bluuebunny/crossref_metadata_2025 huggingface dataset:
Departures from the Boussinesq approximation in laminar Bénard convection
Authors: Paolucci S. , Chenoweth D. R.
Published: July 2002

A two-dimensional computer program has been used to obtain stability and heat transfer results for flow in an air layer heated from below. The gas is contained in a cavity of aspect ratio 20, with rigid conducting top and bottom walls, and rigid insulating side walls. Steady-state results are presented for a temperature difference parameter ε=β̄ΔT/2 as large as 0.6. Consistent with Busse’s analysis [J. Fluid Mech. 30, 625 (1967)], it is found that the critical Rayleigh number and the Nusselt number correlation depend on ε2; furthermore the coefficients for the corrections are given here. For small but non-Boussinesq values of ε, the results are in agreement with the linear theory of Busse and the experimental results of Ahlers [J. Fluid Mech. 98, 137 (1980)].
Hall magnetohydrodynamics: Conservation laws and Lyapunov stability
Authors: Holm Darryl D.
Published: July 2002

Hall electric fields produce circulating mass flow in confined ideal-fluid plasmas. The conservation laws, Hamiltonian structure, equilibrium state relations, and Lyapunov stability conditions are presented here for ideal Hall magnetohydrodynamics (HMHD) in two and three dimensions. The approach here is to use the remarkable array of nonlinear conservation laws for HMHD that follow from its Hamiltonian structure in order to construct explicit Lyapunov functionals for the HMHD equilibrium states. In this way, the Lyapunov stability analysis provides classes of HMHD equilibria that are stable and whose linearized initial-value problems are well posed (in the sense of possessing continuous dependence on initial conditions). Several examples are discussed in both two and three dimensions.
Subgrid modeling for magnetohydrodynamic turbulent shear flows
Authors: Yoshizawa Akira
Published: July 2002

Subgrid modeling for magnetohydrodynamic (MHD) turbulent shear flows is studied with the aid of a statistical result from a multiscale direct-interaction theory. The ensemble-mean MHD correlation function is combined with a filtering procedure to give a subgrid-scale MHD stress. This result leads to a subgrid model of Smagorinsky type, which is applicable to large eddy simulations of earth’s magnetic dynamo and reversed-field pinches of plasma.
Multiple-time-scale modeling of turbulent flows in one-point closures
Authors: Schiestel Roland
Published: July 2002

A particular multiple-scale modeling is developed on the basis of partial integration of spectral evolution equations on wavelength intervals. Several problems of turbulence modeling are considered within this framework such as influence of departure from equilibrium and the return to isotropy process. A simplified version is applied to the prediction of free turbulent shear flows.
The solution of the plasma equation in plane parallel geometry with a Maxwellian source
Authors: Bissell R. C. , Johnson P. C.
Published: July 2002

The plasma equation for a warm collisionless plasma with a Maxwellian particle source is solved in plane parallel geometry. The generalized Bohm criterion is used to identify the plasma–sheath boundary. This kinetic treatment, in common with fluid and cold-ion kinetic models, results in an infinite electric field at the sheath edge. This is in sharp contrast to results from a previous warm-ion kinetic model, by Emmert et al. [Phys. Fluids 23, 803 (1980)], which gave a finite electric field at the sheath edge. Also, the presheath potential given by the present model is greater than that given by Emmert and is in better agreement with fluid results.
Plasma current sustained by fusion charged particles in a field-reversed configuration
Authors: Berk H. L. , Momota H. , Tajima T.
Published: July 2002

The distribution of energetic charged particles generated by thermonuclear fusion reactions in a field-reversed configuration (FRC) are studied analytically and numerically. A fraction of the charged fusion products escapes directly while the others are trapped to form a directed particle flow parallel to the plasma current. It is shown that the resultant current density produced by these fusion charged particles can be comparable to background plasma current density that produces the original field-reversed configuration in a D–3He reactor. Self-consistent equilibria arising from the currents of the background plasma and proton fusion products are constructed where the Larmor radius of the fusion product is of arbitrary size. Reactor relevant parameters are examined, such as how the fusion reactivity rate varies as a result of supporting the pressure associated with the fusion products. A model for synchrotron emission from various pressure profiles is developed and it is shown quantitatively how synchrotron losses vary with different pressure profiles in a FRC.
Dynamics of localized ion-acoustic waves in a magnetized plasma
Authors: Qian S. , Lotko W. , Hudson M. K.
Published: July 2002

The evolution of negative potential pulses in a magnetized plasma is studied. A three-dimensional (3-D) nonlinear ion-acoustic wave equation, including nonstationary effects of reflected electrons, has been derived from the Poisson–Vlasov equations with uniform magnetic field. In the low temperature limit, the equation is the Zakharov–Kuznetsov equation. Computer simulations of 1-D and 2-D versions of the equation have been performed. The studies show that a negative potential pulse can be enhanced by drifting electrons. The growing pulse develops asymmetrically with an oscillatory precursor and a local potential jump resembling the early phase of weak double layer formation. Two-dimensional pulses also show different scale lengths along the magnetic field and in the transverse dimension. Comparisons are made with results from particle simulations and spacecraft observations.
Scalings of implosion experiments for high neutron yield
Authors: Takabe H. , Yamanaka M. , Mima K. , Yamanaka C. , Azechi H. , Miyanaga N. , Nakatsuka M. , Jitsuno T. , Norimatsu T. , Takagi M. , Nishimura H. , Nakai M. , Yabe T. , Sasaki T. , Yoshida K. , Nishihara K. , Kato Y. , Izawa Y. , Yamanaka T. , Nakai S.
Published: July 2002

