bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid/kinetic hybrid moment description of plasmas via a Chapman–Enskog-like approach
Authors: Wang J. P. , Callen J. D.
Published: July 2002

A combined fluid and kinetic description of magnetically confined plasmas is developed via a Chapman–Enskog-like procedure. This approach uses the density, energy, momentum, and heat flow conservation equations, and the kinetic equation to recast the plasma kinetic equation with a full Fokker–Planck collision operator into an equation for F, the departure of the distribution function from a ‘‘dynamic’’ Maxwellian. A density, momentum, and energy conserving collision operator model is adopted in deriving the final form of the recast kinetic equation. Both general and drift-kinetic forms of the kinetic equation for F are presented. Closure of the fluid moment equations through calculation of the anisotropic stress tensor Π and anisotropic heat stress tensor Θ using the kinetic solution for F is discussed.
Hydrogen recycling with multistep and resonance line absorption effects
Authors: Marchand R. , Lauzon J.
Published: July 2002

Recycling of hydrogen at a neutralizer plate in a tokamak divertor is considered, with particular emphasis on the effects of multistep atomic processes and photoexcitation by the resonant Lyman α line. These effects are shown to be significant for parameters relevant to International Thermonuclear ExperimentaL Reactor (ITER) [S. A. Cohen et al., J. Nucl. Mater. 176&177, 909 (1990)].
Pressure gradients and flow patterns in the direct current positive column
Authors: Pekker L. , Ernie D. W.
Published: July 2002

A model is presented for the axial neutral particle pressure gradients and flow patterns in the positive column of a monatomic dc gas discharge. Taking into account both the charge imbalance volume force and charged-particle momentum divergence force components, an analytical expression is derived for the driving volume force acting on the neutral particles. This expression is used to numerically solve for the pressure gradients and flow patterns for both closed and bypass discharge tube configurations. For discharges with sufficiently small current densities (e.g., a 0.2 mA, 1.5 mm radius, 5 Torr He discharge), this model yields a reversal in the pressure gradient from that predicted by previous studies where the charge imbalance volume force had been neglected. For bypass tube configurations, this pressure reversal can result in an on-axis cathode directed neutral flow, as opposed to the previously predicted on-axis anode directed flow.
Free energy in plasmas under wave-induced diffusion
Authors: Fisch Nathaniel J. , Rax Jean-Marcel
Published: July 2002

When waves propagate through a bounded plasma, the wave may be amplified or damped at the expense of the plasma kinetic energy. In many cases of interest, the primary effect of the wave is to cause plasma diffusion in velocity and configuration space. In the absence of collisions, the rearrangement of the plasma conserves entropy, as large-grain structures are mixed and fine-grain structures emerge. The maximum extractable energy by waves so diffusing the plasma is a quantity of fundamental interest; it can be defined, but it is difficult to calculate. Through the consideration of specific examples, certain strategies for maximizing energy extraction are identified.
Radio-frequency-sheath-driven edge plasma convection and interaction with the H mode
Authors: D’Ippolito D. A. , Myra J. R. , Jacquinot J. , Bures M.
Published: July 2002

It is shown that radio-frequency (rf) antenna sheaths can bias the edge plasma potential and drive steady-state convective cells in the scrape-off layer (SOL). The resulting E×B convective flow opposes the direction of the sheared flow in the SOL induced by the radially decaying Bohm sheath potential. A two-dimensional fluid simulation shows that the interaction of the opposing poloidal flows produces secondary vortices, which connect the edge of the confined plasma to the antenna limiters, when the antenna–plasma separation is typically of order a few times the local electron skin depth at the antenna. Estimates for typical tokamak edge parameters suggest that the transit time of particles and energy across these vortices is rapid enough to cause the broadening of SOL density and temperature profiles observed during high-power heating with ion cyclotron range of frequency (ICRF) antennas in monopole phasing. Radio-frequency-sheath-driven convection is also a good candidate to explain the phasing dependence of the global confinement properties of ICRF H modes on the Joint European Torus (JET) [Fusion Technol. 11, 13 (1987)]. A comparison of the JET H-mode data with the theoretical modeling supports this idea and suggests that ICRF convection may be a useful tool to spread the heat deposition in the divertor and to extend the lifetime of the H mode.
A one-dimensional model for lower-hybrid current drive including perpendicular dynamics
Authors: Fuchs V. , Cairns R. A. , Shoucri M. M. , Hizanidis K. , Bers A.
Published: July 2002

The two-dimensional (velocity space) Fokker–Planck equation for lower-hybrid current drive is approximated by its perpendicular moments hierarchy closed in the second moment equation. The closure is derived on the basis of a distribution function composed of a central thermal Maxwellian plus a perpendicularly broadened distribution of fast particles that are diffused into, and pitch-angle scattered out of, the quasilinear plateau region. The resulting one-dimensional model reproduces the relevant features of the solutions obtained from numerically integrating the two-dimensional Fokker–Planck equation. An analytic estimate of the perpendicular temperature on the plateau and the plateau height as a function of spectrum width and position is presented. Also predicted are the current density generated and its figure of merit (the current density per unit power density dissipated).
Collisional dynamics of a strongly magnetized pure electron plasma
Authors: O’Neil T. M. , Hjorth P. G.
Published: July 2002

