bluuebunny/crossref_metadata_2025 huggingface dataset:
Space-charge waves on a relativistic, unneutralized electron beam and collective ion acceleration
Authors: Briggs R. J.
Published: March 2003

The space-charge-wave dispersion characteristics of a thin, hollow, relativistic electron beam is calculated for beam currents near the space-charge limit for propagation of the beam.
Plasma decontamination and energy transport by impurity driven modes
Authors: Coppi Bruno , Rewoldt Gregory , Schep Theo
Published: March 2003

A radial flow of impurities toward the outer edge of a magnetically confined plasma is produced, following an accumulation of impurities at the center of the plasma column, by impurity density gradient driven modes for realistic values of the temperature gradient of the main ion population relative to its density gradient. At the same time an outward transport of the main ion thermal energy is also induced. For mean free paths of the main ion population shorter than the relevant longitudinal wavelengths, these modes can be described by moment equations and are associated with the finite thermal conductivity of the main ions. In the opposite (collisionless) limit, mode-particle resonances replace the effects of finite ion thermal conductivity and in addition, modes that are standing along the magnetic field lines are affected by the magnetic trapping of a fraction of the main ion population. The expression for their growth rate is strongly influenced by the ratio of the effective trapped ion collision frequency to the mode frequency and under realistic conditions, can be independent of mode− particle resonance processes that involve only a relatively small portion of velocity space.
Effects of electrostatic trapping on neoclassical transport in an impure plasma
Authors: Hazeltine R. D. , Ware A. A.
Published: March 2003

Contamination of a toroidally confined plasma by highly charged impurity ions can produce substantial variation of the electrostatic potential within a magnetic surface. The resulting electrostatic trapping and electrostatic drifts, of hydrogen ions and electrons, yields significant alterations in neoclassical transport theory. A transport theory which includes these effects is derived from the drift-kinetic equation, with an ordering scheme modeled on the parameters of recent tokamak experiments. The theory self-consistently predicts that electrostatic trapping should be fully comparable to magnetic trapping, and provides transport coefficients which, depending quadratically upon the temperature and pressure gradients, differ markedly from the standard neoclassical coefficients for a pure plasma.
Experiments on the structure of turbulent shear in pipe flows of water
Authors: Johnson Richard R. , Elkins Rush E. , Lindgren E. Rune , Yoo Jae K.
Published: March 2003

A trace particle method by which three-dimensional turbulent velocity fields can be pictured in great detail both qualitatively and quantiatively is described.
Energy decay of Burgers’ model of turbulence
Authors: Yamamoto K. , Hosokawa I.
Published: March 2003

The previously developed numerical method, using Monte Carlo quadrature, is applied to Burgers’ model of turbulence for many cases of initial energy spectrums. It turns out that the power index of the energy decay ranges continuously from −2/3 to −2, depending on the cases.
Contraction of Z pinches actuated by radiation losses
Authors: Shearer J. W.
Published: March 2003

An equilibrium model of the radial motion of Z pinches is developed, based on the Bennett relation, radiation losses, and Ohmic heating. The highly localized x-ray sources observed in some plasma focus experiments can be explained by the model.
Reattachment of a blownoff supersonic boundary layer subsequent to injection cutoff
Authors: Amr A. , Kassoy D. R.
Published: March 2003

The reattachment of a boundary layer separated by mass addition is studied. A wall injection distribution ṁ=C (2xR)−1/2 is cut off at a finite distance from the leading edge of a hot (cold) flate plate in a uniform supersonic flow. The reattachment is described in terms of an interaction theory. The model consists of a viscous sublayer of O (R−5/12) in thickness, a main inviscid rotational injectant region of O (R−1/3) in thickness, a free-mixing layer O (R−1/2) in thickness, and finally a potential external flow. The shape of the reattaching streamline is determined numerically and an expression for the skin friction at the wall between the cutoff and reattachment points is obtained by a finite difference method. The results indicate that beyond the cutoff point there exists a region proportional to C2 in extent where heat transfer and skin friction are considerably reduced from the value predicted by classical boundary layer.
Shock propagation across deflagrations
Authors: Strachan J. D.
Published: March 2003

The one-dimensional interaction expected when a shock crosses a deflagration is solved. The analysis is complicated by energy source terms which can occur in the refracted shock. The Mach number of the refracted shock is very dependent upon the actual energy absorption mechanism and sample solutions for absorption which is like radiative and chemical heat release are discussed.
Nonlinear tokamak stability
Authors: Strauss H. R.
Published: March 2003

The nonlinear energy theorem of Rosenbluth for helically symmetric tokamak magnetohydrodynamic motions is used to find nonlinear corrections to the linear stability of kink modes. Near marginal stability these corrections are important and give a finite amplitude threshold for instability. Cases are also found in which a mode saturates at low amplitude, in agreement with previous calculations.
Saturation of nonlinear Landau damping by cascading
Authors: Rosen B. , Roycraft T. , Schmidt G.
Published: March 2003

