bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of rectilinear geostrophic vortices in stationary equilibrium
Authors: Bauer Louis , Morikawa George K.
Published: March 2003

A hemispherical barotropic model of global-scale atmospheric motions is studied in a planar approximation using three K0 (Bessel) vortices of equal strength γ and length scale κ−1 to represent the subtropical semipermanent high pressure systems. In equilibrium these anticyclonic vortices are equally spaced on a latitude circle of radius a. One K0 vortex of strength γ0 is used to simulate the polar (cyclonic) vortex; γ0 is chosen so that the equilibrium configuration remains stationary with respect to the rotating earth. Over the range of the length parameter 0⩽κa⩽5, two distinct stable regimes are found: (1) a ’’soft’’-vortex regime, 0?κa<κcr⋅a=1.44763516... and (2) a ’’hard’’-vortex (climatological) regime, κ≳κcr. The nonlinear part of this stability investigation has been carried out mainly by numerical computation.
Marginal instability of turbulent shearing layers and the break point of a jet
Authors: Lessen Martin , Paillet Frederick
Published: March 2003

The postulate that fully turbulent free flows are marginally stable is used to predict the break point of the turbulent jet. The theory is consistent with recent experiments, and is used to improve an existing theory of entrainment.
Poiseuille flow in a pipe with axially symmetric wavy walls
Authors: Lessen Martin , Huang Pao-San
Published: March 2003

The effect of small amplitude wall waviness on the steady flow in a pipe is studied. Friction factors, Reynolds stresses, and mean velocity profiles are obtained for various Reynolds numbers, wall wave amplitudes, and wavenumbers. The results agree qualitatively with the experiments of Batra, Fulford, and Dullien and contradict the classical ’’Moody’’ diagram which represents the laminar friction factor as independent of wall roughness. The mean velocity profiles obtained suggest that wall roughness may cause pipe flow to be unstable to infinitesimal disturbances at a finite Reynolds number.
Nonlinear model of laser supported deflagration waves
Authors: Su F. Y. , Boni A. A.
Published: March 2003

A one-dimensional nonlinear model is formulated to describe the propagation of subsonic waves supported by laser radiation. The model includes the effects of radiation losses, radiative transport in the vacuum ultraviolet, and thermal diffusion. The nonlinear equation is solved by use of the method of quasi-linearization. It is found that the nonlinear effects associated with the equation of state, thermal conductivity, and radiative properties of the gas significantly affect the temperature profile of the wave, but have little influence on the wave velocity. Calculations of the wave velocity for CO2 laser fluxes between 104 and 106 W/cm2 agree well with available experimental data.
Collisional effects on electron heating due to parametric instability
Authors: Denavit J.
Published: March 2003

The energy absorbed in turbulent heating of electrons due to parametric instability in collisionless plasmas has been shown to appear primarily in the form of suprathermal electrons, rather than in a broadening of the main body of the electron distribution function. Computer simulations of the weak collisional regime are presented, in which the dominant heating mechanism remains turbulence caused by instability, and the velocity distribution of the absorbed energy is related to collisions in terms of the ratio η/ηt, where η is the normalized driver field amplitude and ηt is the threshold value of η for the oscillating two-stream instability. For a driver field not too far above threshold, 1<η/ηt≲4, collisional heating remains negligible, but collisions cause a velocity diffusion of cold electrons to velocities where they can contribute to the turbulent heating process. As more electrons take part in this process, the energy gain per electron decreases and the generation of suprathermal electrons is significantly reduced.
Plasma heating at frequencies near the lower hybrid
Authors: Chu C. , Dawson J. M. , Okuda H.
Published: March 2003

The anomalous heating of plasma by large amplitude waves at frequencies close to the lower hybrid frequency has been examined by computer simulation on a two and one-half dimensional electrostatic model. The anomalous heating is due to the turbulent state induced by parametric instability. During the initial stages, the unstable waves propagate in a small cone about the direction of the pump-induced E0×B0 drift. During the development of the instability, the spectrum spreads in angle and creates two-dimensional turbulence in the plane of the self-consistent electric field used in the simulation. The electrons are anomalously heated in the direction parallel to the magnetic field, while the ions are heated in the direction perpendicular to the magnetic field. In the turbulent state, the cross-field energy and momentum transport of electrons are found to be strongly enhanced. The effect of turbulence on the particle transport is discussed.
Experimental studies of lower hybrid wave propagation
Authors: Bellan P. M. , Porkolab Miklos
Published: March 2003

Experimental measurement of the dispersion and damping of externally excited lower hybrid waves are presented. A multiple-ring slow-wave antenna, having 2π/kz=23 cm, is used to excite these waves in the Princeton L3 or L4 linear devices (B=0.5–2.8 kG uniform to ±1% for 1.6 m, n≈1010 cm−3, Te≈3–5 eV, Ti≲0.1 eV, He gas, plasma diameter approximately equal to 10 cm). The waves are localized in a spatial wave packet that propagates into the plasma along a conical trajectory which makes a small angle with respect to the confining magnetic field. Measurements of the dependence of wavelength on frequency are in good agreement with the cold plasma dispersion relation. Measured values of the wave damping are in good agreement with Landau damping by the combination of the main body of the electron distribution and an approximately 30% high energy (Te≈15–30 eV) electron tail.
Propagation properties of density pulses in an ion-beam plasma system
Authors: Michelsen P. , Rasmussen J. Juul , Sato N.
Published: March 2003

