bluuebunny/crossref_metadata_2025 huggingface dataset:
Streamwise structures in a turbulent supersonic boundary layer
Authors: Samimy M. , Arnette S. A. , Elliott G. S.
Published: July 2002

Flow visualizations in a high Reynolds number, Mach 3, fully developed turbulent boundary layer indicate that the upper half of the boundary layer is populated with elongated longitudinal structures. These structures are robust with considerable streamwise but very limited spanwise extent, and are randomly distributed in space and time. Possible mechanisms for the generation of these structures are discussed.
Modulation effects along stability border in Taylor–Couette flow
Authors: Ganske Anette , Gebhardt Thomas , Grossmann Siegfried
Published: July 2002

The stability of time modulated Taylor–Couette flow with co- and counter-rotating cylinders and modulated inner cylinder velocity is investigated. The mean velocities of the inner and outer cylinders are chosen to be in constant relative distance to the stability border of nonmodulated Taylor–Couette flow and the inner cylinder velocity is periodically modulated. The critical modulation amplitude, which leads to linear instability of the modulated laminar flow, is calculated with numerically integrated Floquet theory, as well as with perturbation theory. The critical modulation amplitude as a function of the outer cylinder Reynolds number has a rich structure for counter-rotating cylinders, which is open to experimental test. The critical modulation amplitude decreases monotonically with increasing rotation frequency for corotating cylinders.
Quadrant analysis in a heated-wall supersonic boundary layer
Authors: Deleuze J. , Audiffren N. , Elena M.
Published: July 2002

Quadrant analysis was performed with instantaneous velocities measured by laser Doppler velocimetry in a zero pressure gradient supersonic turbulent boundary layer on an adiabatic or heated plate. The distributions obtained were close to those for subsonic flows, and were not modified by the wall heating. Moreover, this heating made it possible to extend the analysis to a part of the buffer layer that is characteristic of the turbulence process (y+∼15). This led to a differentiation between the effects of Reynolds and Mach numbers in the various regions of the boundary layer.
The forward scattering of sound by vorticity
Authors: Berthet Rémy , Lund Fernando
Published: July 2002

The forward scattering amplitude for the scattering of sound by vorticity appears to diverge. It is pointed out that this divergence occurs only for two-dimensional flows of nonvanishing circulation, in which case it is related to the unphysical assumption of having an infinitely long-range velocity field interacting with an infinitely extended plane wave. Two regularization schemes are possible: In one the velocity field is screened, so as to have an effective finite extent; and in the other the wave is modulated to be of finite amplitude.
Thermodynamic behavior in decaying, compressible turbulence with initially dominant temperature fluctuations
Authors: Cai X. D. , O’Brien E. E. , Ladeinde F.
Published: July 2002

Direct numerical simulation of decaying, isotropic, compressible turbulence in three dimensions is used to examine the behavior of fluctuations in density, temperature and pressure when the initial conditions include temperature fluctuations larger than pressure fluctuations. The numerical procedure is described elsewhere, the initial turbulent Mach number is subsonic, 0.3 to 0.7, and the initial compressible turbulence is characterized as being in one of three states in which the ratios of initial kinetic energy in the compressible modes to total kinetic energy are, respectively, very small, moderate or nearly unity. Only at the lowest values of initial turbulent Mach number and energy ratio do thermodynamic scalings follow the predictions in the literature. For turbulent Mach numbers above 0.3, or for finite values of the kinetic energy ratio, the scalings are more complex. A relationship between turbulent Mach number, compressible pressure and energy ratio, which has been proposed previously for isothermal problems, appears to hold, on average, for the cases computed in this study, all of which are non-isothermal.
Linear and nonlinear development of localized disturbances in zero and adverse pressure gradient boundary-layers
Authors: Bech Knut H. , Henningson Dan S. , Henkes Ruud A. W. M.
Published: July 2002

Localized disturbances in laminar boundary-layers at Reynolds number Reδ*=950 were studied using direct numerical simulation. Instability mechanisms in both an adverse and zero pressure gradient were investigated by introducing three different three-dimensional disturbances. The first disturbance was centered around a pair of oblique waves in Fourier space, the second around a plane wave, while the third was axisymmetric. For small amplitudes, the first disturbance developed into a wave-packet of oblique waves in adverse pressure gradient and into a streaky structure with a trailing wave-packet in a zero pressure gradient. The second disturbance developed into a wave-packet centered around plane waves in both pressure gradients. The third disturbance developed into a wave-packet of plane waves in adverse pressure gradient and, due to the transient growth mechanism, into a streaky structure in a zero pressure gradient. For finite-amplitude plane wave-packets in a zero pressure gradient, a subharmonic secondary instability was observed which subsequently developed into an elongated Λ structure. The secondary instability was less significant in adverse pressure gradient due to the large growth rate of the primary instability. Breakdown was observed as high-frequency oscillations on the spike over the head of the Λ vortices. Providing the initial amplitude was sufficiently large, the vortex pair yielded the fastest route to turbulence. The main growth mechanism in this scenario, in addition to the exponential growth in the adverse pressure gradient case, was the nonlinear excitation of transient growth of streaks by interacting oblique modes. Disturbances dominated by streaks needed substantially larger amplitudes before secondary instability or breakdown occurred. In that case, breakdown was shifted from the spike to an instability of the rear part of the streaks.
Paramagnetic liquid bridge in a gravity-compensating magnetic field
Authors: Mahajan Milind P. , Tsige Mesfin , Taylor P. L. , Rosenblatt Charles
Published: July 2002