A series of experiments focused on high neutron yield has been performed with the Gekko-XII green laser system [Nucl. Fusion 27, 19 (1987)]. Deuterium–tritium (DT) neutron yield of 1013 and pellet gain of 0.2% have been achieved. Based on the experimental data from more than 70 irradiations, the scaling laws of the neutron yield and the related physical quantities have been studied. Comparison of the experimental neutron yield with that obtained by using a one-dimensional fluid code has led to the conclusion that most of the neutrons produced in the stagnation phase of the computation are not observed in the experiment because of fuel–pusher mixing, possibly induced by the Rayleigh–Taylor instability. The coupling efficiency and ablation pressure have been calculated using the ion temperature measured experimentally. A coupling efficiency of 5.5% and an ablation pressure of 50 Mbar have been obtained.
Roll waves on a layer of a muddy fluid flowing down a gentle slope—A Bingham model
Authors: Liu Ko-fei , Mei Chiang C.
Published: July 2002

To study rapid and laminar flow of a mud layer down a gentle slope, a shallow layer of Bingham fluid is considered. Attention is focused on long waves of finite amplitude. Convective inertia is kept in the approximate equations which are simplified by Kármán’s momentum integral approach with an assumed velocity profile. A linearized instability analysis is first carried out for periodic disturbances. The nonlinear development of unstable disturbances is computed numerically to study the formation of periodic shocks, or roll waves. Nonlinear motion caused by external influx to an otherwise steadily flowing mud layer is also investigated.
Flow through arrays of cylinders: Lattice gas cellular automata simulations
Authors: McCarthy J. F.
Published: July 2002

A presentation is given of the results of two-dimensional lattice gas cellular automata (LGA) simulations for calculating the permeability for flow through periodic arrays of cylinders, where the cylinders have either a rhomboid or cruciform cross section. For the case of rhombi, the results are related via a semiempirical shape factor to analytical solutions for flow through arrays of circular cylinders. Finite size effects are studied by comparing simulation results with theory. This calibration of the LGA model then validates its use for the study of flow around cruciforms, for which there is no analytical solution. This has an important application in the mathematical modeling of flow through arrays of dendrites in the mushy region during the solidification of alloy steel.
Chaotic oscillations and breakup of quasigeostrophic vortices in the N-layer approximation
Authors: Marshall J. S.
Published: July 2002

This paper examines the onset of chaotic oscillations in the N-layer model of stratified quasigeostrophic flow due to interaction of N vortex sections in different layers, and then considers the possible role of chaotic self-induced motion in the breakup of quasigeostrophic vortices. In order to investigate the nature of the long-time evolution of the vortex using a low-dimensional dynamical system, attention is restricted to vortices with uniform potential vorticity within the core and sufficiently large values of the stratification parameter that the vortex cross section remains nearly circular with time. Computations using contour dynamics in many-layered systems are performed to check that the deformation of the vortex cross section is small. It is shown that for finite-amplitude perturbations of the vortex centerline, the barotropic interaction between sections of the vortex in different layers can cause the vortex to oscillate chaotically. The approach to chaotic motion is initially investigated using a system with only four layers, in which the vortex is approximated by equal-strength segments of either point vortices or circular patches. The onset of chaos is shown to be sensitive to the stratification parameter, the vortex core radius, and the initial configuration. The consequences of chaos on the vortex evolution and breakup are then demonstrated by numerical computations with a large number of layers for vortices with finite-core area.
Deterministic chaos in the gas inlet pressure of gas–liquid bubbling systems
Authors: Mittoni Louis John , Schwarz Mark Philip , La Nauze Robert David
Published: July 2002

In an attempt to assess the suggestion that submerged gas injection can under certain conditions be described as exhibiting chaotic dynamics, a short analysis has been undertaken on the results of a set of experiments over a limited set of conditions including the variation of chamber volume, injection nozzle diameter, liquid viscosity, and gas flow rate at a temperature of 20 °C. A convenient noninvasive approach is demonstrated using the upstream pressure signal to reconstruct the return map attractors. The classic period doubling sequence leading to irregular behavior is observed only over certain system parameter ranges. The important parameters believed to be responsible for the deterministic chaotic behavior and their relationship are briefly discussed.
An integral equation approach to internal (2-layer) solitary waves
Authors: Evans W. A. B. , Ford M. J.
Published: July 2002

