bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational energy transfer in vibrationally excited acetylene X̃ 1Σg(ν2″=1,J″):ΔJ propensities
Authors: Dopheide R. , Cronrath W. , Zacharias H.
Published: July 2002

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the Ã 1Au(ν3′ = 1)←X̃ 1Σg(ν2″ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.
A cage model of liquids supported by molecular dynamics simulations. I. The cage variables
Authors: Moro Giorgio J. , Nordio Pier Luigi , Noro Massimo , Polimeno Antonino
Published: July 2002

Stochastic cage models require a choice for the cage variables suitable to describe the restoring forces generated by the solvent on the solute. A set of cage variables is introduced from the parametrization of the cage potential which is defined as the solute–solvent interaction energy considered as a function of the solute position for a fixed solvent configuration. This is an operative definition of cage variables that allows their identification at each time step of molecular dynamics simulations. Therefore, quantitative information about the equilibrium properties and the dynamics of cage variables can be extracted from molecular dynamics simulations. This procedure is applied to liquid argon near the triple point, in order to recognize the different processes contributing to the cage diffusion. The equilibrium distribution and the characteristic correlation times are derived as ingredients for the stochastic cage model developed in part II of the work.
Theoretical study of the vibrational spectra of the transition metal carbonyls M(CO)6 [M=Cr, Mo, W], M(CO)5 [M=Fe, Ru, Os], and M(CO)4 [M=Ni, Pd, Pt]
Authors: Jonas Volker , Thiel Walter
Published: July 2002

The harmonic force fields of the title compounds have been calculated at the level of Hartree–Fock (HF) theory, Mo/ller–Plesset second-order perturbation theory (MP2), and gradient-corrected density functional theory (DFT) using all-electron and effective core potential wave functions in conjunction with polarized double- and triple-zeta basis sets. The DFT results are in very good agreement with the available experimental data, whereas the HF results are inadequate and the MP2 results are satisfactory only for the 5d and (partly) the 4d transition metal complexes, but not for the 3d transition metal complexes. The calculated DFT frequencies are accurate enough to suggest reassignments in the vibrational spectrum of Fe(CO)5. In the case of Ru(CO)5, Os(CO)5, Pd(CO)4, and Pt(CO)4 where experimental data are scarce, the DFT predictions may guide future experimental work.
Electronic structure of diatomic boron nitride
Authors: Lorenz M. , Agreiter J. , Smith A. M. , Bondybey V. E.
Published: July 2002

BN molecules, isoelectronic with C2, are produced by laser vaporization of boron nitride and trapped in solid neon. Through Fourier-transform absorption and laser-induced fluorescence spectra, three new electronic states are observed. Unlike C2, BN has a triplet ground state (X 3Π), but with an exceedingly low-lying singlet (a 1∑+) state with 15 cm−1<Te<182 cm−1.
Spinodal decomposition of colloids in the initial and intermediate stages
Authors: Dhont Jan K. G.
Published: July 2002

Spinodal decomposition of colloids in the initial and intermediate stages is described on the basis of the Smoluchowski equation. Hydrodynamic interaction is treated in an approximate way. In the intermediate stage, where a dominant length scale exists, the static structure factor is found to exhibit universal scaling behavior. The corresponding dynamic scaling function is derived from the nonlinear equation of motion for the static structure factor. This scaling function is a much more sharply peaked function of the wave vector than the well-known empirical Furukawa scaling function, which applies to the transition stage where sharp interfaces contribute considerably, giving rise to a Porod tail at larger wave vectors. The wave vector where the static structure factor exhibits a maximum is found to vary with time—in the intermediate stage—like ∼t−α, where the exponent α varies between 0.2 and 1.1, depending on the relative importance of hydrodynamic interaction. Experiments on a microemulsion and binary polymer melt confirm the predicted scaling behavior.
Time-resolved photoelectron angular distributions: A nonperturbative theory
Authors: Seideman Tamar
Published: July 2002

We describe a quantum mechanical framework for the calculation of angle- and energy-resolved photoelectron flux in pump probe scenarios. The theory spans the limits of weak-to-strong field-matter interaction range and the ladder of transitions to all angular momentum states. It allows insight into the type of information that can be extracted from time-resolved photoelectron distributions with regard to the wavepacket dynamics in the probed manifold. In the strong pump case it suggests the possibility of gaining insight also regarding the ionization dynamics, complementary to that extracted from weak-field CW studies. Various limits and several averaged observables are discussed and an efficient computational method is suggested.
Raman spectroscopy of supersonic jets of CO2: Density, condensation, and translational, rotational, and vibrational temperatures
Authors: Maté B. , Tejeda G. , Montero S.
Published: July 2002

