bluuebunny/crossref_metadata_2025 huggingface dataset:
A bounce-averaged Monte Carlo collision operator and ripple transport in a tokamak
Authors: Albert Jay M. , Boozer Allen H.
Published: July 2002

A bounce-averaged Monte Carlo pitch-angle scattering operator presented by Tolliver [Phys. Fluids 28, 1083 (1985)] is discussed and shown to follow directly from a bounce-averaged drift-kinetic equation of Connor and Cordey [Nucl. Fusion 14, 185 (1974)]. This operator is used in a discrete mapping analysis of radial transport of test particles in a slightly rippled tokamak, illustrating time step restrictions for its validity. This treatment of collisions is different than that of Yushmanov [Nucl. Fusion 23, 1599 (1983)] but gives the same results for ripple transport.
Ejection mechanisms in the sublayer of a turbulent channel
Authors: Jiménez Javier , Moin P. , Moser R. , Keefe L.
Published: July 2002

The structure of the vorticity field in the viscous wall layer of a turbulent channel is studied by examining the results of a fully resolved direct numerical simulation. It is shown that this region is dominated by intense three-dimensional shear layers in which the dominant vorticity component is spanwise. The advection and reproduction processes of these structures are examined and shown to be consistent with the classical generation mechanism for two-dimensional Tollmien–Schlichting waves. This process is fundamentally different from the usually accepted mechanism involving hairpin vortices.
A fully toroidal fluid analysis of the magnetohydrodynamic ballooning mode branch in tokamaks
Authors: Andersson P. , Weiland J.
Published: July 2002

A comparatively complete two fluid description of collisionless electromagnetic ballooning modes has been derived. Using an unexpanded ion density response, it has been shown for the first time using a fluid theory that a necessary and sufficient condition for an instability of the magnetohydrodynamic (MHD) branch below the MHD beta limit is the presence of an ion temperature gradient exceeding a threshold. The cause of this instability has been identified and an analytical dispersion relation is given.
An experimental study on wall bounded vortices
Authors: Yamada Hideo , Sun Baoguo , Kakudoh Yuhta , Naitoh Takashi
Published: July 2002

Smoke visualization studies have been made on the flow fields of wall bounded vortices generated by puffing air through an orifice. The characteristics of wall bounded vortices produced by three orifices constructed by placing a dividing plate perpendicular to a circular orifice plate have been investigated. The three orifices were a full circular opening, a three-quarter circular opening, and a semicircular opening. It was found that the vorticity concentrated in the wall shear layer of the three-quarter and semicircular orifices was due to the separation of the boundary layer on the dividing plate and its interaction with the arch shaped vortex generated at the perimeter of the orifice. An interesting observation is that the wall bounded vortex from the semicircular orifice moved in a stable manner, whereas, the vortex ring produced by the circular orifice with the dividing plate at the bottom of the orifice was extremely unstable.
Unsteady flow in a nonuniform channel: A model for wave generation
Authors: Tutty O. R. , Pedley T. J.
Published: July 2002

It is known that a propagating wave train can be generated in unsteady flow in a nonuniform channel. A small amplitude inviscid theory is derived in which the forced displacement of the streamlines is governed by a linearized Korteweg–de Vries equation with time-dependent coefficients. Solutions of this equation for a channel with an asymmetric expansion show a standing wave with a propagating wave front, the amplitude of which is modified by a forced oscillation and by wave packets which travel downstream at a velocity greater than the group velocity for the main wave. An analytical model, based on the assumption of a standing wave, successfully predicts the wavelength of the main wave, the magnitude of the forced oscillation, and the velocity of the wave packets. Predictions from the theory are also compared with numerical solutions of the full Navier–Stokes equations for flow in nonuniform channels; there is agreement on some but not all properties of the wave.
An oblique wave on a shear layer
Authors: Mallier R. , Maslowe S. A.
Published: July 2002

Using Squire’s transformation applied to an oblique wave incident upon a two-dimensional plane parallel flow, it is shown that because the equation for the cross-stream velocity is decoupled from the equation for the streamfunction in the direction of the wave, it is possible to deduce a nonlinear evolution equation for the amplitude of the oblique wave with minimal extra work from the evolution equation for the corresponding two-dimensional wave. Since the decoupled cross-flow velocity obeys the same equation as a passive scalar in a two-dimensional flow, results are used from that problem to show that a finite-amplitude steady-state solution is possible for the oblique case; that solution is related to the two-dimensional solution found by Stuart [J. Fluid Mech. 29, 417 (1967)] which consists of a row of vortices.
Local analysis of the onset of instability in shear flows
Authors: Dušek Jan , Fraunié Philippe , Le Gal Patrice
Published: July 2002

