bluuebunny/crossref_metadata_2025 huggingface dataset:
Turbulent drag reduction in a closed flow system: Boundary layer versus bulk effects
Authors: Cadot Olivier , Bonn Daniel , Douady Stéphane
Published: July 2002

We present an experimental study of drag reduction by polymer additives in a closed turbulent flow with a zero mean velocity at large Reynolds numbers. The study is performed using two different forcing schemes: a smooth forcing, for which the fluid is brought into movement through viscous boundary layers, and a very rough forcing where the fluid is stirred inertially by baffles. For a polymer solution of 30 wppm, a steady drag reduction is only observed for the smooth forcing. In the other case, absolutely no decrease of the turbulent energy dissipation is observed. When drag reduction is observed, the turbulent intensity in the bulk is unaltered upon addition of the polymer. This result demonstrates that, in our experimental system, the reduction of the energy dissipation does not only occur mainly in the boundary layers (as is well known for a long time), but takes place solely in the boundary layers. We also study heterogeneous drag reduction by local injection of a concentrated polymer solution. During and just after the injection, similar effects on the measured pressure histograms are observed for the two forcing schemes. However, these effects are much stronger and last much longer, for the “smooth” forcing than for the “inertial” one. With the use of a hot film probe as a tool to detect the homogeneity of the solution, we can show that these effects are due to the mixing of the concentrated polymer solution. The measured pressure histograms also provide information about coherent structures in the flow. We show that during the mixing of the polymer solution, coherent low-pressure filaments are partially suppressed, also when no drag reduction is present: the coherent structures are thus not directly related to the energy dissipation.
The general motion of a circular disk in a Brinkman medium
Authors: Feng J. , Ganatos P. , Weinbaum S.
Published: July 2002

Solutions of the Brinkman equation for the arbitrary motion of a circular disk are obtained which examine for the first time the effect of particle orientation on the particle drag and torque. Four elementary motions are studied analytically: broadside translation, edgewise translation, rotation about the axis of symmetry, and rotation about the diameter. These motions are closely related to the analogous unsteady oscillation of a disk in Stokes flow [Zhang and Stone, J. Fluid Mech. (1998)]. However, our solution procedure differs in that the problems are formulated using a general solution of the Brinkman equation and are solved by reducing the dual integral equations arising from the mixed boundary conditions in the plane of the disk to a Fredholm integral equation of the second type. Asymptotic results for the drag and torque are derived for both small and large values of the permeability parameter α defined by a/Kp, where a is the radius of the disk and Kp the Darcy permeability. In contrast to the Stokesian motion of a disk, where the drag differs by only a factor of 1.5 for broadside and edgewise translational motion, and is isotropic for rotation about any axis through its center, there is a large difference in the drag and torque with increasing α. In a Brinkman medium, the drag on the disk is proportional to α for edgewise motion and to α2 for broadside motion and the torque is proportional to α2 for out-of-plane rotation and to α for in-plane rotation. For intermediate values of α, the integral equations are solved numerically for the drag and torque exerted by the porous medium on the disk. These results are of importance in probing the microstructure of the porous medium and thus provide a way to test the validity of the effective medium approach.
Screening mechanisms in sedimentation
Authors: Brenner Michael P.
Published: July 2002

This paper considers a mixture of sedimenting particles at low Reynolds numbers and volume fractions. Simple theoretical arguments have long suggested that for a random suspension of particles in an infinite system, the fluctuations in the velocity about the mean should diverge with system size. On the other hand, experiments have shown no such divergence. The primary goal of this paper is to examine the effect of side walls on the predicted divergence in fluctuations, through theory, scaling arguments, and numerical simulations. Side walls lead to important modifications of the standard arguments. A scaling argument (based on wall effects) is presented to rationalize recent experiments by Segré et al. [Phys. Rev. Lett. 79, 2574 (1997)]. The paper also briefly discusses the role of inertia in screening fluctuations in infinite systems when the particle Reynolds number is very low, and also the coupling between the velocity fluctuations and the mean sedimenting velocity. A physical argument suggests that in some circumstances the fluctuations give a leading order correction to the mean sedimenting velocity as a function of volume fraction.
Experimental trajectories of two drops in planar extensional flow
Authors: Tretheway Derek C. , Muraoka Masahiro , Leal L. Gary
Published: July 2002

In this paper we map the experimental trajectories of two deformable drops in planar extensional flow and compare the experimental results with theoretical calculations for spherical drops. We examine the effects that deformation, initial position, and viscosity ratio have on the interaction of two drops and the necessity of incorporating deformation into trajectory calculations, which can be used to estimate the collision rates, the collision efficiencies, and the collision interaction times. For drops which do not come into close contact, the existing theoretical calculations for spherical drops accurately predict the symmetric trajectories and capture the increased hydrodynamic interaction for higher viscosity ratios. For drops which come into close contact, the spherical drop theory accurately predicts the approach and exit trajectories and with a slight empirical modification adequately predicts the interaction times for deformable drops with a Taylor deformation parameter up to 0.22. The experimental results show that for drops with close contact, the collision trajectories are asymmetric and irreversible with a minimum separation between the centers of mass that is less than the minimum separation of two spheres. This minimum separation corresponds to the minor axis of the deformed drop and is not captured by the spherical theory. However, overall, the modified trajectory theory based upon the hydrodynamic mobility for spherical drops does provide a reasonable estimate for the trajectories and the interaction times for two deformable drops in planar extensional flow.
Study of the conditional covariance and variance equations for second order conditional moment closure
Authors: Swaminathan N. , Bilger R. W.
Published: July 2002

