bluuebunny/crossref_metadata_2025 huggingface dataset:
Moving Striations in a Low-Pressure Argon Discharge
Authors: Swain David W. , Brown Sanborn C.
Published: February 2004

Measurements of externally excited moving striations in a low-pressure (0.005-0.03 Torr) argon discharge are presented. The wave propagates with little radial structure, and corresponds to the lowest radial mode. A theory which uses the Boltzmann equation for the electron motion, and fluid equations for the ions gives results in agreement with the experiment. Included in this model are the effects of ion inertia, and of depletion of the high-energy tail of the electron distribution function due to loss of electrons to the tube walls. Results indicate that both effects are important at low pressure. A simple model, which uses fluid equations for electrons and ions, shows the relationship between moving striations and ion acoustic waves.
Nonadiabatic Treatment of Finite Temperature Effects in a Magnetosonic Disturbance
Authors: Kinsinger R. E. , Auer P. L.
Published: February 2004

In a cold plasma, finite magnetosonic disturbances lead to the familiar steady-state solitary pulse of Adlam and Allen. Leading effects of finite plasma pressure on these disturbances are considered using both linear and nonlinear analyses. Earlier limitations imposed by adiabatic considerations have been removed. Within a regime of limited disturbance strength the inclusion of finite temperature leads to a singular perturbation scheme. The nature of pulse distortions due to these effects is discussed both analytically and numerically.
Breaking of Large Amplitude Plasma Oscillations
Authors: Coffey T. P.
Published: February 2004

The existence and structure of large amplitude, stationary, longitudinal plasma oscillations are studied for the case of a simple waterbag distribution of electrons and an immovable background of ions. The analysis employs the one-dimensional Vlasov equation for a plasma of infinite spatial extent. An expression for the maximum amplitude of the oscillations is derived. This maximum amplitude decreases monotonically as the ratio of the electron thermal velocity to the wave phase velocity increases. The structure of the oscillations is expressed analytically in terms of hyperelliptic integrals.
Absorption of Plasma Waves in Collisional Mirror Plasmas
Authors: Cordey J. G.
Published: February 2004

The equations for the equilibrium density and temperature profiles and the self-consistent electric potential are derived for mirror-contained collisional distributions. The equations are solved in two limiting cases: (a) The normal mirror reactor equilibrium where the electrons are contained by the electrostatic potential (eφ≫12mVe2); (b) electron heated plasmas where eφ≪12mVe2. The critical magnetic field scale length in the mirror throat necessary for plasma wave absorption in case (a) is found to be LT>ai[(Te/Ti)(Mi/Me)]1/2 exp (2eφ/3mVe2), and in case (b) LT>ai[(Te/Ti)(Mi/Me)]1/2(R0−1)−1/2. Neither of these conditions should pose a serious problem for the design of mirror reactors.
Collisionless Damping of a Large Amplitude Whistler Wave
Authors: Palmadesso P. , Schmidt G.
Published: February 2004

Nonlinear damping of electromagnetic waves propagating along a uniform magnetic field has been calculated by computing nonlinear trajectories for trapped and resonant untrapped particles while using linear theory to describe the rest. This procedure parallels O'Neil's calculation for electrostatic modes, and the results are qualitatively similar. After an initial linear damping, amplitude oscillations set in and the amplitude quickly approaches a finite constant value. The frequency of the amplitude oscillations is temperature dependent. Phase mixing results from the spread in both parallel and perpendicular velocities, giving rise to a more rapid approach to the asymptotic amplitude than for electrostatic modes.
Wave-Particle Energy Transfer and Wave-Amplitude Limiting Effects for a Flutelike High-Frequency Mode
Authors: Sigmar D. J. , Callen J. D.
Published: February 2004

The change of energy and phase of a particle gyrating in a homogeneous magnetic field and a perpendicularly propagating electrostatic wave is calculated analytically for resonant and nonresonant frequencies in the regime where the wavelength 2πk⊥−1 is of the order of the gyroradius a. This new result provides a quantitative basis for a simple wave-particle interaction model and resolves certain discrepancies between previously obtained results and numerical calculations of the ranges of k⊥a for which instability is possible. The long-time behavior of the particle motion is discussed using a stroboscopic orbit theory. Significant modifications of the particle orbits by a single wave, which necessitate the use of a “strong turbulence” theory, are shown to occur if eδψ/KT⊥∼(2π)1/2(K⊥a)−1/2(ω−lΩ) /ω, where eδψ/KT⊥ is the ratio of wave potential energy to perpendicular particle kinetic energy and l is the harmonic number.
Self-Consistent Calculation of an Explosive Instability
Authors: Hamasaki Seishi , Krall Nicholas A.
Published: February 2004

The effect of an explosive instability on the plasma distribution functions is calculated for a specific initial distribution. It is found that this change in the plasma distribution (a) limits the possible level of field energy and (b) greatly increases the time required for the field to approach this limit compared with the explosion time calculated by ignoring the change in the distribution.
Energy Constants Associated with the Nonlinear Theory of Electromagnetic Instabilities
Authors: Davidson Ronald C. , Hammer David A.
Published: February 2004

