bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of parametric instability near the lower-hybrid frequency
Authors: Porkolab Miklos
Published: February 2004

The dispersion relation for parametric instabilities near the lower-hybrid frequency is derived and analyzed. It is found that for propagation angles cos2θ(mi/me) <̃ 1 resonant decay into ion acoustic (ion-cyclotron) waves does not occur; rather, decay into nonresonant quasi-ion modes and lower-hybrid waves occurs. The driving mechanism for this instability is shown to be analogous to nonlinear Landau damping in perturbation theory. The large amplitude dispersion relation is analyzed numerically for a number of typical experimental regimes, and growth rates and thresholds are obtained for both the purely growing mode and the newly found quasi-ion modes.
Experimental observations of the parametric decay instability
Authors: Yadlowsky Edward J. , Goldan Paul D.
Published: February 2004

The parametric decay instability has been excited in a low-pressure argon plasma characterized by ne ∼ 5 × 108 cm−3, Te ∼ 2 eV, Ti ∼ 0.2 eV. An electron beam probe has been used to measure the frequency and spatial distribution of the fields associated with the pump wave and excited Langmuir and ion waves. The wavenumber matching (K0 = Ke + Ki) and frequency matching (ω0 = ωe + ωi) conditions are well satisfied. If particle losses in the finite geometry are taken into account, the minimum threshold pump electric field is in good quantative agreement with the theoretical prediction ε0Ec2/nkT = 643/9 × γiγe2/ωi2ωe in the limit of γe > ωi ≫ γi.
Spectra and growth rates of a generalized screw pinch
Authors: Vahala George
Published: February 2004

The theory of Weitzner for the bumpy θ pinch is extended to the case of a generalized screw pinch in which the equilibrium magnetic field is B = (εBr, εBθ, Bz). ε represents the slow periodic z variation of the axisymmetric equilibrium and the stability relative to their r variation. Perturbation expansions in ε and in the bumpiness of the flux surfaces are performed on the linearized equations of ideal magnetohydrodynamics. From the theory of Grad there should be four stable continua, two of which are found for modes with ω2 = O (ε2), while for the transverse modes a generalized Suydam criterion for local instability is derived. The (global) growth rates of the most unstable transverse modes are computed numerically for various Bθ and the results are compared to those of the bumpy θ pinch (where Bθ ≡ 0)) as well as to the ordinary screw pinch (where Br ≡ 0).
Bubble growth in dilute polymer solutions
Authors: Ting R. Y. , Ellis A. T.
Published: February 2004

Under a free-fall condition, the growth of individual gas bubbles in dilute polymer solutions was observed experimentally. A retardation in the growth rate was found as compared with the case in water.
Laminar flow in a rectangular duct bounded by a porous wall
Authors: Sparrow E. M. , Beavers Gordon S. , Masha B. A.
Published: February 2004

Velocity field solutions for finite aspect ratio rectangular ducts with one prous bounding wall were obtained and employed to determine friction factors. Comparisons were made with results for the parallel plate channel in order to illuminate published interpretations of experimental data.
Stokes' paradox in second-order fluids
Authors: Huilgol R. R. , Tao L. N.
Published: February 2004

It is shown that Stokes' paradox for incompressible Newtonian fluids also arises in the two-dimensional creeping flows of incompressible second-order fluids. The velocity field in both fluids is again identical for the flow past two or more cylinders, provided the geometries in both flows are identical.
Drop spectrometry of laminar and turbulent jets
Authors: Lafrance Pierre , Aiello G. , Ritter R. C. , Trefil J. S.
Published: February 2004

Drops resulting from the breakup of laminar and turbulent jets are electro-optically measured and drop size histograms are electronically generated. Having large numbers of drops and good statistics, they permit quantitative analysis to an extent not feasible with photographic methods.
Continuous spectra of a cylindrical magnetohydrodynamic equilibrium
Authors: Appert K. , Gruber R. , Vaclavik J.
Published: February 2004

It is shown that, for a cylindrical equilibrium of a perfectly conducting plasma, the magnetohydrodynamic spectrum contains at most two continua.
Two-dimensional growth of a bleaching wave
Authors: Rehm Ronald G.
Published: February 2004

An analytical solution is presented which describes the axisymmetric growth of a high-temperature region heated by laser radiation, known as a bleaching wave. Constant-temperature profiles for the wave are shown for ruby laser radiation and for CO2 laser radiation incident upon a plasma.
Effect of initial ion velocities on the thickness of a planar sheath
Authors: Oliver B. M. , Clements R. M.
Published: February 2004

A straightforward method is presented for calculating the sheath thickness surrounding a planar probe in a collisionless plasma assuming an initial ion velocity. Sheath thicknesses are compared to those calculated from Langmuir's modified planar analysis.
Instability of stratified flows as a result of resonance
Authors: Yih Chia-Shun
Published: February 2004

