bluuebunny/crossref_metadata_2025 huggingface dataset:
Curved Turbulent Mixing Layer
Authors: Margolis D.P. , Lumley J. L.
Published: December 2004

The radial pressure gradient in combination with either a positive or negative radial gradient of angular momentum makes possible the realization of two dynamically different flows. When the angular momentum gradient is positive, the flow is stable; when it is negative, the flow is unstable. Mean velocity, turbulence intensity, and shear correlation measurements were made in both cases. The one-dimensional streamwise energy spectra of the lateral and transverse turbulent velocities were also measured. The turbulent energy balances for both cases reveal that the dominant terms are the production of turbulent energy and its spacial transport by the turbulent velocities. The effect of the centripetal acceleration field is to inhibit the spectral transport of turbulent energy in the unstable case. The dissipation rates are an order of magnitude greater than the dissipation rates in the unstable case. The one-dimensional streamwise energy spectra display a (-5/3) dependence almost two decades long in the unstable case and one decade long in the stable case. Furthermore, the spectra reveal that the constant in the Kolmogorov form of the spectrum is not universal and that it depends on the ratio of the rate of production to the rate of dissipation of turbulent energy. In this study this ratio ranged from 1.5–27, and the values of the Kolmogorov constant varied from 0.4–2.4.
Statistical Properties of the Turbulent Wake behind Hypervelocity Spheres
Authors: Clay W. G. , Herrmann J. , Slattery R. E.
Published: December 2004

Autocorrelation functions and spectra of the gas density fluctuations have been obtained from microdensitometer tracings of schlieren photographs of the turbulent wakes behind hypervelocity spheres ⅜ in. in diameter traveling with a velocity of about 8 kft/sec (≅ Mach 8). Schlieren photographs of wakes 100 to 10 000 body diameters behind the spheres at pressures ranging from 10 to 160 mmHg were taken and densitometer tracings from these films were analyzed. The relationship between the statistical properties of the film density and that of the gas density has been derived. The results show that the normalized autocorrelation functions and spectra of the turbulent wake of spheres are independent both of pressure and of position behind the body. Further, to the accuracy of the measurements, the turbulent wake appears to be statistically isotropic and homogeneous, and the three-dimensional spectrum of the gas density falls off as the 5.5 power of the wavenumber.
Optimum Generation of Ion-Cyclotron Waves in a Cylindrical Two-Ion Plasma
Authors: Chung K. , Rothman M. A. , Sinclair R. M. , Yoshikawa S.
Published: December 2004

Ion-cyclotron waves have been generated in a hydrogen-deuterium plasma (previously formed by Ohmic heating) contained in the Model C stellarator, using an induction coil of finite wavelength. The conditions for optimum generation of waves, found experimentally, were in good agreement with theoretical predictions.
Oscillations of a Plasma Column
Authors: Landau R. W. , Schmidt G.
Published: December 2004

Oscillations in plasma columns exhibit many interesting features not found in infinite uniform plasmas. A study is made on the effects due to retardation of the electromagnetic field which gives rise to a curious behavior of the static mode for long wavelengths and a radiating branch of electrostatic plasma oscillations. The principal aim of this study is to gain an understanding of the physical mechanisms underlying these phenomena. The transition from the finite column to the infinite uniform plasma case is also discussed.
Heat Conduction Experiments in Rarefied Gases between Concentric Cylinders
Authors: Dybbs Alexander , Springer George S.
Published: December 2004

Experiments were performed to measure heat conduction through rarefied gases at rest contained between two concentric cylinders. The test gases used were helium, neon, and argon. The pressure was varied over a range that included free molecule, transition, and temperature jump conditions. The experimentally obtained heat conduction data were used to evaluate the results of the moment method proposed by Lees and Liu. Agreement between this method and the data was found to be within ±2.5% over the entire Knudsen number range covered in the experiments (0.02 < Kn < 50), assuming that the thermal accommodation coefficient at the inner cylinder remained constant over this range at its free molecule value. A comparison was also made between the data and the results of the temperature jump approximation, indicating that the heat conduction can be calculated by the temperature jump approximation within about 1% at Knudsen numbers less than ∼0.02.
Flow Modes in Electric Shock Tubes
Authors: Barach J. P. , Vermillion R. E.
Published: December 2004

Electric shock tube flow has been studied at low pressures, after cessation of the powering discharge, in a 4-in.-diam tube. Three modes of plasma flow down the tube are found: (1) a diffusion type of flow below 10 μ, (2) flow with a single luminous front, and (3) flow with a separated shock front and contact surface. The criterion for the appearance of flow with a separated shock wave appears to be Taylor stability of the contact surface. Since electric shock tube flow is always decelerating strongly, this occurs when the driver density becomes less than that of the shocked gas layer immediately ahead. As a result separated shock flow is more likely the weaker the powering discharge, the higher the ambient pressure, the less the erosion of the electrodes, and the further downstream one observes. These considerations are also found to be consistent with the observed behavior of a medium and a high-pressure shock tube.
Traveling Wave Interaction and Wave Growth in a Dense Plasma
Authors: Messerle Hugo K.
Published: December 2004

Traveling wave interaction for one-dimensional flow is analyzed using small-perturbation theory. This permits a separate treatment of the two major fluctuations, i.e., the harmonic response due to the imposed magnetic wave and the transient response to natural disturbances in the presence of the magnetic wave.