For configurations relevant to transverse electromagnetic instabilities, two independent energy constants are derived within the framework of the fully nonlinear Vlasov-Maxwell equations. It is assumed that the spatial variation in equilibrium and perturbed quantities is perpendicular to a uniform external magnetic field B0. The case B0 = 0 is not excluded, and the spatial variations may be one-or two-dimensional. The energy constants, which are applicable in both the stable and unstable regimes, exhibit precisely how the energy is partitioned between the fields and the individual components of plasma kinetic energy perpendicular and parallel to the propagation direction. The corresponding constants at the quasilinear level of description are also examined.
Two-Rotating-Stream Instability in a Non-Neutral Plasma
Authors: Bogema Bernard L. , Davidson Ronald C.
Published: February 2004

A new surface wave instability is derived for a magnetically confined pure electron gas column with density 34<2ωp02/Ω02<1. The equilibrium configuration consists of two superimposed electron beams aligned parallel to the confining magnetic field B0, and rotating with different azimuthal velocities about the axis of symmetry. The instability, which is of the two-stream form, results from the interaction of surface waves on each of the two superimposed beams.
Turbulence Induced Electron-Ion Cross Field Streaming Instability
Authors: Ott Edward
Published: February 2004

A current flowing across a uniform magnetic field is considered. Turbulently induced anomalous electron collisions allow electrons to diffuse across the magnetic field. The anomalous electron collisions can destabilize a negative energy ion wave and thus cause substantial ion heating. This process may be of interest in collisionless shocks and turbulent heating experiments.
Self-Consistent Weak Turbulence Theory of Resonant Three-Wave Processes
Authors: Caponi Maria E. , Davidson Ronald C.
Published: February 2004

Coupled kinetic equations for the density of plasmas Nkα(t) and spatially averaged distribution function fj(v, t) are presented in the electrostatic approximation for a weakly turbulent, unmagnetized plasma. Discrete particle collisions are neglected, and resonant three-wave processes (ωkα = ωk′β+ωk″γ, k = k′+k″) are assumed to provide the dominant collective interaction mechanism. By taking appropriate velocity moments of the kinetic equations for the particles, rate equations for the changes in (spatially averaged) mean velocity Vj(t) and kinetic energy relative to the mean Kj(t) for each plasma component are derived. The resulting rate equations describe the bulk features of the reaction of the particle distributions (i.e., acceleration and heating rates) to resonant three-wave interactions, and are applicable in both the nonlinearly stable and explosively unstable regimes. As an example corresponding to explosive instability, the self-consistent evolution of well-displaced counterstreaming ion beams in a hot electron background is examined. It is found that as the energy in the field fluctuations increases, the ion beams lose kinetic energy of mean motion, and the electrons and ions gain kinetic energy relative to the mean. This produces significant alterations in the background dielectric properties. Within the context of a simple model for the particle distributions, the region of k space for which the three-wave resonance conditions can be satisfied shrinks to zero volume, which quenches the nonlinear instability.
Instability in a Perpendicular Collisionless Shock Wave for Arbitrary Ion Temperatures
Authors: Lashmore-Davies C. N.
Published: February 2004

Two instability mechanisms which could occur under the conditions found in a perpendicular collisionless shock wave are considered. Both mechanisms are due to the negative energy character of the electron Bernstein waves propagating in the direction of the current flow in the shock. The effect of the voltage jump through the shock, and the density and magnetic field gradients at the shock front are considered. Both instability mechanisms occur only within a definite band of k values. The first mechanism is due to a resonance between the ion acoustic wave and one of the Bernstein harmonics and the second to resonant ions absorbing energy from a negative energy Bernstein mode. The first case requires Te≫Ti whereas the second case can occur for arbitrary values of the ratio Ti/Te, although the maximum effect occurs when this ratio is of order unity.
Instability of Whistler-Dominated Laminar Shocks
Authors: Sloan M. L.
Published: February 2004

A large-amplitude whistler is taken as a model of a collisionless laminar shock and shown to be unstable for perturbations propagating at angles to the whistler. The parameter space of instability is delineated and growth lengths as short as several whistler wavelengths are shown to exist for certain classes of perturbations. The deterioration of a laminar shock structure to a turbulent one, therefore, seems highly probable.
Weak Turbulent Shocks in Magnetized Plasmas
Authors: Pfirsch Dieter
Published: February 2004

An expression for the effective collision frequency is derived in terms of 〈E2〉/4πnκTe. This quantity is shown by similarity considerations to be the same in many different situations. From its value 0.07, and even from values down to (me | mi)2, it is concluded that present quasilinear theory and nonlinear wave equations cannot be used to explain the experimental results. The value 0.07 can be understood as the equipartition between the kinetic energy of the trapped particles and the energy in the electric fields.
Plasma Diffusion in Two Dimensions
Authors: Taylor J. B. , McNamara B.
Published: February 2004