The exact within potential flow integral equation approach of Evans and Ford [Proc. R. Soc. London Ser. A 452, 373 (1996)] for the normal solitary wave, is here generalized to 2-layer, ‘‘internal’’ solitary waves. This differs in its mathematical form from other exact integral equation methods based on the complex velocity potential. For both ‘‘rigid lid’’ (i.e., flat toplayer surface) and ‘‘free-surface’’ boundary conditions, a set of coupled non-linear integral equations are derived by an application of Green’s theorem. For each point on the layer interface(s), these describe functional constraints on the profiles and interface fluid velocity moduli; the exact profiles and velocities being those forms that satisfy these constraints at all such interface points. Using suitable parametric representations of the profiles and interface velocity moduli as functions of horizontal distance, x, and utilizing tailored quadrature methods [Int. J. Comput. Math. B 6, 219 (1977)], numerical solutions were obtained by the Newton–Raphson method that are highly accurate even at large amplitudes. For ‘‘rigid lid’’ boundary conditions, internal wave solutions are presented for layer density and depth ratios typical of oceanic internal wave phenomena as found in the Earth’s marginal seas. Their various properties, i.e., mass, momentum, energy, circulation, phase and fluid velocities, streamline profiles, internal pressures, etc., are evaluated and compared, where possible, with observed properties of such phenomena as reported, for example, from the Andaman Sea. The nature of the limiting (or ‘‘maximum’’) internal wave is investigated asymptotically and argued to be consistent with two ‘‘surge’’ regions separating the outskirts flow from a wide mid-section region of uniform ‘‘conjugate flow’’ as advocated by Turner and Vanden-Broeck [Phys. Fluids 31, 286 (1988)].
Numerical simulation of particle interactions with wall turbulence
Authors: Pan Y. , Banerjee S.
Published: July 2002

This paper presents the results of a numerical investigation of the effects of near-neutral density solid particles on turbulent liquid flow in a channel. Interactions of particles, in a size range about the dissipative length scale, with wall turbulence have been simulated at low volume fractions (average volume fraction less than 4×10−4). Fluid motion is calculated by directly solving the Navier-Stokes equations by a pseudo-spectral method and resolving all scales of motion. Particles are moved in a Lagrangian frame through the action of forces imposed by the fluid and gravity. Particle effects on fluid motion are fed back at each time step by calculating the velocity disturbance caused by the particles assuming the flow around them is locally Stokesian. Particle-particle interactions are not considered. The slightly heavier-than-fluid particles of the size range considered are found to preferentially accumulate in the low-speed streaks, as reported in several other investigations. It is also found that particles smaller than the dissipative length scale reduce turbulence intensities and Reynolds stress, whereas particles that are somewhat larger increase intensities and stress. By examining higher order turbulence statistics and doing a quadrant analysis of the Reynolds stress, it is found that the ejection-sweep cycle is affected—primarily through suppression of sweeps by the smaller particles and enhancement of sweep activity by the larger particles. A preliminary assessment of the impact of these findings on scalar transfer is made, as enhancement of transfer rate is a motivation of the overall work on this subject. For the case investigated, comparison of the calculations with an existing experiment was possible, and shows good agreement.
Suppression of instability in a liquid film flow
Authors: Lin S. P. , Chen J. N. , Woods D. R.
Published: July 2002

The stability of a viscous liquid film flow down an inclined plane that oscillates in the direction parallel to the flow is analyzed by use of a Chebyshev series solution with the Floquet theory. When the inclined plane is stationary, it is known that the onset of the film instability manifests itself as long surface waves [J. Fluid Mech. 554, 505 (1957); Phys. Fluids 6, 321 (1963)] or relatively short shear waves [‘‘Critical angle of shear wave instability in a film,’’ to appear in J. Appl. Mech.; J. Eng. Math. 8, 259 (1974); Phys. Fluids 30, 983 (1987)], depending on the angle of inclination. It is demonstrated that the unstable film can be stabilized by use of appropriate amplitudes and frequencies of the plate oscillation to suppress the shear waves as well as the long waves. The ranges of amplitude and frequency in which the film can be stabilized depend on the flow parameter.
A novel boundary-integral algorithm for viscous interaction of deformable drops
Authors: Zinchenko Alexander Z. , Rother Michael A. , Davis Robert H.
Published: July 2002

A new three-dimensional boundary-integral algorithm for deformable drops moving in a viscous medium at low Reynolds numbers is developed, which overcomes some familiar difficulties with boundary-integral calculations. The algorithm is used to simulate different modes of interaction between drops or bubbles, primarily for buoyancy-driven motion. The present iterative method for mean curvature calculation is found to be more robust and accurate than contour integration schemes. A novel iterative strategy based on combining biconjugate gradient and simple iterations overcomes the poor convergence of “successive substitutions” for drops in very close approach with extreme viscosity ratio. A substantially new variational method of global mesh stabilization solves the problem of mesh degradation with advantageous, soft stability constraints. A curvatureless boundary-integral formulation is also derived and shown to provide, in principle, a more accurate description of the drop breakup than the conventional formulation. The efficiency of these techniques is demonstrated by numerical examples for two drops in gravity-induced motion with high surface resolutions. The present code successfully simulates mutual approach of slightly deformable drops to extremely small separations, as well as their rotation when in “apparent contact,” thus bridging the gap between finite deformation calculations and a recent asymptotic theory for small capillary numbers. Also provided is a 3D simulation of the experimental phenomenon of enhanced bubble coalescence, discovered by Manga and Stone [J. Fluid Mech. 256, 647 (1993); 300, 231 (1995)]. For drops of viscosity comparable to that of the surrounding fluid, it is shown in contrast that breakup is a typical result of hydrodynamic interaction in gravity-induced motion for large and even moderate capillary numbers. The code is readily applicable to any type of an ambient flow and may be adapted to more than two drops.
Analysis and classification of reaction-driven stationary convective patterns in a porous medium
Authors: Subramanian S. , Balakotaiah V.
Published: July 2002

