bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic theory of polymer-mediated interactions between spherical particles
Authors: Chatterjee Avik P. , Schweizer Kenneth S.
Published: July 2002

We develop an analytic integral equation theory for treating polymer-induced effects on the structure and thermodynamics of dilute suspensions of hard spheres. Results are presented for the potential of mean force, free energy of insertion per particle into a polymer solution, and the second virial coefficient between spheres. The theory makes predictions for all size ratios between the spheres and the polymer coil dimension. Based on the Percus–Yevick (PY) closure, the attractive polymer-induced depletion interaction is predicted to be too weak under athermal conditions to induce a negative value for the second virial coefficient, B2cc, between spheres in the colloidal limit when the spheres are much larger than the coil size. A nonmonotonic dependence of the second virial coefficient on polymer concentration occurs for small enough particles, with the largest polymer-mediated attractions and most negative B2cc occurring near the dilute–semidilute crossover concentration. Predictions for the polymer-mediated force between spheres are compared to the results of computer simulations and scaling theory.
An accurate relativistic effective core potential for excited states of Ag atom: An application for studying the absorption spectra of Agn and Agn+ clusters
Authors: Bonačić-Koutecký Vlasta , Pittner Jiřı́ , Boiron Marc , Fantucci Piercarlo
Published: July 2002

A new 11-electron relativistic effective core potential (11e-RECP) for Ag atom based on correlated level of theory and the associated atomic orbital (AO) basis set have been derived which allows for an accurate determination of excited states. This has been verified by comparing the calculated excited states of the dimer with experimental data. Therefore, we applied the new 11e-RECP in the framework of the linear response equation-of-motion coupled-cluster (EOM-CC) method to determine absorption spectra of small Agn=2−4 and Agn=2−4+ clusters. The correlation treatment of 11 electrons per atom and calculations of transition energies and oscillator strengths in a large energy interval allowed us to investigate the influence of d-electrons on the spectroscopic patterns. We have found that d-electrons play a crucial role for accurate predictions of absorption spectra in spite of the fact that they are not always directly involved in the leading excitations contributing to the intense transitions. The calculated spectroscopic patterns for the stable structures are compared with available experimental data.
Photolysis at 266 nm of argon matrix isolated ozone monomer
Authors: Bahou M. , Schriver-Mazzuoli L. , Schriver A.
Published: July 2002

The photodissociation of ozone trapped at high dilution in solid argon has been reinvestigated at different temperatures and various photon flux, combining irradiation at 266 nm and infrared spectroscopy. In argon, recombination of O+O2 is a major pathway and the weak decrease of ozone is due to cage exit of oxygen atoms which is dependent of the temperature and of the initial photon flux. Kinetic curves are well fitted by a double exponential expression and a model based upon two different cage exit pathways is proposed.
A direct procedure for the evaluation of solvent effects in MC-SCF calculations
Authors: Cossi Maurizio , Barone Vincenzo , Robb Michael A.
Published: July 2002

We extended the polarizable continuum solvation model to multi-configurational complete active space as implemented in the Gaussian program. The present formulation allows energy and gradient computations in a unified approach for closed and open shell systems: in this work we present the formal derivation and check the results with energy calculations and geometry optimizations in solution, for ground and excited electronic states.
Instantaneous normal mode theory of diffusion and the potential energy landscape: Application to supercooled liquid CS2
Authors: Li Wu-Xiong , Keyes T.
Published: July 2002

The pure translation (TR) imaginary-frequency (or unstable) instantaneous normal modes (INM), which we have proposed as representative of barrier crossing and diffusion, are obtained for seven densities and eight temperatures of supercooled and near-melting liquid CS2 via computer simulation. The self-diffusion constant D, with a range of over two decades, has been determined previously for these 56 states [Li and Keyes, J. Chem. Phys. 111, 328 (1999)], allowing a comprehensive test of the relation of INM to diffusion. INM theory is reviewed and extended. At each density Arrhenius T-dependence is found for the fraction fu of unstable modes, for the product 〈ω〉ufu of the fraction times the averaged unstable frequency, and for D. The T-dependence of D is captured very accurately by fu at higher densities and by 〈ω〉ufu at lower densities. Since the T-dependence of 〈ω〉u is weak at high density, the formula D∝〈ω〉ufu provides a good representation at all densities; it is derived for the case of low-friction barrier crossing. Density-dependent activation energies determined by Arrhenius fits to 〈ω〉ufu are in excellent agreement with those found from D. Thus, activation energies may be obtained with INM, requiring far less computational effort than an accurate simulation of D in supercooled liquids. Im-ω densities of states, 〈ρuTR(ω,T)〉, are fit to the function a(T)ω exp[−(a2(T)ω/T)a3(T)]. The strong T-dependence of D, absent in Lennard-Jones (LJ) liquids, arises from the multiplicative factor a(T); its activation energy is determined by the inflection-point energy on barriers to diffusion. Values of the exponent a3(T) somewhat greater than 2.0 suggest that liquid CS2 is nonfragile in the extended Angell–Kivelson scheme for the available states. A striking contrast is revealed between CS2 and LJ; a3→2 at low-T in CS2 and at high-T in LJ. The INM interpretation is that barrier height fluctuations in CS2 are negligible at low-T but grow with increasing T, while the opposite is true for LJ.
Symmetric diblock copolymers in thin films. II. Comparison of profiles between self-consistent field calculations and Monte Carlo simulations
Authors: Geisinger T. , Müller M. , Binder K.
Published: July 2002

