bluuebunny/crossref_metadata_2025 huggingface dataset:
The magnetron “threshold voltage” revisited
Authors: Lindsay P. A. , Esterson M. , Chen X.
Published: July 2002

In view of the revival of interest in high-power magnetrons, an important magnetron parameter called the threshold voltage has been rederived for both relativistic and nonrelativistic cases using a novel and simplified approach. The main purpose of the paper is to investigate the physical significance of the threshold voltage and to show that, contrary to a long established opinion, no restriction on the amplitude of oscillations is required. In fact, the name itself is a misnomer, since in practice the threshold voltage is an approximate dc anode voltage at which the tube operates when delivering full power. It is this feature which, in our opinion, makes the threshold voltage so important and useful in practice. A set of numerical computations confirms our analysis and also reveals a form of Lorenz’s “butterfly effect” associated with the perceived start of oscillations.
Comment on “Magnetohydrodynamic simulations of direct current helicity injection for current drive in tokamaks’’ [Phys. Plasmas 3, 1038 (1996)]
Authors: Jarboe T. R. , Nelson B. A. , Shumlak U.
Published: July 2002

We disagree with the conclusions of the previous paper because reaching negative conclusions about helicity injection current drive for tokamaks based on magnetohydrodynamic simulation is unjustified. (AIP)
Ionization-induced frequency up-shift of a high-power microwave interacting with a plasma
Authors: Cros B. , Xu X. , Tsukada T. , Yugami N. , Nishida Y. , Matthieussent G.
Published: July 2002

The propagation of an intense electromagnetic wave (EMW), of frequency 9 GHz, in an underdense, near critical, inhomogeneous plasma (ne⩽1.2×1012 cm−3, Te≃3 eV) leads to an up-shift of the EMW frequency of a few MHz. The injected EMW with power, P0⩽250 kW, and with pulse duration of 1 μs, produces additional ionization of the neutral argon gas (n0≃6.6×1013 cm−3) through the heating of plasma electrons. The increase of plasma density, which can reach during the EMW pulse several tens of percents in relative value, leads to a modulation in time of the spatial phase of the EMW. Experimental results for the evolution in space and in time of density, electric field, and frequency shift are compared to a simple nonlinear model of EMW propagation in a plasma, which takes into account the additional ionization due to the absorption of the EMW.
Magnetohydrodynamic stability of negative central magnetic shear, high pressure (εβpol≫1) toroidal equilibria
Authors: Kleva Robert G.
Published: July 2002

The magnetohydrodynamic (MHD) stability of negative central magnetic shear toroidal equilibria with q>2 everywhere and εβpol≫1 is investigated. Here, q is the safety factor of the equilibrium magnetic field in a torus with inverse aspect ratio ε, and βpol is the ratio of the plasma pressure to the pressure in the poloidal magnetic field. At small εβpol≪1, the elimination of the q=2 resonant surface in a negative shear equilibrium greatly improves resistive MHD stability as compared to equilibria with a monotonic q-profile containing a q=2 resonant surface. However, at large εβpol≫1, the reversal of the central magnetic shear and the elimination of the q=2 resonant surface does not improve MHD stability. The existence, or non-existence, of rational magnetic surfaces has no impact on MHD stability when εβpol≫1. Altering the current profile, and with it the q-profile, does not affect MHD stability when εβpol is no longer small. Stability is not improved by vertical elongation of the plasma in the poloidal plane. The utilization of an external vertical magnetic field to move the magnetic axis in major radius also does not improve MHD stability.
Some remarks on the quasilinear treatment of the stochastic acceleration problem
Authors: Vanden Eijnden E.
Published: July 2002

The range of applicability of the quasilinear theory is discussed through the example of the stochastic acceleration of a test particle in a random electric field. The scheme is able to reproduce the non-standard diffusive behaviors observed in direct numerical simulations.
Continuum plasma flow past a sphere
Authors: Hader M. A. , Jog M. A.
Published: July 2002

Flow of a collision-dominated weakly ionized gas over a solid sphere is investigated in the limit λm≪λD≪rp, where λm is the mean free path, λD is the Debye length, and rp is the sphere radius. Such flow is encountered in plasma heating of metallic/ceramic powders in rf plasma-aided manufacturing processes. The flow Reynolds number (Re) based on sphere diameter is in the intermediate range (Re∼5–100). The continuity, the momentum conservation, and the energy conservation equations for the neutrals and those for the charged particles are simultaneously solved with the Poisson’s equation for the self-consistent electric field. A model for production and recombination of the charged particles is incorporated in the formulation. The surface of the sphere is considered as a sink for the charged particles and is at the floating potential. A finite difference method is employed to solve the governing equations. The flow field, the temperature distributions, the charged particle number density variations are obtained and the heat transport to the sphere surface is determined for a range of flow Reynolds number. The effects of Reynolds number, the far field temperature, and the surface temperature on the electric sheath around the sphere surface and on the heat transport to the sphere, are delineated.
Multipactor with electric field retarding secondary emission
Authors: Riyopoulos Spilios
Published: July 2002

