bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression of nonlinear interchange mode by zonal counterstreaming flow generation
Authors: Takayama Arimichi , Wakatani Masahiro , Sugama Hideo
Published: July 2002

Nonlinear evolution of interchange mode produces both vortex flow and shear flow in a scrape-off layer (SOL) plasma with unfavorable magnetic curvature. When the Rayleigh number becomes the order of 105, the shear flow exceeds the vortex flow and the zonal counterstreaming flow appears in the perpendicular direction to the ambient magnetic field. Simultaneously, the fluctuation level decreases and the associated cross-field transport becomes almost classical. However, since reduction of the saturation level weakens the shear flow generation, an intermittent oscillatory behavior appears and continues. Transport due to the vortex flow measured with a Nusselt number may depend on the Prandtl number for a given Rayleigh number.
Rapid generation of Langmuir wave packets during electron beam–plasma instabilities
Authors: Akimoto K. , Omura Y. , Matsumoto H.
Published: July 2002

A new mechanism for Langmuir wave localization is investigated. Typical computer simulation of weak electron beam–plasma instabilities proceeds as follows: As Langmuir waves grow out of the noise, they react back on the beam particles by trapping. The subsequent phase shift puts the electrons out of phase with the waves. Then the instability is locally quenched, resulting in strong wave modulation, even before saturation of the instabilities. The typical size of wave packets is given by 2πvp/ω̄t, where vp and ω̄t denote the phase speed of the dominant wave and the average bounce frequency, respectively. The time scale for this process is considerably shorter than that of parametric instabilities.
A kinetic model of transient effects in tokamak edge plasmas
Authors: Batishcheva A. A. , Batishchev O. V. , Shoucri M. M. , Krasheninnikov S. I. , Catto P. J. , Shkarofsky I. P. , Sigmar D. J.
Published: July 2002

Fluid descriptions are not adequate for edge plasma studies when the high collision frequency assumption fails in the presence of rapid transient processes. The results of analytic and numerical modeling of some of the non-Maxwellian features of edge plasma transport associated with transient heating and cooling phenomena in the scrape-off layer are presented. It is found that kinetic effects alter the heat conduction coefficient and lead to interesting critical behavior of the distribution function. The numerics are performed with the recently developed time-dependent Fokker–Planck code ALLA.
Low collisionality transport in a torus
Authors: Weitzner Harold
Published: July 2002

Low collisionality transport analysis methods developed earlier [Phys. Plasmas 1, 3942 (1994)] are applied to several problems. Slow time dependence is added to the earlier work on low collisionality steady states. It is shown that two types of time evolution are possible. In one case the energy balance relations adjust instantaneously to time dependent changes, while the flux surface averaged density satisfies an ordinary differential equation in time. Another possible type of solution, which requires self-consistent external energy sources and/or sinks is a flux conserving time evolution in which the safety factor Q moves with the plasma and the magnetic field. The low collisionality solution methods are then applied to the usual neoclassical parameter ordering. Without the use of moment equations to close the system of drift kinetic equations, solvability conditions on higher order drift kinetic equations complete the specification of the solution. The electron energy balance relation drops from the system, so that no electron energy transport anomaly appears. The results are not significantly different from the low collisionality case. Finally, to exhibit the nature of the constraints implicit in the low collisionality analysis procedure, it is shown that the magnetic field in a steady state, low collisionality stellarator must satisfy the condition that the magnetic field must have approximately a tokamak-like symmetry. Specifically, on a flux surface B⋅∇B must be approximately a function of B alone. A more direct solution technique is employed here, compared with the previous work, although certain ambiguities occur. The technique used here greatly simplifies the previous analysis but a solution accurate to N-th order in the expansion in some parameter admits arbitrary N+1-st and higher order additions.
A comparative study of x-ray emission from laser spots in laser-heated hohlraums relative to spots on simple disk targets
Authors: Ze F. , Langer S. H. , Kauffman R. L. , Kilkenny J. D. , Landen O. , Ress D. , Rosen M. D. , Suter L. J. , Wallace R. J. , Wiedwald J. D.
Published: July 2002

In this paper we report the results of experiments that compare the x-ray emission from a laser spot in a radiation-filled hohlraum to that from a similar laser spot on a simple disk target. The studies were done using the Nova laser facility [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)] in its 0.35 μm wavelength, 1 ns square pulse configuration. Focal spot intensities were 2–3.5×1015 W/cm2. X-ray images measured x-ray conversion in a hohlraum and from an isolated disk simultaneously. A laser spot inside a hohlraum emitted more x rays, after subtracting the background emission from the hohlraum walls, than a spot on a disk. Numerical models suggest the enhanced spot emission inside the hohlraum is due to an increase in lateral transport relative to the disk. Filamentation in the hohlraum will also increase the spot size. The models agree fairly well with the results on spot spreading but do not explain the overall increase in conversion efficiency.
The evolution of plane current–vortex sheets
Authors: Dahlburg R. B. , Boncinelli P. , Einaudi G.
Published: July 2002