Presence of transient events like extinction/ignition in turbulent reacting flows increase conditional fluctuations. Thus, one needs to include the conditional fluctuations of reactive scalars, second order quantities, for conditional moment closure calculation when there are transient events and for predictions of sensitive species like NOx. Transport equations for conditional variances Gii and covariances Gij are derived and studied using a direct numerical simulation (DNS) data base. In high Damköhler number situations, chemically reactive and turbulent diffusive processes balance the effect of scalar dissipation–scalar fluctuations correlation on Gij evolution. Additionally, the dissipation of scalar fluctuation becomes important for low Damköhler number situations. Simple models for the different physical processes are proposed and evaluated. Chemical contributions are modeled using a presumed probability density function (PDF) approach which includes second order contributions. The conditional joint PDF of progress variables for the two steps used are observed to be jointly log-normal or jointly Gaussian depending on whether there is extinction or not. The scalar dissipation rate–scalar fluctuations correlation coefficient does not depend on Reynolds or Damköhler numbers. However, the model constant in the classical model for conditional dissipation of scalar fluctuations depends on Damköhler number. Based on the gradient alignment characteristics observed in the DNS, a new model for the above dissipation is proposed using stationary laminar flamelet theory. This model prediction of the above dissipation is better than the classical model prediction.
Nonlinear dynamics of three-dimensional long-wave Marangoni instability in thin liquid films
Authors: Oron Alexander
Published: July 2002

The three-dimensional evolution of the long-wave Marangoni instability of thin liquid films is studied. According to earlier theoretical predictions no continuous steady states exist and the film ruptures. As in two dimensions, the mechanism of fingering is found to be a main route to rupture. A four-fold rotational symmetry of the film interface is retained, when a square periodic domain and the harmonic initial disturbance are used. A use of initial random disturbances in general eliminates the square symmetry of the solution. An increase of the domain size results in a growing complexity of the emerging patterns. In contrast with the dynamics in two dimensions the evolution of the interface in three dimensions and in particular the pattern emerging at rupture may strongly depend on the choice of the initial condition. The two-dimensional evolution of the film is found to be unstable to small three-dimensional random disturbances.
Acceleration and scattering of injected electrons in plasma beat wave accelerator experiments*
Authors: Clayton C. E. , Everett M. J. , Lal A. , Gordon D. , Marsh K. A. , Joshi C.
Published: July 2002

The results from experiments in which a two-frequency CO2 laser is used to beat-excite large-amplitude, relativistic electron plasma waves in a tunnel-ionized plasma are reported. The plasma wave is diagnosed by injecting a beam of 2 MeV electrons and observing the energy gain and loss of these electrons, as well as the scattering and deflection of the transmitted electrons near 2 MeV. Accelerated electrons up to 30 MeV have been observed. The lifetime of the accelerating structure as seen by small-angle Thomson scattering is about 100 ps, whereas the injected electrons are seen to be scattered or deflected by the plasma for several ns, with diffuse scattering occurring 0.5–1 ns after forming the plasma wave and whole beam deflection occurring at later times. A simple model, which includes laser focusing, ionization, transit time, and relativistic saturation effects, suggests that the wave coherence may be short lived while the wave fields themselves persist for a longer time. This may be the reason for the disparate time scales between the Thomson scattering and the electron scattering diagnostic. The whole beam deflection may be evidence for a Weibel-like instability at later times.
Stochastic wave growth
Authors: Robinson P. A.
Published: July 2002

Evolution of waves subject to a randomly varying growth rate is considered and the statistical properties of the waves are calculated in terms of the mean, variance, and correlation time of the growth rate. This enables stochastic growth to be studied without needing full knowledge of the microphysics. However, where the microphysics is understood, this approach also allows it to be easily incorporated into studies of larger-scale phenomena involving stochastic growth. Stochastic differential equations and Fokker–Planck equations are obtained, which describe the wave evolution in the presence of a variety of linear and nonlinear processes and boundary conditions, and it is shown that these phenomena can be diagnosed observationally through their effects on the statistical distribution of the wave field strengths. The results are particularly useful for waves with small dispersion, where they explain the strong wave clumping often observed in nature and emphasize the role of marginal stability in setting the level about which fluctuations occur and in determining their magnitude. Application to type III solar radio bursts illustrates many of the main results and verifies and generalizes earlier conclusions reached using a less rigorous approach. In particular, a new condition for marginally stable propagation of type III solar electron beams is found.
Short-pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity
Authors: Lichters R. , Meyer-ter-Vehn J. , Pukhov A.
Published: July 2002