Numerical calculations of the propagation properties of density pulses through an ion-beam plasma system are presented. Both stable and unstable situations are investigated. The calculations are based on the linearized Vlasov equation, which is solved as an initial value problem. Solutions are presented both for pure velocity modulation and for pure density modulation of the beam. The propagation properties are found to be strongly dependent on the modulation form. In the case of velocity modulation the perturbation grows even under stable conditions, while in the case of density modulation only an instability causes growth.
Classical diffusion in toroidal plasmas
Authors: Pao Young-ping
Published: March 2003

Time-dependent classical diffusion in axisymmetric toridal plasmas with finite aspect ratio and finite beta is examined. The velocity component normal to the flux surfaces can be solved from a partial differentio-integral equation in terms of instantaneous equilibrium quantities. A method for determining the other two velocity components is given which involves the use of convective and viscous terms in the momentum equation. Equations describing the evolution of magnetic field, plasma pressure, and density are presented.
Nonlinear plasma oscillations in terms of multiple-water-bag eigenmodes
Authors: Bertrand P. , Gros M. , Baumann G.
Published: March 2003

The problem of nonlinear electron plasma oscillations is considered, using the water-bag model with N bags, and involving second-order expansions of the electric field. The solutions are readily obtained and are successfully checked by computer experiment.
Cold plasma stabilization of a mirror-confined, hot-electron plasma
Authors: Brown I. G. , Lichtenberg A. J. , Lieberman M. A. , Wyeth N. Convers
Published: March 2003

The stabilization of a hot-electron plasma in a simple mirror geometry is investigated. Controlled quantities of neutral gas generate a cold plasma through hot-electron ionization. Increasing stability is found with increasing gas pressure. The pressure required for stabilization is found to be inversely proportional to the ionization cross section of the type of neutral gas, indicating that the number of cold electrons is the key stabilizing quantity. The stabilization requires the cold electron density to be at least the same order of magnitude as the hot electron density. Line tying to external surfaces does not play a role in the stabilization. A theory in which the electrons, but not the ions, can be treated in the drift approximation is most closely applicable to the experiment. Reasonable agreement between theory and experiment has been obtained.
Emission of longitudinal waves from the interaction of two beams of transverse radiation
Authors: Weibel E. S.
Published: March 2003

When two beams of electromagnetic radiation having finite diameters intersect within a plasma, they generate, in the interaction volume, a second-order charge density which is proportional to the amplitudes of both beams. Provided that a resonance condition is satisfied, this charge density acts as directional antenna emitting longitudinal waves. In this process power is transferred from the beam of higher frequency to the beam of lower frequency with the excess going into the longitudinal waves. The power emitted is P=P1P2/P*, where Pi are the powers of the electromagnetic beams. P* is computed for both Langmuir and sound waves. In the first case this process can probably be used as a means to measure the plasma density locally.
Stability of the ion cyclotron wave for pulsed parallel injection
Authors: Mai L. P. , Horton Wendell
Published: March 2003

The obliquely propagating ion cyclotron wave has three beam-wave resonances for typical two energy component systems. The wave propagating into the injected ion beam Doppler shifts up to the beam cyclotron frequency, and the wave propagating in the direction of the beam Doppler shifts down to zero frequency and, at larger k∥vb, to the negative of the beam cyclotron frequency. The beam density thresholds and the maximum growth rates are given for these three resonant modes.
Principal stage of wake collapse in a stratified fluid: Two-dimensional theory
Authors: Kao Timothy W.
Published: March 2003

A complete theory of the collapse of a wake in the form of a two-dimensional circular mixed region of uniform density in a linearly stratified fluid is given. The initial stage is based on existing theories and the principal stage is based on the concept of a quasi-steady density current in a stratified environment. The two stages are properly matched. The dimensionless time at the initiation of the principal stage is determined theoretically. The theoretical result is in excellent agreement with previous numerical and experimental results.
Leakage and response of waves trapped by round islands
Authors: Lozano C. , Meyer R. E.
Published: March 2003

A study of surface wave trapping by typical islands of small seabed slope is reported. A new formulation is proposed which includes all the established approximations, but also permits treatment of trapping with energy leakage and complex eigenfrequencies. On this basis, we prove existence of eigenvalues with very small leakage and correspondingly large response to excitation by incident waves. Indeed, certain types of trapped wave modes have exponentially small leakage and therefore, a violently resonant character. Two physical effects are found to promote such resonance: It is typical of waves that bear a close local resemblance to coastal edge waves. It also occurs for waves of large radial wavenumber component. Both effects can combine to produce especially strong resonance. The strongly resonant wave modes have a structure more complicated than is commonly considered important, but still clear-cut enough to permit characterization by an eigenvalue problem for an ordinary differential equation. It has three relevant transition points, and extant theory for it is in effect only of first order. However, careful attention to energy conservation makes it possible to extract exponential precision for the imaginary part of the eigenvalues.
Some laminar thermal and plume experiments
Authors: Shlien D. J.
Published: March 2003