Diffusion of plasma in two dimensions is studied in the guiding center model. It is shown that in this model diffusion always exhibits the anomalous 1/B variation with magnetic field. The velocity correlation function and the diffusion coefficient are calculated in detail using functional probabilities. In addition to the 1/B field dependence, the diffusion coefficient is unusual in that it depends weakly on the size of the system. These theoretical results are compared with those from computer experiments and their significance for real plasma is discussed.
Conductivity of a Magnetoplasma from a Convergent Kinetic Equation
Authors: Lai Hon-Ming , Pytte Agnar
Published: February 2004

The convergent classical kinetic equation of Kihara and Aono is used to calculate the collisional part of the ac electrical conductivity of a magnetoplasma to second order in the wave vector. Both dominant (Coulomb logarithm) and subdominant terms are evaluated exactly to first order in the plasma parameter.
Magnetohydrodynamic Stability of a Sharp Boundary Toroidal Plasma
Authors: Bateman Glenn
Published: February 2004

For a toroidally symmetric sharp boundary plasma, the magnetohydrodynamic stability analysis of Lüst, Suydam, Richtmyer, Rotenberg, and Levy is extended to equilibria with a small ratio of azimuthal to longitudinal magnetic fields and a large aspect ratio (α). These equilbria exist only when β<α(field ratio)2/2. In the limit of small β, the stability criteria are identical to those of a straight cylindrical plasma with length 2πα. This is true to leading order in 1/α in spite of mathematical complications arising from toroidal effects. However, as β is increased to its maximum value for field ratios of order 1/α, toroidal effects help stabilize long-wavelength kink modes. The limits considered apply to tokamaks.
Linear and Nonlinear Response of a Plasma Sheath to Radio Frequency Potentials
Authors: Cohen A. J. , Bekefi G.
Published: February 2004

The linear and nonlinear response of a plasma sheath is studied by subjecting it to rf potentials at frequencies corresponding to certain natural modes of oscillation of the sheath-plasma system. The sheath investigated is one that exists around a spherical dipole probe, immersed in an essentially collisionless uniform plasma and excited with sinusoidal signals at frequencies between 50 and 250 MHz. It is shown that the linear rf current-voltage characteristics agree with both the simple sheath model of Mayer and Harp and with the more refined model of DeAngelis and Baldwin. The nonlinear response (which is observed as the generation of higher harmonics) is quantitatively explained as being due to an oscillation of the sheath-plasma boundary induced by the applied rf field. This oscillation, in effect, acts as a time varying sheath capacitance.
Probability of Trapping-State Transitions in a Toroidal Device
Authors: Dobrott D. , Greene J. M.
Published: February 2004

The probabilities of trapping-state transitions of single-particle drift orbits are obtained from a theorem due to Kruskal that is an extension of the usual adiabatic theory. Because such transitions determine the total orbit, they are important to diffusion studies. To apply Kruskal's theorem, we use drift invariants obtained for a large-aspect-ratio stellarator (with l = 1 and l = 3 helical windings) by means of the stellarator expansion are used. In this model, toroidal and helical field modulations are comparable and the total rotational transform is finite. These three effects interact with the particle drift and, with transitions, determine the orbits in the large. When a small horizontal magnetic field is added, the bilateral symmetry of the flux surfaces—and hence the blocked and passing particles—are modified. Localized particles are affected through stochastic transitions, and particle orbits are globally ergodic.
Nonlinear Evolution of Collisionless Electron Beam-Plasma Systems
Authors: Thompson James R.
Published: February 2004

The problem of the nonlinear time evolution of a cold beam-plasma system, for which weak turbulence theory is well known to be inapplicable, is examined under the restrictions to one-dimensional electrostatic oscillations and for systems where the ratio of the beam density to the background plasma density is a small parameter. In this case, it may be shown that the electrostatic field undergoes rapid growth to a state of meta-equilibrium, followed by a slower time development. The mechanism for the nonlinear saturation of this growth is the trapping of beam electrons in the wave troughs of the electrostatic field. The existence of a unique single wave nonlinear Bernstein-Greene-Kruskal stationary state is established for this system and its properties (e.g., its energy content, wavelength, phase velocity) are evaluated. This Bernstein-Greene-Kruskal state is apparently approached closely by the system in the course of its time evolution. The predictions of the theories are compared with computer calculations and are found to be in good agreement.
Collisional Processes at Low Densities in Magnetic Mirror Systems
Authors: Futch A. H. , Damm C. C. , Foote J. H. , Gardner A. L. , Killeen J.
Published: February 2004

The study of collisional processes in plasmas produced by neutral-atom injection into magnetic mirror fields is described. The emphasis is on the many collisional processes which occur as the plasma density increases. Experimental and theoretical results are given. The experimental results are discussed first in terms of a simple model which assumes a Maxwellian electron distribution and a monoenergetic ion component of much higher energy. Analytical solutions may be obtained for this model. Also presented is a more complete theory employing two time-dependent Fokker-Planck equations to describe the behavior of the electron and ion distribution functions. Both models are in good agreement with measured values of the electron temperature and plasma potential. The equilibrium values of these two quantities are found to vary as the 35 power of the ratio of the plasma density to the background-gas density.
Trapping and Confinement of Non-Neutral Hot Electron Clouds in a Magnetic Mirror
Authors: Kapetanakos C. A. , Pechacek R. E. , Spero D. M. , Trivelpiece A. W.
Published: February 2004

