bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonparallel stability of heated two-dimensional boundary layers
Authors: Asrar Waqar , Nayfeh Ali H.
Published: July 2002

The method of multiple scales is used to determine three partial differential equations describing the modulation of the amplitude and complex wavenumbers of three-dimensonal (3-D) waves propagating in two-dimensional (2-D) heated liquid layers. These equations are solved numerically along the characteristics subject to the condition that the ratio of the complex group velocities in the streamwise and transverse directions be real. A new criterion for the most dangerous frequency is proposed. For an n factor of 9, F=25×10−6 is found to be the most dangerous frequency for the Blasius flow. Three-dimensional waves yield lower n factors than 2-D waves, irrespective of the heating distribution. For a power-law heating distribution of the form T=Te+AxN, one cannot make a general statement on the effect of N on the stability. Numerical results are presented that show the n factor to increase with an increase or a decrease in N.
Simulation of the ion-beam-driven drift instability in a magnetic trap. II
Authors: Thomas V. A. , Nevins W. M.
Published: July 2002

Recent experiments on the TMX-U device at the Lawrence Livermore National Laboratory have indicated the possibility of a low-frequency (ω≪Ωci) drift wave being driven unstable by the injection of neutral beams in the thermal barrier region. Nonlocal linear theory is performed for this instability using slab geometry and the electrostatic approximation. Particle simulations are used to verify the linear theory and to recover relevant nonlinear phenomena. Important nonlinear effects include: (1) E×B trapping (2) relaxation of the beam density profiles, and (3) weakly nonlinear electrons (eδφ/Te ≊0.1). These nonlocal simulations are compared with previous simulations of Ref. 1 [Phys. Fluids 28, 2235 (1985)] which used a local approximation for the particle simulations.
Variational formulation for the evolution of a deep-water wavetrain in two space dimensions
Authors: Laedke E. W. , Spatschek K. H.
Published: July 2002

The stability of two-dimensional perturbations of the two-dimensional nonlinear Schrödinger equation for deep-water soliton solutions is investigated. A new variational method is developed that allows calculation of the maximum growth rates close to the cutoff where the conventional variational formulations break down. By a simple numerical evaluation of the variational principle, the growth rate curve is obtained in agreement with previous, purely numerical computations.
Oscillatory and competing instabilities in a nonlinear model for Langmuir circulations
Authors: Moroz Irene M. , Leibovich Sidney
Published: July 2002

A five-mode truncation of the full partial differential equations which describe Langmuir circulations is studied. Instabilities of different kinds are possible depending on the relative strengths of a stabilizing vertical density stratification and a destabilizing vertical Stokes drift gradient. For sufficiently small buoyancy, steady convection obtains, whereas for larger values of the buoyancy oscillatory convection obtains. In addition the competition can create multiple instabilities. Techniques from dynamical systems theory are used to derive equations which describe nonlinear oscillatory convection and the multiple bifurcation, whose character has been explored in the literature. The center manifold and normal form transformations are explained in a direct and elementary manner in order to be clear to readers encountering these methods for the first time. The problem treated may also be interpreted as one of double diffusion, but with boundary conditions not usually considered. The results are discussed in terms of the physics of Langmuir circulations.
Flow under a gate
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

Potential flow of a heavy fluid under a gate is considered. Far downstream the flow is supercritical. The problem is solved numerically by series truncation for various values of the Froude number F. It is found that there are two flows for 1.80≤F≤1.87, one flow for F>1.87, and no flows for F<1.80.
X-ray transmission through a laser-irradiated target
Authors: Ng A. , Adams F. P. , Gazit Y. , Da Silva L.
Published: July 2002

Simultaneous measurements of x-ray intensity and shock breakout at the target rear side showed a significant increase in the postshock x-ray transmission through the target. This was attributed to target decompression by the unloading wave. The results corroborated previously observed delayed radiation heating of the target rear surface. They also indicated that time-integrated x-ray measurements on the target rear side could be affected by target decompression.
Double adiabatic relaxation and stability
Authors: Sun Guo-Zheng , Finn John M.
Published: July 2002

A relaxation model employing double adiabatic magnetohydrodynamics (MHD) is introduced. In this model the neglect of heat flux in the Chew–Goldberger–Low (CGL) equations is treated by considering a very special class of the entropy invariants. The equilibria have uniform temperatures T⊥ and T∥, nearly uniform λ≡j∥/B=μ/σ, where σ is the local firehose parameter, and p=p(B). The density profile is not uniform (unless T⊥=T∥), in qualitative agreement with experiments. The equilibria are shown to be linearly stable, in spite of ‘‘bad’’ curvature, if the firehose and mirror parameters are positive and if the parallel current is not too large. A nonlinear stability criterion involving modified firehose and mirror parameters, and based upon the so-called energy-Casimir method, is also derived.
The stability of imploding detonations: Results of numerical simulations
Authors: Oran Elaine S. , DeVore C. Richard
Published: July 2002