Laboratory observations of the gravitational convection resulting from ’’point’’ source buoyancy (heat) injection in a stationary, unstratified fluid are described. These include various formation stages of buoyant vortex rings (thermals), the effect of rigid boundaries on a single thermal, and the motion of a sequence of two thermals, an infinite sequence of thermals, and the starting, steady, and interrupted plume. In addition, evaporative convection and boiling are briefly described. The stability of the starting plume cap and steady plume are compared by considering the effect of a perturbation of the bulk convective velocity. Perhaps, the main finding of the investigation is evidence that the starting plume cap can be treated as a thermal whose buoyancy increases at a constant rate.
Space–time variations of aberrated optical images through nearly homogeneous isotropic turbulence
Authors: Lin Shao-Chi , Lin Shih-Chun
Published: March 2003

The space–time variations of aberrated optical images formed by propagating an incoherent light source through a nearly homogeneous, isotropic, strongly heated air turbulence generated in the laboratory have been studied using high speed cinematography and one-dimensional sampling of the exposed film transparency. The relative amplitude of image intensity fluctuations at high spatial frequencies is found to increase with decreasing exposure time and also with increasing amplitude of the refractive index fluctuations at a fixed turbulence velocity and spectral distribution. In the range of refractive index fluctuation amplitude accessible to the wind tunnel, the photographic correlation time is barely resolvable at the highest framing rate. The observed time scale is found to be of the same order of magnitude as, but noticeable shorter than, the convective dephasing time deducible from wave propagation theories using previously measured properties of the turbulence field and an extended application of Taylor’s hypothesis.
Response of constant-current and constant-temperature anemometers to artificial turbulence
Authors: Helland K. N. , Van Atta C. W.
Published: March 2003

An artifical fluctuating velocity field similar to the velocity field in grid-generated turbulence has been produced with an electrodynamic shaker to test the ability of constant-current and constant-temperature anemometers to accurately measure higher-order moments of velocity. Evidence was found to suggest that a constant-temperature anemometer may be more suitable than a constant-current anemometer for measurements of odd-order moments of velocity. Both types of anemometers are in good agreement for even-order moments.
Acoustical shadowgraph
Authors: Ho Chih-ming , Kovasznay Leslie S. G.
Published: March 2003

After passing through a turbulent flow field, a coherent collimated acoustic beam will exhibit random fluctuations both in phase and in amplitude. The phenomenon is analogous to the optical shadowgraph and turbulent properties may be determined, e.g., the convection velocity and Taylor microscale. Naturally, the quite different wavelengths and propagation velocities impose different limitations in the two cases.
Fluid dynamics of relativistic blast waves
Authors: Blandford R. D. , McKee C. F.
Published: March 2003

A fluid dynamical treatment of an ultra-relativistic spherical blast wave enclosed by a strong shock is presented. A simple similarity solution describing the explosion of a fixed amount of energy in a uniform medium is derived, and this is generalized to include cases in which power is supplied by a central source and the density of the external medium varies with radius. Radiative shocks, in which the escaping photons carry away momentum as well as energy, are also discussed. Formulas that interpolate between the non- and ultra-relativistic limits are proposed.
Kinetic theory of magnetohydrodynamic Kelvin–Helmholtz instability
Authors: Ohsawa Yukiharu , Nosaki Kazuhiro , Hasegawa Akira
Published: March 2003

The magnetohydrodynamic Kelvin–Helmholtz instability in a low β collisionless plasma with smooth variations in plasma parameters across the boundary is studied. The surface Alfvén wave which is excited by the Kelvin–Helmholtz instability is shown to be mode-converted to the kinetic Alfvén wave (Alfvén wave with finite ion Larmor radius and electron inertia correction). Linear and nonlinear effects of this kinetic Alfvén wave on the Kelvin–Helmholtz instability are discussed. Because the resonant mode conversion enhances the wave amplitude, the nonlinear effect is manifested. In particular, the magnetic field pressure of the kinetic Alfvén wave exerts acceleration on the plasma at the higher density side toward the lower density side and may cause the Rayleigh–Taylor instability at the boundary layer.
Evolution of nonlinear magnetohydrodynamic mode structures
Authors: Jones R.
Published: March 2003

A pinch plasma column in a conducting chamber (a≃0.6) first developes the m=0 concave ’’sausage’’ instability predicted by elementary theory. The shoulder of the solitary wave thickens (a≃0.75), destabilizing a convex sideband constituting transition from solitary wave to periodic mode structure.
Comments on ’’Trapped particle instability’’
Authors: Jahns Gary L.
Published: March 2003

It is noted that a recent theoretical paper on the instability growth rate of electron plasma waves is in agreement with experiment. (AIP)
Preparation of a hot plasma for a high beta tokamak
Authors: Marshall T. C. , Lui H. C.
Published: March 2003

