bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle Diffusion in Turbulent Electric Fields
Authors: Flynn Robert W.
Published: February 2004

The statistics of charged particle dynamics in turbulent plasmas have been studied in a numerical experiment which models a turbulent plasma as a system of uncorrelated plasma waves. The ensemble averaged distribution function roughly “diffuses” in velocity space in both weak and strong field regimes. The experimental data are consistent with Dupree's prediction of a diffusion coefficient having a broadened wave-particle resonance and scaling quadratically in the electric field strength for weak fields, and proportional to E3/2 for stronger fields.
“Universal” Drift Instability in a Collisional Plasma
Authors: Bhadra Dilip K.
Published: February 2004

The effect of electron-ion collisions on the “universal” drift instability in an inhomogeneous plasma, using a Fokker-Planck collision operator is considered and compared with the results obtained using the Bhatnagar, Gross, and Krook model. It is found that for small collision frequency, the BGK model gives a destabilizing effect while the Fokker-Planck model indicates a stabilizing effect. For larger values of collision frequency, growth rates for collisional drift waves increase in both models but the growth rate obtained from the Fokker-Planck model is not nearly as high as would be expected from the results derived from the Krook model calculations. For very large collision frequency, growth rates tend to vanish in both models.
Growth and Decay of One-Dimensional Shock Waves in Fluids with Internal State Variables
Authors: Chen Peter J. , Gurtin Morton E.
Published: February 2004

A differential equation is derived relating the density ρ− and the density gradient ρx− behind a shock wave when the region ahead of the wave has always been at equilibrium. The existence of a critical density gradient λ (generally negative) is established: For a compression shock ρ− will increase, decrease, or remain constant according as ρx− is less than, greater than, or equal to λ. λ is shown to be related to the critical amplitude for acceleration waves; an experimental procedure for determining λ as a function of ρ− is given.
Nonlinear Interaction of a Small Cold Beam and a Plasma
Authors: O'Neil T. M. , Winfrey J. H. , Malmberg J. H.
Published: February 2004

Recently, a simple model was proposed for the nonlinear interaction of a low-density monoenergetic electron beam and a relatively cold infinite homogeneous one-dimensional plasma. The essential feature of this model is the observation that after several e-foldings the bandwidth of the growing waves is so narrow that the electrons interact with a very nearly pure sinusoidal field. In terms of this single wave model, a properly scaled solution of the nonlinear beam-plasma problem which depends analytically on all the basic parameters of the problem (i.e., plasma density, beam density, plasma thermal velocity, and beam drift velocity) is presented. This solution shows that the single wave grows exponentially at the linear growth rate until the beam electrons are trapped. At that time the wave amplitude stops growing and begins to oscillate about a mean value. During the trapping process the beam electrons are bunched in space and a power spectrum of the higher harmonics of the electric field is produced. Both the oscillation in wave amplitude and the power spectrum are given a simple physical interpretation.
Finite Beta Equilibria in Axially Symmetric Toroidal Plasma Containment Devices
Authors: Fisher S. , Killeen J.
Published: February 2004

Numerical methods are applied to solve the guiding-center fluid equations, with isotropic and anisotropic pressure, in an axisymmetric toroidal configuration. The shape and structure of equilibrium magnetic flux surfaces with arbitrary plasma beta and arbitrary toroidal curvature are calculated. A comparison with the vacuum magnetic fields shows that for some pressure distributions the plasma markedly distorts the magnetic field. For other distributions the magnetic mirror instability limits the maximum allowable plasma beta. It is also found that with isotropic pressure the maximum allowable beta is much higher than with slightly anisotropic pressures.
Inertial and Resistive Effects in Toroidal Systems
Authors: Greene J. M. , Johnson J. L. , Weimer K. E. , Winsor N. K.
Published: February 2004

Nearly steady-state toroidal configurations are examined analytically and numerically in a resistive, low-pressure fluid model. An optimal ordering of the physical parameters is introduced in such a way that the problem separates into two distinct time scales, diffusive and acoustic. The dispersion relation, for small oscillations on the acoustic time scale, reduces to a fifth-order algebraic equation coupling rotation, acoustic oscillations, and geodesic modes. In the supersonic regime, two oscillatory, low-frequency modes are unstable, and the rotational and geodesic modes are damped. In the subsonic regime, the rotational mode is always growing, the geodesic modes are damped, and the acoustic modes may or may not be unstable, depending on the parameters. Good agreement is found between the analytic predictions and the results of numerical simulation.
Identification of Resistive Drift Waves in the Spherator, and their Stabilization by Shear
Authors: Pacher Horst D. , von Goeler S.
Published: February 2004