Approximately 1011 electrons are trapped in a single stage 15-kG magnetic mirror and compressed to a volume of roughly 1 cm3. The trapped electron distribution function is peaked at 0.5 MeV and shows a spread of 0.45 MeV. A base pressure of 5×10−8 mm Hg insures negligible neutralization of the electron cloud for about 2 msec. The trapped electron configuration has a low-frequency (2MHz) precession motion about the magnetic axis.
Plasma Containment in Medium and High Shear Stellarators
Authors: Bolton R. A. E. , Hoffmann C. R. J. , Lees D. J. , Medley S. S. , Reynolds P. , Shatford P. A. , White B. M.
Published: February 2004

Plasma from hydrogen-occluded titanium sources is injected into the Proto-Cleo stellarator. This is an l = 3 configuration which has been used with helical windings having 7 or 13 field periods on a torus of minor radius 9-10 cm and a major radius of 40 cm. Plasma is trapped in fields of up to 5 kG. Measurements of density decay rate indicate that containment times are 10-12 times in excess of the Bohm containment time. Fluctuation levels measured at one point appear too low to account for the observed loss rate. The scaling of containment times with magnetic field, collision frequency, and rotational transform is compared with the prediction of equilibrium theory. It is found that although the scaling is approximately correct the value of containment time is up to a factor of 5 too low.
Sheet of Liquid Flowing Down a Wall
Authors: Bowman Thomas E.
Published: February 2004

The no-slip boundary condition results in a leading edge acceleration dependent on velocity distribution across the sheet, but not directly on viscosity. For a one-seventh power distribution, the acceleration of the leading edge must be in the range 0.765-0.875 g.
Effects of Couple Stresses in Fluids on the Creeping Flow Past a Sphere
Authors: Stokes Vijay Kumar
Published: February 2004

Expressions for the velocity and pressure distributions have been derived. It has been shown that, in the presence of couple stresses, the drag increases and that the effect of couple stresses is equivalent to an apparent increase in the viscosity.
Simple Approximate Analysis of the Anomalous Skin Effect in a Plasma
Authors: Lyman F. A. , Poon C. C.
Published: February 2004

The anomalous skin effect is analyzed by Lees' moment method. The results agree reasonably well with those of a previously published exact analysis for a semi-infinite plasma. New results are obtained for a cylindrical plasma.
Parametric Excitation of Ion-Acoustic Waves in a Bounded Plasma
Authors: Yadlowsky E. J. , Abrams R. H. , Ohe T. , Lashinsky H.
Published: February 2004

Parametric excitation of ion-acoustic waves is demonstrated in a Q machine by modulating the electron temperature. The results agree with an analysis based on the Mathieu equation.
Simplified Coupling Coefficient for Bernstein Modes
Authors: Wagers R. S.
Published: February 2004

A new form of the coupling coefficient for nonlinearly interacting Bernstein modes is presented. It has been generalized to include noncollinearly propagating modes and has been reduced to a simple form suitable for computation.
Electromagnetic Modifications of the Electrostatic Dispersion Relation
Authors: Callen J. D. , Guest G. E.
Published: February 2004

Longitudinal plasma waves associated with ion gyroharmonic instabilities couple strongly to transverse waves if ωpe/ck≳1. A suitably modified electrostatic dispersion relation is derived and applied to two electrostatic modes arising in mirror-confined plasmas.
Effect of Transverse Curvature on the Singularity at Separation for a Laminar Boundary Layer
Authors: Buckmaster John
Published: February 2004

The flow close to separation is studied using a coordinate expansion in order to determine the effects of transverse curvature on the structure. The simplest expansion using the Goldstein variables leads to a Stewartson alternative in which either the curvature must vanish or the skin friction must be regular. This difficulty is overcome by the addition of logarithmic terms when the curvature is negative, but is unresolved for positive curvature. The analogy with the flat-plate compressible boundary layer is pointed out (with heat transfer analogous to curvature), and it is concluded that it is misleading to suggest that the difficulties for a hot wall can be resolved by setting the heat transfer equal to zero.
Enhancement of Gas Transfer by Interfacial Ripples
Authors: MacIntyre Ferren
Published: February 2004

When molecular diffusion through a viscous surface boundary layer is the limiting factor in interfacial gas exchange, theory and experiment show that the periodic surface dilation produced by capillary ripples will decrease the average thickness of the boundary layer and increase gas transfer. This increase is independent of boundary layer thinning by turbulence. For surface dilations typical of the sea surface with fully developed capillary ripples, the CO2 transfer may be 3.5 times the flat-surface value.
Stability of Plane Couette-Poiseuille Flow with Uniform Crossflow
Authors: Hains F. D.
Published: February 2004

The stability of plane Couette-Poiseuille flow between porous walls is considered. Injection of fluid through one wall and suction through the opposite wall leads to a nonparallel flow that satisfied the Navier-Stokes equations. Neutral stability diagrams are presented for Poiseuille flow and for Couette-Poiseuille flow. In both cases crossflow produces a significant increase in the critical Reynolds number.
Turbulent Hydrodynamic Line-Stretching: The Random Walk Limit
Authors: Cocke W. J.
Published: February 2004