The links between translational, rotational, and vibrational temperatures of supersonic molecular jets, and their density and degree of condensation, are discussed in terms of quantitative Raman scattering experimental data. Such links are established as the result of energy and momentum conservation laws, and of the collisional regime in the jet. Four representative supersonic expansions of CO2, generated at different stagnation pressures are analyzed, showing the potential of quantitative Raman spectroscopy for a complete characterization of the jet.
Solute-solvent pair distribution functions in highly asymmetric additive hard sphere mixtures
Authors: Henderson Douglas , Chan Kwong-Yu
Published: July 2002

Contact values for the solute-solvent pair distribution function in an additive hard sphere mixture, as computed from the Henderson-Chan (HC) formulas, are compared with the recent Monte Carlo (MC) data and formula of Matyushov and Ladanyi (ML) [J. Chem. Phys. 107, 5815 (1997)]. The agreement is found to be excellent. The negative finding of ML is due to a misprint in one of the HC publications. We find the HC formula to be superior to the ML formula when compared to our MC data [Mol. Phys. 91, 1137 (1997)] for the case where the ratio of diameters is large and the concentration of small spheres is small but not zero. However, more simulation data is needed.
Localized and extended electronic eigenstates in proteins: A tight-binding approach
Authors: Koslowski Thorsten
Published: July 2002

We present a numerical study of the electronic structure of the cytochrome subunit of the photoreaction center of the bacterium Rhodopseudomonas viridis. The electronic structure is described by a tight-binding Hamiltonian, the resulting large secular matrix is diagonalized applying sparse matrix methods. In the highly structured density of states, localized chromophore eigenstates can be observed around the HOMO-LUMO gap. In the low energy range, optical excitations originate from transitions between these states. Eigenfunctions that are extended on a length scale of up to ∼190 Å can be found in the bulk of the valence band density of states. Stretched along α-helices, conduction band eigenstates are characterized by a cylindrical shape. We discuss the role of these states within electron transport and possible extensions of the model presented here.
High order finite difference algorithms for solving the Schrödinger equation in molecular dynamics
Authors: Guantes Raul , Farantos Stavros C.
Published: July 2002

The view of considering global Pseudospectral methods (Sinc and Fourier) as the infinite order limit of local finite difference methods, and vice versa, finite difference as a certain sum acceleration of the pseudospectral methods is exploited to investigate high order finite difference algorithms for solving the Schrödinger equation in molecular dynamics. A Morse type potential for iodine molecule is used to compare the eigenenergies obtained by a Sinc Pseudospectral method and a high order finite difference approximation of the action of the kinetic energy operator on the wave function. Two-dimensional and three-dimensional model potentials are employed to compare spectra obtained by fast Fourier transform techniques and variable order finite difference. It is shown that it is not needed to employ very high order approximations of finite differences to reach the numerical accuracy of pseudospectral techniques. This, in addition to the fact that for complex configuration geometries and high dimensionality, local methods require less memory and are faster than pseudospectral methods, put finite difference among the effective algorithms for solving the Schrödinger equation in realistic molecular systems.
Threshold collision-induced dissociation of anionic copper clusters and copper cluster monocarbonyls
Authors: Spasov Vassil A. , Lee Taeck-Hong , Ervin Kent M.
Published: July 2002

The threshold collision-induced dissociation method is applied to study the fragmentation patterns and to measure the dissociation energies of small anionic copper clusters (Cun−, n=2–8) and their monocarbonyls (CunCO−, n=3–7). For the bare clusters, the main reaction channels are loss of an atom and loss of a dimer. For the copper cluster monocarbonyls, the main channel is loss of CO. Dissociation energies for the loss of an atom from bare copper cluster anions, D0(Cun−1−–Cu), show even–odd alternation. The species with the highest dissociation energy, Cu7−, and the highest carbonyl desorption energy, Cu5CO−, have eight valence electrons, consistent with closed shells in the jellium model. Bond energies are compared with theoretical models.
Frequency dependence of ionic conductivity of electrolyte solutions
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

A theory for the frequency dependence of ionic conductivity of an electrolyte solution is presented. In this theory contributions to the conductivity from both the ion atmosphere relaxation and the electrophoretic effects are included in a self-consistent fashion. Mode coupling theory, combined with time-dependent density functional theory of ion atmosphere fluctuations, leads to expressions for these two contributions at finite frequencies. These expressions need to be solved self-consistently for the frequency dependence of the electrolyte friction and the ion conductivity at varying ion concentrations. In the limit of low concentration, the present theory reduces exactly to the well-known Debye–Falkenhagen (DF) expression of the frequency-dependent electrolyte friction when the non-Markovian effects in the ion atmosphere relaxation are ignored and in addition the ions are considered to be pointlike. The present theory also reproduces the expressions of the frequency-dependent conductivity derived by Chandra, Wei, and Patey when appropriate limiting situations are considered. We have carried out detailed numerical solutions of the self-consistent equations for concentrated solutions of a 1:1 electrolyte by using the expressions of pair correlation functions given by Attard. Numerical results reveal that the frequency dependence of the electrolyte friction at finite concentration can be quite different from that given by the DF expression. With the increase of ion concentration, the dispersion of the friction is found to occur at a higher frequency because of faster relaxation of the ion atmosphere. At low frequency, the real part of the conductivity shows a small increase with frequency which can be attributed to the well-known Debye–Falkenhagen effect. At high frequency, the conductivity decreases as expected. The extensions of the present theory to treat frequency-dependent diffusivities of charged colloid suspensions and conductivity of a dilute polyelectrolyte solution are discussed.
Density functional theory of simple polymers in a slit pore. II. The role of compressibility and field type
Authors: Hooper Justin B. , Pileggi Morgan T. , McCoy John D. , Curro John G. , Weinhold Jeffrey D.
Published: July 2002