The stability of stratified flows consisting of a middle layer of homogeneous fluid in linear shear flow and contiguous upper and lower stratified layers of constant velocities is considered. The density and velocity are continuous throughout. It is shown that there are infinitely many pairs of neutral modes with the phase velocity cr in the range of the velocity which, in general, are not represented by the stability boundary, but which merge as the parameter N (the inverse of the Froude number squared) increases. As N increases further the coalesced neutral modes become modes with complex c, one of which is unstable. The instability may be considered to be caused by the resonance of the original pair of separate modes. In addition, instability of stratified flows is considered for general density and velocity distributions without a layer of constant density and linear velocity, and it is concluded that in that case the modes with complex c on the unstable side of a stability boundary do not continue into any neutral normal modes on the stable side.
Progressive waves of permanent form in continuously stratified fluids
Authors: Yih Chia-Shun
Published: February 2004

A theory for internal progressive waves of permanent form in any continuously stratified fluid is presented, and a calculation for the flow and the wave velocity is carried out for an exponential stratified fluid. The most important conclusion from this calculation, also valid for other weak stratifications, is that the wave velocity always decreases with the amplitude, provided the density gradient is weak and the wavelength is not too short. This conclusion is significant because it entails the existence or nonexistence of solitary waves in weakly stratified fluids. The validity of the Boussinesq approximation and the significance of the well-known exactly linear cases are also discussed.
Thermal convection in the vicinity of a discontinuous change in wall temperature
Authors: Saukin Walter P. , Weinbaum Sheldon , Jiji Latif M.
Published: February 2004

The effects of upstream influence on the thermal field in the vicinity of the leading edge of the new thermal boundary layer that develops within an already well-established thermal and velocity boundary layer at a point of discontinuous thermal boundary conditions are studied. In contrast to ordinary boundary layer theory, the streamwise conduction terms are retained in the Navier-Stokes energy equation for forced convection and locally valid series solutions obtained for the leading edge region.
Non-Gaussian and irreversible events in isotropic turbulence
Authors: Betchov R.
Published: February 2004

The positive spikes observed in the time derivative of turbulent velocity fluctuations constitute the principal non-Gaussian contribution. They are directly related to the irreversible turbulent process. Coincidence between strong spikes detected at four points was measured. The spikes appear in clusters, about as large as the integral scale. The results suggest that they represent crumpled vortex sheets, rather than vortex ligaments or plane vortex sheets.
Detection of the turbulent-nonturbulent interface in slightly heated turbulent shear flows
Authors: LaRue John C.
Published: February 2004

The detection of the interface between turbulent and nonturbulent zones in slightly heated turbulent shear flows on the basis of a temperature signal is discussed. Two detection parameters, a threshold level or gate δθg and a hold time τh, are used. A complete description of the detection process and of the procedure used to select nominal values for the detection param eters is presented. It is shown that the detection process is credible in the sense that the statistics of the intermittency function are nearly independent of the values of the detection parameters. Comparison of the intermittency function statistics with previous results is made.
Shock-wave structure in a partially ionized gas
Authors: Lu C. S. , Huang A. B.
Published: February 2004

The structure of a steady plane shock in a partially ionized gas has been investigated using the Boltzmann equation with a kinetic model as the governing equation and the discrete ordinate method as a tool. The effects of the electric field induced by the charge separation on the shock structure have also been studied. No overshoot of upstream velocity has been observed. Although the three species of an ionized gas travel with approximately the same macroscopic velocity, the individual distribution functions are found to be very different. In a strong shock the atom distribution function may have double peaks, while the ion distribution function has only one peak. Electrons are heated up much earlier than ions and atoms in a partially ionized gas. Because the intereactions of electrons with atoms and with ions are different, the ion temperature can be different from the atom temperature. No undershoot of the upstream temperature has been found. The present results for neutral particles and electrons are generally in fairly good agreement with the continuum solutions at the low Mach number and with the Mott-Smith solutions at the high Mach number. It has been found that the effects of the induced electric field on the shock-wave structure are insignificant for all cases investigated and can be neglected.
Upstream influence in a rarefield hypersonic stream
Authors: Dyer T. Michal , Smith Jerome A.
Published: February 2004

The results of an experimental and theoretical investigation of the flow upstream of a thin foil model aligned with a low density, hypersonic stream are presented. Measurements of number density distributions ahead of the foil for two different foil thicknesses are compared in order to assess the relative importance of leading edge bluntness and aerodynamic surface contributions to the upstream disturbance. Numerical computation of a first collision model for the nearly free molecular flow field confirms the proposed scaling of the flow field. The upstream disturbance can be scaled on λbf, a mean free path for body emitted (wall temperature) molecules traveling into a cloud of free stream molecules.
Predicted spacings in hydrogen-oxygen-argon detonations
Authors: Barthel Harold O.
Published: February 2004

A criterion for predicting the transverse spacing dimensions in gas phase detonations is developed. The criterion depends upon the collision of wave fronts to form new reaction centers which eventually lead to the formation of new cells. Calculated values of transverse spacing over a wide range of pressure, dilution and degree of overdrive in the hydrogen-oxygen-argon system are compared to experimental data for stoichiometric mixtures. The predicted values of spacing with no overdrive are about twice the experimental values. A minimum in the predicted spacing is found with overdrive as well as a cutoff value of overdrive for which the predicted spacing becomes infinite.
Hydrodynamic stability of a laser-driven plasma
Authors: Brueckner Keith A. , Jorna Siebe , Janda Ralph
Published: February 2004