The relative stability of cylindrically imploding shock and detonation waves has been examined using a two-dimensional numerical model. A sequence of increasingly realistic chemistry models is used to explore the effect of model selection on the results. Comparisons with the predictions of Chester–Chisnell–Whitham (CCW) theory for the acceleration of nonreactive shocks and detonations show quantitative agreement between theory and simulation for symmetrically imploding waves. The influence of structural supports in laboratory experiments on the symmetry of imploding waves is simulated by placing an obstacle in the path of the converging flow. Changes in the convergence time, reductions of the peak pressure at implosion, and deviations from symmetry during the implosion induced by the obstacle are greater for detonations than for the corresponding nonreactive shocks, in qualitative agreement with the linearized CCW theory for shocks and Chapman–Jouguet detonations. These conclusions continue to hold when more sophisticated Zel’dovich–von Neumann–Doering or finite-rate chemistry models are assumed. For these models, a substantial amount of new asymmetrical, dynamical structure is evident in the reaction zone behind the leading shock. The results concur with and extend previous theoretical work suggesting that imploding detonation waves are relatively more unstable than nonreactive shocks.
Entrainment effects in vortex flows
Authors: Keller Jakob J.
Published: July 2002

Following recent work of the present author [Phys. Fluids 6, 1524 (1994)], the effects of entrainment of axisymmetric vortex flows are further investigated. Assuming self-similar flow, it is found that, under the influence of entrainment, vortex flows with jet-like cores always approach certain critical flow states, and self-similar solutions cease to exist afterward. It is argued that this apparent paradox can be resolved by accounting for the possibility of vortex breakdown. Furthermore, explanations are presented for the possible appearance of successive vortex breakdown transitions and for the increasingly jet-like character of certain subcritical vortex flows as they approach a cross section of critical flow.
A numerical study of the mixing of a passive scalar in three dimensions in the presence of a mean gradient
Authors: Pumir Alain
Published: July 2002

The mixing of a passive scalar in the presence of a mean gradient is studied in three dimensions by direct numerical simulations. The driving velocity field is either a solution of the three-dimensional (3-D) Navier–Stokes equations, at a microscale Reynolds number in between 20 and 70, and with a Prandtl number varying between 1/8 and 1, or a solution of the Euler equation restricted to a shell of wave numbers, which formally corresponds to an infinite Prandtl number. The probability distribution function (PDF) of the scalar gradients parallel and perpendicular to the direction of the mean gradient are studied. The gradients parallel to the mean gradient have a skewness of order 1 in the range of Péclet number considered. The PDFs are sharply peaked and their maxima correspond to a perfect mixing of the scalar. The PDF of the scalar gradient perpendicular to the mean gradient are reasonably well fit by stretched exponentials. Similar properties are observed for the restricted Euler model. In physical space, the scalar is well mixed in large domains, separated by narrow regions, where very large gradients concentrate. These ‘‘cliffs’’ are found to sit in regions where the flow is hyperbolic, whereas the scalar gradients are much weaker where the flow is elliptic. The present results are generally in agreement with the conclusions reached in a comparable study in two dimensions by Holzer and Siggia (to appear in Phys. Fluids). The stretching acting on the scalar is studied by computing various correlations between scalar gradient and velocity derivatives, as well as the correlations between vorticity and scalar gradient.
Exact solutions for transonic flows of dense gases in two-dimensional and axisymmetric nozzles
Authors: Schnerr G. H. , Molokov S.
Published: July 2002

Flows of dense gases in two-dimensional and axisymmetric nozzles are considered. If during an isentropic expansion the fundamental derivative changes the sign within the flow region, nozzles with two throats and one antithroat are necessary for a shock-free acceleration to supersonic conditions. Exact solutions to the transonic inviscid small-disturbance equation for flows of dense gases in plane and axisymmetric nozzles are obtained. The exact nozzle shapes have only one throat, one antithroat, and an inflection point. In the latter, the minimum or maximum of the Mach number is attained. The flow in a throat–antithroat–throat configuration may be constructed by solving the small-disturbance equation locally for a sequence of points along the isentrope.
Fractal characteristics of isoconcentration surfaces in plumes dispersing in the atmospheric surface layer
Authors: Yee Eugene , Chan R.
Published: July 2002

The geometrical properties of isoconcentration surfaces in a plume dispersing in the atmospheric surface layer are studied using a generalized box-counting method applied to a limited random point set. This method yields the hierarchy of generalized dimensions Dq that can be used to characterize the fractal nature of the plume concentration level sets. The dimension spectra for the concentration level sets are computed from one-dimensional cuts of the concentration field. The concentration level sets are found to be monofractals that can be characterized by one scaling exponent or fractal dimension. The fractal dimension of the level sets is independent of the concentration threshold over a wide range of threshold values. The evolution of the fractal dimension of plume concentration level sets with distance x downwind from the source, cross-wind distance y from the lateral mean-plume centerline, and vertical height z above the ground is examined. At a fixed plume height, the fractal dimension is essentially independent of either x or y. The fractal dimension of the plume isoconcentration surface decreases roughly linearly from a value of 0.7±0.05 near the surface (z≲1 m) to 0.45±0.05 higher up in the plume (z≳8 m). The increased wrinkling of the plume isoconcentration surface near the ground is most likely the result of the increased mean velocity shear and blocking by the surface.
Suppression of the capillary instability in the Rayleigh–Taylor slot problem
Authors: Chen Yi-Ju , Steen Paul H.
Published: July 2002