A two-dimensional model consisting of continuity, momentum, species, and energy balances is considered to analyze the convective patterns that arise due to exothermic reactions occurring in a porous medium. First, the one-dimensional conduction states of the system are classified using singularity theory and the shooting technique. It is observed that there can be either one or three conduction states when the reacting fluid is a gas. Next, we use linear stability analysis to determine the boundary of the parameter values at which the conduction state loses stability leading to convective flows. Pure and mixed-mode convective solutions are then analyzed using local bifurcation theory. The formulas to evaluate the coefficients appearing in the amplitude equations are developed and used to obtain the classification (phase) diagram of the convective flows in the parameter space. The classification is presented in the unique conduction solution region in the presence and absence of mode interactions. The phase diagrams are used to identify the region of parameter values where convection has a detrimental effect on the stability of the system. It is found that the Lewis number (Le), which represents the ratio of thermal to mass diffusivity, has a profound influence on the stability boundaries. For Le>1, the convective solutions may bifurcate subcritically and introduce an ignition point.
Dispersion in two-dimensional periodic porous media. Part I. Hydrodynamics
Authors: Amaral Souto Hélio P. , Moyne Christian
Published: July 2002

In this paper we investigate the hydrodynamics of two-dimensional spatially periodic porous media. The finite volume method is used to compute the flow field in these media with the Navier–Stokes equations. Two particular points are described: how to deal with the periodic boundary conditions on the surface of a unit cell; and how to avoid numerical dispersion. The fluid flow is computed for the Stokes regime and for moderate values of the particle Reynolds number Rep, up to Rep=200. Four types of media are studied. Three of them are “ordered,” with in-line or staggered square cylinders and zigzag medium. The fourth is “disordered,” with randomly distributed square cylinders. The effect of the direction of the average flow velocity is analysed in all these cases. In the Stokes regime, the results for the pressure drop agree with the usual evaluation in terms of permeability. For higher particle Reynolds numbers, the non-linear correction to Darcy’s law is discussed. The correction term for moderate Reynolds number is found to be cubic in the average flow velocity for isotropic media.
The influence of surfactant on two-phase flow in a flexible-walled channel under bulk equilibrium conditions
Authors: Yap Darren Y. K. , Gaver Donald P.
Published: July 2002

A preliminary model of pulmonary airway reopening is developed that includes the physicochemical influence of surfactant under bulk-equilibrium conditions. The airway is modeled following Gaver et al. [J. Fluid. Mech. 319, 25–65 (1996)] as a flexible-walled channel, where walls are membranes under longitudinal tension T, and supported with elasticity E with a stress-free separation distance 2H. The lining fluid has viscosity μ and surface tension γ*. Airway reopening occurs when a semi-infinite bubble of air with pressure Pb* progresses steadily at velocity U and separates the walls. Surfactant exists in the lining fluid (C*) and at the air–liquid interface (Γ*). Bulk equilibrium is assumed (C*=C0) and the kinetic transfer of surfactant between the bulk and interface occurs with a rate k. The equilibrium relationship between Γ* and C* is based upon Henry’s isotherm (Γeq=KC0). The surface tension equation of state, a relationship between γ* and Γ*, is assumed to be linear near Γeq. Marangoni stresses develop from the transport of surfactant at the interface, leading to interfacial rigidification and changes in the airway reopening behavior. The behavior is governed by the following dimensionless parameters: the capillary number Ca=μU/γeq, the surface elasticity number El=−(dγ*/dΓ*)(Γeq/γeq), the modified Stanton number Stλ=(k/K)/(U/H), the wall elastance parameter β=EH2/γeq, the wall tension ratio η=T/γeq, and the surface Pèclet number Peint=UH/Dint. The results indicate that El can have a dual, contrasting influence on the airway reopening behavior. By increasing El through an increase in dγ*/dΓ* (method 1), larger Pb* are predicted from the resulting interfacial surfactant gradients and interfacial rigidification. In contrast, increasing Γeq (method 2) increases El but reduces Pb* due to the global reduction of γ*; however, the reduction in Pb* is augmented by the increasing importance of viscous, elastic, tension and Marangoni stresses. Furthermore, for Stλ>10 the interface remains mobile due to rapid surfactant adsorption and the elimination of Marangoni stresses, which minimizes Pb*. This behavior may be important in the development of improved exogenous surfactants for the treatment of a variety of pulmonary diseases.
Two-dimensional Navier–Stokes simulation of breaking waves
Authors: Chen Gang , Kharif Christian , Zaleski Stéphane , Li Jie
Published: July 2002