The hydrodynamical stability of a laser-driven plasma is investigated by analytically and numerically solving the set of coupled conservation equations of hydrodynamics to first order. The calculations are illustrated by a series of graphs showing the time evolution of the state variables of a laser-driven deuterium-tritium fusion plasma.
Collisional coupling in counterstreaming laser-produced plasmas
Authors: Koopman David W. , Goforth R. R.
Published: February 2004

The collisional processes which transfer momentum between counterstreaming plasmas are reviewed and applied to the example of a laser-produced plasma expanding into a partially ionized background. Experimental measurements of the dependence of the ion flow field on collisional momentum transfer demonstrate the validity of the simplified treatment of collision processes which have been adopted. A numerical model which simulates the laser-plasma interaction with the background confirms the importance of collisions in previous experimental studies of momentum coupling, and provides some insight into the distinction between collisional and collisionless flow regimes.
Resonant transformation of wave polarization due to plasma nonuniformity
Authors: Faulconer D. W. , Messiaen A. M. , Vandenplas P. E.
Published: February 2004

When a transverse electric wave is incident on a plasma column with an axial density gradient, the resultant scattered field is not purely transverse electric, but also has a transverse magnetic component. The relative importance of the latter is strongly enhanced at the main resonance of the plasma cylinder, its absolute magnitude becoming great even for moderate values of the axial density gradient. The present study is limited to a perturbation analysis of the effect. The inclusion of this effect leads to a much closer agreement between theory and experiment in the positive column of a discharge than was the case for a previous theory which only accounted for transverse electric to transverse magnetic transformation due to plasma axial drift along the column.
Relativistic electron dynamics in a cusped magnetic field
Authors: Rhee M. J. , Destler W. W.
Published: February 2004

Single-particle motion of relativistic electrons in a cusped magnetic field has been studied both analytically and experimentally. The Lagrangian formulation is used to solve for the electron trajectories in some simple magnetic cusp configurations. A threshold energy for transmission through the cusp is found, and confirmed experimentally. Two different electron orbit off-centering mechanisms are discussed; one arising from a nonzero radial component of particle velocity on the upstream side of the cusp transition, and another arising from the finite width of the cusp transition. Experiments are reported confirming the existence of these off-centering mechanisms, and the results are compared with theoretical expectations.
Theory of superpinched relativistic electron beams
Authors: Poukey J. W. , Toepfer A. J.
Published: February 2004

Numerical simulation and analytic theory are used to study the z-dependent equilibria of a charge neutralized, relativistic electron beam, subject to an applied axial electric field. The analytic theory consists of nonparaxial envelope equations derived from a finite temperature fluid model for relativistic beams. The application of the theory to the calculation of beam pinching in high-current relativistic electron beam accelerator diodes is discussed. An effect of the applied electric field in the diode is to enhance beam pinching, for configurations relevant to both low- and high-impedance diodes. In certain cases, the highly-pinched beams have more energy in the random motion of the beam particles than in the longitudinal flow.
Propagation and mode conversion of lower-hybrid waves generated by a finite source
Authors: Bellan P. M. , Porkolab M.
Published: February 2004

The propagation of electrostatic plasma waves, and their subsequent conversion into hot plasma waves at the lower hybrid frequency is calculated for realistic density profiles and finite rf sources in a slab geometry. A finite length slow wave source having a potential distribution φ ∼ cosk0z is found to generate spatial oscillations having a well-defined wavelength. These oscillations are confined to regions bounded by conical curves originating at the ends of the source. The axial distance of rf energy propagation to the lower hybrid layer is found to be greater than the radial distance of propagation by a factor of the order (mi/me)1/2. The conversion at the lower hybrid layer of the electrostatic cold plasma waves excited by a finite source into propagating hot plasma waves is calculated. It is shown that collisional damping at the lower hybrid layer may predominate over mode conversion even for relatively low collision frequencies.
Plasma heating calculations using a transform method
Authors: Eltgroth Peter G.
Published: February 2004

The problem of a strongly driven one-dimensional electrostatic plasma is treated by the method of expansion of the distribution function in terms of Hermite polynomials in velocity space. Numerical results for hydrodynamic variables are similar to those found in simulation calculations. The shapes of the ion and electron distribution functions are significantly altered from Maxwellian form during heating.
Effects of finite-bandwidth driver on the parametric instability
Authors: Thomson J. J. , Karush Jack I.
Published: February 2004

A set of model equations for the parametric instability that consist of damped harmonic oscillators coupled with a stochastic oscillatory driver is considered. With the assumption that the autocorrelation time of the stochastic part of the driver is very short, a solution for the average response of the oscillators is derived. This solution holds for arbitrary damping rates, unlike the results of previous authors. Threshold levels and growth rates for the parametric decay and oscillating two-stream instabilities are found as a function of driver bandwidth.
Cylindrical solitons
Authors: Maxon Stephen , Viecelli James
Published: February 2004

The modified Korteweg-de Vries equation appropriate to small amplitude, cylindrically symmetric ion acoustic waves is given. The numerical solution is obtained and compared with recent work on spherical solitons.
Krylov-Bogoliubov-Mitropolsky method for nonlinear wave modulation
Authors: Kakutani T. , Sugimoto N.
Published: February 2004