It is shown that a heating wave in a plasma (say lower hybrid) that decays by a nonlinear Landau damping process can saturate by cascading. No matter how small the (linear) damping rates are the instability saturates without piling up great amounts of energy in the last mode. In the presence of source terms the wave energies approach their equilibrium values. A formal analogy is shown to exist between the present problem and that of interacting animal populations first investigated by Volterra.
Radiation hydrodynamics with opacity enhancement by Doppler shift
Authors: Chan K. L.
Published: March 2003

In a radiative fluid, the Doppler effect due to differential motions of neighboring fluid elements causes photon mean-free-paths to deviate from the static values. The effects of such deviations on the radiative flux and radiation pressure are studied.
Strong field inverse bremsstrahlung via a Lorentz model
Authors: Catto P. J. , Speziale Thomas
Published: March 2003

The commonly employed results of Silin for inverse bremsstrahlung with strong field corrections are obtained in a simple manner from a Lorentz collision operator. The effective collision frequencies for linearly and circularly polarized light are given for arbitrary pump strength.
Nonlinear interactions of focused resonance cone fields with plasmas
Authors: Stenzel R. L. , Gekelman W.
Published: March 2003

A simple yet novel rf exciter structure has been developed for generating remotely intense rf fields in a magnetoplasma. It is a circular line source of radius R in a plane ⊥B0 driven with an rf signal at ω <ωc≃ωp (where ωc, ωp are the electron cyclotron and electron plasma frequency, respectively) which excites two resonance cones with focal points at z=±R cotΘc (Θc is the cone angle). The rf intensity increases away from the exciter to reach a maximum (approximately 20 dB enhancement) at the remote focal points. This arrangement is ideally suited to study nonlinear interactions free of boundary effects. At large applied rf signals, ε0Erf2/nkTe≳0.2, a strong density depression in the focal region (δn/n≳40%) is observed. The density perturbation modifies the cone angle and field distribution. This nonlinear interaction leads to a rapid growth of ion acoustic wave turbulence and a corresponding random rf field distribution in a broadened focal region. The development of the interaction is mapped in space and time.
Structure of sheared turbulent fields
Authors: Gibson Carl H. , Friehe Carl A. , McConnell Steven O.
Published: March 2003

Small scale turbulent temperature and velocity measurements were made in a variety of turbulent shear flows in the atmosphere and laboratory, in air and in water. Nonzero skewness values of the streamwise temperature gradient were observed in every case, with a sign equal to that of the gradient dotted with the cross product of the mean temperature gradient and mean shear vectors. Large temperature jumps were found across sharp shear zones at the outer boundaries of large scale eddies rotating with the mean vorticity of the shear flow, giving a distinctive ramp-like structure to the temperature signals which seems to be a characteristic feature of scalar fields mixed by sheared turbulence. The streamwise gradient of the vertical velocity in the atmospheric boundary layer over the ocean was found to have positive skewness. Thus, both velocity and temperature fields are locally anisotropic in the same sense as the anisotropic mean quantities.
Behavior of a turbulent flow, strongly out of equilibrium, at supersonic speeds
Authors: Gaviglio Jean , Dussauge Jean-Paul , Debieve Jean-François , Favre Alexandre
Published: March 2003

A study is presented of the mechanisms occurring in a turbulent near wake flow at supersonic speed, in which there exist strong pressure and velocity gradients. The role played by the ’’production’’ of turbulent kinetic energy is shown. It is influenced by compressibility and anisotropy. The analysis indicates that compressibility increases production when a sheared flow develops in a positive pressure gradient, and decreases it in the case of an expansion. The experiments give the mean and turbulent characteristics of a near wake flow. The results show that: the levels of vorticity and entropy modes verify the relationship given by the strong Reynolds analogy; the correlation coefficient between modes is not significantly altered by strong gradients and rates of strain; negative production occurs in the expansion zone; as a consequence, there exists a decrease in turbulence measured downstream of this region.
Drag reduction by compliant walls
Authors: Zimmermann G.
Published: March 2003

The turbulent boundary layer flow over a flat plate, smooth and hard except for a finite section, is considered. In this section the plate may be compliant, have a prescribed waviness, or consist of an ’’active wall’’, etc. The section thus constitutes a disturbance to the turbulent boundary layer flow over a completely smooth and hard plate. An exact integral equation for the resulting disturbances of the fluid stresses at the plate is derived. To a reasonable approximation, this integral equation reduces to an explicit integral representation of the stress disturbances at the plate in terms of the undisturbed flow and the properties of the disturbing plate section. Further evaluation then requires the use of measured data. For a certain type of compliant wall a drag reduction of 1%–10% is estimated. PFL
Investigation of pressure fluctuations beneath a turbulent boundary layer by means of an optical method
Authors: Dinkelacker A. , Hessel M. , Meier G. E. A. , Schewe G.
Published: March 2003