The classical Rayleigh–Taylor instability for an interface of finite extent is modified by two independently controlled perturbation effects. A component of gravitational acceleration tangent to the interface is imposed and the interface is subjected to a flow-induced pressure field. The stability of the flat horizontal interface when surface tension holds heavier liquid above ambient gas is considered in a 2D model problem. The two perturbations are realized by tilting the interface to the horizontal and by inducing a flow with shear. It is found that the effect of tilt angle or shear on its own is destabilizing, while together, in the right combination, they can stabilize. It is thereby shown how to extend the stability limit over the classical Rayleigh–Taylor result. The framework for the analysis is the classical unperturbed pitchfork bifurcation (codimension 2). The coefficients in the unfolding are calculated by applying the Lyapunov–Schmidt technique to a pinned deformable interface that holds a shear-induced lubrication flow.
The influence of interacting strain rates on turbulence in convex boundary layers
Authors: Schwarz Andreas C. , Plesniak Michael W.
Published: July 2002

A turbulent boundary layer subjected to multiple, additional strain rates, namely convex curvature coupled with zero, favorable, and adverse streamwise pressure gradients (ZPG, FPG, and APG) was investigated experimentally. The Reynolds stresses were suppressed, with respect to flat plate values, due primarily to the effects of strong convex curvature (δ0/R≊0.10). Combined with curvature, the FPG reduced the strength of the wake component, resulted in a greater suppression of the fluctuating velocity components, and a reduction of the primary Reynolds shear stress relative to the ZPG curved case, whereas the APG counteracted the stabilizing curvature effect. The additional influence of streamwise pressure gradients on the normal stresses is most apparent in the outer part of the boundary layer (y+≥100). However, the primary Reynolds shear stress is impacted by the additional strain rates throughout most of the boundary layer, especially for the strong pressure gradients. Using the uv2 quadrant burst detection method, including grouping of ejections, the normalized mean burst period in the convex boundary layer was found to increase in the presence of FPG and to decrease for APG. The time between bursts increased locally (by as much as 75%) in regions of locally strong acceleration. As the pressure gradient was relaxed to zero downstream, the bursting frequency in the case with stronger initial acceleration remained suppressed compared to that with milder initial acceleration, indicating a strain-rate history dependence.
Nonsolenoidal flow in a liquid diffusion couple
Authors: Perera P. S. , Sekerka R. F.
Published: July 2002

Advective nonsolenoidal (∇⋅v≠0) flow driven by diffusion-induced density changes in strictly zero gravity is studied in a two-dimensional rectangular box. Our model, which is more general than the Oberbeck–Boussinesq model, is a precursor for the study of fluid flow that occurs due to density changes during isothermal interdiffusion in a binary liquid under the influence of stochastic microgravity (g-jitter). We consider perturbation expansions of mass fraction (w) of the second chemical component of a binary solution, pressure (p), velocity (v), and chemical flux (j) with respect to a small parameter α [=ρ0∂(1/ρ)/∂w], where ρ is the density and ρ0 is its value for some average composition. The total barycentric velocity field is given by the sum of an average flow, having a nonzero divergence, and a solenoidal flow derived from a pseudo-stream-function. At first order in α, we obtain a fourth order partial differential equation for this pseudo-stream-function. We solve this equation analytically in a quasi-steady-state approximation for an infinitely long diffusion couple by using transform techniques. We also solve it numerically for the full time-dependent problem for a finite domain. We conclude that such nonsolenoidal flows will dominate for sufficiently small gravity, for which the Oberbeck–Boussinesq approximation will certainly not be valid.
Experimental observations of the mixing transition in a shock-accelerated gas curtain
Authors: Rightley P. M. , Vorobieff P. , Martin R. , Benjamin R. F.
Published: July 2002

Richtmyer–Meshkov instability of a thin curtain of heavy gas (SF6) embedded in air and accelerated by a planar shock wave (Mach 1.2) leads to the growth of interfacial perturbations in the curtain and to mixing. Our experiments produce a phenomenological description of the mixing transition and incipient turbulence during the first millisecond after the shock interaction. Growth of scales both larger and smaller than that of initial perturbations is visually observed and quantified by applying a wavelet transform to laser-sheet images of the evolving gas curtain. Histogram and wavelet analyses show an abrupt mixing transition for a multimode initial perturbation that is not apparent for single-mode perturbations.
A new upper bound on the growth rate of linear instability of baroclinic zonal flows in a two-layer model on a beta-plane
Authors: Kanwar Vinay , Sinha A. K.
Published: July 2002

Temporally growing modes of the linearized equations of motion for the baroclinic zonal flows in the two-layer model on a beta-plane are considered. The classical result for the bound on growth rate derived by Joseph Pedlosky for an arbitrary unstable mode has been improved considerably. Further, a rigorous mathematical proof of Howard’s conjecture which states that the growth rate of an arbitrary unstable wave must approach zero as the wavelength approaches zero, is presented in this case for the first time under the restriction of the boundedness of the basic potential vorticity gradient of the mean zonal current in each layer with respect to the horizontal coordinate in the concerned flow domain.
Shift of peak energy distribution in field-emitted charged particle beams
Authors: Uhm Han S. , Choi E. H. , Cho G. S. , Kang S. O.
Published: July 2002