The possibility of generating a hot, high density plasma in the high beta tokamak configuration [β≲0.3, q (a) ≳1] using a turbulent poloidal current to heat the plasma and adjust the magnetic fields is studied. The physical principles underlying this method are discussed, and it is demonstrated, using a numerical simulation, how they may be implemented.
Theory of anomalous energy flux and deposition in a laser produced plasma
Authors: Manheimer Wallace M. , Colombant Denis , Flynn Robert
Published: March 2003

To study anomalous energy deposition and thermal flux in a collisionless plasma, the steady state fluid equations for the first four moments, density, velocity, temperature and heat flux are examined. The fluid system is coupled to the system of equations for the unstable waves, and the entire system is solved numerically for the case where the anomalous absorption mechanism is the Compton sidescatter instability. Absorption efficiency can be as high as twenty percent, albeit for very long plasma lengths.
Frequency shifts of large-amplitude plasma waves
Authors: Kim Hongjin
Published: March 2003

The time-asymptotic value of the frequency of a large-amplitude, electrostatic, electron plasma wave is studied over a wide range of amplitudes and phase velocities. When the amplitude is relatively small, the asymptotic frequency is less than the linear value, and the frequency shift increases with the square root of the amplitude. At larger amplitudes, the frequency shift increases more slowly, reaches a maximum, and then decreases. The asymptotic frequency finally becomes greater than the linear value.
Measurements of ion cyclotron instability characteristics in a mirror-confined plasma
Authors: Simonen T. C.
Published: March 2003

Frequency spectrum, wavelength, and phase velocity measurements of ion cyclotron oscillations in a mirror-confined plasma are described. The measured potential amplitudes reach, but do not exceed, the electron temperature. Other experiments in this device, not discussed here, reveal a wide range of plasma conditions where these oscillations are not detected.
Excitation of ion cyclotron harmonic waves with an ion beam of high perpendicular energy
Authors: Böhmer H.
Published: March 2003

The interaction of a rotating ion beam with a plasma in an external magnetic field is investigated. The ion beams have an energy component perpendicular to the magnetic field which is large compared with the energy of the background plasma ions. It is shown that the interacton leads to the excitation of ion cyclotron harmonic waves with a growth rate which can be as large as ωci/2. It is also shown, using a theoretical argument, that the growth rate for the excitation of ion cyclotron waves by a purely longitudinal beam is comparatively small.
Rigidly rotating ϑ-pinch equilibria
Authors: Kadish Abraham
Published: March 2003

Properties of rigidly rotating ϑ-pinch equilibria are derived assuming an idealized geometry. The effect of end-shorting is included. Bounds on the possible frequencies of rigid rotation are obtained. It is shown that stability calculations for rotating equilibria in existing straight ϑ-pinch devices must include the effect of Hall current if they are to be relevant.
Local anisotropy in heated and cooled turbulent boundary layers
Authors: Mestayer Patrice G. , Gibson Carl H. , Coantic Michel F. , Patel Anandkumar S.
Published: March 2003

Turbulent temperature as well as streamwise and vertical velocity components have been measured in relatively high Reynolds number (Rλ∼1100) heated and cooled boundary layers of the Air–Sea Interaction Simulation Tunnel at the Institut de Mécanique Statistique de la Turbulence. Spectra and probability distributions are compared with various consequences of Kolmogoroff’s assumption of local isotropy. The spectral functions present a first approximation agreement with the assumption. Yet, evidence of local anisotropy is shown by the values of the skewness of ∂T/∂x1 which are very different from zero, either positive or negative. Possible errors due to cold wire velocity sensitivity are examined. An apparent connection between small scale and large scale anisotropy is discussed.
Stability of modulated plane Couette flow
Authors: von Kerczek Christian
Published: March 2003

The stability of modulated plane Couette flow, in which the channel walls oscillate out-of-phase with each other and also have a mean relative velocity, is analyzed using linear theory. The flow is characterized by the Reynolds number R based on the distance between the channel walls and their mean relative velocity, the nondimensional frequency Ω of the wall oscillations, and the amplitude ratio Δ of the modulation velocity to mean velocity. Numerical solutions show that the decay rates of spatially periodic disturbances are decreased, at certain values of Ω and Δ, from those in unmodulated Couette flow. At fixed Ω the decay rates decrease for Δ<1. The maximum reduction of the decay rates occurs at certain subharmonic frequencies Ω for low R, but for larger R the maximum reduction of the decay rates occurs at higher harmonic tuning.
Thermal collection of aerosol particles on larger liquid droplets
Authors: Phillips Warren F.
Published: March 2003

A mathematical model is presented to predict the collision cross section for aerosol particles on much larger cooler liquid droplets, under the restriction of small Reynolds number. The model includes consideration of inertial, viscous, and thermal forces and is valid for particles of arbitrary Knudsen number. The results are presented in terms of a collection efficiency, defined as the ratio of the collision cross section to the cross section of the droplet.
Scaling and time inversion for the linear mono-energetic Boltzmann equation
Authors: İnönü E.
Published: March 2003