Recent results on the central limit theorem for sums of dependent random variables are used to evaluate the asymptotic rates of turbulent stretching of material line and surface elements in the limit t→∞. The turbulence is assumed to be incompressible and statistically stationary and isotropic, and it is shown that the expectation value of the logarithmic rate of increase of the magnitude of the material elements vanishes at t→∞. Possibilities for application to weak-field magnetohydrodynamic turbulence are noted and some associated difficulties are discussed.
Lognormality of Small-Scale Structure of Turbulence
Authors: Chen W. Y.
Published: February 2004

Velocity derivatives obtained in atmospheric boundary layers over the open ocean are analyzed for correspondence to the lognormal distribution. The squared velocity derivative is found to be not lognormal, but its average over an interval within inertial subrange demonstrates excellent lognormality.
Motion of an Isolated Buoyant Thermal
Authors: Wang C. P.
Published: February 2004

Using a simplified model, the governing equations of the motion of an isolated buoyant thermal are formulated. The general solutions which include both large and small density differences and both the initial accelerating motion and the final decelerating motion, are obtained. The asymptotic behavior for large time is given and compared with experimental results. The effect of various parameters, such as density difference, mass entrainment, and effective drag coefficient are also discussed.
Wakes of Freely Falling Water Drops
Authors: List R. , Hand M. J.
Published: February 2004

The study of 62 wakes of 2.9-mm water drops falling through a cloud of small droplets revealed approximately cylindrical disturbances 5 to 10 drop diameters wide and 1340 diameters long for drop speeds equivalent to 78% of their terminal speed of 8.0 msec−1. The wake disturbances lasted for about 9/16 sec at one place in space, in which time the air was displaced by about 0.5 cm in the direction of the moving drop. An evaluation of the flow pattern, based on velocity measurements showed that the averaged kinetic energy in the wake is extremely small and the work done by the drop on a layer of air with a cross section of the wake must be converted very rapidly into heat. The pattern and the decay of the measurable or gross vorticity, as well as the velocity gradients across the plane of observation are also given. The wake properties seem to agree with those of turbulent wakes; however, the development of a ringlike velocity maximum around the axis of symmetry is also possible. For rain it is concluded that the intensity of the small scale turbulence created by the falling drops is so small that, in general, no interactions are to be expected to affect their heat and mass transfer, their drag, or the collection of droplets by drops falling in the wake of another drop, at least for the wake ranges for which the results are applicable.
Thermal Creep Slip with Arbitrary Accommodation at the Surface
Authors: Loyalka S. K. , Cipolla J. W.
Published: February 2004

An accurate theoretical analysis of the thermal creep slip velocity q̃T,asy for arbitrary α, the fraction of the molecules that are diffusely reflected from the surface, is carried out by applying the method of elementary solutions to the Bhatnagar-Gross-Krook model. Specifically, q̃T,asy is expressed in terms of two power series that, respectively, converge rapidly for α→1 and α→0, and it is found that q̃T,asy depends only slightly on α. Some alternate forms of boundary conditions are also discussed, and it is shown that the thermal creep slip velocity does not depend on αp, the momentum accommodation coefficient. These results confirm the essential accuracy of a very general variational expression given recently by one of the authors. Further, some consequences of the present results in the analysis of the thermal transpiration experiments are also discussed.
Spectroscopic Investigation of Combustion-Driven Spherical Implosion Waves
Authors: Roberts D. E. , Glass I. I.
Published: February 2004

A spectroscopic study was made of imploding shock waves in a hemispherical chamber filled with an O2-H2-He mixture at high pressure (∼10 atm). The waves were generated by reflecting an exploding detonation wave off the periphery of the chamber. The strong focusing of classical imploding shock waves was verified quantitatively, but the temperature of the shock-heated gas near the origin of the chamber was found to be limited to rather low values (∼5000°K). The results generally agree well with calculations employing the artificial-viscosity technique if it is assumed that the minimum shock radius near the origin is limited to the extent of the experimentally observed luminosity region.
Charge Transport by Self-Generated Turbulence in Insulating Liquids Submitted to Unipolar Injection
Authors: Hopfinger E. J. , Gosse J. P.
Published: February 2004

During unipolar (or ambipolar) injection of charge carriers into an insulating liquid, a liquid motion is generally observed. This motion, generated by the charge carriers, is thought to be responsible for the high carrier mobility which is about 2×10−3 cm3 V−1 sec−1 in nitrobenzene, i.e., an order of magnitude higher than the true mobility value (1.9×10−4 cm2 V−1 sec−1). Recent schlieren photographs strongly support the assumption that when the applied voltage is high enough, for nitrobenzene of the order 103-104 V, the “self-generated” motion is turbulent shortly after the voltage is applied. This turbulence invades the undisturbed fluid at practically constant velocity. Neglecting the true carrier mobility and making certain assumptions concerning the turbulent structure, the charge transport and consequently the entrainment can be described to a good approximation by a Lagrangian diffusion process and the charge distribution is governed by a diffusion equation. The latter remains valid during the steady state. The experimental results are found to be in very satisfactory agreement with the theoretical analysis. It is also shown that the turbulent energy is produced by E¯ uq¯, producing kinetic as well as electric energy, the latter being negligible. The turbulence is very intense, u′≈102 cm sec−1, and its scale is only a fraction of the electrode distance. The corresponding turbulence Reynolds number is of the order of Rel≈6×102.
Simple Model for an Ion-Exosphere in an Open Magnetic Field
Authors: Lemaire J. , Scherer M.
Published: February 2004