The Krylov-Bogoliubov-Mitropolsky perturbation method is applied to systems of nonlinear dispersive waves including plasma waves such as ion-acoustic, magneto-acoustic, and electron plasma waves. It is found that long time slow modulation of the complex wave amplitude can be described by the nonlinear Schrödinger equation for a very wide class of nonlinear dispersive waves.
Connection between the linear and nonlinear theory of anomalous reflection from a plasma slab
Authors: Manheimer Wallace M.
Published: February 2004

It is shown that the threshold for absolute backscatter linear instability is the same as the threshold for nonlinear steady state reflection in a plasma slab.
Resonance cones for frequencies below the ion cyclotron frequency
Authors: Kuehl H. H.
Published: February 2004

For frequencies below the ion cyclotron frequency, it is shown that, the potential of an oscillating point charge in a magnetized plasma contains a new resonance cone which is due to the electrostatic ion wave.
Effect of boundary-layer growth on stability of incompressible flat plate boundary layer with pressure gradient
Authors: Wazzan A. R. , Taghavi H. , Keltner Gerlina
Published: February 2004

The stability of an incompressible flat plate boundary layer with pressure gradient (Hartree pressure gradient parameter β = 1.0, 0.6, 0.4, 0.2, 0, −0.1, −0.14, and −0.1988) is computed from the linearized complete small disturbance equations. The analysis is nevertheless a quasiparallel treatment because although boundary layer growth is accounted for, the disturbance wave function is correct only in a strictly parallel flow. It is found that the nonparallel flow effect has a negligible influence on the critical Reynolds number Rδ*−c for 0.4 ≤ β ≤ 1.0, but leads to a decrease in Rδ*−c in the range −0.1988 ≤ β ≤ 0.4. The V terms lead to an increase in Rδ*−c in the range 0.4 ≤ β ≤ 1.0, and to a decrease in the range −0.1988 ≤ β ≤ 0.4. The stream tube stretching term led to a decrease in Rδ*−c in the range 0.4 ≤ β ≤ 1.0, and to an increase in the range 0.4 ≥ β ≥ −0.1988. The effect of the V terms and the stream tube stretching term ∂2U/∂x∂y appear to dominate all other boundary layer growth terms; near Rδ*−c for β = −0.1988 however, the stream tube stretching term and ∂2V/∂x2 appear to be of the same order of magnitude.
Longitudinal vortex instabilities in laminar boundary layers over curved heated surfaces
Authors: Kahawita René A. , Meroney Robert N.
Published: February 2004

An analysis has been performed of penetrative convective instabilities arising from the combined action of thermal and centrifugal buoyancy forces. The objective has been to examine the effect of various mean temperature and velocity profiles on the critical limit and convective penetration of the disturbances. The linearized perturbation equations have been solved employing an approximate technique. The close analogy between stream-line curvature and thermal stratification effects has been demonstrated. It is found that for parallel layers of fluid along curved heated walls, a unique stability curve for neutral disturbances may be obtained if the quantity plotted along the abscissa is Ra + κNG2 where Ra is the Rayleigh number, NG is the Görtler number, and k is a constant which expresses the relative importance of the mean temperature and velocity profiles.
Spatial waves in turbulent jets. Part II
Authors: Chan Y. Y.
Published: February 2004

The nonlinear development of a spatial growth wave in the shear layer of a circular jet is analyzed by an integral method based on energy considerations. The predicted development of the wave is in good agreement with the experimental data obtained previously except at the decaying portion of the flow. It also shows that the jet spreads more rapidly as the strength of the disturbance induced into the jet increases.
Pressure-induced flows at low Rossby numbers
Authors: Bennetts D. A. , Hocking L. M.
Published: February 2004

The flow induced by a pressure gradient along a pipe in a rapidly rotating system is examined for small but nonzero Rossby numbers. The regions of the flow field where inertial terms are important are identified and the flow determined for pipes of rectangular and circular cross section.
Slip, friction, and heat transfer laws in a merged regime
Authors: Cheng Sin-I , Chen J. H.
Published: February 2004

The phenomenological expressions of the slip velocity, slip temperature, the friction, and the heat transfer rate to the surface in the merged flow regime are determined by comparing the results of numerical integration of the Navier-Stokes equations with experimental data over the sharp leading edge of a flat plate. The best agreement is obtained with the two-stream slip models under the surface condition of total diffused reflection. The absolute error bound of the computed results is given as < 15% in the merged regime.
Low concentration two-phase flow near a stagnation point
Authors: Drew Donald A.
Published: February 2004

The flow of low concentrations of particles in a stagnation point fluid flow is studied. Particular attention is paid to the effect of the fluid pressure on the particle motion, and the boundary layer and its effects are considered. Conditions are found for the particles to be sedimented, that is, deposited on the wall. These conditions can be summarized as follows. If the drag is too small, the adverse pressure gradient will dominate the particle motion, and the particles may not approach the wall. If the drag is sufficiently large, the particles will all approach the wall, but may not reach the wall. However, if the inertia of the particles is sufficiently large, the particles will all be deposited on the wall.
Experimental study of shock wave strengthening by a positive density gradient in a cryogenic shock tube
Authors: Rupert V. C.
Published: February 2004