Numerical simulations describing plunging breakers including the splash-up phenomenon are presented. The motion is governed by the classical, incompressible, two-dimensional Navier–Stokes equation. The numerical modeling of this two-phase flow is based on a piecewise linear version of the volume of fluid method. Capillary effects are taken into account such as a nonisotropic stress tensor concentrated near the interface. Results concerning the time evolution of liquid–gas interface and velocity field are given for short waves, showing how an initial steep wave undergoes breaking and successive splash-up cycles. Breaking processes including overturning, splash-up and gas entrainment, and breaking induced vortex-like motion beneath the surface and energy dissipation, are presented and discussed. It is found that strong vorticities are generated during the breaking process, and that more than 80% of the total pre-breaking wave energy is dissipated within three wave periods. The numerical results are compared with some laboratory measurements, and a favorable agreement is found.
Double diffusive instability in an inclined cavity
Authors: Bergeon A. , Ghorayeb K. , Mojtabi A.
Published: July 2002

Double diffusive convection in a rectangular two-dimensional cavity with imposed temperatures and concentrations along two opposite sidewalls is considered. The study is performed for two-dimensional cavities in which the thermal and solutal buoyancy forces have the same magnitude, but are of opposite sign. The influence on the convective instability of the aspect ratio A (height/length) of the cavity and the cavity inclination α with respect to gravity is discussed. The onset of convection is computed for an infinite layer and compared to that for bounded boxes. The study is completed by the continuation of bifurcating solutions. It is found that, due to centrosymmetry, steady bifurcations are either pitchfork or transcritical depending on A and α. However, a primary pitchfork bifurcation is found to create unstable steady solutions, even if it is the first bifurcation. For the aspect ratios we studied, and close to the onset of convection, the stable solutions are mainly one-roll structures that can be destabilized by further interactions with asymmetric solutions created at primary pitchfork bifurcations. For large aspect ratios, additional asymmetric one-roll solutions are created via more complex branch interactions.
Air flow separation over unsteady breaking waves
Authors: Reul N. , Branger H. , Giovanangeli J.-P.
Published: July 2002

The evolution of the airflow instantaneous structure over an unsteady breaking wave propagating in a group is measured in detail using the digital particle image velocimetry technique. It is found that the boundary-layer over a breaking wave, the steepest in the group, separates at a point close to the sharp crest and reattaches in the front slope of the following wave. During breaking, the evolution of the turbulent vorticity is essentially unsteady and the recirculation zone of the separated flow takes the form of a large well-organized vortex. Links between the wave-crest geometry and geometrical features of the separation bubble have been established.
Non-Gaussianity and coherent vortex simulation for two-dimensional turbulence using an adaptive orthogonal wavelet basis
Authors: Farge Marie , Schneider Kai , Kevlahan Nicholas
Published: July 2002

We decompose turbulent flows into two orthogonal parts: a coherent, inhomogeneous, non-Gaussian component and an incoherent, homogeneous, Gaussian component. The two components have different probability distributions and different correlations, hence different scaling laws. This separation into coherent vortices and incoherent background flow is done for each flow realization before averaging the results and calculating the next time step. To perform this decomposition we have developed a nonlinear scheme based on an objective threshold defined in terms of the wavelet coefficients of the vorticity. Results illustrate the efficiency of this coherent vortex extraction algorithm. As an example we show that in a 2562 computation 0.7% of the modes correspond to the coherent vortices responsible for 99.2% of the energy and 94% of the enstrophy. We also present a detailed analysis of the nonlinear term, split into coherent and incoherent components, and compare it with the classical separation, e.g., used for large eddy simulation, into large scale and small scale components. We then propose a new method, called coherent vortex simulation (CVS), designed to compute and model two-dimensional turbulent flows using the previous wavelet decomposition at each time step. This method combines both deterministic and statistical approaches: (i) Since the coherent vortices are out of statistical equilibrium, they are computed deterministically in a wavelet basis which is remapped at each time step in order to follow their nonlinear motions. (ii) Since the incoherent background flow is homogeneous and in statistical equilibrium, the classical theory of homogeneous turbulence is valid there and we model statistically the effect of the incoherent background on the coherent vortices. To illustrate the CVS method we apply it to compute a two-dimensional turbulent mixing layer.
A comparison of kinetic and multifluid simulations of laser-produced colliding plasmas
Authors: Rambo P. W. , Procassini R. J.
Published: July 2002

The amount of interpenetration and stagnation in counterstreaming laser-produced plasmas is investigated via a kinetic model and a multifluid model in one-dimensional planar geometry. Each of these models can evolve multiple ion species in the self-consistent electrostatic field with Coulomb collisional interaction. The two approaches are applied to various colliding plasma systems, ranging from simple homogeneous slowing-down and temperature-equilibration problems to colliding plasmas characteristic of laser irradiated thin foils and thick disks. In particular, direct comparisons are made between the kinetic formulation and the multifluid model with various approximations for the multifluid collisional coupling coefficients.
Toroidal Alfvén eigenmode-induced ripple trapping
Authors: White R. B. , Fredrickson E. , Darrow D. , Zarnstorff M. , Wilson R. , Zweben S. , Hill K. , Chen Yang , Fu Guoyong
Published: July 2002