An optical method has been developed by Emmerling, Meier, and Dinkelacker which allows the investigation of the instantaneous structure of the wall pressure field under a turbulent boundary layer. The main component of this method is a pressure transducer consisting of several hundred small elastic membranes, the displacements of which are recorded by interferometric means with a high-speed camera. The method is explained and examples of measurements in a wind-tunnel are presented. The measurements show the development and convection of pressure patterns of a wide range of sizes (from larger than one boundary layer thickness δ down to 0.1 δ) and with a wide range of convection velocities (from about 0.9 U∞ down to about 0.2 U∞, with U∞ the flow velocity outside the boundary layer). Some speculative considerations are made on what sort of flow configurations might be connected with the observed pressure patterns.
Transitional boundary layer spot in a fully turbulent environment
Authors: Zilberman M. , Wygnanski I. , Kaplan R. E.
Published: March 2003

A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary-layer thicknesses. The scale of the structure is of the order of 10δ in the streamwise direction becoming 2−3δ in the interface region of the boundary layer and is less than 4δ in the spanwise direction, in spite of the fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9U∞. It exhibits features in detailed agreement with those at the outer region of the turbulent boundary layer (interface region) and is consistent with existing two- and three-point space-time correlations.
A study of the structure of a plane stationary computer-simulated free-shear turbulence
Authors: Tsen L. F. , Bouriot M.
Published: March 2003

Computer experiments are carried out on a two-dimensional, time-stationary, turbulent free shear flow. The aim is to visualize in a motion picture the time history of the entire field of all variables that are deemed interesting: velocity, pressure, vorticity, dissipation, etc..., and to observe the connection between these deterministic motions and the corresponding space-time statistics drawn once the ergodicity is admitted. The simple flow behind a truncated base and confined by frictionless sidewalls is considered. The two-dimensional unsteady Navier–Stokes equations in the stream function form are replaced by their finite difference counterpart, using a centered difference scheme in both the space and time derivative and some artifices to secure numerical stability. Realizations are computed at a nominal Reynolds number referred to the base height of Re=300, 3000, and 300 000, for a duration of about T≃800 in reduced time. The one-point one-signal statistics, including probability density, time-correlation, power spectral density, are drawn from these realizations. The Re=300 case exhibits a von Karman vortex street pattern and its statistics are in perfect accordance with a periodic flow. At the two higher Reynolds numbers, the primary vortices break into secondary and higher order vortices. The probability density becomes more and more Gaussian, and, in the power spectrum, more and more energy is transferred from the Strouhal ray and its harmonics to other frequencies. The one-point two-signal statistics are also examined. In particular, the contribution of the successive moving vortices or eddies to the joint probability density of the velocity components u and v shows the way the Reynolds shear stress is generated. Other statistics will also be discussed. Despite the notorious inability of a discretized calculation to resolve the ever smaller scale motions at high Reynolds numbers, it is suggested that numerical simulation can cast light on turbulent processes, especially on those that make use of not readily measured variables. This work was supported by contract DRME 75/354.
Free stream turbulence and shear effects on boundary layer and wake structures
Authors: Luxton R. E. , Antonia R. A. , Ahmad Q. A.
Published: March 2003

The results of experiments first on a turbulent wake of a cylinder imbedded in a uniformly sheared external turbulent flow,1 and second on a zero pressure gradient boundary layer developing under an external uniformly sheared turbulent flow with positive, zero, or negative shear,2 are described and compared. While there are many features which are similar for the wake and for the outer part of the boundary layer, the relative effects of external turbulence level and of free-stream shear are different for the two flows. For the wake it appears that free stream turbulence has little effect on the growth rate, but free stream shear produces a marked asymmetry in the growth rate on the two sides of the wake. (This apparent independence of the two sides of the wake is also evident in the measurements by Fabris3 of the interaction between the wakes of two neighboring cylinders.) In the boundary layer, by contrast, the free stream turbulence has a major effect on the growth rate, large scale structure and entrainment, whilst the free stream shear effects are relatively small. It is possible that the large scale structure proposed by Brown and Thomas4 may still exist in the presence of free stream turbulence, but that the external turbulence increases the amount of ’’jitter’’ making the coherence more difficult to isolate.
The role of large scale structures in turbulent jets
Authors: Laufer J. , Browand F. K. , Petersen R. A.
Published: March 2003

The presence of large scale structures in circular jets is well documented in the literature over a wide range of Reynolds numbers. This presentation explores in more detail the generation and interaction and the role they play in the development of the flow field. It is based on experiments carried out, both in water and air jets, at Reynolds numbers up to 100 000.1,2 It is found, as reported by others, that the frequency of generation, as well as the various modes of the large scale vortices, can be predicted by linear stability theory. However, their further development as they are convected downstream is most difficult to study in detail. Conventional correlation measurements or sampling techniques have proven to be inadequate. In fact, the purpose for doing parallel water and jet experiments is to enable one to develop an adequate experimental method that could provide better information on the interaction process between adjacent structures. In a qualitative way, the measurements so far suggest that the induced flow field of the structures must play an important role in the interaction process and in the flow development. In the region of the potential core there is apparently an upstream influence due to the induced pressure field. This affects and, perhaps, controls such flow characteristics as the length of the potential core and the passage frequency of the structures at the end of the core. By introducing artificial disturbances into the flow further work is being carried out to clarify the above conjecture. This research was supported by the National Aeronautics and Space Administration under Grant NASA-NSG-1221.
Dilute polymer solutions in elongational flow
Authors: Elata C. , Burger J. , Michlin J. , Takserman U.
Published: March 2003

