bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport of impure plasma with arbitrary toroidal rotation
Authors: Wong Seung Kai
Published: July 2002

A previous formulation of neoclassical transport for an axisymmetric plasma with arbitrary toroidal rotation is generalized to plasmas with impurity ions. Calculations in the banana regime for a two-ion species plasma for large aspect circular flux surfaces show significant enhancement of both particle and heat fluxes. The viscous part of the momentum flux remains small. A more detailed treatment of the electron equations reveals the importance of an assumption relating to the balancing of the toroidal torques on the electron fluids which is likely to require modification in practical situations.
Magnetoacoustic shock waves in a relativistic gas
Authors: Majorana A. , Anile A. M.
Published: July 2002

Fast and slow magnetoacoustic shocks are studied in the framework of relativistic magneto-fluid dynamics with the Synge equation of state. An approximate analytical solution is presented in a particular case. The general case is treated by numerical methods.
Stability and finite amplitude natural convection in a shallow cavity with insulated top and bottom and heated from a side
Authors: Kuo Hai Perng , Korpela Seppo A.
Published: July 2002

The stability of laminar natural convection in a shallow cavity has been studied theoretically. The flow is driven by a horizontal temperature gradient between isothermal vertical sidewalls of the cavity, the top and bottom of which are insulated. It was found that for a Prandtl number (Pr) less than 0.033, shear instability causes stationary transverse cells to be formed in the flow. For larger values of Prandtl number the instability sets in as oscillating longitudinal rolls in the range 0.033<Pr<0.2; and as stationary longitudinal rolls for larger values of Pr. The importance of three-dimensional disturbances was investigated for Pr=0.02 and they were found to be less critical than two-dimensional ones. Finite amplitude motions of the stationary transverse cells were simulated by solving the nonlinear equations numerically with the use of pseudospectral methods. This simulation supports the calculations of the onset of the instability by linear theory, and explains why the Nusselt number decreases when secondary flows are present.
The influence of surfactant on the bubble motion in Hele–Shaw cells
Authors: Park Chang-Won , Maruvada S. R. K. , Yoon Do-Young
Published: July 2002

The surfactant influence on the bubble motion in a Hele–Shaw cell was studied experimentally. In order to differentiate the cases with and without the surfactant influence, the motion of air bubbles and water drops driven by silicone oil was investigated. The water drops contained a predetermined amount of an organic surfactant (sodium dodecyl sulfate), so that the dependence of their motion on the surfactant concentration could be studied systematically. In case of air bubbles in silicone oil, surfactants were likely to have negligible influence, although they might be present as contaminants, and their translational velocities were observed to be close to the prediction of Taylor and Saffman [Q. J. Mech. Appl. Math. 12, 265 (1959)]. The bubble shapes were also in accordance with available theories for a surfactant-free system. The water drops, on the other hand, behaved very differently, in that the translational velocities were smaller by an order of magnitude and their shapes were very unusual. These observations are apparently consistent with those of Kopf-Sill and Homsy [Phys. Fluids 31, 18 (1988)], and the present study suggests that the perplexing observations by them may be due to the influence of surface-active contaminants.
On the Lagrangian nature of the turbulence energy cascade
Authors: Meneveau Charles , Lund Thomas S.
Published: July 2002

The spatial and temporal evolution of turbulence kinetic energy at different scales is studied using direct numerical simulations of isotropic turbulence. To explicitly follow the energy during the cascade process in physical space, a Lagrangian correlation coefficient between local kinetic energy at different scales is computed. This correlation is found to peak only after a Lagrangian time delay that is an increasing function of the scale separation. It is shown that a characteristic length reduction of a factor of 2 is achieved approximately after the local eddy-turnover time scale. The results show that the view of spatially localized eddy structures transferring their kinetic energy to smaller scales appears to be, on average, quite realistic.
On the relationship between stochastic Lagrangian models of turbulence and second-moment closures
Authors: Pope S. B.
Published: July 2002

A detailed examination is performed of the relationship between stochastic Lagrangian models—used in PDF methods—and second-moment closures. To every stochastic Lagrangian model there is a unique corresponding second-moment closure. In terms of the second-order tensor that defines a stochastic Lagrangian model, corresponding models are obtained for the pressure-rate-of-strain and the triple-velocity correlations (that appear in the Reynolds-stress equation), and for the pressure-scrambling term in the scalar flux equation. There is an advantage in obtaining second-moment closures via this route, because the resulting models automatically guarantee realizability. Some new stochastic Lagrangian models are presented that correspond (either exactly or approximately) to popular Reynolds-stress models.
Effect of pressure gradient on first mode of instability in compressible boundary layers
Authors: Masad Jamal A. , Zurigat Yousef H.
Published: July 2002