Some consequences are derived from the existence of a generalized scale transformation and a special 1+1 dimensional Lorentz type transformation for the linear, mono-energetic Boltzmann equation. The scale transformation gives a kind of renormalization group which leads to scaling properties for transport parameters such as the diffusion length. The Lorentz transformation is connected with time inversion and the averaging property of the collision integral in the Boltzmann equation. It provides the basis for a previously noted exact relation between the solutions of two homogeneous stationary transport equations which essentially differ by the existence of a backward delta distribution in the scattering kernel.
Diffusion coefficient scaling in a ε→1 plasma
Authors: Navratil G. A. , Post R. S.
Published: March 2003

When the low frequency plasma dielectric constant, ε, was reduced below two by decreasing the density in a collisionless hydrogen plasma with a purely poloidal magnetic field, a change in the scaling of the diffusion coefficient was observed in agreement with the ε→1 limit for vortex diffusion.
Stationary solutions of resonant nonlinear three wave interactions involving growing and damped waves
Authors: Anderson D. , Bondeson A.
Published: March 2003

An investigation is made of the existence and stability of stationary solutions of a system describing the resonant interaction between one linearly unstable pump wave and two damped decay waves. An explicit stability criterion is found as well as expressions for the steady state excitation levels.
Trapped particles and harmonic generation
Authors: Hobbs W. E. , DeGroot J. S.
Published: March 2003

A novel fluid-particle algorithm has been used to study the evolution of continuously generated ion acoustic waves numerically. The code permits the simultaneous analysis of the two nonlinearities involved: wave steepening or harmonic generation, a fluid effect; and wave damping and amplitude oscillations, a particle effect. For a temperature ratio 10<Te/Ti<30, the behavior of low amplitude long wavelength waves is well described by a Korteweg–de Vries equation augmented to include a time dependent damping term.
Nonlinear plasma waves excited near resonance
Authors: Cohen Bruce I. , Kaufman Allan N.
Published: March 2003

The nonlinear resonant response of a uniform plasma to an external plane-wave field is formulated in terms of the mismatch Δnl between the driving frequency and the time-dependent, complex, nonlinear normal mode frequency at the driving wavenumber. This formalism is applied to computer simulations of this process, yielding a deduced nonlinear frequency shift. The time dependence of the nonlinear phenomena, at frequency Δnl and at the bounce frequency of the resonant particles, is analyzed. The interdependence of the nonlinear features is described by means of energy and momentum relations.
Filamentary instabilities of rotating electron beams
Authors: Molvig Kim , Benford Gregory , Condit William C.
Published: March 2003

A rotating, relativistic electron beam is highly unstable if its net current is neutralized by a background plasma. The beam breaks up into a series of concentric doughnuts (ring tearing). The external magnetic field cannot inhibit this k=kz perturbation. Growth rates are calculated in the thin beam approximation. Numerical simulations bear out the linear growth times. Growth saturates when the relativistic electrons are sufficiently thermalized to satisfy a pinch equilibrium condition, and they oscillate in the magnetic wells formed by the field perturbations. An additional feature of the mode in this geometry is a radial electrostatic field which is necessary to drive the cross-field current.
Magnetic compression of charge-neutralized ion beams
Authors: Mosher David
Published: March 2003

Magnetic compression of a cold, charge-neutralized ion beam due to passage through a slowly varying magnetic-mirror field is studied. Conditions on the field and beam parameters are derived which permit the beam to be compressed to order-of-magnitude higher current densities while preserving the laminar nature of the flow. Effects associated with rotation, beam convergence, and velocity shear in the incident beam, and those due to self-fields are included in the analysis. Utilization of compressed beams extracted from reflex triodes and ion diodes for heating of thermonuclear plasmas is discussed.
Self-similar motion of laser fusion plasmas. Absorption in an unbounded plasma
Authors: Barrero A. , Sanmartín Juan R.
Published: March 2003

The one-dimensional motion generated in a cold, infinite, uniform plasma of density n0 by the absorption, in a certain plane, of a linear pulse of energy per unit time and area φ=φ0t/τ, 0<t⩽τ, is considered; the analysis allows for thermal conduction and viscosity of ions and electrons, their energy exchange, and an electron heat flux limiter. The resulting motion is self-similar and governed by a single nondimensional parameter α∝ (n02τ/φ0)2/3. Detailed asymptotic results are obtained for both α≪1 and α≫1; the general behavior of the solution for arbitrary α is discussed. The analysis can be extended to the case of a plasma initially occupying a half-space, and throws light on how to optimize the hydrodynamics of laser fusion plasmas. Known approximate results corresponding to motion of a plasma submitted to constant irradiation (φ) are recovered in the present work under appropriate limiting processes.
Coupling of lower-hybrid radiation at the plasma edge
Authors: Morales G. J.
Published: March 2003

The mode-conversion process encountered in the lower-hybrid heating scheme at the edge of the plasma, where the frequency of the external radiation is comparable to the local value of the electron plasma frequency is considered. The problem is formulated in terms of a modulational representation of the electric field that permits the description of the full space-time evolution of the process. The numerical study of the model equations confirms that a backward wave is excited at the edge and that it leads to a net energy flow directed toward the interior of the plasma. For small external power levels, the flow is characterized by the progagation of a leading-edge pulse whose arrival at a given spatial location marks the onset of the steady state. At large external power levels, the self-consistent modification of the density profile by the ponderomotive force is found to quench the mode-conversion process, thus causing a significant reduction in the amount of rf energy that can be coupled from the external source to the interior of the plasma.
Self-trapped laser beams in plasma
Authors: Lam Juan Francisco , Lippmann Bernard , Tappert Frederick
Published: March 2003