Simple tangent, hard site chains near a hard wall are modeled with a density functional (DF) theory that uses the direct correlation function, c(r), as its “input.” Two aspects of this DF theory are focused upon: (1) the consequences of variations in c(r)’s detailed form; and (2) the correct way to introduce c(r) into the DF formalism. The most important aspect of c(r) is found to be its integrated value, ĉ(0). Indeed, it appears that, for fixed ĉ(0), all reasonable guesses of the detailed shape of c(r) result in surprisingly similar density distributions, ρ(r). Of course, the more accurate the c(r), the better the ρ(r). As long as the length scale introduced by c(r) is roughly the hard site diameter and as long as the solution remains liquid-like, the ρ(r) is found to be in good agreement with simulation results. The c(r) is used in DF theory to calculate the medium-induced potential, UM(r), from the density distribution, ρ(r). The form of UM(r) can be chosen to be one of a number of different forms. It is found that the forms for UM(r)which yield the most accurate results for the wall problem are also those which were suggested as accurate in previous, related studies.
Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI
Authors: Zimmer F. , Ballone P. , Maier J. , Parrinello M.
Published: July 2002

The equilibrium concentration of ionic and electronic charge carriers in ionic crystals as a function of temperature, concentration of dopants, and chemical environment is phenomenologically well understood as long as these point defects can be considered sufficiently dilute. However, there are cases, usually at temperatures close to the melting point, where the defects appear in higher concentrations. In these cases interactions come into play and cause anomalous increases in the conductivity or even phase transitions. Recently Hainovsky and Maier showed that for various Frenkel disordered materials this anomalous conductivity increase at high temperature can be described by a cube root term in the chemical potential of the defects. This quasi-Madelung approach does not only allow ionic conductivities and heat capacities to be computed, it also leads to a phenomenological understanding of the solid–liquid or superionic transition temperatures. In the present study we analyze this approach on the atomistic level for AgI: The defect concentrations as well as defect energies, including excess energies, are computed as a function of temperature by molecular-dynamics and Monte Carlo simulations based on a classical semiempirical potential. The simulations support the cube-root model, yield approximately the same interaction constants and show that the corrections in the chemical potential are of an energetic nature. In agreement with structural expectations, the simulations reveal that two different kinds of interstitials are present: Octahedral interstitials, which essentially determine the ionic transport at higher temperature, and tetrahedral ones, which remain substantially associated with the vacancies. It is shown how these refinements have to be introduced into the cube root.
Photodissociation of cyclic HF complexes: Pentamer through heptamer
Authors: Oudejans L. , Miller R. E.
Published: July 2002

Photofragment angular and final rotational state distributions have been measured, corresponding to the vibrational predissociation of HF polymers [(HF)N N=5–7]. The present experiments confirm the theoretical prediction that, for clusters in this size range, the only open dissociation channel corresponds to the “evaporation” of a single HF monomer. The pump–probe experiments reported here indicate that the resulting HF monomer is produced with only modest rotational excitation. Rotational temperatures of 180 and 140 K have been determined for the evaporated HF monomer following dissociation of the HF pentamer and hexamer, respectively. Energy conservation in these photodissociation studies provides upper limits for the dissociation energies (D0), namely, 2941 cm−1 and 2854 cm−1 for the pentamer and hexamer, respectively.
Nonabelian progressive waves
Authors: Oh C. H. , Teh Rosy
Published: July 2002

We present two families of nonabelian wave solutions of the Yang–Mills field equations, some of the previous known solutions occur as special cases of our solutions.
Wavelets associated with representations of the affine Weyl–Heisenberg group
Authors: Torrésani Bruno
Published: July 2002

The affine Weyl–Heisenberg group (generated by time and frequency translations, and time dilations) is considered, and some associated resolutions of the identity are derived. As a result, it will be shown that they can all be obtained from those associated with the affine group and the Weyl–Heisenberg group by frequency translations of the analyzed function and the analyzing and reconstructing wavelets.
Padé interpolation: Methodology and application to quarkonium
Authors: Leung C. N. , Murakowski J. A.
Published: July 2002

