bluuebunny/crossref_metadata_2025 huggingface dataset:
Rippled shock front solutions for testing hydrodynamic stability simulations
Authors: Munro David H.
Published: July 2002

The response of a shock front to arbitrary small perturbations can be calculated analytically. Such rippled shock front solutions are useful for determining the accuracy of hydrodynamic simulation codes such as lasnex [Comments Plasma Phys. Controlled Fusion 2, 51 (1977)], which are used to compute perturbation growth in inertial fusion targets. The lasnex fractional errors are of order κ2L2, where κ is the transverse wavenumber of the perturbation, and L is the largest zone dimension. Numerical errors are about 25% for a calculation using 26 zones per transverse wavelength.
New contributions to transit-time damping in multidimensional systems
Authors: Robinson P. A.
Published: July 2002

The existence of two previously unrecognized contributions to transit-time damping in systems of more than one dimension is demonstrated and discussed. It is shown that these contributions cannot be treated by one-dimensional analyses unless it is assumed that the gradient of the field perpendicular to itself always vanishes. Such an assumption is unjustified in general and the new contributions can dominate damping by fast particles in more general situations. Analytic expressions obtained using a Born approximation are found to be in excellent agreement with numerical test-particle calculations of transit-time damping for a variety of field configurations. These configurations include those of a resonance layer and of a spherical wave packet, which approximates a collapsing wave packet in a strongly turbulent plasma. It is found that the fractional power absorption can be strongly enhanced in non-slablike field configurations.
Strong Langmuir turbulence generated by electron beams: Electric-field distributions and electron scattering
Authors: Robinson P. A. , Newman D. L.
Published: July 2002

Strong-turbulence theory is used to predict the statistics of intense Langmuir fields generated when an electron beam is injected into a plasma, and the theory of transit-time interactions is then used to calculate the beam scattering caused by the turbulent waves. The theory predicts that the distribution of field strengths will have a Gaussian tail corresponding to fields in nonlinearly collapsing coherent wave packets that are near the arrest of their collapse by damping. The functional form of the tail of the field distribution is determined by the statistical distribution of wave packets at the time of their formation, whereas its exponent depends on the arrest scale. Comparison of numerical calculations of this exponent with experimental measurements confirms the Gaussian form of the tail and implies that collapse is arrested at a scale of (16±5)λD, where the peak electrostatic energy density is of the same order as the thermal energy density, in good agreement with independent particle-in-cell calculations. Transit-time calculations of beam scattering in strong turbulence yield rms energy changes in good agreement with experimental values and mean energy changes that are well within the experimental limits. These results support the validity of the recently developed scaling theory of strong turbulence, and the predicted form of transit-time interactions with coherent wave packets.
Characteristics of the toroidal relaxed state of reversed field pinches and comparison with experimental results
Authors: Jain K. K.
Published: July 2002

A toroidal equilibrium of a reversed field pinch based on Taylor’s Bessel function model (BFM) is investigated. Characteristics of the toroidal relaxed state for a few reversed field pinch devices are presented in detail. It is shown that the toroidal relaxed state agrees with experimental observations not only in the inner region, like BFM, but also to a good extent in the outer region of a reversed field pinch. Inclusion of toroidal effects in the BFM leads to a decrease of the toroidal reversed flux and an increase of the minor radius at which the toroidal field reverses. The toroidal relaxed state has ‘‘μ’’ decreasing in the outer region when calculated using experimental procedure and a saturation of the on-axis value of the pinch parameter Θ. Toroidal effects and finite plasma pressure lead to enhancement of the pinch parameter Θ. The results in this paper show that the observed departure of experiment from the BFM is partly due to the toroidal geometry.
Ion velocity distributions at the tokamak edge
Authors: Pitts R. A.
Published: July 2002

This paper compares the form of f(v∥) arising from the warm-ion, kinetic models of Emmert et al. [Phys. Fluids 23, 803 (1980)] and Bissel and Johnson [Phys. Fluids 30, 779 (1987)] with experimentally measured distributions from the DITE tokamak obtained with a retarding field analyzer (RFA). The results show that the commonly adopted assumption of a distribution accelerated through the sheath potential and approximating to a Maxwellian in the high-energy tail is reasonable. In this respect, Emmert’s distribution is more appropriate. In addition, the common practice of neglecting the presheath effect on the distribution is generally justified in analyzing the RFA characteristic, but may incur large error under some circumstances. While the comparison also illustrates how differing assumptions regarding the source function can have a reasonably strong influence on the ion velocity distribution at walls and limiters, the integrated distributions required for comparison with RFA data are sufficiently similar that distinguishing between the models experimentally is not generally possible. The problems of resolving these differences experimentally are discussed and suggestions are proposed for further experiments.
The validity of Bohm’s sheath criterion in rf discharges
Authors: Riemann K.-U.
Published: July 2002