Local analysis of the onset of instability in flows that are not exactly parallel is considered. Corrections to the Orr–Sommerfeld equation arising as a consequence of the nonparallelism of the unperturbed flow are studied. The quasiparallel hypothesis is quantified on a model of Gaussian plane wave packets. It appears that the characteristic length scales of the downstream dependence of flow field characteristics must be substantially larger than the inverse of the wave number characterizing the instability. A new eigenvalue problem describing the propagation of these Gaussian wave packets is written. The relation between the marginal and absolute instability analysis for marginal Reynolds numbers is discussed. For flows varying slowly in the downstream direction, closed-form corrections of the Orr–Sommerfeld equation terms taking account of the x variation of the flow field and of the extension of the propagating wave packets are derived. A first-order perturbation theory correction of the Orr–Sommerfeld dispersion relation is proposed, allowing the reduction of the calculation of nonparallel corrections of the local instability quantities to quadratures. The proposed theory is applied to two important cases: the Blasius boundary layer and the cylinder wake. For the Blasius boundary layer the basic condition of applicability of the quasiparallel theory is found to be satisfied. However, the nonparallel correction of the critical Reynolds number is found to be non-negligible and provides a good agreement with experimental results. In the cylinder wake case direct bidimensional simulation results are used to assess the downstream variation of the flow field characteristics. The characteristic length scale of this variation in the near wake is found to be of the order of unity, which is also the magnitude of the wave numbers characterizing the local absolute instabilities in this region. Hence, the Orr–Sommerfeld analysis and any corrections based on the propagation of plane waves in the wake can hardly be expected to provide more than qualitative results.
On the dynamical origin of asymptotic t2 dispersion of a nondiffusive tracer in incompressible laminar flows
Authors: Mezić Igor , Wiggins Stephen
Published: July 2002

Using an elementary application of Birkhoff’s ergodic theorem, necessary and sufficient conditions are given for the existence of asymptotically t2 dispersion of a distribution of nondiffusive passive tracer in a class of incompressible laminar flows. Nonergodicity is shown to be the dynamical mechanism giving rise to this behavior.
Velocity measurements of the laminar flow through a rotating straight pipe
Authors: Lei U. , Lin M. J. , Sheen H. J. , Lin C. M.
Published: July 2002

Some measurements have been obtained for the axial velocity of the fully developed laminar flow in a circular straight pipe with radius a, which is rotating with constant angular speed Ω about an axis perpendicular to its own axis. A diode laser LDA system was mounted together with a circulating pipe flow system on a rotating table for the experiment. According to previous analyses and calculations, there exist four types of axial velocity distributions that result from the various effects of the secondary flow on the main stream via the convection and Coriolis effect for different values of R( = wm′a/ν) and RΩ(=Ωa2/ν), where wm′ is the mean axial velocity and ν is the kinematic viscosity of the fluid. The present study provides experimental validation for the previous theoretical and numerical analyses. Experiments have also been carried out for studying the asymptotic nature of the slow flow in a rapidly rotating pipe (RΩ≫1 and RΩ≫R) and the rapid flow in a slowly rotating pipe (RRΩ≫1 and R≫RΩ).
Visualization of transitional pipe flow using the photochromic tracer method
Authors: Zalzal P. , Ojha M. , Ethier C. R. , Cobbold R. S. C. , Johnston K. W.
Published: July 2002

Two noninvasive measurement techniques are described for investigating transitional flow in a rigid straight tube. Photochromic trace recordings, representing fluid displacement profiles, were made within the core and at the leading and trailing edges of turbulent slugs generated by a disturbance at the tube inlet. Consistent photochromic trace recordings were made possible through the discovery that Doppler ultrasound can be used to detect the passage of turbulent slugs, thereby allowing precise timing of the photochromic traces. Upstream and downstream Doppler probes enabled the leading and trailing edge velocities of slugs to be measured and also enabled the laminar/turbulent transition regions to be investigated in detail. Trailing edge interface velocities were in excellent agreement with existing data, while leading edge velocities agreed less well. Turbulence generation at the trailing edge appeared to be associated with a localized vortical structure which originated in the near wall region and then grew toward the tube centerline. The generation of this vortex is postulated to be induced by a shear layer instability. Mean velocity profiles obtained from within the core of turbulent slugs were similar to those obtained for fully developed turbulent flow.
Particle interactions in diffusiophoresis in nonelectrolyte gradients
Authors: Keh Huan J. , Luo Shih C.
Published: July 2002

An analytical study is presented for the diffusiophoretic motion of two colloidal spheres in a constant gradient of a nonionic solute using a method of reflections. The particles are oriented arbitrarily relative to the gradient, and they are allowed to differ in radius and in surface properties. The range of the interaction between the solute and the particle surfaces is assumed to be small compared to the radius of each particle and to the gap width between the particles, but the polarization effect in the thin particle–solute interaction layers caused by the strong adsorption of the solute is incorporated. A normal flux of the solute and a slip velocity of the fluid at the outer edge of the thin diffuse layer are used as the boundary conditions for the fluid domain outside the diffuse layers. The method of reflections is based on an analysis of the solute concentration and fluid velocity disturbances generated by a single sphere placed in an arbitrarily varying concentration field. The quasisteady results for two-sphere interactions are correct to O(r−712), where r12 is the distance between the particle centers. For the special case of spheres oriented along the undisturbed concentration gradient, our results are in perfect agreement with the numerical solution obtained using a boundary collocation method. Based on a microscopic model, the results for two-sphere interactions are used to find the effect of the volume fraction of particles of each type on the average diffusiophoretic velocity in a bounded suspension. For a suspension of identical spheres, this average velocity is decreased as the particle volume fraction is increased. In general, the effect of particle interactions on diffusiophoresis is much weaker than that on sedimentation.
Computation of unsteady laminar flow over a flexible two-dimensional membrane wing
Authors: Smith Rick , Shyy Wei
Published: July 2002