The effect of a pressure gradient on the first mode of instability of compressible subsonic and supersonic boundary layers is investigated using linear stability theory. A pressure gradient is studied that generates potential-flow Mach number distributions at the edge of the boundary layer of the form Me=cxn, where c is a constant and x is the dimensionless streamwise distance. Variations are calculated for the maximum growth rates of three-dimensional first-mode waves with different edge Mach numbers and different levels of both adverse and favorable pressure gradients. A favorable pressure gradient is shown to have a stabilizing effect on first-mode waves. However, at high edge Mach numbers, a favorable pressure gradient becomes less effective in stabilizing first-mode waves. The frequencies and streamwise and spanwise wave numbers that correspond to the maximum growth rates of first-mode waves decrease as the pressure gradient becomes more favorable at all Mach numbers when the Reynolds number R=1500 and at Me≥2 when R=600. Setting the Prandtl number to unity significantly increases the maximum growth rates of first- and second-mode waves at high Mach numbers compared with setting it to the realistic value for air of 0.72. Predicted transition in flow over a flat plate using the N-factor criterion is found to be due to first-mode waves up to free-stream Mach numbers of 6–6.5.
Effect of surface tension on the stability of a binary fluid layer under reduced gravity
Authors: Chen C. F. , Chen C. C.
Published: July 2002

A temperature gradient imposed across a binary fluid layer with a nonzero Soret coefficient will induce a solute concentration gradient. The ratio of these two gradients is proportional to the separation ratio χ, a property of the fluid. Similarly, the ratio of the thermal and solutal Marangoni numbers, which are nondimensional increments in surface tension due to changes in temperature and concentration, is also proportional to the separation ratio. As a consequence, the stability of a given binary fluid layer with a free surface under zero gravity depends only on the temperature difference, ΔT, imposed across the layer or, equivalently, on the thermal Marangoni number, M, albeit the dependence, is rather complicated. When the gravity is nonzero but of small magnitude, such that the buoyancy effects are not dominant, the stability characteristics of the layer are functions of two parameters, M and R, the thermal Rayleigh number. In this paper, the stability of such a binary layer under zero and reduced gravity by means of linear stability analysis is studied. Results show that the nature of the instability depends on the product χK, where K is a material constant=(α/αS)(γS/γ), with α and αS denoting the volumetric expansion coefficient due to temperature and solute concentration, respectively, and γ and γS the rate of change of surface tension with respect to temperature and solute concentration, respectively. Both χ and K can assume positive and negative values. Under zero gravity, instability at the critical value of M onsets in steady convection if χK<0 and in oscillating convection if χK≳0. For a layer that is being heated from below and K≳0, the steady instability in the case of χK<0 can be rendered stable by subjecting the layer to a gravity of small magnitude. But for χK≳0, the effect of gravity is always destabilizing.
On the spatial structure of global modes in wake flow
Authors: Zielinska B. J. A. , Wesfreid J. E.
Published: July 2002

Numerical simulations of wake flow behind an equilateral triangular obstacle are presented. The form of global modes and their dependence on the Reynolds number found in this study are in accordance with recent experimental results of Goujon–Durand et al. [Phys. Rev. E 50, 308 (1994)]. A scaling law of the amplitude oscillating with the fundamental frequency corresponding to the maximum of the global mode is found to agree with the Landau model in a range of Reynolds Re numbers larger than in previous studies. The position of the maximum amplitude of the fundamental modes scales as (Re−Rec)−1/2. The amplitude of the second harmonic of the longitudinal component of the velocity as well as the correction to the mean flow have different critical behavior than the velocity components oscillating with fundamental frequency. During linear growth the position of the maximum of the global modes is constant and moves only in the nonlinear regime. The effects of the blockage and the boundary conditions on the side walls on the form of the global modes are discussed.
Ignition dynamics of a diffusion flame rolled up in a vortex
Authors: Thévenin Dominique , Candel Sébastien
Published: July 2002

This article deals with the dynamics of ignition for a diffusion flame rolled up in a vortex. The flame is formed at the interface between a cold fuel and a hot oxidizer. The calculations are carried out in the thermodiffusive approximation, where all the volumetric expansion effects are neglected. Regimes of ignition are obtained for different vortex Reynolds numbers and oxidizer temperatures. By varying this last parameter, one observes three possible ignition modes: a diffusion flame ignition at moderate air temperatures, a premixed ignition in the core at high temperatures, and a mixed mode involving premixed and diffusion flames in the intermediate temperature range. Results of calculations are interpreted on the basis of one-dimensional asymptotic analysis of unsteady strained laminar diffusion flames.
Viscous sublayer flow visualizations at Rθ≂1 500 000
Authors: Klewicki J. C. , Metzger M. M. , Kelner E. , Thurlow E. M.
Published: July 2002

Plan view flow visualization experiments were conducted in the atmospheric surface layer that flows over the Great Salt Lake Desert at the U.S. Army Dugway Proving Ground, Dugway, Utah. Measurements were acquired on a nonconductive, polyethylene platform made flush with the desert floor. Surface conditions upstream of the measurement site were flat, devoid of vegetation, and because of the dried mud/clay/salt composition, essentially dust free. Local surface variations ranged between 1 and 3 mm, which corresponded to three to ten viscous units during the experiments. Flow visualizations were accomplished by continuously injecting theatrical fog through a tangential slit covering a smoke reservoir buried under the platform. During the visualizations, the atmospheric surface layer flow was near neutral thermal stability. Flow velocities at 2.0 m above the surface maintained directional constancy, with a magnitude of about 1.5 m/s. A single element hot-wire probe positioned near y+=3.4 was used to measure the wall shear. Visualization results indicate the existence of the pocket and streak motions seen at much lower Reynolds number. The average inner normalized streak spacing was found to be λ+=93.1, with a positively skewed, nearly lognormal distribution. The average maximum inner normalized pocket width was found to be w+=127.2, with a positively skewed distribution. The average time between pockets was determined to be T+=36.6. Comparisons are made with existing low Reynolds number results, and a brief discussion is provided regarding the physics underlying the present observations.
Nonlinear transport for a dilute gas in steady Couette flow
Authors: Garzó V. , López de Haro M.
Published: July 2002