No interaction takes place when both wave and plasma velocity are equal. The interaction increases as the relative velocity increases, and there are definite resonance peaks for the harmonic fluctuations at the relative sound speeds. The resonance is normally damped; however, the perturbation of the electrical parameters such as the electrical conductivity can have a destabilizing effect.

Transient disturbances which travel at the relative magneto acoustic speed are normally damped when moving along the wave. However, as the relative speed is raised, these disturbance waves can be made to grow. Stability criteria are established, defining regions of instability for the relative speed. It is shown that the electrical conductivity and the Hall factor are primarily responsible for instability due to their critical dependence on the fluctuating thermodynamic quantities in magnetohydrodynamic converters.
Unstable Precession under the Influence of Drag Forces
Authors: Furth H. P.
Published: December 2004

Coherent precession of plasma particles in ▿B is generally subjected to drag forces associated with viscous or resistive energy dissipation in background plasma. The dissipated energy can be supplied by continual displacement of the processing entity along — ▿B. Precession in mirror machines therefore tends to be unstable against drag, while the converse is true for minimum-B geometries. A heuristic mechanical model is first described. The precession of the E-layer is next shown to be drag-unstable if one uses the ``super-particle model,'' which does not allow for a finite spread in particle precession frequencies. Similar results are obtained for ordinary mirror-machine plasmas in the two limits that are ordinarily considered stable by virtue of the high-density and low-density finite-Larmor-radius conditions.
Collisional Damping of Transverse Plasma Oscillations
Authors: Buti B. , Jain R. K.
Published: December 2004

The effect of Coulomb collisions on the transverse electron oscillations in a fully ionized hot plasma is studied by means of the Fokker-Planck equation. The damping coefficient for the electron-ion collisions γi turns out to be much greater than the one for the electron-electron collisions γe. To first order in electron-collision frequency, these are given by, γi = [νc/3(2π)12][ωp2/(c2k2+ωp2)]·[1+(2.075k2υ02)/(c2k2+ωp2)−k2υ02ωp2/(c2k2+ωp2)2],γe=(41/16)[νc/3π12][ωp2/(c2k2+ωp2)][k2υ02/(c2k2+ωp2)],where v0 and ωp are the electron thermal speed and the plasma frequency, respectively.
Asymptotic Theory of a Photoionization Chamber
Authors: Cohen Ira M.
Published: December 2004

A photoionization chamber consists of two parallel plates between which a test gas is photoionized. The charged particles thus produced then diffuse to the plates where they recombine. This process is analyzed in detail when the fraction of ionization is very low and the mean free path for charged-neutral encounters is very small compared to the chamber width. Particular attention is paid to the physically important limit of sufficiently large charged particle densities to make the Debye length small compared with any macroscopic dimension. In this limit the plasma is quasi-neutral in the body of the chamber. Near the walls, however, quasi-neutrality breaks down and a new scaling of the variables is necessary to account for the changed orders of magnitude of the properties in the sheaths. The sheath equations are reduced to one previously solved by the author (in connection with a spherical electrostatic probe problem). The sheath solution and quasi-neutral plasma solution are asymptotically matched in their region of common validity. Thus, charged particle and electric field distributions are found and typical curves are given. From the tabulated data, current-voltage characteristics of the photoionization chamber may be constructed. Three sample characteristics are given with a little discussion of their interpretation.
Measurements of a Precursor Electron Front
Authors: Barach J. P. , Sivinski J. A.
Published: December 2004

An electron current front of a few thousand amperes has been produced in the expansion chamber of an electric shock tube. It precedes the shock and follows the (much faster) precursor breakdown wave. The electron front velocity of up to 2 × 107 cm/sec is linear with applied voltage gradient and gas conductivity, as measured by double probes. As the operating parameters are varied over a wide range the electron temperature varies from 600 to 8000°K, but the number density in the front remains about 2 × 1013 cm−3. The front heats the gas and increases the speed of the subsequent shock by a factor of about two. This field-driven wave may exist as a precursor in other shock tubes and must be distinguished from breakdown and hot electron diffusion precursor waves already reported by other authors.
Uniform Motion of Vortex System
Authors: Pollock Franklin
Published: December 2004

The motion of systems of vortices in inviscid fluids that move uniformly without change of structure is considered. The Euler equations are cast in a form that selects out only uniform motion, and the resulting time-independent equation is examined. It is shown in this case that the requirement that there exist well-behaved solutions insures that a vortex pair moves with the velocity given by the classical linear theory.
Class of Exact Solutions in Nonlinear Kinetic Theory
Authors: Sirovich Lawrence
Published: December 2004

A class of exact solutions of the one-dimensional steady Krooked equation is obtained. The collision frequency of the latter is left arbitrary. The solutions found depict flows widely removed from equilibrium.
Definition of Burning Velocity and a Geometric Interpretation of the Effects of Flame Curvature
Authors: Fristrom R. M.
Published: December 2004