A theoretical model for energy spread and shift of peak energy distribution in a charged particle beam is developed. The beam may expand radially and axially as it propagates downstream. Charged particles placed randomly in the beam possess different potentials, which are converted to different kinetic energies as the beam propagates and expands. This randomness introduces energy spread in the beam. It is also shown that the shift of peak energy distribution in beam particles is proportional to the one-third power of the beam current, which is the manifestation of random particle locations. The theoretical predictions of the energy-distribution shift agree remarkably well with experimental data.
Formation of field-reversed ion rings in a magnetized background plasma
Authors: Omelchenko Yu. A. , Sudan R. N.
Published: July 2002

In typical field-reversed ion ring experiments, an intense annular ion beam is injected across a magnetic cusp into neutral gas immersed in a solenoidal magnetic field. In anticipation of a new experimental thrust to create strong field-reversed ion rings the beam evolution is investigated in a preformed background plasma on a time scale greater than an ion cyclotron period, using a new two and a half-dimensional (21/2-D) hybrid, particle-in-cell (PIC) code FIRE, in which the beam and background ions are treated as macro-particles and the electrons as a massless fluid. It is shown that under appropriate conditions axial beam bunching occurs in the downstream applied field and a compact field-reversed ring is formed. It is observed that the ring is reflected in a ramped magnetic field. Upon reflection its axial velocity is very much less than that expected from a single particle model due to the transfer of the mean axial momentum to the background ions. This increases the time available to apply a pulsed mirror for trapping the ring experimentally.
Nonlinear saturation of the parallel velocity shear instability
Authors: Daughton William , Migliuolo Stefano
Published: July 2002

The linear stability and eventual nonlinear saturation of electrostatic instabilities due to the shear in a plasma flow in the direction parallel to the magnetic field are described, analytically, via collisional fluid equations. Saturation is provided by coupling the linearly unstable eigenmode to a linearly damped nonlinear harmonic (double wave number). These modes are shown to saturate only when the linear instability driving term is below a critical value. The saturation amplitude is a function of the ratio of the ‘‘ideal’’ growth rate to the diamagnetic frequency and can easily reach levels of 25% or more.
Two-body similarity and its violation in tokamak edge plasmas
Authors: Catto Peter J. , Knoll D. A. , Krasheninnikov S. I.
Published: July 2002

Scaling laws found under the assumption that two-body collisions dominate can be effectively used to benchmark complex multi-dimensional codes dedicated to investigating tokamak edge plasmas. The applicability of such scaling laws to the interpretation of experimental data, however, is found to be restricted to the relatively low plasma densities (<1019 m−3) at which multistep processes, which break the two-body collision approximation, are unimportant.
Beam space-charge modes in a free-electron laser
Authors: Willett Joseph E. , Aktas Yildirim , Bolon Bruce
Published: July 2002

Propagation of space-charge waves in a Raman free-electron laser is analyzed. The device under consideration consists of a cylindrical metallic waveguide filled with a relativistic electron beam which passes through a helical wiggler magnetic field and a uniform axial magnetic field. A solution of the fluid equations and Gauss’s law is developed in the form of truncated Fourier and Fourier–Bessel series. The dispersion relation is derived and the combined effects of the waveguide boundary and the wiggler and axial magnetic fields on cyclotron-like waves are studied numerically. In the limit of infinite waveguide radius, the dispersion relation yields a well-known expression in which the combined wiggler and axial fields determine an effective plasma frequency. In the limit of zero wiggler field, it reduces to the dispersion relation for axisymmetric space-charge waves which, in the beam frame, are purely electrostatic.
Particle simulation of ion extraction from plasma by a radio frequency resonance method
Authors: Matsui Tetsuya , Tsuchida Kazuki , Tsuda Shinji , Suzuki Kazumichi , Shoji Tatsuo
Published: July 2002

The mechanism of ion extraction from plasma by rf resonance has been studied by one-dimensional particle simulation. The plasma-sheath resonance under the weak magnetic field occurs at the theoretically predicted frequency in the simulation and it has durability. In the rf period at the resonance, the large electric field penetrates into the plasma and the electrons move collectively due to a polarization drift and E×B drift. Two processes are proposed for the ion extraction mechanism. In the first, the rectified electron current in the resonance causes the plasma potential to be higher. Consequently, ions are extracted to a pair of parallel plate electrodes, which sandwich the plasma. In the second, the time-averaged electric field in the plasma region causes ions to accelerate to both electrodes. This means that the restriction of the plasma shielding effect is overcome by the time-averaged electric field.
Current sheet formation due to nonlinear kink modes in periodic and line-tied configurations
Authors: Lionello Roberto , Schnack Dalton D. , Einaudi Giorgio , Velli Marco
Published: July 2002