For a pure electron plasma in a sufficiently strong magnetic field, there is a many-electron adiabatic invariant which constrains the collisional dynamics. For the case of a uniform magnetic field, the adiabatic invariant is the total kinetic energy associated with the electron velocity components that are perpendicular to the magnetic field (i.e., ∑j mv2j⊥ /2). Were the adiabatic invariant an exact constant of the motion, no exchange of energy would be possible between the parallel and the perpendicular degrees of freedom, and the plasma could acquire and maintain two different temperatures, T∥ and T⊥. However, an adiabatic invariant is not strictly conserved. In the present case, each collision produces an exponentially small exchange of energy between the parallel and the perpendicular degrees of freedom, and these act cumulatively in such a way that T∥ and T⊥ relax to a common value. This paper provides a calculation of the equipartition rate.
Coherent structures in a turbulent boundary layer. Part 1: Generation of ‘‘artificial’’ bursts
Authors: Gad-el-Hak Mohamed , Hussain A. K. M. Fazlé
Published: July 2002

In an effort to better understand the physics and structure of coherent events in a turbulent boundary layer, an attempt is made to produce ‘‘artificial’’ bursts. These are generated in a turbulent boundary layer developed on a flat plate towed in an 18 m water channel and thus with negligible freestream turbulence. The burstlike events are produced by either withdrawing near-wall fluid from two minute holes separated in the spanwise direction or by pitching a miniature delta wing that is flush-mounted to the wall. Either of these two actions generates a hairpinlike vortex and low-speed streak that resemble naturally occurring structures. The resulting sequence of events that occur at a given location can be uniquely controlled, thus allowing detailed examination via phase-locked measurements and flow visualization. In this paper, the proof of concept of artificial burst generation is established by comparing the artificial events with natural, random bursts using flow visualization and hot-film signals.
An interpretation of the Yakhot–Orszag turbulence theory
Authors: Kraichnan Robert H.
Published: July 2002

Yakhot and Orszag have recently developed a theory of turbulence [J. Sci. Comput. 1, 3 (1986)] based on dynamic renormalization-group (RNG) techniques. They predict parameters of the Kolmogorov inertial range and then successfully use eddy-viscosity formulas from the inertial-range theory in computations of shear flows. In the present paper a critical analysis of the Yakhot–Orszag theory is offered based on comparison with a simple perturbative model. The latter appears to parallel much of the physical and operational content of the lowest order of the Yakhot–Orszag theory, without using RNG methods. The essence is as follows: (1) the dynamics of modes in the inertial and dissipation ranges are assumed to be dominated by interactions more-or-less local in wavenumber that are modeled by a white-noise force acting against an effective viscosity; and (2) the effective viscosity is estimated by extrapolation from the small contributions of interactions very nonlocal in wavenumber (distant interactions). In common with the Yakhot–Orszag theory, the only explicit contacts with the Navier–Stokes equation are overall energy conservation by nonlinear terms and the interaction coefficients of highly nonlocal wave vector triads.
The motion of small spherical particles in a cellular flow field
Authors: Maxey M. R.
Published: July 2002

In an earlier paper, Maxey and Corrsin [J. Atmos. Sci. 43, 1112 (1986)] studied the motion of small aerosol particles settling under gravity through an infinite, periodic, cellular flow field subject to the effects of a Stokes drag force and inertia of the particles. Particle inertia was shown to have an important influence on the motion: No permanent suspension in the flow occurred, particles generally settled more rapidly than in still fluid, and the particle paths merged into isolated asymptotic trajectories. This study is continued for particles that are not necessarily much denser than the surrounding fluid but vary in density. Two basic responses are identified: an aerosol response for particles denser than the fluid, similar to that mentioned, and a bubble response for particles less dense. For both, particle accumulation is still a recurring feature. Results of numerical simulations are discussed, together with the stability of equilibrium points and the role of particle or fluid inertia.
Analysis of energy transfer in direct numerical simulations of isotropic turbulence
Authors: Domaradzki J. Andrzej
Published: July 2002

The energy transfer between wavenumber bands is calculated from the results of direct numerical simulations of isotropic, decaying turbulence. The results are consistent with the notion of the energy cascade induced by the interactions among wavenumbers of comparable lengths. The nonlocal interactions are also present and give rise to the inverse energy cascade.
A comparison of two dynamic subgrid closure methods for turbulent thermal convection
Authors: Wong V. C. , Lilly D. K.
Published: July 2002

Two dynamic subgrid-scale (SGS) closure methods for turbulent thermal convection are described. The first method assumes the dissipation rate equals the SGS energy production rate that includes a troublesome buoyancy term, while the second method avoids this complication with a simplifying scale analysis. Tests with large-eddy simulations (LES) of thermal convection reveal that the second method is computationally efficient, and produces results agreeing with direct numerical simulation (DNS) data, as well as values predicted by the inertial subrange theory. Within the LES, the SGS representation is locally and dynamically adjusted to match the statistical structure of the smallest resolvable eddies.
A vortex ring impinging on a solid plane surface—Vortex structure and surface force
Authors: Chu Chin-Chou , Wang Chi-Tzung , Chang Chien-Cheng
Published: July 2002