The burning velocity, v0, has been a subject of controversy for curved flames. This curvature is the macroscopic result of stream tube area variation. The flow can not be one-dimensional, but for small curvatures it is quasi-one-dimensional. As usually defined, v0, varies with curvature due to ducting. However, if the primary reaction zone is narrow, the velocity there, vr, is approximately independent of curvature. vr is a function of the reaction rate and is related to v0 by the density ratio ρr/ρ0 and the stream tube area ratio A0, a geometric factor. Thus, the burning velocity in the absence of curvature, v0*, can be defined by v0* = vrρr/ρ0 = v0A0. It can be derived from measurements on flame tips or expanding flame kernels, and appears to be reproducible and independent of external variables such as geometry, inlet flow and flame curvature. The concept was substantiated by measurements on the microstructure of matched flames of differing curvature and by quantitatively correlating the variation of v0 with flame curvature in flame tips, expanding flame kernels and turbulent flamelets.
Singularities of the Green's Function for a Collisionless Magnetoplasma
Authors: Nuttall J.
Published: December 2004

A method is given for determining the singularities of the Green's function Gij(r, ω) for a uniform Maxwellian plasma immersed in a magnetic field and described by the linearized Vlasov equation. The positions of the singularities determine the asymptotic time dependence of the electric field. In this paper, only the singularities near the plasma frequency ωp and the Pythagoras frequency ωΔ ≡ (ωb2 + ωp2)½ are studied in detail.
First- and Second-Order Landau Damping in Maxwellian Plasmas
Authors: Denavit J.
Published: December 2004

Longitudinal electron oscillations in Maxwellian plasmas give a linear (first-order) electric field which is the superposition of damped oscillations. The complex frequency of lowest damping which yield the long time behavior of the solution has been determined by Landau and Jackson. The other frequencies, which indicate the time required for the time-asymptotic solution to be reached, are evaluated in the paper for arbitrary values of the ratio r = wavelength/Debye length. These frequencies form two strings of isolated points extending to infinity along the diagonals in the lower half frequency plane. The amplitude and phase angle corresponding to each frequency are found for an initial perturbation sinusoidal in space and Maxwellian in velocity. The results show that for r ≥ 10, the asymptotic solution is very good after the first cycle. Nonlinear (second-order) corrections are evaluated using the method of Montgomery. They give second space harmonics with a damped oscillatory time behavior. The frequencies, amplitudes and phase angles are explicitly given. For r → ∞ the results match those of the zero temperature solution.
Verification of Hot Plasma Theory for Microwave Propagation
Authors: Meservey E. B. , Schlesinger S. P.
Published: December 2004

An analysis is presented of an experimentally detected resonance at the second harmonic of the electron cyclotron frequency. Phase shift and attenuation measurements show reasonable agreement with the predictions of hot plasma theory. In this theory a dispersion relation is derived from a perturbation solution of the linearized collisionless Boltzmann equation with the velocity distribution function of the unperturbed plasma assumed to be Maxwellian. In the experiment 4-mm microwaves were propagated in the extraordinary mode (E ⊥ B0) across a highly ionized essentially uniform plasma of 10-cm diameter confined in a highly homogeneous (0.05%) magnetic field in the Model C Stellarator. Conditions existing for the measurements are: 5≲Te≲25 eV;ωp≲ω/2;10−5≲νcoll/ω<5×10−4;Ω≅ω/2 (Ω is the electron cyclotron frequency). Under these conditions multiple harmonic anomalies as detected by other investigators were not observed.
Plasma Rotation in a Hollow-Cathode Discharge
Authors: Morse David L.
Published: December 2004

Langmuir probe measurements of the external plasma generated by a hollow-cathode discharge indicate the presence of a ``flutelike'' density perturbation, or ``vane'' of plasma, rotating with a left-handed sense about the magnetic field. The velocity of rotation is found to be closely related to the Hall drift speed of particles in the measured outward radial electric and axial magnetic fields. Azimuthal electric fields are found whose net effect is to increase the outward radial transport of plasma across the magnetic field. A discussion of this phenomenon in terms of a previously described theory is given.
Low-Density Theta Pinch Experiments
Authors: Goldman L. M. , Kilb R. W. , Pollock H. C. , Reynolds J. A.
Published: December 2004

Deuterium plasmas, produced by a theta pinch device, with initial pressure of 0.01 Torr, have been studied by means of neutron measurements, photography, magnetic probes, pick-up loops, spectroscopy, soft x-ray absorption, and by the introduction of impurity ions. Most of the plasmas were created in the absence of trapped fields. Typical plasma conditions obtained are: n ∼ 1016 cm−3, Te ∼ 300 eV, neutron rates of 1012 sec−1 and ion energies of several keV. The plasma shows no growing instabilities if no reversed field is present. There is particle loss through the end mirrors, with low-energy ions lost preferentially as in classical mirror confinement.
Motion of a Sphere in a Rotating Fluid at Small Reynolds Numbers
Authors: Davis Philip K.
Published: December 2004

