bluuebunny/crossref_metadata_2025 huggingface dataset:
Linear instability of a short wave propagating in a long internal wave
Authors: Jou Wen-Huei
Published: July 2002

The linear inviscid instability of the flow field created by a long wave propagating along a thin mixing layer is considered here. The main interest is the modulation of the unstable short wave by the slowly varying unsteady and inhomogeneous shear flow. Based on the description of the long wave flow field developed by the author [Phys. Fluids 30, XXXX (1987)], a multiple-scale expansion scheme is employed for the stability problem. A series of perturbation problems is derived for the evolution of the secondary unstable waves in space and time. It is found that if μ is the ratio between the thickness of the mixing layer and the wavelength of the long wave, the phase speed of the long wave is O(μ−1/2) as compared to that of the marginally unstable short wave. Hence, the appropriate slow variables for time and space are T=μ1/2t and X=μx, respectively. The important consequence of this scaling is the absence of a cumulative ray-focusing effect. The amplitude equation in the slow scale is solved, and the higher-order growth rate is determined. The unsteady background waves contribute to the growth rate in the form of a perturbation of O(μ1/2). This perturbation is destabilizing on the back sides of the wave crests and troughs while stabilizing on the front sides.
Bifurcations of steady forced flows in spectral models of rotating fluids
Authors: Gardini L. , Lupini R. , Tebaldi C.
Published: July 2002

In systems of ordinary differential equations representing the truncation to any order of the spectral, barotropic vorticity equation for two-dimensional flows in rotating fluids, subject to constant forcing and dissipation, large values of absolute rotation inhibit the occurrence of equilibrium states other than the basic, forced regime, and promote transitions to periodic oscillations, via Hopf bifurcation.
Light-gasdynamic-induced refractive index changes and beam propagation in fast-pumped high-power photodissociation lasers
Authors: Witte K. J. , Krug J.
Published: July 2002

In high-power photodissociation lasers such as the iodine laser, spatially inhomogeneous pump light deposition causes temperature and pressure gradients. These gradients set the gaseous laser medium into motion, thereby changing its density and thus also its refractive index. These perturbations may seriously degrade the laser beam quality. A pulse propagation model is presented, which for the first time incorporates the gasdynamic disturbances in an appropriate manner. Thus any deterioration of the beam quality resulting from gasdynamic effects and diffraction can be precisely evaluated. The model is shown to compare well with experiment. It is also suited for predicting the performance of high-power amplifier chains.
Phenomenological modeling of Mirnov oscillations
Authors: Arter Wayne
Published: July 2002

A phenomenological model of Mirnov oscillations is constructed, based on very general ideas about the behavior of systems close to instability. Although the model is only a first approximation, the agreement with experiment is encouraging.
Relativistic electron beam heating of a hydrogen plasma in open confinement systems: Theoretical model
Authors: Gupta G. P. , Vijayan T. , Rohatgi V. K.
Published: July 2002

A one-dimensional theoretical model for predicting the heating of a hydrogen plasma in open confinement systems by a relativistic electron beam is presented. Direct energy transfer of beam electrons via interaction with large amplitude waves of the two-stream instability and Ohmic dissipation of plasma return current caused by classical and anomalous resistivities are considered as power input terms. For loss terms, various atomic processes and heat conduction mechanisms are considered. In the light of observed changes in the average scattering angle of the beam inside the plasma, criteria deciding the character of beam–plasma interaction and the estimation of direct power transfer are discussed. The numerical results are presented with a reference to the results of the beam–plasma heating experiments reported in the literature. Better agreement is observed between the experiment and the present analysis.
Analysis and computer simulation of confined ring vortices driven by falling sprays
Authors: Marx K. D.
Published: July 2002

When a liquid spray is injected into the upper region of a closed container, it will entrain air as it falls to the bottom under the influence of gravity. In some cases, the resulting flow field assumes the form of one or more large ring vortices. In this paper, the droplet drag mechanism by which this systematic motion is sustained in steady state is studied theoretically and numerically. It is shown that the structure of the airflow is approximately that of a solution to the inviscid fluid equations. The airflow velocity is determined by the equilibrium of energy exchanged between the air and the spray in different regions of the flow. A simple formula for estimating the order of magnitude of the flow velocity is derived. Numerical calculations that solve for the coupled flow of the gas and the interacting spray are presented for cases of practical interest. The effect of varying the droplet diameter distribution and the mass flow rate of the spray is investigated.
Metastable states of a cylinder wake adjacent to a free surface
Authors: Sheridan J. , Lin J.-C. , Rockwell D.
Published: July 2002

For the same Froude number and depth of submergence beneath a free surface, the wake of a cylinder exhibits two admissible states. The first state involves a jet-like flow generally attached to the free surface; it gives rise to a large-amplitude, quasistationary wave. In the second state, the jet is detached from the free-surface, which exhibits only mild distortion. The critical feature of the transformation between these two states involves the formation of a separated vorticity layer from the free-surface and its interaction with the vorticity layer from the surface of the cylinder. This transformation can occur spontaneously over a time scale much longer than the Kármán period.
Exact relation between spatial mean enstrophy and dissipation in confined incompressible flows
Authors: Raynal Florence
Published: July 2002