A series of experiments is described using solutions of various concentrations of Polyox coagulant flowing through porous beds of spherical particles. Results are reported, similar to those obtained by James and McLaren, which show a substantial increase in drag. The onset of this drag increase from solvent flow seems to occur at a constant Deborah number multiplied by the square root of the concentration. The flow in a bundle of symmetrical convergent-divergent channels which may serve as a model for a porous medium is analyzed. The presence of elongated particles in the fluid will alter the flow pattern in such a channel from that given by the Stokes’ solution. Assuming the particles to be aligned parallel to the mean flow direction, this flow re-arrangement will attenuate the effect of the particles on the dissipation of mechanical energy. Extended molecules may be misaligned with the streamlines in the diverging section of the channel, which could explain the large increase in drag observed with dilute polymer solutions in porous beds.
Drag reduction in two phase and polymer flows
Authors: Lumley John L.
Published: March 2003

The basic dynamics of turbulent boundary layers of several media is described qualitatively: suspensions of several types and polymer solutions. Despite the considerable differences in these media, it is argued that a number of the flows are affected only in the buffer layer, and drag reduction can result if behavior in the sublayer and buffer layer differ. In polymer solutions, it is argued that molecular expansion is responsible for the difference, and experimental evidence of this expansion is presented, and compared with calculations.
Drag reduction scale-up criteria
Authors: Savins J. G. , Seyer F. A.
Published: March 2003

Available drag reduction correlations are presented and their utility for engineering scale-up to a scale-up from a correlation based on friction velocity and the availability of some turbulent flow data with the process fluid of interest is compared. A further purpose is to investigate the consequences of polymer modification of the mean flow profile in the outer layer. Emphasis is placed on accuracy and ability to portray the drag reducing activity of complex systems. In the case of very dilute polymeric systems in large conduits, very accurate predictions may be needed to assess the performance of a particular additive. Similarly, equations capable of detailed predictions of observed behavior can provide needed insight into the mechanism of drag reduction. Inasmuch as the semi-logarithmic resistance law, e.g., the Seyer and Metzner model of turbulent viscoelastic flow, provides a scale-up criterion significantly more complicated to use than a number of other models, an engineering approximation to this model is described. It is shown that for accurate predictions to be made at modest levels of drag reduction activity, the model must be modified to compensate for thickening of the sublayer.
Laser Doppler velocimetry studies of early turbulence
Authors: Zakin J. L. , Ni C. C. , Hansen R. J. , Reischman M. M.
Published: March 2003

Laser Doppler anemometry is employed to measure the mean velocity and turbulence intensity in the early turbulence regime of the flow of a dilute polymer solution through a small-diameter tube. (This regime is initiated at an onset Reynolds number in the subcritical range, above which the polymer additive increases drag, and continues through the transitional Reynolds number range, where a continuous transition to turbulent flow with reduced drag is observed.) When the onset flow rate for early tubulence is exceeded, the mean velocity profile becomes progressively flatter with increasing flow rate. That is, a gradual transition from a laminar profile at onset to a more ’’turbulent-like’’ profile takes place. Turbulence intensities at the tube centerline are about one half those of a fully developed turbulent flow. The inference is drawn from these results that the onset of eary turbulence corresponds to the flow becoming hydrodynamically unstable due to viscoelastic effects introduced by the polymer additive and that the character of the flow is intermediate between laminar and fully developed turbulent flow.
Laser light absorption due to self-generated magnetic fields
Authors: Kruer W. L. , Estabrook Kent
Published: March 2003

It is shown that self-generated magnetic fields can play a significant role in laser light absorption. Even normally incident light is then resonantly absorbed via the excitation of upper hybrid oscillations. Computer simulations and theoretical estimates for this absorption and the concomitant harmonic generation are given for parameters characteristic of some recent experiments.
Spectra of ion acoustic plasma instability
Authors: Ilić Dragan B.
Published: March 2003

An experimental study of ion acoustic plasma turbulence in a collision-dominated positive column is presented. Linear theory predictions, which include ion Landau damping and the effects of a dc electric field, are shown to agree with measured values of convective growth rates. At high levels of turbulence, interferometric methods are not useful for studying the wave phase characteristics. Instead, the detailed spectral density for two-dimensional wave propagation, S (k,ω), is obtained by performing a two-dimensional spatial Fourier transform on sampled values of the frequency sweeps of the complex cross-power spectral density. The instability is found to be essentially one-dimensional, with the waves propagating along the direction of the electron drift velocity. The spectra are integrated numerically, and the energy density obtained is discussed in terms of two nonlinear models of saturated state of the instability. Time- and frequency-resolved power spectra are measured by sampling techniques, and compared to the predictions of linear theory. Temporal evolution of the phase characteristics is measured by a novel diagnostic technique.
Spectral distribution of wave power in wideband beam plasma turbulence
Authors: Greenly John B. , Cavalli Alexander , Walsh J. E.
Published: March 2003