The slow steady rise of a solid sphere along the axis of a uniformly rotating viscous liquid is studied when the Reynolds number, based on the translational velocity along the axis of rotation, is less than unity. The terminal velocity of a sphere rising slowly along the axis of a uniformly rotating viscous liquid has been determined experimentally. Spheres of varying sizes with a density less than that of the fluid were released at the bottom of a rotating cylinder filled with castor oil. The subsequent terminal velocities along the axis of rotation were measured. It has been found that the rise velocity decreases from the Stokes velocity as the angular speed of rotation is increased. The governing equations show that this is due to the Coriolis acceleration.
Solution for the Inertial Energy Spectrum of Isotropic Turbulence
Authors: Eschenroeder Alan Q.
Published: December 2004

A dynamic equation for quasi-stationary, inertially dominated, isotropic turbulence is formulated on the basis of a modified Obukhoff transfer term. By means of an elementary transformation, a self-similar solution for the energy spectrum is obtained. The closed-form expression for an inertial range in the quasi-equilibrium regime obeys previously derived functional laws for large and small wavenumbers. Matching the analytical result with an experimentally determined coefficient in the large wavenumber limit gives consistent scaling of terms over the energy-containing eddy range of the spectrum.
Structure of Radiation-Resisted Shock Waves with Vibrational Nonequilibrium
Authors: Emanuel George
Published: December 2004

The continuum equations of gasdynamics are used to study the effect of thermal radiation and vibrational nonequilibrium on the structure of a shock wave. Emphasis is on a description of the gasdynamic phenomena involved rather than on numerical results. For example, a Mollier diagram interpretation, useful for understanding radiation-resisted shock waves, is given. The principal results stem from an analysis of the singular-point structure far upstream and far downstream in the flow. This analysis indicates in a heuristic fashion that a unique solution exists. It also shows that two parameters, a Mach number and the ratio of a radiation length to a relaxation length, primarily govern the qualitative behavior of the flow. Thus, for a specific range of values of these parameters, the vibrational energy overshoots its final equilibrium value. For a different range, a net amount of radiant energy is transferred from the hot gas behind the adiabatic shock to the gas far downstream.
Some Exact Solutions in Magnetohydrodynamics with Astrophysical Applications
Authors: Clark Alfred
Published: December 2004

The equations describing a class of exact solutions in magnetohydrodynamics are given. The solutions considered are those for which the magnetic field is everywhere parallel to a fixed direction in space. Several velocity fields satisfying the equations are discussed briefly. One example discussed in detail serves as a model for the time development of a vertical magnetic field in a flow resembling the solar supergranulation flow. For this problem, it is shown that the characteristic time scale for finite conductivity diffusion effects to be important is much shorter than the usual dimensional scale of (length)2 divided by diffusivity. In particular, it is shown that under typical conditions of solar supergranulation flow, the time scale for diffusion is comparable with the observed lifetimes of the supergranulation cells.
Validity of the Kadomtsev—Nedospasov Approximation for a Simple Geometry Plasma
Authors: Ivash Eugene V.
Published: December 2004

The problem of the instability of a weakly ionized plasma between two infinite parallel plates in crossed uniform electric and magnetic fields has been considered for a simple set of boundary conditions. The primary purpose has been to investigate the validity of the Kadomtsev—Nedospasov type of simplifying approximation for the perturbed particle density and potential for a relatively simple case. It has been found that the approximation in the form introduced by Simon is valid under some, though not all plasma conditions. The effect of ionization on stability has also been investigated, and may be appreciable.
Plasma Vortices in the Coaxial Plasma Accelerator
Authors: Bostick W. H. , Prior W. , Farber E.
Published: December 2004

Considerations of conservation of mass and momentum flow in the plasma coaxial accelerator make it apparent that plasma eddies or vortices should be produced behind the current sheet. Measurements of axial and radial electric fields of radial current density and ion flow density, show experimentally that these vortices are present.
Effect of Finite Ion and Electron Temperatures on the Ion-Cyclotron Resonance Instability
Authors: Soper G. K. , Harris E. G.
Published: December 2004

The ion-cyclotron resonance instability theory as first developed by Harris is extended to include finite ion and electron temperature effects. With anisotropic ion and electron temperatures the dispersion relation due to Harris is subjected to numerical analysis and stability zone boundaries in the two-dimensional parameter space representing essentially the magnetic field strength and the electron density is obtained. The effect of a uniform rotation of the ions with respect to the electrons is also included. Comparison of the results of this analysis with some experimental observations is possible.
Measurement of Drag of Cylinders and Spheres in a Couette-Flow Channel
Authors: Kohlman David L. , Mollo-Christensen Erik
Published: December 2004

An investigation was carried out to determine the range of Reynolds numbers for which flow shear does not affect drag in a Couette flow, and to measure the wall effects on the drag of cylinders and spheres. A moving belt Couette-flow apparatus is described, and observations of mean flow are given.
Temperature Relaxation in a Fully Ionized Plasma
Authors: Wu C.-S. , Klevans E. H. , Primack J. R.
Published: December 2004