An exact non-trivial relation between spatial mean enstrophy and dissipation in closed incompressible flows is derived analytically, which also applies to unbounded fluid domains when the velocity decays to zero sufficiently rapidly at infinity. The quantity 〈ω2−σ2〉 is shown to depend only on the boundary conditions, and it is pointed out that in many confined flows, the equality 〈ω2〉=〈σ2〉 is not satisfied.
Flow due to a periodic array of point forces, and the motion of small particles within a cylindrical tube of arbitrary cross section
Authors: Coulliette C. , Pozrikidis C.
Published: July 2002

The properties and computation of Stokes flow due to a periodic array of point forces exerted in the interior of a fluid-filled cylindrical tube with an arbitrary cross-sectional shape are discussed. It is shown that the relationship between the pressure drop and the axial flow rate occurring when the point forces have a component parallel to the generators can be deduced immediately from a knowledge of the velocity profile corresponding to unidirectional pressure-driven flow. A boundary-integral method for computing the associated Green’s function of Stokes flow is developed and implemented in a numerical procedure that exploits the cylindrical boundary geometry to improve the accuracy of the results and efficiency of the computations. Streamline patterns of the flow within tubes with circular, elliptical, and nearly square shapes are presented and discussed with reference to flow reversal. In the limit as the separation between the point forces becomes increasingly larger than the typical size of the cross section of the tube, we recover the flow due to a solitary point force, and the numerical result are in agreement with those derived by previous authors for the particular case of a tube with a circular cross-sectional shape. The flow due to the point forces is then coupled with the boundary integral representation to develop asymptotic expansions for the surface stress, force, torque, and higher moments of the traction exerted on a small suspended particle that belongs to a periodic array. Each particle may translate and rotate while the ambient fluid undergoes pressure-driven flow. The coefficients of the asymptotic expansion are related to the non-singular part of the Green’s function and its spatial derivatives, evaluated at the location of the point force. These quantities are computed and plotted for several cross-sectional shapes.
One-dimensional models for slender axisymmetric viscous liquid bridges
Authors: García F. Javier , Castellanos Antonio
Published: July 2002

A set of one-dimensional models, previously derived for liquid jets, is generalized to viscous liquid bridges by applying suitably modified boundary conditions at the anchoring disks. A linear analysis for small-amplitude perturbations around the cylindrical static solution is performed. The oscillation frequencies and growth factors so obtained are compared to the already known linear three-dimensional results for a wide range in both the slenderness and viscosity. The relative error of each model is studied in terms of the typical axial length. Good agreement is found for slender enough bridges. The existence of boundary layers for weakly dissipative liquid bridges in the context of one-dimensional models is also discussed.
A model for eastward and westward jets in laboratory experiments and planetary atmospheres
Authors: Marcus P. S. , Lee C.
Published: July 2002

Flows in a rotating annular tank [J. Sommeria, S. D. Meyers, and H. L. Swinney, Nonlinear Topics in Ocean Physics, edited by A. Osborne (North Holland, Amsterdam, 1991); Nature (London) 337, 58 (1989); T. H. Solomon, W. J. Holloway, and H. L. Swinney, Phys. Fluids A 5, 1971 (1993); J. Sommeria, S. D. Meyers, and H. L. Swinney, Nature (London) 331, 689 (1989)] with a sloping bottom (that simulates a barotropic atmosphere’s Coriolis force with a topographic β-effect [J. Pedlosky, Geophysical Fluid Dynamics, 2nd ed. (Springer, Berlin, 1986)]) produce eastward and westward jets, i.e., azimuthal flows moving in the same or opposite direction as the annulus’ rotation. Flows are forced by pumping fluid in and out of two concentric slits in the bottom boundary, and the direction of the jets depends on the direction of the pumping. The eastward and westward jets differ, with the former narrow, strong, and wavy. The jets of Jupiter and Saturn have the same east–west asymmetry [P. S. Marcus, Ann. Rev. Astron. Astro. 431, 523 (1993)]. Numerical simulations show that the azimuthally-averaged flow differs substantially from the non-averaged flow which has sharp gradients in the potential vorticity q. They also show that the maxima of the eastward jets and Rossby waves are located where the gradients of q are large, and the maxima of the westward jets and vortex chains are located where they are weak. As the forcing is increased the drift velocities of the two chains of vortices of the eastward jet lock together; whereas the two chains of the westward jet do not. Inspired by a previously published, [P. S. Marcus, Ann. Rev. Astron. Astro. 431, 523 (1993)] piece-wise constant-q model of the Jovian jets and based on numerical simulations, a new model of the experimental flow that is characterized by regions of undisturbed flow and bands of nearly uniform q separated by sharp gradients is presented. It explains the asymmetry of the laboratory jets and quantitatively describes all of the wave and vortex behavior in the experiments including the locking of the vortex chains of the eastward jet. The simulations and new model contradict the predictions of a competing, older model of the laboratory flow that is based on a Bickley jet; this raises concerns about previous calculations of Lagrangian mixing in the laboratory experiments that used the Bickley model for the fluid velocity. The new model, simulations and laboratory experiments all show that jets can be formed by the mixing and homogenization of q. The relevance of this to the jets of Jupiter is discussed.
A note on subharmonic instabilities
Authors: Schulze T. P.
Published: July 2002