A highly turbulent beam plasma steady state which completely annihilates the beam has been studied experimentally. A large fraction of the entering beam power appears in a wideband spectrum of longitudinal electrostatic waves. A scaling law relating power spectral density to velocity space particle flux has been derived and applied to this interaction. Its predictions are in excellent agreement with experimental behavior.
Inertial motion of a continuum
Authors: Faller Alan J.
Published: March 2003

The Burgers equation ut+uux=νuxx represents pure inertial motion except for the effects of viscosity ν. If ν=0, this equation becomes u̇=0 and describes the inertial motion of a one-dimensional continuum until the time of formation of discontinuities, or ’’shocks’’. The study of pure inertial motion is extended to an arbitrary number of dimensions. Starting from some initial state of motion each parcel of the continuum may or may not have the intrinsic ability to form a shock, this property being a function of the symmetrical part of the velocity gradient matrix in the vicinity of the parcel. This study determines the conditions for which a parcel will, in fact, form a shock, the time that is required, and the temporal development of the full velocity gradient matrix and of coordinate invariants of the flow such as the divergence and the vorticity.
Spectroscopic study of combustion-driven implosions
Authors: Roig Randy A. , Glass I. I.
Published: March 2003

Time and spatially resolved studies have been made of imploding shock waves in a 20 cm hemispherical chamber filled with 2H2+O2 mixtures at high initial pressures (7–56 atm). Implosion pulse durations and maximum temperatures were found to be (10–4) μsec and (4500–6000) K, respectively. Peak pressure for a 7 atm initial pressure experiment was estimated to be (12.5±1.0) ×103 atm. The focus of the imploding shock was examined by using an image converter camera in both streak and framing modes. No major instabilities or symmetry defects were noted. The results may be of interest for laser fusion applications.
Third-order resonant wave interactions
Authors: Lekoudis Spyridon G. , Nayfeh Ali H. , Saric William S.
Published: March 2003

An analysis is presented of third-order resonant interactions in capillary-gravity waves. When dissipation is accounted for, the theory predicts the amplitude of the third harmonic near the resonant frequency and at the early stage of the interaction. A more detailed account of the viscous effects is needed when these effects dominate the interaction.
Log-normality of temperature dissipation in a turbulent boundary layer
Authors: Antonia R. A. , Sreenivasan K. R.
Published: March 2003

Using measurements of all three components of the temperature dissipation χ in a laboratory boundary layer, the measured probability density p (χr) of χr, or χ averaged over a distance r, is found to be closely log-normal over a significant range of r. The variance σ2 of lnχr follows the relation σ2=A+μln(L/r), with μ=0.35, where L is an integral scale of turbulence. High order moments, up to order 5, of χr show a power-law variation with r/L. With increasing order of the moment, the power-law exponents become increasingly smaller than the corresponding values implied by assumed log-normality of p (χr) but are consistent with the bounds given by Novikov’s theory. It is suggested that the observed close agreement with log-normality of p (χr) may be misleading when sufficiently high order moments of χr are considered.
Large scale motions in electrical discharges
Authors: Uberoi Mahinder S. , Chow Chuen-Yen
Published: March 2003

Mass motions of a plasma are numerically analyzed for a discharge from a central small screen electrode of circular cross section and two other screen electrodes covering the two ends of a circular tube. In effect, there are two oppositely directed discharges from the central electrode and this simplifies the analysis. Rotational electromagnetic forces cause two large eddies, each with a region of high velocity or a jet on either side of the central electrode. The case is further computed where the plasma is allowed to flow from one end to the other through the tube and the screen electrodes. The plasma entering the end screen electrode flows around the eddy lying between the electrode and the central electrode. This provides a recirculating and confined motion associated with the first discharge. As the plasma flows farther downstream, it is drawn to the axis of the second discharge and becomes a part of its jet.
Influence of diffusion on the resistive tearing mode
Authors: Dobrott D. , Prager S. C. , Taylor J. B.
Published: March 2003

Although resistive diffusion is much slower than the growth of resistive instabilities, the conventional neglect of diffusion in tearing mode calculations is incorrect. The proper criterion for neglect of diffusion in resistive instability calculations, which is not satisfied for tearing modes, is ω≫v/δ, where v is the resistive diffusion velocity and δ is the resistive layer thickness. The effect of diffusion is calculated in the limit of large and small ωδ/v for the plane slab model, and new expressions for growth rate and stability boundary are obtained. The diffusion appears to have a stabilizing effect.
Evidence for profile steepening in laser-irradiated plasmas
Authors: Phillion D. W. , Lerche R. A. , Rupert V. C. , Haas R. A. , Boyle M. J.
Published: March 2003