The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ2=1017–1019 W cm−2 μm2 for normal and oblique incidence and different polarizations. Fully relativistic one-dimensional particle-in-cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.
Multimegawatt cyclotron autoresonance accelerator
Authors: Hirshfield J. L. , LaPointe M. A. , Ganguly A. K. , Yoder R. B. , Wang Changbiao
Published: July 2002

Means are discussed for generation of high-quality multimegawatt gyrating electron beams using rf gyroresonant acceleration. TE111-mode cylindrical cavities in a uniform axial magnetic field have been employed for beam acceleration since 1968; such beams have more recently been employed for generation of radiation at harmonics of the gyration frequency. Use of a TE11-mode waveguide for acceleration, rather than a cavity, is discussed. It is shown that the applied magnetic field and group velocity axial tapers allow resonance to be maintained along a waveguide, but that this is impractical in a cavity. In consequence, a waveguide cyclotron autoresonance accelerator (CARA) can operate with near-100% efficiency in power transfer from rf source to beam, while cavity accelerators will, in practice, have efficiency values limited to about 40%. CARA experiments are described in which an injected beam of up to 25 A, 95 kV has had up to 7.2 MW of rf power added, with efficiencies of up to 96%. Such levels of efficiency are higher than observed previously in any fast-wave interaction, and are competitive with efficiency values in industrial linear accelerators. Scaling arguments suggest that good quality gyrating megavolt beams with peak and average powers of 100 MW and 100 kW can be produced using an advanced CARA, with applications in the generation of high-power microwaves and for possible remediation of flue gas pollutants.
Nonlinear response of driven systems in weak turbulence theory
Authors: Berk H. L. , Breizman B. N. , Fitzpatrick J. , Pekker M. S. , Wong H. V. , Wong K. L.
Published: July 2002

A method is presented for predicting the saturation levels and particle transport in weakly unstable systems with a discrete number of modes. Conditions are established for either steady-state or pulsating responses when several modes are excited for cases where there is and there is not resonance overlap. The conditions for saturation and the associated transport are discussed. Depending on parameters, the saturation level can be low, with only a small fraction of the available free energy released to waves and with no global transport, or the saturation level can be quite high, with almost complete conversion of free energy to wave energy coupled with rapid transport.
Turbulent transport in low-β plasmas
Authors: Nielsen A. H. , Pécseli H. L. , Rasmussen J. Juul
Published: July 2002

Low-frequency electrostatic fluctuations are studied experimentally in a low-β plasma, with particular attention to their importance for the anomalous plasma transport across magnetic field lines. The presence of large coherent structures in a turbulent background at the edge of the plasma column is demonstrated by a statistical analysis. The importance of these structures for the turbulent transport is investigated. The study is extended by a multichannel conditional analysis to illustrate detailed properties and parameter dependences of the turbulent transport.
Nonlinear gyrokinetic equations for turbulence in core transport barriers
Authors: Hahm T. S.
Published: July 2002

An energy-conserving set of the nonlinear electrostatic gyrokinetic Vlasov and Poisson equations is derived for the first time in the presence of equilibrium E×B velocity uE∼vTi, via phase-space Lagrangian Lie-perturbation theory. In this general formulation, only the basic small parameter ε with ω/Ω∼k∥/k⊥∼ε and δn/n0∼1/k⊥L∼ε, is used, while no device-specific expansion has been made. Here, L is the equilibrium scale length. For application to microturbulence in tokamak core transport barriers, an additional small ordering parameter δB≡Bθ/B≪1 is utilized. This leads to a useful form of the nonlinear gyrokinetic system which is applicable to a realistic situation in which the gradient lengths of the equilibrium radial electric field and pressure are of the same order as the ion poloidal gyroradius. The ordering for fluctuations is also modified to δn/n0∼εδB≪1/k⊥L∼δB for a better description of sub-mixing-length level fluctuations. uE/vTi∼δB and ρθi∼Lp put the pressure-gradient contribution to Er and the toroidal-flow contribution to Er at the same order. δB∼ε is shown to be a maximal ordering for studying the E×B flow shear suppression of turbulence.
Performance of a vacuum arc centrifuge with a nonuniform magnetic field
Authors: Hole M. J. , Simpson S. W.
Published: July 2002

The effect of a variable magnetic field on behavior in the rotation region of a vacuum arc centrifuge is analyzed using a first order perturbation of the fluid equations describing the plasma. It is found that the plasma contracts and rotates more rapidly as it moves into an increasing magnetic field. Under conditions typical of current devices, the separative performance is predicted to improve significantly.
Experimental investigation of three-component magnetic reconnection by use of merging spheromaks and tokamaks
Authors: Ono Y. , Inomoto M. , Okazaki T. , Ueda Y.
Published: July 2002

A laboratory experiment of magnetic reconnection has been developed in the Tokyo University Spherical Torus (TS-3) [Y. Ono et al., Phys. Fluids B 5, 3691 (1993)] merging device, using two colliding plasma toroids with cohelicity and counterhelicity. The conventional two-component reconnection was extended experimentally to three-component reconnections by introducing a new field component BX parallel to the X-line, an external force and a reconnection-driven global equilibrium transition. Selective ion heating accompanied by a field-aligned jet was documented during the counterhelicity reconnection without BX, indicating its direct energy-conversion into the ion thermal energy. Ion heating, current-sheet resistivity and reconnection rate all increase significantly with decreasing BX and with increasing the external force, indicating three-component and driven effects of reconnection. The anomalous sheet-current dissipation and the ion heating are both found to depend on whether the current-sheet is compressed shorter than the ion gyroradius or not.
Equilibrium pellet and liquid jet shape under high ablation pressures
Authors: Parks P. B. , Rosenbluth M. N.
Published: July 2002