Magnetic levitation was used to stabilize cylindrical columns of a paramagnetic liquid in air between two solid supports. The maximum achievable length to diameter ratio Rmax was ∼(3.10±0.07), very close to the Rayleigh–Plateau limit of π. For smaller R, the stability of the column was measured as a function of the Bond number, which could be continuously varied by adjusting the strength of the magnetic field.
Convection in rotating annulus: An asymptotic theory and numerical solutions
Authors: Zhang K. , Greed G. T.
Published: July 2002

Thermal instabilities in the form of two-dimensional convection rolls in a rotating annulus with the flat rigid ends and a moderate gap are investigated by both asymptotic and numerical methods. It is shown that the thin Ekman boundary layers at the ends of the annulus, to which the convection rolls attach, play an active controlling role. An asymptotic theory for an asymptotically large Taylor number T is developed to obtain complete analytical solutions of the convection rolls, indicating at leading order Rc=R0+C(T1/4/λ), ω=0, where λ is the aspect ratio, ω is the frequency, Rc is the critical Rayleigh number with the presence of the Ekman boundary layers and R0 is the critical Rayleigh number without the influence of the boundary layers. While R0 can be determined exactly by using Bessel functions as the eigenfunction, constant C is obtained by matching the interior convection rolls to explicit solutions of the Ekman boundary layers. In the corresponding numerical analysis, convection solutions in a rotating annulus are calculated up to T=108 with λ=1. The analytical and numerical convection solutions are then compared to show a remarkable quantitative agreement when T>106.
Intermittency and eddy viscosities in dynamical models of turbulence
Authors: Benzi R. , Biferale L. , Succi S. , Toschi F.
Published: July 2002

The dependence of intermittent inertial properties on ultraviolet eddy viscosity closures is examined within the framework of shell models of turbulent flows. Inertial intermittent exponents turn out to be fairly independent on the way energy is dissipated at small scales.
Dynamic inverse modeling and its testing in large-eddy simulations of the mixing layer
Authors: Kuerten J. G. M. , Geurts B. J. , Vreman A. W. , Germano M.
Published: July 2002

We propose new identities for dynamic subgrid modeling in large-eddy simulation involving an explicit filter and its inverse. Exact defiltering of a class of numerical realizations of the top-hat filter is developed. The approach is applied to large-eddy simulation of the temporal mixing layer. Smagorinsky’s model is adopted as base model and the results are compared to the standard dynamic eddy-viscosity model as well as to filtered DNS (direct numerical simulation) results. The difference between the results of the two models for the present application is found to be quite small. This is explained by performing a sensitivity analysis with respect to the dynamic coefficient, which hints towards a “self-restoring” response underlying the observed robustness of the physical predictions. Using DNS data the validity of the assumption that the model coefficients are independent of filter width is tested and found to favor the inverse modeling procedure. The computational effort of the dynamic inverse model is 15% smaller than of the standard dynamic eddy-viscosity model.
Recent progress in laser fusion research at Osaka University: Uniformity and stability issues*
Authors: Nishihara K. , Murakami M. , Azechi H. , Jitsuno T. , Kanabe T. , Katayama M. , Miyanaga N. , Nakai M. , Nakatsuka M. , Tsubakimoto K. , Nakai S.
Published: July 2002

Improvement of laser irradiation uniformity in the Gekko XII system [IEEE J. Quantum Electron. QE-17, 1639 (1981)], both single beam pattern and power balance, is discussed. Substantial reduction of laser absorption nonuniformity is obtained for spherical harmonic modes greater than 15 by introducing spectrally dispersed amplified spontaneous emission. No perturbation growth is observed in flat foils accelerated by spectrally dispersed amplified spontaneous emission. Dependences of laser absorption uniformity on beam pattern and power imbalance are investigated in detail. The design goal of the power imbalance in the precision Gekko XII system is 3% in peak to valley, and laser absorption nonuniformity is estimated to be a few percent. Growth of hydrodynamic instabilities is analyzed for high convergence ratio implosions in the precision Gekko XII system.
Chaotic particle dynamics in a periodic focusing quadrupole magnetic field
Authors: Qian Qian , Davidson Ronald C. , Chen Chiping
Published: July 2002

The Hamiltonian for a test particle is used to study the two-dimensional transverse particle dynamics in a non-neutral charged particle beam propagating through a periodic quadrupole focusing field. The self-electric and self-magnetic fields produced by the beam space charge and current are taken into account. The Lie point-symmetry group method and Noether’s theorem are applied to study the integrability of the equations of motion. It is concluded that the particle orbits are regular only for the case of a uniform density beam, and become chaotic when the beam density is nonuniform. Numerical computations, including Poincaré surface-of-section plots and Lyapunov exponents, have been performed to confirm the analytical results. A new numerical scheme, called the numerical irreversibility method, is proposed as an alternative approach to determine the integrability of Hamiltonian systems.
Experimental study of a supersonic low-pressure nitrogen plasma jet
Authors: Robin L. , Vervisch P. , Cheron B. G.
Published: July 2002