The linear and nonlinear evolution of the plane current–vortex sheet, with a basic magnetic field given by B0(y)=tanh yêz, and a basic velocity field given by W0(y)=V tanh Ryêz, is examined. The discovery of an ideal instability in a large region of parameter space previously found to be stable is reported. In this paper numerical evidence is presented that this parameter regime is in fact highly unstable, with growth rates exceeding those of the modes existing in the region of parameter space previously found to be unstable. An examination of the perturbation energy balance indicates that enhanced energy transfer from the basic velocity field to the perturbed velocity and magnetic fields is responsible for the enhanced growth rate. This occurs due to processes absent from both the resistive and Kelvin–Helmholtz instabilities. Nonlinearly it is found that magnetic reconnection can occur on an ideal time scale in certain cases. These faster instabilities lead to a more violent cascade of excitation in the streamwise direction, as evidenced by the rapid formation of higher harmonics of the initial disturbance. A nonlinear saturation due to increased correlation of the perturbed velocity and magnetic field occurs for all cases.
Tilt stabilization by cycling ions crossing magnetic separatrix in a field-reversed configuration
Authors: Nishimura Kazumi , Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

The stabilization of the tilt disruption in a field-reversed configuration is investigated by means of a three-dimensional particle simulation. The growth rate of tilting instability decreases as the plasma beta value at magnetic separatrix βsp increases, while it is slightly affected by the finite ion Larmor radius parameter s̄ and the hollowness parameter of an equilibrium current profile D for low βsp and moderately kinetic (2⩽s̄⩽5) plasmas. It is found that the number flux of ions crossing the separatrix repeatedly increases with increasing βsp and the crossing motion of ions plays a role in leading to the tilt stabilization by disturbing the unstable tilting motion.
Ion plateau transport near the tokamak magnetic axis
Authors: Shaing K. C. , Hazeltine R. D.
Published: July 2002

Conventional neoclassical transport theory does not pertain near the magnetic axis, where orbital variation of the minor radius and the poloidal field markedly change the nature of guiding-center trajectories. Instead of the conventional tokamak banana-shaped trajectories, near-axis orbits, called potato orbits, are radially wider and lead to distinctive kinetic considerations. Here it is shown that there is a plateau regime for the near-axis case; the corresponding potato-plateau ion thermal conductivity is computed.
Sheath over a rough surface in a tilted magnetic field
Authors: Cohen Ronald H. , Ryutov Dmitri D.
Published: July 2002

In this paper a sheath near the wall whose surface is “rough,” containing random bumps with a height comparable to the distance between them is considered. The magnetic field is assumed to form a shallow angle with the wall (as in a tokamak divertor) and the scale of nonuniformities is assumed greater than the ion gyroradius. The following new effects caused by surface roughness are identified: (1) The fraction of the surface “wetted” by the plasma becomes much smaller than for a flat wall. (2) The ion subsheath is broadened and the diamagnetic current in it acquires a complex three-dimensional structure. (3) Cross-field nonuniformities of the plasma potential appear in the bulk plasma.
Toroidally localized and nonlocalized ballooning instabilities in a stellarator
Authors: Cuthbert P. , Lewandowski J. L. V. , Gardner H. J. , Persson M. , Singleton D. B. , Dewar R. L. , Nakajima N. , Cooper W. A.
Published: July 2002

It is shown that the coexistence of toroidally nonlocalized ideal-hydromagnetic ballooning instabilities, with a quasidiscrete spectrum, and toroidally localized ballooning instabilities with a broad continuous spectrum, as predicted by Dewar and Glasser [Phys. Fluids 26, 3038 (1983)] can be realized in a Mercier-unstable equilibrium case modeling the Large Helical Device (LHD) [A. Iiyoshi et al., Fusion Technol. 17, 148 (1990)] with a broad pressure profile. The quasidiscrete, interchange branch corresponds to extended modes that can be understood on the basis of a ripple-averaged ballooning equation, whereas the broad-continuum, ballooning branch corresponds to modes localized along a flux tube. The physical origin of the two branches is discussed.
Driven plasma rotation in the Compact Auburn Torsatron
Authors: Thomas E. , Knowlton S. , Gandy R. , Cooney J. , Pritchard D. , Pruitt T.
Published: July 2002

Poloidal plasma rotation has been driven in the Compact Auburn Torsatron (CAT) [R. F. Gandy et al., Fusion Technol. 18, 281 (1990)]. In these studies, a biased electrode is used to establish the rotation. Currents of up to 1 A are injected into hydrogen, helium, and argon plasmas from a biased electrode placed inside of the last closed magnetic flux surface of the CAT device. The response of the plasma to the injected current is measured with Langmuir, Mach, and emissive probes. Poloidal plasma velocities of up to 10 km/s in hydrogen plasmas are measured by the Mach probe. The plasma density and electron temperature increased during the driven rotation, and there are indications of improved particle confinement. The experimental measurements show generally good agreement with a model of plasma rotation by Coronado and Talmadge. Experiments also show that the sign of the injected current (i.e., the direction of the radial electric field) has a significant impact on the effect of the driven rotation. A reduction in the particle transport is observed only for radially inward electric fields.
The dynamic form factor for ion-collisional plasmas
Authors: Zheng Jian , Yu C. X. , Zheng Z. J.
Published: July 2002

Based on the fluctuation-dissipation theorem, this work is concerned with the dynamic form factor of Thomson scattering for ion-collisional plasmas. The main effort is to calculate the ion susceptibility by analytically solving the linearized ion Fokker–Planck equation through the Chang–Callen 13-moment approach. Comparison to other theories shows that this theory can provide an accurate prediction of the position and width for ion-acoustic peaks and a proper description of the entropy wave with some underestimation of its damping. The theory may be useful for explaining experimental data and making measurement more precise.
Acceleration of injected electrons in a laser wakefield experiment
Authors: Dorchies F. , Amiranoff F. , Malka V. , Marquès J. R. , Modena A. , Bernard D. , Jacquet F. , Miné Ph. , Cros B. , Matthieussent G. , Mora P. , Solodov A. , Morillo J. , Najmudin Z.
Published: July 2002