The structure of lamellar phases of symmetric AB diblock copolymers in a thin film is investigated. We quantitatively compare the composition profiles and profiles of individual segments in self-consistent field calculations with Monte Carlo simulations in the bond fluctuation model for chain length N=32 and χN=30. Three film thicknesses are investigated, corresponding to parallel oriented lamellae with 2 and 4 interfaces and a perpendicular oriented morphology. Taking account of capillary waves, we find good quantitative agreement between the Monte Carlo simulations and the self-consistent field calculations. However, the fluctuations of the local interfacial position are strongly suppressed by confinement and mutual interactions between lamellae.
Photoelectron circular dichroism of the randomly oriented chiral molecules glyceraldehyde and lactic acid
Authors: Powis Ivan
Published: July 2002

The differing interaction of left and right circularly polarized light with chiral molecules is shown to lead to different angular distributions of the photoelectrons created by photoionization of a given enantiomer, even when the target molecules are randomly oriented. Numerical calculations are presented to demonstrate the magnitude of this effect for the C3H6O3 structural isomers lactic acid and glyceraldehyde, including two different conformations of the latter. Circular dichroism in the angular distributions (CDAD) of the valence electrons of these biomolecules is most pronounced close to threshold, but tends to vanish as the electron kinetic energy approaches 20 eV and above. CDAD signals are predicted to range, typically, from 10% to 40% and sometimes to more than 60% of the differential cross section.
The evolution of perturbations of the renormalized long wave equation
Authors: Easwaran C. V. , Majumbar S. R.
Published: July 2002

The evolution of the solitary wave solution of the perturbed renormalized long wave equation ut+6uux−uxxt=εu is considered using two timing and matched asymptotic expansions. As in the case of the perturbed KdV equation, it is found that behind the slowly varying solitary wave there are two distinct regions, a near tail and a far tail. The far tail is given by an exponentially decaying expression in contrast to the KdV far tail which is oscillatory.
General form of the equation of motion for a point charge
Authors: Lozada Abraham
Published: July 2002

In this paper, using a distribution theory, the question of the compatible phenomenology associated with the electromagnetism of a classical point charge is addressed. The compatibility is restricted to the conservation laws of energy momentum and angular momentum. A general form of the equation of motion for a point charge is derived. Renormalization is not required. Previous results are discussed. It is shown that a distribution approach yields a deep insight into the problem.
Thermodynamic perfect fluid. Its Rainich theory
Authors: Coll Bartolomé , Ferrando Joan Josep
Published: July 2002

The conditions for a relativistic perfect fluid to admit a thermodynamic scheme are considered, and the necessary and sufficient requirements for a metric to define a thermodynamic perfect fluid space-time are given.
The Hamiltonization problem from a global viewpoint
Authors: Perlick Volker
Published: July 2002

Owing to the so-called Lie–Koenigs theorem, any (finite-dimensional) dynamical system can be put into Hamiltonian form locally. To that end, the introduction of dummy variables is not necessary, provided that the original system is even dimensional and that we allow for an explicitly time-dependent symplectic structure. In this paper, inquiry is made as to which conditions are needed for a universality statement similar to the Lie–Koenigs theorem to hold true globally. In addition, the passage to the Lagrangian formalism is discussed, and the results are illustrated by three examples.
A spectral transform for the intermediate nonlinear Schrödinger equation
Authors: Pelinovsky Dmitry E. , Grimshaw Roger H. J.
Published: July 2002

A new spectral transform system is introduced to solve the initial-value problem for the intermediate nonlinear Schrödinger (INLS) equation describing envelope waves in a deep stratified fluid. The spectral system is a combination of the Zakharov–Shabat linear system and a local Riemann–Hilbert problem in a strip of the complex plane. The inverse scattering transform technique is developed and the Bäcklund–Darboux transformation, soliton solutions and an infinite number of conservation laws are constructed. It is shown that all these properties of the INLS equation are closely related to those of the intermediate long-wave equation.
An algebraic method to construct the Darboux matrix
Authors: Cieśliński Jan
Published: July 2002