Experimental evidence of a steepened electron density profile near critical density has been obtained by studying the light scattered by targets (10-μm thick disks and 100-μm diam glass spherical microshells filled with deuterium and tritium gas) illuminated by linearly polarized, 1.06-μm light. Scale lengths on the order of 1 μm have been inferred both from the polarization of the reflected light and from the azimuthal asymmetry (asymmetry about the beam axis) of the time-integrated scattered light with respect to the laser electric field. Azimuthally asymmetric heating of the microshells targets is indicated both by x-ray micrographs and by the spatial distribution of the plasma blowoff.
Numerical simulation of the nonlinear convective and oscillating instabilities in a beam plasma system
Authors: Goldstein B. , Carr W. , Rosen B. , Seidl M.
Published: March 2003

Plasma simulation models have been developed which replicate the fundamental features of the convective and oscillating instability in the nonlinear weak beam regime. The oscillating instability is characterized in this regime by a randomly fluctuating electric field which is caused by the presence of a fresh, strong beam in a region of space in which the electric field is large. Nonlinear behavior of the background plasma need not be present during the production of the oscillating instability. The convective instability develops into a quasilinear quiescent state during its nonlinear evolution because the nonlinear behavior of the system is confined to a diffused beam.
Parametric instabilities due to lower-hybrid radio frequency heating of tokamak plasmas
Authors: Porkolab Miklos
Published: March 2003

The linear theory of parametric instabilities relevant to radiofrequency heating near the lower-hybrid frequency of tokamak-type plasmas is presented. The dispersion relations are analyzed numerically to all orders of the ion Larmor radius. The complete range of unstable spectra is obtained for 1<ω0/ωlh<5, for both deuterium and hydrogen plasmas. As the pump wave propagates from the edge of the plasma toward its interior, the transition from resonant decay to decay into quasi-modes is demonstrated. The effects of inhomogeneities upon the threshold for parametric decay, such as density gradients, finite pump width, and magnetic shear, are obtained. The relevance of these results to recent tokamak experiments is discussed briefly.
Platelet aggregate formation in a region of separated blood flow
Authors: Parmentier E. Marc , Morton William A. , Petschek Harry E.
Published: March 2003

Microscopic observations of the flow of fresh canine blood in a separated region formed by a step in a polyurethane surface have been made. These observations show the formation of large clumps of freely floating aggregates of platelets. These clumps can easily leave the region and may be representative of disorders occurring in the distal circulation when artificial devices are used in the bloodstream. Measurements of the growth rate of these aggregates are compared with a theoretical analysis of the rate of platelet collisions. Some criteria defining conditions under which such free floating aggregates occur are also suggested.
Test particle study of nonlinear wave-particle interaction in the magnetosonic mode: Pure sinusoidal wave model
Authors: Matsumoto Hiroshi
Published: March 2003

A numerical study of the motions of test particles in a sinusoidal magnetosonic (hydromagnetic fast) wave is presented. The magnetic amplitude of the wave was varied from 1% to 40% of the external magnetic field strength. The motions of 400 particles are calculated in each computer run by time integration of the equation of motion, without the use of the guiding center approximation. The particles are initially distributed uniformly in space with a specified velocity distribution; the space and velocity distributions then evolve in time. For small wave amplitudes, the numerical results agree well with the small amplitude kinetic theory of collisionless Landau damping of hydromagnetic waves. In large amplitude cases, nonlinear effects such as particle trapping, saturation and oscillation characteristics of the average kinetic energy, and the diffusion of particles in velocity space are observed. The physical interpretation of these numerical results is discussed in terms of the guiding center approximation.
Magnetohydrodynamic stability of highly elongated axisymmetric equilibria
Authors: Grossmann W. , Tataronis J. A. , Weitzner Harold
Published: March 2003

The magnetohydrodynamic stability of diffuse, axisymmetric toroidal plasma configurations with large elongation in the direction of the axis of symmetry and large aspect ratio is investigated. In an asymptotic expansion the elongation and aspect ratio are related and both are made large. This expansion reduces the equilibrium relations in leading order to an ordinary differential equation readily susceptible to numerical solution or, in some cases, analytical solution. The same expansion procedure is also applied to the linearized stability analysis and both partial differential equations and a Rayleigh–Ritz quotient for the eigenvalues and eigenvector result. Belt pinch and doublet shapes for various current density profiles are examined for internal and external modes for both high and low β forms of the equilibrium. Growth rates and eigenfunctions for the discrete modes are given and general features of the spectrum are discussed.
Magnetohydrodynamic equilibrium of plasma confined in a spherical microwave cavity
Authors: Woods C. H.
Published: March 2003

Methods of producing a spatially uniform, time-averaged pressure over the surfaces of an overdense plasma confined in a spherical electromagnetic cavity are discussed. It is found that excitation of the cavity at only two eigenfrequencies is required, whereas a method suggested nearly twenty years ago requires three nondegenerate frequencies.
Theory for laser simulation of hypervelocity impact
Authors: Pirri Anthony N.
Published: March 2003