A novel application of the Padé approximation is proposed in which the Padé approximant is used as an interpolation for the small and large coupling behaviors of a physical system, resulting in a prediction of the behavior of the system at intermediate couplings. This method is applied to quarkonium systems, and reasonable values for the c and b quark masses are obtained.
Radial motion of highly conducting sphere in magnetic field
Authors: Gürcan Ö. D. , Mirnov V. V. , Üçer D.
Published: July 2002

Radial motion of a highly conducting sphere in external magnetic field is considered. It both perturbs the external magnetic field and generates an electric field. Exact analytic solution has been obtained previously for a uniformly expanding sphere. In the present paper a new exact solution is derived which is valid not only for expansion but for contraction as well. It allows us to calculate analytically the total electromagnetic energy irradiated by the sphere involved in periodical radial motion with arbitrary velocity.
Symmetries of asymptotically flat axisymmetric space–times with null dust
Authors: Gönna U. von der , Pravdová A.
Published: July 2002

Symmetries of space–times with null dust field as a source compatible with asymptotic flatness are studied by using the Bondi–Sachs–van der Burg formalism. It is shown that in an axially symmetric space–time with null dust field in which at least locally a smooth null infinity in the sense of Penrose exists, the only allowable additional Killing vector forming with the axial one a two-dimensional Lie algebra (the axial and the additional Killing vector are not assumed to be hypersurface orthogonal) is a supertranslational Killing vector and the gravitational field is then nonradiative (the Weyl tensor has a nonradiative character).
Resonant magnetic properties of gadolinium–gallium garnet single crystals
Authors: Bedyukh A. R. , Danilov V. V. , Nechiporuk A. Yu. , Romanyuk V. F.
Published: July 2002

The results of experimental investigations of resonant magnetic properties of gadolinium–gallium garnet (GGG) single crystals at temperatures 4.2–300 K in the frequency range 1.6–9.3 GHz are considered. It is found that magnetic losses in GGG are determined by the initial splitting of energy levels for gadolinium ions in the garnet crystal lattice and by the dipole broadening. The width and shape of the electron paramagnetic resonance (EPR) line in the GGG crystal, whose asymmetry is manifested most strongly at low frequencies, can be explained by the influence of these factors. Magnetic losses in GGG increase with frequency and upon cooling. It is found that the EPR linewidth increases considerably with decreasing temperature due to the presence of rapidly relaxing impurities.
Numerical analysis of a flow induced in a rarefied gas between noncoaxial circular cylinders with different temperatures for the entire range of the Knudsen number
Authors: Aoki Kazuo , Sone Yoshio , Yano Takeru
Published: July 2002

A flow induced by the temperature field in a rarefied gas contained between two noncoaxial circular cylinders at rest with different uniform temperatures is analyzed numerically on the basis of the linearized Boltzmann–Krook–Welander equation with a diffuse reflection boundary condition for the entire range of the Knudsen number. The computation is carried out for the case where the radius of the inner cylinder is one-half and the eccentricity is one-fourth of the radius of the outer cylinder. It is demonstrated that two symmetric circulating flows are established, the direction of which does not depend on the Knudsen number. The force acting on the inner cylinder and the energy transmitted to it are also obtained, and their variations with the Knudsen number are investigated in detail. The magnitude of the force takes its maximum at a moderate Knudsen number, and the reversal of the direction of the force, which is predicted from the existing analyses for the two extreme cases of small and large Knudsen numbers, takes place at a small Knudsen number.
A numerical study of flow separation and reattachment on a blunt plate
Authors: Tafti D. K. , Vanka S. P.
Published: July 2002

A two-dimensional time-dependent numerical study of separating and reattaching flow over a blunt plate is described. Four Reynolds numbers, Re=150, 250, 300, and 1000, are studied. The first three are in the steady flow regime and calculated values of reattachment lengths compare well with experimental data. For Re=1000, the separated shear layer becomes unsteady with the formation of spanwise vortices. These vortices coalesce and are shed periodically from the reattachment region. Although the resulting flow field is known to be three dimensional, the current two-dimensional calculation is able to predict important flow properties. Calculated time-dependent properties such as vortex shedding frequency and convection velocities compare well with experimental data. The present study is a precursor to a three-dimensional simulation.
Kinematic alignment effects in turbulent flows
Authors: Jiménez Javier
Published: July 2002

It is shown that, in the neighborhood of a vortex whose maximum vorticity is large with respect to that in the surrounding flow, the two principal strains with the largest absolute values lie in the equatorial plane, so that the vorticity is automatically aligned to the intermediate eigenvector. This purely kinematic effect is offered as an explanation for the alignment properties recently reported for the velocity and pressure derivatives in turbulent flows. The model is compared with experimental evidence from numerical simulations. The observed ratio between the principal strains is also related to the properties of a two-dimensional Burgers’ vortex.
Short wave instability of core-annular flow
Authors: Chen Kang Ping
Published: July 2002

An asymptotic expression for the dispersion relation is derived in the short wave limit for core-annular flow of two fluids when the surface tension parameter is of order one.
Photophoresis of a spherical particle in a rarefied gas
Authors: Beresnev Sergey , Chernyak Vladimir , Fomyagin Gavriil
Published: July 2002