In the limit of a small Debye length λD the plasma boundary layer is split into a thin space-charge sheath and a quasineutral presheath. In dc discharges, the well-known Bohm criterion expresses a necessary condition for the formation of a stationary sheath. With the increasing importance of rf discharges there has arisen some confusion on the validity of the sheath condition for rf sheaths. It is shown that Bohm’s sheath criterion holds without modification in rf discharges with arbitrary frequency ratio ω0 /ωpi (ion plasma frequency).
Stabilization of axisymmetric modes in plasma–vacuum equilibria by current reversal
Authors: Lortz D. , Petroff P. , Spies G. O.
Published: July 2002

Two-dimensional plasma–vacuum equilibria are constructed by solving a free-boundary problem for small deviations from a circular cross section, and their two-dimensional stability is studied by examining that eigenmode that reduces to the marginal rigid shift in the circular limit (all other two-dimensional modes are stable). The vacuum is assumed to extend to infinity (no wall), N axial external currents are assumed to be symmetrically placed far away from the plasma, and the plasma current density is assumed to be a linear function of the poloidal magnetic flux. It is found that instability always results for N=2 (elliptical corrugation), but that stability may result for N≥3 if the axial plasma current density changes sign across some pressure surface. It is concluded that axially symmetric modes in toroidal equilibria with large aspect ratio can be stabilized by current reversal, even if no conducting wall surrounds the vacuum.
Electron acoustic surface waves in a two-electron component plasma
Authors: Bharuthram R. , Misthry S. S. , Yu M. Y.
Published: July 2002

Surface electron acoustic waves on a two-electron component plasma in a semi-infinite half-space are investigated. The dispersion relation of these waves differs considerably from that of the surface electron plasma waves.
Pressure induced islands in three-dimensional toroidal plasma
Authors: Cary John R. , Kotschenreuther M.
Published: July 2002

The production of magnetic islands by plasma pressure in three-dimensional toroidal systems is analyzed. Far from the rational surfaces, a procedure based on linearization in the plasma pressure applies. This yields the solution in terms of δ-function currents at the surface. These currents are found by a nonlinear analysis valid in the vicinity of the island. The result is a set of coupled nonlinear equations determining the island widths. Scaling is found by using the approximation of nearly circular flux surfaces. The results indicate that for typical stellarators, which have a small ratio ιl0/m0 of field line rotational transform to coil rotational transform, the island size in the low-β limit depends dramatically on whether a magnetic well is present. In this case, if a magnetic well is present, islands are insignificant; in contrast, if a magnetic hill is present, island overlap occurs for arbitrarily low pressure.
Effects of particle trapping on ion-cyclotron resonance heating in a toroidal plasma
Authors: Anderson D. , Lisak M. , Pekkari L.-O.
Published: July 2002

The effect of particle trapping on the velocity space diffusion caused by ion-cyclotron resonance heating is considered. A bounce-averaged Fokker–Planck equation is derived, which determines the distribution function of the heated ions. Trapped particle effects lead to significant changes in the rf-driven diffusion operator as compared to previously used models.
The rewetting of an inclined solid surface by a liquid
Authors: Silvi N. , Dussan V E. B.
Published: July 2002

When a thin layer of liquid flowing down an inclined solid surface becomes unstable, a dry region may appear. It is of general engineering interest to identify the parameters which control the ability of the liquid to rewet the solid so that a uniform film of liquid can be reestablished. The role played by the contact angle in the rewetting process is investigated by performing a specific set of experiments of the type introduced by Huppert [(Nature 300, 427 (1982)]. It is found that a small static advancing contact angle promotes rewetting, while a large value of the angle does not.
Laminar flows in circular pipes whose cross-sectional area varies slowly in the axial direction
Authors: Terrill Robert M.
Published: July 2002

Analytical solutions of the Navier–Stokes equations for flows in circular pipes whose cross-sectional area varies slowly in the axial direction are given. Specific shapes of the pipe wall are determined to give axial velocity profiles that are independent of the Reynolds number.
Steady flow past a sluice gate
Authors: Naghdi P. M. , Vongsarnpigoon L.
Published: July 2002

The problem of steady two-dimensional flow, under the action of gravity, of an incompressible inviscid fluid sheet past a sluice gate is reexamined with particular reference to the theoretical prediction of the contraction ratio, i.e., the ratio of the far downstream height to the sluice opening. The problem is formulated here by a direct approach with the use of the theory of a directed fluid sheet, which includes appropriate jump conditions arising in a natural way within the scope of the basic theory. The determination of the contraction ratio is effected by a novel procedure in the utilization of the jump conditions at the gate. Detailed numerical calculations are made for two different sluice openings and these provide very good agreement with the experimental results of Benjamin [J. Fluid Mech. 1, 227 (1956)].
Global dynamics and rapid collapse of an isolated current-sheet system enclosed by free boundaries
Authors: Ugai M.
Published: July 2002