The observation of resistive drift waves in plasmas produced in the sheared magnetic field of the spherator is reported. The plasmas were produced by electron cyclotron heating, at a density of 1010−1011/cm3 and electron temperatures ≤1 eV. An investigation of oscillation amplitude demonstrates the existence of a stable regime (relative amplitude ≤0.5%) in toroidal fields near the condition of maximum shear (poloidal field and toroidal field approximately equal). For the purpose of further identification of the modes, experiments were carried out in a marginally unstable regime, resulting in nonturbulent single modes. An approximate theory of nonlocal normal modes using an average radial width is derived, and is confirmed by comparison in a special case with a more accurate calculation taking into account the radial dependence. This theory predicts the expected wave parameters for the most unstable modes. The wave parameters of the single modes are measured for the case of argon, and are found to be in excellent agreement with the frequencies and wavelengths predicted for the most unstable modes. Finally, the theory predicts that plasmas of low electron temperature, low ion mass, sharp density gradient, and high density should be the most unstable to resistive drift waves. A series of experiments in which these parameters were varied agrees with these predictions.
Generalization of Reflected Shock Calculations
Authors: Kim Yong Wook
Published: February 2004

A generalized method of reflected shock calculations, without the usual assumption of zero flow speed, is shown to remove the outstanding discrepancy between the calculated and observed temperatures in the spectroscopic regime.
Measurements of the Dispersion Relation for Drift Waves in a Cesium Plasma
Authors: Obiki T. , Takahashi N. , Itatani R.
Published: February 2004

Dispersion curves of drift waves in a cesium plasma column obtained experimentally with the use of rotational electric fields show different characteristics from that of the slab model.
Heating of Pinch Devices with Lasers
Authors: Vlases George C.
Published: February 2004

A simple thermodynamic model is used to determine the change in plasma density and temperature and in confining magnetic field strength when a θ or Z pinch is subjected to strong laser heating.
Use of Radioactive Test Ions in the Measurement of the Spatial Distribution of Plasma Ion Drift Speeds
Authors: Dreicer Harry , Henderson Dale B. , Mosher David
Published: February 2004

Radioactive test ions have been used to measure the axial ion drift speed distribution in a single-ended Q machine. In the main plasma region these drift speeds are found to be significantly smaller than predicted by the sheath model.
Force on a Rigid Sphere in an Incompressible Inviscid Fluid
Authors: Rubinow S. I. , Keller Joseph B.
Published: February 2004

The force exerted by an incompressible inviscid fluid upon a stationary rigid sphere is calculated for an arbitrary potential flow incident upon the sphere. The result is applied to a sphere in the steady flow produced by a line vortex.
Modulation of Thermal Convection Instability
Authors: Rosenblat S. , Tanaka G. A.
Published: February 2004

The linear stability problem for a fluid in a classical Bénard geometry, when the temperature gradient has both a steady and a time-periodic component, is considered. The modulating effect of the oscillatory gradient on the stability characteristics of the basic configuration is examined. It is found that, in general, there is enhancement of the critical value of a suitably defined Rayleigh number.
Radiatively and Collisionally Structured Shock Waves Exhibiting Large Emission-Convection Ratio
Authors: Farnsworth Archie V. , Clarke Joseph H.
Published: February 2004

Radiatively and collisionally structured, normal shock waves in a monatomic, nonequilibrium, ionizing gas are solved for waves in which the emission-convection ratio varies from a small value in the preshock flow to a very large value in the postshock flow. The gas model adopted for the mixture allows nonequilibrium ionization and spectral radiative emission and absorption. Two sequential regions are identified: (1) One which displays far upstream an abrupt isothermal “ionization front” and then subsequent heating and adjustment, and next (2) a uniform asymptotic region in which occurs a thick, hot, unusual precursor, a frozen and adiabatic discontinuity, and a very thin tail. The second region permits asymptotic simplifications leading to rational uncoupling of precursor and tail and simplification of spectral difficulties. The first region affords no spectral simplification, and nonrational spectral approximations are employed.
Detonation Zone Observations in Nitromethane Solutions
Authors: Mallory H. Dean
Published: February 2004

Solutions of nitromethane in acetone which are too dilute to support a self-sustained detonation can be forced to react by a strong booster. In such cases the wave front is composed of an induction zone followed by a turbulent reaction zone and a region of turbulence decay. As the nitromethane concentration is increased, the induction zone disappears and reaction appears to begin at the wave front. At the highest concentrations no wave structure could be resolved and the reaction time may be subnanosecond with the booster system used.
Finite-Beta Instabilities Inherent to a Mirror Confined Plasma
Authors: Himmell Lewis C.
Published: February 2004

A class of microinstabilities which is likely to exist in plasmas confined in magnetic mirror geometries is examined. These waves propagate across the magnetic field in a homogeneous plasma at multiples of the ion cyclotron frequency. The instabilities are found when all electromagnetic terms are included in the theory. They exist even when β≪1 (perpendicular ion beta) since there are still parametric regions where electromagnetic contributions are larger than the electrostatic terms. Growth rates for a value of β of 0.01 are of the order of several percent of the ion cyclotron frequency even for loss-cone distributions that are reasonably well randomized.
Nonlinear Damping on an Adiabatic Plasma Species
Authors: Drummond W. E. , Sloan M. L.
Published: February 2004