A theory is developed to determine the parameters for simulation of the hypervelocity impact of small particles on a surface by focusing a high power pulsed laser upon that surface. It is proposed that the early time history of hypervelocity impact effects can be simulated by requiring that the laser induced surface pressure equal the surface impact pressure, equating the laser pulse time to a particle/surface interaction time, and setting the focal spot diameter equal to the particle diameter. Calculations of impact pressure versus projectile velocity are performed using an idealized one dimensional model for shock wave propagation. A simplified theory for the interaction of a highly focused pulsed laser beam with a surface is developed to obtain the laser induced surface pressure. Results of a sample calculation for water droplets impacting a graphite surface between 3 and 6 km/sec indicate that laser intensities of 5×1010–5×1011 W/cm2 are required. The corresponding laser energies are strongly particle size and shape dependent and range from 0.01 J with a 5 nsec pulse time for 20 μ radius particles to 2 J with a 20 nsec pulse time for 80 μ radius particles. A qualitative comparison of data for high power laser produced craters and hypervelocity impact produced craters is also presented.
Velocity space diffusion of ions in controlled turbulence
Authors: Itatani Ryohei , Yasaka Yasuyoshi , Fukumasa Osamu
Published: March 2003

The velocity space diffusion of ions due to ion wave turbulence is investigated experimentally using an ion wave echo. Experiments are so arranged that the streaming perturbation due to the first excited wave propagates through the localized region where the ion wave turbulence is excited, to interact with the second excited wave. The resultant second order ion wave echo damps in comparison with the case of no turbulence. The variation of the echo amplitude as the power of the spectrum of the turbulence is changed is observed, which is in quantitative agreement with the theoretical prediction. It is also revealed that the turbulent waves are far more effective than the monochromatic waves in increasing the effective collisions.
Parametric excitation of equilibrium surface ripples on magnetically confined nonneutral plasma columns
Authors: Mahaffey R. A. , Trivelpiece A. W.
Published: March 2003

The parametric excitation of equilibrium surface ripples on a drifting, magnetically confined nonneutral plasma column is investigated by passing it through a spatially periodic magnetic field. Measurements of the relationship between the ripple amplitude and the pump frequency, the number of pump periods, and the pump amplitude verify most of the features predicted theoretically. Measurements also verify the predicted relationship between the rigid rotor angular velocity of the plasma column and the amplitude of these equlibrium ripples. Rotation of the column opposite to the direction that an isolated electron would gyrate in the same magnetic field and rotation at an angular velocity greater than the cyclotron frequency are observed. The relation between the ripple amplitude and the propagation characteristics of space-charge waves is discussed.
Nonlinear cascade models for fully developed turbulence
Authors: Bell Thomas L. , Nelkin Mark
Published: March 2003

The cascade model of Desnyansky and Novikov is extended to include energy flow inward as well as outward from a given k-space shell. The stationary solutions for the energy spectrum are of the 1941 Kolmogoroff form, ε2/3 k−5/3 f (k/kd), but the scaling function f (x) has a nonanalytic dependence on a parameter C of the model which represents the relative strength of inward and outward energy flow in k space. For C<1, f (0) is finite, and the energy dissipation rate ε is nonzero in the limit of zero viscosity. For C ≳1, f (x) behaves as x−ζ for small x with ζ=2 log C/log 2. This leads to an inertial range exponent for the energy spectrum of 5/3+ζ, and to an ε (ν) which goes slowly to zero as the viscosity, ν, goes to zero. The model may play the role of a mean field theory for strong turbulence in which fluctuations leading to intermittency are neglected. The parameter C would then be a smooth function of the spatial dimensionality. A comparison with other recent work indicates that the crossover phenomenon when C=1 occurs between two and three dimensions.
Physics of a fusion plasma boundary layer
Authors: Jensen B. K.
Published: March 2003

A theoretical and computational study has been made of plasma phenomena occurring when a hot, dense plasma containing a transverse magnetic field is brought into sudden contact with a cold metal wall; thermal and magnetic boundary layers develop. The time evolution of the plasma temperature, pressure, the charged and neutral particle concentration, magnetic and electric field strengths, and the plasma current density in the neighborhood of the solid surface are investigated. The rate of energy transfer from the plasma to the wall is calculated, and the conditions under which wall surface melting occurs are estimated. The physical conditions previously studied experimentally by Feinberg, are calculated, and the predicted rate of energy transfer from the plasma to the wall is found to be in good agreement.
Realizability inequalities in turbulent flows
Authors: du Vachat Régis
Published: March 2003

It is shown that the various moments of a one- (multi-) dimensional variable must obey special inequalities, where the random variable can be taken as the turbulent fluctuation of either velocity components or temperature or pollutant density. These inequalities called realizability inequalities, are derived from the well-known problem of moments. The inequalities are specified for the case of the quasi-normal approximation (vanishing of fourth-order cumulants) or of other approximations (vanishing of cumulants of order k, 4⩽k?2n, where n is an integer). An example of the use of such inequalities is given for the case of the quasi-normal approximation.
Slightly rarefied gas flow over a specularly reflecting body
Authors: Sone Yoshio , Aoki Kazuo
Published: March 2003