The self-consistent behavior of an isolated current-sheet system enclosed by free boundaries is numerically studied in the framework of magnetohydrodynamics. Initiated by a disturbance, the middle current sheet drastically collapses into a thin layer in a collisionless plasma. It is argued that an anomalous resistance should limit the extreme current concentration. When the current sheet, initially of width 2L, becomes thin, an anomalous resistivity η is introduced that increases with the relative electron–ion drift velocity ‖VD‖ when ‖VD‖ exceeds a threshold value VC. It is found that, when the reconnection flow has just been coupled to the anomalous resistivity, the reconnection growth rate [normalized by (L/VA)−1 ] becomes extremely large and scales as (∂η/∂‖VD‖)0.6V2.2C (normalization is based on the initial configuration); accordingly, the fast reconnection mechanism is set up on a short time scale and leads to the rapid collapse of the system at large. The initial growth of the fast reconnection is shown to be in good agreement with the underlying physical mechanism of the modified tearing instability.
Two-stream instability in convergent geometry
Authors: Gratton Fausto T. , Gnavi Graciela
Published: July 2002

The problem of the instability of counterstreaming beams of charged particles is extended to cylindrical and spherical geometries. For well-focused configurations it can be solved by complex contour integral representations. The effects of the convergence of the flow and the density gradient along the trajectories of the particles are considered. The linear spectrum for the cylindrical case is obtained, together with the proof that the solution has finite energy and satisfies two physical matching conditions through the origin. The properties of the special functions which solve this problem are presented. Although the density of the ideally focused model diverges as 1/r at the origin, the growth rate of the instability, for a system of radius R, is given by ω2pR/V02ξn, where V0 is the beam velocity, ξn are the zeros of the Bessel function of zeroth order, and the plasma frequency ωp is evaluated at one-half the average density of particles.
Interpretation of plasma impurity deposition probes. Analytic approximation
Authors: Stangeby P. C.
Published: July 2002

Insertion of a probe into the plasma induces a high speed flow of the hydrogenic plasma to the probe which, by friction, accelerates the impurity ions to velocities approaching the hydrogenic ion acoustic speed, i.e., higher than the impurity ion thermal speed. A simple analytic theory based on this effect provides a relation between impurity fluxes to the probe Γimp and the undisturbed impurity ion density nimp, with the hydrogenic temperature and density as input parameters. Probe size also influences the collection process and large probes are found to attract a higher flux density than small probes in the same plasma. The quantity actually measured, cimp, the impurity atom surface density (m−2) net-deposited on the probe, is related to Γimp and thus to nimp by taking into account the partial removal of deposited material caused by sputtering and the redeposition process.
Bubble motion in a Hele–Shaw cell
Authors: Kopf-Sill Anne R. , Homsy G. M.
Published: July 2002

The shapes and motion of immiscible bubbles in a Hele–Shaw cell driven by the motion of the surrounding fluid were studied. Six classes of steady shapes, some of which are remarkable, were observed. Multiple steady states exist over some ranges of parameters and the shape as a function of speed may slow hysteresis. The observed translational velocities do not agree with available theory, but some of the shapes are in qualitative agreement with those computed by Tanveer [Phys. Fluids 29, 3537 (1986)].
Self-focusing and filamentation of laser light in high Z plasmas
Authors: Jones R. D. , Mead W. C. , Coggeshall S. V. , Aldrich C. H. , Norton J. L. , Pollak G. D. , Wallace J. M.
Published: July 2002

Self-focusing and filamentation of short wavelength laser light in high Z plasmas of interest to laser fusion are discussed. It is found that self-focusing behavior is very dependent on the details of the characteristics of the laser beam, the plasma conditions, and the energy transport processes. Laser light absorption and self-focusing are strongly competitive processes. At. 0.26 μm wavelength the collisional absorption is often so great that there is no intensity amplification of the beam despite the fact that strong self-focusing is present. Wide variations are found in laser light penetration, affected by several factors. Diverging optics reduce the likelihood of self-focusing. Large scale length density gradients have little effect on focusing behavior. The self-focusing behavior is very dependent on beam shape. Large scale hot spots can have a significant effect on whole beam self-focusing early in the pulse. The behavior of small scale hot spots can be qualitatively different than the standard picture. The calculations indicate that small scale hot spots do not achieve a steady state in some cases. Sound waves cause chaotic interactions among neighboring hot spots. It is found that sub-beam size structures are also generated when nonlocal thermodynamic equilibrium (non-LTE) radiation and atomic physics are used in the calculations. The nature of the heat flux and thermoelectric magnetic field generation are examined with a kinetic model. Stimulated Raman backscattering levels in self-focused light are significantly reduced for short wavelengths and high Z plasmas Landau damping plays an important role in determining the Raman levels. Implications for suprathermal electron production, symmetric illumination, x-ray conversion efficiency, and laser light absorption are discussed.
Tokamak m=1 magnetohydrodynamic calculations in toroidal geometry using a full set of nonlinear resistive magnetohydrodynamic equations
Authors: Charlton L. A. , Carreras B. A. , Holmes J. A. , Lynch V. E.
Published: July 2002