An electron plasma wave (EPW) has been excited by a short laser pulse (5 J, 400 fs) via the laser wakefield (LWF) mechanism. At the LWF quasi-resonance condition, the 3 MeV injected electrons have been accelerated with a maximum energy gain of 1.5 MeV. The maximum longitudinal electric field is estimated to be 1.5 GV/m. It has been observed that electrons deflected during the interaction, can scatter on the walls of the experimental chamber and fake a high energy signal. A special effort has been given in the electron detection to separate the accelerated electrons signal from the background noise. The experimental data are confirmed with numerical simulations, demonstrating that the energy gain is affected by the EPW radial electric field. The duration of the EPW inferred by the number of accelerated electrons and by the numerical simulations is of the order of 1–10 ps.
Magnetohydrodynamic calculations with a nonmonotonic q profile and equilibrium, sheared toroidal flow
Authors: Held E. D. , Leboeuf J. N. , Carreras B. A.
Published: July 2002

The linear and nonlinear stability of a nonmonotonic q profile is examined using a reduced set of magnetohydrodynamic (MHD) equations with an equilibrium, sheared toroidal flow. The reversed shear profile is shown to be unstable to a rich variety of resistive MHD modes including pressure-driven instabilities and tearing instabilities possessing a tearing/interchange character at low Lundquist number, S, and taking on a double/triple tearing structure at high S. Linear calculations show that the destabilizing effect of toroidal velocity shear on tearing modes is enhanced at finite pressure. In addition, this velocity shear decreases the stabilizing effect of finite pressure seen previously for tearing modes at high S. Nonlinear calculations show the generation of a large, m=1, n=0, Reynolds-stress-driven poloidal flow in the absence of significant flow damping. Calculations in which the poloidal flow was heavily damped show that sub-Alfvénic, sheared toroidal flows have a minimal effect on weakly coupled, localized instabilities.
Propagation and damping of m=+1 and m=−1 helicon modes in an inhomogeneous plasma column
Authors: Krämer M.
Published: July 2002

The effect of the density gradient on the dispersion of the m=+1 and m=−1 helicon modes is investigated both theoretically as well as numerically. In particular, attention is focussed on the helicon wave damping that is closely related to the rf power absorption and the Poynting flux. It is shown that the propagation characteristics of the m=−1 mode changes drastically when the density gradient is sufficiently strong. This mode exhibits a cutoff that depends on the gradient scale length and the plasma parameters. In the regime where both helicon modes propagate, the excitation of the m=−1 mode is unlikely as it is strongly damped. The results give evidence that the density gradient effect is decisive for the formation of helicon discharges, which are predominantly sustained by the m=+1 mode.
Observations of low-frequency mode in a multicomponent plasma with negative ions
Authors: Handique B. , Bailung H. , Das G. C. , Chutia Joyanti
Published: July 2002

Based on linear perturbation theory, a dispersion relation for low-frequency (LF) electrostatic waves has been derived in a multicomponent plasma containing negative ions. To relate the theoretical observations with experiments on the LF wave, the slower mode has been studied extensively for various plasma models. The experiment was carried out in Ar, F, and electron plasma in a double plasma device. The experimental observations on LF electrostatic waves yield the qualitative frequency variations due to the presence of negative ions. In the region of lower negative ion concentration, the mode decreases with the addition of a higher percentage of negative ion concentration while later, in the higher region of negative ion concentration, the variation shows the asymptotic behavior with negative ion concentration. Both the observations, theoretical and experimental, have shown a good agreement with each other. Moreover, a theoretical model of plasma with heavy negative ions has also been presented.
Flow-curvature effect on linear Rayleigh–Taylor instability in magnetized plasmas
Authors: Chakrabarti Nikhil
Published: July 2002

The effect of sheared poloidal flow on the linear Rayleigh–Taylor (RT) instability of magnetized plasma is reexamined. In the weak curvature flow limit (V⊥0″≪1) the linear RT mode stabilization effect is recovered. In the strong flow curvature limit (V⊥0″≫1) it is shown that the RT mode may not stabilize.
Two-dimensional implosion of liners
Authors: Lisitsyn Igor V. , Katsuki Sunao , Akiyama Hidenori
Published: July 2002

A two-dimensional implosion of liners is studied. This is instead of the conventional one-dimensional cylindrical implosion of Z pinches, where there is a wire array or a gas-puff liner, which has a cylindrical shape and an uniform mass distribution along the z axis. Two-dimensional Z-pinch compression can be realized utilizing either a spherically shaped wire array or a double gas-puff liner with a special mass distribution along the z-axis. The results obtained from computer simulations show that both kinds of liners can be compressed in both the z and the r directions. This results in a uniform and a high-power heating of the spherical dynamic hohlraum placed inside the liner. The spherical, two-dimensional liner implosion increases the power density at the hohlraum surface, improves the symmetry of the capsule irradiation and helps partially in quenching of the Rayleigh–Taylor instability.
Raman scattering in a partially filled plasma waveguide near cyclotron resonance
Authors: Maraghechi B. , Naseri N.
Published: July 2002