A systematic study of the effects of emission under opposing rf electric field in multipactors is performed including external magnetic field. It is shown that the small, 1 eV or less, kinetic energy of the emitted secondaries suffices to overcome opposing ac electric fields of the order kV/cm at the moment of emission, owing to the high frequency that the field changes sign. Thus, the static space charge limitation (no emission under the opposing field) does not apply, and ac driven avalanches occur for resonant impact phases with retarding electric field. Hence (a) the limit in the parameter space where multipactor occurs is pushed towards much higher ac fields, where the resonant impact phase is negative. (b) For mostly negative impact phases the space charge remains confined in a tight bunch during multipactor saturation. At lower ac fields, where the resonant impact phases are positive, the finite emission energy has a small overall effect.
Damping of perpendicular magnetosonic pulses in a two-ion-species plasma
Authors: Dogen Daiju , Toida Mieko , Ohsawa Yukiharu
Published: July 2002

The damping of finite-amplitude magnetosonic pulses propagating perpendicular to a magnetic field in a collisionless plasma containing two ion species (majority light ions and minority heavy ions) is studied through theory and simulation. A magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front; this cross-filed motion of heavy ions then generates a long-wavelength perturbation behind the original pulse. Because of these processes the original pulse is damped. The damping rate of a solitary pulse is theoretically obtained. It decreases with increasing amplitude. The damping is further investigated by means of a one-dimensional, fully electromagnetic code based on a three-fluid model. The theoretically obtained damping rate is in good agreement with the simulation result. Also, it is confirmed that small-amplitude periodic waves are not damped.
Sheath and presheath structure in the plasma–wall transition layer in an oblique magnetic field
Authors: Beilis I. I. , Keidar M.
Published: July 2002

In this paper we present a hydrodynamic study of a current carrying transition layer that separates the plasma from electrically conducting or insulating walls. The plasma is placed in a magnetic field that could be parallel to, or intersects obliquely with the walls. The self-consistent model of the smooth presheath–sheath transition of the near-wall plasma layer developed here for a finite Debye radius to ion Larmor radius ratio (Ψ) is based on a previously developed model of the quasineutral plasma presheath [Phys. Plasmas 4, 3461 (1997)]. The potential distribution in the presheath is found to have a positive maximum with respect to the plasma–presheath interface. In the case of a wall with a floating potential, the value of the maximum decreases with the incidence angle θ of the magnetic line force, and approaches zero when θ=2°. The presheath thickness generally increases with the incidence angle, from about the electron Larmor radius up to the ion Larmor radius, and depends on the electron to ion current ratio, ion velocity at the plasma–presheath interface, and Hall parameter. Analysis of presheath-sheath transition when Ψ is in the range of 10−2 to 10−4 shows that (1) the electric field at the presheath–sheath interface is finite, (2) the critical ion velocity at the sheath edge is about (0.6–0.9) of ion sound speed, depending on the plasma parameters, and generally increases with the ion to electron current ratio. The current–voltage characteristic of the transition layer does not depend on θ, the magnetic field incidence angle, for θ⩾5°.
Nonlinear tearing mode study using the “almost ideal magnetohydrodynamics (MHD)” constraint
Authors: Ren C. , Jensen T. H. , Callen J. D.
Published: July 2002

Tearing modes of finite amplitude are studied numerically using the “almost ideal magnetohydrodynamics (MHD)” constraint, which is a modification of the ideal MHD constraints that allows reconnection of the magnetic field lines. For the one-dimensional initial equilibria studied here, the stability criterion is found to be the same as that of the linear tearing mode theory, namely Δ′<0. The nonlinear saturation level of the mode can also be determined; it is found to be smaller than that estimated from Δ′(Wsat)=0 [R. B. White et al., Phys. Fluids 20, 800 (1977)].
Grain charge in dusty plasmas
Authors: de Angelis U. , Forlani A.
Published: July 2002

Measurements [Barkan et al., Phys. Rev. Lett. 73, 3093 (1994)] of the grain charge in dusty plasmas have shown that it can be significantly reduced with respect to the single grain charge. This can have important consequences on the still open problem of strong vs weak coupling in many experiments with dusty plasmas. A simple charging model, based on the linearized Vlasov equations for the plasma particles, gives results in good agreement with the measurements and can be used to determine the grain charge in many experimental situations.
Small amplitude solitons in a magnetized dusty plasma with two-ion species
Authors: Malik H. K. , Bharuthram R.
Published: July 2002

Small amplitude dust acoustic solitons are studied in a magnetized plasma having two-ion species (hot and cool), electrons, and negatively charged dust grains. The charged dust grains are magnetized. It is found that both compressive and rarefactive solitons can occur in such a plasma system. The effects of the magnetic field strength, wave-propagation angle, particle densities, and temperatures on the soliton characteristics are investigated.
Dynamics of narrow electron streams in magnetized plasmas
Authors: Reitzel K. J. , Morales G. J.
Published: July 2002