Owing to the nonspherical nature of the heat deposition in the pellet ablation cloud by energy loss of incident plasma electrons streaming parallel to the uniform magnetic field, a nonuniform pressure distribution develops at the pellet surface. This can lead to deformation of “soft” cryogenic pellets exposed to high temperature and high density magnetized plasmas. The effect of deformation on the burning rate and stability of the condensed phase is evaluated for pellets and liquid jets.
Parametric studies of the Hall current plasma thruster
Authors: Ashkenazy J. , Raitses Y. , Appelbaum G.
Published: July 2002

The Hall current plasma thruster accelerates a plasma jet by an axial electric field and an applied radial magnetic field in an annular ceramic channel. A relatively large current density (>0.1 A/cm2) can be obtained as the acceleration mechanism is not limited by space charge effects. Such a device can be used as a small rocket engine on board spacecraft with the advantage of a large jet velocity compared to conventional rocket engines (10000–30000 m/s versus 2000–4800 m/s). An experimental Hall thruster was constructed and operated in a broad range of operating conditions and under various configuration variations. Electrical, magnetic and plasma diagnostics, and as well accurate thrust and gas flow rate measurements, have been used to investigate the dependence of thruster behavior on the applied voltage, gas flow rate, magnetic field, channel geometry and wall material. The studies conducted so far have demonstrated a significant effect of channel material on thruster electrical characteristics and the existence of an optimal channel length for a given flow rate. Representative results highlighting these findings are presented.
Modification of Langmuir solitons due to ion sound dispersion
Authors: Karpman V. I.
Published: July 2002

The modifications of Langmuir solitons caused by the dispersion of the ion sound are investigated. They include the soliton resonant radiation, soliton deformation, and the decrease of the soliton amplitude and velocity because of the radiation. The results of the previous paper [Karpman and Schamel, Phys. Plasmas 4, 120 (1997)] are confirmed and completed.
Pycnonuclear reactions in dense astrophysical and fusion plasmas
Authors: Ichimaru Setsuo , Kitamura Hikaru
Published: July 2002

Rates of pycnonuclear reactions in ultradense fluids and solids are evaluated by taking account of recent progress in quantum-statistical formulations of the equations of state and phase transitions in dense matter. New theoretical results are summarized for the reaction rates and the enhancement factors, obtained through systematic studies of the screening effects by relativistic and nonrelativistic electrons, as well as of the thermodynamic properties of dense matter resulting from internuclear cohesion in electron-screened Coulombic fluids and solids; the results prove to be a significant improvement over those in a previous review [S. Ichimaru, Rev. Mod. Phys. 65, 255 (1993)]. On the basis of these theoretical developments, coupled with renovated experiments in ultrahigh-pressure metal physics, outstanding issues of nuclear reactions in stellar and planetary interiors and in terrestrial settings are explored, with inertial-confinement-fusion experiments, and for a novel scheme of fusion studies in dense liquid-metallic proton–deuteron mixtures.
Nonlinear evolution of the Rayleigh–Taylor and Richtmyer–Meshkov instabilities
Authors: Dimonte Guy
Published: July 2002

Scaled experiments on the nonlinear Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities are described for two limiting conditions. First, at high Reynolds number, the mixing layer is found to grow self-similarly ∼αiAgt2 for a constant acceleration and as a power law tθi for an impulsive acceleration g=Uδ(t). The growth coefficients αi and exponents θi are measured over a comprehensive range of Atwood number A. Second, with non-Newtonian materials, the critical wavelength and amplitude for RT instability associated with the shear modulus and tensile yield of the material is observed over a variety of conditions. The results are applied to naturally occurring supernova explosions and volcanic islands.
Magneto-hydro-dynamic limits in spherical tokamaks
Authors: Hender T. C. , Allfrey S. J. , Akers R. , Appel L. C. , Bevir M. K. , Buttery R. J. , Gryaznevich M. , Jenkins I. , Kwon O. J. , McClements K. G. , Martin R. , Medvedev S. , Nightingale M. P. S. , Ribeiro C. , Roach C. M. , Robinson D. C. , Sharapov S. E. , Sykes A. , Villard L. , Walsh M. J.
Published: July 2002