A joint numerical and experimental investigation has been performed to study a vortex ring normally impinging on a solid plane surface. Specific emphasis is placed upon the vortex structure and the associated surface force at Reynolds numbers between 500 and 2000. Along the line of Chang [Proc. R. Soc. London Ser. A 437, 517 (1992)], the force was viewed to be contributed by fluid elements with nonvanishing vorticity in the flow. This viewpoint enables the paper to provide a unique opportunity to shed light on the detailed mechanism which causes a small net surface force during the impingement process. It is found that the small net surface force results from the cancellation of a force pushing on the surface, due to the primary vortex ring, and a suction force, due to the induced boundary layer. Nevertheless, both the pushing and suction forces are much larger than that of the net surface force by several orders of magnitude. This fact provides proper directions for flow manipulation regarding forces exerted on the surface.
The wetting of a plane surface by a fluid
Authors: Hocking L. M.
Published: July 2002

The spreading of a thin, planar drop of fluid that completely wets a solid surface is described, from an initial state in which the drop has a compact shape through its final approach to an infinitesimally thin film extending to infinity. Because the slope of the drop surface is everywhere small, the lubrication approximation can be used, and the effects of capillarity, viscosity, and intermolecular forces are all included. It is shown that a simple model for the intermolecular forces allows a mathematically acceptable solution to be found, without the need to invoke a large-distance cutoff and without violating the small-slope requirement of lubrication theory.
Surfactant-driven spreading of a liquid on a vertical surface
Authors: He S. , Ketterson J. B.
Published: July 2002

The spreading behavior of a liquid on the surface of a solid substrate is greatly changed by the presence of a molecular layer of organic material (a surfactant) on the liquid surface. In this work, we studied the spreading of water covered by a monolayer of valinomycin on a vertical glass slide, using an apparatus for Langmuir–Blodgett film deposition. The rate of spreading strongly depends on the surfactant concentration, and the spreading front is highly unstable: it bifurcates while spreading, forming tree-like patterns.
Temporal instability of plane gas sheets in a viscous liquid medium
Authors: Li Xianguo , Bhunia Avijit
Published: July 2002

This paper reports a linear temporal instability analysis of an incompressible plane gas sheet in a quiescent viscous liquid medium of infinite expanse. Results indicate that there exist two unstable modes of disturbance waves, sinuous and varicose, and surface tension always reduces, while the relative velocity between the gas and liquid phases and the gas density always enhance instability development. For both unstable modes, the presence of liquid viscosity increases the instability limit, which is however independent of the absolute value of viscosity. It is also shown that the sinuous mode becomes stable when the gas Weber number, defined as the ratio of aerodynamic forces to surface tension forces, is less than the critical value of one. At slightly larger gas Weber numbers, liquid viscosity exhibits dual effects—it may enhance or suppress the growth of unstable disturbances, depending on specific flow conditions. However, for sinuous mode at high Weber numbers and varicose mode at any Weber numbers, liquid viscosity always reduces disturbance growth rates and dominant wave numbers. Unlike the case for plane liquid sheets, varicose mode controls the instability process for all Weber number ranges and for both inviscid and viscous liquids, and only at high Weber numbers, do varicose and sinuous modes become almost equally important. It is further found that the wave propagation velocity for both unstable modes is much smaller than the gas velocity at the mode of maximum instability, implying that the disturbance waves appear almost stationary rather than travelling-wave type, in contrast with the plane liquid sheet results.
Homogenization theory applied to soil vapor extraction in aggregated soils
Authors: Ng Chiu-On , Mei Chiang C.
Published: July 2002

The macroscale equations governing the convective diffusion of a volatile organic compound during soil vapor extraction in the vadose zone composed of aggregated soils are derived systematically. Although the macroscale effective equations of flow and chemical transport can be developed by intuitive arguments, a formal derivation is presented by the theory of homogenization in order to prescribe precisely the underlying assumptions and the degree of accuracy, and to prepare grounds for further extensions to three-scaled media. Specifically the basic assumptions are (i) the soil matrix is composed of a nearly periodic array of spherical aggregates, (ii) within an aggregate water is immobile because of capillarity and viscous forces, (iii) the aggregates are much smaller than the macroscale, and (iv) diffusion in the aggregates is much weaker than that in the pore air. Local equilibrium is confirmed as a limiting case of the present theory when the aggregate diffusion time is much shorter than the global advection time.
Dynamics of small, spherical particles in vortical and stagnation point flow fields
Authors: Raju N. , Meiburg E.
Published: July 2002

The transport in vortical and stagnation point flow fields is analyzed for particles across the entire range of density ratios, based on the Maxey–Riley equation [Phys. Fluids 26, 883 (1983)] without history effects. For these elementary flow fields, the governing equations simplify substantially, so that analytical progress can be made towards quantifying ejection/entrapment trends and accumulation behavior. For a solid body vortex, the analysis shows that optimal ejection or entrapment occurs for all density ratios, as the difference between inward and outward forces reaches a maximum for intermediate values of the Stokes number. The optimal Stokes number value is provided as a function of the density ratio. Gravity is shown to shift accumulation regions, without affecting the entrapment or ejection rates. For a point vortex flow, the existence of up to three different regimes is demonstrated, which are characterized by different force balances and ejection rates. For this flow, optimal accumulation is demonstrated for intermediate Stokes numbers. The stagnation point flow gives rise to optimal accumulation for heavy particles, whereas light particles do not exhibit optimal behavior. The analysis furthermore indicates that nonvanishing density ratios give rise to a finite Stokes number regime in which the particle motion is oscillatory. Above and below this regime, the motion is overdamped.
Low-dimensional models of subcritical transition to turbulence
Authors: Baggett Jeffrey S. , Trefethen Lloyd N.
Published: July 2002