A supersonic nitrogen arc plasma jet expanded in a low-pressure test chamber is studied. Radial measurements of temperatures and densities are performed in the expanding region over various sections from the nozzle exit. Nitrogen is highly dissociated in the arc chamber and the plasma is far from equilibrium. Electron temperature and density, as well as gas velocity, are measured by means of parallel and crossed electrostatic probes. Measurements of selected atomic and molecular line intensities are performed in the 250–850 nm spectral range to obtain number densities of various N2, N2+ and N excited levels. The rotationally resolved emission spectrum from the (0–0) band of the N2+ first negative system (B2Σ+u→X2Σ+g) is analyzed and compared to synthetic spectra to evaluate the rotational temperature within the flow field. The kinetic scheme of ionic recombination of N+ and N2+ is examined. A recombination path of the N2(C3Πu) electronic state via the N2(C″5Πu) state is proposed for the N(2D0)+N(4S0) nitrogen recombination.
Collective behavior of non-neutral plasma in a Kingdon trap
Authors: Robertson Scott , Alexander David
Published: July 2002

Pancake-like clouds of non-neutral plasma confined in near-circular orbits in a cylindrical electrostatic (Kingdon) trap will have similarities to planetary rings, galaxies, and accretion disks. For example, the rotation is sheared, interparticle collisions cause radial spreading, and there are spiral density waves and bending waves similar to those observed in the rings of Saturn. The dispersion relations have one term with a change of sign as a consequence of the difference in sign between the Coulomb force and the gravitational force. Disks may be subject to negative mass instabilities if there is insufficient spread in the particle velocities. Charged dust particles have a sufficiently long confinement time to allow experimental observations of these waves and instabilities.
Theory of competition between synchronous and nonsynchronous modes in a magnicon output cavity
Authors: Fliflet Arne W. , Gold Steven H.
Published: July 2002

In the magnicon amplifier, a scanning electron beam drives a synchronous fast-wave interaction in a cylindrical output cavity. The output cavity is designed to support a synchronous transverse magnetic (TM) waveguide mode, usually the TM210 mode. However, a number of other transverse electric (TE) or TM modes can be excited in the cavity via a nonsynchronous, gyrotron-type interaction. To investigate the possibility of competition from these nonsynchronous modes, a multimode gyrotron simulation theory and code have been adapted to the magnicon configuration. The gyrotron theory and corresponding code have been generalized to include a synchronous TM mode as well as nonsynchronous TE modes. Proper phase averaging between the modes, and between the modes and the beam electrons, is critical to accurate mode competition calculations. In nonsynchronous interactions this is achieved by averaging with respect to electron entrance time and the orbit guiding center angle. The synchronous mode interaction is invariant with respect to these two averages; however, it is affected by the scanning angle spread, which is included via a third average over scanning angles. Calculations have been carried out to model a second-harmonic X-band magnicon experiment, which is currently underway at the Naval Research Laboratory (NRL). The output cavity has been optimized for the TM210 mode at 11.4 GHz or twice the drive frequency (ωd=5.7 GHz). The principal competing mode is the TE121 mode. The simulations show that nonsynchronous mode interactions are suppressed by the synchronous interaction if the scanning angle spread is sufficiently small (≤90° in the NRL configuration).
Observation of wave-induced chaotic radial transport in a laboratory terrella experiment
Authors: Warren H. P. , Mauel M. E. , Brennan D. , Taromina S.
Published: July 2002

The wave-induced chaotic radial transport of energetic electrons has been observed in a laboratory terrella, the Collisionless Terrella Experiment (CTX) [H. P. Warren and M. E. Mauel, Phys. Plasmas 2, 4185 (1995)]. In the experiment electron-cyclotron-resonance heating (ECRH) is used to create a localized population of trapped energetic electrons which excite drift-resonant electrostatic fluctuations. Measurements with multiple high-impedance floating potential probes are used to determine the amplitude, frequency, and mode structure of the observed fluctuations. Energetic electron transport is observed with gridded particle detectors. Poincaré surfaces of section indicate that increases in the flux of energetic electrons to the detectors occur only when fluctuations which meet the conditions for global chaos are present. Quasilinear transport simulations do not reproduce several important features of the experimental measurements. In contrast, Hamiltonian simulations reproduce many of the salient temporal characteristics of the experimental measurements and indicate that the persistence of phase-space correlations plays an important role in the energetic electron transport observed in the experiment.
Comments on ‘‘Analysis of electron cyclotron current drive using neoclassical Fokker–Planck code without bounce-average approximation’’ [Phys. Plasmas 2, 4570 (1995)]
Authors: Westerhof E. , Giruzzi G. , Harvey R. W. , O’Brien M. R. , Peeters A. G.
Published: July 2002

The conclusion reached in ref. 1 that the 3-D Fokker-Planck code verses the 2-D bounce-averaged code does not yield the correct current density is prove to be wrong. It is aruged that the results for the 3-D code are very close to those of the 2-D code that employs a bounce-average procedure.(AIP)
Velocity shear generated Alfvén waves in electron–positron plasmas
Authors: Rogava Andria D. , Mahajan S. M. , Berezhiani Vazha I.
Published: July 2002