In this analytical and computer simulation study we explore the dynamics of a narrow electron stream embedded in a magnetized plasma. The transverse dimension of the stream is envisioned to be on the order of the electron skin depth, as is appropriate to several problems of current interest (e.g., auroral beams, reconnection). Within the layer the drift velocity exceeds the thermal velocity, and thus the Buneman instability is excited. The method of matched asymptotic expansions is used to describe the linear stage of the instability for a nonuniform drift profile. It predicts a lowering of the growth rate and a rapid decrease in wave amplitude at the spatial location where the beam mode resonance is encountered. A particle-in-cell (PIC) simulation is used to verify the predictions of the analysis and to illustrate the important nonlinear behavior. It is found that the rapid flash of the Buneman instability excites a lower-hybrid wave, causes strong perpendicular ion acceleration, and results in a region of depleted density. The PIC study also considers a quasisteady situation that models the effect of a shear Alfvén wave of narrow transverse extent.
Chaotic oscillations enhanced by magnetosonic waves in plasma-filled traveling-wave tubes
Authors: Bliokh Yu. P. , Liubarskii M. G. , Podobinskii V. O. , Fainberg Ya. B. , Nusinovich G. S. , Kobayashi S. , Carmel Y. , Granatstein V. L.
Published: July 2002

A theory of plasma-filled traveling-wave tubes (TWTs) is developed in which the effect of magnetosonic waves excited in plasma by the operating wave is taken into account. These waves are excited by the ponderomotive force caused by the radial inhomogeneity of the axial component of the electric field of the operating wave. In the simplest case considered in the paper, this effect leads to an additional reactive nonlinearity in the wave envelope equation. This leads to a shrinkage of the region of stable oscillations in TWTs with the feedback causing the self-excitation; at the same time, the region of stochastic oscillations becomes larger. The radiation spectrum of stochastic oscillations in plasma-filled TWTs in which magnetosonic waves are excited is much wider and more continuous than the spectrum of stochastic oscillations in vacuum TWTs with the same feedback.
Magnetosonic eigenmodes near the magnetic field well in a spherical torus
Authors: Gorelenkova M. V. , Gorelenkov N. N.
Published: July 2002

The structure and spectrum of magnetosonic Alfvén eigenmodes in a spherical torus in the presence of a magnetic field well are studied. Analytical solution for eigenmodes localized in the well is obtained and compared with the numerical one. The possibility of using the eigenmode spectrum measurements for reconstructing the magnetic field well, and, thus, the central magnetic safety factor profile, is discussed.
Effect of neutrals on scrape-off-layer and divertor stability in tokamaks
Authors: D’Ippolito D. A. , Myra J. R.
Published: July 2002

The influence of ion–neutral interactions (charge exchange, elastic scattering) on scrape-off-layer (SOL) stability is studied in the eikonal limit for a single-null X-point geometry typical of tokamak divertors. Instability drives due to curvature and to the ion–neutral drag effect are included in the model. The ion–neutral interaction terms are highly localized near the divertor plates; these terms are stabilizing for typical parameters and large enough to affect the SOL ballooning-interchange stability in the absence of resistivity. It is shown that the growth rate of ideal curvature-driven modes is significantly reduced by the ion–neutral interaction terms; the growth rate of resistive ballooning modes is not affected much by the neutrals, because resistivity allows the mode to disconnect from the divertor region. In both cases, the X-point geometry significantly affects the stability. An ion–neutral drag instability localized near the plates is only found in a small region of parameter space. Conditions for the existence of this instability in X-point geometry are discussed.
Equivalent electric charge of photons in magnetized plasmas
Authors: Shukla P. K. , Tsintsadze N. L. , Mendonça J. T. , Stenflo L.
Published: July 2002

Explicit expressions for the induced electric charge of photons in magnetized plasmas are derived. The electric charges arise due to the electric fields that are created by the photon pressure.
Localized nonlinear structures of intense electromagnetic waves in two-electron-temperature electron–positron–ion plasmas
Authors: Shatashvili N. L. , Rao N. N.
Published: July 2002

Nonlinear propagation of intense electromagnetic waves in a hot electron–positron relativistic plasma containing a small fraction of cold electron–ion component has been investigated by deriving a generalized Schrödinger–Boussinesq system of coupled equations. The latter includes self-nonlinearity in the electromagnetic field amplitude due to the relativistic effects, and describes the coupling between the high-frequency electromagnetic wave and low-frequency electron–acoustic wave arising from the cold plasma component. For stationary propagation of finite amplitude waves, only supersonic solitons are found to exist, while for the quasineutral, linear low-frequency plasma response both sub- as well as supersonic solitons can occur. The relevance of these results to astrophysical situations is pointed out.
Collisionless reconnection in instabilities and due to external forcing
Authors: Valori G. , Grasso D. , de Blank H. J.
Published: July 2002