In the past five years, working largely independently, five groups of researchers have proposed low-dimensional models of the behavior of parallel shear flows at high Reynolds numbers. These models are compared, and it is found that they are more similar than their authors have recognized. Among other similarities, most of them exhibit a threshold amplitude ε=O(Rα) as R→∞ for some α<−1, where R is the Reynolds number, for perturbations of the laminar state that may excite transition to turbulence. The reason for this behavior in each case is an interaction of non-normal linear effects with quadratic nonlinearities.
Intermittency in the two-dimensional inverse cascade of energy: Experimental observations
Authors: Paret Jérôme , Tabeling Patrick
Published: July 2002

An extensive experimental study of the two-dimensional inverse energy cascade is presented. The experiments are performed in electromagnetically driven flows, using thin, stably-stratified layers. Complete instantaneous velocity fields are measured using particle imaging velocimetry techniques. Depending on the bottom-wall friction, two different regimes are observed: when the friction is low, the energy transferred from the forcing scale towards large scales accumulates in the lowest accessible mode, leading to a mean rotation of the flow and to an energy spectrum displaying a sharp peak at the minimum wave-number. This condensation is accompanied by the emergence of a very strong vortex around which the rotation is organized. At higher frictions, the inverse energy cascade conjectured by Kraichnan [Phys. Fluids 10, 1417 (1967)] is observed and is found to be stationary, homogeneous and isotropic, with a Kolmogorov constant consistent with numerical estimates. This inverse cascade does not appear to be characterized by the presence of strong coherent vortices. The characteristic size of the latter is of the order of the injection scale. Their statistical properties tend to show that the cascade is rather driven by a clustering mechanism involving same sign vortices rather than a sequence of merging events producing larger and larger vortices. Intermittency effects are also investigated for the inverse cascade range. It is found that, within experimental errors, there is no intermittency in the inverse cascade range of two-dimensional turbulence and that the statistics of velocity increments, either longitudinal or transverse, are close to Gaussian. These results constitute the first experimental study of intermittency in two-dimensional turbulence as well as the first observation of normal scaling in a field of research which has been increasingly concerned with anomalous exponents.
Lagrangian tetrad dynamics and the phenomenology of turbulence
Authors: Chertkov Michael , Pumir Alain , Shraiman Boris I.
Published: July 2002

A new phenomenological model of turbulent fluctuations is constructed by considering the Lagrangian dynamics of four points (the tetrad). The closure of the equations of motion is achieved by postulating an anisotropic, i.e., tetrad shape dependent, relation of the local pressure and the velocity gradient defined on the tetrad. The nonlocal contribution to the pressure and the incoherent small scale fluctuations are modeled as Gaussian white “noise.” The resulting stochastic model for the coarse-grained velocity gradient is analyzed approximately, yielding predictions for the probability distribution functions of different second- and third-order invariants. The results are compared with the direct numerical simulation of the Navier–Stokes. The model provides a reasonable representation of the nonlinear dynamics involved in energy transfer and vortex stretching and allows the study of interesting aspects of the statistical geometry of turbulence, e.g., vorticity/strain alignment. In a state with a constant energy flux (and K41 power spectrum), it exhibits the anomalous scaling of high moments associated with formation of high gradient sheets—events associated with large energy transfer. An approach to the more complete analysis of the stochastic model, properly including the effect of fluctuations, is outlined and will enable further quantitative juxtaposition of the model with the results of the direct numerical simulations.
A priori analyses of three subgrid-scale models for one-parameter families of filters
Authors: Pruett C. David , Adams Nikolaus A.
Published: July 2002

The decay of isotropic turbulence in a compressible flow is examined by direct numerical simulation (DNS). A priori analyses of the DNS data are then performed to evaluate three subgrid-scale (SGS) models for large-eddy simulation (LES): an eddy-diffusivity model (M1) [J. Fluid Mech. 238, 1 (1992)], a stress-similarity model (M2) [J. Fluid Mech. 275, 83 (1994)], and a gradient model (M3) [Theor. Comput. Fluid Dyn. 8, 309 (1996)]. The models exploit one-parameter second- or fourth-order filters of the Pade type, which permit the cutoff wave number kc to be tuned independently of the grid increment Δx. The modeled (M) and exact (E) SGS-stresses are compared component-wise by correlation coefficients of the form C(E,M) computed over the entire three-dimensional fields. In general, M1 correlates poorly against exact stresses (C<0.2), M3 correlates moderately well (C≈0.6), and M2 correlates remarkably well (0.8<C<1.0). Specifically, correlations C(E,M2) are high provided the grid and test filters are of the same order. Moreover, the highest correlations (C≈1.0) result whenever the grid and test filters are identical (in both order and cutoff). Finally, present results reveal the exact SGS stresses obtained by grid filters of differing orders to be only moderately well correlated. This implies for LES that the model cannot be specified independently of the filter.
An approach to wall modeling in large-eddy simulations
Authors: Nikitin N. V. , Nicoud F. , Wasistho B. , Squires K. D. , Spalart P. R.
Published: July 2002