The parametric instability of a predominantly transverse magnetic mode in a waveguide, partially filled with a magnetized plasma, is considered. Raman backscattering of the pump wave off the space-charge wave leads to the scattered radiation. Waveguide eigenmodes relevant to the pump wave and the excited waves are discussed. Formulas are derived for the temporal and spatial growth rates of the backscattered wave and scatterer wave with the pump wave frequency near the electron cyclotron frequency. A numerical study indicates that the phase-matching conditions permit the approach to cyclotron resonance with substantial enhancement of the growth rates. Growth rates show considerable dependence on the ratio of the plasma radius to the waveguide radius a/R and their maximum values are found to be for a/R≈0.3. The growth rate found from the electromagnetic treatment was about 20% less than the one calculated with electrostatic assumption on the space-charge wave. It is also shown that the electromagnetic effect of the space-charge wave has considerable influence on the maximum growth rate near cyclotron resonance by preventing the pump wave from approaching the resonance. Possible use of the EH waveguide mode as a pump for a free-electron laser is discussed. The results of this theory are found to be comparable with the results of previous theoretical and experimental investigations.
Shear flow effect on ion temperature gradient vortices in plasmas with sheared magnetic field
Authors: Chakrabarti Nikhil , Rasmussen Jens Juul
Published: July 2002

The effect of velocity shear on ion temperature gradient (ITG) driven vortices in a nonuniform plasma in a curved, sheared magnetic field is investigated. In absence of parallel ion dynamics, vortex solutions for the ITG mode are studied analytically. It is shown that under certain conditions the coupled equations for potential and pressure exhibit special tripolar vortex-like structures. For the general case, however, parallel ion dynamics is included and the equation describing the stationary ITG vortex has the structure of a nonlinear Poisson-type equation. Analytical as well as numerical solutions of this equation are presented for various possible cases. It is shown that, for a critical value of the velocity shear asymmetric dipolar vortices can arise which are strongly modified as a localized vortex chain at resonance. For strong velocity shear these structures are destroyed and ultimately lead to a dominating monopolar form. The effects of magnetic shear indicate it may destroy these structures.
High Mach number mix instability experiments of an unstable density interface using a single-mode, nonlinear initial perturbation
Authors: Farley D. R. , Peyser T. A. , Logory L. M. , Murray S. D. , Burke E. W.
Published: July 2002

The growth of an unstable density interface from a nonlinear, single-mode, two-dimensional initial perturbation (10 μm amplitude; 23 μm wavelength) has been studied experimentally using a miniature shock tube attached to a gold hohlraum irradiated by the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)]. The initial perturbation was machined into a brominated plastic ablator (1.22 g/cm3) adjacent to a low density carbon foam (0.10 g/cm3). Upon laser illumination of the hohlraum and x-ray ablation of the plastic, a strong shock wave (Mach∼30) propagated across the perturbed density interface causing the onset of the Richtmyer–Meshkov (RM) instability. The interface subsequently experienced a relatively weak Rayleigh–Taylor (RT) unstable deceleration. The nonlinear growth of the mixing layer was obtained from time-resolved radiography of the x-ray transmission through the shock tube, and the decompression-corrected results were compared to published incompressible models, including a Lagrangian energy formulation. The experimental data were also compared with the results of two-dimensional numerical simulation.
Interaction between an Alfvén wave and a particle undergoing acceleration along a magnetic field
Authors: Khazanov George V. , Khabibrakhmanov Ildar K. , Krivorutsky E. N.
Published: July 2002

The energy exchange between a plasma particle and an Alfvén wave propagating along a magnetic field is considered. It is shown that, in the presence of an accelerating force parallel to the background magnetic field, there exists a new channel for nonresonant energy transfer between the wave and the particle.
Bernstein–Greene–Kruskal chain of drift-tearing vortices
Authors: Jovanović D. , Pegoraro F.
Published: July 2002

Collisionless magnetic reconnection in the driftwave plasma regime, ρs,i≫de, is studied using a drift-kinetic description of electrons and fluid ions. A fully nonlinear Bernstein–Greene–Kruskal (BGK) stationary solution is constructed which has the form of a quasi-three-dimensional chain of electron holes coupled to hydrodynamic vortices. This stationary structure is obtained by setting up a plateau in the electron distribution function, followed by the trapping and consecutive depletion of resonant particles, and by the removal of the singular current layers by their short-circuiting via electron gyrations.
Particulates: An origin of GaAs oval defects grown by molecular beam epitaxy
Authors: Weng Shang-Lin , Webb C. , Chai Y. G. , Bandy S. G.
Published: July 2002

We present micrographic evidence that particulates which inadvertently adhere to the wafer surface during substrate preparation result in the formation of oval defects in GaAs layers grown by molecular beam epitaxy (MBE). With elimination of particulate contamination, we simultaneously reduce the total defect density in 1-μm-thick GaAs MBE layers from a few thousand to about 300 cm−2. Further evidence indicates that most of these remaining defects also originate from the surface particulates which adhere during the process of wafer transfer and layer growth.
Observation of power instability and multimode behavior in a far-infrared free-electron laser
Authors: Amir Avner , Elias Luis R. , Gregoire Dan J. , Hu R. James , Kotthaus Jörg P. , Ramian Gerald , Stern Avinoam
Published: July 2002