A simple model for an ion-exosphere with an open magnetic field is set up. The ions move under the influence of (1) the gravitational field, (2) the monotonic decreasing static magnetic field, and (3) the electrostatic potential due to a small charge separation. Neglecting collisions and particle drift across the magnetic field lines the particles can be classified into four classes: ballistic, escaping, trapped, and incoming particles. For each class the number density, the particle flux, the momentum fluxes, and the energy flux are calculated as a function of the electrostatic potential. Finally, it is shown how this potential can easily be computed by considering two basic physical conditions: (1) the quasineutrality, and (2) the zero current condition.
Structure of the Plasma Boundary Layer at a Conducting Sphere
Authors: Ekbote D. D. , Schott L.
Published: February 2004

The spatial distributions of the ion density and the space potential in the boundary layer at a conducting sphere of 2.5-cm radius are measured with a small movable cylindrical probe. The measured density distributions are in good agreement with the theory of Lam in the collisionless case and with the diffusion model in the collision-dominated limit. The existence of a near-Boltzmann distribution of the electrons in the collisionless quasineutral layer is demonstrated by a plot of the logarithm of the ion density versus space potential. The electron temperature obtained from this plot, from the floating potential of the sphere, and from the characteristic of the small probe differ from each other by at most 12%. A new method to detect deviations from the Boltzmann distribution in the transitional layer is tested. The ion densities measured by using the sphere as a probe agree within 13% with those obtained with the small cylindrical probe in the collisionless case.
Deviation from a Maxwellian Velocity Distribution in Low-Density Plasmas
Authors: Gould R. J. , Thakur R. K.
Published: February 2004

The problem of the electron velocity distribution for a plasma in a steady state is investigated. The principal deviation from Maxwellian results from inelastic collisions of electrons with atoms and ions wherein the excited level depopulates predominantly by radiating a photon. A simple analytic solution to the Boltzmann equation is obtained for the case where we are in the tail of the distribution (this allows simplification of the Fokker-Planck Coulomb operator) and for the case where the inelastic cross section varies as 1/E, where E is the kinetic energy of the incident electron. A generalization is made to the case where many atomic levels can be excited, if the cross section for each varies as 1/E. In applying the results to astrophysical plasmas it is found that the non-Maxwellian corrections are always small. For example, the relative corrections to inelastic collision rates for the primordial hydrogen-helium plasma are less than 10−3. In gaseous nebulae the typical corrections are even smaller (about 10−6). For some cosmic x-ray sources and for stellar coronae the corrections are also of this order (10−6).
Langmuir Probe and Microwave Measurements of the Properties of Streaming Plasmas Generated by Focused Laser Pulses
Authors: Koopman David W.
Published: February 2004

Measurements have been made on density, velocity, electron temperature, and ion temperature in streaming plasmas generated by allowing the high-density, high-temperature material formed by a Q-switched laser pulse focused on a solid target to expand into vacuum. Langmuir probe and Lecher-wire microwave systems for this application have been developed and their reliable operation has been verified. Typical conditions at distances 15-50 cm from the target are found to be: Te≃1-12 eV, ne = ΣniZi≃1010-1012/cm3, Ti < 10 eV, peak flow velocity U ≃ 10-20 cm/μsec, the exact values depending on target materials, laser power, and position. Photoionization and charge exchange in a low-density background gas have been observed.
Finite-β Stabilization of the Collisionless Trapped Particle Instability
Authors: Rosenbluth M. , Sloan M. L.
Published: February 2004

A particularly dangerous mode of instability in confined plasmas is the trapped particle mode which is highly insensitive to magnetic shear and average favorable curvature. The study of the collisionless trapped particle mode is extended to include the effects of finite β = 8πnT / B2. It is found that the “magnetic well” dug by plasma diamagnetism is highly effective in stabilizing the collisionless trapped-particle mode. It is found that for magnetohydrodynamically stable systems the collisionless trapped-particle mode becomes stable when the diamagnetic well is sufficient to give all particles average favorable drift. It would appear from this result that, in general, the diamagnetic well should be an effective stabilizing agent against all nonflutelike microinstabilities.
Plasmons and the Linear and Nonlinear Two-Stream Instabilities
Authors: Fukai J. , Harris E. G.
Published: February 2004