We present an effective procedure to construct the 1-soliton Darboux matrix. Our approach, based on the Zakharov–Shabat–Mikhailov’s dressing method, is especially useful in the case of non-canonical normalization and for non-isospectral linear problems. The construction is divided into two steps. First, we represent a given linear problem as a system of some algebraic constraints on two matrices. In this context we introduce and discuss invariants of the Darboux matrix. Second, we derive the Darboux matrix demanding that it preserves the algebraic constraints. In particular, we consider in details the restrictions imposed by various reduction groups on the form of the Darboux matrix.
Geometries of quantum states
Authors: Petz Dénes , Sudár Csaba
Published: July 2002

The quantum analog of the Fisher information metric of a probability simplex is searched and several Riemannian metrics on the set of positive definite density matrices are studied. Some of them appeared in the literature in connection with Cramér–Rao-type inequalities or the generalization of the Berry phase to mixed states. They are shown to be stochastically monotone here. All stochastically monotone Riemannian metrics are characterized by means of operator monotone functions and it is proven that there exist a maximal and a minimal among them. A class of metrics can be extended to pure states and a constant multiple of the Fubini–Study metric appears in the extension.
Negative binomial and multinomial states: Probability distributions and coherent states
Authors: Fu Hong-Chen , Sasaki Ryu
Published: July 2002

Following the relationship between probability distribution and coherent states, for example the well known Poisson distribution and the ordinary coherent states and relatively less known one of the binomial distribution and the su(2) coherent states, we propose interpretation of su(1,1) and su(r,1) coherent states in terms of probability theory. They will be called the negative binomial (multinomial) states which correspond to the negative binomial (multinomial) distribution, the non-compact counterpart of the well known binomial (multinomial) distribution. Explicit forms of the negative binomial (multinomial) states are given in terms of various boson representations which are naturally related to the probability theory interpretation. Here, we show fruitful interplay of probability theory, group theory, and quantum theory.
Higher symmetries and exact solutions of linear and nonlinear Schrödinger equation
Authors: Fushchych W. I. , Nikitin A. G.
Published: July 2002

A new approach for the analysis of partial differential equations is developed which is characterized by a simultaneous use of higher and conditional symmetries. Higher symmetries of the Schrödinger equation with an arbitrary potential are investigated. Nonlinear determining equations for potentials are solved using reductions to Weierstrass, Painlevé, and Riccati forms. Algebraic properties of higher order symmetry operators are analyzed. Combinations of higher and conditional symmetries are used to generate families of exact solutions of linear and nonlinear Schrödinger equations.
Optical absorption edge in α–Fe2O3: The exciton–magnon structure
Authors: Galuza A. I. , Beznosov A. B. , Eremenko V. V.
Published: July 2002

Transmission spectra of synthetic and natural hematite (α-Fe2O3) crystals are measured at temperatures 10, 25, and 300 K in the wavelength range 500–1100 nm, and the absorption spectra are computed. Pure exciton and exciton–magnon d–d transition bands are revealed, the corresponding wavelengths at 10 K being λ0=1020 nm and λ1=965 nm respectively. The half-widths and oscillator forces are g0=84 cm−1, f0=4×10−9, g1=60 cm−1, f1=1.4×10−7 for 10 K, g0=85 cm−1, f0=5×10−9, g1=110 cm−1, f1=2.1×10−7 for 25 K. The mechanisms of band formation for weakly allowed d–d transitions in hematite are analyzed.
Electromagnetic stirring
Authors: Moffatt H. K.
Published: July 2002

It is well known that an alternating magnetic field (either single phase or multiphase) applied to a conductor, whether solid or fluid, will induce electric currents in the conductor, and hence a Lorentz force distribution. This Lorentz force is in general rotational, and if the conductor is fluid, it is set in motion. Thus the magnetic field acts as a nonintrusive stirring device and it can, in principle, be engineered to provide any desired pattern of stirring. Stirring may also be effected through the interaction of a steady current distribution driven through a fluid and the associated magnetic field. When the field frequency is high, the Lorentz force is confined to a thin electromagnetic boundary layer, and the net effect of the magnetic field is to induce either a tangential velocity or a tangential stress just inside the boundary layer. The distribution of velocity or stress is related to the structure of the applied field. Symmetric configurations may lead to patterns of stirring in which the streamlines lie on toroidal surfaces; more generally, however, the streamline pattern is chaotic. Examples of both types are presented, and applications are discussed. A somewhat related problem arises if electrically conducting particles are suspended in a nonconducting fluid, the whole being subjected to the same type of alternating magnetic field. Currents are now induced in the particles, and each particle experiences a force and a couple. When several particles are in suspension, the movement of each is influenced by the presence of the others, and a problem somewhat analogous to the problem of interacting point vortices arises. For a suspension of conducting particles, inhomogeneity of the field leads to inhomogeneity of the concentration of particles, and a bulk flow is, in general, induced. The general principles underlying this behavior are briefly summarized.
High poloidal beta equilibria in the Tokamak Fusion Test Reactor limited by a natural inboard poloidal field null
Authors: Sabbagh S. A. , Gross R. A. , Mauel M. E. , Navratil G. A. , Bell M. G. , Bell R. , Bitter M. , Bretz N. L. , Budny R. V. , Bush C. E. , Chance M. S. , Efthimion P. C. , Fredrickson E. D. , Hatcher R. , Hawryluk R. J. , Hirshman S. P. , Janos A. C. , Jardin S. C. , Jassby D. L. , Manickam J. , McCune D. C. , McGuire K. M. , Medley S. S. , Mueller D. , Nagayama Y. , Owens D. K. , Okabayashi M. , Park H. K. , Ramsey A. T. , Stratton B. C. , Synakowski E. J. , Taylor G. , Wieland R. M. , Zarnstorff M. C. , Kesner J. , Marmar E. S. , Terry J. L.
Published: July 2002