Measurements of the time dependence and the frequency spectrum of the output power in the far-infrared free-electron laser at the University of California at Santa Barbara are reported. In typical light pulses of 20–50 μs we have observed unexpected oscillations of the laser output with a characteristic period of 5 μs. At the same time, the laser frequency swept over a discrete set of frequency modes, separated by 1.3 GHz. We also present measurements of the gain and loss of the optical mode and discuss the problem of the accelerator terminal voltage drop in a single electron beam pulse with relation to the light spectrum.
Projection printing of gold micropatterns by photochemical decomposition
Authors: Baum Thomas H. , Marinero Ernesto E. , Jones Carol R.
Published: July 2002

Gold micropatterns have been generated by the laser photolysis of dimethylgold (III) acetylacetonate, Me2Au(acac), in the vapor phase. Linewidths as fine as 2 μm were readily obtained utilizing a simple optical projection system comprised of a 4× projection lens, a lithographic photomask, and the UV output from an excimer laser. The single-step, dry process for selectively producing metal patterns is highlighted.
Magnetic properties of cobalt nitride thin films
Authors: Matsuoka Morito , Ono Ken’ichi , Inukai Takashi
Published: July 2002

Cobalt nitride (CoNx) thin films with a large perpendicular magnetic anisotropy are synthesized by reactive sputtering. The perpendicular magnetic anisotropy is induced by substrate heating during deposition, annealing after deposition, or ion bombardment during deposition.
Direct evidence of recrystallization rate enhancement during rapid thermal annealing of phosphorus amorphized silicon layers
Authors: Adekoya W. O. , Ali M. Hage , Muller J. C. , Siffert P.
Published: July 2002

The solid phase epitaxial regrowth of implanted phosphorus amorphized 〈100〉 silicon during rapid thermal annealing (RTA) in the temperature range 475–600 °C has been studied using Rutherford backscattering and channeling measurements. Within the temperature range 475–550 °C, our results show an enhancement of growth rate (Vg) by a factor of 4 when compared with values reported in literature for processing in a conventional furnace, with an activation energy of 2.7±0.2 eV. For temperatures above 550 °C, a reduction of this enhancement effect sets in, with a fall in the enhancement factor from 4 at 550 °C to about 1.5 at 600 °C. Estimation of the absolute error in temperature measurement in the RTA furnace shows that the accelerated kinetics cannot be completely explained by such errors as they are small compared to the enhancement rate. These results therefore give evidence of RTA induced enhancement of growth rate during solid phase recrystallization of implantation-amorphized silicon.
Molecular beam epitaxial growth and optical properties of InAs1−xSbx in 8–12 μm wavelength range
Authors: Yen M. Y. , Levine B. F. , Bethea C. G. , Choi K. K. , Cho A. Y.
Published: July 2002

We have successfully grown InAs1−xSbx by molecular beam epitaxy over the complete compositional range of 0<x<1 on InAs substrates. The band gaps have been measured using optical absorption and cut-off wavelengths as long as 12.5 μm have been obtained.
Unpinning of the Fermi level on GaAs by flowing water
Authors: Ives N. A. , Stupian G. W. , Leung M. S.
Published: July 2002

Unpinning of the Fermi level on GaAs (100) surfaces by photochemical reactions resulting from simultaneous exposure of specimens to flowing water and light was recently reported. We discuss here a series of experiments carried out to provide further information on the changes in surface electronic structure responsible for unpinning of the Fermi level under these conditions. The present work supports the conclusion that the surface states which pin the Fermi level are associated with elemental arsenic and arsenic sesquioxide (As2O3). Effects of each of these two species on pinning are distinguished experimentally. We find that, in addition to photochemical reactions, exposure to flowing water alone can result in Fermi level unpinning under certain conditions. The oxygen content of the wash water and the specimen preparation are shown to be important variables.
Laser blow-off plasma propagating in low-pressure gas
Authors: Bakos J. S. , Ignácz P. N. , Szigeti J. , Kovács J.
Published: July 2002

The change of the properties of the plasma ball created in the laser blow-off process is investigated in interaction with residual gas and buffer gas particles during its flight in space. The main process of the interaction is the collision leading to plasma particle loss. The center of mass velocity and the temperature of the ball weakly depend on the buffer gas density because the plasma is mainly collisionless.
Synthesis of crafted optical pulses by time domain modulation in a fiber-grating compressor
Authors: Haner M. , Warren W. S.
Published: July 2002

We report a method for direct generation of voltage programmable, arbitrarily shaped laser pulses of subpicosecond duration and roughly 100 fs resolution. A waveguide intensity modulator was fabricated and incorporated into a fiber-grating pulse compressor to shape temporally the chirped pulse before compression. Optical pulses with sin(x)/x and square envelopes are synthesized to demonstrate the technique.
NiAl/n-GaAs Schottky diodes: Barrier height enhancement by high-temperature annealing
Authors: Sands T. , Chan W. K. , Chang C. C. , Chase E. W. , Keramidas V. G.
Published: July 2002