For an infinite cold plasma consisting of several species in relative motion a Hamiltonian is derived in terms of creation and annihilation operators for plasmons. From a quantum-mechanical point of view the two-stream instability may be described as the creation of pairs of plasmons, one with positive and the other with negative energy. The Hamiltonian derived includes both the linear and nonlinear interactions. In order to discuss the nonlinear instability a canonical (Bogolyubov) transformation is performed to diagonalize the linear part of the Hamiltonian. It is shown that the nonlinear interactions may cause the explosive instabilities, even though the plasma is stable for the linear instability.
Dispersal of a Localized Density Fluctuation in a Uniform Magnetoplasma
Authors: Greenwald R. A. , Shea D. W. J.
Published: February 2004

The dispersal of a localized density fluctuation in a uniform magnetoplasma is studied using the macroscopic fluid equations. Expressions are obtained for the Fourier-Laplace transforms of the perturbed density, potential field, and flux distributions. These expressions are inverse transformed, using a combination of analytic and numeric techniques. The resulting curves which display the real space and time development of the system are then compared with magnetic-field-free ambipolar diffusion. One finds that diffusion along the magnetic field, θ = 0°, is much more rapid than ambipolar diffusion, that when the ion motion is not affected by the magnetic field, diffusion at θ = 90° proceeds at approximately the ambipolar rate, and that the electron and ion fluxes are not ambipolar.
Noise Broadening of Trapped-Particle Echoes
Authors: Williams E. A. , Oberman C.
Published: February 2004

The effects of noise (caused by collisions or weak turbulence) on the production of echoes in magnetically trapped particles is discussed using a propagator formalism. Provided the effective collision time is long compared with a typical bounce time, the echoes are found to be broadened but not destroyed.
Radiation Temperature of a Bounded Nonthermal Plasma. I. Theory
Authors: Wright B. L. , Bekefi G.
Published: February 2004

The departure of the radiation temperature Tr from the kinetic temperature of the emitting electrons can be used as a measure of the distribution of electron velocities f(v). This relationship between Tr and f(v), using the theory of fluctuations of random currents is explored theoretically. The method is particularly suited to radiation from relatively small, bounded plasmas of laboratory size where the traditional theory of radiative transport of wave packets becomes questionable.
Radiation Temperature of a Bounded Nonthermal Plasma. II. Experiment
Authors: Wright B. L. , Bekefi G.
Published: February 2004

Time-resolved measurements of the radiation temperature Tr in a weakly ionized plasma column subjected to an axial magnetic field are reported. The experiments are made at frequencies near the electron cyclotron frequency during and in the early afterglow of a pulsed argon discharge. The observed departures of Tr from the kinetic temperature are used in the determination of the distribution of electron velocities f(v). A comparison of the steady-state distribution with computations based on an appropriate kinetic model shows good agreement and establishes the validity of this experimental method of measuring f(v). In addition, the measured time dependence of f(v) allows the inference of the rates of electron-atom cooling and electron-electron relaxation, during the early afterglow of the pulsed discharge.
Simulation of Pseudowaves and of Plasma Sheath Formation about a Grid by Computer Solution of the Ion Vlasov Equation
Authors: Estabrook K. , Widner M. , Alexeff I. , Jones W. D.
Published: February 2004

The time evolution of an ion distribution function is computed in the sheath about a negatively pulsed transparent grid. The algorithm details the ion flow in the potential well when the pulse is on and the burst of ions ejected after the pulse potential is removed.
Influence of Fluctuations in Local Dissipation Rates on Turbulent Scalar Characteristics in the Inertial Subrange
Authors: Van Atta C. W.
Published: February 2004

Inertial subrange expressions for the structure functions of a turbulent passive scalar field are derived taking into account the fluctuations in dissipation rates of scalar and velocity fields.
Very Rapid, Isothermal, Two-Species Reactions in Final Period Turbulence
Authors: O'Brien E. E.
Published: February 2004

Asymptotic decay rates for mean concentration and root-mean-square fluctuations in concentration are derived for a decaying second-order, isothermal two-species reaction in final period turbulence.
Solution for Turbulent Correlations Using Multipoint Distribution Functions
Authors: Fox R. L.
Published: February 2004

A solution for the time evolution of the velocity correlation function in homogeneous isotropic turbulence is obtained using multipoint distribution functions. The dependence of the velocity correlation function on the fluid parameters is calculated for a simple initial condition.
Straight-Line Orbit Approximation in Periodic Systems
Authors: Berk H. L. , Pearlstein L. D.
Published: February 2004

It is shown that the plasma response function, in a system whose particle orbits are periodic, is accurately approximated by the infinite medium response function for real frequencies when the bounce time is sufficiently spread.
Boundary Layer Induced by a Potential Vortex
Authors: Burggraf Odus R. , Stewartson Keith , Belcher Ralph
Published: February 2004