Recent operation of the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Research 1, 51 (1986)] has produced plasma equilibria with values of Λ≡βp eq+li/2 as large as 7, εβp dia≡2μ0ε〈p⊥〉/〈〈Bp〉〉2 as large as 1.6, and Troyon normalized diamagnetic beta [Plasma Phys. Controlled Fusion 26, 209 (1984); Phys. Lett. 110A, 29 (1985)], βNdia≡108〈βt⊥〉aB0/Ip as large as 4.7. When εβp dia≳1.25, a separatrix entered the vacuum chamber, producing a naturally diverted discharge that was sustained for many energy confinement times, τE. The largest values of εβp and plasma stored energy were obtained when the plasma current was ramped down prior to neutral beam injection. The measured peak ion and electron temperatures were as large as 24 and 8.5 keV, respectively. Plasma stored energy in excess of 2.5 MJ and τE greater than 130 msec were obtained. Confinement times of greater than 3 times that expected from L-mode predictions have been achieved. The fusion power gain QDD reached a value of 1.3×10−3 in a discharge with Ip=1 MA and εβp dia=0.85. A large, sustained negative loop voltage during the steady-state portion of the discharge indicates that a substantial noninductive component of Ip exists in these plasmas. Transport code analysis indicates that the bootstrap current constitutes up to 65% of Ip. Magnetohydrodynamic (MHD) ballooning stability analysis shows that, while these plasmas are near, or at the βp limit, the pressure gradient in the plasma core is in the first region of stability to high-n modes.
Alfvén shock trains
Authors: Malkov M. A. , Kennel C. F. , Wu C. C. , Pellat R. , Shapiro V. D.
Published: July 2002

The Cohen–Kulsrud–Burgers equation (CKB) is used to consider the nonlinear evolution of resistive, quasiparallel Alfvén waves subject to a long-wavelength, plane-polarized, monochromatic instability. The instability saturates by nonlinear steepening, which proceeds until the periodic waveform develops an interior scale length comparable to the dissipation length; a fast or an intermediate shock then forms. The result is a periodic train of Alfvén shocks of one or the other type. For propagation strictly parallel to the magnetic field, there will be two shocks per instability wavelength. Numerical integration of the time-dependent CKB equation shows that an initial, small-amplitude growing wave asymptotes to a stable, periodic stationary wave whose analytic solution specifies how the type of shock embedded in the shock train, and the amplitude and speed of the shock train, depend on the strength and phase of the instability. Waveforms observed upstream of the Earth’s bowshock and cometary shocks resemble those calculated here.
The use of neutral beam heating to produce high performance fusion plasmas, including the injection of tritium beams into the Joint European Torus (JET)*
Authors: Thompson E. , Stork D. , de Esch H. P. L. ,   the JET Teama)
Published: July 2002

The neutral beam injection (NBI) system of the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] has proved to be an extremely effective and flexible heating method capable of producing high performance plasmas and performing a wide range of related physics experiments. High fusion performance deuterium plasmas have been obtained in the hot-ion (HI) H-mode regime, using the central particle fueling and ion heating capabilities of the NBI system in low target density plasmas, and in the pellet enhanced plasma (PEP) H-mode regime, where the good central confinement properties of pellet fueled plasmas are exploited by additional heating and fueling as well as the transition to H mode. The HI H-mode configuration was used for the First Tritium Experiment (FTE) in JET in which NBI was used to heat the plasma using 14 D0 beams and, for the first time, to inject T0 using the two remaining beams. These plasmas had a peak fusion power of 1.7 MW from deuterium–tritium (D–T) fusion reactions. The capability for injection of a variety of beam species (H0, D0, 3He0, and 4He0) has allowed the study of confinement variation with atomic mass and the simulation of α-particle transport. Additionally, the use of the NBI system has permitted an investigation of the plasma behavior near the toroidal β limit over a wide range of toroidal field strengths.
Observation of parametric instabilities in the lower-hybrid range of frequencies in the high-density tokamak
Authors: Takase Y. , Porkolab M. , Schuss J. J. , Watterson R. L. , Fiore C. L. , Slusher R. E. , Surko C. M.
Published: July 2002