A metallurgically stable and laterally uniform contact to n-GaAs with an enhanced barrier height (0.99 V) and an ideality factor of 1.10 has been achieved with a NiAl bimetallic metallization. This barrier height, as measured by the forward current-voltage technique after annealing for 20 s at 650 °C, is higher than the reported barrier heights of refractory metallizations to n-GaAs. Auger electron spectroscopy (AES) sputter profiles reveal an Al-Ga exchange reaction after high-temperature (500–950 °C) rapid thermal annealing. From these results, the barrier height enhancement is attributed to the formation of an Al1−xGaxAs layer at the NiAl/n-GaAs interface. The thermal stability and low electrical resistivity of the NiAl phase, the enhanced barrier height on n-GaAs, and the ease of patterning the as-deposited Ni/Al/Ni layered structure by lift-off techniques make NiAl a very promising gate contact material for GaAs metal-semiconductor field-effect transistors and related devices.
Correlated spontaneous emission between two longitudinal modes in an extended-cavity semiconductor laser
Authors: Ohtsu M. , Liou K.-Y.
Published: July 2002

Recently, correlated spontaneous emission in a laser of three-level atoms has been analyzed theoretically and has been demonstrated experimentally using a three-level He-Ne Zeeman laser. We demonstrate that correlated spontaneous emission is also feasible in a semiconductor laser with a long extended cavity. As a result, the heterodyned spectral width between two lasing modes in a grating extended-cavity laser was reduced to below the spontaneous emission noise level. The laser can be used for high-precision optical-heterodyne sensing.
Strains in Si-on-SiO2 structures formed by oxygen implantation: Raman scattering characterization
Authors: Olego D. J. , Baumgart H. , Celler G. K.
Published: July 2002

Low-temperature Raman scattering measurements were carried out to characterize Si-on-SiO2 structures formed by oxygen implantation and subsequent furnace or lamp annealing. The experiments were conducted with 413.1 nm laser light to probe only the thin Si layers at the top of the structures. The Raman spectra of the furnace-annealed samples are red shifted and broadened when compared with a virgin Si surface. The shifts and broadenings decrease with increasing annealing temperatures but they are still present in samples annealed above 1250 °C for 3 h. No shifts or broadenings affect the Raman peaks of the layers, which were lamp annealed at 1405 °C for half an hour. The red shifts indicate that the recrystallized Si layers are under tensile strains, whose origin is attributed to oxide precipitates. Quantitative estimates of the strains and associated stresses are obtained from the measured Raman shifts.
Archaeal dUTPase enhances PCR amplifications with archaeal DNA polymerases by preventing dUTP incorporation
Authors: Hogrefe Holly H. , Hansen Connie J. , Scott Bradley R. , Nielson Kirk B.
Published: July 2002

We discovered a thermostable enzyme from the archaeon
Pyrococcus furiosus
(
Pfu
), which increases yields of PCR product amplified with
Pfu
DNA polymerase. A high molecular mass (>250 kDa) complex with PCR-enhancing activity was purified from
Pfu
extracts. The complex is a multimer of two discrete proteins, P45 and P50, with significant similarity to bacterial dCTP deaminase/dUTPase and DNA flavoprotein, respectively. When tested in PCR, only recombinant P45 exhibited enhancing activity. P45 was shown to function as a dUTPase, converting dUTP to dUMP and inorganic pyrophosphate.
Pfu
dUTPase improves the yield of products amplified with
Pfu
DNA polymerase by preventing dUTP incorporation and subsequent inhibition of the polymerase by dU-containing DNA. dUTP was found to accumulate during PCR through dCTP deamination and to limit the efficiency of PCRs carried out with archaeal DNA polymerases. In the absence of dUTP inhibition, the combination of cloned
Pfu
DNA polymerase and
Pfu
dUTPase (
PfuTurbo
DNA polymerase) can amplify longer targets in higher yield than
Taq
DNA polymerase.
In vivo
, archaeal dUTPases may play an essential role in preventing dUTP incorporation and inhibition of DNA synthesis by family B DNA polymerases.
Efficient bacterial transcription of DNA nanocircle vectors with optimized single-stranded promoters
Authors: Ohmichi Tatsuo , Maki Angèle , Kool Eric T.
Published: July 2002

We describe experiments aimed at establishing whether circular
single-stranded DNAs can form promoters for bacterial transcription
from small folded motifs.
In vitro
selection
experiments were carried out on circular 103-nt DNA libraries encoding
40-nt randomized sequences as well as self-processing hammerhead
ribozymes. Rounds of rolling circle transcription, reverse
transcription–PCR, and recyclization were carried out to optimize
transcription efficiency. Sequences were identified that are 80-fold
more actively transcribed than the initial library by
E.
coli
RNA polymerase (RNAP). The selected motifs were found to
be more active than canonical
E. coli
promoters in the
same context. Experiments also demonstrated that a single-stranded
pseudopromoter identified by this selection can be transplanted to
other circular DNA contexts and retain transcriptional activity.
Results suggest that the promoter is localized in a short (≈40 nt)
hairpin, which is smaller than canonical
E. coli
promoters. To test whether this pseudopromoter was active in bacterial
cells, a synthetic DNA nanocircle vector encoding a ribozyme targeted
to a site in the
mar
A drug resistance gene was
constructed to contain an optimized single-stranded promoter. It is
shown that this DNA circle can act as a “Trojan horse” in
E. coli
, being actively transcribed by the cellular RNAP
and producing ribozymes that cleave a sequence in the
mar
A drug resistance gene. The use of optimized
single-stranded promoters in combination with synthetic nanocircle DNA
vectors represents a potentially useful way to engender the
synthesis of biologically active RNAs in living cells.
Attenuation of thermal nociception and hyperalgesia by VR1 blockers
Authors: García-Martínez Carolina , Humet Marc , Planells-Cases Rosa , Gomis Ana , Caprini Marco , Viana Felix , De la Peña Elvira , Sanchez-Baeza Francisco , Carbonell Teresa , De Felipe Carmen , Pérez-Payá Enrique , Belmonte Carlos , Messeguer Angel , Ferrer-Montiel Antonio
Published: July 2002