The times for relaxation of anisotropic ion or electron temperatures to isotropy, and for relaxation of different ion and electron temperatures to equilibrium, are calculated using the plasma kinetic equation including dynamic shielding. It is shown that, in contradiction to the results (or assumptions) of previous analyses, electron—ion interactions can be more important than interactions between particles of the same species in the relaxation of an anisotropic distribution. The meaning of relaxation time is discussed and a new time for relaxation is introduced for the problem of the relaxation of electron and ion temperatures that are initially greatly different. Detailed numerical computations are presented for various cases of special interest.
``Universal'' Overstability of a Resistive, Inhomogeneous Plasma
Authors: Chen Francis F.
Published: December 2004

An inhomogeneous plasma with finite resistivity in a straight or curved magnetic field is subject to instabilities of electrostatic drift waves with small but finite k∥. The influence on these instabilities of various factors such as finite β, resistive and viscous damping, and finite Larmor radius is investigated; and explicit, experimentally useful expressions for the frequency and growth rate are given.
Oscillations of a Capillary Jet in the Presence of a Uniform Magnetic Field
Authors: Trehan S. K. , Malik S. K. , Dikshit M. S.
Published: December 2004

The oscillations of a capillary jet of finite electrical conductivity in the presence of a uniform axial magnetic field are investigated taking into account the presence of Hall current in Ohm's law. The dispersion relation is obtained as the exact solution of the characteristic value equations governing the general perturbations. The special case of axisymmetric perturbations (m = 0) is examined in detail. In particular for the case of high resistivity, it is found that the system does not become overstable and that the Hall current contributes an effect only in the second order in an expansion in terms of a small parameter κ = H2k/8πηωc, where H is the magnetic field, k the wavenumber of the perturbation, η the resistivity, and ωc is the ion cyclotron frequency.
High-Frequency Velocity Space Instabilities
Authors: Gruber S. , Klein M. W. , Auer P. L.
Published: December 2004

The electrostatic dispersion relation is examined for high-frequency instabilities in a collisionless, infinite, magneto-active plasma with electrons having non-Maxwellian velocity distributions. Growing waves are found, and the complex frequency solutions are calculated. The variation of wave growth with electron density, angle of propagation, and frequency is calculated for several distributions of electron velocity. Calculations of the spectrum when the electrons have an axial velocity spread are included.
Ionization Rates behind Shock Waves in Argon
Authors: Morgan Edward J. , Morrison Robert D.
Published: December 2004

The ionization rate behind shock waves in argon has been recalculated allowing for excitation and ionization by atom-atom collisions. Good agreement is obtained between these calculations and the experimental results of others for very pure argon. For pure argon, the initial rate of ionization is controlled by the rate of excitation from the ground state by means of atom-atom collisions. The atom-atom cross sections are several percent of the corresponding electron-atom cross sections. When the degree of ionization builds up to some value greater than 10−4, electron-atom collisions dominate, and then the ionization rate is controlled by the rate of energy transfer between ions and electrons by means of elastic collisions. The effect of impurities is considered and a relatively crude theory is proposed to explain the experimental observations. It is shown that one must be careful to distinguish between ionization times and luminosity delay times.
Flow Field of a Bunsen Flame according to Source Sheet Approximation
Authors: Uberoi Mahinder S. , Zimmerman Alan
Published: December 2004

A volume source sheet plus a uniform flow is used to represent the flow field of a two-dimensional Bunsen flame when the density ratio of the unburned to burned gases is 5.5. The unknown flame shape is determined by the requirement that the velocity normal and relative to it, i.e., the local flame speed is constant. The flow field is computed by the method of squares and compares favorably with that obtained experimentally.
Stability of a Shear Flow in an Unstable Layer
Authors: Eisler Thomas J.
Published: December 2004

The stability with respect to small perturbations of a piecewise linear velocity profile of the half-jet (or shear layer) type in a layer with an unstable entropy gradient has been studied. The maximum instabilities are found to be either in the plane of the velocity profile or perpendicular to it depending on a critical Richardson number which is quite insensitive to the depth of the layer.
Mass-Transfer Method of Measuring Wall Shear Stress in Supersonic Flow
Authors: Wazzan Ahmed R. , Smith A. M. O. , Lind R. C.
Published: December 2004

If a bright, polished surface is covered with a thin layer of transparent liquid and viewed in monochromatic light, the thickness of the layer can be determined with high precision by counting interference fringes. By this means, the local rate of evaporation can be determined. If, for example, air is blowing past a suitable model in a wind tunnel, the mass-transfer coefficient can be determined from measurements of the rate of evaporation. Then the local skin friction can be determined by utilizing the analogy between mass transfer and skin friction. This method has previously been investigated at low speed and is now extended to supersonic flows. Further studies and experiments at Mach numbers 1.6 and 2.8 on a small flat plate wind-tunnel model in laminar flow are described. As with the earlier subsonic tests, agreements between experiment and theory is good.
Wave Propagation across a Two-Fluid Plasma Density Discontinuity
Authors: Haynes Katherine A. , Kahn David
Published: December 2004