The strengthening of a shock wave propagating through an isobaric region of increasing density was determined. A new experimental configuration consisting of a pressure driven shock tube mounted vertically with its test section partially immersed in a cryogenic bath was used in this study. The shock wave created by the diaphragm rupture propagated past a region of uniform low density through a strong local density gradient and finally into a high-density zone. The Mach number of the shock emerging from the variable density region was measured and compared with results from approximate theoretical models. Mach numbers considerably higher than predicted were obtained and suggested the existence of shock strengthening processes not hypothesized in the previous theories but significant in this experimental configuration. Preliminary computations showed that multiple interactions between waves generated within the density gradient and flow nonuniformities due to the shock formation mechanism were of sufficient magnitude to explain the observed Mach numbers. This study contributes to the understanding of shock tube operation in the presence of a density gradient and provides a tool useful for experimentation in a cryogenic environment.
Subshock criterion for switch-on shock waves
Authors: Leonard B. P.
Published: February 2004

Conditions are established for determining whether the downstream state of a switch-on hydromagnetic shock wave will be subsonic, sonic, or supersonic, thus giving an explicit criterion for the existence of a gasdynamic subshock within the overall structure. The analysis is made for the general case of an ionizing shock wave with nonzero electric field. The results include the well-known magnetohydrodynamic case in the limit of zero-electric field. Post-shock temperatures are investigated, and it is found that it may be possible for an ionizing switch-on shock to exist with a nonzero electric field without involving a gas-dynamic subshock. In this case, the structure consists of an initial photoionization layer followed by magnetohydrodynamic compression giving a high-temperature post-shock region, the radiation from which drives the upstream photoionization.
Hydrodynamic stability of a rotating liner
Authors: Barcilon A. , Book D. L. , Cooper A. L.
Published: February 2004

The Rayleigh-Taylor instability is investigated for a nonsteady basic state. A model of a magnetically imploded cylindrical metallic liner compressing an axial magnetic field is constructed and used as the basis of a linear stability analysis. The liner, idealized to be without energy loss mechanisms, can be given an initial rotation about its axis. Analytic and numerical techniques are used to study the stability of flutelike (∼eimφ) irrotational perturbations about this state. Stability is quantified in terms of the tendency of the liner to disrupt or to encroach toward the axis, and is determined as a function of mode number m, the form of initial disturbance, liner thickness and the amount of rotation. It is shown that thickening the liner tends to stabilize against both encroachment and disruption, while increasing rotational velocity tends to stabilize against encroachment. Implications for experimental designs are discussed, in particular for experiments with deep compressions (large ratio of initial to final liner radius) of possible interest in the Linus concept of a pulsed power device.
Electrostatic precession of electron layers. Part I: Long E layers
Authors: Condit W. C. , Lovelace R. V.
Published: February 2004

An analysis is made of the coherent electrostatic precession of an unneutralized astron E layer. This precession is analogous to a slow E × B drift of the relativistic electrons of the E layer in the applied axial magnetic field and in the electric field arising from the image charges on the conducting vessel walls. The analysis assumes an E layer with axial length much longer than its radius, a precession frequency much smaller than the relativistic cyclotron frequency, coherent motion of the individual electrons, and small amplitude perturbations. The electrostatic precession is predicted to occur at a much higher frequency than that of the magnetic precession observed for neutralized E layers. The precessional motion is in the direction of the electron cyclotron motion and therefore is expected to be unstable to dissipative forces.
Electrostatic precession of electron layers. Part II: Precession of finite length E layers
Authors: Porter G. D. , Stallard B. W.
Published: February 2004

The precessional stability of un-neutralized, finite-length E layers is examined both theoretically and experimentally. The theoretical analysis assumes the layer is a rigid cylindrical thin shell. Experimental measurements are consistent with the assumption of rigidity. It is shown that in the absence of a toroidal field, an un-neutralized layer is unstable for all amplitudes. This is in contrast with a neutralized layer, which is stable above a critical amplitude. However, the precessional mode is stabilized by the application of a toroidal field. Experimental measurements of the precession frequency show agreement with theory within a factor of two. The discrepancy is attributed to the finite thickness of the layer.
Propagation studies using modulated relativistic electron beams
Authors: Prono D. S.
Published: February 2004

The propagation of intense relativistic electron beams in a gas can be hampered by the low-frequency hose instability. Modulating the beam current, however, enhances the stability of the beam in certain pressure regions. These regions are characterized by the basic parameters of beam radius and plasma channel conductivity profile. Techniques unique to the measurement of these and other plasma parameters are reported and new experimental results are presented. These results pertain to a 5-MeV electron beam of 300-nsec duration. The average beam current ranges from 100 to 300 A, modulated at 120 MHz with peak currents two to three times the average. The neutral gas is hydrogen or nitrogen ranging in pressure from 0.1 to 5 Torr.
Linear ion acoustic waves in a density gradient
Authors: Doucet H. J. , Jones W. D. , Alexeff I.
Published: February 2004