Channel flow with friction Reynolds number Reτ as high as 80 000 is treated by large-eddy simulation at a moderate cost, using the subgrid-scale model designed for detached-eddy simulations. It includes wall modeling, and was not adjusted for this flow. The grid count scales with the logarithm of the Reynolds number. Three independent codes are in fair agreement with each other. Reynolds-number variations and grid refinement cause trades between viscous, modeled, and resolved shear stresses. The skin-friction coefficient is too low, on the order of 15%. The velocity profiles contain a “modeled” logarithmic layer near the wall and some suggest a “resolved” logarithmic layer farther up, but the two layers have a mismatch of several units in U+.
Electron temperature measurements from optically gray third harmonic electron cyclotron emission in the DIII-D tokamak
Authors: Austin M. E. , Ellis R. F. , James R. A. , Luce T. C.
Published: July 2002

In many high-performance discharges in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] high electron density prevents determining the electron temperature from the second harmonic electron cyclotron emission (ECE). A technique for obtaining central electron temperatures from optically gray third harmonic ECE is presented that does not require knowledge of the reflective properties of the vessel wall. The temperature values derived from ECE spectra measured with an absolutely calibrated Michelson interferometer agree with independent measurements by Thomson scattering. As part of this work, a method of determining the optical depth of third harmonic frequencies in a low aspect ratio tokamak is also demonstrated. The optical depth measurements are in agreement with calculations correct to first order in the Larmor radius.
The role of nonlinear beating currents on parametric instabilities in magnetoplasmas
Authors: Kuo S. P.
Published: July 2002

A general coupled mode equation for the low-frequency decay modes of parametric instabilities in magnetoplasmas is derived. The relative importance of the nonlinear contributions from the ponderomotive force, nonlinear beating current, and anisotropic effect to the parametric coupling is then manifested by the coupling terms of the equation. It is first shown in the unmagnetized case, that the contribution of the nonlinear beating current is negligibly small because of the small coefficient (i.e., weight) of this current contribution, instead of the beating current itself. It then follows that the weight of the beating current contribution increases significantly in the magnetized case, and consequently, this contribution to the parametric coupling is found to be important, as exemplified by two specific examples.
The neoclassical theory of poloidal flow damping in a tokamak
Authors: Morris R. C. , Haines M. G. , Hastie R. J.
Published: July 2002

The damping rate of the poloidal flow uθ in a tokamak is determined in the banana regime as an initial value problem. The bounce averaged drift kinetic equation is solved analytically for early times and numerically for longer time scales of the order of the ion–ion collision time τii. Initial conditions are chosen for the ion distribution function fi(t=0) describing states with similar flows uθ(t=0), but varying structures in pitch angle velocity space. At early times an analytical treatment shows that the damping characteristics of uθ(t) depend sensitively on whether or not the ions resposible for the flow are close to the trapped–passing boundary. Initial decay is shown to be of the form duθ/dt∼(νiiε/t)1/2. A numerical treatment then confirms this early time result and extends the solution to the long term asymptotic decay, which is found to be independent of the initial preparation of the system. This long term evolution is also found to tend to independence of inverse aspect ratio ε as t→0.
Dust-acoustic soliton in a dusty plasma
Authors: Ma J. X. , Liu Jinyuan
Published: July 2002

It is shown that a dusty plasma can admit dust-acoustic solitons on a very slow time scale involving the motion of dust grains, whose charge is self-consistently determined by local electron and ion currents. The solitons exist for a range of velocities and the peak amplitude increases almost linearly with the velocity.
Observation of multiple mechanisms for stimulating ion waves in ignition scale plasmas
Authors: Kirkwood R. K. , MacGowan B. J. , Montgomery D. S. , Afeyan B. B. , Kruer W. L. , Pennington D. M. , Wilks S. C. , Moody J. D. , Wharton K. , Back C. A. , Estabrook K. G. , Glenzer S. H. , Blain M. A. , Berger R. L. , Hinkel D. E. , Lasinski B. F. , Williams E. A. , Munro D. , Wilde B. H. , Rousseaux C.
Published: July 2002

The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have a significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of stimulated Brillouin scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersects, ion waves can be excited at the “beat” frequency and wave number of the intersecting beams, causing the sidescatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et al., Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by stimulated Raman scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et al., Phys. Rev. Lett. 77, 2706 (1996)].
Effects of various forces on the distribution of particles at the boundary of a dusty plasma
Authors: Liu Jin-yuan , Ma J. X.
Published: July 2002

The distribution and suspension of dust particles under the action of electrostatic, gravitational, ion-drag and neutral collision forces are investigated near the boundary of a dusty plasma. It is shown that the competition among the forces results in spatial oscillations (multi-layer) of the particle distribution. For sub-micron grains the ion-drag has a significant effect on the grain dynamics while for micrometer sized grains the gravity quickly dominates over other forces. The effect of the neutral gas flux is to enhance or diminish that of the gravity while the effect of the neutral viscosity is to shift the profile toward the wall. Under the force balance, the particles are suspended in a narrow region with sharp boundaries within the sheath.
Low frequency modes in strongly coupled dusty plasmas
Authors: Kaw P. K. , Sen A.
Published: July 2002