When a fluid system is subject to time-periodic forcing, it is well known that it may exhibit both harmonic and subharmonic instabilities, the classic example being Faraday oscillations. When the forcing is confined to a periodic shearing motion, however, it has been observed that the subharmonic response is absent. The underlying mathematical feature that unifies these systems is a conjugate-translation symmetry [A. C. Or, J. Fluid Mech. 335, 213 (1997)]. We show that any subharmonic solutions of periodically driven systems with conjugate-translation symmetry must have Floquet multipliers with multiplicity greater than one. The effect of this constraint is that subharmonic solutions are very difficult to locate within the system’s parameter space and, more importantly, that phase locking cannot occur for such systems.
Confinement of dust particles in a double layer
Authors: Barkan A. , Merlino R. L.
Published: July 2002

Negatively charged, micron-sized dust grains have been electrostatically confined in the high-potential region of an anode double layer formed in a single-ended Q-machine plasma. The levitated dust grains are large enough to be visually observed in the scattered light from the plasma source. The various forces on a dust grain in a plasma, i.e., gravitational, electric, ion drag, and neutral drag are estimated based on the measured plasma and dust parameters. The possibility of observing a Coulomb lattice in such a setup is discussed.
Theory of pressure-induced islands and self-healing in three-dimensional toroidal magnetohydrodynamic equilibria
Authors: Bhattacharjee A. , Hayashi T. , Hegna C. C. , Nakajima N. , Sato T.
Published: July 2002

The role of singular currents in three-dimensional toroidal equilibria and their resolution by magnetic island formation is discussed from both analytical and computational points of view. Earlier analytical results are extended to include small vacuum islands, which may, in general, have different phases with respect to pressure-induced islands. In currentless stellarators, the formation of islands is shown to depend on the resistive parameter DR, as well as the integrated effect of global Pfirsch–Schlüter currents. It is demonstrated that the pressure-induced ‘‘self-healing’’ effect, recently discovered computationally, is also predicted by analytical theory.
Electrodynamics as a problem of eigenvalues. II. Variational method of calculation of electromagnetic field oscillations in dispersive medium
Authors: Yudin Lev A. , Korenev Igor L. , Kapchinsky Michael I. , Efimov Sergey P.
Published: July 2002

This part of the paper deals with a variational approach. The general scheme is given and two examples are presented: free oscillations of an electromagnetic field in a cavity filled with an inhomogeneous dispersive resonant medium and wave propagation in a longitudinally homogeneous plasma cylinder in an infinitely strong magnetic field. A comparison shows good agreement of results of a variational approach with results of an exact numerical solution. Possible ways of improving accuracy are discussed.
Non-perturbative three-dimensional wave solutions for cylindrically bounded plasmas
Authors: Yu M. Y. , Stenflo L.
Published: July 2002

Finite amplitude three-dimensional waves in a cold plasma cylinder bounded by a dielectric are considered. Exact evolution equations governing the waves are obtained and studied numerically. It is shown that the surface and volume modes are strongly but selectively coupled. The waves can also appear temporally as simple or envelope solitons.
Attainment of high plasma pressures by a large number of channeled pinch discharges intersecting in one point
Authors: Winterberg F.
Published: July 2002

If a large number of laser- or charged particle beam-channeled gas embedded Z-pinch discharges, with alternating directions of the currents for adjacent pinch discharges, are made to intersect in one point, one obtains a quasispherical plasma configuration likely to be stable against the m=1 kink instability. Because gas-embedded Z-pinch discharges are diffuse, they should also be stable against the m=0 sausage instability and all other higher than m=1 instabilities. In the proposed plasma configuration the pressure falls off radially with an 1/r2 dependence. If, for example, the plasma pressure at the periphery of a 25 cm plasma sphere is 100 atm≊108 dyn/cm2, corresponding to a magnetic field of ∼5×104 G, the plasma pressure at a radius of ∼0.3 cm from the center of the sphere would have risen to 700 000 atm, with the magnetic field at this distance equal to ∼4×106 G. At a thermonuclear temperature of ∼109 K, the plasma particle number density at this distance would be ∼3×1018 cm−3, with a correspondingly high thermonuclear reaction rate. The feasibility of such a plasma configuration for the controlled release of thermonuclear energy then only depends on the radial heat conduction losses in the presence of a strong perpendicular magnetic field. It appears that these losses can be made smaller than the heat released by the thermonuclear reaction.
Nonlinear magnetohydrodynamic detonation: Part I
Authors: Hurricane O. A. , Fong B. H. , Cowley S. C.
Published: July 2002