On the basis of the linearized gas-kinetic model equations solution (the BGK and the S model), a force affecting a unilaterally illuminated spherical particle in a rarefied gas is calculated. Numerical calculations of the photophoretic force and the particle motion velocity are carried out for the whole range of Knudsen numbers taking into account the particle’s thermophysical, optical, and accommodation properties. The results obtained are compared to the known theoretical and experimental data.
Hydrodynamic, translational diffusion in fiber suspensions subject to simple shear flow
Authors: Rahnama Mani , Koch Donald L. , Iso Yoichi , Cohen Claude
Published: July 2002

Translational, hydrodynamic diffusivities for fibers and fluid particles in a suspension of fibers undergoing simple shear flow have been calculated. The hydrodynamic interactions between the fibers were treated using slender-body theory. The diffusivities were obtained by averaging over all possible interactions. Both dilute (nl3≪1) and semidilute (nl3≫1 but nl2d≤1) suspensions were studied. Here, n is the number of fibers per unit volume, l is the fiber length, and d is the fiber diameter. In a complementary experimental study, the positions of tracer particles were observed in an index of refraction and density matched semidilute fiber suspension in a cylindrical Couette cell. The effective diffusivity of small tracer spheres that approximated ideal fluid particles exhibited an approximate linear growth with fiber concentration in qualitative agreement with the theoretical predictions. The experimentally measured diffusivities were smaller than the theoretical predictions by about 20%–45%.
Measurements of absorption and Brillouin sidescattering from planar plasmas produced by 0.53 μm laser light
Authors: Drake R. P. , Turner R. E. , Lasinski B. F. , Campbell E. M. , Kruer W. L. , Williams E. A. , Kauffman R. L.
Published: July 2002

A study of the absorption and scattering of laser light by planar, laser-produced plasmas is reported. Up to 4 kJ of 0.53 μm laser light irradiated thick, gold targets in 1 nsec pulses at intensities up to 4×1015 W/cm2. The absorption was determined from measurements of the scattered light by an array of about 100 detectors. Stimulated Brillouin sidescattering, identified by the angular distribution and spectrum of the scattered light, exceeded 10% of the laser energy for intensities above 1015 W/cm2. The observations are compared to those of previously reported experiments. It is suggested that a combination of collisional absorption, enhanced backscattering, and convective sidescattering produces the observed dependence on laser intensity.
Stimulated Raman scattering from a density maximum in a laser-produced plasma
Authors: Barr H. C. , Boyd T. J. M. , Mackwood A. P.
Published: July 2002

Thresholds and growth rates for stimulated Raman scattering from a plasma density maximum, corresponding to laser–target burnthrough experiments, are obtained by numerical solution of the full-wave Raman scattering equations. This supplements and extends to new regimes the WKB analysis of Rosenbluth [Phys. Rev. Lett. 29, 565 (1972)] and the quasiglobal analysis of Williams and Johnston [Phys. Fluids B 1, 188 (1989)]. The results show unexpected scalings with laser intensity well above threshold, and permit absolutely unstable forward scattering with thresholds comparable to those for backward scattering.
Safety factor on the axis of a tokamak during Ohmically heated sawtoothing discharges from a localized measurement of circular polarization of the Lii 6708 Å line
Authors: Huang L. K. , Finkenthal M. , Wróblewski D. , Moos H. W. , West W. P. , Thomas D. M. , Thomas M. P. , Yang X. Z. , Phillips P. E. , Wootton A. J. , Bengtson Roger D. , Boedo J. A. , Bravenec R. V. , Chen J. Y. , Cima G. , Gentle K. W. , McCool S. C. , Richards B. U. , Rowan W. L. , Sing D. C. , Wenzel K. W.
Published: July 2002

Analysis of the circular polarization of the Lii 6708 Å line emitted from a monoenergetic lithium beam provides a spatially resolved measurement of the poloidal magnetic field on the Texas experimental tokamak [Nucl. Technol. Fusion 1, 479 (1989)]. Determined from the measured magnetic field, the safety factor on the magnetic axis during Ohmically heated sawtoothing discharges is near unity.
Comment on ‘‘Energy and nonlinearity considerations for the enhanced plasma wave model of Raman scattering’’ [Phys. Fluids B 1, 1073 (1989)]
Authors: Drake R. P.
Published: July 2002

The interpretation of some recent experiments [Phys. Rev. Lett. 60, 1018 (1988); Phys. Fluids 30, 1795 (1988)] by Simon and Short [Phys. Fluids B 1, 1073 (1989), referred to as SS4] is discussed. The predicted angular distribution is almost narrow enough to correspond to the data discussed in SS4, but not to correspond to data from previous experiments having similar plasma-wave damping but much smaller density-gradient scale length. Examination of the time dependence of the Raman spectrum in all the data, rather than only the single case discussed in SS4, shows that the hydrodynamic behavior, rather than the vanishing of quarter critical, determines the time of maximum emission. Finally, it is notable that the analysis of the power flow in SS4 requires that most of the laser energy be converted into intense electron beams with narrow velocity distributions having intensities of 20× the average laser power. Though not impossible, this seems unlikely.
Study of instabilities in long scale-length plasmas with and without laser-beam-smoothing techniques
Authors: Willi O. , Afshar-rad T. , Coe S. , Giulietti A.
Published: July 2002