Linear magnetohydrodynamic (MHD) modes in a cold, nonrelativistic electron–positron plasma shear flow are considered. The general set of differential equations, describing the evolution of perturbations in the framework of the nonmodal approach is derived. It is found, that under certain circumstances, the compressional and shear Alfvén perturbations may exhibit large transient growth fueled by the mean kinetic energy of the shear flow. The velocity shear also induces mode coupling, allowing the exchange of energy as well as the possibility of a strong mutual transformation of these modes into each other. The compressional Alfvén mode may extract the energy of the mean flow and transfer it to the shear Alfvén mode via this coupling. The relevance of these new physical effects to provide a better understanding of the laboratory e+e− plasmas is emphasized. It is speculated that the shear-induced effects in the electron–positron plasmas could also help solve some astrophysical puzzles (e.g., the generation of pulsar radio emission). Since most astrophysical plasmas are relativistic, it is shown that the major results of the study remain valid for weakly sheared relativistic plasmas.
The effect of partial poloidal wall sections on the wall stabilization of external kink modes
Authors: Ward D. J.
Published: July 2002

An analysis of the effect on the wall stabilization of external kink modes due to toroidally continuous gaps in the resistive wall is performed. The effects both with and without toroidal rotation are studied. For a high-β equilibrium, the mode structure is localized on the outboard side. Therefore, outboard gaps greatly increase the growth rate when there is no rotation. For resistive wall stabilization by toroidal rotation, the presence of gaps has the same effect as moving the wall farther away, i.e., destabilizing for the ideal plasma mode, and stabilizing for the resistive wall mode. The region of stability, in terms of wall position, is reduced in size and moved closer to the plasma. However, complete stabilization becomes possible at considerably reduced rotation frequencies. For a high-β, reverse-shear equilibrium both the resistive wall mode and the ideal plasma mode can be stabilized by close fitting discrete passive plates on the outboard side. The necessary toroidal rotation frequency to stabilize the resistive wall mode using these plates is reduced by a factor of three compared to that for a poloidally continuous and complete wall at the same plasma-wall separation.
The interaction of fast alpha particles with pellet ablation clouds
Authors: McChesney J. M. , Parks P. B. , Fisher R. K. , Olson R. E.
Published: July 2002

The energy spectra of energetic confined alpha particles are being measured using the pellet charge exchange method [R. K. Fisher, J. S. Leffler, A. M. Howald, and P. B. Parks, Fusion Technol. 13, 536 (1988)]. The technique uses the dense ablation cloud surrounding an injected impurity pellet to neutralize a fraction of the incident alpha particles, allowing them to escape from the plasma where their energy spectrum can be measured using a neutral particle analyzer. The signal calculations given in the above-mentioned reference disregarded the effects of the alpha particles’ helical Larmor orbits, which causes the alphas to make multiple passes through the cloud. Other effects such as electron ionization by plasma and ablation cloud electrons and the effect of the charge state composition of the cloud, were also neglected. This report considers these issues, reformulates the signal level calculation, and uses a Monte-Carlo approach to calculate the neutralization fractions. The possible effects of energy loss and pitch angle scattering of the alphas are also considered.
Magnetohydrodynamic waves linear evolution in parallel shear flows: Amplification and mutual transformations
Authors: Chagelishvili G. D. , Chanishvili R. G. , Lominadze J. G. , Tevzadze A. G.
Published: July 2002

Evolution of three-dimensional magnetohydrodynamic (MHD) waves [fast magnetosonic (FMW), slow magnetosonic (SMW) and Alfvén waves] is studied in unbounded parallel flows with uniform shear of velocity and uniform magnetic field directed along the flow. The energy exchange between the MHD waves and background flow is explored. This process is noticeably different for each type of wave and is characterized by the unusual (algebraic) behavior of the linear amplification processes. Another novelty is shown in the wave linear evolution process — the coupling of MHD waves and their mutual transformations are originated in a limited time interval for a wide range of systems (flow and waves) parameters. Significant transformation of Alfvén waves into FMW may take place (depending on the parameters of the system) if the former has been initially generated in shear flow. It is possible to reveal these results by employing the nonmodal linear approach which has been extensively used in the study of evolution of perturbations in shear flows since the beginning of the 1990s. The change in the understanding of flow turbulence due to the coupling of the MHD modes is discussed. Namely, the usual consideration of just Alfvén wave turbulence in some astrophysical flows is not always sufficient for a complete analysis-not only should Alfvén waves be “ingredients’’ of turbulence, but magnetosonic waves, as well. In this case MHD turbulence should be of a “mixed’’ type.
Antiscreening channels for ion–ion collisional excitations in dense plasmas
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Plasma-screening effects are investigated on antiscreening channels for excitation of a one-electron target by a one-electron projectile in dense plasmas. The interaction Hamiltonian in dense plasmas is obtained by an appropriate form of the nonspherical Debye–Hückel potentials. Semiclassical straight-line trajectory method is applied to the ion projectile path in order to visualize the antiscreening transition probability in dense plasmas as a function of the impact parameter and the collision energy. The plasma screening effects reduce the transition probabilities, especially, in the dipole–dipole transition. The maximum position of the transition probability is receding from the target nucleus with increasing projectile energy for the dipole–dipole and dipole–quadrupole excitations. However, in the quadrupole–quadrupole excitation, the maximum position is getting closer to the target nucleus with increasing projectile energy. The quadrupole transitions (dipole–quadrupole and quadrupole–quadrupole) show the second maximum which is due to the double peaks of the 2s radial distribution function.
A Lyapunov functional for stability of ideal magnetohydrodynamic systems in arbitrary motion
Authors: Berge Gerhard
Published: July 2002