Toroidal Alfvén eigenmodes are shown to be capable of inducing ripple trapping of high-energy particles in tokamaks, causing intense localized particle loss. The effect has been observed in TFTR [R. Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)].
Large Mach number ion acoustic rarefactive solitary waves for a two electron temperature warm ion plasma
Authors: Ghosh S. S. , Ghosh K. K. , Sekar Iyengar A. N.
Published: July 2002

An exact analytical form of Sagdeev pseudopotential has been derived for a two electron temperature warm ion plasma, from which ion acoustic rarefactive solitary wave solutions could be investigated for a wide range of different plasma parameters, viz., ion temperature (σ), cold to hot electron temperature ratio (β), and initial cold electron concentration (μ). Explicitly large Mach numbers have been obtained for increasing hot to cold electron temperature ratios, and an analytical condition for the upper bound of the Mach number has been derived for such a rarefactive solitary wave. It is found that the width of these waves obey Korteweg–de Vries soliton-type behavior only for small amplitudes (i.e., eφ/Teff<1) while for large amplitudes, the width of the rarefactive solitary waves increases with increasing amplitude.
Two-dimensional radiation-magnetohydrodynamic simulations of SATURN imploding Z pinches
Authors: Hammer James H. , Eddleman James L. , Springer Paul T. , Tabak Max , Toor Arthur , Wong Keith L. , Zimmerman George B. , Deeney Chris , Humphreys Russ , Nash Thomas J. , Sanford Thomas W. L. , Spielman Rick B. , De Groot John S.
Published: July 2002

Z-pinch implosions driven by the SATURN device [D. D. Bloomquist et al., Proceedings of the 6th Institute of Electrical and Electronics Engineers (IEEE) Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] at Sandia National Laboratory are modeled with a two-dimensional radiation magnetohydrodynamic (MHD) code, showing strong growth of the magneto-Rayleigh–Taylor (MRT) instability. Modeling of the linear and nonlinear development of MRT modes predicts growth of bubble-spike structures that increase the time span of stagnation and the resulting x-ray pulse width. Radiation is important in the pinch dynamics, keeping the sheath relatively cool during the run-in and releasing most of the stagnation energy. The calculations give x-ray pulse widths and magnitudes in reasonable agreement with experiments, but predict a radiating region that is too dense and radially localized at stagnation. We also consider peaked initial density profiles with constant imploding sheath velocity that should reduce MRT instability and improve performance. Krypton simulations show an output x-ray power ≳80 TW for the peaked profile.
Electron temperature fluctuations in drift-resistive ballooning turbulence
Authors: Zeiler A. , Drake J. F. , Biskamp D.
Published: July 2002

Three-dimensional nonlinear simulations of collisional plasma turbulence are presented to model the behavior of the edge region of tokamak discharges. Previous work is extended by including electron temperature fluctuations T̃e. The basic paradigm that turbulence and transport are controlled by resistive ballooning modes in low temperature plasma and nonlinearly driven drift wave turbulence in higher temperature regimes persists in the new system. Parallel thermal conduction strongly suppresses the ability of the electron temperature gradient ∇Te to drive the turbulence and transport everywhere except the very low temperature edge of the resistive ballooning regime. As a consequence, over most of the resistive ballooning regime only the density gradient drives the turbulence and the temperature fluctuations are convected as a passive scalar. In the drift wave regime only the density gradient acts to drive the nonlinear instability and the temperature fluctuations have a relatively strong stabilizing influence on the turbulence due to an enhanced damping of density and potential fluctuations resulting from local electron heating.
Anomalous width variations for ion acoustic rarefactive solitary waves in a warm ion plasma with two electron temperatures
Authors: Ghosh S. S. , Sekar Iyengar A. N.
Published: July 2002

Anomalous width–amplitude variations were observed in large amplitude rarefactive solitary waves which show increasing width with increasing amplitude, contrasting the usual reciprocal relation between the square of the width and the amplitude, beyond a certain value of the plasma parameters [S. S. Ghosh, K. K. Ghosh, and A. N. Sekar Iyengar, Phys. Plasmas, 3, 3939 (1996)]. For the limiting maximum amplitude, the “increasing width” solitary wave tends to a double layer-like solution. The overall variation was found to depend crucially on the specific parameter space. From a detailed investigation of the above behavior, a plausible physical explanation has been presented for such increases in the width. It is found that the ions’ initial kinetic energies and the cold electron concentration within the perturbed region play a significant role in determining the observed width–amplitude variation. This contradicts the investigation of Sayal, Yadav, and Sharma [Phys. Scr. 47, 576 (1993)].
Characterization of an atmospheric pressure plasma generated by a plasma torch array
Authors: Koretzky E. , Kuo S. P.
Published: July 2002

Based on a capacitively coupled electrical discharge scheme, it is demonstrated that an array of plasma torches can be lit up simultaneously to form a dense plasma layer in the open air by a single ac power source. The number of torches is only limited by the power handling capability of the source. The measured v–i characteristic of the discharge indicates that the torch is operating in the diffuse arc mode. Experiments have been performed to explore the effect of the plasma torches on the propagation of microwaves in an X-band rectangular waveguide by passing the plasma of the torches through holes in the top and bottom walls of the waveguide. The results show that the plasma torches can effectively attenuate microwaves. The wave–plasma interaction process is also analyzed numerically. The plasma parameters deduced from the theoretical model, by matching the numerical results with those of experiments, are shown to agree well with the experimental measurements.
Nonlinear coupling of rotating magnetic island triplets
Authors: Coelho R. M. , Lazzaro E. , Nave M. F. , Serra F.
Published: July 2002