The asymptotic behavior (for small Knudsen number) of the steady flow of a rarefied gas over a specularly reflecting body with arbitrary but smooth shape is investigated on the basis of the linearized Boltzmann–Krook–Welander equation. As applications of the theory, a sphere in a uniform flow of gas and that in a gas at rest with a uniform temperature gradient are considered. The velocity and temperature fields around the spheres and the drag and thermal forces acting on the sphere are obtained. The formulae of the forces are compared with those given by various approximate methods and those for other types of boundary conditions.
Transport of a multiple-ion species plasma in the Pfirsch–Schlüter regime
Authors: Hirshman S. P.
Published: March 2003

The classical parallel friction coefficients, which relate the collisional friction forces to the flow of particles and heat along the magnetic field, are calculated for a multiple ion species plasma. In the short mean-free-path regime, the neoclassical Pfirsch–Schlüter transport coefficients for a toroidally confined multispecies plasma are computed in terms of the classical friction coefficients. The dependence of the neoclassical cross-field transport on the equilibration of the parallel ion temperature profiles is determined.
Poloidal profiles and transport during turbulent heating
Authors: Mascheroni P. L.
Published: March 2003

The current penetration stage of a turbulently heated tokamak is modeled. The basic formulae are written in slab geometry since the dominant anomalous transport has a characteristic frequency much larger than the bounce frequency. Thus, the basic framework is provided by the Maxwell and fluid equations, with classical and anomalous transport. Quasi-neutrality is used. It is shown that the anomalous collision frequency dominates the anomalous viscosity and thermal conductivity, and that the convective wave transport can be neglected. For these numerical estimates, the leading term in the quasi-linear series is used. During the current penetration stage the distribution function for the particles will depart from a single Maxwellian type. Hence, the first objective was to numerically compare calculated poloidal magnetic field profiles with measured, published poloidal profiles. The poloidal magnetic field has been calculated using a code which handles the anomalous collision frequency self-consistently. The agreement is good, and it is concluded that the current penetration stage can be satisfactorily described by this model.
Plasma density in an electrostatically plugged spindle cusp
Authors: Hayward R. L. , Dolan T. J.
Published: March 2003

The theoretical plasma density and confinement time for an electrostatically plugged spindle cusp experiment (R=0.16 m, Br=0.3 T, Bz=0.5 T) are compared with experimental data. The plasma is produced by injection of a 5 mA, 1 keV electron beam into H2 at 10−5 Torr. The electron density and confinement time, measured with a 20 GHz interferometer, are on the order of 2×1016 m−3 and 50 μsec.
Plasma ringing associated with pulsed resonance cones
Authors: Bellan P. M.
Published: March 2003

A temporal wave packet of frequency ω0(ωci, ωpi≪ω0≪ωce, ωpe) applied to a finite antenna in a magnetically confined plasma, is shown to excite an electrostatic field consisting of (1) a short-duration resonance cone and (2) a wave-like disturbance which follows afterward. This latter disturbance, which is related to the lower frequency mode of the delta-function induced transients discussed by Simonutti, persists long after the resonance cone. The phase factor associated with the disturbance is independent of ω0, and its constant-phase surfaces are lines which emanate from the antenna and make a small angle ϑ with respect to the confining magnetic field B. ϑ decreases with increasing time so that ’’wavelengths’’ perpendicular to B also decrease with increasing time. The correspondence of the fields obtained by wave-packet excitation to those obtained by both cw and delta-function excitations is discussed and experimental results are reported.
Vlasov-fluid theory of short-wavelength instabilities of a sharp-boundary screw pinch
Authors: Turner Leaf
Published: March 2003

The physics of instabilities of a sharp-boundary screw pinch is analyzed using the Vlasov-fluid model of Freidberg. An ordering especially applicable to the study of stabilized Z pinches is used. The equations of the Vlasov-fluid screw pinch differ from those of an ideal magnetohydrodynamic screw pinch in two crucial respects: a frequency-dependent compressibility function γM(ω) and a magnetoviscosity term that can induce finite-Larmor-radius stabilization. The domain of marginal stability of the Vlasov-fluid screw pinch is identical to that of the ideal magnetohydrodynamic screw pinch, in agreement with the results of Freidberg. Even when finite-Larmor-radius effects are ignored, the ratio (Vlasov-fluid growth rate)/(ideal magnetohydrodynamic growth rate) is less than unity in all cases studied. When β is finite, this ratio must approach zero as marginal stability is approached. In addition, for unstable modes with m≳1, finite-Larmor-radius stabilization effects in the Vlasov-fluid model can be significant when β‖ω/kvth‖≫ 1 and ‖ω/kvth‖≪ 1, where vth is the ion thermal speed and k is the wavenumber. In contrast, these effects are always insignificant in a magnetohydrodynamic model even when a phenomenological magnetoviscosity term is included! Numerical results are presented for m=1 and m=2 instabilities of a stabilized Z pinch. This analysis may help to explain the apparent stability of the Kurchatov reversed-field experiment.