A moment theory for the nonlinear quasi-optical equation with saturation is applied to self-trapped laser beams in plasma for the ponderomotive nonlinearity. The results differ quantitatively from those obtained form the paraxial ray theory but do agree qualitatively in that there is a minimum beam radius of order c/ωp, a threshold ’’critical’’ power, no upper limit to the power that can be transmitted, and the self-trapped beams are stable.
Magnetic compression of ion rings
Authors: Weibel E. S.
Published: March 2003

The ion ring is described by the total number of ions, N; its major and minor radii, R and r; and the ion and electron ring currents Ii and Ie. It is assumed that the toroidal structure maintains itself and that the aspect ratio R/r is large. The evolution of these variables for a prescribed external field is then governed by a set of ordinary differential equations. If the ions alone produce the ring current and if the external field varies slowly, the equations can be integrated analytically to yield simple transcendental relations which determine the radius R, the aspect ratio R/r, the field reversal factor, and the kinetic and thermal energies as functions of the external field B. For a few cases these relations are given in graphic form for normalized, dimensionless variables. Both the field reversal factor and the aspect ratio increase during compression but, it is shown that these quantities approach constant values even when B increases without limit.
Two temperature effects in the laser heating of short plasma columns
Authors: Pietrzyk Z. A. , Scudder D. W. , Nelson G.
Published: March 2003

Plasmas heated uniformly by inverse bremsstrahlung are found to approach a constant electron-ion temperature difference as a function of only the laser wavelength and intensity. The characteristic time for that approach is found.
Nonlinear stability of a viscous film with respect to three-dimensional side-band disturbances
Authors: Krishna M. V. G. , Lin S. P.
Published: March 2003

The analysis of Lin is extended to investigate the nonlinear stability of a liquid film with respect to three-dimensional side-band disturbances. Near the upper branch of the linear-stability curve where the amplification ci is O (ε2), ε being proportional to the ratio of the amplitude to the film thickness, the nonlinear evolution of initially infinitestimal three-dimensional disturbances of a finite band width is shown to obey the nonlinear Schrödinger equation. Near the lower branch of the neutral curve, the nonlinear evolution is stronger. An equation is derived which describes this strong nonlinear development of relatively long three-dimensional waves. It is shown that the supercritically stable, finite amplitude, long monochromatic wave is stable to three-dimensional side-band disturbances under modal interaction if the bandwidth is less in magnitude than ε.
Effect of velocity profile upon the asymptotic contraction ratio of horizontal jets
Authors: Aiello Gerald A.
Published: March 2003

Jets of Newtonian liquid issuing from circular tubes at high Reynolds number are known to contract. The effect of the length of the tube or position of a constriction in the tube, upon the asymptotic contraction ratio is investigated.
Turbulent diffusion from a quasi-kinematical point of view
Authors: Salu Yehuda , Montgomery David
Published: March 2003

An estimate for the coefficient of self-diffusion can be derived whenever the Eulerian velocity spectrum is known. The result is more general than those previously obtained. A comparison is made with computed test-particle diffusion in an inviscid two-dimensional Navier–Stokes fluid.
Breakdown waves in argon and nitrogen
Authors: Scott Robert P. , Fowler R. G.
Published: March 2003

Extensive measurements are presented of initial electric breakdown wave speed in nitrogen and argon as a function of local electric field at the wavefront. The results are in basic agreement with the electron fluid dynamic theory of the phenomenon. The apparent gas density dependence over and above the E/p dependence predicted by theory, that has been found by other investigators, is confirmed and has been traced to observational limitations. The results indicate that photo-ionizing processes are not needed to explain the effects observed over most of the pressure range studied.
Experimental study of the post-implosion phase of a theta pinch
Authors: Commisso R. J. , Griem Hans R.
Published: March 2003

Using 90° Thomson scattering and magnetic probes, the time and space resolved electron temperature and density and magnetic field profiles were obtained for a small, 15 kJ theta pinch. At times after the implosion phase, t≳0.45–0.5 μsec, the plasma is characterized by a high β, 0.5≲β≲1.0, Te/Ti<1, and vEϑ≲ vi (where vEϑ is the E×B drift speed and vi is the ion thermal speed). The current sheath width, ΔLs, and plasma resistivity in the sheath ηeff, were obtained from the measured temperature, density, and field profiles at these late times. The results show that ΔLs≲ 3c/ωpi≫c/ωpe, ηeff≈100 η⊥, and the effective collision frequency, ν′eff, ω1h≲ν′eff≪ωpi, where η⊥ is the classical, Spitzer–Härm resistivity, and ω1h is the lower hybrid frequency. These results indicate that plasma turbulence is an important factor when considering heating and transport, even in the low drift velocity regime. Theoretical estimates based on the lower-hybrid-drift instability are in reasonably good agreement with the measured resistivity and ion heating rate.
Onset of natural convection in a box of water-saturated porous media with large temperature variation
Authors: Zebib A. , Kassoy D. R.
Published: March 2003