The sudden release of magnetic free energy, as occurs in spectacular solar flare events, tokamak disruptions, and enigmatic magnetospheric substorms, has long defied any acceptable theoretical explanation. Usual attempts at explaining these explosive events invoke magnetic reconnection and/or ideal magnetohydrodynamic (MHD) instability. However, neither of these two mechanisms can explain the fast time scales without nonlinear destabilization. Recently, Cowley et al. [Phys. Plasmas 3, 1848 (1996)] have demonstrated a new mechanism for nonlinear explosive MHD destabilization of a line tied Rayleigh–Taylor model. In this paper, this picture is generalized to arbitrary magnetic field geometries. As an intermediate step, the ballooning equation in a general equilibrium is derived including the effects of magnetic field curvature, shear, and gravity. This equation determines the linear stability of the plasma configuration and the behavior of the plasma displacement along the magnetic field line. The nonlinear equation which determines the time and spatial dependence, transverse to the equilibrium magnetic field, of the plasma displacement is obtained in fifth order of the expansion. The equations show that explosive behavior is a natural and generic property of ballooning instabilities close to the linear stability boundary.
Driving laser pulse evolution in a hollow channel laser wakefield accelerator
Authors: Volfbeyn P. , Lee P. B. , Wurtele J. , Leemans W. P. , Shvets G.
Published: July 2002

An analytic and numerical study of the coupling between a high-intensity laser pulse and a plasma wake is presented, in the context of laser wakefield acceleration in a hollow channel. Laser wavelength reddening and pulse length shortening are qualitatively described using simple scaling laws. One-dimensional self-consistent equations are derived and numerically solved to provide a more detailed description of the laser pulse evolution for the case of propagation in a uniform plasma. These equations are extended to treat the case of laser pulse propagation in a hollow channel. The coupling between the plasma and the laser pulse is calculated using energy conservation. The model obtained provides a simple method for inferring the plasma wake characteristics from measurement of changes in phase and amplitude of the driving laser pulse.
Bäcklund transformations and Painlevé analysis: Exact soliton solutions for strongly rarefied relativistic cold plasma
Authors: Khater A. H. , Callebaut D. K. , Shamardan A. B. , Ibrahim R. S.
Published: July 2002

The relativistic nonlinear self-consistent equations for a strongly rarefied plasma with stationary ions is considered. The resulting system of equations is reduced to a single sinh-Gordon equation. Using the Bäcklund transformations technique and Painlevé analysis, a set of exact soliton solutions is obtained for stationary and nonstationary equations that describes the charge density equilibrium configuration.
Instability of ion flows in bounded dusty plasma systems
Authors: Rosenberg M. , Shukla P. K.
Published: July 2002

The effect of longitudinal boundaries on electrostatic instabilities driven by an ion beam in a dusty plasma is considered. In analogy with instabilities in electron-ion plasma diodes, in this case both the low-frequency ion-space-charge waves and ion-dust waves can be unstable. The growth rates and ion current thresholds are estimated. The present investigation should help to design a large scale dusty plasma device to study these low-frequency analogs of instabilities in bounded beam-plasma systems.
The ideal magnetohydrodynamic continuous spectra of a cylindrical plasma: Solution of the initial value problem
Authors: Tataronis J. A. , Torasso R. , Salat A.
Published: July 2002

The initial value problem of the linearized equations of compressible magnetohydrodynamics in cylindrical screw-pinch geometry is treated. The initial value terms that arise in the Laplace-transformed equations are retained. The formation of continuous spectra is examined, including possible effects of the initial value terms. In accordance with earlier findings, the well-known shear Alfvén and cusp continua are found. No evidence of continua tied to the fast and slow magnetosonic waves is found.
Scaling supernova hydrodynamics to the laboratory
Authors: Kane J. , Arnett D. , Remington B. A. , Glendinning S. G. , Bazan G. , Drake R. P. , Fryxell B. A. , Teyssier R. , Moore K.
Published: July 2002

Supernova (SN) 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. Initial results were reported in J. Kane et al. [Astrophys. J. 478, L75 (1997) and B. A. Remington et al., Phys. Plasmas 4, 1994 (1997)]. The Nova laser is used to generate a 10–15 Mbar shock at the interface of a two-layer planar target, which triggers perturbation growth due to the Richtmyer–Meshkov instability, and to the Rayleigh–Taylor instability as the interface decelerates. This resembles the hydrodynamics of the He-H interface of a Type II supernova at intermediate times, up to a few ×103 s. The scaling of hydrodynamics on microscopic laser scales to the SN-size scales is presented. The experiment is modeled using the hydrodynamics codes HYADES [J. T. Larson and S. M. Lane, J. Quant. Spect. Rad. Trans. 51, 179 (1994)] and CALE [R. T. Barton, Numerical Astrophysics (Jones and Bartlett, Boston, 1985), pp. 482–497], and the supernova code PROMETHEUS [P. R. Woodward and P. Collela, J. Comp. Phys. 54, 115 (1984)]. Results of the experiments and simulations are presented. Analysis of the spike-and-bubble velocities using potential flow theory and Ott thin-shell theory is presented, as well as a study of 2D versus 3D differences in perturbation growth at the He-H interface of SN 1987A.
Nonlinear effects of a beam interacting with a single damped wave
Authors: Stoltz Peter H. , Cary John R.
Published: July 2002