The full magnetohydrodynamic evolution of kink instabilities in cylindrical geometry is computed. The equilibria investigated serve as generic models of coronal loops. The effects of both periodic and line-tying boundary conditions at the axial ends of the cylinder are compared and contrasted. The net axial current, which is distributed internally to the loop, can be varied from case to case. It is found that one effect of the line-tying boundary condition is that the minimum length for the onset of a kink instability is increased. For line-tied loops, resonant surfaces do not exist, though linear analysis shows that there may or may not occur quasi-resonant regions, i.e., regions of strong gradients, which, however, are confined to the loop apex, far from the line-tied boundaries. When such a region is present for the linear mode, the formation and nonlinear development of current layers is confined to the central region of the loop. In a case where no such regions exist for the linear mode (corresponding to k⋅B≠0 in the periodic cylinder), current concentrations appear two thirds of the way from the center of the loop. This effect disappears in the axially periodic case. The consequences of these results for solar physics are discussed.
Dispersion characteristics of waves in a waveguide with an annular plasma column
Authors: Mirzanejhad S. , Maraghechi B.
Published: July 2002

A detailed analysis of waves in a cylindrical waveguide containing an annular plasma column is presented. Dispersion relations for four families of electromagnetic and electrostatic modes are derived and solved numerically to study the characteristics of azimuthally symmetrical and nonsymmetrical eigenmodes. The general result which is obtained for the annular plasma is reduced to be used for the thin annular and solid configurations. Figures are presented to illustrate the dependence of frequencies on the plasma radii and the cyclotron frequency. Interaction of the dispersion curves of HE waveguide modes with cyclotron modes is demonstrated graphically. The presented results may be used in the studies of free-electron type devices for generation of high-power electromagnetic radiation.
Particle acceleration in three-dimensional reconnection regions: A new test particle approach
Authors: Schopper Rüdiger , Birk Guido T. , Lesch Harald
Published: July 2002

Magnetic Reconnection is an efficient and fast acceleration mechanism by means of direct electric field acceleration parallel to the magnetic field. Thus, acceleration of particles in reconnection regions is a very important topic in plasma astrophysics. In this paper we show that the conventional analytical models and numerical test particle investigations can be misleading concerning the energy distribution of the accelerated particles, since they oversimplify the electric field structure by the assumption that the field is homogeneous. These investigations of the acceleration of charged test particles are extended by considering three-dimensional field configurations characterized by localized field-aligned electric fields. Moreover, effects of radiative losses are discussed. The comparison between homogeneous and inhomogeneous electric field acceleration in reconnection regions shows dramatic differences concerning both, the maximum particle energy and the form of the energy distribution.
Transport in accretion disks
Authors: Hawley John F. , Balbus Steven A.
Published: July 2002

Astrophysical accretion disks are powered by the release of gravitational potential energy as gas spirals down onto a compact star or black hole. The dynamics and evolution of accretion disks depend upon how angular momentum is transported outward from one fluid element to another. The nature of this process was unclear for many years. Since the early 1990s, however, considerable progress has been made in understanding how turbulence arises and transports angular momentum in astrophysical accretion disks. Accretion disks are generally highly conducting plasmas; the equations governing their evolution are those of ideal magnetohydrodynamics. Although a hydrodynamical disk would be locally stable, the combination of a weak subthermal magnetic field and outwardly decreasing differential rotation rapidly generates magnetohydrodynamical turbulence via a remarkably simple linear instability. Thus, turbulent accretion disks are fundamentally magnetohydrodynamical in nature.
Edge convection driven by externally applied potentials
Authors: D’Ippolito D. A. , Myra J. R.
Published: July 2002

A theoretical model of convection in collisional tokamak edge and scrape-off-layer plasmas is described. In the linear theory, any mechanism for poloidal and toroidal symmetry breaking of the equilibrium will drive E×B flows; this result stems from the parallel thermal and pressure forces in Ohm’s law. In the nonlinear theory, the quadratic coupling of the perturbations leads to quasilinear-type fluxes in the vorticity, density, and temperature equations. If the convection is strong enough, these fluxes lead to an ambipolarity constraint on the equilibrium electric field and to increased transport of particles and energy. The theory shows qualitative agreement with some tokamak experiments in which potential perturbations are externally driven by radio frequency antennas.
Ideal magnetohydrodynamic flow around a blunt body under anisotropic pressure
Authors: Erkaev Nikolai V. , Biernat Helfried K. , Farrugia Charles J.
Published: July 2002

The plasma flow past a blunt obstacle in an ideal magnetohydrodynamic (MHD) model is studied, taking into account the tensorial nature of the plasma pressure. Three different closure relations are explored and compared with one another. The first one is the adiabatic model proposed by Chew, Goldberger, and Low. The second closure is based on the mirror instability criterion, while the third depends on an empirical closure equation obtained from observations of solar wind flow past the Earth’s magnetosphere. The latter is related with the criterion of the anisotropic ion cyclotron instability. In the presented model, the total pressure, defined as the sum of magnetic pressure and perpendicular plasma pressure, is assumed to be a known function of Cartesian coordinates. The calculation is based on the Newtonian approximation for the total pressure along the obstacle and on a quadratic behavior with distance from the obstacle along the normal direction. Profiles of magnetic field strength and plasma parameters are presented along the stagnation stream line between the shock and obstacle of an ideal plasma flow with anisotropy in thermal pressure and temperature.
Studies of laser-generated ultrasonic waveforms at different orientations
Authors: Scruby C. B.
Published: July 2002