The onset of convection in a box of water-saturated porous media is considered. The effects of viscosity variation due to large temperature differences are included. Critical conditions for three box geometries which are representative of the problem are reported: a thin vertical slab, a flat box, and a tall slender column. The critical Rayleigh number is found to be substantially reduced from the value predicted by constant viscosity calculation. The horizontal planform of the preferred cellular motion is found to be independent of the temperature difference, being a function of the aspect ratios of the box only. The liquid motion is faster near the lower hot surface since the viscosity decreases with depth. The case of very tall slender columns led to the study of a fourth-order ordinary differential equation with a turning point at the boundary.
Theoretical derivation of laser induced plasma profiles
Authors: Lee K. , Forslund D. W. , Kindel J. M. , Lindman E. L.
Published: March 2003

The hydrodynamic modification of an expanding plasma by coherent radiation for polarization out of the plane of incidence is investigated theoretically in the absence of parametric and modulational instabilities. The pondermotive force of the incident wave modifies the plasma flow from that of a rarefaction wave into a subsonic flow in the overdense region with a transition to supersonic flow at less than critical density. A constraint imposed at the sonic transition point allows a complete determination of the equilibrium as observed in simulations. The theory predicts that the average flow in the subcritical density region scales as the square root of the incident power.
Geometric optics in space and time varying plasmas. II
Authors: Bernstein Ira B. , Baldwin D. E.
Published: March 2003

The theory of geometric optics in space and time varying plasmas is extended to include internal sources. The calculation proceeds by deriving the far field for a point monochromatic source. This is matched asymptotically to the geometric optics field, and the result used to generate the field due to a distributed source and the velocity space diffusion tensor for a fluctuating source. The general initial value problem for a source free plasma is solved in the geometric optics approximation, and the associated wave kinetic equation is derived.
Experimental determination of the diffusion rate in a mirror machine by means of trapped particle echoes
Authors: Chatelier M. , Samain A.
Published: March 2003

Echoes associated with the bounce motion of ions are observed in a mirror fusion device. The echoes are initiated by means of two sucessive perturbations of the confining magnetic field. The characteristic time for echo cancellation under diffusion in velocity space is measured. This time is somewhat less than the predicted classical time for Coulomb collisions. The discrepancy may be accounted for either by rf turbulent fields at ωci or by impurities both of which provide an enhancement of the diffusion coefficient.
Diffusion coefficient scaling in the Wisconsin levitated octupole
Authors: Drake J. R. , Greenwood J. R. , Navratil G. A. , Post R. S.
Published: March 2003

Measurements of cross-field diffusion were made in the levitated octupole using two independent methods. First, the magnitude and scaling of D⊥ with density, n, and field strength, B, were determined by direct measurements of the particle flux onto an internal ring surface. Second, the shape of the density profile and the time decay of the density at the separatrix were compared with solutions of a diffusion equation model. Both methods indicated that D⊥ was proportional to n−1/2 and independent of B. The plasma was created by gun injection into the purely poloidal magnetic field and the initial parameters were ne∼5 ×109 cm−3, Ti∼20 eV, Te∼5 eV. The observed diffusion scaling can be understood within the framework of vortex diffusion in a high-dielectric constant plasma if the spectrum of waves is enhanced in energy over thermal levels with an effective temperature, T*, of 104 eV.
Effects of curvature on unsteady free convection past a circular cylinder
Authors: Gupta A. S. , Pop I.
Published: March 2003

The curvature effects on unsteady free convection during the initial period of a suddenly heated circular cylinder immersed in a viscous liquid are studied. It is shown that the curvature leads to an increase in skin friction as well as heat transfer rate from the surface of the cylinder.
Final stage of a falling plate
Authors: Wang Chang-Yi
Published: March 2003

The final stage of one plate falling onto another, with a fluid in between, is found to be highly viscous. The mean height decays as t−1/2. For an elliptic plate, perturbation of the inclination decays as t−β/2, where 2⩽β⩽5. Thus, floating always occurs. PFLDA/JAN 7
Helicity conservation in a parametric scattering instability in a magnetic field
Authors: Tajima T.
Published: March 2003

It is found that the helicity of a photon in a plasma is conserved in the process of the parametric scattering instability along the magnetic field. This fact may be utilized to help diagnose the scattering instabilities by an intense laser in a plasma. The growth rates of these instabilities are given as modifications to those without a static magnetic field.
Coalescence instability of magnetic islands
Authors: Finn J. M. , Kaw P. K.
Published: March 2003