Transport properties of a dilute gas subjected to arbitrarily large velocity and temperature gradients (steady planar Couette flow) are determined. The results are obtained from the so-called ellipsoidal statistical (ES) kinetic model, which is an extension of the well-known BGK kinetic model to account for the correct Prandtl number. At a hydrodynamic level, the solution is characterized by constant pressure, and linear velocity and parabolic temperature profiles with respect to a scaled variable. The transport coefficients are explicitly evaluated as nonlinear functions of the shear rate. A comparison with previous results derived from a perturbative solution of the Boltzmann equation as well as from other kinetic models is carried out. Such a comparison shows that the ES predictions are in better agreement with the Boltzmann results than those of the other approximations. In addition, the velocity distribution function is also computed. Although the shear rates required for observing non-Newtonian effects are experimentally unrealizable, the conclusions obtained here may be relevant for analyzing computer results.
Self-similarity and mixing characteristics of turbulent mixing layers starting from laminar initial conditions
Authors: Ansari Amid
Published: July 2002

Direct numerical simulations of two turbulent mixing layers starting from low- and high-gradient laminar profiles have been performed, to assess the transitional and long time characteristics of these flows. The simulations include a passive scalar with Schmidt number of 0.7, and were conducted using a grid resolution of 512×256×257 implemented in parallel on 128 nodes of the Intel Paragon supercomputer. The two mixing layers at their final turbulent condition achieved Reynolds number of ≈11 000. It is found that although the self-similarity appears to have been achieved early in the evolution of these flows, long time development is necessary for the mixing characteristics and the statistical content of these flows to become similar to the mixing layers that start from turbulent boundary layers.
Forces on a slowly rotating, rough cylinder in a Couette device containing a dry, frictional powder
Authors: Tardos Gabriel I. , Khan M. Irfan , Schaeffer David G.
Published: July 2002

In the present work, a fine, dry powder was sheared in a Couette device: i.e., sheared between concentric vertical cylinders. The torque generated on the rough, inner cylinder was measured as this inner wall was rotated. Our experiments provided evidence that, in a column of granular material undergoing continuous shearing, normal and shear stresses increase linearly with depth. In other words, Janssen’s analysis ceases to apply if granular material is continuously sheared.
Inviscid two dimensional vortex dynamics and a soliton expansion of the sinh-Poisson equation
Authors: Chow K. W. , Ko N. W. M. , Leung R. C. K. , Tang S. K.
Published: July 2002

The dynamics of inviscid, steady, two dimensional flows is examined for the case of a hyperbolic sine functional relation between the vorticity and the stream function. The 2-soliton solution of the sinh-Poisson equation with complex wavenumbers will reproduce the Mallier-Maslowe pattern, a row of counter-rotating vortices. A special 4-soliton solution is derived and the corresponding flow configuration is studied. By choosing special wavenumbers complex flows bounded by two rigid walls can result. A conjecture regarding the number of recirculation regions and the wavenumber of the soliton expansion is offered. The validity of the new solution is verified independently by direct differentiation with a computer algebra software. The circulation and the vorticity of these novel flow patterns are finite and are expressed in terms of well defined integrals. The questions of the linear stability and the nonlinear evolution of a finite amplitude disturbance of these steady vortices are left for future studies.
Magnetohydrodynamic flow between insulating shells rotating in strong potential field
Authors: Starchenko Sergey V.
Published: July 2002

We consider the motion of a spherical annulus of fluid driven by the slightly differential rotation of the confining spherical shells. The novelty of our analysis is the imposition of a strong axisymmetric potential magnetic field on a conducting annulus bounded by insulators. The strength of the field is measured by its ability to dominate the dynamics of the fluid shell, being much larger than the viscous and Coriolis forces. The fluid flow is structured such as to minimize the effect of the strong field and the associated induction currents. The thrust of the analysis is the identification of regions of the fluid shell in which the azimuthal flow and generated field are relatively uniform. These regions are bounded by shear layers in which the azimuthal flow and field vary rapidly. Thickness of the shears is about a square root of a Hartmann layer thickness. These Hartmann boundary layers control main streams in the magnetohydrodynamic flow. An imposed axial magnetic field produces two rigid-body rotating regions that have different rotation rates outside and inside the axial cylinder tangent to the inner sphere. The azimuthal magnetic field is proportional to the cylindrical radius and is expelled from the outside of the tangent cylinder. For an imposed dipole magnet, similar regions with rigid-body rotation are divided by a shear that has a shape of a lobe touching the outer equator. Very weak meridional fluid flux is inversely proportional to a cube of the imposed magnetic field for these degenerated cases. In a quadrupole magnetic field, two lobe-like regions are symmetric with respect to the shell’s equator, touch the outer sphere, and rotate with the inner sphere. Outside these regions, differential rotation is valuable and a weak meridional fluid flux is inversely proportional to a square of the imposed magnetic field as in the general case.
Two-fluid boundary layer stability
Authors: Özgen S. , Degrez G. , Sarma G. S. R.
Published: July 2002