The operational limits observed in spherical tokamaks, notably the small tight aspect ratio tokamak (START) device [A. Sykes et al., Nucl. Fusion 32, 694 (1992)], are consistent with those found in conventional aspect ratio tokamaks. In particular the highest β achieved (∼40%) is consistent with an ideal magneto-hydro-dynamic (MHD) Troyon type limit, the upper limit on density is well described by the Greenwald density (πa2n̄e/Ip∼1) and the normalized current (Ip/aBt) is limited such that q95≳2. Stability calculations indicate scope for increasing both normalized β and normalized current beyond the values so far achieved, although wall stabilization is generally needed for low-n modes. In double null configurations current terminating disruptions occur at each of the operational boundaries, though the current quench tends to be slow at the density limit and disruptions at high β may be due to the low q. In early limiter START discharges, before the divertor coils were installed, disruptions rarely occurred. Instead internal reconnection events which have all the characteristics of a disruption except the current quench occurred. These various disruptive behaviors are explained in terms of a model in which helicity is conserved during the disruption. Due to the low toroidal field beam ions in START, and α particles in a ST power plant, are super-Alfvénic. This gives the possibility for toroidal Alfvén eigenmodes (TAEs) to occur and such modes are frequently observed in START neutral beam injection (NBI) discharges, but seem to be benign. The features of these observed TAEs are shown to be in agreement with MHD calculations.
Self-induced transparency and self-induced opacity in laser-plasma interactions
Authors: Goloviznin V. V. , Schep T. J.
Published: July 2002

The interaction of a strong electromagnetic wave with a layer of overdense plasma is considered under conditions that allow for self-induced transparency (SIT). In a certain range of the amplitude of the incoming wave, the corresponding nonlinear one-dimensional (1D) set of equations is shown to admit two competing steady-state solutions that satisfy the same boundary conditions. One of those solutions describes just the well-known SIT regime of propagation, the other one corresponds to the full reflection of the incident wave. Their coexistence indicates the possibility of hysteresis-like effects in ultraintense laser interactions with overdense plasmas. 1D Particle-In-Cell simulations confirm analytical results and suggest the dominance of the opaque solution over the entire parameter window where it is possible. Fast longitudinal heating of plasma electrons occurs then in a natural way as a result of the breakup of the opaque solution above some threshold in laser intensity.
Laser-induced photochemical etching of SiO2 studied by x-ray photoelectron spectroscopy
Authors: Yokoyama S. , Yamakage Y. , Hirose M.
Published: July 2002

Thermally grown SiO2 on Si and quartz wafers have been photochemically etched in an NF3 gas containing 0–4 mol % hydrogen by using an ArF excimer laser (193 nm in wavelength) irradiation. In situ x-ray photoelectron spectroscopy of the etched surface at each step of the photochemical reaction and in situ infrared absorption measurements of the etching gas have revealed the elementary process of etching reactions on the surface and the resulting products in the gas phase.
Femtosecond solitary waves in optical fibers—beyond the slowly varying envelope approximation
Authors: Christodoulides D. N. , Joseph R. I.
Published: July 2002

We present a theoretical calculation which demonstrates that optical solitary waves can propagate in an optical fiber well into the femtosecond regime. Dispersive effects up to fourth order have been considered and more importantly, the ‘‘slowly varying envelope’’ approximation is not invoked. The optical fiber which can support such a wave must exhibit negative fourth-order dispersion ( βIV0e <0) and must be used close to appropriate ‘‘operating’’ points. Use of such waves in optical fiber communication systems may finally result in the full exploitation of the bandwidth of light.
Low-temperature silicon epitaxy by ultrahigh vacuum/chemical vapor deposition
Authors: Meyerson B. S.
Published: July 2002

We have successfully demonstrated the use of a novel chemical vapor deposition technique, ultrahigh vacuum/chemical vapor deposition, to deposit homoepitaxial silicon layers of high crystalline perfection at low temperatures (T≥750 °C). Rutherford backscattering and transmission electron microscopy showed the transition to epitaxial silicon growth took place in the range 700–750 °C, and secondary ion mass spectrometry showed typical oxygen and carbon levels to be near the detection limits of the technique 1016–1017 cm−3. In addition, abrupt dopant transitions have been demonstrated, with B levels dropping four orders of magnitude, 1019–1015 B/cm3, in the first 1000 angstroms of an intrinsic epilayer.
Delta-doped ohmic contacts to n-GaAs
Authors: Schubert E. F. , Cunningham J. E. , Tsang W. T. , Chiu T. H.
Published: July 2002

A new type of nonalloyed ohmic contact to GaAs is realized by molecular beam epitaxy. The ohmic characteristic of the metal-semiconductor junction is obtained by placing a highly δ-doped donor layer a few lattice constants away from the metal-semiconductor interface of the contact and thus keeping the tunneling barrier extremely thin. The current-voltage characteristic of the δ-doped contacts is strictly linear. The measured contact resistance is in the 10−6 Ω cm2 range. Theoretical analysis of the tunneling current through the triangular barrier predicts contact resistances in the range 10−7–10−9 Ω cm2. In spite of the high doping concentration (≂1021 cm−3) the surface morphology of the sample shows no degradation.
Time-resolved reflectivity measurements during explosive crystallization of amorphous silicon
Authors: Bruines J. J. P. , van Hal R. P. M. , Boots H. M. J. , Polman A. , Saris F. W.
Published: July 2002