Experimental results are presented for the interaction of an intense laser beam in large preformed, homogeneous underdense plasmas. Parametric instabilities, including filamentation, stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), have been extensively studied. In this paper comparisons are made between coherent and smoothed laser interaction beams for their effectiveness in suppressing these instabilities. Direct observations of the enhanced breakup of the coherent interaction beam due to filamentation are presented. Measurements of the transverse scale lengths of the hot spots, and the level of the intensity modulations of the filamentary structures are given. Further, direct experimental observations of SRS and SBS generated in filaments are presented. These provided conclusive evidence that this instability provides the primary mechanism for the generation of SRS and SBS under such plasma conditions. With the smoothed laser interaction beams, using random phase plates (RPP) and induced spatial incoherence (ISI), the level of filamention was significantly reduced. The backscattered fractions of SRS and SBS with respect to the irradiance of the interaction beam were measured for coherent, RPP and ISI. Reduction of typically a factor of 20 and 500 were recorded using RPP and ISI, respectively, in the backscattering levels of SRS and SBS when compared with the coherent interaction beam. The reductions are attributed primarily to the effectiveness of the beam-smoothing techniques in suppressing the filamentation instability.
Penetration and expulsion of magnetic fields in plasmas due to the Hall field
Authors: Fruchtman Amnon
Published: July 2002

The axial penetration of an azimuthal magnetic field into a short-duration hollow cylindrical plasma is studied. When the process is so fast that the ion motion is small and the plasma dissipative resistivity, electron inertia, and pressure are small, the evolution of the magnetic field is governed by the Hall field. When the radial current flows inward, the magnetic field penetrates in the form of a Hall-induced shock wave with a narrow current channel. When outward, the magnetic field does not penetrate the plasma. Moreover, in the latter case the magnetic field is expelled from an initially magnetized plasma. The increase and decrease of the magnetic field intensity in the cylindrical plasma are shown to result naturally from the frozen-in law.
Test particle motion in the cyclotron resonance regime
Authors: Ginet Gregory P. , Albert Jay M.
Published: July 2002

Test particles moving in the field of an electromagnetic wave propagating in a background magnetic field can gain significant energy when the wave parameters and particle energy are such that the cyclotron resonance condition is satisfied. Central to the acceleration process and long time scale periodic behavior is the coherent accumulation over many cyclotron orbits of a small change in energy during each orbit, a result of the circularly polarized component of the wave electric field. Also important is the small change in the relative wave phase during each orbit resulting from relativistic variations of the cyclotron frequency and wave-induced streaming along the background magnetic field. The physical mechanisms underlying cyclotron resonance acceleration are explored using a set of heuristic mapping equations (the PMAP) describing changes in the particle momentum and relative wave phase. More accurate (but less transparent) descriptions of the particle motion are pursued in the context of orbit-averaged Hamiltonian theory. A discrete set of mapping equations for the slowly varying canonical action and angle are derived (the QMAP) but are found to generate inaccurate solutions in certain regions of phase space when the resonance number l is such that {↓l}↓=1 and the particles are initially cold. These difficulties are avoided by constructing a continuous time orbit-averaged Hamiltonian and solving the resultant canonical equations of motion. Assuming the momentum is small relative to mc (where m is the particle mass and c is the speed of light), details of the distribution of particle trajectories in the action-angle phase space for {↓l}↓=1 and {↓l}↓=2 are presented and criteria for the existence of orbits oscillatory in angle are derived.
Local shear in general magnetic stellarator geometry
Authors: Waltz R. E. , Boozer A. H.
Published: July 2002

A nonlinear ballooning mode formulation in general magnetic coordinates is developed for the cold ion electrostatic drift wave problem in stellarators. It is shown that local ripple shear, not global average shear, radially localizes microturbulent modes which are located along the field lines by local ripple curvature wells. It is argued that microturbulence in stellarators depends dominantly on local ripple shear and curvature and may be rather insensitive to global (or average) shear and curvature.
Drift-Alfvén vortices
Authors: Shukla P. K. , Yu M. Y. , Varma R. K.
Published: July 2002

The solitary vortex solution is shown to exist for the set of nonlinear equations describing low-frequency fluid motion in a magnetized plasma.
Capillary-gravity waves generated in a viscous fluid
Authors: Pramanik A. K. , Majumdar S. R.
Published: July 2002