The phenomenon of nonlinear Landau damping is investigated for damping on an adiabatic species. It is shown that the cancellation which leads to the (κλD)4 dependence for damping on a hydrodynamic species no longer occurs; rather, the four-wave resonance term (driven spectra term) of the governing matrix element dominates the resonant three-wave term (induced spectra term), leading to the conjecture that, for a plasma with an adiabatic constituent, nonlinear Landau damping on that constituent may well dominate the nonlinear mode coupling processes. A detailed investigation of such damping is conducted for the case of two equidense zero temperature ion beams streaming through a hot, adiabatic electron plasma. A nonlinear Landau damping instability on the electrons is found which cannot be limited in the usual quasilinear sense. The instability serves to heat the electrons driving the system to a linear ion-ion streaming instability.
Nonlinear Theory of Macroinstabilities and Enhanced Transport in Plasmas
Authors: Weinstock Jerome , Williams Robert H.
Published: February 2004

A method is described for solving the magnetohydrodynamic equations for nonlinear plasma waves and turbulence due to macroinstabilities. It is argued that the dominant nonlinear effect for macroinstabilities is the scattering of fluid elements of charge by plasma waves. The consequent wave-fluid interaction causes an enhanced diffusion coefficient in the continuity equation and an identical enhanced viscosity coefficient in the momentum equation. This enhanced plasma transport is analogous to the eddy transport and Reynolds stress of neutral turbulence. A nonlinear dispersion relation is obtained from the linear dispersion relation by simply adding the enhanced diffusion term to the frequency. It is shown that linear macroinstabilities are stabilized by the enhanced diffusion, and viscosity, terms. As examples, the enhanced diffusion coefficients, and density and electric field fluctuations are calculated for resistive instabilities.
Effect of Condensation in the Boundary Layer on Mass Transfer from a Rotating Disk. I. Theoretical
Authors: Omberg R. P. , Olander D. R.
Published: February 2004

When vaporization occurs in the presence of a temperature gradient in the boundary layer, homogeneous nucleation and condensation of droplets may occur. This process is studied theoretically in a rotating disk flow system for iron vaporizing into an atmosphere of cold argon. The usual boundary-layer conservation equations are provided with appropriate source or sink terms to account for the conversion of the gaseous diffusing species (monomer) to drops of liquid and vice versa. Conservation equations for the monomer and for drops of all sizes are required. The source and sink terms are developed from the classical theory of homogeneous nucleation. Simultaneous solution of the coupled monomer and droplet equations provides insight into the enhancement of the vaporization rate due to condensation and also into the structure of the nucleation, droplet growth, and re-evaporation cycle which was found to take place in the boundary layer on the disk.
Decay of Homogeneous Turbulence from a Given Initial State
Authors: Deissler Robert G.
Published: February 2004

The homogeneous turbulence problem is formulated by first specifying the multipoint velocity correlations or their spectral equivalents at an initial time. Those quantities, together with the correlation or spectral equations are then used to calculate initial time derivatives of correlations or spectra. The derivatives are in turn used in time series to calculate the evolution of turbulence quantities with time. When the problem is treated in this way, the correlation equations are closed by the initial specification of the turbulence, and no assumption is necessary for closing those equations. An exponential series, which is an iterative solution of the Navier-Stokes equations, gave much better results than a Taylor power series when used with the limited available initial data. In general, the agreement between theory and experiment was good.
Arbitrary Amplitude Magnetoacoustic Waves under Gravity: An Exact Solution
Authors: Chiu Y. T.
Published: February 2004

Exact Riemann wave solutions to the magnetohydrodynamic equations for a compressible medium under the influence of a gravitational field are obtained. The method of solution consists of an application of Hamburger's subsidiary equations previously found useful in the analysis of such three-dimensional waves in compressible hydrodynamics under gravity. The resultant magnetoacoustic waves cannot be classified as slow and fast modes, and their propagation characteristics are found to be quite different from those in a uniform medium. In particular, the phenomenon of large amplitude wave ducting is examined. Certain wave properties correspond exceedingly well with the observed fine structure of waves propagating away from solar flares.
Interaction of a Helium Plasma with an Inhomogeneous Transverse Magnetic Field
Authors: Marković P. D. , Scott F. R.
Published: February 2004

A dynamic helium plasma, produced by a conical z-pinch gun, was used to study interactions with an inhomogeneous transverse magnetic field. Magnetic probe measurements show that up to a critical value of the field Bc the plasma penetrates the region occupied by the field by completely removing it. An increase in the plasma electron temperature and density is seen. For fields higher than the critical value Bc, plasma penetrates in the form of thin sheets oriented along the magnetic field lines. The observed effects are compatible with the available theory if resistive effects are taken into account.
Comparison of Numerical Solutions of the Vlasov Equation with Particle Simulations of Collisionless Plasmas
Authors: Denavit J. , Kruer W. L.
Published: February 2004

Numerical solutions of the Vlasov and Poisson equations for several physically significant problems are presented and compared with the results of particle simulations. The numerical solutions of the Vlasov equation are based on the Fourier-Fourier transform method. The spatial representation includes up to 85 modes and is capable of representing strong nonlinear effects. The particle simulations are based on a multipole expansion of finite-size particles about their nearest grid point location. Special techniques are used to suppress the noise and to accurately control the initial conditions of the plasmas so that quantitative comparisons with Vlasov solutions can be made. Close quantitative agreement between the results of the two simulation techniques is observed. The problems considered are one-dimensional, with periodic boundary conditions, and involve (1) two-stream instabilities, with equal and unequal electron beams, and (2) large-amplitude electron oscillations, with sideband instabilities.
Electron Cyclotron Heating in Toroidal Octupoles
Authors: Sprott J. C.
Published: February 2004