Computations of the harmonically forced, unsteady viscous flow over a flexible, two-dimensional membrane wing are presented. The aeroelastic problem is nondimensionalized and a set of six basic dimensionless parameters is derived that govern the physical problem. The computational investigation is facilitated by distinguishing three distinct classes of problems—the constant tension, elastic, and inextensible membrane problems—which are associated with limiting cases of the dimensionless parameter set. A pressure-based method for the incompressible Navier–Stokes equations written in general time-dependent curvilinear coordinates is adopted as the flow solver. The computations were performed at a Reynolds number of 4×103, which is near the upper limit of the laminar flow regime. The periodic appearance and collapse of recirculation zones, along with an attendant adjustment in membrane configuration, results in an aeroelastic response, which may not be characterized as a simple harmonic response at the free-stream forcing frequency. Ostensibly, the computations are designed to simulate the behavior of a marine sail in a wind gust; however, the simulation of a harmonically forced free-stream flow also proves to be a useful vehicle for demonstrating some of the more generic features of membrane wing mechanics.
A numerical turbulence model for multiphase flows in the protoplanetary nebula
Authors: Champney Joëlle M. , Dobrovolskis Anthony R. , Cuzzi Jeffrey N.
Published: July 2002

It is thought that planets form from solid particles in a flattened, rotating, 99% gaseous nebula. These grains gradually coagulate into millimeter-to-meter sized aggregates which settle toward the midplane of the nebula. It is widely believed that the resulting dense layer eventually becomes gravitationally unstable and collapses into ‘‘planetesimals.’’ A new numerical model is presented to simulate the predominant processes (gravitation, vertical convection, and shear-driven turbulence) during the stage while the particulate material is still dispersed about the midplane of the nebula. In our previous work, particles were assumed to be spheres of a single radius; in the present work, particles are spheres of different radii. Results indicate that neither a broad nor a narrow distribution of particle sizes is likely to become gravitationally unstable.
On the two-dimensional stability of the axisymmetric Burgers vortex
Authors: Prochazka A. , Pullin D. I.
Published: July 2002

The stability of the axisymmetric Burgers vortex solution of the Navier–Stokes equations to two-dimensional perturbations is studied numerically up to Reynolds numbers, R=Γ/2πν, of order 104. No unstable eigenmodes for azimuthal mode numbers n=1,..., 10 are found in this range of Reynolds numbers. Increasing the Reynolds number has a stabilizing effect on the vortex.
Reevaluation of the experimental support for the Kolmogorov refined similarity hypothesis
Authors: Thoroddsen S. T.
Published: July 2002

Experiments in high Reynolds number turbulence call into question the experimental verification of Kolmogorov’s refined similarity hypothesis. Previous experimental support for the hypothesis is shown to be distorted by the severe simplifications used in estimating the turbulence energy dissipation, which has invariably been based only on the streamwise gradient of the streamwise velocity component. However, when the dissipation is estimated by the streamwise gradient of a transverse velocity component the dependence between ‖Δur‖ and (rεr*)1/3 all but disappears. These results suggest that previous experimental support for the hypothesis may have been premature.
The role of time-varying gravity on the motion of a drop induced by Marangoni instability
Authors: Rednikov Alexei Ye. , Kurdyumov Vadim N. , Ryazantsev Yuri S. , Velarde Manuel G.
Published: July 2002

We study the influence of a time-dependent force on the translatory convective instability of a drop due to surface stresses (Marangoni effect). This effect and its associate flows inside and outside the drop are induced by solute transfer from within the drop to the drop surface and its consumption there in an isothermal chemical reaction. In particular, we show that for a gravity field sinusoidally varying in time, the drop rather than aligning with the direction of the force tends to move in a direction orthogonal to it.
Experimental investigation of the instabilities in a fluidized bed origin of the pressure fluctuations
Authors: M’chirgui A. , Tadrist H. , Tadrist L.
Published: July 2002

This paper presents an experimental investigation of the hydrodynamic instabilities inside a self-excited fluidized bed. Pressure fluctuation and bed height measurements have been reported. In the slugging regime (above a critical gas flow rate), the bulk is found to exhibit a regular and periodic macroscopic pattern (large numbers of particles moving collectively in a seemingly organized manner). This state is characterized by nearly signoidal pressure fluctuations and regular bed height oscillations. As the gas flow increases the bulk motion tends toward large oscillations with intermittent smaller ones. The pressure fluctuations above the bed surface and the bed height oscillations are found to be intimately correlated. We report experimental evidence that the pressure fluctuations observed in a fluidized bed under and above its surface are caused by the oscillations of the bed height. The pressure wave initiates at the surface of the bed and propagates both upward and downward.
A diffuse-interface description of internal waves in a near-critical fluid
Authors: Anderson D. M. , McFadden G. B.
Published: July 2002