A self-consistent nonlinear theory of the longitudinal dynamics of a low density beam interacting with a single damped wave is developed. In this paper, the model is applied to the coasting beam-cavity system of accelerator physics, but it also applies to beam-plasma systems and traveling wave tubes. Motivating the theory are numerical simulations showing different beam behaviors in the nonlinear regime depending on the amount of wave damping. For highly damped systems, breakoff and energy loss of a self-formed bunch from the beam is observed. This bunch breakoff and energy loss is the cause of the overshoot phenomenon of accelerator physics; furthermore this overshoot does not contradict the Keil–Schnell criterion, as the beam is far from a Gaussian distribution. An expression for the amount of cavity damping necessary for bunch breakoff is derived. Finally, using a single-particle model, an expression for the rate of energy loss of the bunch in terms of the cavity damping is derived.
Franz–Keldysh electrorefraction and electroabsorption in bulk InP and GaAs
Authors: Van Eck T. E. , Walpita L. M. , Chang W. S. C. , Wieder H. H.
Published: July 2002

Franz–Keldysh electrorefraction and electroabsorption were measured for semi-insulating InP and GaAs at several electric fields and several wavelengths near the absorption edge. For InP, the experimental results are described well by an effective mass approximation theory with a correction that accounts for the exponential absorption tail. It is shown that, at least in InP, Franz–Keldysh electrorefraction is under some conditions much stronger than the Pockels effect.
Limited reaction processing: Growth of III-V epitaxial layers by rapid thermal metalorganic chemical vapor deposition
Authors: Reynolds S. , Vook D. W. , Gibbons J. F.
Published: July 2002

We have demonstrated a new technique for III-V epitaxial layer growth combining rapid thermal processing and metalorganic chemical vapor deposition. This technique yields enhanced layer thickness control and abrupt interfaces while maintaining a high growth rate (≥10 Å/s). Multilayer structures have been grown with smooth, featureless surfaces and good electrical quality (Nd=2×1016 cm−3, μn=3000 cm2/V s) using trimethylarsenic and trimethylgallium.
Instability mechanism in hydrogenated amorphous silicon thin-film transistors
Authors: Schropp R. E. I. , Verwey J. F.
Published: July 2002

The degradation of hydrogenated amorphous silicon under an applied field is studied in an amorphous silicon thin-film transistor. A possible mechanism for metastable defect creation due to trapping of electrons at weak bonds together with a bond-switching event is investigated. The energy for the bond-switching process is assumed to be supplied thermally. The rate equation is set up and it is shown that this new model for defect creation is capable of describing the experimentally observed slow field-effect current transients at various temperatures.
Electro-optical characteristics of a new liquid-crystal display with an improved gray-scale capability
Authors: Leenhouts F. , Schadt M. , Fromm H.-J.
Published: July 2002

A new liquid-crystal display (LCD) is presented with a much better gray-scale capability than twisted nematic LCD’s and a comparably good contrast. In the not-energized state the coloration of the display is weak, enabling full color applications. Due to its improved gray-scale capability which results from the shallow slope of the electro-optical characteristics, the display is particularly suited for active matrix applications.
GaInAsP/InP double heterostructure lasers emitting at 1.5 μm grown by chemical beam epitaxy
Authors: Tsang W. T.
Published: July 2002

Double heterostructure lasers of GaxIn1−xAs1−yPy lattice matched to InP and emitting at 1.55 μm have been grown by chemical beam epitaxy (CBE). Broad-area lasers fabricated from these wafers had pulsed room-temperature threshold current densities Jth and differential quantum efficiencies that are similar to the best results obtained from wafers grown by liquid phase epitaxy and metalorganic chemical vapor deposition. The lowest Jth obtained was 1 kA/cm2 with an active layer thickness of 0.14 μm. Differential quantum efficiencies were ∼15–18% per facet. Lasing was obtained with these broad-area lasers up to 106 °C with relatively weak degradation of quantum efficiency. Excellent device uniformity and wafer-to-wafer reproducibility were obtained with CBE. The method appears at this yet early stage to be an attractive approach to GaInAsP epitaxy.
Interdiffusion behavior of HgTe-CdTe junctions
Authors: Tang Mei-Fan S. , Stevenson David A.
Published: July 2002

Interdiffusion coefficients (D̃) were measured from 600 to 300 °C for HgTe-CdTe diffusion couples. The influence of the binary composition, temperature, and Hg overpressure on D̃ has been established. Significant differences have been found in the behavior of D̃ in two temperature regions. A dual mechanism is proposed for diffusion at higher temperatures (T≥450 °C), with a vacancy mechanism and an interstitial mechanism dominating at low and high x value regions, respectively, and an interstitial mechanism predominating at lower temperatures. The proposed model is confirmed by electrical property, tracer diffusion, and theoretical studies, and provides a basis for predicting the interdiffusion behavior at even lower temperatures.
1-ns pulse-generating, compact, low-pressure N2 lasers
Authors: Ishikawa Kazuhito , Muto Shinzo , Matsuzawa Hidenori
Published: July 2002

Effects of a pressurized spark switch on N2 laser output have been observed optically with quartz fibers. The output power increased with decreasing the gap of the spark switch while keeping pgd=0.51 kg/cm (pg is the N2 gas pressure in the spark switch and d is the gap distance) and pN2=40 Torr (pN2 is the pressure of the laser medium gas). One nanosecond pulses with peak powers of 120 kW have been generated for the first time for low-pressure operating N2 lasers.
Quasiparticle trapping and the quasiparticle multiplier
Authors: Booth N. E.
Published: July 2002