Collisionless magnetic reconnection due to electron inertia is numerically investigated in two-dimensional, externally forced systems and unstable configurations. A common characteristic of reconnection in the two cases, associated with the effects of electron inertia and temperature, is a faster than exponential scale collapse. This collapse creates structures that are much narrower than the inertial reconnection layer, such as a very localized, X-shaped current distribution. The nonlinear evolution at this small scale is largely independent of large scale features such as the initial and boundary conditions which constitute the differences between driven and unstable cases. However, only in forced reconnection cases ion viscosity is found to stop the scale collapse of the current. High numerical resolution made the detailed investigation of small scale structure formation and continued scale collapse possible.
Plasma harmonic emission from laser interactions with dense plasma
Authors: Ondarza-Rovira R. , Boyd T. J. M.
Published: July 2002

A nonlinear procedure is used to account for the mechanism of laser harmonic generation to high orders and plasma emission in laser-dense plasma interactions. A Lagrangian model is used to describe harmonic generation in terms of the oscillatory behavior of an electron plasma. It is shown that the effect of pulse shaping constitutes an important issue for the generation and enhancement of harmonics. It is found that in the presence of electron density perturbations further harmonics beyond the order that corresponds to the upper shelf plasma density can be excited. Computations were performed for mildly relativistic electrodynamics and time scales where wave breaking effects do not take place. Using particle-in-cell (PIC) simulations, plasma emission was observed for the interaction of laser light with super-critical density plasmas with spectral characteristics resembling those found from the fluid model.
Trapping and excitation of modes in the magnetotail
Authors: Mills Katharine J. , Wright Andrew N.
Published: July 2002

A slab model of the magnetotail lobes is studied, and the trapping and excitation of fast magnetoacoustic modes is considered. The tail lobe is taken to be uniform with the magnetic-field stretching parallel to the flow in the magnetosheath. The modes are assumed to be trapped between free magnetopause boundaries. The ideal magnetohydrodynamic (MHD) equations are used to study the effects of linear perturbations to this equilibrium state. It is found that the plasma parameters in this region mean that fast magnetoacoustic waves are unlikely to be Kelvin–Helmholtz (KH) unstable for realistic flow speeds, and in fact these modes are likely to be leaky, losing energy across the magnetopause boundary. However, the decay rates of modes propagating both with and against the magnetosheath flow are found to be small for reasonable plasma parameters and this boundary may be reasonably assumed to be reflecting for the purposes of wave guide theory. Slow modes are found to have very small growth rates and since in the magnetotail lobes the plasma beta is very low, they are not expected to be important.
Positron acceleration by a laser pulse in a plasma
Authors: Du Chunguang , Xu Zhizhan
Published: July 2002

The motion of a charged particle in the fields associated with a circularly polarized laser pulse propagating in a plasma is analytically studied. In contrast to an electron, a positron can be accelerated by the leading edge of the pulse to very high energy, even if it is initially at rest, and ultrahigh intensity of the pulse is not necessary. The pulse threshold intensity ath2 is always less than an upperlimit amax2=6 in the long pulse (L≫λp) case.
Recent developments in high-power magnicons for particle accelerators
Authors: Nezhevenko Oleg
Published: July 2002

Continued progress in high energy and nuclear physics demands a new generation of particle accelerators, which in turn requires new high-power, high efficiency rf sources. One of the attractive candidates for this role is the magnicon–microwave amplifier with circular deflection of an electron beam. Magnicons have shown great potential with both high efficiency and high power. A first magnicon to have demonstrated these qualities was built and tested in the 80s in Novosibirsk. A power of 2.6 MW was obtained at 915 MHz with a pulse width of 30 μs and an electronic efficiency of 85%. The latest results in magnicon research, as well as the status of current magnicon projects, are presented in this paper. Examples of both operating magnicons and those under development include decimeter wave (1.3 GHz) amplifiers with a power level up to 10 MW in millisecond pulses; frequency-doublers in the centimeter wave range (7 GHz and 11.4 GHz) at a power of 50–60 MW, and a 40 MW frequency-tripler in the millimeter range (34.3 GHz), with microsecond pulses.
Acoustic phenomena in electrostatic dusty plasma shear flows
Authors: Poedts Stefaan , Khujadze George R. , Rogava Andria D.
Published: July 2002