Microwave frequencies of 0.4-24 GHz at powers up to 100 kW have been used to heat electrons in regions of cyclotron resonance in a supported and in a levitated toroidal octupole magnetic field. A theoretical model has been developed to predict the heating rate of a cold, tenuous plasma in an arbitrary, nonuniform, magnetic field. The model predicts strong heating at places where ∇‖B = 0 at resonance, and heating efficiencies approaching 100% for sufficiently high densities (ωp2>ω2/Q). Scintillator probes have been used to verify the predicted localized heating and to measure the density at which total absorption occurs. Gun injected plasmas and microwave produced plasmas with n∼109 cm−3 and kTe∼5 eV are heated to ≲ 1 KeV in the supported octupole and to ≲ 10 KeV in the levitated octupole. Upper off-resonance heating is also observed.
Initiation of Instability in Micropolar Fluids
Authors: Liu C. Y.
Published: February 2004

The classical theory of fluid stability as applied to micropolar fluids is used to study, in a few limiting cases, the delayed initiation of instability due to the presence of particles in the fluid.
Direct Measurements of Dipolar Radial Electric Fields in a Resonating Plasma Column
Authors: Parbhakar Kanwal J. , Gregory Brian C.
Published: February 2004

The dipolar resonances of a cylindrical plasma column are investigated using a small movable dipole antenna probe inside the plasma. The frequency is kept constant and the radial electric field is measured both for constant discharge current and for when the discharge current is swept. Measurements using a hybrid ring indicate that the electromagnetic waves are coupled to the electroacoustic waves at the critical point where the excitation frequency equals the plasma frequency. The waves traveling in the overdense region are evanescent, but the waves traveling in the underdense region are reflected at the sheath region, thus forming standing longitudinal waves. The Tonks-Dattner resonances are essentially due to the formation of these standing waves. In addition to the standing longitudinal waves, another resonance, the so-called main or dielectric resonance is also present. Running plasma waves with phase velocities ranging between zero and some finite value, propagating toward the discharge tube wall are observed for the main resonance and for the Tonks-Dattner resonances as well. The experimental results are compared with theory and are in good agreement only for the Tonks-Dattner resonances; for the main resonance the agreement is rather poor.
Electron Plasma Waves and Free-Streaming Electron Bursts
Authors: Ikezi H. , Barrett P. J. , White R. B. , Wong A. Y.
Published: February 2004

The response of electrons in a collisionless plasma to a fast-rising voltage step is investigated experimentally and theoretically. At small amplitudes the step generates a wave train, the leading edge of which propagates along the column at the long-wavelength electron plasma wave velocity. At large step amplitudes a fast burst of electrons is observed, with kinetic energy roughly proportional to the step amplitude, followed by a nonlinear electron plasma wave signal which shows solitonlike behavior.
Chemical Instabilities. VI. Hydrodynamic Stability of the Dissociating Fluid A2⇋2A
Authors: Bdzil J. , Frisch H. L.
Published: February 2004

The stability characteristics of the dissociating fluid A2⇋2A experiencing bulk reactions are re-examined. The results of this analysis show qualitative agreement with those obtained for the case of chemical surface reactions.
Diffusion Equation for a Pair of Brownian Particles
Authors: Aguirre J. L. , Murphy T. J.
Published: February 2004

A diffusion equation for a pair of Brownian particles is derived by a Chapman-Enskog-type method from a two-particle kinetic equation. The diffusion tensors are given explicitly in terms of the friction tensors, and are evaluated hydrodynamically in the long-range limit.
Influence of Rotational Energy on a Beam-Plasma Interaction
Authors: Gruber S.
Published: February 2004

An experiment is discussed in which a beam of rotating electrons interacts with a plasma in a static magnetic field. The rotational energy is caused to vary in a continuous manner. It is found that the saturated intensity varies exponentially with the transverse energy of the electron beam.
Plasma Transport in Toroidal Confinement Systems
Authors: Rosenbluth M. N. , Hazeltine R. D. , Hinton F. L.
Published: February 2004

The neoclassical theory of plasma transport in axisymmetric, toroidal confinement systems, is developed by means of a variational principle for the rate of irreversible entropy production. The variational principle derived here employs the full Fokker-Planck collision operator, including both like and unlike species collisions. Using the variational principle, all the relevant neoclassical transport coefficients are systematically evaluated in the “banana” regime of small collisional frequency, to lowest order in the inverse aspect ratio. These results include both the “diagonal” and “cross” coefficients for the particle fluxes, ion and electron heat flux, and electric current. By combining the transport coefficients with appropriate moments of the drift equation, a closed set of equations which accurately summarize the predictions of neoclassical theory in the banana regime is obtained. The significance of these equations, in particular with regard to recent tokamak experiments, is discussed briefly.
High Rayleigh Number Convection in an Enclosure—A Numerical Study
Authors: Quon Charles
Published: February 2004