The ultrasonic waveforms generated by a pulsed laser incident at two positions on a steel plate are studied as a function of laser energy. At the 45° position there is a steady increase in compression wave amplitude with energy when passing from thermoelastic to ablation regimes, in contrast to the epicenter where there is a large increase in amplitude and change of pulse shape. It is concluded that the thermoelastic and ablation sources are comparable in generation efficiency per unit laser energy, except close to the epicenter, and that the chief effect of raising the power density is to vary the angular distribution of ultrasonic energy.
Bound-exciton-related fine structure in charge transfer spectra of InP:Fe detected by calorimetric absorption spectroscopy
Authors: Juhl A. , Hoffmann A. , Bimberg D. , Schulz H. J.
Published: July 2002

Low-temperature calorimetric absorption spectra (CAS) of semi-insulating InP:Fe show three types of a novel fine structure related to Fe3+/Fe2+ charge transfer transitions. Absorption lines near 0.77 eV are assigned unambiguously to formation of excitons bound to Fe3+ centers. Fine structure of CAS spectra at 0.79 and 1.14 eV is caused by transitions of valence electrons to various 5E and 5T2 levels of the Fe2+ ion, respectively. In addition, there is some indication of a donor-type charge transfer process Fe3+/Fe4+ at 1.1722 eV. The CAS results presented here allow a detailed description of the energy level scheme of InP:Fe.
Single longitudinal mode operation of Er-doped 1.5-μm InGaAsP lasers
Authors: van der Ziel J. P. , Oberg M. G. , Logan R. A.
Published: July 2002

We attribute the single longitudinal mode operation of some Er-doped 1.5-μm InGaAsP lasers, observed here, to inhomogeneities in the active layer resulting from the Er. These results do not rule out the possibility of gain narrowing by the Er when Er is properly incorporated in the active layer.
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u study of the molecular beam epitaxy of CoSi2 on (111) Si by transmission electron microscopy and diffraction
Authors: Gibson J. M. , Batstone J. L. , Tung R. T.
Published: July 2002

The growth of CoSi2 has been studied by deposition and reaction of Co on clean Si (111) surfaces in situ in a modified ultrahigh vacuum transmission electron microscope. On deposition of nominally 20 Å Co at room temperature, strong interaction between Si and Co occurs yielding an epitaxial film which is believed to be a hexagonally distorted form of the Co2Si structure. On heating to ∼350 °C the growth of epitaxial CoSi is observed, which transforms to CoSi2 at ∼450 °C. Silicon-rich phases propagate by lateral motion of phase boundaries. Substantial pinhole density arises in films only after annealing at higher temperatures. The B orientation dominates under similar annealing conditions. In contrast to NiSi2, the growth of cobalt silicide films on Si is dominated by bulk phase formation, with interfacial energy minimized in all cases by epitaxy.
On epitaxial growth of diamond films on (100) silicon substrates
Authors: Narayan J. , Srivatsa A. R. , Peters M. , Yokota S. , Ravi K. V.
Published: July 2002

We have investigated characteristics of polycrystalline diamond thin films formed by plasma-enhanced chemical vapor deposition method on silicon substrates using Raman spectroscopy, analytical and high-resolution transmission electron microscopy techniques. Grains with average size 1 μm in diameter were observed in these films. The Raman spectra from these films contain the strongest peak at 1335 cm−1, providing the characteristic signature for sp3 (diamond) bonding. The broad peak centered around 1550 cm−1 is believed to be due to some graphitic bonding. From detailed high-resolution images and microdiffraction, films were characterized to be cubic diamond with a lattice parameter of 3.56 Å. Diamond crystallites with fivefold external morphologies were also observed. The large crystallites in the films exhibited preferential texture in 〈011〉 type orientations. These crystallites were found to be twinned in {111} planes. The large 〈011〉 crystallites exhibited matching in {111} or {200} lattice planes of diamond with {022} planes of silicon. This is in agreement with our previous work on the growth of Ni on MgO, which showed that textured growth can occur by matching a set of lattice planes in the absence of matching of lattice constants.
Engineering a terbium-binding site into an integral membrane protein for luminescence energy transfer
Authors: Vázquez-Ibar José Luis , Weinglass Adam B. , Kaback H. Ronald
Published: July 2002