The linearized initial-value problem of capillary-gravity waves generated by a moving oscillatory surface pressure distribution in a viscous incompressible fluid of infinite depth is solved. It is found that viscosity apart from introducing a damping factor into the amplitude of each wave plays an important role in the critical case. While the solution in the inviscid fluid becomes singular for certain values of the parameters of the problem, the solution in viscous fluid remains valid for all values of the parameters, though the amplitudes are relatively large in the critical case.
Electromagnetic radiation from strong Langmuir turbulence
Authors: Akimoto K. , Rowland H. L. , Papadopoulos K.
Published: July 2002

A series of computer simulations is reported showing the generation of electromagnetic radiation by strong Langmuir turbulence. The simulations were carried out with a fully electromagnetic 2 1/2 -dimensional fluid code. The radiation process takes place in two stages that reflect the evolution of the electrostatic turbulence. During the first stage while the electrostatic turbulence is evolving from an initial linear wave packet into a planar soliton, the radiation is primarily at ωe. During the second stage when transverse instabilities lead to the collapse and dissipation of the solitons, 2ωe and ωe radiation are comparable, and 3ωe is also present. The radiation power at ω=2ωe is in good agreement with theoretical predictions for electromagnetic emissions by collapsing solitons.
Direct simulation Monte Carlo for thin-film bearings
Authors: Alexander Francis J. , Garcia Alejandro L. , Alder Berni J.
Published: July 2002

The direct simulation Monte Carlo (DSMC) scheme is used to study the gas flow under a read/write head positioned nanometers above a moving disk drive platter (the slider bearing problem). In most cases, impressive agreement is found between the particle-based simulation and numerical solutions of the continuum hydrodynamic Reynolds equation which has been corrected for slip. However, at very high platter speeds the gas is far from equilibrium, and the load capacity for the slider bearing cannot be accurately computed from the hydrodynamic pressure.
Kinematic effects on local energy dissipation rate and local enstrophy in fluid turbulence
Authors: Chen Hudong , Chen Shiyi
Published: July 2002

A simple analysis shows that for incompressible isotropic turbulence there exists a generic inequality between the energy dissipation rate and enstrophy whenever the moment correlations of the velocity field is quasi-normal. This indicates that the cause of the difference of spatial fluctuations between the two quantities is at least in part kinematic.
The effect of mobility gradients on viscous instabilities in miscible flows in porous media
Authors: Loggia D. , Rakotomalala N. , Salin D. , Yortsos Y. C.
Published: July 2002

The onset of viscous instability and the subsequent fingering have been mostly studied under conditions of a sharp mobility contrast. In stable miscible flows in porous media, however, mobility gradients of variable extent can develop due to hydrodynamic dispersion. Graded mobility and density profiles will affect instabilities, either by mitigating (in the case of a monotonic variation) or by enhancing (nonmonotonic variation) the rates of growth. The mitigating effect was exploited by Claridge and Gorrel and Homsy for the optimal design of graded mobility banks. Hickernell and Yortsos showed that in the linear stability limit, the growth rate is controlled by the maximum of the logarithmic derivative of the mobility profile. We present an experimental study of miscible displacements in porous media to study effects of mobility gradients in viscous instability. We take advantage of molecular diffusion in miscible displacements to create a mobility gradient zone and subsequently initiate the instability by increasing the displacement velocity. A theoretical analysis based on linear stability is used to analyze the experimental results.
Effects of granular additives on transition boundaries between flow states of rimming flows
Authors: Boote O. A. M. , Thomas P. J.
Published: July 2002

An experimental study of the rimming flow established inside a partially fluid-filled cylinder rotating around a horizontal axis of rotation is described. For the first time effects of granular additives on transition boundaries between flow states adopted by the fluid for different experimental conditions are studied. For the granule-free fluid and low filling levels we confirm results of previous authors showing that the ratio of viscous stresses and gravitational force remains constant along the transition boundaries considered. For higher filling levels our new data indicate, however, that the gravitational force becomes increasingly more important. For the solid–liquid two-phase flow our data reveal that even small amounts of granular additives can have a significant effect on a suitable parameter defined to characterize the transition boundaries. Granular additives can lead to the stabilization of states and to the extension of the parameter range over which certain states can be observed. It is shown that the origin of the observed effects appears to be associated with an increased bulk density of the solid–liquid flow. For high granule concentrations a pattern of equally-spaced circumferential granular bands is observed to form on the inner cylinder wall. It is speculated that these bands form as a consequence of the mechanism which has been referred to as shear-induced migration/diffusion in the literature in the past. It appears that the granule-band pattern has not been observed previously for the flow investigated here.
Large scale drag representation in simulations of two-dimensional turbulence
Authors: Sukoriansky Semion , Galperin Boris , Chekhlov Alexei
Published: July 2002