Explosive crystallization of Cu implanted amorphous silicon during irradiation by a 32-ns FWHM ruby laser pulse has been studied using time-resolved reflectivity measurements and Rutherford backscattering spectrometry. From interferences in the reflectivity, the position and the velocity of the self-propagating melt have been deduced as a function of time. A maximum average velocity of 13±2 m/s has been obtained. The reflectivity behavior indicates the presence of crystalline nuclei in the melt.
Picosecond excite-and-probe absorption measurement of the intra-2E
g
E3/2-state vibrational relaxation time in Ti3+:Al2O3
Authors: Gayen S. K. , Wang W. B. , Petričević V. , Yoo K. M. , Alfano R. R.
Published: July 2002

The kinetics of vibrational transitions in the 2EgE3/2 electronic state of Ti3+:Al2O3 is studied by exciting higher lying vibrational levels of the state by a 527-nm picosecond pump pulse, and monitoring the subsequent growth of population in the zero vibrational level and lower lying vibrational levels by a 3.9-μm picosecond probe pulse. An upper limit of 3.5 ps for intra-2EgE3/2 -state vibrational relaxation time is estimated.
GaAs-AlAs monolithic microresonator arrays
Authors: Jewell J. L. , Scherer A. , McCall S. L. , Gossard A. C. , English J. H.
Published: July 2002

Monolithic optical logic devices 1.5–5 μm across are defined by ion-beam assisted etching through a GaAs/AlAs Fabry–Perot structure grown by molecular beam epitaxy. They show reduced energy requirements (more than an order of magnitude smaller than the unetched heterostructure), uniform response over small arrays, negligible crosstalk at 3 μm center-center spacing, ∼150 ps recovery time, and thermal stability at 82 MHz operating frequency. All experiments were performed at room temperature.
Imaging of MoS2 by scanning tunneling microscopy
Authors: Stupian Gary W. , Leung Martin S.
Published: July 2002

We report imaging of molybdenum disulfide by scanning tunneling microscopy (STM) in air. MoS2, a layer lattice material, is an interesting compound scientifically and is technologically important as a solid lubricant and as a catalyst. Images with atomic spatial resolution were formed only at negative (−0.8 to −1.9 V) sample bias, i.e., by electrons tunneling from the sample into the tip. Our observations are consistent with an electronic model in which the valence electrons are located in bonding or nonbonding orbitals largely confined within the S-Mo-S ‘‘sandwich’’ layers which make up the MoS2 lattice. There are no sigma or pi sulfur bonding orbitals in the basal plane from which electrons can tunnel. The STM image is produced by electrons originating from the filled molybdenum dz2 orbitals. These orbitals do not participate significantly in the formation of chemical bonds. They do project sufficiently far spatially above the basal plane in the presence of an applied electric field to permit formation of a STM image.
Low-loss InGaAs/InP multiple quantum well optical electroabsorption waveguide modulator
Authors: Koren U. , Miller B. I. , Koch T. L. , Eisenstein G. , Tucker R. S. , Bar-Joseph I. , Chemla D. S.
Published: July 2002

An optical electroabsorption waveguide modulator is described based on the quantum-confined Stark effect in an InGaAs/InP multiple quantum well waveguide. The optical modulator has a high on/off ratio (47:1) with very low insertion loss (2.9 dB) and a 3-dB modulation bandwidth of 3.0 GHz at 0.1 mW optical input power.
Effect of arsenic source on the growth of high-purity GaAs by molecular beam epitaxy
Authors: Chand Naresh , Miller R. C. , Sergent A. M. , Sputz S. K. , Lang D. V.
Published: July 2002

We report growth of ultrapure GaAs by molecular beam epitaxy (MBE) and show that the arsenic source has a dramatic effect on the purity. With every thing else the same, by changing the arsenic source from 6N grade double refined chunks to a 7N grade slug which closely fits the 35 cm3 crucible, the deep electron trap and the residual acceptor densities were reduced by nearly two orders of magnitude. When intentionally doped with Si to an electron density of 3×1013 cm−3, the measured mobility at 77 K was 205 600 cm2 V−1 s−1, which increased to 294 700 cm2 V−1 s−1 at 42 K. These mobilities are the highest ever observed in intentionally n-doped MBE GaAs. Low-temperature photoluminescence studies of both undoped and Si-doped GaAs layers grown with the As slug show a marked decrease in acceptor- and defect-related luminescence over that observed when the As chunks are used. Our study conclusively proves that the major source of residual acceptor impurities and typical MBE GaAs traps observed by deep level transient spectroscopy is the chemical impurities present in the arsenic source, although the exact nature of these impurities is yet to be identified. These impurities or their complexes with the native crystal defects, and not the native defects alone, are responsible for the deep traps.
Gain switching of semiconductor injection lasers
Authors: Lau K. Y.
Published: July 2002

The limiting factors in short pulse generation by gain switching of semiconductor lasers are analyzed using an approach parallel to conventional Q switching analysis. The dependence of the gain-switched pulse width and pulse energy on various input parameters such as drive current amplitude, bias current, and spontaneous emission factor, etc., can be predicted in a simple manner.
Silicon epitaxial growth on (100) patterned oxide wafers at 800 °C by ultralow-pressure chemical vapor deposition
Authors: Yew Tri-Rung , O Kenneth , Reif Rafael
Published: July 2002