The stability of a periodic magnetic island structure using the ideal magnetohydrodynamic equations has been investigated. An instability is found which describes the tendency toward coalescence of parallel currents in the neighboring islands. It is expected that this instability will proceed at a fast hydromagnetic rate as long as the forces driving the instability can overcome the stabilizing forces due to the compression of the magnetic field between the islands. Beyond that phase, resistivity is expected to dominate the tendency toward island coalescence. Island coalescence of this kind can explain why in the observation of tearing mode instabilities in tokamaks, only the modes with minimum values of m and n are seen.
Quenching of the current-driven ion-wave instability in the trapped-electron regime in a toroidal plasma
Authors: Arunasalam V. , Okabayashi M. , Hawryluk R. J. , Suckewer S.
Published: March 2003

An analysis is presented of an experimental study of the scattering of microwaves by density fluctuations due to the current-driven ion acoustic wave instability in a toroidal plasma confinement device, the FM-1 spherator. It is found that the instability exists only in the collisional regime (where νef≳ωbe) and it is inhibited (i.e., quenched by two orders of magnitude) in the trapped-electron banana regime (where νef<ωbe). Further, it is found that for two different values of the ratio of the trapped to untrapped electron number density (i.e., for both 70% and 40% mirror trapping) this inhibition always starts to occur when νef ≈ωbe. Finally, it is shown that the nature of this inhibition is in general agreement with the recent theoretical notion that in the banana regime the entire Ohmic heating current is carried only by the freely circulating electrons.
Proper equation for angular momentum of trailing vortices
Authors: Uberoi Mahinder S.
Published: March 2003

It is shown that the invariably neglected radial convection and the associated axial convection of angular momentum are the most important terms in the equation for the change of angular momentum of a trailing vortex.
Thermal instability of stochastically modulated flows
Authors: Antar Basil N.
Published: March 2003

A study of the Bénard problem under the influence of a random gravitational field. It is shown that under these conditions the fluid may be slightly destabilized.
Turbulent fluctuations in the viscous wall region for Newtonian and drag reducing fluids
Authors: Hanratty Thomas J. , Chorn Larry G. , Hatziavramidis Dimitrios T.
Published: March 2003

An electrochemical method, which is the mass transfer analog of the hot film anemometer, has been developed to study flow fluctuations in the immediate vicinity of a wall. Electrodes mounted flush with the wall are the cathode of an electrolysis cell. At high enough voltages, the electric current is controlled by the rate of mass transfer to the cathode and the concentration of the reacting species at the surface is zero. These techniques allow the measurement of two components of the fluctuating velocity gradient at the wall. Results have been obtained on the structure of turbulence close to a wall and on the influence of drag reducing polymers on this structure.
Drag reduction experiments with polystyrene with some implications for the mean velocity profile
Authors: Gustavsson L. H.
Published: March 2003

Results from drag reduction experiments with polystyrene in toluene are presented. Measurements were performed with a polymer sample with a molecular weight of 2×106 at different concentrations (c). Assuming a new sublayer portion of the velocity profile of the same form as that proposed by Virk in his elastic sublayer model, it is found that the thickness of this layer is proportional to c0.76. The dependence of the onset wall shear stress on molecular weight was investigated for molecular weights of 2×106, 4.1×106, and 7.1×106. Because of degradation, the molecular weight range used was not sufficient to determine whether onset of drag reduction correlates with length or time scale.
Spatial structure of the viscous sublayer in drag-reducing channel flows
Authors: Oldaker D. K. , Tiederman W. G.
Published: March 2003

The effect of dilute, long-chain, polymer solutions upon the spatial structure of the viscous sublayer in fully developed, turbulent, two-dimensional channel flows was determined. The longitudinal, low-speed streaks characteristic of the sublayer were made visible by seeping dye through a transverse slot in one wall. Motion pictures yielded a detailed description of the streak formation process and revealed two major differences in the sublayer structure when drag reduction occurs. At a given nondimensional distance from the wall, the average transverse spacing of the streaks increases as the amount of drag reduction increases. The average streak spacing within the viscous sublayer is not a function of the distance from the wall in water flows and flows at lower levels of drag reduction. At high levels of drag reduction, the average spacing varies within the sublayer increasing as the wall is approached.
Polymer solution and fiber suspension rheology and their relationship to turbulent drag reduction
Authors: Metzner Arthur B.
Published: March 2003

It is shown that the rheological behavior of dilute polymeric solutions can be described, at least semi-quantitatively, by means of an equation containing a single viscosity coefficient and a single time constant. This expression and the Batchelor equation for suspension rheology are used to predict the major drag-reduction phenomena observed experimentally. It is shown that mixed polymer-fiber systems may be much more effective in their drag reduction capabilities than is either kind of additive when used alone. This latter prediction finds its qualitative verification in drag coefficients which are several-fold lower than those observed at the asymptote of ’’maximum’’ drag reduction in polymer solutions.
Turbulent boundary layer bearing silt in suspension
Authors: Nihoul Jacques C. J.
Published: March 2003

Essential characteristics of a turbulent boundary layer, bearing silt flocks in suspension, are described and interpreted with the help of a simple steady state model calibrated for the test region of the Math Modelsea project off the Northern Belgian Coast under near critical conditions : no net flux of particles through the bottom boundary. The model emphasizes the existence between the viscous sublayer and the classical Prandtl–Kármán logarithmic layer, of an ’’elastic sublayer’’ where gravity acting on the suspended load provides the necessary restoring force.