We present a diffuse-interface treatment of the internal gravity waves which have been observed experimentally by Berg et al. in xenon near its thermodynamic critical point. The results are compared with theoretical predictions by Berg et al. that were obtained using separate models above and below the critical temperature Tc. The diffuse-interface model applies both above and below Tc, and is formulated by using the density as an order parameter. The diffuse interface is represented as a transition zone of rapid but smooth density variation in the model, and density gradients appear in a capillary tensor, or Korteweg stress term, in the momentum equation. We obtain static density profiles, compute internal wave frequencies and compare with the experimental data and theoretical results of Berg et al. both above and below the critical temperature. The results reveal a singularity in the diffuse-interface model in the limit of incompressible perturbations.
A dynamical model for turbulence. VI. Two-dimensional turbulence
Authors: Canuto V. M. , Dubovikov M. S. , Wielaard D. J.
Published: July 2002

We apply a recent turbulence model to study 2-D turbulence. As in previous models, e.g., EDQNM, our model with a purely local transfer for both energy and enstrophy is not compatible with the conservation laws of energy and enstrophy that characterize 2-D turbulence. In contrast with previous two-point closure models that abandoned the Kolmogorovian spirit of locality, the present model satisfies both energy and enstrophy conservation laws with transfers that are local within the corresponding inertial regimes but not outside. We derive the dynamical equations governing the evolution of the spectra for energy, enstrophy, palinstrophy, and skewness. Results for both forced and unforced turbulence compare satisfactorily with DNS data.
Thermocapillary interaction between a solid particle and a liquid-gas interface
Authors: Leshansky A. M. , Golovin A. A. , Nir A.
Published: July 2002

When a hot solid particle is submerged into an ambient fluid near a free liquid-gas interface, a non-uniform temperature field around the particle produces surface tension gradients at the interface and generates thermocapillary flow in the surrounding fluid. This flow sets the particle itself in motion towards or away from the interface. In the first part of the paper, the interaction between a hot spherical solid particle and a plane undeformable liquid-gas interface is studied. The velocity of the thermocapillary induced motion of the solid particle is proportional to the surface tension gradient at the liquid-gas interface, and is calculated in the approximation of the Stokes flow and a zero Peclet number as a function of the separation distance between the particle and the interface. The asymptotic cases of both small and large separation distances are studied. In the second part of the paper, the interaction between a hot solid sphere and a gas bubble submerged into an ambient fluid is studied in the limiting case when the separation distance between them tends to zero. The velocity of a pairwise migration of a particle and a bubble in contact is calculated as a function of their radius ratio. The asymptotic values of the individual velocities of a solid particle and a gas bubble in near contact are also computed. The relative velocity of their motion towards each other is found to be proportional to the separation distance between them. In the third part of the paper, we investigate the effect of gravity on the thermocapillary driven motion of a solid particle for the two cases. It is shown that due to the thermocapillary interaction, a particle can move against the buoyancy forces.
The instability of a vortex tube in a weak external shear and strain
Authors: Arendt Steve , Fritts David C.
Published: July 2002

We investigate the stability of a straight vortex tube of constant vorticity in the presence of external shearing and straining flows orthogonal to the axis of the vortex tube. We find that the tube is unstable at degenerate wavenumbers (i.e., wavenumbers at which two Kelvin wavemodes have the same frequency). The maximum growth rate of the instability is weighted by the sum of the shear magnitude and the strain magnitude. The presence of the shear also shifts the wavenumber of the instability.
Two- and three-dimensional large-eddy simulations of a transitional separation bubble
Authors: Wilson Peter G. , Pauley Laura L.
Published: July 2002

In the present study we use two- and three-dimensional large-eddy simulations to examine the role of small-scale turbulence within a transitional separation bubble studied experimentally by Gaster (AGARD Conference Proceedings No. 4, 1966, pp. 813–854). In addition, several large-eddy simulation parameters and models are studied to show their effect on the computations. The inclusion of a small-scale turbulence model in the two-dimensional computations leads to an increase in the time-averaged separation bubble length and a slight reduction in the peak of the pressure coefficient distribution near reattachment. Increasing the filter width or increasing the Smagorinsky coefficient reduces the peak in the pressure coefficient distribution but also decreases the pressure coefficient within the pressure plateau. The two-dimensional LES accurately predicts the time-averaged bubble length of Gaster but does not accurately describe the experimental wall pressure distribution within the bubble. Three-dimensional LES computations allow the generation of vortex shedding and Görtler vortices within the separated region. A computation without a subgrid scale model allows the Görtler vortices to grow in strength and eliminate the boundary layer separation. The application of a subgrid scale model reduces the strength of Görtler vortices and spanwise vortex shedding. This produces a bubble size and time-averaged wall pressure distribution which compare favorably with experiment. Little difference is seen between the results using the constant coefficient and dynamic coefficient models.
Numerical simulations of a lobed fuel injector
Authors: Strickland J. H. , Selerland T. , Karagozian A. R.
Published: July 2002