Both the experimental and theoretical behavior of linear ion acoustic waves in density gradients formed in a collisionless discharge plasma have been studied. The experiment and the theory both show a strong spatial growth of the density perturbation produced by the wave when the wave propagates in the direction of increasing density and a damping when the wave propagates in the direction of decreasing density. Theoretically, the growth and damping rates are found to be proportional to n01/2, where n0 is the local unperturbed density. By using the measured density profile, good agreement is found between experiment and the linearized fluid theory. Although the wave amplitude n1, itself, decreases as the wave propagates into a region of lower density, the relative amplitude n1/n0 increases. This can be expected to lead to wave steepening and shock-like behavior, as noted previously by others. The work reported here is mainly concerned with the range where the wavelength is smaller than the characteristic length of variation of the plasma density.
Collisional effects on ion-acoustic waves in a nonequilibrium plasma
Authors: Huang Tsang-Yi , Chen Liu , Hasegawa Akira
Published: February 2004

For a nonisothermal plasma (Te ≫ Ti), there is a range of wavenumbers, λe−1 < k < λe−1εT−3/2, such that electrons and ions are weakly and strongly collisional, respectively. Here, λe is the electron-ion mean free path and εT = Ti/Te. Using a hybrid description, the ion-acoustic dispersion relation corresponding to this wavenumber regime is derived and analyzed. It is found that the dominant collisional effects are due to electron-ion collisions and ion viscosity. The electron-ion collision smears out the wave-particle resonance of slow electrons. Consequently, it reduces the electron Landau damping rate by an order εe1/2 (εe = 1/kλe) and increases the phase velocity by an order εe3/4. The ion viscosity increases the damping rate by an order εT5/2/εe.
Stimulated Compton scattering and self-focusing in the outer regions of a laser-fusion plasma
Authors: Ott Edward , Manheimer Wallace M. , Klein H. H.
Published: February 2004

The collective processes which can occur in the outer regions of a laser-fusion plasma are investigated theoretically and with numerical simulations. The most important processes are induced Compton scatter and self-focusing. Compton scatter can rapidly heat the plasma electrons. The self-focus instability filaments the laser light.
Trapped-electron drift instability caused by bad curvature
Authors: Yoshikawa S. , Okabayashi M.
Published: February 2004

The effect of curvature on the trapped electron drift instability is considered. If the particles are trapped in bad curvature regions, the instability is seen to have a growth rate comparable to that of the temperature gradient instability treated by Kadomtsev and Pogutse. On the other hand, particles trapped in good curvature regions have a stabilizing influence, although a lower growth rate mode (which has a node at the magnetic field minimum) can still exist.
High-amplitude torsional oscillations of a sphere in a viscous fluid
Authors: Folse R. F. , Strahan A. E. , Nail J. M.
Published: February 2004

Measurements have been made of the period and logarithmic decrement of a torsionally oscillating sphere in viscous fluids as a function of the amplitude of oscillation. Amplitude effects were observed which were contrary to the predictions derived from the theory of Carrier and Di Prima.
Prediction of high order moments of turbulent temperature derivatives for large Reynolds numbers
Authors: Antonia R. A. , Van Atta C. W.
Published: February 2004

An analysis based on a consideration of the structure functions of passive turbulent temperature fluctuations explains the experimental Reynolds number dependence of the flatness factor of temperature derivatives reasonably well.
Transverse velocities in fully developed turbulent duct flows
Authors: Fillo J. A.
Published: February 2004

Based on the mechanical energy equation, integrals relating the vorticity and turbulent production terms are established for fully developed turbulent duct flows.
Electron temperature and density measurements by laser scattering in turbulently heated plasma
Authors: DeSilva A. W. , Fujita J. , Kawabe T. , Kondo K. , Uyama T.
Published: February 2004

Electron temperature and density have been measured by Thomson scattering prior to onset of turbulence in a linear turbulent heating experiment. Heating is faster on the outer boundary than in the column center and appears to be classical until the time when resistivity suddenly rises.
Impurity transport in the Pfirsch-Schlüter regime
Authors: Rutherford P. H.
Published: February 2004

It is shown that the classical inward diffusion of high-Z impurities in toroidal plasmas is enhanced by the Pfirsch-Schlüter effect. Numerical transport coefficients are evaluated. Typically, both density and temperature gradients are found to produce inward impurity diffusion.
Absorption of the left wave near the electron cyclotron frequency
Authors: Ferrari L. A. , McQuade A. W. , LaHaye R. J.
Published: February 2004

Strong absorption of the left circularly polarized electromagnetic wave near the electron cyclotron frequency is observed. The time at which the absorption begins depends on the peak power of the wave.
Dairy pasture responses to soil physical properties
Authors: Drewry J. J. , Littlejohn R. P. , Paton R. J. , Singleton P. L. , Monaghan R. M. , Smith L. C.
Published: February 2004