The linear stability and nonlinear evolution of the resistive m=1 mode in tokamaks is studied using a full set of resistive magnetohydrodynamic (MHD) equations in toroidal geometry. The modification of the linear and nonlinear properties of the mode by a combination of strong toroidal effects and low resistivity is the focus of this work. Linearly there is a transition from resistive kink to resistive tearing behavior as the aspect ratio and resistivity are reduced, and there is a corresponding modification of the nonlinear behavior, including a slowing of the island growth and development of a Rutherford regime, as the tearing regime is approached. In order to study the sensitivity of the stability and evolution to assumptions concerning the equation of state, two sets of full nonlinear resistive MHD equations (a pressure convection set and an incompressible set) are used. Both sets give more stable nonlinear behavior as the aspect ratio is reduced. The pressure convection set shows a transition from a Kadomtsev reconnection at large aspect ratio to a saturation at small aspect ratio. The incompressible set yields Kadomtsev reconnection for all aspect ratios, but with a significant lengthening of the reconnection time and development of a Rutherford regime at an aspect ratio approaching the transition from a resistive kink mode to a tearing mode. The pressure convection set gives an incomplete reconnection similar to that sometimes seen experimentally. The pressure convection set is, however, strictly justified only at high beta.
Enhanced sedimentation of suspensions in porous media
Authors: Marmur Abraham , Bar Sigalit
Published: July 2002

It is demonstrated that the sedimentation rate of a suspension can be enhanced by a factor of about 2–8 if the suspension is placed inside a porous medium. Experiments were performed with suspensions of red blood cells inside glass tubes packed with glass beads, both under the effect of gravity and in a centrifuge. The enhancement factor is shown to depend on the packing of the porous medium, on the volumetric concentration of the suspension, and on the rigidity of the red blood cells. It seems independent of the material of which the beads are made.
Skewness of the generalized centrifugal force divergence for a joint normal distribution of strain and vorticity components
Authors: Hua Bach Lien
Published: July 2002

This note attempts to connect the skewness of the probability distribution function (PDF) of pressure, which is commonly observed in two-dimensional turbulence, to differences in the geometry of the strain and vorticity fields. This paper illustrates analytically the respective roles of strain and vorticity in shaping the PDF of pressure, in the particular case of a joint normal distribution of velocity gradients. The latter assumption is not valid in general in direct numerical simulations (DNS) of two-dimensional turbulence but may apply to geostrophic turbulence in presence of a differential rotation (β effect). In essence, minus the Laplacian of pressure is the difference of squared strain and vorticity, a quantity which is named the generalized centrifugal force divergence (GCFD). Square strain and vorticity distributions follow chi-square statistics with unequal numbers of degrees of freedom, when one assumes a joint normal distribution of their components. Squared strain has two degrees of freedom and squared vorticity only one, thereby causing a skewness of the PDF of GCFD and hence of pressure.
On tornado-like vortex flows
Authors: Keller Jakob J.
Published: July 2002

In a first step, the problem of inviscid axisymmetric flow with buoyancy is investigated. It is found that both supercritical and subcritical vortex flows depart from their critical flow states if they are dominated by buoyancy. In a second step, the effects of entrainment are also taken into account. However, the investigation is restricted to nearly self-similar flows. Both supercritical and subcritical vortex flows approach their critical flow states if they are dominated by entrainment. However, the superimposed effects of buoyancy and entrainment may lead to an equilibrium close to the critical state. It is argued that the existence of such an equilibrium is the direct reason for the appearance of certain violent vortex flows in nature, including tornadoes and fire storms. The weather conditions that may lead to the appearance of tornadoes and the different forms of appearance are also discussed.
Generation of ring vortices in axisymmetric spin-down: A numerical investigation
Authors: Valentine Daniel T. , Miller Keith D.
Published: July 2002

In this paper the sequence of finite-amplitude events that occurs prior to the onset of Taylor–Görtler (TG) vortices in axisymmetric spin-down to rest is described. It is conjectured that the TG vortices are induced by these events, which include internal waves that propagate along the axis of rotation and transient vortex breakdown. The cascade of events is led by an internal ‘‘solitary wave’’ from each end wall. These waves are induced by the end-wall (Bödewadt) boundary layers with concomitant Ekman ‘‘blowing.’’ The passage of the solitary wave causes a transient vortex breakdown followed by a train of internal waves. The combined effect of these phenomena induces disturbances in the sidewall boundary layer that grow and lead to the formation of TG vortices. The TG vortices that originate adjacent to the midplane of the cylinder in the sidewall (Rayleigh) boundary layer dominate the flow field at an intermediate time interval following the onset of spin-down. Favorable comparisons with computationally determined onset times and subsequent evolution of TG vortices reported in the literature support the prediction that naturally occurring finite-amplitude phenomena induce these vortices.
The singularity at the tip of the rising plane bubble: The case of nonzero surface tension
Authors: Daripa Prabir
Published: July 2002

In the past pointed bubbles have been obtained numerically in the presence of surface tension. In this paper it is proven that if such pointed bubbles do exist in the presence of surface tension, then the singularity at the corner must be an irregular singular point. The generality and significance of the result are discussed.
Propagation and transport properties of dipolar vortices on a γ plane
Authors: Velasco Fuentes O. U.
Published: July 2002