The stability properties of an ideal magnetohydrodynamic (MHD) fluid (compressible) in an arbitrary state of motion is explored. A stability condition is formulated in general terms based on the concept of a Lyapunov functional for the system which can be taken to be the Hamiltonian. Special consideration is given to stationary systems where the fluid is bounded by a surface on which the normal component of the fluid velocity is zero. For this case a necessary and sufficient condition for stability in terms of an energy principle is formulated. This can be considered to be a generalization of the classic MHD energy principle. A comparison to normal mode solutions is made. Systems being subjected to forced oscillations around a static equilibrium that may be unstable are of prime interest for dynamic stabilization problems. The present theory also includes results relevant to this type of system, although no details of such problems are presented. The emphasis is on general theory, however, one example of an ordinary fluid which is rotating is discussed in order to get a better understanding of the theoretical results.
Deuterium-tritium simulations of the enhanced reversed shear mode in the Tokamak Fusion Test Reactor
Authors: Mikkelsen D. R. , Manickam J. , Scott S. D. , Zarnstorff M. C.
Published: July 2002

The potential performance, in deuterium-tritium plasmas, of a new enhanced confinement regime with reversed magnetic shear [enhanced reversed shear (ERS) mode] is assessed. The equilibrium conditions for an ERS mode plasma are estimated by solving the plasma transport equations using the thermal and particle diffusivities measured in a short duration ERS mode discharge in the Tokamak Fusion Test Reactor [F. M. Levinton et al., Phys. Rev. Lett. 75, 4417 (1995)]. The plasma performance depends strongly on Zeff and neutral beam penetration to the core. The steady-state projections typically have a central electron density of ∼2.5×1020 m−3 and nearly equal central electron and ion temperatures of ∼10 keV. In time-dependent simulations the peak fusion power, ∼ 25 MW, is twice the steady-state level. Peak performance occurs during the density rise when the central ion temperature is close to the optimal value of ∼15 keV. The simulated pressure profiles can be stable to ideal magnetohydrodynamic instabilities with toroidal mode number n=1,2,3,4 and ∞ for βnorm up to 2.5; the simulations have  βnorm⩽2.1. The enhanced reversed shear mode may thus provide an opportunity to conduct alpha physics experiments in conditions similar to those proposed for advanced tokamak reactors.
Spatio-temporal dynamics of the resonantly excited relativistic plasma wave driven by a CO2 laser
Authors: Lal A. K. , Gordon D. , Wharton K. , Clayton C. E. , Marsh K. A. , Mori W. B. , Joshi C. , Everett M. J. , Johnston T. W.
Published: July 2002

The dynamics of a relativistic plasma wave (RPW) resonantly excited by a two frequency CO2 laser pulse and the effects of this wave on a co-propagating relativistic electron beam were studied through experiments and supporting simulations. The amplitude of the RPW and its harmonics were resolved in time and space with a Thomson scattering diagnostic. In addition, the plasma wave amplitude-length product and temporal duration were independently measured through time and frequency resolved forward scattering. The transverse electric and magnetic fields associated with the RPW were studied by the scattering of a 2 MeV electron beam, and the eventual heating of the plasma after the breakup of the RPW was measured from the x-ray radiation spectrum. The experiments and simulations show that the RPW reaches a peak amplitude of approximately 30%, with the amplitude limited by plasma blowout driven by the radial ponderomotive forces of the plasma wave.
Evolution of paired luminous rings in capacitive radio-frequency hydrogen discharges
Authors: Sakawa Y. , Hori M. , Shoji T. , Sato T.
Published: July 2002

The temporal and spatial evolution of paired luminous rings is observed in pulsed capacitive radio-frequency (rf) hydrogen discharges. The time-resolved axial profile of the light intensity indicates that the outermost ring pairs near the electrodes start to appear earlier than the inner ones, and that only the left-side (right-side) rings of ring pairs turn on when the rf voltage applied to the left-side (right-side) electrode is positive. The physical mechanism to create the paired rings seems to be similar to that of the standing striations in dc glow discharges.
Current-driven dust ion-acoustic instability in a collisional dusty plasma with a variable charge
Authors: Annou R.
Published: July 2002

The analysis of the excitation of the dust ion-acoustic (DIA) waves in a collisional dusty plasma by Merlino [IEEE Trans. Plasma Sci. 25, 60 (1997)] has been extended to include the dust charge fluctuations. A damping term arises in the growth rate. It is due to the grain charge variations.
Observation of the cross-excitation instability in a relativistic backward wave oscillator
Authors: Grabowski C. , Schamiloglu E. , Abdallah C. T. , Hegeler F.
Published: July 2002

Experimental observation of the cross-excitation instability in a relativistic backward wave oscillator is reported. The transition between single frequency operation and the cross-excitation regime is measured. The initial mode excited in the cross-excitation regime has a relatively low efficiency, while the second mode has considerably higher efficiency and a slightly higher frequency. The two modes are observed to be beating for a considerable fraction of the radiated microwave pulse. These results are consistent with earlier theoretical predictions for the onset and voltage scaling for this instability.
Wave propagation and absorption simulations for helicon sources
Authors: Mouzouris Y. , Scharer J. E.
Published: July 2002