Superconductors and in particular superconducting tunnel junctions can be used to detect phonons, electromagnetic radiation, x rays, and nuclear particles by the mechanism of Cooper-pair breaking to produce excess quasiparticles and phonons. We show that the sensitivity can be increased by a factor of 100 or more by trapping the quasiparticles in another superconductor of lower gap in the region of the tunnel junction. Moreover, if the ratio of the gap energies is >3 a multiplication process can occur due to the interaction of the relaxation phonons. This leads to the concept of the quasiparticle multiplier, a device which could have wider applications than the Gray effect transistor or the quiteron.
Investigation of GaAs/AlxGa1−xAs and InyGa1−yAs/GaAs superlattices on Si substrates
Authors: Reddy U. K. , Ji G. , Huang D. , Munns G. , Morkoç H.
Published: July 2002

We have studied the optical properties of lattice-matched GaAs/AlxGa1−xAs and InyGa1−yAs/GaAs strained-layer superlattices grown on Si substrates using the photoreflectance technique. These preliminary results show that good quality III-V epilayers can be grown on Si. The experimental data were compared with calculations based on the envelope-function approximation and fitted to the third-derivative functional form of reflectance modulation theory.
Observation of quantum-size effects in optical transmission spectra of PbTe/Pb1−xEuxTe superlattices
Authors: Ishida Akihiro , Matsuura Shuji , Mizuno Makoto , Fujiyasu Hiroshi
Published: July 2002

Optical transmission spectra of PbTe/Pb1−xEuxTe (x=0.05) superlattices were measured at 300 K. The superlattices were prepared on thick Pb1−xEuxTe buffers grown on KCl (100) substrates using the hot wall epitaxy technique. Clear steplike absorptions corresponding to the interband electron transitions between subbands (n=1,2,3 to n=1,2,3) were observed for the first time for the PbTe/Pb1−xEuxTe superlattice. The experimental absorption edges agreed very well with theoretical ones calculated assuming the conduction-band offset is equal to that of the valence band.
Growth of YBa2Cu3Ox single crystals
Authors: Kaiser D. L. , Holtzberg F. , Scott B. A. , McGuire T. R.
Published: July 2002

We report a set of conditions for crystal growth of the high-temperature superconductor YBa2Cu3Ox. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.
High-fluence ion damage effects in Ar-implanted (InGa)As/GaAs strained-layer superlattices
Authors: Myers D. R. , Arnold G. W. , Hills C. R. , Dawson L. R. , Doyle B. L.
Published: July 2002

We have characterized (InGa)As/GaAs strained-layer superlattices (SLS’s) that have been implanted with 300 keV argon ions at room temperature for fluences sufficient to induce precipitous stress relief in the implanted composite. SLS compositional modulation is lost due to ion beam mixing over much of the ion range; however, the implanted regions remain crystalline despite extensive damage near the mean ion range. High-density dislocation networks are found near the substrate-buffer interface after precipitous stress relief and are correlated with the occurrence of the effect.
Observation of first-order Freedericksz transition in a nematic film induced by electric and optical fields
Authors: Chen Shu-Hsia , Wu J. J.
Published: July 2002

The observation of optical and electric first-order Freedericksz transitions in a homeotropically aligned nematic liquid crystal film induced by electric and optical fields, respectively, is reported. The experimental results are in good agreement with theoretical predictions.
Protein structure elucidation from NMR proton densities
Authors: Grishaev Alexander , Llinás Miguel
Published: July 2002

The NMR-generated foc proton density affords a template to which the molecule has to be fitted to derive the structure. Here we present a computational protocol that achieves this goal. H
N
atoms are readily recognizable from
1
H/
2
H exchange or
1
H/
15
N heteronuclear single quantum correlation (HSQC) experiments. The primary structure is threaded through the unassigned foc by leapfrogging along peptidyl amide H
N
s and the connected H
α
s. Via a Bayesian approach, the probabilities of the sequential connectivity hypotheses are inferred from likelihoods of H
N
/H
N
, H
N
/H
α
, and H
α
/H
α
interatomic distances as well as
1
H NMR chemical shifts, both derived from public databases. Once the polypeptide sequence is identified, directionality becomes established, and the foc N and C termini are recognized. After a similar procedure, side chain H atoms are found, including discriminated cis/trans proline loci. The folded structure then is derived via a direct molecular dynamics embedding into mirror image-related representations of the foc and selected according to a lowest energy criterion. The method was applied to foc densities calculated for two protein domains, col 2 and kringle 2. The obtained structures are within 1.0–1.5 Å (backbone heavy atoms) and 1.5–2.0 Å (all heavy atoms) rms deviations from reported x-ray and/or NMR structures.
Induction of p57
KIP2
expression by p73β
Authors: Bálint Éva , Phillips Andrew C. , Kozlov Serguei , Stewart Colin L. , Vousden Karen H.
Published: July 2002