Relationships between relative seasonal pasture yields (total, spring, or summer/autumn) and soil physical properties were investigated. Dairy farm data were collected from trials over 7 years. There were few significant response relationships between total relative yield and each soil physical property at each soil depth, from 0 to 20 cm. Macroporosity (air-filled porosity; pores >30 μm), percentage of pores >300 μm, and total porosity at both 0–5 and 5–10 cm soil depths all had highly significant positive relationships with spring relative yield, with the best fit being for macroporosity. Summer/autumn relative yield had significant but negative relationships with macroporosity and the percentage of pores >300 μm at both 0–5 and 5–10 cm soil depths, and saturated hydraulic conductivity at 0–5 cm. Summation of yield data into seasonal totals gave more meaningful relationships with macroporosity, compared with summation over several individual yield harvests within a season. Macroporosity at 0–5 and 5–10 cm was the most useful indicator for predicting spring and summer/autumn pasture yields, but none of the physical measurements was a reliable indicator of total yield.
The metabolic rate and thermal conductance of the eastern barred bandicoot (Perameles gunnii) at different ambient temperatures
Authors: Ikonomopoulou M. P. , Rose R. W.
Published: February 2004

We investigated the metabolic rate, thermoneutral zone and thermal conductance of the eastern barred bandicoot in Tasmania. Five adult eastern barred bandicoots (two males, three non-reproductive females) were tested at temperatures of 3, 10, 15, 20, 25, 30, 35 and 40°C. The thermoneutral zone was calculated from oxygen consumption and body temperature, measured during the daytime: their normal resting phase. It was found that the thermoneutral zone lies between 25°C and 30°C, with a minimum metabolic rate of 0.51 mL g–1 h–1 and body temperature of 35.8°C. At cooler ambient temperatures (3–20°C) the body temperature decreased to approximately 34.0°C while the metabolic rate increased from 0.7 to 1.3 mL g–1�h–1. At high temperatures (35°C and 40°C) both body temperature (36.9–38.7°C) and metabolic rate (1.0–1.5 mL g–1 h–1) rose. Thermal conductance was low below an ambient temperature of 30°C but increased significantly at higher temperatures. The low thermal conductance (due, in part, to good insulation, a reduced body temperature at lower ambient temperatures, combined with a relatively high metabolic rate) suggests that this species is well adapted to cooler environments but it could not thermoregulate easily at temperatures above 30°C.
LacZ
-expressing olfactory ensheathing cells do not associate with myelinated axons after implantation into the compressed spinal cord
Authors: Boyd J. G. , Lee J. , Skihar V. , Doucette R. , Kawaja M. D.
Published: February 2004

Studies have shown that implanting olfactory ensheathing cells (OECs) may be a promising therapeutic strategy to promote functional recovery after spinal cord injury. Several fundamental questions remain, however, regarding their
in vivo
interactions in the damaged spinal cord. We have induced a clip compression injury at the T10 level of the spinal cord in adult rats. After a delay of 1 week, OECs isolated from embryonic day 18 rats were implanted into the cystic cavity that had formed at the site of injury. Before implantation, OECs were infected with a LacZ-expressing retrovirus. At 3 weeks after implantation,
LacZ
-expressing OECs survived the implantation procedure and remained localized to the cystic cavity. At the electron microscopic level, the cystic cavity had clusters of
LacZ
-expressing OECs and numerous Schwann cells lacking LacZ expression. Although labeled OECs made no direct contact with axons, unlabeled Schwann cells were associated with either a single myelinated axon or multiple unmyelinated axons. Positively labeled OEC processes often enveloped multiple Schwann cell-axon units. These observations suggest that the role of OECs as the primary mediators of the beneficial effects on axon growth, myelination, and functional recovery after spinal cord injury may require re-evaluation.
The glyceryl ester of prostaglandin E2mobilizes calcium and activates signal transduction in RAW264.7 cells
Authors: Nirodi Chaitanya S. , Crews Brenda C. , Kozak Kevin R. , Morrow Jason D. , Marnett Lawrence J.
Published: February 2004

Glyceryl prostaglandins (PG-Gs) are generated by the oxygenation of the endocannabinoid, 2-arachidonylglycerol, by cyclooxygenase 2. The biological consequences of this selective oxygenation are uncertain because the cellular activities of PG-Gs have yet to be defined. We report that the glyceryl ester of PGE2, PGE2-G, triggers rapid, concentration-dependent Ca2+accumulation in a murine macrophage-like cell line, RAW264.7. Ca2+mobilization is not observed after addition of PGE2, PGD2-G, or PGF2α-G but is observed after addition of PGF2α. Moreover, PGE2-G, but not PGE2, stimulates a rapid but transient increase in the levels of inositol 1,4,5-trisphosphate (IP3) as well as the membrane association and activation of PKC. PGE2-G induces a concentration-dependent increase in the levels of phosphorylated extracellular signal regulated kinases 1 and 2 through a pathway that requires the activities of PKC, IP3receptor, and phospholipase C β. The results indicate that PGE2-G triggers Ca2+mobilization, IP3synthesis, and activation of PKC in RAW264.7 macrophage cells at low concentrations. These responses are independent of the hydrolysis of PGE2-G to PGE2.
The role of protein tyrosine phosphatase 1B in Ras signaling
Authors: Dubé Nadia , Cheng Alan , Tremblay Michel L.
Published: February 2004