A two-dimensional (2-D), finite-difference computer code is developed to examine helicon antenna coupling, wave propagation, collisionless Landau, and collisional heating mechanisms. The code calculates the electromagnetic wave fields and power absorption in an inhomogeneous, cold, collisional plasma. The current distribution of the launching antenna, which provides the full antenna spectra, is included in the model. An iterative solution that incorporates warm plasma thermal effects has been added to the code to examine the contribution of collisionless (Landau) wave absorption by electrons. Detailed studies of the wave fields and electron heating profiles at low magnetic fields (B0<100 G), where both Trivelpiece–Gould (TG) and helicon (H) modes are present, are discussed. The effects of the applied uniform magnetic field (B0=10–1000 G), 2-D (r,z) density profiles (ne0=1011–1013 cm−3), neutral gas pressures of 1–10 mTorr and the antenna spectrum on collisional and collisionless wave field solutions and power absorption are investigated. Cases in which the primarily electrostatic (TG) surface wave dominates the heating and the power is absorbed near the edge region and cases in which the propagating helicon wave transports and deposits its energy in the core plasma region are examined.
Temporal evolution of dust grain-charge in a plasma
Authors: Yamaguchi H. , Nejoh Y.-N.
Published: July 2002

Temporal and spatial evolution of the dust-charge in a low temperature plasma is demonstrated by numerical calculation. It is shown that the dust-charge number at the steady state obtained by the present calculation is in good agreement with that derived from an experiment of a direct current glow discharge. By changing the conditions, such as plasma and dust densities and ion and electron temperatures, their effects on the dust-charge are clarified by the present model. These are useful results for understanding the charging process of micron-sized dust grains in plasmas.
Decontamination of chemical and biological warfare (CBW) agents using an atmospheric pressure plasma jet (APPJ)
Authors: Herrmann H. W. , Henins I. , Park J. , Selwyn G. S.
Published: July 2002

The atmospheric pressure plasma jet (APPJ) [A. Schütze et al., IEEE Trans. Plasma Sci. 26, 1685 (1998)] is a nonthermal, high pressure, uniform glow plasma discharge that produces a high velocity effluent stream of highly reactive chemical species. The discharge operates on a feedstock gas (e.g., He/O2/H2O), which flows between an outer, grounded, cylindrical electrode and an inner, coaxial electrode powered at 13.56 MHz rf. While passing through the plasma, the feedgas becomes excited, dissociated or ionized by electron impact. Once the gas exits the discharge volume, ions and electrons are rapidly lost by recombination, but the fast-flowing effluent still contains neutral metastable species (e.g., O2*, He*) and radicals (e.g., O, OH). This reactive effluent has been shown to be an effective neutralizer of surrogates for anthrax spores and mustard blister agent. Unlike conventional wet decontamination methods, the plasma effluent does not cause corrosion and it does not destroy wiring, electronics, or most plastics, making it highly suitable for decontamination of sensitive equipment and interior spaces. Furthermore, the reactive species in the effluent rapidly degrade into harmless products leaving no lingering residue or harmful by-products.
Physics of one-dimensional capsule designs for the National Ignition Facility
Authors: Bradley P. A. , Wilson D. C.
Published: July 2002

This article describes a suite of 250, 280, and 350 eV copper-doped Be [Be(Cu)] capsule designs for the National Ignition Facility [Paisner et al., Laser Focus World 30, 75 (1994)] and compare these to previous Be(Cu) and bromine-doped CH plastic [CH(Br)] capsule designs for 300 and 330 eV drives. These capsule designs are constrained to have the same deuterium-tritium (DT) fuel mass as the 300 and 330 eV designs so that differences in yield are due to differences in capsule compression before ignition. The one-dimensional (1-D) calculations show that the fuel ρr reaches a maximum value when about 20–30 μm of ablator material is left behind, and this amount of ablator material provides the best trade-off between maximizing the fuel ρr, the implosion velocity, and the calculated clean yield. The results of this paper add optimized 1-D capsule designs that operate at drive temperatures of 250, 280, and 350 eV and they complement the established 300 eV CH(Br) ablator and the 330 eV Be(Cu) ablator designs.
Growth of CdTe on GaAs by organometallic vapor phase heteroepitaxy
Authors: Ghandhi Sorab K. , Taskar Nikhil R. , Bhat Ishwara B.
Published: July 2002

Organometallic vapor phase heteroepitaxy of CdTe on (100) oriented GaAs substrates is described in this letter. It is shown that high quality CdTe layers can be grown on GaAs substrates over the temperature range 350–440 °C by this process. Growth under different temperatures and reactant partial pressures is described. Conditions are outlined for obtaining optimum photoluminescence properties, with suppression of the defect level associated with the cadmium vacancy. Electron channeling data are presented to indicate that excellent epitaxy is achieved by this process. The orientation of the epitaxial layer is found to be the same as that of the substrate.
Control of a natural permeable CoSi2 base transistor
Authors: Tung R. T. , Levi A. F. J. , Gibson J. M.
Published: July 2002

We report results on the fabrication of a natural permeable base transistor in a Si/CoSi2/Si heterostructure using molecular beam epitaxy (MBE). No photolithography is required, the transistor action being controlled by MBE growth conditions through the density and size of natural openings in the silicide base. The electrical characteristics of devices processed from such heterostructures are intimately related to the presence of these openings. Common base current gains in the range 0.01–0.95 have been observed and correlated with the size and density of the openings.
Fast write and erase surface resistance induced nematic liquid crystal display
Authors: Monkade M. , Fuliang Zang , Martinot-Lagarde P. , Durand G.
Published: July 2002