The p53-related protein p73 has many functions similar to that of p53 including the ability to induce cell-cycle arrest and apoptosis. Both p53 and p73 function as transcription factors, and p73 activates expression of many genes that also are regulated by p53. Despite their similarities, it is evident that p53 and p73 are not interchangeable functionally, with p73 playing a role in normal growth and development that is not shared by p53. In this paper we describe the ability of p73β but not p53 to activate expression of the cyclin-dependent kinase inhibitor
p57
KIP
and
KvLQT1
, two genes that are coregulated in an imprinted region of the genome. Our results suggest that p73 may regulate expression of genes through mechanisms that are not shared by p53, potentially explaining the different contributions of p53 and p73 to normal development.
Oncogenic
ras
activates the ARF-p53 pathway to suppress epithelial cell transformation
Authors: Lin Athena W. , Lowe Scott W.
Published: July 2002

Chemically induced skin carcinomas in mice are a paradigm for
epithelial neoplasia, where oncogenic
ras
mutations
precede
p53
and
INK4a
/
ARF
mutations during the
progression toward malignancy. To explore the biological basis for
these genetic interactions, we studied cellular responses to oncogenic
ras
in primary murine keratinocytes. In wild-type
keratinocytes,
ras
induced a cell-cycle arrest that
displayed some features of terminal differentiation and was accompanied
by increased expression of the p19
ARF
,
p16
INK4a
, and p53 tumor suppressors. In
ARF
-null keratinocytes,
ras
was unable to
promote cell-cycle arrest, induce differentiation markers, or properly
activate p53. Although oncogenic
ras
produced a
substantial increase in both nucleolar and nucleoplasmic
p19
ARF
, Mdm2 did not relocalize to the nucleolus or to
nuclear bodies but remained distributed throughout the nucleoplasm.
This result suggests that p19
ARF
can activate p53 without
overtly affecting Mdm2 subcellular localization. Nevertheless, like
p53
-null keratinocytes,
ARF
-null
keratinocytes were transformed by oncogenic
ras
and
rapidly formed carcinomas
in vivo
. Thus, oncogenic
ras
can activate the ARF-p53 program to suppress
epithelial cell transformation. Disruption of this program may be
important during skin carcinogenesis and the development of other
carcinomas.
Links between replication and recombination in
Saccharomyces cerevisiae
: A hypersensitive requirement for homologous recombination in the absence of Rad27 activity
Authors: Debrauwère Hélène , Loeillet Sophie , Lin Waka , Lopes Judith , Nicolas Alain
Published: July 2002

The
RAD27
gene of
Saccharomyces cerevisiae
encodes a 5′-3′ flap exo/endonuclease, which plays an important role during DNA replication for Okazaki fragment maturation. Genetic studies have shown that
RAD27
is not essential for growth, although
rad27
Δ mutants are temperature sensitive. Moreover, they exhibit increased sensitivity to alkylating agents, enhanced spontaneous recombination, and repetitive DNA instability. The conditional lethality conferred by the
rad27
Δ mutation indicates that other nuclease(s) can compensate for the absence of Rad27. Indeed, biochemical and genetical analyses indicate that Okazaki fragment processing can be assured by other enzymatic activities or by alternative pathways such as homologous recombination. Here we present the results of a screen that makes use of a synthetic lethality assay to identify functions required for the survival of
rad27
Δ strains. Altogether, we confirm that all genes of the Rad52 recombinational repair pathway are required for the survival of
rad27
Δ strains at both permissive (23°C) and semipermissive (30°C) temperatures for growth. We also find that several point mutations that confer weaker phenotypes in mitotic than in meiotic cells (
rad50S
,
mre11s
) and additional gene deletions (
com1/sae2
,
srs2
) exhibit synthetic lethality with
rad27
Δ and that
rad59
Δ exhibits synergistic effects with
rad27
Δ. This and previous studies indicate that homologous recombination is the primary, but not only, pathway that functions to bypass the replication defects that arise in the absence of the Rad27 protein.
Immunolocalization of UDP-glucose:glycoprotein glucosyltransferase indicates involvement of pre-Golgi intermediates in protein quality control
Authors: Zuber Christian , Fan Jing-yu , Guhl Bruno , Parodi Armando , Fessler John H. , Parker Carol , Roth Jürgen
Published: July 2002

The UDP-glucose:glycoprotein glucosyltransferase (GT) is a protein folding sensor and glycosyltransferase that constitutes an important component of the protein quality control machinery. With the use of quantitative immunogold electron microscopy, we established the subcellular distribution of GT in rat liver and pancreas and
Drosophila melanogaster
salivary gland as well as cell lines and correlated it with that of glucosidase II, calreticulin, and pre-Golgi intermediate markers. Labeling for GT, as well as for glucosidase II and calreticulin, was found in the endoplasmic reticulum (ER), including nuclear envelope and pre-Golgi intermediates located between ER and Golgi apparatus, and in the cell periphery. In the rough ER, labeling for GT was inhomogeneous, with variously sized labeled and unlabeled cisternal regions alternating, indicative of a meshwork of quality control checkpoints. Notably, labeling intensity for GT was highest in pre-Golgi intermediates, corresponding to twice that of rough ER, whereas the Golgi apparatus exhibited no specific labeling. These results suggest that protein quality control is not restricted to the ER and that the pre-Golgi intermediates, by virtue of the presence of GT, glucosidase II, and calreticulin, are involved in this fundamental cellular process.
Phenotypic analysis of mice expressing exclusively apolipoprotein B48 or apolipoprotein B100.
Authors: Farese R V , Véniant M M , Cham C M , Flynn L M , Pierotti V , Loring J F , Traber M , Ruland S , Stokowski R S , Huszar D , Young S G
Published: July 2002