The role and mechanism of nonlinear coupling of finite size rotating magnetic islands in a tokamak are analyzed in a finite dimensional model obtained from the continuum Reduced Magnetohydrodynamic (RMHD) description of resistive tearing modes. General properties of the electrodynamic interaction of modes of different helicities are found and a phase space analysis of the effects of inertia and eddy currents in the tokamak wall is presented. The results obtained are relevant for the definition of strategies of feedback control of resistive instabilities and a possible help for the interpretation of tokamak performances linked to the onset of a spectrum of magnetic perturbations.
Stability of short wavelength tearing and twisting modes
Authors: Waelbroeck F. L.
Published: July 2002

The stability and mutual interaction of tearing and twisting modes in a torus is governed by matrices that generalize the well-known Δ′ stability index. The diagonal elements of these matrices determine the intrinsic stability of modes that reconnect the magnetic field at a single resonant surface. The off-diagonal elements indicate the strength of the coupling between the different modes. A method is presented for evaluating the elements of these matrices, in the limit of short wavelength, from the free energy driving radially extended ballooning modes. This method is applied to calculate the tearing and twisting Δ′ for a model high-beta equilibrium with circular flux surfaces.
Generation of plasma rotation by ion cyclotron resonance heating in tokamaks
Authors: Chang C. S. , Phillips C. K. , White R. , Zweben S. , Bonoli P. T. , Rice J. E. , Greenwald M. J. , deGrassie J.
Published: July 2002

A physical mechanism for generation of a plasma rotation and radial electric field by ion cyclotron resonance heating (ICRH) is presented in a tokamak geometry. By breaking the omnigenity of resonant ion orbits, ICRH can induce a nonambipolar minor-radial transport of resonant ions. This yields a radial charge separation, a modification to radial electric field Er, and the generation of plasma rotation. It is estimated that the ICRH fast-wave power available in the present-day tokamak experiments can be large enough to give a significant modification to plasma rotation.
Observation of ionization fronts in low density foam targets
Authors: Hoarty D. , Willi O. , Barringer L. , Vickers C. , Watt R. , Nazarov W.
Published: July 2002

Ionization fronts have been observed in low density chlorinated foam targets and low density foams confined in gold tubes using time resolved K-shell absorption spectroscopy. The front was driven by an intense pulse of soft x-rays produced by high power laser irradiation. The density and temperature profiles inferred from the radiographs provided detailed measurement of the conditions. The experimental data were compared to radiation hydrodynamics simulations and reasonable agreement was obtained.
Effect of dust charge inhomogeneity on linear and nonlinear dust–acoustic wave propagation
Authors: Singh S. V. , Rao N. N.
Published: July 2002

Propagation of linear as well as nonlinear dust–acoustic waves (DAWs) in an inhomogeneous dusty plasma consisting of electrons, ions, and dust particles is investigated by taking into account equilibrium dust charge inhomogeneity along with the density inhomogeneity of the plasma and dust particles. For the linear case with harmonic perturbations, coupled equations for self-consistently determining the wave amplitude and the wave number have been derived. On the other hand, nonlinear DAWs are shown to be governed by a Korteweg–de Vries (KdV) type of evolution equation having variable coefficients arising due to charge and density inhomogeneities. Qualitatively, the amplitudes of the linear and nonlinear DAWs are found to decrease (increase) as the waves propagate into regions of increasing (decreasing) dust charge, which is similar to the behavior found in the case with density inhomogeneity alone. Quantitatively, the amplitude of the dust number density perturbation in the linear wave is proportional to qd0−3/2 nd0−1/2 for a cold dusty plasma, while in the nonlinear regime it scales as qd0−1 nd0−1/4. Approximate analytical solutions of the KdV equation have been obtained by making use of a suitable set of coordinate transformations.
Linear stability of an accelerated, current carrying wire array
Authors: Hammer J. H. , Ryutov D. D.
Published: July 2002

The linear stability of an array of a large number of thin wires is considered. The wires form a cylindrical surface, accelerated toward the axis under the action of a current. Stability equations are derived and a complete classification of the modes is presented. In agreement with Felber and Rostoker [Phys. Fluids 24, 1049 (1981)], it is shown that there exist two types of modes: medial modes, with deformation in the rz plane, and lateral modes, with only azimuthal deformation. For a given axial wave number, k, the most unstable medial mode has all the wires moving in phase similar to an axisymmetric mode for a continuous shell, whereas the most unstable lateral perturbation has opposite displacements of neighboring wires. Lateral modes are of particular interest because they may remain unstable for parameters where medial modes are stable. Numerical analysis of the dispersion relation for a broad range of modes is presented. Some limiting cases are discussed. It is shown that k1/2 scaling holds until surprisingly high wave numbers, even exceeding the inverse interwire distance. In the long-wavelength limit, the wires behave as a continuous shell with strong anisotropy of the electrical conductivity, i.e., high along the wires and vanishing across the wires. The results differ considerably from the modes of a thin, perfectly conducting shell. In particular, a new “zonal flow” mode is identified.
Characteristic behavior of the sheath formation in thermal plasma
Authors: Das G. C. , Singha Bornali , Chutia Joyanti
Published: July 2002