This letter presents for the first time the results of low-temperature (800 °C) silicon epitaxial growth on (100) patterned oxide wafers by ultralow-pressure chemical vapor deposition. The epitaxial layers were deposited at 6 mTorr of SiH4 following an in situ argon sputter cleaning step. The films on the exposed single-crystal silicon islands were epitaxial with high structural quality, while those on the oxide were polycrystalline with columnar grains. The growth rate of epitaxial silicon was 2.5 times that of polycrystalline silicon. The transition region between the epitaxial and polycrystalline silicon deposited is very sharp, confined to a boundary which is parallel to the growth direction, and located on the edges of the exposed silicon islands.
Noise reduction technique for scanning tunneling microscopy
Authors: Abraham David W. , Williams C. C. , Wickramasinghe H. K.
Published: July 2002

Noise stemming from mechanical vibration, electronic noise, or low frequency (1/f power spectrum) inherent in the tunneling process, often limits the resolution, speed, or range of application of scanning tunneling microscopy (STM). We demonstrate a technique for minimizing the effect of these noise sources on the STM image. In our method, the tunneling tip is vibrated parallel to the sample surface at a frequency f0, above that of the feedback response frequency. Two signals are obtained simultaneously: the conventional topography, and a differential image corresponding to the amplitude of current modulation at f0. The resultant ac signal can be simply related to the normal STM topographic image, with significant improvement in the signal-to-noise ratio.
Determinants of cluster distribution in the molecular epidemiology of tuberculosis
Authors: Murray Megan
Published: July 2002

Recently developed molecular techniques have revolutionized the epidemiology of tuberculosis. Multiple studies have used these tools to examine the population structure of
Mycobacterium tuberculosis
isolates in different communities. The distributions of clusters of
M. tuberculosis
isolates in these settings may variously reflect social mixing patterns or the differential fitness of specific clones of the organism. We developed an individual-based microsimulation of tuberculosis transmission to explore social and demographic determinants of cluster distribution and to observe the effect of transmission dynamics on the empiric data from molecular epidemiologic studies. Our results demonstrate that multiple host-related factors contribute to wide variation in cluster distributions even when all strains of the organism are assumed to be equally transmissible. These host characteristics include interventions such as chemotherapy, vaccination and chemoprophylaxis, HIV prevalence, the age structure of the population, and the prevalence of latent tuberculosis infection. We consider the implications of these results for the interpretation of cluster studies of
M. tuberculosis
as well as the more general application of microsimulation models to infectious disease epidemiology.
An advanced glycation endproduct cross-link breaker can reverse age-related increases in myocardial stiffness
Authors: Asif Mohammad , Egan John , Vasan Sara , Jyothirmayi Garikiparthy N. , Masurekar Malthi R. , Lopez Santos , Williams Chandra , Torres Ramon L. , Wagle Dilip , Ulrich Peter , Cerami Anthony , Brines Michael , Regan Timothy J.
Published: July 2002

Decreased elasticity of the cardiovascular system is one of the
hallmarks of the normal aging process of mammals. A potential
explanation for this decreased elasticity is that glucose can react
nonenzymatically with long-lived proteins, such as collagen and lens
crystallin, and link them together, producing advanced glycation
endproducts (AGEs). Previous studies have shown that aminoguanidine, an
AGE inhibitor, can prevent glucose cross-linking of proteins and the
loss of elasticity associated with aging and diabetes. Recently, an AGE
cross-link breaker (ALT-711) has been described, which we have
evaluated in aged dogs. After 1 month of administration of ALT-711, a
significant reduction (≈40%) in age-related left ventricular
stiffness was observed [(57.1 ± 6.8
mmHg⋅m
2
/ml pretreatment and 33.1 ± 4.6
mmHg⋅m
2
/ml posttreatment (1 mmHg = 133 Pa)].
This decrease was accompanied by improvement in cardiac function.
Postexposure vaccination massively increases the prevalence of γ-herpesvirus-specific CD8+T cells but confers minimal survival advantage on CD4-deficient mice
Authors: Belz Gabrielle T. , Stevenson Philip G. , Castrucci Maria R. , Altman John D. , Doherty Peter C.
Published: July 2002

Mice that lack CD4+T cells remain clinically normal for more than 60 days after respiratory challenge with the murine γ-herpesvirus 68 (γHV-68), then develop symptoms of a progressive wasting disease. The γHV-68-specific CD8+T cells that persist in these I-Ab−/−mice are unable to prevent continued, but relatively low level, virus replication. Postexposure challenge with recombinant vaccinia viruses expressing γHV-68 lytic cycle epitopes massively increased the magnitude of the γHV-68-specific CD8+population detectable by staining with tetrameric complexes of MHC class I glycoprotein + peptide, or by interferon-γ production subsequent toin vitrorestimulation with peptide. The boosting effect was comparable for γHV-68-infected I-Ab−/−and I-Ab+/+mice within 7 days of challenge, and took more than 110 days to return to prevaccination levels in the I-Ab+/+controls. Although the life-span of the I-Ab−/−mice was significantly increased, there was no effect on long-term survival. A further boost with a recombinant influenza A virus failed to improve the situation. Onset of weight loss was associated with a decline in γHV-68-specific CD8+T cell numbers, though it is not clear whether this was a cause or an effect of the underlying pathology. Even very high levels of virus-specific CD8+T cells thus provide only transient protection against the uniformly lethal consequences of γHV-68 infection under conditions of CD4+T cell deficiency.
Endocardial cushion and myocardial defects after cardiac myocyte-specific conditional deletion of the bone morphogenetic protein receptor ALK3
Authors: Gaussin Vinciane , Van de Putte Tom , Mishina Yuji , Hanks Mark C. , Zwijsen An , Huylebroeck Danny , Behringer Richard R. , Schneider Michael D.
Published: July 2002