The stability of a two-fluid boundary layer is investigated. A boundary layer shears a second fluid that is bounded by the wall and the shearing fluid. The eigenvalue problem governing the linear stability of the configuration is solved using an efficient shooting-search method. Besides the Tollmien-Schlichting mode (hard mode) found in the classical hydrodynamical stability theory an additional Yih-mode (interfacial mode) exists due to the two-fluid interface. Effects of viscosity and density stratifications, thickness of the bounded fluid, gravity, surface tension as well as the non-Newtonian character of the lower fluid on the stability characteristics are determined. The interfacial mode is found to be very sensitive against viscosity stratification. However, with a highly viscous liquid layer, the system approaches a single-layer behavior. The shear-thinning non-Newtonian liquid layer is observed to have a stabilizing effect for both of the modes. Surface tension is stabilizing for short waves for the interfacial mode but a more complex effect was observed for the hard mode. Gravity is stabilizing with a favorable density stratification. Density stratification alone is destabilizing for low and moderate values of this parameter but becomes stabilizing for higher values. When the external boundary layer profile is turbulent, the interfacial mode is more likely to be observed in an experiment. Agreement of the obtained results with experimental, theoretical and numerical results reported in the literature is good. This is encouraging as the study is intended for solving the stability characteristics of de/anti-icing fluid-air systems and comparing the results with the experimental data when they become available.
Rotatory oscillations of arbitrary axi-symmetric bodies in an axi-symmetric viscous flow: Numerical solutions
Authors: Tekasakul P. , Tompson R. V. , Loyalka S. K.
Published: July 2002

The problem of the rotatory oscillation of an axi-symmetric body in an axi-symmetric, viscous, incompressible flow at low Reynolds number has been studied. In contrast to the steady rotation of a body, which involves solving the Laplace equation, the study of an oscillating body requires solution of the Helmholtz equation which results from the simplification of the unsteady Stokes equations. In the present work, we have numerically evaluated the local stresses and torques on a selection of free, oscillating, axi-symmetric bodies in the continuum regime in an axi-symmetric viscous incompressible flow. The Helmholtz equation was solved by a Green’s function technique. The accuracy of the technique is tested against known solutions for a sphere, a prolate spheroid, a thin disk, and an infinitely long cylinder. Good agreements have been obtained. Finite cylinders have been studied and the edge correction factors for the circular disk geometry, that are basic to oscillating disk viscometers, have been calculated. It has been found that the calculated edge correction factors, based on the ratios of the real parts of the actual torques (calculated from this work) to the ideal torques, agree to within 1% to 10% with the reported values obtained by Clark et al. [Physica A 89, 539 (1977)] using the theory of Kestin and Wang [J. Appl. Mech. 24, 197 (1957)]. However, since the ratios of the real parts and the ratios of the imaginary parts of the torques do not coincide, the edge correction factors depend upon which ratio is used.
A PDF model for dispersed particles with inelastic particle–wall collisions
Authors: Devenish B. J. , Swailes D. C. , Sergeev Y. A. , Kurdyumov V. N.
Published: July 2002

A simple model is proposed for the phase-space distribution of dispersed, high-inertia particles in a turbulent boundary layer. This model includes a boundary condition describing inelastic particle–wall collisions. Two models for the normal coefficient of restitution are considered. The simpler model treats this coefficient as a constant, and this is shown to lead to a singular distribution of particle velocities at the boundary. A numerical scheme for treating this model is presented, and results from this are compared with those obtained from particle random-walk simulations. An asymptotic analysis is given for this singular model; the asymptotic behavior of the phase-space distribution shows that the one-dimensional steady state distribution exists only for relatively high values of the coefficient of restitution, ε>0.5. The more detailed model for the coefficient of restitution treats this coefficient as a function of the normal impact velocity. Numerical results from this model are also given and compared with those obtained by simulation. For both models the particle number density and particle fluctuation energy at the wall, required to formulate boundary conditions for the “macroscopic” two-fluid models, are calculated as functions of model parameters. The results also illustrate the phenomenon of particle segregation towards the wall in turbulent gas-particulate suspensions, i.e., the formation of the near-wall dense layer of particles; the thickness of this layer is also determined as a function of model parameters.
Free decay of turbulence and breakdown of self-similarity
Authors: Eyink Gregory L. , Thomson David J.
Published: July 2002