Recent studies of nonmodal phenomena in two-component plasma flows revealed that the velocity shear induces a number of new effects both in electrostatic and magnetized shear flows. It can be expected that dusty plasmas also host shear-modified and shear-induced modes of collective behavior, which may be found by means of the nonmodal approach and which are inaccessible by means of the standard normal mode analysis. In this paper, considering the simple electrostatic dusty plasma case, a general mathematical formalism is developed for studying how velocity shear affects the evolution of dust-acoustic waves (DAWs) and ion-acoustic waves (IAWs). In the limiting (very low-frequency) case when Boltzmann distributions are used both for the electrons and the ions it is found that the velocity shear enables the extraction of kinetic energy of the background flow by the dust-acoustic waves. It is also shown that the velocity shear leads to the appearance of a new collective mode of the dust particles—shear dust vortices. In the general case it is demonstrated that the velocity shear couples DAWs and IAWs and under suitable conditions may cause their mutual transformation into each other. The flow also sustains shear ion-dust vortices—nonperiodic patterns, which may eventually acquire oscillating features and generate both DAWs and IAWs. The inverse regime, which is called evanescence of acoustic waves, can also occur: the initial blend of DAWs and IAWs can fade away degenerating into the nonperiodic, evanescent perturbation.
Frequency dependence of tokamak plasma response to the externally applied rotating helical field
Authors: Kobayashi M. , Kojima H. , Zhai K. , Takamura S.
Published: July 2002

The frequency dependence of the tokamak plasma response to the externally applied rotating helical field was investigated by measuring the radial profile of the perturbation field with small magnetic probes which were inserted in the plasma. The experimental results are discussed, directly comparing with the generally accepted linear theory, and taking into account the E×B drift and diamagnetic effect of the plasma. It was found that the experimental results are in good agreement with the simple linear analysis based on the single helicity approximation in a cylindrical geometry, except for the ergodic region, which comes from the overlapping of several sideband components of the perturbation. It is also found that the radial component of the perturbation field is still amplified inside the resonance surface (r<rs), even when a partial shielding takes place at r≈rs.
Nondestructive detection of Rayleigh wave dispersion in beryllium
Authors: Weglein R. D. , Hanafee J. E.
Published: July 2002

The presence of machining damage (MD) in beryllium may be determined nondestructively in the acoustic microscope via the acoustic material signature (AMS). Rayleigh wave dispersion is attributed to substantial modification of the macroelastic properties in the surface region due to the machining process. Comparison of acoustic material signatures at 35 MHz obtained from chemically etched (undamaged) and machining-damaged (MD) specimens show substantial (3%) Rayleigh velocity reduction in the latter. Damage variation with depth may be ascertained via AMS mesurements over a wide frequency range.
Optogalvanic study of excited H atoms in a dc glow discharge
Authors: May Randy D.
Published: July 2002

A cw dye laser operating at 6563 Å has been used to measure Doppler broadened absorption profiles of atomic hydrogen in a glow discharge using optogalvanic detection. In the cathode fall region, neutral hydrogen atoms were observed having kinetic energies >140 eV. In the negative glow and positive column average kinetic energies correspond more nearly to thermal velocities.
Amorphous silicon produced by a new thermal chemical vapor deposition method using intermediate species SiF2
Authors: Matsumura Hideki , Tachibana Hiroyuki
Published: July 2002

Amorphous silicon (a-Si) films are deposited at about 320 °C by a new thermal chemical vapor deposition method. In this method, the gas mixture of intermediate species SiF2 and H2, decomposed thermally by the catalytic reaction, is used as a material gas. It is found that the photosensitivity of the a-Si film for AM1 of 100 mW/cm2 exceeds over 106 and that the spin density is as low as 1.5×1016 cm−3 for the film deposited with a rate of several Å/s.
Monolithically integrated thermoelectric controlled laser diode
Authors: Dutta N. K. , Cella T. , Brown R. L. , Huo D. T. C.
Published: July 2002

The fabrication and performance characteristics of a monolithically integrated thermoelectric controlled laser diode are described. The thermoelectric element is the n-InP substrate. The lasers (λ∼1.51 μm InGaAsP) have threshold currents of ∼20 mA and operate kink free to >10 mW/facet. A variation of active region temperature of ± 2.5 °C has been achieved using 50 mA of thermoelectric controller current. The observed frequency tuning rate associated with this temperature shift is ∼0.5 GHz/mA. The device is useful for applications that require a high degree of frequency stability or small frequency tuning. Some potential lightwave system applications are in single-frequency transmission systems, coherent transmission systems, optical amplifiers, resonant external cavity modulators, and injection locking.
Simultaneous optical pulse compression and wing reduction
Authors: Halas N. J. , Grischkowsky D.
Published: July 2002

We report the compression of picosecond optical pulses with a simultaneous reduction of the pulse wings by using a combination of both the self-phase modulation and nonlinear birefringence effects in a modified optical-fiber pulse compressor.
Rapid thermal annealing of high concentration, arsenic implanted silicon single crystals
Authors: Larsen A. Nylandsted , Shiryaev S. Yu. , So/rensen E. Schwartz , Tidemand-Petersson P.
Published: July 2002

Rapid thermal annealing of arsenic implanted 〈100〉 silicon single crystals has been studied by Rutherford backscattering/channeling spectrometry, and Hall effect/resistivity measurements, combined with layer removal. Redistribution of the implanted arsenic was followed as a function of anneal time (6–60 s including temperature rise time) and implanted peak concentration (3–10×1020 cm−3) at temperatures of 1050 and 1090 °C. The maximum concentration of electrically active arsenic was found to be 2–3×1020 cm−3 independent of anneal time and implanted peak concentration. Fast arsenic redistribution was observed to take place within the first 20 s of annealing. Complete arsenic activation occurred by means of rapid redistribution to the solubility limit.
Observation of, and bias-dependent annealing of, a paramagnetic defect possibly unique to thermally grown SiO2
Authors: Carlos W. E.
Published: July 2002