The reflection and refraction of waves across a density discontinuity in a two-fluid fully ionized plasma are investigated by means of the coupled linearized equations of hydrodynamics and Maxwell's equations. This extension from the previously considered electronic plasma to the two-fluid gas shows that there is energy coupling between the acoustic modes of propagation as well as the acoustic-electromagnetic coupling found before. The electron acoustic-transverse and transverse-electron acoustic wave coupling found in previous studies is reproduced here, the heavy ions having little effect on this energy conversion; however, we find also the presence of electron-ion, ion-electron, ion-transverse, and transverse-ion coupling in the two-fluid plasma. Numerical results show that all these coupling mechanisms become most efficient as the wave frequency approaches the plasma frequency in the region of the transmitted wave.
Electrical and Thermal Properties of Plasmas
Authors: Meador W. E. , Staton L. D.
Published: December 2004

General theoretical expressions in terms of commonly used transport coefficients are derived and analyzed for the current density and heat flux within a partially ionized gas. Features of the development include the following: an examination of some of the relations between space charge and the Navier-Stokes hydrodynamic equations of change ending with the conclusion that only a quasi-neutral gas is consistent with linear flux theory; an explanation in terms of irreversible thermodynamics of the factor of three separating Ahtye and Spitzer-Härm thermal conductivities; a subsequent demonstration of the numerical equivalence between certain transport calculations based on the Boltzmann and Fokker-Planck kinetic equations; and a discussion and evaluation of the methods of Yos and Thomas. In addition, it is shown that, apart from a thermal diffusion term, expected to be small, the field-reduced thermal conductivity is that of Muckenfuss and Curtiss with a Butler-Brokaw reaction contribution tacked on. This means that the latter form, provided Boltzmann's equation is still reliable, is just as good for ionizing gases as for any other reacting mixture.
Investigation of a Plasma from an Occluded Gas Cold Plasma Source
Authors: Steinhaus James F. , Oleson Norman L. , Barr William L.
Published: December 2004

Results are given for various measurements made on a plasma from a hydrogen-loaded titanium washer mounted at a dc magnetic mirror (∼6 kG), the power being supplied to the source by a variable length pulse line (15–635 μsec). Spectroscopic studies indicated the presence of H, Ti+, Ti++, Ti+++, O+, O++, Al, Al+, Al++, and Ta+. Doppler shift measurements showed ion velocities from 5 × 105 to 2 × 106 cm/sec. Microwave cutoff measurements provided time history curves of electron density and indicated that the maximum density exceeded 3 × 1014 electrons per cm3. Profile measurements using a thermistor close to the aperture of the plasma gun revealed that 10% of the particles were neutrals. Probe measurements showed that the electron temperature decreased during the pulse from 15 to 1 eV and from 3 to <1 eV for old and new source washers, respectively.
Coupling of Ion-Acoustic to Neutral-Acoustic Waves at a Diffuse Boundary
Authors: Kahalas Sheldon L. , Parker Lee W.
Published: December 2004

The coupling of ion-acoustic (longitudinal plasma) waves to neutral-acoustic (sound) waves is treated at a diffuse boundary consisting of a partially ionized plasma of spatially varying composition. Collisions among the electrons, ions, and neutrals are represented by linear velocity terms. The effect of collisions is shown, for low frequencies, to give rise to coupling between an ion—electron (plasma) mode and a neutral (sound) mode. An approximate coupling coefficient is derived, based on an assumed analytic form for the ion—neutral collision frequency.
Positron annihilation lifetime and coincidence-gated Doppler broadening studies of FeS2 nanostructures
Authors: Kar S. , Chaudhuri S. , Nambissan P. M. G.
Published: December 2004

Positron lifetimes were measured in different nanosystems of FeS2—granular samples, ribbons, oxidized ribbons, rods, and a mixture of wires and tubes. The initial characterization and size determination of the synthesized samples were carried out using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. To identify the positron trapping sites, the 511–511keV gamma rays coincidence-gated Doppler-broadened spectra were recorded and it appeared that the trapping of positrons took place mainly in the vacancies created by the absence of Fe2+ ions. The positron lifetimes in the nanogranular sample were conspicuously larger compared to those in the coarse-grained bulk due to trapping and annihilation at the grain surfaces. The difference in the measured lifetimes in the nanoribbons from those of the nanogranular samples is explained as due to the annihilation of positrons in the additional trapping sites available at the inter-ribbon-layered cavities. Positron annihilation in nanotubes differed distinctly from solid nanorods because of the trapping of positrons in the inner channel surfaces of the former and a significant fraction of the positrons formed orthopositronium inside these open holes.
Yellow and infrared self-frequency conversions in GdAl3(BO3)4:Nd3+
Authors: Brenier Alain , Tu Chaoyang , Zhu Zhaojie , Li Jianfu , Wu Baichang
Published: December 2004