It has been generally assumed, since the work of von Kármán and Howarth in 1938, that free decay of fully-developed turbulence is self-similar. Here we present a simple phenomenological model of the decay of three-dimensional incompressible turbulence, which predicts breakdown of self-similarity for low-wavenumber spectral exponents n in the range nc<n<4, where nc is some threshold wavenumber. Calculations with the eddy-damped quasi-normal Markovian approximation give the value as nc≈3.45. The energy spectrum for this range of exponents develops two length-scales, separating three distinct wavenumber ranges.
Low-frequency vibrations in a near-critical fluid
Authors: Jounet Arnaud , Mojtabi Abdelkader , Ouazzani Jalil , Zappoli Bernard
Published: July 2002

The response of a near-critical fluid to low-frequency vibrations is investigated by means of numerical simulations. Its characteristics are first established by one-dimensional analysis. It is shown that the strong thermo-mechanical coupling occurring in the boundary layers tends to make the fluid oscillate homogeneously at low frequencies, and with a larger amplitude than in a normal gas. The numerical results obtained in this first part are found to confirm earlier predictions made in pioneering theoretical work. Then, the study is extended to a two-dimensional configuration. In a square cavity, the wall shear stresses developing along the longitudinal boundaries do not affect the one-dimensional regime, since the viscous layer present in these areas behaves like the Stokes boundary layer. By contrast, thermostatting these boundaries, like the others, generates local curvature of the stream lines. The fluid response to the homogeneous acceleration field then takes some more pronounced two-dimensional patterns, but remains driven by the strong alternating expansions and retractions of the fluid in the thermal boundary layers, which are specific to near-critical fluids.
Effects of confinement on the temporal instability of gas jets injected in viscous liquids
Authors: Subramaniam K. , Parthasarathy R. N.
Published: July 2002

A temporal linear stability analysis of an inviscid incompressible gas jet injected into a coflowing viscous liquid confined within a tube was conducted to study the growth of small disturbances that lead to the breakup of the gas jet. For low-viscosity liquids, the confinement of the liquid jet did not affect the growth rates of the gas-jet disturbances. However, for high viscosity liquids, confining the liquid jet results in a significant increase in the disturbance growth rate and the range of unstable wave numbers. Also, for small ratios of the liquid jet diameter to gas jet diameter, the change in disturbance growth rate with Weber number is small. The effect of Weber number becomes significant only at large values of the diameter ratio.
An analytical and numerical investigation of ion acoustic waves in a two-ion plasma
Authors: Vu H. X. , Wallace J. M. , Bezzerides B.
Published: July 2002

The ion acoustic dispersion relation for a plasma containing two distinct ion species is investigated over a wide range of plasma conditions. An approximate general analytic solution to the dispersion relation has been found, and is shown, by comparison to accurate numerical solutions of the individual modes, to be remarkably precise. This solution provides for the first time a systematic account of the totality of ion acoustic modes of the two-ion system. It has been found that ion acoustic modes consist of two types of modes: (a) at least one, and, at most, two weakly damped modes for which ‖ωI/ωR‖≪1, and (b) an infinity of critically damped modes for which ωI/ωR≂−1. The critically damped modes are organized into two distinct categories: (a) modes for which ‖ω‖/k≳vF (vF is the thermal speed of the fast ion species); and (b) modes for which vS<‖ω‖/k<vF (vS is the thermal speed of the slow ion species). The critically damped modes with ‖ω‖/k≳vF are further organized into three distinct classes: (1) modes with phase speeds characterized by vF, (2) modes with phase speeds characterized by vFvS/√v2F−v2S, and (3) modes with phase speeds characterized by vS. The critically damped modes with vS<‖ω‖/k<vF belong to a single class, and are characterized by vS. Generally, it is found that there are one, or, at most, two modes with relatively small damping, while most of the remaining modes are too strongly damped to be physically realized. It has also been found possible to maximize the ion acoustic damping in a two-ion plasma by a judicious choice of the relative ion concentrations. More specifically, an admixture of the lighter ion species will maximize the damping coefficient over a wide range of plasma conditions. This is important for certain gas-filled, inertial-fusion targets of current interest, where it is desirable to minimize the stimulated Brillouin backscatter process by maximizing the damping.
Pseudo-three-dimensional simulation on stability of thin shell target implosion
Authors: Taguchi Toshihiro , Mima Kunioki
Published: July 2002

Pseudo-three-dimensional simulation method has been developed to analyze implosion stability of thin shell target in inertial confinement fusion. Nonlinear motion of a thin spherical surface driven by the pressure simulates the implosion of a shell target filled with fuel gas. The simulation shows that acceleration and deceleration of the shell give rise to the Rayleigh–Taylor instability and the instability prevents the target from being compressed uniformly. The results of the simulation show how the maximum volume compression ratio and breakup time of the target depend upon the initial perturbation amplitude. The new simulation scheme has also been applied to another shape of closed surfaces. The implosion phenomenon of the surface of torus is shown as an example. The toroidal target implosion is found to be more stable against the Rayleigh–Taylor instability than that of spherical targets.
Long pulse experiments on the Advanced Toroidal Facility
Authors: Jernigan T. C. , Bigelow T. S. , Colchin R. J. , Dyer G. R. , England A. C. , Fehling D. T. , Greenwood D. E. , Klepper C. C. , Murakami M. , Overbey D. R. , Rasmussen D. A. , Schaich C. R. , Simpkins J. E. , White J. A. , Wilgen J. B. , Wilson C. T. , Yarber J. L. , Komori A. , Morimoto S. , Motojima O. , Sagara A. , Sato M. , Yamada H.
Published: July 2002

The Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] is the world’s largest stellarator. It was designed and built to demonstrate high beta, steady-state operation in a toroidal confinement system. During its final operating period ATF achieved pulse lengths of over one hour (4667 s). The objectives of these experiments were (1) investigation of plasma performance at times that are long compared to the plasma/wall equilibrium time; (2) determination of plasma control and wall conditioning techniques; and (3) adaptation of plasma diagnostic and data acquisition systems to long-pulse operation. Other experiments have also extended earlier studies of dimensionless-parameter plasma confinement scaling. By employing two discrete electron cyclotron heating (ECH) frequencies (28 and 35 GHz), and by simultaneously modulating the ECH power, magnetic field, and plasma density, it has been possible to maintain fixed plasma beta and collisionality while modulating the normalized gyroradius.
Interaction of crossed laser beams with plasmas
Authors: Eliseev V. V. , Rozmus W. , Tikhonchuk V. T. , Capjack C. E.
Published: July 2002

The parametric interaction of two crossed collimated laser beams with ion plasma modes has been studied. The underlying process is a density grating that is created by the two laser beams. The Bragg diffraction that is produced enhances forward stimulated Brillouin scattering (SBS) which results in a time dependent energy exchange between the two beams. A diversity of other SBS processes which depend on the symmetry of driving laser beams are also discussed.
Evaluation of the modified plasma dispersion function for half-integral indices
Authors: Summers Danny , Thorne Richard M. , Matsumoto Hiroshi
Published: July 2002

Space and astrophysical plasmas typically possess particle distribution functions with a power-law tail (in energy) that are well modeled by generalized Lorentzian distributions with an associated spectral index κ. Dispersion equations for linear waves of any mode in a plasma described by a Lorentzian-type particle distribution involve the modified plasma dispersion function Zκ*, a special function analogous to the plasma dispersion function Z that arises when the particle distribution is Maxwellian. The function Zκ*, originally defined by Summers and Thorne [Phys. Fluids B 3, 1835 (1991)] for integral values of κ, was recently generalized to real values of κ by Mace and Hellberg [Phys. Plasmas 2, 2098 (1995)]. In the present paper, a general formula is derived for the modified plasma dispersion function Zκ* corresponding to half-integral values of κ, and simple, explicit closed-form expressions are given for the functions Z3/2*, Z5/2*, Z7/2*, Z9/2*, and Z11/2*. These results complement the simple, closed-form expressions for the functions Zκ*, for κ=1, 2, 3,..., that already exist in the literature.
Experimental and theoretical investigations of a rectangular grating structure for low-voltage traveling wave tube amplifiers
Authors: Joe J. , Louis L. J. , Scharer J. E. , Booske J. H. , Basten M. A.
Published: July 2002

A periodic, rectangular grating slow-wave-structure is considered for forward and backward wave low-voltage (⩽10 kV) Ku-band traveling wave tube (TWT) amplifiers. For forward wave operation, it is required that the ratio of groove depth, d, to grating period, p, be large (i.e. d/p⩾5) while small values of d/p allow backward wave operation. For large d/p, skin effect losses in the grating slots are large and can substantially reduce the growth rate produced by the beam-slow wave interaction. Phase and amplitude measurements of the grating structures utilizing a slotted line and a fast Fourier transform (FFT) analysis have been carried out. The results show that the measured dispersion relations for both shallow (d/p= 0.446) and deep groove gratings (d/p= 7.43) agree very well with the theoretical dispersion relations. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solenoidal magnetic field. Tuning curve and single particle backward wave gain measurements are presented and discussed.
Improved gas-filled hohlraum performance on Nova with beam smoothing
Authors: Kauffman Robert L. , Powers L. V. , Dixit S. N. , Glendinning S. G. , Glenzer S. H. , Kirkwood R. K. , Landen O. L. , MacGowan B. J. , Moody J. D. , Orzechowski T. J. , Pennington D. M. , Stone G. F. , Suter L. J. , Turner R. E. , Weiland T. L. , Richard A. L. , Blain M. A.
Published: July 2002

Gas-filled hohlraums are presently the base line ignition target design for the National Ignition Facility. Initial Nova [E. M. Campbell et al. Rev. Sci. Instrum. 57, 2101 (1986).] experiments on gas-filled hohlraums showed that radiation temperature was reduced due to stimulated Brillouin and stimulated Raman scattering losses and that implosion symmetry had shifted compared with vacuum hohlraums and calculations. Subsequent single beam experiments imaging thermal x-ray emission showed the shift is due to laser–plasma heating dynamics and filamentation in a flowing plasma. Experiments using a single beam have shown that scattering losses and effects of filamentation are reduced when the beam is spatially smoothed with a random phase plate or kinoform phase plate. Scattering is further reduced to less than 5% of the incident laser energy when temporal smoothing is added.
Minimum requirement for second law violation: A paradox revisited
Authors: Sheehan D. P. , Means J. D.
Published: July 2002