The dynamics and transport properties of dipolar vortices on a γ plane (a plane where the Coriolis parameter has a quadratic variation with the latitude) are studied using the modulated point-vortex model. Similarly to the β-plane case, different regimes are found for the evolution of a single dipole, depending on the initial direction of propagation α0. Two steadily translating couples exist: The one rotating eastward (α0=0) has a stable trajectory and the one rotating westward (α0=π) is unstable. For initial angles in the range 0<α0<π, the couple moves along sine-like, 8-shaped and cycloid-like trajectories. In all solutions the dynamically relevant variables (the latitude and the direction of propagation) are periodic. The advection equations of passive particles in the dipole’s velocity field can be exactly written in the form of a periodically perturbed integrable Hamiltonian system. The study of transport is performed using a ‘‘dynamical-systems theory’’ approach. The entrainment and detrainment of fluid as a function of γ and α0 are computed exactly from some invariant curves in the Poincaré map and approximately by using the Melnikov function. The exchange of mass increases with both increasing γ and α0, while the rate at which this occurs has a maximum for some α0 and increases with γ. A major difference in particle spreading exists between dipoles which exactly return to their initial position after an integer number of oscillations and dipoles that do not. In the former case the Poincaré map shows broad areas of unstirred fluid coinciding with the maximum radial displacement in the dipole’s meandering trajectory.
Probability distribution functions for Navier–Stokes turbulence
Authors: Giles M. J.
Published: July 2002

The probability distribution function (PDF), P(Δur), of a velocity difference, Δur, over a distance r in incompressible fluid turbulence is derived from the Navier–Stokes equations using the underlying functional probability distribution. Two different types of approximation are used to evaluate the resulting functional integral for P(Δur). The first is based on the saddle-point technique. It is used to examine the non-Gaussian features of P(Δur) and to demonstrate, in particular, that its tail has the characteristic exponential form associated with intermittency. The second approximation is developed for the purpose of deriving the anomalous scaling exponents of the structure functions from P(Δur). It represents P(Δur) as the integral with respect to the spatially averaged dissipation rate, εr, of the product of the PDF of Δur conditioned to a particular εr, and the PDF of εr. These are coarse-grained PDFs, obtained using the renormalization group. The former is approximately Gaussian, whereas the latter is given by a constrained Gaussian functional integral, which is evaluated approximately using a transformation that enables it to be represented in terms of a Poisson process. This approach yields the scaling exponents in the form discussed by She and Lévêque [Phys. Rev. Lett. 72, 336 (1994)], and also gives the complete third-order structure function exactly.
Streaming generated in a liquid bridge due to nonlinear oscillations driven by the vibration of an endwall
Authors: Lee C. P. , Anilkumar A. V. , Wang T. G.
Published: July 2002

It has been observed that streaming can be generated in a liquid bridge supported by two endwalls, with one wall vibrating to excite its capillary oscillations [Anilkumar et al., J. Appl. Phys. 73, 4165 (1993); Mollot et al., J. Fluid Mech. 255, 411 (1993)]. The finding has been applied to suppress thermocapillary convection in crystal growth using the float zone technique [Grugel et al., J. Cryst. Growth 142, 209 (1994)]. In this work we shall explain the mechanism that drives the streaming, for low streaming velocities, in terms of an ‘‘acceleration shear’’ of the free surface of the liquid column. The results compare favorably with experiments with long columns. We have found that for a low viscosity liquid, the streaming pattern changes sign when the oscillation is adjusted from one side of a resonant peak to the other. This allows for more flexibility in the application of the streaming to counteract thermocapillary convection, during crystal growth in a float zone. We have also found that for low viscosities, the streaming velocity is inversely proportional to viscosity.
Fluid particle dispersion in homogeneous turbulent shear flow
Authors: Shen Peiqing , Yeung P. K.
Published: July 2002

The dispersion of Lagrangian fluid particle pairs in homogeneous turbulent shear flow is studied by a direct numerical simulation using 256×1282 grid points. It is emphasized that anisotropy requires the effects of both magnitude and orientation of the initial separation vector on two-particle statistics to be considered separately, especially at early times when these statistics are primarily determined by the Eulerian structure of the velocity gradient fluctuations. The effects of initial particle positions and pair separation on displacement and dispersion statistics are examined in relation to classical theory. Particle-pair dispersion is found to be most effective in the streamwise direction, and especially so for particle pairs initially separated in the direction of the mean velocity gradient. Similarly, two-particle velocity correlations are anisotropic, being strongest in the streamwise component but weakest in the cross-stream component. The particle-pair separation distance probability density function differs significantly from a Gaussian form used in most dispersion models for air-quality applications. It is hoped that the new results will be helpful for extending two-particle stochastic models to anisotropic turbulent flows.
Three-dimensional intrinsic convection in dilute and dense dispersions of settling spheres
Authors: Bruneau D. , Feuillebois F. , Bławzdziewicz J. , Anthore R.
Published: July 2002

The three-dimensional intrinsic convection in a monodisperse dispersion of spheres settling in a vertical container of arbitrary cross section is calculated using the simple model of point forces with excluded volume near the walls, proposed by Bruneau et al. [Phys. Fluids 8, 2236 (1996)]. An exact solution of the model equations for a container with rectangular cross-section shows that corners have no significant influence on the convection. A dense suspension is modeled by assuming an equilibrium particle distribution in the near wall region. It is predicted that as a result of near wall ordering, the intrinsic convection decreases with increasing particle volume fraction.
Drop breakup in three-dimensional viscous flows
Authors: Cristini Vittorio , Bławzdziewicz Jerzy , Loewenberg Michael
Published: July 2002