Numerical modeling of the nonreactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates strong streamwise vorticity, producing locally high strain rates which can enhance the molecular mixing of reactants while delaying ignition in a controlled manner. Vortex element modeling is used to simulate flow field evolution and fuel element mixing characteristics for this lobed fuel injector. Quantitative predictions for vorticity generation and qualitative results for streamwise rollup compare well qualitatively with recent experimental investigations of this flow field [Smith et al., Phys. Fluids 9, 667 (1997)]. Parametric studies of the effects of lobe amplitude-to-wavelength ratio, lobe angle, and lobe shape for given flow conditions suggest that geometrical features may be optimized to enhance mixing and control reaction processes.
Observed drag crisis on a sphere in flowing He I and He II
Authors: Smith Michael R. , Hilton David K. , Van Sciver Steven W.
Published: July 2002

The pressure distribution on the surface of a sphere has been measured in flowing He I and He II as a function of Reynolds number. The drag coefficient was extracted by integrating the pressure distribution, using some assumptions about symmetry of the flow field. Drag coefficients are plotted against Reynolds number for both He I and He II against classical data for both smooth and nonsmooth spheres. Latest results in He II suggest that the drag crisis occurs at a Reynolds number of approximately 2×105, in fair agreement with classical data.
Linear control and estimation of nonlinear chaotic convection: Harnessing the butterfly effect
Authors: Bewley Thomas R.
Published: July 2002

This paper examines the application of linear optimal/robust control theory to a low-order nonlinear chaotic convection problem. Linear control feedback is found to be fully effective only when it is switched off while the state is far from the desired equilibrium point, relying on the attractor of the system to bring the state into a neighborhood of the equilibrium point before control is applied. Linear estimator feedback is found to be fully effective only when (a) the Lyapunov exponent of the state estimation error is negative, indicating that the state estimate converges to the uncontrolled state, and (b) the estimator is stable in the vicinity of the desired equilibrium point. The aim in studying the present problem is to understand better some possible pitfalls of applying linear feedback to nonlinear systems in a low-dimensional framework. Such an exercise foreshadows problems likely to be encountered when applying linear feedback to infinite-dimensional nonlinear systems such as turbulence. It is important to understand these problems and the remedies available in a low-dimensional framework before moving to more complex systems.
On the derivation of hydrodynamics from the Boltzmann equation
Authors: Esposito R. , Lebowitz J. L. , Marra R.
Published: July 2002

We review the main ideas on the derivation of hydrodynamical equations from microscopic models. The Boltzmann equation, which is a good approximation for the evolution of rare gases, provides a useful tool to test these ideas in mathematically controllable situations such as the Euler and incompressible Navier–Stokes limits, which we describe in some detail. We also discuss the heuristics and some rigorous results available for stochastic particle systems.
Effects of gravity on turbulent nonpremixed flames
Authors: Elghobashi S. , Zhong R. , Boratav O.
Published: July 2002

The paper examines the effects of buoyancy on the behavior of turbulent nonpremixed flames using the method of direct numerical simulation. We examine the effects of the interactions between turbulence, chemical reaction, and buoyancy on the fields of vorticity, turbulence kinetic energy, scalar dissipation, and reaction rate. In the buoyant flame, the baroclinic torque creates strong counter-rotating vortices saddling the flame surface. These vortices result in a significantly more wrinkled flame surface than in the nonbuoyant case. One of the effects of wrinkling is that it reduces the distances between the isosurfaces of the mixture fraction F, thus augmenting the local ∇F and the scalar dissipation εF, and consequently the reaction rate ṙT. The zones of maximum ṙT coincide with those of maximum tangential extensional strain. Our results show a direct correlation between εF and ṙT for both the buoyant and nonbuoyant flames.
Pulsed currents carried by whistlers. II. Excitation by biased electrodes
Authors: Urrutia J. M. , Stenzel R. L. , Rousculp C. L.
Published: July 2002

The transport of time-dependent current between electrodes in contact with a large laboratory magnetoplasma is examined experimentally. Single electrodes biased with respect to the chamber wall or pairs of electrically floating electrodes are used to produce pulsed currents (ωci≪2π/Δt≪ωce). The associated magnetic field vector, B(r,t), is measured in space and time, and the total current density is calculated from J(r,t)=∇×B(r,t)/μ0. The current front is found to propagate at a characteristic wave speed, which does not depend on current amplitude or polarity. The transient current spreads across B0 within a conical region, which depends on source geometry and plasma parameters. It is shown by Fourier transforming B(r,t) into B(k,ω) that the transient fields consist of a spectrum of oblique low-frequency whistler waves. In Fourier space, the inductive and space charge electric fields are calculated from Faraday’s law and the assumption that Etot=Eind+Esc along B0 is negligible. Inverse transforming yields E(r,t). The transient wave fields (B,J,E) exhibit multiple induction effects and the formation of space charges. The results are relevant to pulsed Langmuir probes, beams, and antennas as well as moving steady-state magnetic/current sources such as particle collectors on spacecraft and magnetized asteroids (e.g., Gaspra).
Study of wave–particle interaction from the linear regime to dynamical chaos in a magnetized plasma*
Authors: Fasoli A. , Skiff F. , Tran M. Q.
Published: July 2002