The observation of a variant of the Si E′ center which is possibly unique to thin films of SiO2 is reported. This center which has not been reported for bulk a-SiO2 is detected after x irradiation of films grown on (111) and (100) wafers of silicon. The center contains two threefold coordinated Si atoms with one having axial symmetry and the other having orthorhombic symmetry. Evidence indicates that this defect is very near the Si-SiO2 interface. The annealing behavior of the defect has a bias dependence which is very similar to that observed for fixed oxide charge in metal-oxide-semiconductor devices.
Electroelastic effect in layer acoustic mode propagation along ZnO films on Si substrates
Authors: Palmieri L. , Socino G. , Verona E.
Published: July 2002

The influence of a bias electric field on the phase velocity of acoustic layer modes propagating along a ZnO overlay on a Si substrate is analyzed. Measurements of the fractional change in phase velocity as a function of the applied electric field were performed on both Rayleigh and Sezawa modes. The strength of the electroelastic effect has been experimentally determined for two samples obtained by different sputtering runs. An evident dependence of this parameter on the acoustic mode and on the quality of the sputtered film was observed. The maximum value of 15.8×10−6 μm/V was attained by the Sezawa wave in one of the specimens, while a weaker effect was observed in the other cases.
Effects of transparent electrode resistance on the performance characteristics of electrochemichromic cells
Authors: Kaneko H. , Miyake K.
Published: July 2002

The performance characteristics of WO3 electrochemichromic cells have been studied in connection with the electrical resistance of transparent electrodes and a lumped resistor connected to the electrodes in series. The sheet resistance of transparent electrodes used and the resistance of lumped resistor used are in the range of 10–1250 Ω/⧠ and 10–2500 Ω, respectively. The thickness of the WO3 film is ∼2400 Å or ∼4300 Å, and the electrolyte is 1 N H2SO4 aqueous solution containing 10 vol % glycerol. The current pulse form and the change in optical transmittance of experimental cells under operation were observed, and the operating characteristics of electrochemichromic cells have been found to clearly depend on the sheet resistance of transparent electrodes and/or the resistance of a lumped resistor connected serially.
Tunneling in In0.53Ga0.47As-InP double-barrier structures
Authors: Vuong T. H. H. , Tsui D. C. , Tsang W. T.
Published: July 2002

We report the first observation of tunneling through In0.53Ga0.47As-InP double-barrier structures at 77 and 4.2 K. The tunneling peaks occur at voltages close to the predicted values of the subband levels of the quantum well. The observed values are symmetrical about zero bias, unlike the results for similar structures in the AlGaAs-GaAs system. The measured peak-to-valley ratios are low, being near to unity. This result is attributed to the presence of a large leakage current caused by conduction at the edges of the devices. We also report the observation of minima in the conductance curves due to the resonance levels in the continuum of states above the quantum well.
Dominance of surface recombination current in planar, Be-implanted GaAs p-n junctions prepared by rapid thermal annealing
Authors: de Lyon T. J. , Casey H. C. , Timmons M. L. , Hutchby J. A. , Dietrich D. H.
Published: July 2002

Rapid thermal annealing with incoherent light was used to fabricate planar, Be-implanted p-n junctions in GaAs. Diodes of varying diameter were used to determine if the residual implant damage would cause the space-charge recombination current to dominate the surface recombination current. Unpassivated diodes are dominated by surface recombination over the 20–150 μm range of diameters investigated. Passivation of diode structures with a surface layer of high-resistivity Al0.4Ga0.6As grown by metalorganic chemical vapor deposition resulted in a significant reduction of surface recombination current and permitted the measurement of the space-charge recombination current. The space-charge recombination current for these diodes was found to be similar in value to that previously reported for Zn-diffused GaAs junctions.
Injection locking and single-mode fiber coupling of a 40-element laser diode array
Authors: Goldberg L. , Weller J. F.
Published: July 2002

Near-diffraction-limited (0.13° wide) single-lobe operation of a 40-element AlGaAs array emitting 510 mW cw is obtained by external injection locking, with 11.0 mW of injected power incident on the array. The injected and the array output beams were tilted relative to the facet normal to allow for spatial separation of the two beams and efficient coupling into a single-mode fiber; 150 mW was coupled into a 5-μm core, polarization holding fiber.
Optical anisotropy in compositionally modulated Cu-Ni films by spectroscopic ellipsometry
Authors: Flevaris N. K. , Logothetidis S.
Published: July 2002