Vanilloid receptor subunit 1 (VR1) appears to play a critical role in the transduction of noxious chemical and thermal stimuli by sensory nerve endings in peripheral tissues. Thus, VR1 antagonists are useful compounds to unravel the contribution of this receptor to pain perception, as well as to induce analgesia. We have used a combinatorial approach to identify new, nonpeptidic channel blockers of VR1. Screening of a library of trimers of
N
-alkylglycines resulted in the identification of two molecules referred to as DD161515 {
N
-[2-(2-(
N
-methylpyrrolidinyl)ethyl]glycyl]-[
N
-[2,4-dichlorophenethyl]glycyl]-
N
-(2,4-dichlorophenethyl)glycinamide} and DD191515 {[
N
-[3-(
N
,
N
-diethylamino)propyl]glycyl]-[
N
-[2,4-dichlorophenethyl]glycyl]-
N
-(2,4-dichlorophenethyl)glycinamide} that selectively block VR1 channel activity with micromolar efficacy, rivaling that characteristic of vanilloid-related inhibitors. These compounds appear to be noncompetitive VR1 antagonists that recognize a receptor site distinct from that of capsaicin. Intraperitoneal administration of both trialkylglycines into mice significantly attenuated thermal nociception as measured in the hot plate test. It is noteworthy that these compounds eliminated pain and neurogenic inflammation evoked by intradermal injection of capsaicin into the animal hindpaw, as well as the thermal hyperalgesia induced by tissue irritation with nitrogen mustard. In contrast, responses to mechanical stimuli were not modified by either compound. Modulation of sensory nerve fibers excitability appears to underlie the peptoid analgesic activity. Collectively, these results indicate that blockade of VR1 activity attenuates chemical and thermal nociception and hyperalgesia, supporting the tenet that this ionotropic receptor contributes to chemical and thermal sensitivity and pain perception
in vivo
. These trialkylglycine-based, noncompetitive VR1 antagonists may likely be developed into analgesics to treat inflammatory pain.
Heat shock protein 90 catalyzes activation of the prekallikrein-kininogen complex in the absence of factor XII
Authors: Joseph Kusumam , Tholanikunnel Baby G. , Kaplan Allen P.
Published: July 2002

Bradykinin is a major mediator of swelling in C1 inhibitor deficiency as well as the angioedema seen with ACE inhibitors and may contribute to bronchial hyperreactivity in asthma. Formation of bradykinin occurs in the fluid phase and along cell surfaces requiring interaction of factor XII, prekallikrein, and high
M
r
kininogen (HK). Recent data suggest that activation of the kinin-forming cascade can occur on the surface of endothelial cells, even in the absence of factor XII. We sought to further define this factor XII-independent mechanism of kinin formation. Both cytosolic and membrane fractions from endothelial cells possessed the ability to catalyze prekallikrein conversion to kallikrein, and activation depended on the presence of HK and zinc ion. We fractionated the cytosol by ion exchange chromatography and affinity chromatography by using corn trypsin inhibitor as ligand. The fractions with peak activity were subjected to SDS gel electrophoresis and ligand blot with biotinylated corn trypsin inhibitor, and positive bands were sequenced. Heat shock protein 90 (Hsp90) was identified as the protein responsible for zinc-dependent prekallikrein activation in the presence of HK. Zinc-dependent activation of the prekallikrein-HK complex also depended on addition of either α and β isoforms of Hsp90 and the activation on endothelial cells was inhibited on addition of polyclonal Ab to Hsp90 in a dose-dependent manner. Although the mechanism by which Hsp90 activates the kinin-forming cascade is not understood, this protein represents the cellular contribution to the reaction and may become the dominant mechanism in pathologic circumstances in which Hsp90 is highly expressed or secreted.
Fabrication of aligned microstructures with a single elastomeric stamp
Authors: Tien Joe , Nelson Celeste M. , Chen Christopher S.
Published: July 2002

The fabrication of complex patterns of aligned microstructures has required the use of multiple applications of lithography. Here we describe an approach for microfabrication that encodes the two-dimensional spatial information of several photomasks onto a single elastomeric stamp by mapping each photomask onto distinct heights on the surface of the stamp. Pressing the stamp against a surface collapses the topography of the stamp such that each recessed layer contacts the surface in stepwise sequence; the greater the applied pressure, the larger the area of the stamp that contacts the surface. After contact of each new layer with the surface, we use techniques of soft lithography (microcontact printing, microfluidics, and patterning through membranes) to pattern the surfaces that contact the stamp and those that do not with inorganic, organic, or living materials. Microfabrication through the use of multilevel stamps provides a promising alternative to conventional lithography for the construction of multicomponent, aligned surfaces; these structures may find use as components of microfluidic devices or biological patterns.
Theory for normal and impaired experience-dependent plasticity in neocortex of adult rats
Authors: Beňušková Lubica , Rema Velayudhan , Armstrong-James Michael , Ebner Ford F.
Published: July 2002