A new three-dimensional boundary integral algorithm is presented that is capable of simulating the process of drop breakup in viscous flows. The surface discretization is fully adaptive, thus providing accurate resolution of the highly deformed drop shapes that are characteristic of breakup events. Our algorithm is used to study drop breakup in shear flow and in buoyancy; the predictions are compared with experimental observations.
Viscous fingering in a magnetic fluid. I. Radial Hele-Shaw flow
Authors: Flament C. , Pacitto G. , Bacri J.-C. , Drikis I. , Cebers A.
Published: July 2002

Viscous fingering phenomena in a circular geometry are studied for a magnetic fluid submitted to a perpendicular magnetic field. Air is injected at the center of a Hele-Shaw cell filled with a viscous magnetic fluid. The instability of the interface between the air and the magnetic fluid is favored by the presence of a magnetic field. More precisely, the threshold of the instability is magnetic field dependent. The patterns obtained for high values of the magnetic field with a low injection rate are similar to those obtained in the absence of an external field and at high flow rates. We also give a linear analysis for the stability of radial flow under the influence of a magnetic field. This calculation provides us with an understanding of the magnetic field effect.
Direct numerical simulation and subgrid analysis of a transitional droplet laden mixing layer
Authors: Miller Richard S. , Bellan Josette
Published: July 2002

Direct numerical simulations of a temporally developing, droplet laden mixing layer undergoing transition to mixing turbulence are conducted. The formulation includes complete two-way couplings of mass, momentum, and energy. As many as 18×106 grid points are used to discretize the Eulerian gas phase equations and up to 5.7×106 initially polydisperse evaporating droplets are tracked in the Lagrangian reference frame. The complete transition to mixing turbulence is captured for several of the higher Reynolds number simulations and it is observed that increasing the droplet mass loading ratio results in a more “natural” turbulence characterized by increased rotational energy and less influence of the initial forcing perturbations. An increased mass loading also results in increased droplet organization within the layer. An a priori subgrid analysis is then conducted which shows that neglecting subgrid velocity fluctuations in the context of large eddy simulations may result in significant errors in predicting the droplet drag force for Stokes numbers St∼1 (with the flow time scale based on the mean velocity difference and initial vorticity thickness). Similar possible errors of lesser magnitude are also observed for the droplet heat flux and evaporation rate when thermodynamic subgrid fluctuations are neglected. An extension of the eddy interaction model commonly used in Reynolds-averaged simulations is then proposed in order to account for the missing subgrid information. Probability density functions (PDFs) of the subgrid fluctuations calculated across homogeneous planes are shown to be highly intermittent, particularly near the laminar–turbulent boundaries of the mixing layer. However, the actual subgrid PDFs calculated locally are much less intermittent and may be adequately modeled by the Gaussian distribution throughout the majority of the mixing layer. A scale similarity model is then employed to predict both the velocity and thermodynamic subgrid variances. The similarity model is well correlated with the actual subgrid variances and shows good agreement in predicting the local fluctuation intensities when a filter width-dependent model constant is used. The subgrid fluctuation variances acting on the droplets are then shown to be well modeled if the Eulerian subgrid variance model is interpolated to the droplet locations.
A subgrid-scale resistivity for magnetohydrodynamics
Authors: Theobald Michael L. , Fox Peter A. , Sofia Sabatino
Published: July 2002

In this paper an investigation is made of dissipation of small scale magnetic fields embedded in a turbulent gas flow using magnetohydrodynamic (MHD) simulations, under conditions suggested by the outer layers of the Sun, where heat is transported by turbulent convection (as estimated by fluid and magnetic Reynolds numbers), and dissipation is occurring on small scales. The large eddy simulation approach is utilized along with a model for the unresolved scales. Although a significant amount of work on modeling the small scales has been done for hydrodynamic turbulence, the application to MHD has received little attention. This paper specifically deals with the small scale dissipation of magnetic fields and thus a so-called subgrid-scale resistivity is postulated and then tested using a series of two-dimensional numerical simulations. An attempt is made to clarify the advantages and uncertainties of using this new formulation of turbulent resistivity in MHD and suggest areas for improvement and further application.
Response to ‘‘Comment on ‘Dielectric energy versus plasma energy, and Hamiltonian action-angle variables for the Vlasov equation’ ’’ [Phys. Fluids B 4, 3038 (1992)]
Authors: Morrison P. J. , Pfirsch D.
Published: July 2002

The energy density expression Best presents is not (when integrated) equal to the energy of a linear perturbation in Vlasov theory. The exact expression is given by either Eq.(42) or Eq.(48) of Ref.2. The authors comment on this and other discrepancies. (AIP)
Filamentary double layers
Authors: Theisen W. L. , Carpenter R. T. , Merlino R. L.
Published: July 2002

