recombination into Rydberg states of an anomalously low principal quantum number. In addition, many of these effects establish themselves very early in the plasma evolution ( similar ns) before the present experimental observations begin.

Molecular dynamics simulations of shocked benzene

The behavior of benzene at high temperatures and pressures is studied using nonequilibrium molecular dynamics. The interatomic forces were generated using linear-scaling tight-binding electronic structure theory on systems containing 128 and 576 molecules. The shock Hugoniot, calculated directly from the simulations without predetermining the equation of state, is compared with experiment. Piston velocities of 4 km/s or greater result in a pressure-induced polymerization. This transition is consistent with the bend in the experimental measurements of shock versus piston velocity.

Recent Developments in the Theory of Electron Scattering by Highly Polar Molecules

Publisher Summary This chapter summarizes recent work in the theory of electron collisions as applied to highly polar molecules. In the discussion of both theory and calculations presented in the chapter, the emphasis is on elastic scattering and rotational excitation; however, vibrational excitation is also discussed briefly. The chapter presents a general overview of the unique features of scattering by polar molecules with a view to motivating and providing direction for the discussion to follow. The chapter is also devoted to a synthesis of the formal theoretical hardware, to numerical techniques that are essential to scattering calculations, and to an adequate description of the interaction potential. The chapter emphasizes on the approaches that employ the most powerful and general techniques. Discussions and comparisons of some of the recent applications of this machinery, the various approximations made thereto, and the general physical picture thus developed are presented in the chapter.

First-principles equation of state of polystyrene and its effect on inertial confinement fusion implosions

Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial confinement fusion (ICF) capsules using CH/CH-based ablators. With first-principles calculations, we have investigated the extended EOS of CH over a wide range of plasma conditions (ρ=0.1to100g/cm(3) and T=1000 to 4,000,000 K). When compared with the widely used SESAME-EOS table, the first-principles equation of state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of cryogenic target implosions on OMEGA using the FPEOS table of CH have predicted ∼30% decrease in neutron yield in comparison with the usual SESAME simulations. This is attributed to the ∼5% reduction in implosion velocity that is caused by the ∼10% lower mass ablation rate of CH predicted by FPEOS. Simulations using CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered light from ICF implosions.

Average atom transport properties for pure and mixed species in the hot and warm dense matter regimes

The Kubo-Greenwood formulation for calculation of optical conductivities with an average atom model is extended to calculate thermal conductivities. The method is applied to species and conditions of interest for inertial confinement fusion. For the mixed species studied, the partial pressure mixing rule is used. Results including pressures, dc, and thermal conductivities are compared to ab initio calculations. Agreement for pressures is good, for both the pure and mixed species. For conductivities, it is found that the ad hoc renormalization method with line broadening, described in the text, gives best agreement with the ab initio results. However, some disagreement is found and the possible reasons for this are discussed.

Attosecond Two-Photon Interferometry for Doubly Excited States of Helium

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible noncoincident single-electron spectra. We analyze the interferometric signal in terms of a semianalytical model which is validated by a numerical solution of the time-dependent two-electron Schrödinger equation in its full dimensionality.

Quantum Molecular Dynamics calculations of radiative opacities

We show that Quantum Molecular Dynamics provides a powerful tool to extend and benchmark current opacities libraries into the complex regime of warm dense matter. In this regime, the medium can be constituted of electrons, protons, atoms and molecules, while plasma and many-body effects can not be treated as perturbations. Among the most notable features of this new approach for calculating Rosseland mean opacities is the ability to obtain a consistent set of material (equation-of- state), optical and electrical properties for various mixtures from the same simulation.

Electronic excitation of the b 3Σu state of H2 by electron impact in the linear algebraic approach

The authors have performed two-state close-coupling calculations on the X 1 Sigma g to b 3 Sigma u transition for e-H2 scattering within the linear algebraic, effective optical potential approach. The strong orthogonality constraint between bound and continuum orbitals is relaxed by including correlation-type configurations. These correlation terms prove to be very important in the 2 Sigma g and 2 Sigma u symmetries and lead to an increase in the total cross section of a factor of two over earlier approaches, which neglected these terms. The authors obtain good agreement with recent R-matrix and Schwinger variational calculations.