Parametric decay processes have been studied using rf probes and CO2 laser scattering during the lower-hybrid wave heating and current drive experiments in the Alcator C tokamak. The most important process is believed to be the nonresonant decay into ion-cyclotron quasimodes and/or that into electron-Landau quasimodes. At lower densities ω0/ωlh(0)>2 [where ω0 is the frequency of the injected pump wave and ωlh(0) is the lower-hybrid frequency evaluated at the plasma center], where efficient current drive and electron heating are obtained, parametric decay is absent or very weak. At higher densities [ω0/ωlh(0) ≲2] strong parametric decay is observed which correlates well with ion tail formation near the plasma edge. Above these densities the electron tail is no longer formed and no significant heating or current drive have been obtained. Parametric decay may be responsible, at least partially, for loss of wave power near the plasma periphery.
Effects of quadratic drag on convection in a saturated porous medium
Authors: Nield D. A. , Joseph Daniel D.
Published: July 2002

The effects of inertia (involving a drag which is quadratic in the velocity) on convection in a fluid-saturated porous medium are considered. It is shown that the effect of quadratic drag is physically significant for natural convection, at realistic values of the Rayleigh number, in a thin layer of a medium whose overall Prandtl number is small. The qualitative effect of quadratic drag on the global stability of the conduction regime, and on bifurcation into the convection regime, is reported. Convection in an inclined slab of material is also discussed.
Diagnostic value of odd-integer half-harmonic emission from laser-produced plasmas
Authors: Seka W. , Afeyan B. B. , Boni R. , Goldman L. M. , Short R. W. , Tanaka K. , Johnston T. W.
Published: July 2002

The diagostic value of odd-integer half-harmonic emission from laser-produced plasmas is evaluated in the light of recent spectral measurements on UV irradiation experiments. It has been found that under certain conditions a sharp, slightly red-shifted feature is observed in the ω/2 spectra. This feature has been identified with a particular mode of the 2ωp decay instability for which one of the plasmon wave vectors vanishes. This feature is eminently well suited for coronal electron temperature measurements. Another half-harmonic (blue-shifted) feature is more easily observed and may serve as a secondary—though less accurate—temperature diagnostic. In contrast, the spectral splitting of the (3)/(2) harmonic emission proved to be ill suited for temperature diagnostics because of its sensitivity to irradiation and observation geometry. Either ω/2 or 3ω/2 emission is, however, a good qualitative indicator for the presence of the 2ωp decay instability although quantitative inferences on the level of the 2ωp decay instability are not possible at this point.
Radial diffusion of energetic tail ions driven by electromagnetic waves of ion-cyclotron range of frequencies in bumpy torus and tokamak geometry
Authors: Chang C. S.
Published: July 2002

Radial particle and energy transport of the high-energy tail ions created by fundamental resonance heating of ion-cyclotron range of frequency waves is studied for tokamak and bumpy torus geometry. The lowest-order distribution function for the high-energy ions is calculated in a two-dimensional velocity space, and all the diffusion coefficients are explicitly evaluated.
The paradox of the inverted temperature profiles between an evaporating and a condensing surface
Authors: Cercignani C. , Fiszdon W. , Frezzotti A.
Published: July 2002

The effect of internal degrees of freedom and of a class of boundary conditions on inverted temperature profiles between evaporating and condensing parallel plates is considered. It is found that such inverted profiles are present also in the case of polyatomic species and for most (but not all) modifications of the boundary conditions.
Consequence of filamentation on stimulated Raman scattering
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

In a plasma with long density scale length, a laser beam breaks up into a number of filaments, each having a size a∼c/ωp. The process of stimulated Raman scattering in these filaments involves a backscattered light wave of radial extent ∼a and a Langmuir wave of extent b∼(aλD)1/2≪a. The strong localization of the interaction region tends to severely suppress the growth rate. However, the filamentation-induced enhancement of pump intensity opposes this effect. At power densities of a few times 1014 W/cm2 at 1 μ filamentation does not seem to have a strong effect on stimulated Raman scattering.
The temperature paradox in the kinetic theory of evaporation
Authors: Hermans L. J. F. , Beenakker J. J. M.
Published: July 2002

It is shown that the inverted temperature profile between an evaporating and a condensing surface, which is a controversial result of kinetic theory, cannot be excluded by nonequilibrium thermodynamics.
Finite beta effects on ion temperature gradient driven modes
Authors: Dong J. Q. , Guzdar P. N. , Lee Y. C.
Published: July 2002

A set of coupled integral equations is derived to investigate the finite beta effects on the slab ion temperature gradient driven modes for a collisionless plasma in a sheared magnetic field. Low beta plasma is considered so that only the parallel component of perturbed vector potential Ã∥ and the perturbed scalar potential φ̃ are involved. Comprehensive parametric studies show that finite beta of several percent could have significant stabilization effects on the slab modes under investigation.
The m=1 convection cell and sawteeth in tokamaks
Authors: Denton Richard E. , Drake J. F. , Kleva Robert G.
Published: July 2002