Receptors for bone morphogenetic proteins (BMPs), members of the transforming growth factor-β (TGFβ) superfamily, are persistently expressed during cardiac development, yet mice lacking type II or type IA BMP receptors die at gastrulation and cannot be used to assess potential later roles in creation of the heart. Here, we used a Cre/lox system for cardiac myocyte-specific deletion of the type IA BMP receptor, ALK3. ALK3 was specifically required at mid-gestation for normal development of the trabeculae, compact myocardium, interventricular septum, and endocardial cushion. Cardiac muscle lacking ALK3 was specifically deficient in expressing TGFβ2, an established paracrine mediator of cushion morphogenesis. Hence, ALK3 is essential, beyond just the egg cylinder stage, for myocyte-dependent functions and signals in cardiac organogenesis.
A novel microbial habitat in the mid-ocean ridge subseafloor
Authors: Summit Melanie , Baross John A.
Published: July 2002

The subseafloor at the mid-ocean ridge is predicted to be an excellent microbial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These characteristics also make it a good analog for potential subsurface extraterrestrial habitats. Subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy-rich, and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low-temperature (≈15°C), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs isolated from low-temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the Endeavour Segment, Juan de Fuca Ridge, were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genusPyrococcusand three groups corresponding to the genusThermococcus. Of these three groups, one was composed solely of sulfide-derived organisms, and the other two related groups were composed of subseafloor organisms. There was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats, and therefore this phylogenetic distinction indicates a selective force operating between the two habitats. Hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles; some potential differences between these habitats include fluid flow stability, metal ion concentrations, and sources of complex organic matter.
A mammalian homolog ofunc-53is regulated byall-transretinoic acid in neuroblastoma cells and embryos
Authors: Merrill R. A. , Plum L. A. , Kaiser M. E. , Clagett-Dame M.
Published: July 2002

The vitamin A metabolite,all-transretinoic acid (atRA), plays an important role in neuronal development, including neurite outgrowth. However, the genes that lie downstream of atRA and its receptors in neuronal cells are largely unknown. By using the human neuroblastoma cell line, SH-SY5Y, we have identified an atRA-responsive gene (RAINB1: retinoic acid inducible in neuroblastoma cells) that is induced within 4 h after exposure of SH-SY5Y cells to atRA. RAINB1 mRNA is highly expressed in the nervous system (10.5- to 11-kb transcript) in both developing embryos and adults. Its expression is perturbed in developing rat embryos exposed to excess or insufficient atRA. RAINB1 is present on chromosome 11 and is spread over 38 exons, resulting in a putative ORF of 2,429 amino acids. The RAINB1 protein shows high similarity to a gene inCaenorhabditis elegans,unc-53, that is required for axonal elongation of mechanosensory neurons, suggesting that these proteins are orthologs. Thus, RAINB1 may represent a critical downstream gene in atRA-mediated neurite outgrowth.
Impaired neural tube closure, axial skeleton malformations, and tracheal ring disruption in TRAF4-deficient mice
Authors: Régnier Catherine H. , Masson Régis , Kedinger Valérie , Textoris Julien , Stoll Isabelle , Chenard Marie-Pierre , Dierich Andrée , Tomasetto Catherine , Rio Marie-Christine
Published: July 2002

TRAF4 belongs to the tumor necrosis factor receptor-associated factor (TRAF) family of proteins but, unlike other family members, has not yet been clearly associated to any specific receptor or signaling pathway. To investigate the biological function of TRAF4, we have generated
traf4
-deficient mice by gene disruption. The
traf4
gene mutation is embryonic lethal but with great individual variation, as approximately one third of the homozygous mutant embryos died
in utero
around embryonic day 14, whereas the others reach adulthood. Surviving mutant mice manifest numerous developmental abnormalities; notably, 100% of homozygous mutant mice suffer respiratory disorder and wheezing caused by tracheal ring disruption. Additional malformations concern mainly the axial skeleton, as the ribs, sternum, tail, and vertebral arches are affected, with various degrees of penetrance.
Traf4
-deficient mice also exhibit a high incidence of spina bifida, a defect likened to neural tube defects (NTD) that are common congenital malformations in humans. Altogether, our results demonstrate that TRAF4 is required during embryogenesis in key biological processes including the formation of the trachea, the development of the axial skeleton, and the closure of the neural tube. Considering the normal expression pattern of TRAF4 in neural tissues, we can conclude that TRAF4 participates in neurulation
in vivo
.