The onset of chaotic ion dynamics in electrostatic waves is investigated in a linear magnetized plasma. The existence of a threshold for heating in agreement with the predictions of single particle Hamiltonian theories and the fast time scale for velocity space diffusion are observed in the experiment. Measurements of test-particle dynamics indicate an exponential separation of initially close ion trajectories for amplitudes above the heating threshold. Ion orbits in the different wave–particle interaction regimes are inferred using an optical tagging diagnostic method, which provides the two point correlation function in phase space.
Comments on ‘‘Theory and three-dimensional simulation of light filamentation in laser-produced plasmas’’ [Phys. Fluids B 5, 2243 (1993)]
Authors: Epperlein E. M. , Short R. W.
Published: July 2002

A comment on the theory and simulation of light filamentation in laser-produced plasma is presented. The authors’ model differs from previous such models principally in the treatment of thermal conductivity. (AIP)
Computer studies on noncoplanar slow and intermediate shocks associated with the sheared fast reconnection mechanism
Authors: Ugai M. , Shimizu T.
Published: July 2002

It was recently found that noncoplanar slow shocks stood in the sheared fast reconnection configuration. Hence, the present one-dimensional magnetohydrodynamics (MHD) simulations with high numerical resolution study the temporal dynamics of MHD shocks, from a slow shock to a weak intermediate shock, that are placed in a noncoplanar situation. It is shown that for any case the noncoplanar shock structure can be sustained by physical dissipations involved. The resulting noncoplanar slow shock structure is, both qualitatively and quantitatively, in good agreement with the two-dimensional shock transition layer associated with the sheared fast reconnection mechanism. The one-dimensional noncoplanar slow or (subfast) intermediate shock structure is eventually bifurcated into an intermediate wave and a coplanar slow shock as a result of magnetic field rotation. In general, any stable shock must be coplanar, and in actual systems strictly coplanar boundary conditions ahead of and behind a shock cannot be provided nor sustained. Hence we propose a criterion, required for a stable shock to be realized, such that the (coplanar) shock must survive and hence be derived as an eventual solution in noncoplanar situations. It is argued that the present simulation results as well as the previous ones should be interpreted and reconsidered on the basis of this criterion.
Drift wave propagation as a source of plasma edge turbulence: Toroidal theory
Authors: Mattor Nathan
Published: July 2002

Recently [N. Mattor and P. H. Diamond, Phys. Rev. Lett. 72, 486 (1994)] it was suggested that large-amplitude turbulence in edge plasmas [ñ/n0∼O(1)] could originate from the core, propagating outward into the regions of low density and temperature. Here this theory is expanded, using a toroidal geometry. It is shown that a wave launched in the core region can propagate over a substantial portion of the radius, reaching the edge from the core. It is shown that a large increase of plasma flow from core to edge causes wave reflection, cutting off the source of edge turbulence. Experimental evidence is discussed, with particular attention to subtleties of global mode structure in a torus. Possible new experimental tests of this theory are discussed. Details are given of a new theory to explain the observed correlation between edge shear flows and improved confinement (‘‘H’’ modes), which are compared to experiment.
Stimulated scattering of electromagnetic waves in collisional dusty plasmas
Authors: Shukla P. K. , Vladimirov S. V.
Published: July 2002

The modulational instability of high-frequency electromagnetic waves in collisional dusty plasmas has been investigated, taking into account the effects of dust charge fluctuations in the dynamics of quasistationary plasma slow motions. It is found that inclusion of the dust charge perturbation leads to novel dissipative modulational instabilities when Joule heating nonlinearity dominates over the ponderomotive force. The results of our investigation should be useful in understanding the phenomena of enhanced density and temperature fluctuations that could be triggered by intense electromagnetic waves in space and astrophysical plasmas.
Current–voltage characteristic of a partially ionized plasma in cylindrical geometry
Authors: Lebowitz Joel L. , Rokhlenko Alexander
Published: July 2002

The properties of a partially ionized plasma in a long cylindrical tube subject to a uniform axial electric field are investigated. The plasma is maintained by an external ionizing source balanced by bulk and surface recombinations. Collisions between neutrals, whose density greatly exceeds the density of charged particles, and of neutrals with ions, are sufficiently effective for their velocity distribution to be close to a Maxwellian with the same uniform temperature, independent of the external field. The behavior of the plasma is described by a collisional two-fluid scheme with charge neutrality in the interior of the tube. Approximate nonlinear equations for the hydrodynamical moments are obtained from a Boltzmann equation in which electron–neutral, electron–ion, and electron–electron collisions are all important. It is found that under certain circumstances the current and the temperature of the electrons undergo a drastic change, with hysteresis, as the electric field is varied.
Novel symmetry tuning in Nova hohlraums using axial gold disks
Authors: Amendt Peter , Murphy T. J. , Hatchett S. P.
Published: July 2002