A paradox has been posed that challenges the second law of thermodynamics in a plasma blackbody environment [D. P. Sheehan, Phys. Plasmas 2, 1893 (1995)]. Laboratory experiments testing critical aspects of the paradox have supported theoretical predictions and have failed to resolve the paradox in favor of the second law. In this article, the paradox is sharpened and expanded in scope by identifying the crux of the paradox and the general requirements for second law violation. It is found that for an electrically conducting probe immersed in a blackbody plasma and connected to ground through a load, the general requirement for second law violation is Vf≠0, where Vf is the plasma floating potential. This requirement can be met by many natural and manmade plasmas.
Can large-scale magnetic fields survive during the pre-recombination era of the universe?
Authors: Lesch H. , Birk G. T.
Published: July 2002

The influence of a dense photon field on the generation of electric currents and magnetic fields in the early phases of the universe is considered. It is shown that due to the strong interaction of charged particles and photons, any directed relative drift velocity of positive and negative charges is immediately thermalized and currents are shut off. Large-scale magnetic fields caused by phase transitions evolve only under the influence of the expanding plasma, which is strongly coupled to the photons—an effect well known as Silk-damping. Immediately after the epoch of baryon–antibaryon annihilation, this damping mechanism results in a radiation dominated electric conductivity that is mainly determined by the proton–photon collisions. After the next phase transition (electron–positron annihilation), the electron–photon collisions provide the effective damping. Whereas the electric conductivity due to Coulomb collisions scales as the temperature T3/2, the photon-driven conductivity scales as T−3. The corresponding magnetic diffusion time scale in very early phases of the universe is too fast to allow for any significant macroscopic magnetic field.
Global gyrokinetic simulation of tokamak transport
Authors: Furnish G. , Horton W. , Kishimoto Y. , LeBrun M. , Tajima T.
Published: July 2002

A kinetic simulation code based on the gyrokinetic ion dynamics in global general metric (including a tokamak with circular or noncircular cross-section) has been developed. This gyrokinetic simulation is capable of examining the global and semi-global driftwave structures and their associated transport in a tokamak plasma. We investigate the property of the ion temperature gradient (ITG) or ηi(ηi≡∂ ln Ti/∂ ln ni) driven drift waves in a tokamak plasma. The emergent semi-global drift wave modes give rise to thermal transport characterized by the Bohm scaling.
Korteweg-de Vries solitons in inhomogeneous plasma
Authors: Duan Wenshan , Zhao Jingbao
Published: July 2002

Transmission and reflection of a soliton in an inhomogeneous plasma are investigated. Both the transmission and the reflection can be described by the KdV (Korteweg-de Vries) equations for the discontinuous case. The numbers and magnitudes for transmitted and reflected solitons are analytically given. The transmission can be described by the KdV-type equation for continuous varying inhomogeneous plasma, and the quasi-solitons is also analytically given.
On the existence of small amplitude double layers in an electron–positron–ion plasma
Authors: Kakati H. , Goswami K. S.
Published: July 2002

A theoretical model is presented to investigate the double layers, associated with the kinetic Alfvén waves, in a magnetized electron–positron–ion plasma with a small but finite value of β, and small-amplitude double layer solutions are obtained. The existence of small-amplitude double layers requires that the density of ions is appreciably larger than that of positrons at equilibrium. The properties of the double layers are determined by the ratio between the number densities of positrons and ions at equilibrium, the direction cosines defining the moving frame, as well as the electron to positron temperature ratio.
Statistical characterization of fluctuation wave forms in the boundary region of fusion and nonfusion plasmas
Authors: Sánchez E. , Hidalgo C. , López-Bruna D. , Garcı́a-Cortés I. , Balbı́n R. , Pedrosa M. A. , van Milligen B. , Riccardi C. , Chiodini G. , Bleuel J. , Endler M. , Carreras B. A. , Newman D. E.
Published: July 2002

The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion (tokamaks and stellarators) and nonfusion plasmas. Fluctuations in ion saturation current and floating potential have a near-Gaussian character in the proximity of the velocity shear layer (rsh). However, fluctuations deviate from a Gaussian distribution when moving inside of the plasma edge (rrsh). Furthermore, fluctuations show sporadic pulses that are asymmetric in time. The present analysis shows a coupling of those pulses and the averaged flow in the shear layer region.
Optical monitoring of photoacoustic pulse propagation in silicon wafers
Authors: Sontag H. , Tam A. C.
Published: July 2002

We demonstrate a new optical and noncontact method for direct and fast detection of photoacoustic pulse propagation in thin silicon wafers. A probe beam deflection technique is used to monitor both longitudinal and Lamb wave propagations from which information on the elastic constants and the sample orientation can be obtained. Results obtained for wafers of different orientations and doping levels are compared with previous contact measurements.
Domain inversion effects in Ti-LiNbO3 integrated optical devices
Authors: Thaniyavarn S. , Findakly T. , Booher D. , Moen J.
Published: July 2002