Luminescence resonance energy transfer with a lanthanide like Tb3+as donor is a useful technique for estimating intra- and intermolecular distances in macromolecules. However, the technique usually requires the use of a bulky chelator with a flexible linker attached to a Cys residue to bind Tb3+and, for intramolecular studies, an acceptor fluorophor attached to another Cys residue in the same protein. Here, an engineered EF- hand motif is incorporated into the central cytoplasmic loop of the lactose permease ofEscherichia coligenerating a high-affinity site for Tb3+(K\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{Tb}^{3+}}}\end{equation*}\end{document}≈ 4.5 μM) or Gd3+(K\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{Gd}^{3+}}}\end{equation*}\end{document}≈ 2.3 μM). By exciting a Trp residue in the coordination sequence, Tb3+bound to the EF-hand motif is sensitized specifically, and the efficiency of energy transfer to strategically placed Cys residues labeled with fluorophors is measured. In this study, we use the technique to measure distance from the EF-hand in the central cytoplasmic loop of lactose permease to positions 179 or 169 at the center or periplasmic end of helix VI, respectively. The average calculated distances of ≈23 Å (position 179) and ≈33 Å (position 169) observed with three different fluorophors as acceptors agree well with the geometry of a slightly tilted α-helix. The approach should be of general use for studying static and dynamic aspects of polytopic membrane protein structure and function.
Threefold symmetric trimetallic macrocycles formed via the Weak-Link Approach
Authors: Ovchinnikov Maxim V. , Holliday Bradley J. , Mirkin Chad A. , Zakharov Lev N. , Rheingold Arnold L.
Published: July 2002

The synthesis and characterization of a new threefold symmetric hemilabile phosphino-alkylthioether ligand are described. This ligand can be used in combination with Rh(I) and Ir(I) precursor complexes to prepare 40-membered macrocyles with threefold symmetry via the Weak-Link Approach. Synthesis and characterization of two such structures are reported along with a single crystal x-ray diffraction analysis of one of the key intermediates in the case of Rh(I). This is a demonstration of the viability of the Weak-Link Approach for preparing structures other than bimetallic macrocycles and suggests that it could be generalized for a wide range of higher symmetry structures through appropriate hemilabile ligand design.
Aberrant B cell receptor signaling fromB29(Igβ,CD79b) gene mutations of chronic lymphocytic leukemia B cells
Authors: Gordon Melinda S. , Kato Roberta M. , Lansigan Frederick , Thompson Alexis A. , Wall Randolph , Rawlings David J.
Published: July 2002

Chronic lymphocytic leukemia (CLL) B cells characteristically exhibit low or undetectable surface B cell receptor (BCR) and diminished responses to BCR-mediated signaling. These features suggest that CLL cells may have sustained mutations affecting one or more of the BCR proteins required for receptor surface assembly and signal transduction. Loss of expression and mutations in the critical BCR protein B29 (Igβ, CD79b), are prevalent in CLL and could produce the hallmark features of these leukemic B cells. Because patient CLL cells are intractable to manipulation, we developed a model system to analyzeB29mutations. Jurkat T cells stably expressing μ, κ, and mb1 efficiently assembled a functional BCR when infected with recombinant vaccinia virus bearing wild-typeB29. In contrast, a B29 CLL mutant protein truncated in the transmembrane domain did not associate with μ or mb1 at the cell surface. Another B29 CLL mutant lacking the C-terminal immunoreceptor tyrosine activation motif tyrosine and distal residues brought the receptor to the surface as well as wild-type B29 but showed significant impairment in anti-IgM-stimulated signaling events including mitogen-activated protein kinase activation. These findings demonstrate thatB29mutations previously identified in CLL patients can affect BCR-dependent signaling and may contribute to the unresponsive B cell phenotype in CLL. Finally, the features of theB29mutations in CLL predict that they may be generated by somatic hypermutation.
Genetic documentation of filial cannibalism in nature
Authors: DeWoody J. Andrew , Fletcher Dean E. , Wilkins S. David , Avise John C.
Published: July 2002

Cannibalism is widespread in natural populations of fishes, where the stomachs of adults frequently contain conspecific juveniles. Furthermore, field observations suggest that guardian males routinely eat offspring from their own nests. However, recent genetic paternity analyses have shown that fish nests often contain embryos not sired by the nest-tending male (because of cuckoldry events, egg thievery, or nest piracy). Such findings, coupled with the fact that several fish species have known capabilities for distinguishing kin from nonkin, raise the possibility that cannibalism by guardian males is directed primarily or exclusively toward unrelated embryos in their nests. Here, we test this hypothesis by collecting freshly cannibalized embryos from the stomachs of several nest-tending darter and sunfish males in nature and determining their genetic parentage by using polymorphic microsatellite markers. Our molecular results clearly indicate that guardian males do indeed consume their own genetic offspring, even when unrelated (foster) embryos are present within the nest. These data provide genetic documentation of filial cannibalism in nature. Furthermore, they suggest that the phenomenon may result, at least in part, from an inability of guardians to differentiate between kin and nonkin within their own nests.
Nonrandom mating inDrosophila melanogasterlaboratory populations derived from closely adjacent ecologically contrasting slopes at “Evolution Canyon”
Authors: Korol Abraham , Rashkovetsky Eugenia , Iliadi Konstantin , Michalak Pawel , Ronin Yefim , Nevo Eviatar
Published: July 2002