We model experience-dependent plasticity in the cortical
representation of whiskers (the barrel cortex) in normal adult rats,
and in adult rats that were prenatally exposed to alcohol. Prenatal
exposure to alcohol (PAE) caused marked deficits in
experience-dependent plasticity in a cortical barrel-column. Cortical
plasticity was induced by trimming all whiskers on one side of the face
except two. This manipulation produces high activity from the intact
whiskers that contrasts with low activity from the cut whiskers while
avoiding any nerve damage. By a computational model, we show that the
evolution of neuronal responses in a single barrel-column after this
sensory bias is consistent with the synaptic modifications that follow
the rules of the Bienenstock, Cooper, and Munro (BCM) theory. The BCM
theory postulates that a neuron possesses a moving synaptic
modification threshold, θ
M
, that dictates whether the
neuron's activity at any given instant will lead to strengthening or
weakening of its input synapses. The current value of θ
M
changes proportionally to the square of the neuron's activity averaged
over some recent past. In the model of alcohol impaired cortex, the
effective θ
M
has been set to a level unattainable by the
depressed levels of cortical activity leading to “impaired”
synaptic plasticity that is consistent with experimental findings.
Based on experimental and computational results, we discuss how
elevated θ
M
may be related to (
i
) reduced
levels of neurotransmitters modulating plasticity, (
ii
)
abnormally low expression of
N-
methyl-
d
-aspartate receptors (NMDARs), and
(
iii
) the membrane translocation of
Ca
2+
/calmodulin-dependent protein kinase II
(CaMKII) in adult rat cortex subjected to prenatal alcohol exposure.
The kinase-deficient Src acts as a suppressor of the Abl kinase for Cbl phosphorylation
Authors: Shishido Tomoyuki , Akagi Tsuyoshi , Ouchi Toru , Georgescu Maria-Magdalena , Langdon Wallace Y. , Hanafusa Hidesaburo
Published: July 2002

The kinase activity of Abl is known to be regulated by a putative trans-acting inhibitor molecule interacting with the Src homology (SH) 3 domain of Abl. Here we report that the kinase-deficient Src (SrcKD) directly inhibits the tyrosine phosphorylation of Cbl and other cellular proteins by Abl. We found that both the SH2 and SH3 domains of SrcKD are necessary for the suppressor activity toward the Abl kinase phosphorylating Cbl. To suppress the Cbl phosphorylation by Abl, the interaction between the SH3 domain of SrcKD and Cbl is required. This interaction between SrcKD and Cbl is regulated by a closed structure of Cbl. The binding of Abl to the extreme carboxyl-terminal region of Cbl unmasks the binding site of SrcKD to Cbl. This results in a ternary complex that inhibits the Abl-mediated phosphorylation of Cbl by steric hindrance. These results illustrate a mechanism by which the enzymatically inactive Src can exert a biological function
in vivo
.
Noise in neurons is message dependent
Authors: Cecchi Guillermo A. , Sigman Mariano , Alonso José-Manuel , Martínez Luis , Chialvo Dante R. , Magnasco Marcelo O.
Published: July 2002

Neuronal responses are conspicuously variable. We focus on one particular aspect of that variability: the precision of action potential timing. We show that for common models of noisy spike generation, elementary considerations imply that such variability is a function of the input, and can be made arbitrarily large or small by a suitable choice of inputs. Our considerations are expected to extend to virtually any mechanism of spike generation, and we illustrate them with data from the visual pathway. Thus, a simplification usually made in the application of information theory to neural processing is violated: noise is not independent of the message. However, we also show the existence of error-correcting topologies, which can achieve better timing reliability than their components.
Functional interactions of HslV (ClpQ) with the ATPase HslU (ClpY)
Authors: Ramachandran Ravishankar , Hartmann Claudia , Song Hyun Kyu , Huber Robert , Bochtler Matthias
Published: July 2002

HslVU is a bacterial homolog of the proteasome, where HslV is the protease that is activated by HslU, an ATPase and chaperone. Structures of singly and doubly capped HslVU particles have been reported, and different binding modes have been observed. Even among HslVU structures with I-domains distal to HslV, no consensus mode of activation has emerged. A feature in the
Haemophilus influenzae
HslVU structure, insertion of the C termini of HslU into pockets in HslV, was not seen in all other structures of the enzyme. Here we report site-directed mutagenesis, peptide activation, and fluorescence experiments that strongly support the functional relevance of the C terminus insertion mechanism: we find that mutations in HslV that disrupt the interaction with the C termini of HslU invariably lead to inactive enzyme. Conversely, synthetic peptides derived from the C terminus of HslU bind to HslV with 10
−5
M affinity and can functionally replace full HslU particles for both peptide and casein degradation but fail to support degradation of a folded substrate. Thus, the data can be taken as evidence for separate substrate unfoldase and protease stimulation activities in HslU. Enhanced HslV proteolysis could be due to the opening of a gated channel or allosteric activation of the active sites. To distinguish between these possibilities, we have mutated a series of residues that line the entrance channel into the HslV particle. Our mutational and fluorescence experiments demonstrate that allosteric activation of the catalytic sites is required in HslV, but they do not exclude the possibility of channel opening taking place as well. The present data support the conclusion that the
H. influenzae
structure with I-domains distal to HslV captures the active species and point to significant differences in the activation mechanism of HslV, ClpP, and the proteasome.