Significant degradation in the cumulative electro-optic effect attributable to domain inversion has been observed in integrated optical devices fabricated by Ti diffusion in c+-LiNbO3. The c− surface was found to be considerably more immune to domain inversion than the c+ surface under similar diffusion conditions. Domain inversion was found to be dependent on diffusion temperature, time, and Ti concentration. A large increase in the Vπ voltage was observed in Mach–Zehnder interferometers fabricated on c+ substrates compared to that of c− substrates under the same fabrication conditions. The effect of domain inversion on crosstalk in directional coupler switches is also discussed.
Monolithic integration of GaAs/AlGaAs modulation-doped field-effect transistors and N-metal-oxide-semiconductor silicon circuits
Authors: Fischer R. , Henderson T. , Klem J. , Kopp W. , Peng C. K. , Morkoç H. , Detry J. , Blackstone S. C.
Published: July 2002

We have demonstrated for the first time the compatibility of GaAs with Si N-channel metal-oxide-semiconductor (NMOS) transistors by successfully fabricating GaAs/AlGaAs modulation-doped field-effect transistors (MODFET’s) on a Si wafer containing NMOS devices. The MODFET’s with 2 μ gate length on 6 μ channels exhibited transconductances of 120 and 180 mS/mm at 300 and 77 K, respectively. The NMOS devices exhibited little if any performance degradation in going through the GaAs growth and fabrication process. These results show that the monolithic integration of GaAs devices with Si devices is possible, which may add a new dimension to the already exploding world of electronics.
Array mode selection utilizing an external cavity configuration
Authors: Yaeli Joseph , Streifer William , Scifres Donald R. , Cross Peter S. , Thornton Robert L. , Burnham Robert D.
Published: July 2002

We report operation of a ten-stripe, gain-guided, phase-locked diode laser in an external cavity configuration. The laser radiates in a single narrow (1°) lobe. Such lasers generally lase in the highest order array mode, L=10, which radiates in a twin-lobe far-field pattern. With one antireflection-coated facet and a slit spatial filter, the laser has been operated in the L=1, 2, 3 or 10 array modes. A theoretical explanation of the spatial filter function is included.
Material reaction and silicide formation at the refractory metal/silicon interface
Authors: Rubloff G. W. , Tromp R. M. , van Loenen E. J.
Published: July 2002

Medium energy ion scattering studies show that material reactions other than interfacial silicide formation dominate the low-temperature reactivity of the Ti/Si interface. This provides an explanation for the anomalous kinetics and considerable extent (100 Å or more) of reaction of refractory metals with Si, which are observed well below the temperatures normally associated with silicide growth in thicker films.
33.6 T dc magnetic field produced in a hybrid magnet with Ho pole pieces
Authors: Rubin L. G. , Brandt B. L. , Weggel R. J. , Foner S. , McNiff E. J.
Published: July 2002

A dc magnetic field of 33.6±0.3 T was produced in a hybrid magnet whose central field was enhanced by a 3.5 T contribution from holmium pole pieces. The working (sample) volume was a 2-mm gap between the 19-mm-diam×32-mm-long pole pieces tapered to 12-mm-diam faces. The magnetic field was measured with high-field/low-temperature Hall-effect sensors whose reproducibility and linearity made practical an extrapolation technique for fields above 30 T. Three superconducting samples with previously measured upper critical fields were used as calibration check points.
Defect formation in thermal SiO2 by high-temperature annealing
Authors: Hofmann K. , Rubloff G. W. , McCorkle R. A.
Published: July 2002

The formation of voids (or holelike defects) in 500-Å-thick thermal oxide films on Si(100) due to thermal decomposition of SiO2 during vacuum annealing at high temperatures (>1050 °C) has been studied as a function of temperature and time. The defect size distribution is sharply peaked and the density of the defects is essentially independent of annealing time. These observations suggest strongly that the void formation process is initiated at defect sites which are already present after oxidation. The kinetics of oxide void growth suggest the presence of a nucleation stage of the reaction prior to void growth.
NiSi2 precipitation in nickel-implanted silicon films
Authors: Cammarata R. C. , Thompson C. V. , Tu K. N.
Published: July 2002

We report on the formation of nickel silicide in nickel-implanted amorphous silicon thin films. We have found that during annealing, precipitates of NiSi2 form in the interior of the film. This is in contrast with results for interfacial reactions between nickel films and silicon, where the first phases to appear are Ni2Si and NiSi on amorphous silicon, and Ni2Si on crystalline silicon. We suggest that these results reflect differences in surface energies and their effects on silicide nucleation.
Efficiency of oxygen plasma cleaning of reactive ion damaged silicon surfaces
Authors: Oehrlein Gottlieb S. , Scilla Gerald J. , Jeng Shwu-Jen
Published: July 2002

It is demonstrated with the use of in situ x-ray photoemission spectroscopy, secondary ion mass spectrometry, and transmission electron microscopy, that the commonly practiced in situ oxygen plasma/hydrofluoric acid dip treatment of reactive ion damaged silicon surfaces is insufficient in removing all reactive ion etching (RIE) related contaminants and damage. For CHF3/CO2 RIE the residual modifications are shown to be fluorine and carbon contamination and deeper lying modifications, e.g., hydrogen-induced extended Si defects. An enhanced silicon oxidation rate during air exposure has been observed for post-RIE-treated silicon, which correlates to the amount of residual fluorine.