Apolipoprotein (apo)-B is found in two forms in mammals: apo-B100, which is made in the liver and the yolk sac, and apo-B48, a truncated protein made in the intestine. To provide models for understanding the physiologic purpose for the two forms of apo-B, we used targeted mutagenesis of the apo-B gene to generate mice that synthesize exclusively apo-B48 (apo-B48-only mice) and mice that synthesize exclusively apo-B100 (apo-B100-only mice). Both the apo-B48-only mice and apo-B100-only mice developed normally, were healthy, and were fertile. Thus, apo-B48 synthesis was sufficient for normal embryonic development, and the synthesis of apo-B100 in the intestines of adult mice caused no readily apparent adverse effects on intestinal function or nutrition. Compared with wild-type mice fed a chow diet, the levels of low density lipoprotein (LDL)-cholesterol and very low density lipoprotein- and LDL-triacylglycerols were lower in apo-B48-only mice and higher in the apo-B100-only mice. In the setting of apo-E-deficiency, the apo-B100-only mutation lowered cholesterol levels, consistent with the fact that apo-B100-lipoproteins can be cleared from the plasma via the LDL receptor, whereas apo-B48-lipoproteins lacking apo-E cannot. The apo-B48-only and apo-B100-only mice should prove to be valuable models for experiments designed to understand the purpose for the two forms of apo-B in mammalian metabolism.
Identification of novel genes in Drosophila reveals the complex regulation of early gene activity in the mesoderm.
Authors: Casal J , Leptin M
Published: July 2002

Two zygotic genes, twist and snail, are indispensable for the correct establishment of the mesoderm primordium in the early Drosophila embryo. They are also needed for morphogenesis and differentiation of the mesoderm. Both genes code for transcription factors with different, albeit complementary, functions. Therefore, to understand the early development of the mesoderm, it will be necessary to identify and study the genes regulated by twist and snail. We have searched for downstream genes using a subtractive cDNA library enriched in sequences expressed in the mesoderm. We have isolated sequences that correspond to 13 novel early mesoderm genes. These novel genes show a variety of expression patterns and also differ in their dependence on twist and snail functions. This indicates that the regulation of early gene activity in the mesoderm is more complex than previously thought.
Significant competitive advantage conferred by meiosis and syngamy in the yeast Saccharomyces cerevisiae.
Authors: Birdsell J , Wills C
Published: July 2002

The presumed advantages of genetic recombinations are difficult to demonstrate directly. To investigate the effects of recombination and background heterozygosity on competitive ability, we have performed serial-transfer competition experiments between isogenic sexual and asexual strains of the yeast Saccharomyces cerevisiae. The members of these diploid pairs of strains differed only in being heterozygous (sexual) or homozygous (asexual) at the mating type or MAT locus. Competing pairs had either a completely homozygous or a heterozygous genetic background, the latter being heterozygous at many different loci throughout the genome. A round of meiotic recombination (automixis) conferred a large and statistically significant enhancement of competitive ability on sexual strains with a heterozygous genetic background. By contrast, in homozygous background competitions, meiosis decreased the sexual strains' initial relative competitive ability. In all cases, however, the sexual strains outcompeted their isogenic asexual counterparts, whether meiotic recombination had occurred or not. In some genetic backgrounds, this was due in part to an overdominance effect on competitive advantage of heterozygosity at the MAT locus. The advantage of the sexual strains also increased significantly during the course of the homozygous background competitions, particularly when meiosis had occurred. This latter effect either did not occur or was very weak in heterozygous background competitions. Overall, sexual strains with heterozygous genetic backgrounds had a significantly higher initial relative competitive ability than those with homozygous backgrounds. The advantage of mating type heterozygosity in this organism extends far beyond the ability to recombine meiotically.
Prime factors
Authors: Ge Liming
Published: July 2002

We use Voiculescu’s free probability theory to prove the existence
of prime factors, hence answering a longstanding problem in the theory
of von Neumann algebras.
Complex formation between stage-specific oocyte factors and a
Xenopus
mRNA localization element
Authors: Mowry Kimberly L.
Published: July 2002

It is a long-standing proposal that localization of maternal
factors in eggs can provide the basis for pattern formation in the
early embryo. The localized information can be stored as RNA, one
example being Vg1 RNA, which is localized exclusively to the vegetal
hemisphere of
Xenopus
oocytes and eggs. Localization of
Vg1 mRNA is directed by a 340-nt sequence element contained within its
3′ untranslated region. To understand the mechanism of localization, I
have tested whether factors from the oocyte interact specifically with
the RNA localization sequence. Results presented here show that a set
of oocyte proteins form complexes with the localization element both
in vitro
and
in vivo
. These proteins are
specifically enriched in the stages of oogenesis during which
localization occurs and recognize sub-elements of the RNA localization
element that are essential for localization
in vivo
.
These data suggest that formation of a localization-specific
RNA–protein complex may be the first step in directing Vg1 mRNA to its
subcellular destination.