A pair of axial gold disks is used to reduce the capsule view of the two laser-entrance-holes in hohlraums irradiated by the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. By varying the disk radii a novel means of tuning the x-ray drive flux asymmetry is provided. Predicted implosion asymmetry is experimentally validated over a wide range of disk radii. Although simulations suggest increased x-ray drive onto the capsule in the presence of gold disks, neutron-based diagnostics are inconclusive in this regard.
Plasma–maser instability of electromagnetic radiation in the presence of an electrostatic beat wave
Authors: Sarma S. N. , Nambu M. , Vladimirov S. V.
Published: July 2002

The plasma–maser instability of the electromagnetic radiation in the presence of electrostatic lower hybrid turbulence is studied, considering the contribution of the electrostatic beat wave. It is found that the plasma–maser contribution from the electrostatic beat wave is (ωpe/Ωe)2 times that from the electromagnetic beat wave, here Ωe and ωpe are, respectively, the electron cyclotron and plasma frequency.
On the conical refraction of hydromagnetic waves in plasma with anisotropic thermal pressure: General consideration
Authors: Tsiklauri David
Published: July 2002

A phenomenon analogous to the conical refraction well known in crystalo-optics and crystaloacoustics is considered for the magnetohydrodynamical waves in a collisionless plasma with anisotropic thermal pressure. Imposing the most general condition for the existence of the phenomenon, the angle of the conical refraction is calculated, which appeared to be dependent on the ratio of the Alfvén velocity and sound speed measured in the perpendicular direction with respect to the external magnetic field. Feasible ways to experimentally demonstrate the phenomenon are discussed and a novelty brought about by the general consideration is outlined.
Collisionless particle orbits in a torsatron with the complicated magnetic-field harmonic spectrum. I. Particle orbits topology
Authors: Smirnova M. S.
Published: July 2002

Charged particle orbits are investigated in a torsatron with the complicated magnetic-field harmonic spectrum when distant, high-order harmonics are present. The structure of the local magnetic wells in such a torsatron is considered. Conditions for the appearance of the additional local magnetic wells along the field line (toroidal-ripple wells) due to the distant harmonics effect are obtained. It is shown that in a torsatron, the particles which are trapped in such wells have the largest radial deviations among all types of trapped particles. The analytical conditions under which the toroidal-ripple wells are dangerous for the particle confinement (criteria of the strong toroidal-field ripple) are derived.
Quasi-thermal fluctuations in a beam-plasma system
Authors: Lund E. J. , Treumann R. A. , LaBelle J.
Published: July 2002

The quasi-thermal electrostatic field fluctuations of a stable unmagnetized electron beam-plasma system is considered. Both the beam and the background plasma are modeled as isotropic Maxwellians. Both the normal modes of the plasma and the contributions from other modes are considered. The dependence of these fluctuations on beam velocity, density, and temperature are examined. The implications of these waves for real beam-plasma systems are also discussed.
Scalings, spectra, and statistics of strong wave turbulence
Authors: Robinson P. A.
Published: July 2002

A two-component model of strongly nonlinear wave turbulence is developed for a broad class of systems in which high-frequency electrostatic waves interact with low-frequency sound-like waves. In this model coherent nonlinear wave packets form and collapse amid a sea of incoherent background waves. It is shown that three classes of turbulence exist, typified by Langmuir, lower-hybrid, and upper-hybrid turbulence. Balance between power input to incoherent waves, and dissipation at the end of collapse determines power-law scalings of turbulent electrostatic energy density, density fluctuations, length and time scales. Knowledge of the evolution of collapsing packets enables probability distributions of the magnitudes of electric fields and density fluctuations to be calculated, yielding power-law dependences. Wavenumber spectra of collapsing waves and associated density fluctuations are also calculated and shown to have power-law forms. Applications to Langmuir, lower-hybrid, and upper-hybrid waves are discussed. In the Langmuir case the results agree with earlier theory and simulations, with one exception, which is consistent only with earlier simulations. In the lower-hybrid and upper-hybrid cases, the results are consistent with the few simulations to date.
Expansion and nonlinear relaxation of a strongly magnetized non-neutral electron plasma due to elastic collisions with background neutral gas
Authors: Davidson Ronald C. , Moore David A.
Published: July 2002