Resistive magnetohydrodynamic (MHD) simulations of sawtooth activity in tokamaks are presented. An m=1 convection cell driven by the resistivity gradient forms in the center of the column and causes the sawteeth to die away. The nonlinear properties of this convection cell are calculated analytically. Periodic sawteeth are obtained when the parallel thermal conduction is sufficiently large.
Stability of low-shear tokamaks
Authors: Waelbroeck F. L. , Hazeltine R. D.
Published: July 2002

It has been suggested that the recently observed fast sawtooth crashes are caused by a low-shear, pressure-driven ideal instability. This hypothesis is investigated, using asymptotic methods to solve the toroidal mode equations for a class of equilibria characterized by a low-shear central region in which q−1 is small, separated from the wall by a region with finite shear. A dispersion relation that differs significantly from previous results is obtained. An explicit expression for the growth rate is given for a model q profile.
Three dimensionalization of the stratified mixing layer
Authors: Caulfield C. P. , Peltier W. R.
Published: July 2002

A theoretical analysis of the stability of a stratified, two-dimensional Kelvin–Helmholtz billow against three-dimensional perturbations is presented. This predicts the three-dimensional spectrum to be dominated by a shear aligned convective instability, which is localized in the region surrounding the billow core. The results of a direct numerical simulation of the evolution of the three-dimensional stratified mixing layer fully verify the dominance of this convective mode in the mixing transition. The origin of the streamwise streaks of vorticity, which precede turbulent collapse in a stratified shear layer, is thereby explained.
Investigations of liquid surface rheology of surfactant solutions by droplet shape oscillations: Theory
Authors: Tian Yuren , Holt R. Glynn , Apfel Robert E.
Published: July 2002

A theoretical analysis is presented for the shape oscillations of drops suspended in air. For drops of surfactant solution, the oscillation frequency is basically determined by the surface tension; the free-damping constant depends on the surface viscoelasticities. Different types of surfactant mass transfer at the droplet surface produce different surface rheological behaviors. Analytical approximate solutions for free-oscillation frequency and damping constant are derived by a perturbation method as functions of surface compositional elasticity, surface dilatational viscosity, and surface shear viscosity. These solutions are verified with exact numerical solutions. The existence of a second oscillation mode due to surface elasticity is illustrated. The phase relationships between the external driving forces and the droplet shapes for forced oscillations are discussed. It is found that at the 90° phase shift, the driving frequency and the slope of the phase diagram are equivalent to the free-oscillation frequency and damping constant.
Quantitative experimental and numerical investigation of a vortex ring impinging on a wall
Authors: Fabris Drazen , Liepmann Dorian , Marcus Daniel
Published: July 2002

A joint experimental and computational methodology is developed and applied to investigate a vortex ring impinging normally on a wall. The method uses digital particle image velocimetry to make planar flow measurements, which are then used to initialize a second-order finite difference calculation. The experiment and the simulation are compared at later times and agree extremely well. The ring undergoes two rebounds from the wall and continues to expand. During the approach to the wall, peak vorticity grows by 50% due to vortex stretching. Peak vorticity strengths of the secondary and tertiary vortices formed from the shedding boundary layer are 40% and 20% of the primary. In addition, a ring with a Gaussian core is simulated and compared to demonstrate the benefits of using realistic initial conditions.
Consistent initial conditions for the DNS of compressible turbulence
Authors: Ristorcelli J. R. , Blaisdell G. A.
Published: July 2002

Relationships between diverse thermodynamic quantities appropriate to weakly compressible turbulence are derived. It is shown that for turbulence of a finite turbulent Mach number there is a finite effect of compressibility. A methodology for generating initial conditions, consistent with finite Mach number, for the fluctuating pressure, density, and dilatational velocity is given. Use of these initial conditions gives rise to a smooth development of the flow, in contrast to cases in which these fields are specified arbitrarily or set to zero. Comparisons of the effect of different types of initial conditions are made using direct numerical simulation of decaying isotropic turbulence.
Can repulsive forces lead to stable patterns in thin liquid films?
Authors: Erneux Thomas , Gallez Dominique
Published: July 2002

Thin films subject to both attractive and repulsive forces are modeled by a long wave equation for the film thickness. By using bifurcation techniques, we show analytically that stable stationary and periodic structures are possible.
Stability of a traveling roll system in a rotating disk flow
Authors: Schouveiler L. , Le Gal P. , Chauve M. P.
Published: July 2002

The stability of a traveling roll system, which results from the development of a primary instability of the flow between a stationary and a rotating disk, is experimentally studied. The characteristics of this traveling pattern and of the bifurcation from which it results are obtained. We show in particular that the band of the stable roll modes is limited by the Eckhaus secondary instability.
Shear instability inhibition in a cylinder wake by local injection of a viscoelastic fluid
Authors: Cadot O. , Lebey M.
Published: July 2002