Ecological differentiation of natural populations ofDrosophila melanogaster, Drosophila simulans, and another drosophilid,Zaprionus tuberculatus, in “Evolution Canyon,” Mount Carmel, Israel, is well established. The fitness complex ofD. melanogasterincludes oviposition temperature preferences, tolerance to high temperature, drought stress and starvation, and different longevity patterns. This remarkable differentiation has evolved despite small interslope distances (only 100–400 m), within easy dispersal distance. The differences between populations are those expected from genetic adaptation to local microclimates. How such differentiation could evolve and be maintained despite the likelihood of genetic exchange between populations is a challenging question. We hypothesized that interslope microclimatic differences caused strong differential selection for stress tolerance, accompanied by behavioral differentiation (habitat choice and reduced migration rate), reinforced by sexual isolation. Here we report highly significant mate choice by flies from different slopes of the canyon, with preference for sexual partners originating from the same slope. No preferences were found when the sexual partners belonged to different isofemale lines from the same slope.
Protein threading by learning
Authors: Chang Iksoo , Cieplak Marek , Dima Ruxandra I. , Maritan Amos , Banavar Jayanth R.
Published: July 2002

By using techniques borrowed from statistical physics and neural
networks, we determine the parameters, associated with a scoring
function, that are chosen optimally to ensure complete success in
threading tests in a training set of proteins. These parameters provide
a quantitative measure of the propensities of amino acids to be buried
or exposed and to be in a given secondary structure and are a good
starting point for solving both the threading and design problems.
ATPase activity of Escherichia coli Rep helicase crosslinked to single-stranded DNA: implications for ATP driven helicase translocation.
Authors: Wong I , Lohman T M
Published: July 2002

To examine the coupling of ATP hydrolysis to helicase translocation along DNA, we have purified and characterized complexes of the Escherichia coli Rep protein, a dimeric DNA helicase, covalently crosslinked to a single-stranded hexadecameric oligodeoxynucleotide (S). Crosslinked Rep monomers (PS) as well as singly ligated (P2S) and doubly ligated (P2S2) Rep dimers were characterized. The equilibrium and kinetic constants for Rep dimerization as well as the steady-state ATPase activities of both PS and P2S crosslinked complexes were identical to the values determined for un-crosslinked Rep complexes formed with dT16. Therefore, ATP hydrolysis by both PS and P2S complexes are not coupled to DNA dissociation. This also rules out a strictly unidirectional sliding mechanism for ATP-driven translocation along single-stranded DNA by either PS or the P2S dimer. However, ATP hydrolysis by the doubly ligated P2S2 Rep dimer is coupled to single-stranded DNA dissociation from one subunit of the dimer, although loosely (low efficiency). These results suggest that ATP hydrolysis can drive translocation of the dimeric Rep helicase along DNA by a "rolling" mechanism where the two DNA binding sites of the dimer alternately bind and release DNA. Such a mechanism is biologically important when one subunit binds duplex DNA, followed by subsequent unwinding.
Cell-surface receptors for retroviruses and implications for gene transfer.
Authors: Miller A D
Published: July 2002

Retroviruses can utilize a variety of cell-surface proteins for binding and entry into cells, and the cloning of several of these viral receptors has allowed refinement of models to explain retrovirus tropism. A single receptor appears to be necessary and sufficient for entry of many retroviruses, but exceptions to this simple model are accumulating. For example, HIV requires two proteins for cell entry, neither of which alone is sufficient; 10A1 murine leukemia virus can enter cells by using either of two distinct receptors; two retroviruses can use different receptors in some cells but use the same receptor for entry into other cells; and posttranslational protein modifications and secreted factors can dramatically influence virus entry. These findings greatly complicate the rules governing retrovirus tropism. The mechanism underlying retrovirus evolution to use many receptors for cell entry is not clear, although some evidence supports a mutational model for the evolution of new receptor specificities. Further study of factors that govern retrovirus entry into cells are important for achieving high-efficiency gene transduction to specific cells and for the design of retroviral vectors to target additional receptors for cell entry.
Chemical specificity and physical properties of the lipid bilayer in the regulation of protein kinase C by anionic phospholipids: evidence for the lack of a specific binding site for phosphatidylserine.
Authors: Mosior M , Golini E S , Epand R M
Published: July 2002

The association of protein kinase C (PKC) with membranes was found not to be specific for phosphatidyl-L-serine (PS). In particular, a synthetic phospholipid, dansyl-phosphatidylethanolamine, proved to be fully functional in the association of PKC with lipid bilayers and in mediating the interaction of this enzyme with diacylglycerol. Dansyl-phosphatidylethanolamine was also able to activate the enzyme in a Ca2+-dependent fashion. Differences in the ability to bind and activate PKC observed for an array of anionic lipids were not larger than alterations caused by changes in acyl chain composition. Thus, although different lipids interact to different extents with PKC, there are no specific binding sites for the PS headgroup on the enzyme. We found that lipids with a greater tendency to form inverted phases increased the binding of PKC to bilayers. However, these changes in lipid structure cannot be considered separately from the miscibility of lipid components in the membrane. For pairs of lipids with similar acyl chains, the dependence on PS concentration is sigmoidal, while for dissimilar acyl chains there is much less dependence of binding on PS concentration. The results can be explained in terms of differences in the lateral distribution of components in the membrane.