The expansion and nonlinear relaxation of a non-neutral electron plasma due to elastic collisions with constant collision frequency νen between the plasma electrons and a low-density background neutral gas are investigated theoretically. The model treats the electrons as a strongly magnetized fluid (ω2pe/ω2ce≪1) with isothermal temperature T=const immersed in a uniform magnetic field B0êz. The model also assumes an axisymmetric plasma column (∂/∂θ=0) with negligible axial variation (∂/∂z=0). Introducing the scaled radial coordinate ρ=r/r0(t), and the density profile shape function η(ρ,t)=[2πr20(t)/N]n(r,t), a diffusion equation is derived that describes the nonlinear evolution and relaxation of η(ρ,t) in response to electron collisions with the neutral gas. A very important consequence of the present analysis is that electron–neutral collisions (νen≠0) cause the electron density profile to relax to a (slowly expanding) quasiequilibrium state with identical profile shape ηeq(ρ) to that which would be produced by relaxation to thermal equilibrium by electron–electron collisions in the absence of background neutral gas (νen=0).
Intuitive and rigorous derivation of spontaneous emission and Landau damping of Langmuir waves through classical mechanics
Authors: Escande D. F. , Zekri S. , Elskens Yves
Published: July 2002

Classical mechanics provides the intuitive and unified description of spontaneous emission, Landau growth and damping of Langmuir waves, the cold beam–plasma instability, and van Kampen modes. This is done by studying the interaction between M weak modes of a plasma without resonant particles and N quasiresonant particles, which leads to an exactly solvable high-dimensional Floquet problem. Growth corresponds to an eigenmode of the system, whereas damping requires statistical averaging. Both imply synchronization of near-resonant particles with waves, and the corresponding force on individual particles is computed explicitly.
Kinetic modeling of scrape-off layer plasmas
Authors: Kupfer K. , Harvey R. W. , Sauter O. , Schaffer M. J. , Staebler G. M.
Published: July 2002

Electron transport along open field lines in the diverted scrape-off layer of a tokamak is studied numerically via a kinetic Fokker–Planck approach. The method allows calculation of the distribution function in a situation where large parallel temperature gradients are maintained by collisional relaxation and, at the same time, superthermal electrons stream freely from the midplane of the plasma to the target/sheath boundary. The method also allows calculation of the self-consistent electrostatic field associated with parallel gradients in the distribution function, as well as the potential drop across the target/sheath boundary, where the latter is calculated to enforce appropriate boundary conditions at the target, although the sheath itself is not resolved. The kinetic results are compared to classical fluid results for the case of a simple (nonradiative) divertor. The kinetic solutions exhibit an enhanced superthermal electron population in the vicinity of the target, which results in a larger sheath energy transmission factor, a lower bulk electron temperature, and a smaller sheath potential drop. The sheath potential largely determines the energy with which ions impact the target, thereby affecting the rate of target erosion. Ionization rates and radiation rates from impurities in the vicinity of the target also depend strongly on the local electron temperature and can be sensitive to superthermal tails.
Forced reconnection and mode locking in rotating cylindrical plasmas
Authors: Wang Xiaogang , Bhattacharjee A.
Published: July 2002

The problem of mode locking due to forced magnetic reconnection in rotating cylindrical plasmas is revisited. Forced reconnection is characterized by very large values of the parameter Δ′, which makes the constant-ψ approximation generally inapplicable in the linear and early nonlinear regimes. The nonlinear dynamics of rotating non-constant-ψ islands is distinguished by the persistence of current sheets spanning Y points. Mode locking due to a suddenly imposed error field is discussed. Temporal dynamics and locking thresholds that differ significantly from the predictions of the constant-ψ theory [R. Fitzpatrick and T. C. Hender, Phys. Fluids B 3, 644 (1991)] are obtained. The predictions of the present theory are compared with experimental tokamak observations.
Plasma screening effects on eikonal phase for electron-ion elastic collisions in weakly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

Eikonal approximation is applied to investigate elastic electron-ion collisions in weakly coupled plasmas. Plasma screening effects on eikonal phase are investigated for eikonal differential elastic scattering cross sections. The electron-ion interaction potential in weakly coupled plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter and the plasma parameters. In the first-order eikonal approximation, the dynamic plasma screening effect is identical to the static screening effect obtained by the Debye–Hückel potential. The eikonal differential elastic cross section substantially decreases with an increase in the projectile energy and increases as the plasma screening effect decreases through the Debye length. The plasma screening effects are more significant for large impact parameters.
The evolution of a plane jet in a neutral sheet
Authors: Dahlburg R. B. , Boncinelli P. , Einaudi G.
Published: July 2002

The linear and nonlinear evolution of the plane magnetized jet, a magnetohydrodynamic configuration consisting of a plane fluid jet embedded in a neutral sheet, is examined. At low Alfvén number (A=ratio of the characteristic Alfvén speed to the characteristic flow speed), two ideally unstable modes are found that correspond to the sinuous and varicose modes of the fluid plane jet. Increasing A leads to a stabilization of both of these modes. For large A there is a separate resistively unstable mode. The ideal varicose mode and the resistive varicose mode are distinct modes with similar properties in a given range of A. A magnetohydrodynamic generalization of the Howard semicircle theorem indicates that a strong enough magnetic field will damp out all ideal modes. The stability properties of the two modes are studied in terms of their perturbation energy balances. The nonlinear evolution is quite different for the three modes in terms of time scales and of the properties and spatial location of the small-scale structures that strongly modify the initial configuration. The two ideal modes have in common the capability of accelerating the fluid initially at rest in a much more efficient way than the resistive one.