The influence of strong correlations on low frequency collective modes in a dusty plasma is investigated. The dust dynamics is modeled by the generalized hydrodynamics description. For the well known dust acoustic mode, strong correlations lead to new dispersive corrections, an overall reduction of the frequency and phase velocity and the existence of parameter regions where ∂ω/∂k<0. A novel result is the possibility of sustaining a low frequency transverse mode—a dust shear mode—in which the correlation energy acts as an effective bulk modulus. The influence of ion streaming and collisional interaction with a background of neutrals on the modes are also studied and it is shown that the longitudinal modes may be driven unstable by ion streaming.
Drift wave test particle transport in reversed shear profile
Authors: Horton Wendell , Park Hyoung-Bin , Kwon Jae-Min , Strozzi D. , Morrison P. J. , Choi Duk-In
Published: July 2002

Drift wave maps, area preserving maps that describe the motion of charged particles in drift waves, are derived. The maps allow the integration of particle orbits on the long time scale needed to describe transport. Calculations using the drift wave maps show that dramatic improvement in the particle confinement, in the presence of a given level and spectrum of E×B turbulence, can occur for q(r) profiles with reversed shear. A similar reduction in the transport, i.e., one that is independent of the turbulence, is observed in the presence of an equilibrium radial electric field with shear. The transport reduction, caused by the combined effects of radial electric field shear and both monotonic and reversed shear magnetic q profiles, is also investigated.
High-gain direct-drive target design for laser fusion
Authors: Bodner S. E. , Colombant D. G. , Schmitt A. J. , Klapisch M.
Published: July 2002

A new laser fusion target concept is presented with a predicted energy gain of 127 using a 1.3 MJ KrF laser. This energy gain is sufficiently high for an economically attractive fusion reactor. X rays from high- and low-Z materials are used in combination with a low-opacity ablator to spatially tune the isentrope, thereby providing both high fuel compression and a reduction of the ablative Rayleigh–Taylor instability.
Growth of diamond thin films by electron assisted chemical vapor deposition
Authors: Sawabe Atsuhito , Inuzuka Tadao
Published: July 2002

Diamond thin films have been formed by the newly proposed electron assisted chemical vapor deposition on SiC with a high growth rate (3∼5 μm/h). The obtained films have good crystallinity in the sense of electron and x-ray diffraction. Vicker’s hardness of the films is about 9000 kg/mm2. The influence of the electron bombardment on the initial island density on the substrate surface and on the decomposition of the reactant gases (CH4 and H2) is discussed relating to the growth process of the films.
Strain measurement of epitaxial CaF2 on Si (111) by MeV ion channeling
Authors: Hashimoto Shin , Peng J.-L. , Gibson W. M. , Schowalter L. J. , Fathauer R. W.
Published: July 2002

Planar strain in CaF2 films, grown by molecular beam epitaxy at 700 °C on (111) Si substrates, has been measured by MeV 4He+ ion channeling. For CaF2 films thinner than 200 nm, the planar strain was found to be tensile. No strain was observed for films thicker than 200 nm. The observed tensile strain cannot be explained by a simple pseudomorphic growth model since the planar strain would be compressive due to the larger lattice constant of CaF2 relative to Si. A plausible explanation of the results is that defects are nucleated at the growth temperature to relieve stress. These defects then result in a tensile planar strain as the sample is cooled down after growth due to the large difference in thermal expansion coefficients between CaF2 and Si.
Nitrogen related doping with implant Si3N4 formation in Si
Authors: Davies D. Eirug , Adamski Joseph A. , Kennedy E. F.
Published: July 2002

A heavily conducting region has been observed on implanting nitrogen into silicon for forming a buried dielectric layer. The conduction predominantly occurs in the surface silicon layer adjacent to the higher nitrogen content isolating region. The doping, ≳1018 cm−3 and exceeding previously observed nitrogen related doping, is relatively stable at customary 1150–1200 °C processing temperatures and its elimination requires annealing ≳1300 °C.
Morphological studies of oval defects in GaAs epitaxial layers grown by molecular beam epitaxy
Authors: Matteson S. , Shih H. D.
Published: July 2002

The detailed morphology of defects occurring in GaAs layers grown by molecular beam epitaxy (MBE) was determined by high resolution scanning electron microscopy and scanning Auger spectroscopy under various substrate preparation and MBE growth conditions. It was observed that surface defects commonly identified as oval defects are of two varieties: particulate-originated defects and liquid-gallium-originated defects. The former type was shown to be sensitive to the cleanliness of the surface, while the latter type was shown to be determined primarily by the growth conditions. In addition, we found the use of an arsenic cracking source eliminated the liquid-gallium-originated defects.
Production of YBa2Cu3O7−y superconducting thin films i
n s
i
t
u by high-pressure reactive evaporation and rapid thermal annealing
Authors: Lathrop D. K. , Russek S. E. , Buhrman R. A.
Published: July 2002