Spectroscopic ellipsometry studies (1.66–5.5 eV) of Cu-Ni thin films, containing short (0.62 and 4.13 nm) lattice-commensurate modulation wavelengths, have revealed a strong anisotropy of the dielectric function. These observations are discussed, in accordance with theoretical predictions for superlattices, in terms of the coherency strains. Other possible sources of anisotropy are also discussed in conjunction with other studies.
Polarization control in ridge-waveguide-laser diodes
Authors: Amann Markus-Christian
Published: July 2002

The polarization dependence of the gain/current relation and threshold current of quasi-index-guided laser diodes is analyzed for the case of λ=1.3 μm InGaAsP-InP ridge-waveguide lasers. Thereby it is shown that three different regimes for the stripe width and the lateral effective index discontinuity can be distinguished where one modal polarization (TE or TM) predominates. The significance of this finding on laser design and polarization control is discussed, and a comparison is performed on experimental results.
High-power (710 mW cw) single-lobe operation of broad area AlGaAs double heterostructure lasers grown by metalorganic chemical vapor deposition
Authors: Sakamoto Masamichi , Kato Yoji
Published: July 2002

Broad area, conventional double heterostructure lasers were fabricated by metalorganic chemical vapor deposition. Continuous wave output power level of 710 mW and stable, single-lobed far-field patterns with a beam divergence as narrow as 2.1° (2.2 times diffraction limit) at 50 mW and 2.9° at 200 mW for a 50-μm-wide laser were obtained.
Improved activation of Mg+ and As+ dual implants in GaAs by capless rapid thermal annealing
Authors: Masum Choudhury A. N. M. , Armiento C. A.
Published: July 2002

The activation of high dose Mg+ implants (1×1015 cm−2, 100 keV) in GaAs using capless rapid thermal annealing has been improved by the co-implantation of As+. This technique reduces the outdiffusion of the implanted Mg, which can adversely affect the activation of shallow, high dose implants. Compared with an activation of 18% for an implant of Mg+ only, the co-implantation of As+ has increased the activation to as much as 61% with concomitant sheet resistance of 136 Ω/⧠. The placement of the As+ implant with respect to the position of the Mg+ profile has been determined to play a role in the activation efficiency. This technique has been applied to the formation of thick p+ regions with high surface carrier concentrations, which has important applications in device fabrication for reduction of contact resistances.
Fabrication of ion-exchanged channel waveguides directly into integrated circuit mask plates
Authors: Honkanen S. , Tervonen A. , von Bagh H. , Salin A. , Leppihalme M.
Published: July 2002

Optical channel waveguides have been fabricated into integrated circuit lithography mask plates by silver-sodium ion exchange. The process combines the advantages of Ag thin-film ion sources and the accurate and reliable patterning of chromium films on mask plates. The process is potentially suitable for cheap mass production, since no lithography is needed during the actual waveguide fabrication. The tailoring of waveguide cross sections by modifying the electric field distribution inside the glass is also presented.
Rapid thermal annealing of Si-implanted GaAs with trimethylarsenic overpressure
Authors: Reynolds S. , Vook D. W. , Opyd W. G. , Gibbons J. F.
Published: July 2002

We have developed a novel rapid thermal processor to perform annealing of ion-implanted GaAs in a trimethylarsenic overpressure. This has allowed study of arsenic ambient annealing in a time/temperature regime not accessible with an arsine furnace. We have compared Si implant activation efficiency and surface degradation for arsenic ambient and proximity capped anneals. The arsenic ambient gives consistently higher activation efficiency with better surfaces.
Chemical beam epitaxially grown InP/InGaAsP interference mirror for use near 1.55 μm wavelength
Authors: Tai K. , McCall S. L. , Chu S. N. G. , Tsang W. T.
Published: July 2002

An interference mirror for use near 1.55 μm wavelength was constructed using InP and lattice-matched InGaAsP on an InP substrate with chemical beam epitaxy methods. The maximum reflectivity was 92.5% at 1.46 μm. The design value was 94.75% at 1.55 μm. The mirror was used as an output mirror in a synchronously pumped mode-locked Tl:KCl color center laser.
Reflection electron microscopic observation of high-temperature grown GaAs surfaces of molecular beam epitaxy
Authors: Shimizu N. , Muto S.
Published: July 2002

We used reflection electron microscopy (REM) to observe GaAs (001) surfaces grown at 700 °C by molecular beam epitaxy. The REM images revealed such interesting features as high step density and large undulations that occurred even though we interrupted epitaxial growth for 3 min, in an attempt to produce a flat surface. A cross section of the large undulation showed a structure consisting almost entirely of plateaus, 0.1–2 μm wide and several monolayers high but less than 30 Å. Small steps distributed uniformly across the tops of these plateaus appeared comparatively straight in the [11̄0] direction but appeared to ‘‘zigzag’’ in the [110] direction—a phenomenon we think may have been caused by Ga surface migration anisotropy.
Effect of strain on the band structure of GaAs and In0.2Ga0.8As
Authors: Hwang J. , Shih C. K. , Pianetta P. , Kubiak G. D. , Stulen R. H. , Dawson L. R. , Pao Y.-C. , Harris J. S.
Published: July 2002