The wake behind a circular cylinder in water is visualized when colored solutions are injected through rows of holes pierced in the cylinder. When these solutions are viscoelastic, a drastic change of the shape of the wake is observed compared to that observed for water injections. The aspect ratio of the wake is decreased, the wavelength of the vortices is increased, and a large region of slow fluid motion is developed behind the cylinder. It is shown that these observations are consistent to a shear instability inhibition due to a local elasticity.
Motion of three vortices near collapse
Authors: Leoncini X. , Kuznetsov L. , Zaslavsky G. M.
Published: July 2002

A system of three point vortices in an unbounded plane has a special family of self-similarly contracting or expanding solutions: during the motion, the vortex triangle remains similar to the original one, while its area decreases (grows) at a constant rate. A contracting configuration brings three vortices to a single point in a finite time; this phenomenon known as vortex collapse is of principal importance for many-vortex systems. Dynamics of close-to-collapse vortex configurations depends on the way the collapse conditions are violated. Using an effective potential representation, a detailed quantitative analysis of all the different types of near-collapse dynamics is performed when two of the vortices are identical. We discuss time and length scales, emerging in the problem, and their behavior as the initial vortex triangle is approaching an exact collapse configuration. Different types of critical behaviors, such as logarithmic or power-law divergences are exhibited, which emphasize the importance of the way the collapse is approached. Period asymptotics for all singular cases are presented as functions of the initial vortice’s configurations. Special features of passive particle mixing by near-collapse flows are illustrated numerically.
The role of dissipation in the theory and simulations of homogeneous plasma turbulence, and resolution of the entropy paradox
Authors: Krommes John A. , Hu Genze
Published: July 2002

The role of dissipation in the theory and simulations of homogeneous plasma slices is analyzed with the goal of understanding the ‘‘entropy paradox,’’ which is that a certain positive-definite functional of the perturbed distribution function increases without bound in some situations even though the potentials appear to have achieved a steady state. Confusion arises from an interchange of the limits t→∞ and η→0, where η is a measure of dissipation. It is argued that it is never strictly correct to neglect η; the averaged dissipation approaches a nonzero limit (proportional to the averaged flux) even as η→0. An exactly soluble model is worked out to illustrate the point. In collisionless particle simulations, the particle and heat fluxes may nevertheless saturate with their correct values. The relations of kinetic and fluid entropy balances are discussed with the aid of (1) the Terry–Horton model for collisionless drift waves, and (2) a simple model of the ion-temperature-gradient-driven mode. The rationale for simulations of homogeneous slices of plasma is given, with particular emphasis being placed on the relationship of dissipation in such slices to dissipation in a complete physical domain.
Laser guiding in an axially nonuniform plasma channel
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

A plasma channel produced by a short laser pulse is axially nonuniform due to self-defocusing of the laser. When a delayed second laser pulse propagates through the channel, diffraction and refraction effects do not balance each other exactly, resulting in periodic beam radius variation with the distance of propagation.
More on core-localized toroidal Alfvén eigenmodes
Authors: Berk H. L. , Van Dam J. W. , Borba D. , Candy J. , Huysmans G. T. A. , Sharapov S.
Published: July 2002

A novel type of ideal toroidal Alfvén eigenmode, localized in the low-shear core region of a tokamak plasma, is shown to exist, whose frequency is near the upper continuum of the toroidal Alfvén gap. This mode converts to a kinetic-type toroidal Alfvén eigenmode above a critical threshold that depends on aspect ratio, pressure gradient, and shear. Opposite to the usual ideal toroidal Alfvén eigenmode, this new mode is peaked in amplitude on the small-major-radius side of the plasma.
Design and modeling of ignition targets for the National Ignition Facility
Authors: Haan Steven W. , Pollaine Stephen M. , Lindl John D. , Suter Laurance J. , Berger Richard L. , Powers Linda V. , Alley W. Edward , Amendt Peter A. , Futterman John A. , Levedahl W. Kirk , Rosen Mordecai D. , Rowley Dana P. , Sacks Richard A. , Shestakov Aleksei I. , Strobel George L. , Tabak Max , Weber Stephen V. , Zimmerman George B. , Krauser William J. , Wilson Douglas C. , Coggeshall Stephen V. , Harris David B. , Hoffman Nelson M. , Wilde Bernhard H.
Published: July 2002

Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x-ray irradiation, and to use a low-Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions.
Plasma sheath in a tilted magnetic field: Closing of the diamagnetic currents; effect on plasma convection
Authors: Cohen Ronald H. , Ryutov Dmitri D.
Published: July 2002