A room-temperature nematic liquid crystal cell is prepared in the homeotropic geometry using dielectrically positive pentyl cyanobiphenyl. The indium tin oxide electrodes are treated to increase locally their surface resistance. At high frequencies (ω>ωc) the border of these regions behaves as capacitance edge, creating an oblique field which ‘‘writes.’’ At low frequencies (ω<ωc) the field is back normal to the electrodes and ‘‘erases’’ rapidly. A 50-μs erasing time has been achieved for E∼10 V/μm and ωc/2π=104 Hz. The practical smallest response time of the device is ∼(8ωc)−1. Sub-μs writing and erasing are possible.
Measurement of radiative, Auger, and nonradiative currents in 1.3-μm InGaAsP buried heterostructure lasers
Authors: Olshansky Robert , LaCourse Joanne , Chow Terrance , Powazinik William
Published: July 2002

Frequency response measurements are used to determine the carrier lifetime of 1.3-μm InGaAsP buried heterostructure lasers between 1 mA and threshold. The data confirm previous results on the radiative and Auger recombination coefficients and reveal the presence of a nonradiative current which dominates at low currents and contributes 4 mA at threshold.
Hydrogen neutralization of chalcogen double donors in silicon
Authors: Pensl G. , Roos G. , Holm C. , Sirtl E. , Johnson N. M.
Published: July 2002

Hydrogen neutralization of chalcogen (S, Se, and Te) double-donor centers in single-crystal silicon is demonstrated with deep level transient spectroscopy. The deep-donor chalcogen concentration can be reduced by greater than a factor of 100, while in the same samples the phosphorus shallow-donor concentration decreases by only a small percentage. Both electronic levels of the double donors were fully removed by hydrogenation and recovered with an anneal at 500 °C.
Surface photovoltage spectroscopy of surface states on indium phosphide
Authors: Byun Y. , Wessels B. W.
Published: July 2002

Indium phosphide photoelectrodes have been studied in situ using surface photovoltage spectroscopy. The observed spectra were a strong function of electrode surface conditions. Electronic properties of the chemically induced surface states correlated well with previously reported values determined by surface photovoltage spectroscopy measurements in vacuum. Metal deposits of Cu and Ag were found to induce deep surface states at 0.9 eV above the valence-band edge.
Semiconductor figure of merit for nonlinear directional couplers
Authors: Wright E. M. , Koch S. W. , Ehrlich J. E. , Seaton C. T. , Stegeman G. I.
Published: July 2002

We present numerical calculations of the material figure of merit for a variety of bulk semiconductors at room temperature. For efficient switching to be possible in a half-beat-length nonlinear directional coupler, a minimum material requirement is that the figure of merit should exceed unity.
The mouse
Nkx-1.2
homeobox gene: Alternative RNA splicing at canonical and noncanonical splice sites
Authors: Rovescalli Alessandra Cecilia , Cinquanta Mario , Ferrante Jacqueline , Kozak Christine A. , Nirenberg Marshall
Published: July 2002

A mouse homeobox gene,
Nkx-1.2
, (previously termed
Sax-1)
that is closely related to the
Drosophila NK-1/S59
gene was cloned, and genomic DNA and cDNA were sequenced. Nine
Nkx-1.2
cDNA clones were found that correspond to three species of
Nkx-1.2
mRNA that are formed by alternative splicing at conventional 5′ donor and 3′ acceptor splice sites; however, seven cDNA clones were found that correspond to three species of
Nkx-1.2
mRNA from testes that have novel TG/AC 5′ and 3′ splice sites. The consensus splice sequences are: 5′ donor, CC↓TGGAAG; 3′ acceptor, ACTTAC↓. Predicted amino acid sequences suggest that some transcripts may be translated into proteins that lack part or all of the homeodomain. At least three bands of
Nkx-1.2
mRNA were found in RNA from the testes.
Nkx-1.2
mRNA was shown to be present in postmeiotic germ cells of the testis and in mature spermatozoa.
Nkx-1.2
mRNA also was found in regions of the adult cerebral cortex, hippocampus, diencephalon, pons/medulla, and cerebellum.
Nkx-1.2
mRNA was found in embryos in highest abundance in 10-day embryos; the mRNA levels decrease during further development.
Nkx-1.2
mRNA also was found in discrete zones of the embryonic mesencephalon and myelencephalon.
Preservation of myocardial β-adrenergic receptor signaling delays the development of heart failure after myocardial infarction
Authors: White David C. , Hata Jonathan A. , Shah Ashish S. , Glower Donald D. , Lefkowitz Robert J. , Koch Walter J.
Published: July 2002