Seven finite difference computations for convection in a square cavity at Rayleigh number ∼O(106) have been carried out for a variety of dynamical boundary conditions, Rayleigh numbers, and Prandtl numbers. The results indicate that various horizontal dynamical boundary conditions virtually make no difference to the main boundary-layer flow although free vertical boundaries double the velocity in the boundary layer adjacent to them. The scaled fields in the vertical boundary layers are independent of the Rayleigh number, and to a large extent, independent of the Prandtl number, when the two vertical boundary layers are distinct. The results compare reasonably well with Elder's experiments.
Spatial Electron Cyclotron Damping
Authors: Olson Craig L.
Published: February 2004

Spatial electron cyclotron damping in an infinite uniform Vlasov plasma is examined. It is found that the plasma response to a steady-state transverse excitation consists of several terms (dielectric-pole, free-streaming, and branch-cut), but that the cyclotron-damped pole term is the dominant term for z>l≈c/ωce provided (ωpe/ωce)2(c/a)≫1 (where c is the velocity of light, a is the electron thermal velocity, ωpe is the electron plasma frequency, and ωce is the electron cyclotron frequency). If the latter inequality does not hold, then the free-streaming and branch-cut terms persist well past z = c/ωce as ω1 approaches ωce, making experimental measurement of cyclotron damping essentially impossible. Considering only (ωpe/ωce)2 × (c/a)≫1, the effects of collisions and of a finite-width excitation are investigated. Criteria are established concerning collisional damping, measurable damping length sizes, and allowed uncertainty in the magnetic field B0. Results of numerical calculations, showing the regions in the appropriate parameter spaces that meet these criteria, are presented. From these results, one can determine the feasibility of, or propose parameter values for, an experiment designed to measure spatial electron cyclotron damping. It is concluded that the electron temperature Te should be at least 1 eV, and preferably 10 eV or higher, for a successful experiment.
Effects of Inhomogeneities in Plasma Density on the Beam-Plasma Instability and Landau Damping
Authors: Lin A. T. , Rowe J. E.
Published: February 2004

A numerical computer simulation using a charge sheet model is employed to investigate the beam-plasma instability and Landau damping in a one-dimensional, finite-length, inhomogeneous plasma. The results confirm the theoretical predictions that the inhomogeneities in plasma density distribution reduce the spatial growth rate of the beam-plasma instability and increase the Landau damping rate.
Turbulence Model for Boundary Layers near Walls
Authors: Ng K. H. , Spalding D. B.
Published: February 2004

A turbulence model is proposed for the prediction of boundary-layer flows near walls. Two differential equations are solved: one for the kinetic energy of turbulence, and one for its length scale. The local effective viscosity in the flow is taken as proportional to the product of the length and the square root of the energy. The constants appearing in the equations are determined by reference to experimental data. Four cases of self-similar flow are predicted with the model and found to compare favorably with the relevant experimental data. Satisfactory predictions for more general flows are also reported.
Reaction Zone-Shock Front Coupling in Detonations
Authors: Barthel Harold O.
Published: February 2004

Some new observations are reported of the delayed effect of chemical reaction on the trajectory of an isolated triple point in a detonation. The implications of these observations relative to the self-sustenance of finite amplitude transverse waves on a propagating detonation front are developed. A new criterion for the minimum size of a detonation cell is found, based on the requirement that a reaction-generated compression wave must overtake the triple point within the first half of the cell. This feature and certain other acoustic wave propagation phenomena are incorporated in a new, qualitatively quite detailed model of a detonation cell.
Measurement of Plasma Properties from the Cross Spectrum of Thermal Density Fluctuations
Authors: Geissler K. H. , Greenwald R. A. , Calvert W.
Published: February 2004

The relation between the density cross-correlation function and the cross spectrum of fluctuations in the electron saturation currents drawn by two Langmuir probes is used to measure basic plasma properties. The measured quantities using this approach are the ambipolar diffusion coefficient and the electron density. It is found that the method has sufficient sensitivity to determine changes in the ionic composition. These measurements are compared with plasma parameters obtained by using conventional probe techniques. Discrepancies are shown to exist and the source of these discrepancies is discussed.
Stability of Liquid Interfaces in Porous Media
Authors: Nayfeh Ali Hasan
Published: February 2004

An analysis is presented for the stability of liquid interfaces moving with uniform velocities in homogeneous isotropic porous media. The effects of viscosity, surface tension, and body forces are taken into account. The first part of this investigation concerns the linear stability of a finite depth liquid layer bounded by two semi-infinite liquids. It is found that the critical conditions separating stable from unstable disturbances are independent of the depth of the liquid layer. For a sufficiently large flow velocity, the system is stable if, across each interface, the viscosity decreases in the direction of flow, and it is unstable if viscosity increases in the direction of flow across at least one interface. The second part treats the nonlinear frontal stability of two semi-infinite liquids. The results show that the nonlinearity is destabilizing owing to the decrease of the surface tension force. Deviations from Darcy's law are found to be stabilizing when the flow is from the denser to the lighter fluid and vice versa.
Theory of Second-Order Stress in a Dense Gas
Authors: Theodosopulu Mary , Dahler John S.
Published: February 2004