A plasma on open field lines that intersect endplates at a shallow angle, as in a tokamak with a toroidal limiter or poloidal divertor, is considered. A microscopic picture is developed, which shows the closure of the electron diamagnetic and electric drift currents approaching the wall by parallel (to B) electron current. A proper account of the radial variation of the ion ‘‘skin’’ current in the ion gyrosheath is made. The resulting sheath current-voltage characteristics are derived. Two applications are considered: the toroidal potential variations induced by a wavy limiter or divertor surface, which can lead to significant plasma convection, and the determination of the boundary conditions appropriate for macroscopic instabilities in such a configuration.
The structure of three-dimensional magnetic neutral points
Authors: Parnell C. E. , Smith J. M. , Neukirch T. , Priest E. R.
Published: July 2002

The local configurations of three-dimensional magnetic neutral points are investigated by a linear analysis about the null. It is found that the number of free parameters determining the arrangement of field lines is four. The configurations are first classified as either potential or non-potential. Then the non-potential cases are subdivided into three cases depending on whether the component of current parallel to the spine is less than, equal to or greater than a threshold current; therefore there are three types of linear non-potential null configurations (a radial null, a critical spiral and a spiral). The effect of the four free parameters on the system is examined and it is found that only one parameter categorizes the potential configurations, whilst two parameters are required if current is parallel to the spine. However, all four parameters are needed if there is current both parallel and perpendicular to the spine axis. The magnitude of the current parallel to the spine determines whether the null has spiral, critical spiral or radial field lines whilst the current perpendicular to the spine affects the inclination of the fan plane to the spine. A simple method is given to determine the basic structure of a null given M the matrix which describes the local linear structure about a null point.
Transport processes and entropy production in toroidal plasmas with gyrokinetic electromagnetic turbulence
Authors: Sugama H. , Okamoto M. , Horton W. , Wakatani M.
Published: July 2002

Transport processes and resultant entropy production in magnetically confined plasmas are studied in detail for toroidal systems with gyrokinetic electromagnetic turbulence. The kinetic equation including the turbulent fluctuations are double averaged over the ensemble and the gyrophase. The entropy balance equation is derived from the double-averaged kinetic equation with the nonlinear gyrokinetic equation for the fluctuating distribution function. The result clarifies the spatial transport and local production of the entropy due to the classical, neoclassical and anomalous transport processes, respectively. For the anomalous transport process due to the electromagnetic turbulence as well as the classical and neoclassical processes, the kinetic form of the entropy production is rewritten as the thermodynamic form, from which the conjugate pairs of the thermodynamic forces and the transport fluxes are identified. The Onsager symmetry for the anomalous transport equations is shown to be valid within the quasilinear framework. The complete energy balance equation, which takes account of the anomalous transport and exchange of energy due to the fluctuations, is derived from the ensemble-averaged kinetic equation. The intrinsic ambipolarity of the anomalous particle fluxes is shown to hold for the self-consistent turbulent electromagnetic fields satisfying Poisson’s equation and Ampère’s law.
Magnetic reconnection in a magnetohydrodynamic plasma
Authors: Kulsrud Russell M.
Published: July 2002

Magnetic reconnection is important because of its connection with the topology of field lines. In general, a change in topology means a change of equilibrium, and a release of energy, such as occurs in solar flares. In the context of the solar flare two models for magnetic reconnection, the Sweet–Parker and the Petschek mechanism are presented. The pros and cons of these two models are presented. The role of anomalous resistivity in the Sweet–Parker model is discussed. The bearing of a laboratory experiment and a boundary layer analysis of the problem are described.
Active control of 2/1 magnetic islands in a tokamak
Authors: Navratil G. A. , Cates C. , Mauel M. E. , Maurer D. , Nadle D. , Taylor E. , Xiao Q. , Reass W. A. , Wurden G. A.
Published: July 2002

Closed and open loop control techniques were applied to growing m/n=2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse (HBT-EP) [J. Fusion Energy 12, 303 (1993)]. HBT-EP combines an adjustable, segmented conducting wall (which slows the growth or stabilizes ideal external kinks) with a number of small (6° wide toroidally) driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.
On the heating mode transition in high-frequency inductively coupled argon discharge
Authors: Seo Sang-Hun , Hong Jung-In , Bai Keun-Hee , Chang Hong-Young
Published: July 2002

In high-frequency inductively coupled argon discharges with a planar-type coil the phenomena of discharge mode transition (E–H mode transition) are investigated. Experimental observation is done at the low pressure of 10 mTorr and the high frequency of 19 MHz over a range of rf power, 40–525 W. First of all, the discharge mode transition is observed through a change of luminous intensity. This transition is found to occur at the relatively high power of about 280 W compared with the mode transition in a 6.5 MHz discharge. Also, some distinctive features are compared to low-frequency discharges during this transition. In particular, during the E–H mode transition the apparent changes of plasma potential are observed and the sudden variation of plasma potential is proposed as an important factor that indicates the change of power coupling. The features of the discharge mode transition in high-frequency discharge are discussed by considering the power coupling at each mode by measurements of the electron energy distribution functions.