A numerical computation of the laminar boundary layer on a fixed circular disk of radius a whose axis is concentric with that of a vortex having circulation Γ is described. The computations were started at the edge of the disk and continued inward toward the axis until the properties of the terminal flow became evident. A two-layer asymptotic expansion was formulated for the solution of the boundary-layer equations near the axis, and the terminal-flow properties revealed by the analysis are shown to be in excellent agreement with the numerical results. The structure of the terminal boundary layer consists of an inner layer next to the surface with thickness O[(ν/Γ)1/2r] in which the flow is primarily radial, and an outer layer with thickness O[(ν/Γ)1/2a] of predominantly inviscid nature in which the flow recovers to the external potential vortex. The mass flux in the outer layer does not vanish as r→0, indicating that the boundary layer must erupt from the surface at r=0in the manner envisioned by Moore.
Effect of Condensation in the Boundary Layer on Mass Transfer from a Rotating Disk. II. Experimental
Authors: Omberg R. P. , Olander D. R.
Published: February 2004

The vaporization rate of chromium metal into an atmosphere of cold helium gas was measured in a rotating disk flow geometry. Gas conditions were 1 atm pressure and temperatures of 300 and 600°K. The disk temperature was varied from 1630 to 1760°K. At the lowest disk temperatures, the rates appeared to approach the values predicted by isothermal convection-diffusion theory. At higher temperatures, the measured rates were considerably greater than those predicted by simple mass transfer considerations and were reduced when the helium temperature was increased. These observations suggest that condensation of metal vapor in the boundary layer was accelerating the vaporization rate. At the highest disk temperature investigated, the observed rate was a factor of 10 greater than the rate predicted from isothermal rotating disk theory. A difference of this magnitude is expected for kinetically unhindered condensation in the boundary layer (i.e., the metal partial pressure follows the temperature profile according to the vapor pressure curve).
A Fundamental Derivative in Gasdynamics
Authors: Thompson Philip A.
Published: February 2004

The quantity which is here called the fundamental derivative has been defined as the nondimensional form Γ≡12ρ3c4(∂2Υ/∂P2)s. The relation of Γ to other thermodynamic variables is discussed. It is already known that the existence of conventional compression shocks requires Γ>0. It is shown that other dynamic behavior of compressible fluids is fixed by the sign of Γ. Particular emphasis is given to phenomena corresponding to negative Γ. These phenomena include the area variation of a transonic passage, the form of a Prandtl-Meyer wave, the behavior of adiabatic flow with friction, and nonlinear wave propagation. Formulas and numerical values are given for Γ in various substances.
Kinetic Theory of an Optically Pumped Gas
Authors: Cipolla J. W. , Morse T. F.
Published: February 2004

Kinetic equations are presented which successfully incorporate the effects of atomic line radiation on the spatial and time evolution of the velocity distributions of a radiating gas. The equations are applied to the steady-state optical excitation of a free molecular slab of two level atoms in a model of the optically excited laser amplifier. Iteration on the coupled kinetic equations and equation of radiative transfer is used to construct a first approximation to the velocity distributions in the optically thick limit. This corresponds to integral iteration in kinetic theory. By performing certain quadratures over the first-order velocity distribution for particles in the excited or upper lasing state, it is possible to calculate the inversion density as well as all the radiative properties of the gas. An asymptotic theory is given for the density in the limit of weak external radiation and for two wall reflectance conditions. Also an interesting bimodal behavior of the emission profile is seen. The lineshape of the emitted radiation is constant in the line core for about one Doppler width, then it reaches a sharp peak before a Gaussian decay in the far wings.
A Thermodynamical Theory of Fluid Suspensions
Authors: Kline Kenneth A. , Allen Stuart J.
Published: February 2004

A development is given of a thermodynamical theory of fluid suspensions of deformable particles. Assuming particles are spherical when undeformed and become ellipsoidal when sheared, linear constitutive equations are extracted from the general theory and restrictions on the material coefficients which appear in these linear constitutive equations are deduced by thermodynamical considerations. Applying this linear theory to steady shear flow of solutions for which particle interactions are negligible, it is found, solely by use of the thermodynamical restrictions on material coefficients, that the theory qualitatively predicts observed behavior in steady shear flow of dilute solutions of random coiling macromolecules, such as solutions of polyisobutylene. In particular, the resulting expressions for apparent viscosity t21/K and normal stress difference t11 − t22 are exactly those known to characterize experimental results for K≤103sec−1. For low shear rates K, the predicted value for the second normal stress difference t22−t33 in terms of t11−t22 is exactly as measured experimentally. It is suggested that consideration of particle interaction effects will be necessary to accurately predict t22−t33 at higher shear rates.
Calculated Flow and Energy Distribution Following Underwater Detonation of a Pentolite Sphere
Authors: Sternberg H. M. , Walker W. A.
Published: February 2004

The artificial viscosity method is used to calculate the flow following the detonation of a centrally initiated pentolite sphere in fresh water at sea level, up to the time the main shock in the water is 100 charge radii from the center. Pressure, particle velocity, and temperature versus distance at various times are obtained; also peak pressures, time constants, and pressure versus time at fixed positions. Partial steam formation in the water close to the gas bubble is shown to be possible but unimportant at the distances covered. The partition and distribution of kinetic and internal energies in the water and the gas sphere, and the energy dissipated by shock heating, are found. The calculated dissipated energy is 33% of the total energy released in the detonation when the shock front is 10 charge radii from the center, and 40.5% when the shock front is 100 charge radii from the center.