Numerical simulations of isotropic, homogeneous, forced and dissipative two-dimensional (2D) turbulence in the energy transfer subrange are complicated by the inverse cascade that continuously propagates energy to the large scale modes. To avoid energy condensation in the lowest modes, an energy sink, or a large scale drag is usually introduced. With a few exceptions, simulations with different formulations of the large scale drag reveal the development of strong coherent vortices and steepening of energy and enstrophy spectra that lead to erosion and eventual destruction of Kolmogorov–Batchelor–Kraichnan (KBK) statistical laws. Being attributed to the intrinsic anomalous fluctuations independent of the large scale drag formulation, these coherent vortices have prompted conjectures that KBK 2D turbulence in the energy subrange is irreproducible in long term simulations. Here, we advance a different point of view, according to which the emergence of coherent vortices is triggered by the inverse energy cascade distortion directly attributable to the choice of a large scale drag formulation. We subdivide the computational modes into explicit and implicit, or supergrid scale (SPGS), which are the few lowest wave numbers modes that adhere to KBK statistics. Then, we introduce a new concept of the large scale drag—rather than being an energy sink, it accounts for the energy and enstrophy exchange between the explicit and SPGS modes. The new SPGS parameterization was used in both direct numerical simulations (DNS) and large eddy simulations (LES) in a doubly periodic box setting. It was found that the new technique enables both DNS and LES to reach a steady state preserved for many large scale eddy turnover times. For the entire time of integration, the flow field remained structureless and in good agreement with the KBK statistical laws. We conclude that homogeneous, isotropic, forced, dissipative 2D turbulence in the energy subrange is statistically stable, does not produce coherent structures, and obeys the KBK statistical laws for as long as its inverse energy cascade remains undisturbed. The proposed new technique of computing the intermediate modes while the statistics of the largest scales is known may find a wide range of applications.
Filtered subgrid-scale models
Authors: Sagaut P. , Comte P. , Ducros F.
Published: July 2002

Filtered subgrid-scale models are derived, which are less sensitive to the low-frequency modes than their original counterparts. They are obtained thanks to the definition of a high-pass discrete test filter. After presenting the theoretical derivation of these models, some a posteriori tests on the incompressible channel flow case are presented.
Monte Carlo analysis of macroscopic fluctuations in a rarefied hypersonic flow around a cylinder
Authors: Stefanov Stefan K. , Boyd Iain D. , Cai Chun-Pei
Published: July 2002

From consideration of the length scales characteristic of molecular and turbulent phenomena, it is proposed that flow instabilities and structural motions should be generated under certain rarefied, hypersonic flow conditions. This proposal is investigated using the direct simulation Monte Carlo (DSMC) method for hypersonic, rarefied flow over a cylinder. The overall mean flow field contains a number of regions including an undisturbed free stream, a bow shock, a recompression in the wake, and a recirculation zone behind the cylinder. Analysis of the fluctuations in velocity and number density predicted by the DSMC technique for this flow is conducted. An important aspect of this analysis is the clarification of the nature of the observed fluctuations. It is found that different types of fluctuations are found in the bow shock and in the wake. These are attributed to different types of physical phenomena. In the bow shock, large fluctuations are caused by the macroscopic field gradients in the shock front. In the wake, one can also observe an area with enhanced scatter of the mean flow characteristics caused by a macroscopic structural motion. The calculated instantaneous flow fields show small oscillations of the vortices immediately downstream of the cylinder for a Mach number of 26 and a Knudsen number of 0.0025. When the Knudsen number is reduced to 0.001, an irregular secondary flow motion containing vortex structures is exhibited in the far wake.
Hall magnetohydrodynamic modeling of a long-conduction-time plasma opening switch
Authors: Huba J. D. , Grossmann J. M. , Ottinger P. F.
Published: July 2002

The dynamics of long-conduction-time (τc∼1 μs) plasma opening switches (POS) is studied using magnetohydrodynamic (MHD) theory, including the Hall term. Plasma switches with initial electron densities of ne=1014–1016 cm−3 are modeled; these densities are appropriate to recent experiments carried out at the Naval Research Laboratory using the Hawk generator (800 kA, 1.2 μs). The conduction times obtained from the simulation studies are in the range τc≂0.4–2.0 μs. The POS plasma is strongly redistributed by the penetrating magnetic field. As the field penetrates, it pushes the plasma both axially and radially (i.e., toward the anode and cathode). In the higher-density regime (ne≳1015 cm−3), Hall effects do not play a significant role. The magnetic field acts as a snowplow, sweeping up and compressing the plasma as it propagates through the POS plasma. In the lower-density regime (ne<1015 cm−3), Hall effects become important in two ways: the conduction time is less than that expected from ideal MHD, and the POS plasma becomes unstable as the magnetic field penetrates, leading to finger-like density structures. The instability is the unmagnetized ion Rayleigh–Taylor instability and is driven by the magnetic force accelerating the plasma. The structuring of the plasma further decreases the conduction time and causes the penetrating magnetic field to have a relatively broad front in comparison to EMHD simulations (i.e., Vi=0). The simulation results are consistent with experimental data for conduction currents 300–800 kA.