The phenomenological coefficients associated with stress tensor terms proportional to the square of the velocity gradient have been computed for dense gases composed of rigid spheres and of particles interacting with a square-well potential. Most of the calculations reported here were obtained from a generalization of the moment method which is appropriate to a dense gas. A detailed comparison between theory and experiment leads to the following conclusions: (i) Reiner's torsional flow observations for gases and for liquid toluene are not attributable to second-order stress. At low densities kinetic theory predicts an effect with the opposite algebraic sign from that observed experimentally. At high densities the theory predicts a reversal of sign. At all densities the magnitudes of the theoretical and experimental effects differ by several orders of magnitude. (ii) Oliver and MacSporran's measurements of jet thrust due to liquids issuing from circular orifices differ by many orders of magnitude from the predictions of kinetic theory. However, at high densities there is agreement with regard to the algebraic sign of the effect. (iii) Oliver and MacSporran also studied jet thrust from a slot, but were unable to observe any effect with gases. Kinetic theory is in agreement with this.
Standing Striations as Solutions of the Pekarek Equation
Authors: Lee D. A. , Garscadden A.
Published: February 2004

Boundary conditions suggested by observations are used with a partial differential equation equivalent to Pekarek's striation model to construct a linear boundary value problem for the description of time-independent ionization disturbances in the finite positive column. Solutions of this problem display several features of standing striations.
Precession of a Strong Astron E Layer
Authors: Lovelace R. V. , Sudan R. N.
Published: February 2004

Precession of weak E layers observed in the astron is compatible with the “superparticle” model according to which the electrons of the layer move as a rigid body at the precession rate of a single electron in the external magnetic fields. The superparticle assumption is investigated by perturbation of a Vlasov equilibrium E layer of arbitrary strength. A slow precessional motion of the layer is found with a frequency equal to the single electron external field precession rate in the weak E-layer limit. For strong E layers with loading factors ζ∼1, the precession frequency is reduced from the external field rate, due to the self-field of the layer, by an appreciable factor of the order of 1 + (R/2a)ζ, where R is the radius and a is the thickness of the layer. It is shown that there may be damping of the E-layer motion arising from the distribution in energy and/or momentum of the electrons.
Collapse of One-Dimensional Cavities in Compressible Liquids
Authors: Biasi L. , Prosperetti A. , Tozzi A.
Published: February 2004

The collapse of a one-dimensional cavity in a compressible, inviscid liquid is treated exactly. Results of numerical computations for some specific examples are presented.
Rarefied Gas Flow through a Slit
Authors: Hegna Harwood A. , Berman A. S.
Published: February 2004

The transition from near free molecule flow to slow viscous continuum flow through a slit has been studied experimentally. The measurements have been compared with the results of a theoretical analysis of the expected flow through a slit with zero length in the flow direction.
Linearized Analysis of Magnetohydrodynamic Channel Entrance Flow
Authors: Hwang C. C.
Published: February 2004

A linearized analysis is employed for solving the magnetohydrodynamic channel entrance flow with parabolic velocity at the entry. The results are compared with those of the finite difference method.
Bifurcation of Convection in Internally Heated Fluid Layers
Authors: Schwiderski E. W.
Published: February 2004

A Lyapunov-Schmidt expansion was used to investigate the bifuraction phenomenon of convection in internally heated fluid layers of infinite extent. The introduction of a fundamental periodicity vector as a substitute for the missing sidewall boundary values permitted a complete analysis and classification of the total spectrum of bifurcations in periodic planforms of the linearized stability problem. Numerous new convection flows of physically significant cell patterns were discovered. The successive integration led to well-known integrability conditions which were interpreted as amplitude and phase equilibrium requirements necessary to control resonance between the linear and nonlinear convection forces. A detailed analysis of this resonance mechanism brought about a rather intriguing scheme of parameter points at which generally existent solutions become virtually knocked out. The replacing convective solutions bifurcate from the conductive integral in a subcritical or supercritical manner.
Spherical Source Flow with a Finite Back Pressure
Authors: Brook J. W. , Hamel B. B.
Published: February 2004

The interaction of a hypersonic spherical source flow with a rarefied background is investigated using the Boltzmann equation with model collision terms. Employing multifluid descriptions of the flow, it is found that two length scales, Rp and R∞, are important in describing the interaction as the background becomes increasingly rarefied. Background scattering occurs on the scale of Rp and jet scattering on the scale of R∞. Matched asymptotic expansions of the variables in the two regions are developed in terms of the parameter ε = Rp/R∞→0, and first-order composite solutions constructed. These solutions describe the transition away from the perfect vacuum limit of the expanding flow.
Gas Heating Effects in the Constriction of a High-Pressure Glow Discharge Column
Authors: Rogoff Gerald L.
Published: February 2004