We have demonstrated two self-frequency conversions from the bifunctional laser and optical nonlinear GdAl3(BO3)4:Nd3+ crystal. For the first one, the laser operated in a dual-wavelength mode in the F3∕24→I11∕24 (at 1062nm) and F3∕24→I13∕24 (at 1338nm) channels, under a fixed wavelength pumping (at 747.7nm) and with a fixed crystal orientation (28.5°). Then the self-sum frequency mixing 1∕1062+1∕1338→1∕592 process occurred, leading to a weak yellow beam at 592nm. In the second self-frequency conversion, the Nd3+:GdAl3(BO3)4 laser operated in a single wavelength mode at 1062nm. Then the self-difference frequency mixing process 1∕λP−1∕1062=1∕λI occurred at a 29.9° phase matching angle. The idler wavelength λI was tunable in the 2450–2600nm range, due to the tunability of the λP pump wavelength. The highest yield was 0.12%. New Sellmeier formulas valid at least up to 2600nm have been established.
Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates
Authors: Darakchieva V. , Paskova T. , Paskov P. P. , Monemar B. , Ashkenov N. , Schubert M.
Published: December 2004

We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase epitaxy (MOVPE) templates and on epitaxial lateral overgrown MOVPE templates are characterized by Raman scattering, high-resolution x-ray diffraction, and reciprocal space mapping. The strain relaxation in the films versus their thickness was found to proceed similarly in the GaN samples grown using the two types of templates but the strain saturates at different nonzero levels. The lattice parameters of relatively thin HVPE-GaN free-standing quasisubstrates indicate that no total strain relaxation is achieved after the sapphire removal. The lattice parameters of the thick quasisubstrates grown on different templates are not affected by the separation process and are found to have values very close to the reference strain-free lattice parameters of GaN powder.
Excitation power dynamics of photoluminescence in InGaN∕GaN quantum wells with enhanced carrier localization
Authors: Kazlauskas K. , Tamulaitis G. , Mickevičius J. , Kuokštis E. , Žukauskas A. , Cheng Yung-Chen , Wang Hsiang-Cheng , Huang Chi-Feng , Yang C. C.
Published: December 2004

Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in InxGa1−xN∕GaN multiple quantum wells (x∼0.15) was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75meV in 2nm wells to about 250meV in 4nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak (0.5MV∕cm) built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest (4nm) wells. Increased incident photon energy resulted in an additional blueshift of the PL peak, which was explained by a nonthermalized distribution of localized carriers and/or carrier localization in the interface region. Our results are consistent with a concept of emission from partially relaxed large In-rich regions with internal band potential fluctuations, which are enhanced with increasing the growth time.
Quasi-Debye microwave loss as an intrinsic limitation of microwave performance of tunable components based on SrTiO3 and BaxSr1−xTiO3 ferroelectrics
Authors: Astafiev K. F. , Tagantsev A. K. , Setter N.
Published: December 2004

A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 perovskite ferroelectrics at microwave frequencies is presented. The field dependence and the size of the field-induced quasi-Debye loss contribution in SrTiO3 and Ba0.6Sr0.4TiO3 systems have been evaluated in the simple approximation of the isotropic phonon spectra of these materials. Good agreement between experimental and simulated results for the field dependence of dielectric loss for SrTiO3 ferroelectric material is observed for the weak dc fields [tunability n=ε(0)∕ε(E)⪡2]. A significant feature in the field dependence of dielectric loss at microwave frequencies, a pronounced maximum, is predicted for SrTiO3 at higher values of the bias field (n≅2) whereas in Ba0.6Sr0.4TiO3 the field dependence of the loss is found to be monotonic. The influence of the quasi-Debye loss contribution on microwave performance of SrTiO3 and Ba0.6Sr0.4TiO3 ferroelectric materials is analyzed. It is concluded that the contribution of this mechanism is large enough to affect the performance of tunable microwave devices based on SrTiO3 ferroelectric materials, whereas in the case of Ba0.6Sr0.4TiO3 ferroelectrics the contribution of the quasi-Debye loss mechanism is found to be relatively small.
The Shadow Knows: A Primer on the Informational Structure of Cast Shadows
Authors: Casati Roberto
Published: December 2004

I list relevant properties of cast shadows that the visual system could exploit (although it may not exploit). The study concerns the various types of information that can be extracted from shadows by systems that are significantly like ours (including embodied artificial systems) in an environment that is significantly like ours.
Estimations of vapour pressure deficit and crop water demand in APSIM and their implications for prediction of crop yield, water use, and deep drainage
Authors: Wang Enli , Smith Chris J. , Bond Warren J. , Verburg Kirsten
Published: December 2004