Protein tyrosine phosphatase (PTP) 1B has been implicated as a negative regulator of multiple signaling pathways downstream of receptor tyrosine kinases. Inhibition of this enzyme was initially thought to potentially lead to increased oncogenic signaling and tumorigenesis. Surprisingly, we show that platelet-derived growth factor-stimulated extracellular-regulated kinase signaling in PTP1B-deficient cells is not significantly hyperactivated. Moreover, these cells exhibit decreased Ras activity and reduced proliferation by way of previously uncharacterized pathways. On immortalization, PTP1B-deficient fibroblasts display increased expression of Ras GTPase-activating protein (p120RasGAP). Furthermore, we demonstrate that p62Dok (downstream of tyrosine kinase) is a putative substrate of PTP1B and that tyrosine phosphorylation of p62Dok is indeed increased in PTP1B-deficient cells. Consistent with the decreased Ras activity in cells lacking PTP1B, introduction of constitutively activated Ras restored extracellular-regulated kinase signaling and their proliferative potential to those of WT cells. These results indicate that loss of PTP1B can lead to decreased Ras signaling, despite enhanced signaling of other pathways. This finding may in part explain the absence of increased tumor incidence in PTP1B-deficient mice. Thus, PTP1B can positively regulate Ras activity by acting on pathways distal to those of receptor tyrosine kinases.
Preferential oxidation of the second phosphatase domain of receptor-like PTP-α revealed by an antibody against oxidized protein tyrosine phosphatases
Authors: Persson Camilla , Sjöblom Tobias , Groen Arnoud , Kappert Kai , Engström Ulla , Hellman Ulf , Heldin Carl-Henrik , den Hertog Jeroen , Östman Arne
Published: February 2004

Protein tyrosine phosphatases (PTPs) constitute a large enzyme family with important biological functions. Inhibition of PTP activity through reversible oxidation of the active-site cysteine residue is emerging as a general, yet poorly characterized, regulatory mechanism. In this study, we describe a generic antibody-based method for detection of oxidation-inactivated PTPs. Previous observations of oxidation of receptor-like PTP (RPTP) α after treatment of cells with H
2
O
2
were confirmed. Platelet-derived growth factor (PDGF)-induced oxidation of endogenous SHP-2, sensitive to treatment with the phosphatidylinositol 3-kinase inhibitor LY294002, was demonstrated. Furthermore, oxidation of RPTPα was shown after UV-irradiation. Interestingly, the catalytically inactive second PTP domain of RPTPα demonstrated higher susceptibility to oxidation. The experiments thus demonstrate previously unrecognized intrinsic differences between PTP domains to susceptibility to oxidation and suggest mechanisms for regulation of RPTPs with tandem PTP domains. The antibody strategy for detection of reversible oxidation is likely to facilitate further studies on regulation of PTPs and might be applicable to analysis of redox regulation of other enzyme families with active-site cysteine residues.
Low acetylcholine during slow-wave sleep is critical for declarative memory consolidation
Authors: Gais Steffen , Born Jan
Published: February 2004

The neurotransmitter acetylcholine is considered essential for proper functioning of the hippocampus-dependent declarative memory system, and it represents a major neuropharmacological target for the treatment of memory deficits, such as those in Alzheimer's disease. During slow-wave sleep (SWS), however, declarative memory consolidation is particularly strong, while acetylcholine levels in the hippocampus drop to a minimum. Observations in rats led to the hypothesis that the low cholinergic tone during SWS is necessary for the replay of new memories in the hippocampus and their long-term storage in neocortical networks. However, this low tone should not affect nondeclarative memory systems. In this study, increasing central nervous cholinergic activation during SWS-rich sleep by posttrial infusion of 0.75 mg of the cholinesterase inhibitor physostigmine completely blocked SWS-related consolidation of declarative memories for word pairs in human subjects. The treatment did not interfere with consolidation of a nondeclarative mirror tracing task. Also, physostigmine did not alter memory consolidation during waking, when the endogenous central nervous cholinergic tone is maximal. These findings are in line with predictions that a low cholinergic tone during SWS is essential for declarative memory consolidation.
Linear-After-The-Exponential (LATE)–PCR: An advanced method of asymmetric PCR and its uses in quantitative real-time analysis
Authors: Sanchez J. Aquiles , Pierce Kenneth E. , Rice John E. , Wangh Lawrence J.
Published: February 2004

Conventional asymmetric PCR is inefficient and difficult to optimize because limiting the concentration of one primer lowers its melting temperature below the reaction annealing temperature. Linear-After-The-Exponential (LATE)–PCR describes a new paradigm for primer design that renders assays as efficient as symmetric PCR assays, regardless of primer ratio. LATE-PCR generates single-stranded products with predictable kinetics for many cycles beyond the exponential phase. LATE-PCR also introduces new probe design criteria that uncouple hybridization probe detection from primer annealing and extension, increase probe reliability, improve allele discrimination, and increase signal strength by 80–250% relative to symmetric PCR. These improvements in PCR are particularly useful for real-time quantitative analysis of target numbers in small samples. LATE-PCR is adaptable to high throughput applications in fields such as clinical diagnostics, biodefense, forensics, and DNA sequencing. We showcase LATE-PCR via amplification of the cystic fibrosis
CF
Δ
508
allele and the Tay-Sachs disease
TSD 1278
allele from single heterozygous cells.