The properties of double layers produced in current filaments have been studied. The experiments were performed in a magnetized triple plasma device in which the diameter of the central plasma column could be varied. The scaling of the double layer potential drop Vdl with the parameter jd2, where j is the current density in the double layer and d is the double layer thickness, was determined for several values of the current filament radius R0. For relatively large values of R0, the one-dimensional (Langmuir) scaling was obeyed, Vdl∼(jd2)2/3. When R0 was decreased, a departure from the strictly one-dimensional scaling was observed, with Vdl becoming less and less dependent on the parameter jd2.
Guiding-center equations for electrons in ultraintense laser fields
Authors: Moore Joel E. , Fisch Nathaniel J.
Published: July 2002

The guiding-center equations are derived for electrons in arbitrarily intense laser fields also subject to external fields and ponderomotive forces. Exhibiting the relativistic mass increase of the oscillating electrons, a simple frame-invariant equation is shown to govern the behavior of the electrons for sufficiently weak background fields and ponderomotive forces. The parameter regime for which such a formulation is valid is made precise, and some predictions of the equation are checked by numerical simulation.
Ion acoustic solitons in a plasma with finite temperature drifting ions: Limit on ion drift velocity
Authors: Malik H. K. , Singh Sanjay , Dahiya R. P.
Published: July 2002

Propagation of ion acoustic solitons in a plasma consisting of finite temperature drifting ions and nondrifting electrons has been studied. It is shown that in addition to the electron inertia and weak relativistic effects, the ion temperature also modifies the soliton behavior. By including the finite ion temperature, limit for the ion drift velocity u0 for which the ion acoustic solitons are possible, is obtained. The solitons can exist for vTe≤u0≤‖u0 max‖, where vTe is the electron thermal velocity and u0 max is the maximum value of the ion drift velocity. The maximum value of this velocity is decided by the ion temperature. Under the limiting conditions, the Korteweg–deVries (KdV) equation is derived for one-dimensional ion acoustic soliton. Expressions are obtained for the soliton phase velocity, peak soliton amplitude, soliton width, and the soliton energy. The present results correspond to those of the previous investigations under appropriate plasma conditions.
Parametric effect of a spatially periodic voltage depression on operation of Čerenkov sources of electromagnetic radiation
Authors: Nusinovich G. S. , Vlasov A. N.
Published: July 2002

In microwave sources of coherent Čerenkov radiation the electrons usually propagate near the rippled wall of a slow-wave structure. These ripples cause the periodic modulation of electron potential depression, and therefore, lead to periodic modulation of electron axial velocities. Since the period of this electrostatic pumping is the period of the slow-wave structure the parametric coupling of electrons to originally nonsynchronous spatial harmonics of the microwave field may occur. This effect can be especially important for backward-wave oscillators (BWO’s) driven by high current, relativistic electron beams. In the paper both linear and nonlinear theories of the relativistic resonant BWO with periodic modulation of electron axial velocities are developed and results illustrating the evolution of the linear gain function and the efficiency of operation in the large-signal regime are presented.
Neoclassical kinetic theory near an X point: Plateau regime
Authors: Solano E. R. , Hazeltine R. D.
Published: July 2002

Traditionally, neoclassical transport calculations ignore poloidal variation of the poloidal magnetic field. Near an X point of the confining field of a diverted plasma, the poloidal field is small, causing guiding centers to linger at that poloidal position. A study of how neoclassical transport is affected by this differential shaping is presented. The problem is solved in general in the plateau regime, and a model poloidal flux function with an X point is utilized as an analytic example to show that the plateau diffusion coefficient can change considerably (factor of 2 reduction). Ion poloidal rotation is proportional to the local value of Bpol but otherwise it is not strongly affected by shaping. The usual favorable scaling of neoclassical confinement time with plasma current is unaffected by the X point.
Mercier criterion in tokamaks with anisotropic energetic particle component
Authors: Zheng L.-J. , Tsai S.-T.
Published: July 2002

Analytical equilibria for axisymmetric plasmas with an anisotropic energetic-particle component are obtained by expansions in the inverse aspect ratio and core- and hot-plasma betas. Upon the analytical equilibria, the energetic-particle-modified Mercier criterion [L.-J. Zheng and S.-T. Tsai, Phys. Fluids B 4, 1416 (1992)] is evaluated. A drift-nonreversed energetic-particle component is shown to make stabilizing terms to the criterion, which originate from the additional Shafranov shift of magnetic surfaces and the stabilizing kinetic energy contributed by the energetic particles. The results confirm that interchange modes in tokamaks can be stabilized by properly introducing drift-nonreversed energetic particles.
β limit disruptions in the Tokamak Fusion Test Reactor
Authors: Fredrickson E. D. , McGuire K. , Chang Z. , Janos A. , Bell M. , Budny R. V. , Bush C. E. , Manickam J. , Mynick H. , Nazikian R. , Taylor G.
Published: July 2002