The plasma sheath equation, based on the quasipotential analysis, has been derived in a plasma taking finite ion temperature into account with a view to showing the characteristic behavior of the sheath formation in front of the electron absorbing wall. The derived Sagdeev potential equation, under small amplitude approximation, describes the Bohm sheath criterion, which enables one to find the required condition for the formation of sheath in plasmas. Later, the sheath equation is completely solved numerically by using the fourth-order Runge–Kutta method to yield the characteristic features of the potential variation in the sheath region. The main goal is to determine the sheath thickness and the potential variation in the sheath region and to compare the variation with those in cold ion plasma. Again, in the higher potential region, an improved Child’s law has been derived and the potential variation has been estimated numerically, which shows that the improved Child’s law improves the results as compared to that by Child’s law.
Nonlinear propagation of self-guided ultra-short pulses in ionized gases
Authors: Bergé L. , Couairon A.
Published: July 2002

The nonlinear propagation regimes of femtosecond pulses in noble gases and air are analyzed from an extended nonlinear Schrödinger equation coupled with the inertial response of an electron plasma created by multiphoton process and avalanche ionization. By means of integral relations supplemented by two variational approaches, it is shown that the electron density produced by photo-ionization defocuses the beam and arrests the self-focusing promoted by the Kerr response of the gas, so that a balance between both these effects can be realized for incident pulses with sufficiently high input power. Theoretical descriptions of self-guided beams emphasize the distortions caused by multiphoton sources in the temporal pulse profiles and they are confronted with direct numerical simulations. Dissipative effects as multiphoton absorption and the delayed Kerr response induced by air are finally discussed.
A collisionless self-organizing model for the high-confinement (H-mode) boundary layer
Authors: Mahajan S. M. , Yoshida Z.
Published: July 2002

It is shown that in a collisionless two-fluid model, a combination of the Hall term and fluid vorticity can lead to the formation of a self-organized singular layer that displays the essential observational features of the thin shear layer associated with the high-confinement (H-mode) tokamak discharges: the layer width is of the order of a poloidal gyroradius, the poloidal velocity of the order of poloidal Mach number unity, and an electrostatic potential (yielding a negative electric field) of the order of the edge plasma temperature.
Schottky barrier diodes on 3C-SiC
Authors: Yoshida S. , Sasaki K. , Sakuma E. , Misawa S. , Gonda S.
Published: July 2002

Schottky barrier contacts have been made on n-type 3C-SiC epitaxially grown by chemical vapor deposition, and their characteristics were studied by the capacitance and photoresponse measurements. By evaporating Au onto chemically etched surfaces of 3C-SiC, good quality Schottky barrier junctions have been obtained. The barrier height determined by the capacitance measurements is 1.15 (±0.15) eV, while the height by the photoresponse measurements is 1.11 (±0.03) eV. These values are about a half of the energy band gap Eg at room temperature.
Passive and active mode locking of a semiconductor laser without an external cavity
Authors: Lau K. Y. , Ury I. , Yariv A.
Published: July 2002

This letter describes the first attempt to passively and actively mode lock a discrete semiconductor laser, i.e., one not coupled to an external cavity. Beat notes of the longitudinal modes of a 1.97-mm-long GaAlAs laser have been observed at 17.7 GHz. The spectral width of the beat note was approximately 100 kHz. Stable passive mode locking has been observed under appropriate operating conditions. Active mode locking by an externally injected microwave signal was also achieved.
Propagation delays and transition times in pulse-modulated semiconductor lasers
Authors: Tucker R. S. , Wiesenfeld J. M. , Downey P. M. , Bowers J. E.
Published: July 2002

An experimental and theoretical study of large-signal switching transients in directly modulated semiconductor lasers shows that the main parameters affecting high-speed switching are the small-signal relaxation oscillation frequency and the optical on/off ratio of the pulses. Simple expressions agree well with measured data and can be used to obtain an estimate of the maximum achievable modulation bit rate.
Organometallic vapor phase epitaxial growth of InP using new phosphorus sources
Authors: Larsen C. A. , Chen C. H. , Kitamura M. , Stringfellow G. B. , Brown D. W. , Robertson A. J.
Published: July 2002

Two organophosphorus compounds, isobutylphosphine and tertiarybutylphosphine, have been investigated for their possible use as precursors in the organometallic vapor phase epitaxy process. They are the first nonhydride compounds to be used as phosphorus sources. Pyrolysis studies show that the first decomposition products are phosphine and various organic compounds. The phosphine then pyrolyzes to give phosphorus. The materials are less pyrophoric and less toxic than phosphine, and so are safer to use. The compounds have been used to grow epitaxial layers of InP on InP and GaAs substrates. The layers exhibit photoluminescence and electrical properties which are similar to those of layers grown with phosphine.