A high-pressure reactive evaporation process has been used to produce smooth YBa2Cu3O7−y high Tc superconducting films without the necessity of a post-deposition oven anneal cycle. Augmented in some cases by rapid thermal annealing, the process has yielded films with zero resistance transition temperatures above 80 K, and critical current densities above 106 A/cm2 at 4.2 K on both cubic zirconia and SrTiO3 substrates.
Resolution of amorphous silicon thin-film transistor instability mechanisms using ambipolar transistors
Authors: van Berkel C. , Powell M. J.
Published: July 2002

Bias stress measurements on amorphous silicon-silicon nitride ambipolar thin-film transistors give clear evidence for the co-existence of two distinct instability mechanisms: the metastable creation of states in the a-Si:H layer and charge trapping in the a-SiN:H layer. The creation of metastable states in the a-Si:H is found to dominate at low positive bias, while charge trapping in the nitride dominates at larger positive bias and negative bias.
Preparation of superconducting YBa2Cu3Ox thin films by oxygen annealing of multilayer metal films
Authors: Tsaur B-Y. , DiIorio M. S. , Strauss A. J.
Published: July 2002

Superconducting thin films consisting predominantly of YBa2Cu3Ox have been prepared by oxygen annealing of metal films formed by using electron beam evaporation to deposit a three-layer sequence of Cu, Ba, and Y in nominally stoichiometric proportions, then repeating the sequence five times. For the best superconducting films, which were prepared on yttria-stabilized cubic zirconia substrates, the onset of superconductivity occurred at 94 K and zero resistivity was observed at 72 K. For the best films prepared on sapphire substrates, the corresponding temperatures were 95 and 40 K.
Magnetron sputtering and laser patterning of high transition temperature Cu oxide films
Authors: Scheuermann M. , Chi C. C. , Tsuei C. C. , Yee D. S. , Cuomo J. J. , Laibowitz R. B. , Koch R. H. , Braren B. , Srinivasan R. , Plechaty M. M.
Published: July 2002

High Tc Y-Ba-Cu-O films have been prepared by dc magnetron sputtering of metal alloy targets. To circumvent the negative ion effect, two alloy targets, YCu and BaCu, are sputtered in an argon atmosphere with an oxygen spray near the substrate. Films deposited on sapphire with onsets at 92 K and a 6° transition width (10–90%) have been achieved using this technique. These films have been successfully patterned with the technique of laser ablation.
Physical origin of the D
X center
Authors: Bourgoin J. C. , Mauger A.
Published: July 2002

When intervalley mixing effects are large enough, they induce a sharp shallow deep instability for a substitutional impurity. We show that this is the case in GaAs for the ground state associated with the L valleys, which becomes located in the forbidden gap under hydrostatic pressure or in GaAlAs alloys, whereas the donor states associated to the Γ and X valleys remain shallow. This result accounts for the behavior of the so-called DX center.
Cloning and characterization of a cAMP-specific phosphodiesterase (TbPDE2B) from
Trypanosoma brucei
Authors: Rascón Ana , Soderling Scott H. , Schaefer Jonathan B. , Beavo Joseph A.
Published: July 2002

Here we report the cloning, expression, and characterization of a cAMP-specific phosphodiesterase (PDE) from
Trypanosoma brucei
(TbPDE2B). Using a bioinformatic approach, two different expressed sequence tag clones were identified and used to isolate the complete sequence of two identical PDE genes arranged in tandem. Each gene consists of 2,793 bases that predict a protein of 930 aa with a molecular mass of 103.2 kDa. Two GAF (for cGMP binding and stimulated PDEs,
Anabaena a
denylyl cyclases, and
Escherichia coli
FhlA) domains, similar to those contained in many signaling molecules including mammalian PDE2, PDE5, PDE6, PDE10, and PDE11, were located N-terminal to a consensus PDE catalytic domain. The catalytic domain is homologous to the catalytic domain of all 11 mammalian PDEs, the
Dictyostelium discoideum
RegA, and a probable PDE from
Caenorhabditis elegans
. It is most similar to the
T. brucei
PDE2A (89% identity). TbPDE2B has substrate specificity for cAMP with a
K
m
of 2.4 μM. cGMP is not hydrolyzed by TbPDE2B nor does this cyclic nucleotide modulate cAMP PDE activity. The nonselective PDE inhibitors 3-isobutyl-1-methylxanthine, papaverine and pentoxifyline are poor inhibitors of TbPDE2B. Similarly, PDE inhibitors selective for the mammalian PDE families 2, 3, 5, and 6 (erythro-9-[3-(2-hydroxynonyl)]-adenine, enoximone, zaprinast, and sildenafil) were also unable to inhibit this enzyme. However, dipyridamole was a reasonably good inhibitor of this enzyme with an IC
50
of 27 μM. cAMP plays key roles in cell growth and differentiation in this parasite, and PDEs are responsible for the hydrolysis of this important second messenger. Therefore, parasite PDEs, including this one, have the potential to be attractive targets for selective drug design.
On the activation of soluble guanylyl cyclase by nitric oxide
Authors: Bellamy Tomas C. , Wood John , Garthwaite John
Published: July 2002