A disruptive β limit (β=plasma pressure/magnetic pressure) is observed in high-performance plasmas in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Plasma Phys. Controlled Nuclear Fusion 1, 421 (1987)]. The magnetohydrodynamic character of these disruptions differs substantially from the disruptions in high-density plasmas (density limit disruptions) on TFTR. The high β disruptions can occur with less than a millisecond warning in the form of a fast growing precursor. The precursor appears to be an n=1 kink strongly coupled through finite β effects and toroidal terms to higher m components. It does not have the ‘‘cold bubble’’ structure found in density limit disruptions. The n=1 kink, in turn, appears to excite a ballooning-type mode that may contribute to the thermal quench.
Asymmetry and thermal effects due to parallel motion of electrons in collisionless magnetic reconnection
Authors: Tanaka Motohiko
Published: July 2002

Fast collisionless reconnection of magnetic flux loops by the macro-particle simulation code shows significant asymmetry of the plasma flow under an ambient toroidal magnetic field. The parallel motion of electrons induced by the reconnection electric field is found to produce large density and toroidal magnetic field inhomogeneities of a quadrupole shape, δn/n0∼0.3, unlike the m=1 mode. The divergence of the plasma flow is locally not identical to zero with each species, ∇⋅V(s)≠0 (s=e,i), due to the electron spatial movement along the magnetic field. This internal structure results in a thick current layer and enhances the reconnection process. A plasmoid that impedes magnetic reconnection is created when the parallel mass diffusivity of electrons arising from their thermal motion is suppressed (the fluid limit). The reconnection rate becomes a smoothly increasing function of the ion mass and an inverse of the toroidal magnetic field, the latter of which being due to the compressional effect. The rate is drastically reduced when the ion Larmor radius far exceeds the ion skin depth.
Plasma-screening effects on bremsstrahlung in electron–ion Coulomb scatterings in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma-screening effects are investigated for the bremsstrahlung in electron–ion Coulomb scatterings in strongly coupled plasmas using the classical trajectory method. In strongly coupled plasmas, the electron–ion interaction potential is obtained by the ion-sphere model potential. The classical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the differential bremsstrahlung radiation cross section as a function of the impact parameter and ion-sphere radius. The results show that the plasma-screening effects for the differential bremsstrahlung radiation cross section are increased with increasing the ion-sphere radius. The differential bremsstrahlung radiation cross sections are decreasing as the radiation energy increases. However, for a small ion-sphere radius the radiation cross sections are almost unchanged with increasing or decreasing the radiation energy.
Spatiotemporal evolution of parametric instabilities driven by short laser pulses: One-dimensional analysis
Authors: McKinstrie C. J. , Turano E. J.
Published: July 2002

The spatiotemporal evolution of parametric instabilities such as stimulated Raman scattering and stimulated Brillouin scattering is studied analytically, within the framework of a one-dimensional model. New solutions of the model equations are found that illustrate the evolution of forward, sideward, and backward instabilities. The evolution of instabilities that are initiated by fluctuations within the plasma has two distinct phases. The first phase is characterized by exponential functions, whereas the second phase is characterized by Bessel functions. For times that are shorter than the time taken for the leading edge of the laser pulse to cross the plasma, near-forward instabilities dominate. For longer times near-sideward instabilities dominate.
Collisionless energy coupling to electrons by helicon waves in the near field of an antenna
Authors: Borg G. G. , Kamenski I. V.
Published: July 2002

Linear kinetic damping mechanisms of helicon waves in helicon wave driven plasma sources are re-examined. It is demonstrated that a reversal of the phase velocity of the wave in the near field of an antenna can lead to a transient overshoot in the rate of work done by the wave. This overshoot occurs when the magnitude of the phase velocity is a few times the thermal speed and Landau damping is the dominant kinetic absorption mechanism. These conditions often occur in low field and low density sources. The overshoot in the rate of work done by the wave may be an order of magnitude larger than the far-field Landau absorption rate. The effect of collisions is also assessed.
Comment on the thermoelectric term
Authors: Saleem H.
Published: July 2002

It is shown that the thermoelectric term which appears as a source of fluctuating magnetic field B1 in unmagnetized nonuniform plasmas in the limit ωpe≪ω≪ωpe [where ωpi(ωpe) are ion (electron) plasma oscillation frequencies, respectively] is of smaller order. Therefore, the thermoelectric term does not seem to contribute to magnetic fluctuations linearly. The theory of magnetic field generation based on ion dynamics predicts the same order of magnitude of B1 which is experimentally observed.
On self-consistent stationary propagation of relativistically coupled electromagnetic and electrostatic waves. II. Effects of electron–positron pair creation
Authors: Tsintsadze Levan N. , Kusano Kanya , Nishikawa Kyoji
Published: July 2002

Effects of electron–positron pair creation on the stationary one-dimensional propagation of relativistically coupled electromagnetic and electrostatic waves are studied based on the self-consistent model presented in the previous paper for an electron–ion plasma [L. N. Tsintsadze and K. Nishikawa, Phys. Plasmas 3, 511 (1996)]. The pair-created particles are treated as a cold electron–positron plasma at their creation point. In addition to the results similar to those obtained for the electron–ion plasma, a new type of instability of the finite amplitude plane wave is found which is purely growing in the wave frame near threshold. A novel solution describing an envelope shock which represents a wake-field excitation by a solitary electromagnetic pulse is obtained by taking into account the trapping of the pair-created particles in the upstream region.