When the heart fails, there is often a constellation of biochemical alterations of the β-adrenergic receptor (βAR) signaling system, leading to the loss of cardiac inotropic reserve. βAR down-regulation and functional uncoupling are mediated through enhanced activity of the βAR kinase (βARK1), the expression of which is increased in ischemic and failing myocardium. These changes are widely viewed as representing an adaptive mechanism, which protects the heart against chronic activation. In this study, we demonstrate, using
in vivo
intracoronary adenoviral-mediated gene delivery of a peptide inhibitor of βARK1 (βARKct), that the desensitization and down-regulation of βARs seen in the failing heart may actually be maladaptive. In a rabbit model of heart failure induced by myocardial infarction, which recapitulates the biochemical βAR abnormalities seen in human heart failure, delivery of the βARKct transgene at the time of myocardial infarction prevents the rise in βARK1 activity and expression and thereby maintains βAR density and signaling at normal levels. Rather than leading to deleterious effects, cardiac function is improved, and the development of heart failure is delayed. These results appear to challenge the notion that dampening of βAR signaling in the failing heart is protective, and they may lead to novel therapeutic strategies to treat heart disease via inhibition of βARK1 and preservation of myocardial βAR function.
The chemokine growth-related gene product β protects rat cerebellar granule cells from apoptotic cell death through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors
Authors: Limatola Cristina , Ciotti Maria Teresa , Mercanti Delio , Vacca Fabrizio , Ragozzino Davide , Giovannelli Aldo , Santoni Angela , Eusebi Fabrizio , Miledi Ricardo
Published: July 2002

Cultured cerebellar granule neurons are widely used as a cellular model to study mechanisms of neuronal cell death because they undergo programmed cell death when switched from a culture medium containing 25 mM to one containing 5 mM K+. We have found that the growth-related gene product β (GROβ) partially prevents the K+-depletion-induced cell death, and that the neuroprotective action of GROβ on granule cells is mediated through the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type of ionotropic glutamate receptors. GROβ-induced survival was suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione, which is a specific antagonist of AMPA/kainate receptors; it was not affected by the inhibitor ofN-methyl-d-aspartate receptors, 2-amino-5-phosphonopentanoic acid, and was comparable to the survival of granule cells induced by AMPA (10 μM) treatment. Moreover, GROβ-induced neuroprotection was abolished when granule cells were treated with antisense oligonucleotides specific for the AMPA receptor subunits, which significantly reduced receptor expression, as verified by Western blot analysis with subunit-specific antibodies and by granule cell electrophysiological sensitivity to AMPA. Our data demonstrate that GROβ is neurotrophic for cerebellar granule cells, and that this activity depends on AMPA receptors.
Ordered intracellular RecA–DNA assemblies: A potential site of
in vivo
RecA-mediated activities
Authors: Levin-Zaidman Smadar , Frenkiel-Krispin Daphna , Shimoni Eyal , Sabanay Ilana , Wolf Sharon G. , Minsky Abraham
Published: July 2002

The inducible SOS response increases the ability of bacteria to cope with DNA damage through various DNA repair processes in which the RecA protein plays a central role. Here we present the first study of the morphological aspects that accompany the SOS response in
Escherichia coli
. We find that induction of the SOS system in wild-type bacteria results in a fast and massive intracellular coaggregation of RecA and DNA into a lateral macroscopic assembly. The coaggregates comprise substantial portions of both the cellular RecA and the DNA complement. The structural features of the coaggregates and their relation to
in vitro
RecA–DNA networks, as well as morphological studies of strains carrying RecA mutants, are all consistent with the possibility that the intracellular assemblies represent a functional entity in which RecA-mediated DNA repair and protection activities occur.
Deciphering the design of the tropomyosin molecule
Authors: Brown Jerry H. , Kim Kyoung-Hee , Jun Gyo , Greenfield Norma J. , Dominguez Roberto , Volkmann Niels , Hitchcock-DeGregori Sarah E. , Cohen Carolyn
Published: July 2002

The crystal structure at 2.0-Å resolution of an 81-residue N-terminal fragment of muscle α-tropomyosin reveals a parallel two-stranded α-helical coiled-coil structure with a remarkable core. The high alanine content of the molecule is clustered into short regions where the local 2-fold symmetry is broken by a small (≈1.2-Å) axial staggering of the helices. The joining of these regions with neighboring segments, where the helices are in axial register, gives rise to specific bends in the molecular axis. We observe such bends to be widely distributed in two-stranded α-helical coiled-coil proteins. This asymmetric design in a dimer of identical (or highly similar) sequences allows the tropomyosin molecule to adopt multiple bent conformations. The seven alanine clusters in the core of the complete molecule (which spans seven monomers of the actin helix) promote the semiflexible winding of the tropomyosin filament necessary for its regulatory role in muscle contraction.
p70S6 kinase signals cell survival as well as growth, inactivating the pro-apoptotic molecule BAD
Authors: Harada Hisashi , Andersen Jens S. , Mann Matthias , Terada Naohiro , Korsmeyer Stanley J.
Published: July 2002

Cytokines often deliver simultaneous, yet distinct, cell growth and
cell survival signals. The 70-kDa ribosomal protein S6 kinase (p70S6K)
is known to regulate cell growth by inducing protein synthesis
components. We purified membrane-based p70S6K as a kinase responsible
for site-specific phosphorylation of BAD, which inactivates this
proapoptotic molecule. Rapamycin inhibited mitochondrial-based
p70S6K, which prevented phosphorylation of Ser-136 on BAD and blocked
cell survival induced by insulin-like growth factor 1 (IGF-1).
Moreover, IGF-1-induced phosphorylation of BAD Ser-136 was abolished in
p70S6K-deficient cells. Thus, p70S6K is itself a dual pathway kinase,
signaling cell survival as well as growth through differential
substrates which include mitochondrial BAD and the ribosomal subunit
S6, respectively.