Numerical calculations have been made to examine effects of rapid gas heating and subsequent gas density changes on the current density distribution in a high-pressure positive column. Radial profiles of gas temperature, radial expansion velocity, density, and pressure and of electron current density are calculated as functions of time for a simplified model of a glow discharge column. Initially, the glow is diffuse and the background gas is uniform. Considered specifically is a hydrogen discharge with an initial pressure of 500 Torr, an initial current of 6.0 A, a constant axial electric field of 10 kV/cm, and a radius of ∼0.75 mm. The gas dynamics are described by conservation equations for mass, momentum, and energy in a viscous, thermally conductive, compressible fluid and by the equation of state for a perfect gas. The dependence of the ionization rate on the electric field to gas density ratio makes the electron current density a sensitive function of the gas density. At 60 nsec the gas temperature near the axis is sufficiently high for the onset of rapid thermal dissociation in an axial core, and effects of gas transport are significant. The axial gas density is decreased ∼10%; the axial current density is twice its initial value; and the column is partially constricted. Calculated results are consistent with properties of a similar discharge studied experimentally by Cavenor and Meyer, particularly at the time of the formation of a filamentary channel, observed experimentally after the first stage of the glow-to-arc transition.
Experimental Study of the Luminous Front Produced by a Coaxial Plasma Accelerator
Authors: Wisler David C. , Nakagawa Y.
Published: February 2004

A plane luminous front is produced in hydrogen by channeling the hot gas of the dense plasma focus formed in front of a coaxial accelerator into a small tube. The reflection of this luminous front by a solid wall is examined by using pressure and electron density probes and streak photographs. The radiation of the luminous front is examined in detail by means of time-resolved spectroscopic measurements of the line emissions of Hα, Hπ, Hγ, and Hδ, and the continuum emission across the Balmer series limit. Then, the population number densities of various excited states as well as the number density and temperature of electrons are determined. It is shown that, although a shock front propagates ahead of the luminous front, the observed luminosity results from the expanding hot driver gas and not from the shock heated gas. It is further shown that a satisfactory over-all agreement between the results of the spectroscopic measurements and the theoretical solutions of the statistically steady-state rate equations is obtained with a correction for the self-absorption of the Hα line emission.
Current Sheet Collapse in a Plasma Focus
Authors: Jalufka Nelson W. , Lee Ja H.
Published: February 2004

Collapse of the current sheets in a plasma focus has been recorded simultaneously through slits parallel and perpendicular to the symmetry axis in the streak mode. The dark period following the collapse is due to the plasma moving out of the field of view. Microdensitometric measurements of intensity variation also support this conclusion. A large anisotropy is also found in the x-ray radiation pattern. Effects of different vacuum vessels were investigated.
Counterstreaming Ion Instability for Arbitrary Angles
Authors: Landau Ronald W.
Published: February 2004

The fully electromagnetic dispersion relation for two equidense counterstreaming ion beams flowing almost perpendicular to a magnetic field has been derived. The beams flow through a warm stationary electron background treated using the Vlasov equation. The wavelengths must therefore be much smaller than the ion Larmor radius, but are assumed to be much greater than the electron Larmor radius. There is then an instability that occurs when [β/(1+2β)]≦(m−/m+)(υ¯2/V02) (υ¯ and V0 are the electron and ion velocities). It is found that in the region of fastest growth and for small β the instability growth drops sharply when the angular deviation of the wave vector from the direction perpendicular to the B field is greater than (m−/8m+)1/2 for charged ions of mass m+ or ∼12° for protons. The transition to the zero field electrostatic case is also obtained by going to the β≫1 limit.
Simulation of Ion-Acoustic Wave and Pseudowave Generation in a Plasma
Authors: Estabrook Kent , Alexeff Igor
Published: February 2004

The unexpected effects of a grid in attempts to launch ion-acoustic waves are discussed. A time varying potential on a grid was found to inherently produce ion bursts (pseudowaves) at all voltages and frequencies used. The voltages φ ranged from low (eφ=κTi), to medium (eφ=κTe), to high (eφ≫κTe) in cases where the electron-to-ion temperature ratio was large (Te/Ti=30). The frequencies f used ranged from below to above the ion plasma frequency fpi(0.25fpi≤f≤2fpi). Ion-acoustic waves, however, could be clearly observed only for low excitation voltages and for frequencies at and below the ion plasma frequency. In the computer simulation, the grid is treated as a one-dimensional source of potential, and particle absorption and scattering by the grid wires is neglected. A one-dimensional ion distribution function F(x, v, t) is found by numerically integrating the nonlinear ion Vlasov-Poisson equations by a finite difference method for phase space. Electrons are treated as a massless neutralizating fluid, and the electron density is computed by balancing the electric field against the thermal electron pressure in a self-consistent fashion [ne=n0 exp (eφ)(κTe)−1].
Trapping of Electrons in Magnetic Mirrors due to Enhanced Fluctuations
Authors: Roque C. R. , Gregory B. C.
Published: February 2004

A Fokker-Planck equation for an electron beam interacting with a turbulent plasma is derived. The Fokker-Planck coefficients are estimated using quasilinear theory. The equation is then applied to the problem of calculating the fraction of beam electrons trapped in a magnetic mirror device. The conditions under which the trapping compensated for the losses due to classical scattering into the loss cone are considered.
Analysis of Hot-Electron Confinement by Electron Cyclotron Plugging
Authors: Ikegami H. , Aihara S. , Hosokawa M.
Published: February 2004