Soluble guanylyl cyclase (sGC) is the major cellular
receptor for the intercellular messenger nitric oxide (NO) and mediates
a wide range of physiological effects through elevation of
intracellular cGMP levels. Critical to our understanding of how NO
signals are decoded by receptive cells and translated into a useful
physiological response is an appreciation of the molecular and kinetic
details of the mechanism by which NO activates sGC. It is known that NO
binds to a haem prosthetic group on the receptor and triggers a
conformational change that increases the catalysis of cGMP synthesis by
several hundred-fold. The haem is covalently attached to sGC at His-105
of the β1 subunit, and it was thought previously that activation of
sGC by NO occurs in two steps: binding of NO to the haem to form a
biliganded state and then rupture of the bond to His-105 triggering an
increase in catalytic activity. A recent investigation of the kinetics
of sGC activation [Zhao, Y., Brandish, P. E., Ballou, D. P.
& Marletta, M. A. (1999)
Proc. Natl. Acad. Sci.
USA
, 96, 14753–14758], however, proposed an additional
mechanism by which NO regulates sGC activity, namely, by influencing
the rate of cleavage of the His-105 bond. The existence of a second
(unidentified) NO-binding site on the enzyme was hypothesized and
suggested to be fundamental to cellular NO-signal transduction. Here,
we show that it is unnecessary to postulate any such additional
mechanism because the results obtained are predicted by the simpler
model of sGC activation with a single NO-binding event.
Muscle-derived hematopoietic stem cells are hematopoietic in origin
Authors: McKinney-Freeman Shannon L. , Jackson Kathyjo A. , Camargo Fernando D. , Ferrari Giuliana , Mavilio Fulvio , Goodell Margaret A.
Published: July 2002

It has recently been shown that mononuclear cells from murine skeletal muscle contain the potential to repopulate all major peripheral blood lineages in lethally irradiated mice, but the origin of this activity is unknown. We have fractionated muscle cells on the basis of hematopoietic markers to show that the active population exclusively expresses the hematopoietic stem cell antigens Sca-1 and CD45. Muscle cells obtained from 6- to 8-week-old C57BL/6-CD45.1 mice and enriched for cells expressing Sca-1 and CD45 were able to generate hematopoietic but not myogenic colonies
in vitro
and repopulated multiple hematopoietic lineages of lethally irradiated C57BL/6-CD45.2 mice. These data show that muscle-derived hematopoietic stem cells are likely derived from the hematopoietic system and are a result not of transdifferentiation of myogenic stem cells but instead of the presence of substantial numbers of hematopoietic stem cells in the muscle. Although CD45-negative cells were highly myogenic
in vitro
and
in vivo
, CD45-positive muscle-derived cells displayed only very limited myogenic activity and only
in vivo
.
Polycystin 1 is required for the structural integrity of blood vessels
Authors: Kim Keetae , Drummond Iain , Ibraghimov-Beskrovnaya Oxana , Klinger Katherine , Arnaout M. Amin
Published: July 2002

Autosomal dominant polycystic kidney disease (ADPKD), often caused by mutations in the PKD1 gene, is associated with life-threatening vascular abnormalities that are commonly attributed to the frequent occurrence of hypertension. A previously reported targeted mutation of the mouse homologue of PKD1 was not associated with vascular fragility, leading to the suggestion that the vascular lesion may be of a secondary nature. Here we demonstrate a primary role of PKD1 mutations in vascular fragility. Mouse embryos homozygous for the mutant allele (

Pkd1
L

) exhibit s.c. edema, vascular leaks, and rupture of blood vessels, culminating in embryonic lethality at embryonic day 15.5. Kidney and pancreatic ductal cysts are present. The
Pkd1
-encoded protein, mouse polycystin 1, was detected in normal endothelium and the surrounding vascular smooth muscle cells. These data reveal a requisite role for polycystin 1 in maintaining the structural integrity of the vasculature as well as epithelium and suggest that the nature of the PKD1 mutation contributes to the phenotypic variance in ADPKD.
Destabilization of Ca2+-free gelsolin may not be responsible for proteolysis in Familial Amyloidosis of Finnish Type
Authors: Ratnaswamy Gayathri , Huff Mary E. , Su Andrew I. , Rion Severine , Kelly Jeffery W.
Published: July 2002

Mutations at position 187 in secreted gelsolin enable aberrant proteolysis at the 172–173 and 243–244 amide bonds, affording the 71-residue amyloidogenic peptide deposited in Familial Amyloidosis of Finnish Type (FAF). Thermodynamic comparisons of two different domain 2 constructs were carried out to study possible effects of the mutations on proteolytic susceptibility. In the construct we consider to be most representative of domain 2 in the context of the full-length protein (134–266), the D187N FAF variant is slightly destabilized relative to wild type (WT) under the conditions of urea denaturation, but exhibits aTmidentical to WT. The D187Y variant is less stable to intermediate urea concentrations and exhibits aTmthat is estimated to be ≈5°C lower than WT (pH 7.4, Ca2+-free). Although the thermodynamic data indicate that the FAF mutations may slightly destabilize domain 2, these changes are probably not sufficient to shift the native to denatured state equilibrium enough to enable the proteolysis leading to FAF. Biophysical data indicate that these two FAF variants may have different native state structures and possibly different pathways of amyloidosis.