Vapour pressure deficit (VPD) has a significant effect on the amount of water required by the crop to maintain optimal growth. Data required to calculate the mean VPD on a daily basis are rarely available, and most models use approximations to estimate it. In APSIM (Agricultural Production Systems Simulator), VPD is estimated from daily maximum and minimum temperatures with the assumption that the minimum temperature equals dew point, and there is little change in vapour pressure or dew point during any one day. The accuracy of such VPD estimations was assessed using data collected every 15 min near Wagga Wagga in New South Wales, Australia. Actual vapour pressure of the air ranged from 0.5 to 2.5 kPa. For more than 75% of the time its variation was less than 20%, and the maximum variation was up to 50%. Daytime mean VPD ranged from 0 to 5.3 kPa. Daily minimum temperature was found to be a poor estimate of dew point temperature, being higher than dew point in summer and lower in winter. Thus the prediction of vapour pressure was poor. Vapour pressure at 0900 hours was a better estimate of daily mean vapour pressure. Despite the poor estimation of vapour pressure, daytime mean VPD was predicted reasonably well using daily maximum and minimum temperatures. If the vapour pressure at 0900 hours from the SILO Patched Point Dataset was used as the actual daily mean vapour pressure, the accuracy of daytime VPD estimation was further improved. Simulations using historical weather data for 1957–2002 show that such improved accuracy in daytime VPD estimation slightly increased simulated crop yield and deep drainage, while slightly reducing crop water uptake. Comparison of the APSIM RUE/TE and CERES-Wheat approaches for modelling potential transpiration revealed differences in crop water demand estimated by the two approaches. Although the differences had a small effect on the probability distribution of simulated long-term wheat yield, water uptake, and deep drainage, this finding highlights the need for a scientific re-appraisal of the APSIM RUE/TE and energy balance approaches for the estimation of crop demand, which will have implications for modelling crop growth under water-limited conditions and calculation of water required to maintain maximum growth.
Comparative wood anatomy of Eucryphia wilkiei and E. jinksii (Cunoniaceae): two narrow endemics from Queensland, Australia
Authors: Poole Imogen , Barnes Richard W.
Published: December 2004

Qualitative and quantitative wood anatomical data are given for two narrow Queensland endemic Eucryphia species, E. wilkiei B.Hyland and Eucryphia jinksii P.I.Forst. Comparisons of wood anatomy of all extant Eucryphia taxa show that E. jinksii and E. wilkiei are distinct from each other, and other Eucryphia species. However, for both species characters relating to perforation plates, helical thickening (E. wilkiei only) and fibres are shared with the South American species, whereas the presence of crystals in the axial parenchyma is shared only with the Australian species. These data suggest that, based on wood anatomy, E. jinksii and E. wilkiei are basal among Australian species.
TSC2 modulates actin cytoskeleton and focal adhesion through TSC1-binding domain and the Rac1 GTPase
Authors: Goncharova Elena , Goncharov Dmitry , Noonan Daniel , Krymskaya Vera P.
Published: December 2004

Tuberous sclerosis complex (TSC) 1 and TSC2 are thought to be involved in protein translational regulation and cell growth, and loss of their function is a cause of TSC and lymphangioleiomyomatosis (LAM). However, TSC1 also activates Rho and regulates cell adhesion. We found that TSC2 modulates actin dynamics and cell adhesion and the TSC1-binding domain (TSC2-HBD) is essential for this function of TSC2. Expression of TSC2 or TSC2-HBD in TSC2−/− cells promoted Rac1 activation, inhibition of Rho, stress fiber disassembly, and focal adhesion remodeling. The down-regulation of TSC1 with TSC1 siRNA in TSC2−/− cells activated Rac1 and induced loss of stress fibers. Our data indicate that TSC1 inhibits Rac1 and TSC2 blocks this activity of TSC1. Because TSC1 and TSC2 regulate Rho and Rac1, whose activities are interconnected in a reciprocal fashion, loss of either TSC1 or TSC2 function may result in the deregulation of cell motility and adhesion, which are associated with the pathobiology of TSC and LAM.
Identification of Cell Cycle-regulated Genes in Fission Yeast
Authors: Peng Xu , Karuturi R. Krishna Murthy , Miller Lance D. , Lin Kui , Jia Yonghui , Kondu Pinar , Wang Long , Wong Lim-Soon , Liu Edison T. , Balasubramanian Mohan K. , Liu Jianhua
Published: December 2004

Cell cycle progression is both regulated and accompanied by periodic changes in the expression levels of a large number of genes. To investigate cell cycle-regulated transcriptional programs in the fission yeast Schizosaccharomyces pombe, we developed a whole-genome oligonucleotide-based DNA microarray. Microarray analysis of both wild-type and cdc25 mutant cell cultures was performed to identify transcripts whose levels oscillated during the cell cycle. Using an unsupervised algorithm, we identified 747 genes that met the criteria for cell cycle-regulated expression. Peaks of gene expression were found to be distributed throughout the entire cell cycle. Furthermore, we found that four promoter motifs exhibited strong association with cell cycle phase-specific expression. Examination of the regulation of MCB motif-containing genes through the perturbation of DNA synthesis control/MCB-binding factor (DSC/MBF)-mediated transcription in arrested synchronous cdc10 mutant cell cultures revealed a subset of functional targets of the DSC/MBF transcription factor complex, as well as certain gene promoter requirements. Finally, we compared our data with those for the budding yeast Saccharomyces cerevisiae and found ∼140 genes that are cell cycle regulated in both yeasts, suggesting that these genes may play an evolutionarily conserved role in regulation of cell cycle-specific processes. Our complete data sets are available at http://giscompute.gis.a-star.edu.sg/~gisljh/CDC .