Plugging and confinement of hot-electron plasma by microwaves making use of the electron cyclotron resonance is studied. Experimentally, the plugging is observed to make a plasma decay time several times longer than that without the plugging microwave power, and the mechanism is analyzed theoretically.
Polarizability of Singly Ionized Argon
Authors: Bristow M. P. F. , Glass I. I.
Published: February 2004

The polarizability (specific refractivity or Gladstone-Dale constant) of singly ionized argon was determined experimentally using dual-frequency laser interferometry over a significant wavelength range.
Observations of Precursor Ionization in Electromagnetic T Tubes
Authors: McRae Thomas G. , Leadon B. M.
Published: February 2004

Experimental measurements of electron number densities in the precursor region of Mach 5 shocks in xenon have been conducted. The ionization is attributed to 1200-3000-Å radiation and is due primarily to the presence of impurities in the vacuum system.
Upper-Critical Conditions for Resistance-Free Magnetosonic Shocks
Authors: Mason Rodney J.
Published: February 2004

It is demonstrated theoretically that the upper-critical Mach number for resistance-free magnetosonic shocks drops rapidly from 3.06 as the initial ion temperature is raised from zero. This is substantiated by the results of numerical shock-overtaking experiments.
Continuum Measurements on a Hydrogen Plasma
Authors: Engelhardt W.
Published: February 2004

By measurement of the continuous emission in the visible and uv from an ionizing hydrogen plasma, it is shown that the main contribution to the continuum is due to the molecular continuum emitted by a small amount of 2% of H2 molecules.
Reply to Comments by M. R. Foster
Authors: Singh S. N.
Published: February 2004

A reply is made to comments by Foster on a paper “Laminar Boundary Layer on a Rotating Sphere” by S. Singh, [Phys. Fluids 13, 2452 (1970)].
Temperature Field Structure in Strongly Heated Buoyant Thermals
Authors: Lin Shao-Chi , Tsang Leslie , Wang C. P.
Published: February 2004

Laboratory measurements on the temperature field structure in rising turbulent thermals are presented. The relatively small size (about 10 cm initial diameter), but strongly heated thermal bubbles are generated by pulsed arc discharges of about 103 J energy in otherwise undisturbed air. Temperature field measurements are made by stationary and rotating fine-wire thermometers which probe through the rising bubble. The thermal front is found to be sharp at the top and around the side of the bubble, but become diffused near the bottom, suggesting thermal wake formation. The large amplitude fluctuation, strong gradient, and rich harmonic content of the interior temperature field indicate that the mass entrainment process penetrates deeply into the bubble, and turbulent mixing proceeds at all depths. By invoking passive scalar mixing theories and assuming local equilibrium spectral transfer, the averaged turbulence kinetic energy dissipation rate within the bubble ε can be estimated from the diffusive cutoff wavenumber apparent in these preliminary experiments to be of the order of 0.03 (U3/D), where U and D are, respectively, the instantaneous rise velocity and mean diameter of the bubble.
Characteristics of the Unsteady Shock-Induced Laminar Boundary Layer on a Flat Plate
Authors: Cook William J. , Chapman Gary T.
Published: February 2004

The unsteady shock-induced laminar boundary layer on a flat plate is theoretically studied for shock speeds ranging from 1.12 to 9 km/sec. Boundary layer flows for real air, assumed to be in thermochemical equilibrium, are analyzed by extending Lam's work on shock-induced laminar boundary layers to equilibrium dissociated and ionized flows. A complete description of the unsteady nature of the boundary layer is presented in terms of heat transfer and several boundary layer thickness quantities as functions of a single time-position variable. Boundary layer development is considered for two points of view. Time-dependent boundary layer development and approach to steady state for any fixed position on the plate is described, as is the configuration of the boundary layer with position on the plate at any point in time. Quantities presented in graphical form permit accurate values for descriptive boundary layer quantities to be easily obtained for post shock pressures at or near 1 atm. Results indicate that in shock-tube experiments where observations of the flow are made through a boundary layer that influences experimental measurements, it is desirable to employ a flat plate to generate a boundary layer that minimizes the boundary layer thickness and diminishes or eliminates boundary layer unsteadiness.
First-Order Perturbation Theory for the Distribution Functions of a Dense Fluid
Authors: Brelvi S. W. , O'Connell J. P.
Published: February 2004

A first-order expansion of the Percus-Yevick functional is derived for the properties of a real fluid as a perturbation about a hard-sphere reference fluid. The resulting expression is an integral equa ion relating the real fluid distribution functions and intermolecular potential to those of the reference fluid. Use of a realistic approximation in calculating the integral in the Ornstein-Zernike equation leads to real-fluid distribution functions that are in closer agreement with molecular dynamics results than those of the usual first-order Percus-Yevick equation.
Radiative Decay behind Strong Shocks
Authors: Oppenheim Alan
Published: February 2004

The effect of radiation on the state of a gas behind a strong shock is studied with a different radiation model than used by Koch and Gross. The formalism and assumptions are otherwise the same as in their paper.