The valence-band structures of 50-Å-thick layers of GaAs(001) in tension and In0.2Ga0.8As(001) in compression have been determined using angle-resolved photoemission spectroscopy. Our studies show that the Δ3+Δ4 bands and the Δ1 band respond differently to the strain perturbation. For strained GaAs(001), the Δ3+Δ4 bands (Px+Py-like) are shifted up in energy by a maximum amount of 0.3 eV, while in contrast, the Δ1 band (Pz-like) is shifted down by about 0.1 eV. For strained In0.2Ga0.8As(001), the band shifts are in the opposite direction, consistent with the opposite strain conditions. For both materials, the strain-induced changes cannot be characterized simply by rigid band shifts, but rather exhibit significant wave vector dependence. This results in a reduction of the effective mass of the Δ3+Δ4 bands for both GaAs and In0.2Ga0.8As.
Reflection mass spectrometry of As incorporation during GaAs molecular beam epitaxy
Authors: Tsao J. Y. , Brennan T. M. , Hammons B. E.
Published: July 2002

An apertured and cryoshrouded mass spectrometer, which measures line-of-sight molecular fluxes from the surface, has been incorporated into a GaAs molecular beam epitaxy system. The spectrometer is simple to implement, yet is a powerful real-time growth diagnostic. We have used the spectrometer to measure transient and steady-state As incorporation from As4 during bilayer-by-bilayer growth of GaAs. We find, interestingly, that (1) the incorporation coefficient does not oscillate significantly; (2) transient incorporation coefficients depend on surface reconstruction, and may be higher than 0.5 at high Ga fluxes; and (3) in the absence of a Ga flux, excess Ga on the surface need not imply an incorporation coefficient of 0.5.
High-peak-power low-threshold AlGaAs/GaAs stripe laser diodes on Si substrates grown by migration-enhanced molecular beam epitaxy
Authors: Kim Jae-Hoon , Nouhi Akbar , Radhakrishnan Gouri , Liu John K. , Lang Robert J. , Katz Joseph
Published: July 2002

A high-peak-power low-threshold AlGaAs/GaAs double-heterostructure stripe laser diode on Si substrates grown by hybrid migration-enhanced molecular beam epitaxy (MEMBE) and metalorganic chemical vapor deposition (MOCVD) has been demonstrated for the first time. These devices showed the highest peak powers of up to 184 mW per facet reported so far for double-heterostructure stripe laser diodes on Si substrates, room-temperature pulsed threshold currents as low as 150 mA, and differential quantum efficiencies as high as 30% without mirror facet coating. An intrinsic threshold current density has been estimated to be about 2 kA/cm2 when taking current spreading and lateral diffusion effects into account. Low dislocation density shows that MEMBE can be a useful method to grow high quality GaAs and AlGaAs/GaAs layers on Si substrates by combining with MOCVD.
Sequence evolution and copy number of Ty1-
copia
retrotransposons in diverse plant genomes
Authors: Navarro-Quezada Aura , Schoen Daniel J.
Published: July 2002

Sequence evolution of the reverse transcriptase (RT) gene in
retrotransposons belonging to the Ty1-
copia
class was
studied in 11 plant species. Phylogenetic reconstruction of the
evolutionary history of RT sequences indicated a strong pattern of
purifying selection, manifested as high ratios of third to first plus
second codon position substitutions, and low ratios of nonsynonymous
substitutions per nonsynonymous site to synonymous substitutions per
synonymous site, especially in internal portions of the element
phylogenies. Evidence of purifying selection was most pronounced in
plant species with low estimated copy numbers of
Ty1-
copia
elements. This finding is consistent with the
hypothesis that high element turnover rates (e.g., caused by high rates
of element loss and selection against high element copy number) favors
elements capable of transposition. Simulations of RT sequence evolution
were conducted to help verify the logical validity of this conclusion.
The results argue that it is incorrect to assume that low copy numbers
of transposable elements are the product of reduced levels of element
activity.
Immunization of
Aotus
monkeys with a functional domain of the
Plasmodium falciparum
variant antigen induces protection against a lethal parasite line
Authors: Baruch Dror I. , Gamain Benoit , Barnwell John W. , Sullivan JoAnn S. , Stowers Anthony , Galland G. Gale , Miller Louis H. , Collins William E.
Published: July 2002

Immunity to
Plasmodium falciparum
in African children has been correlated with antibodies to the
P. falciparum
erythrocyte membrane protein 1 (PfEMP1) variant gene family expressed on the surface of infected red cells. We immunized
Aotus
monkeys with a subregion of the Malayan Camp variant antigen (MCvar1) that mediates adhesion to the host receptor CD36 on the endothelial surface and present data that PfEMP1 is an important target for vaccine development. The immunization induced a high level of protection against the homologous strain. Protection correlated with the titer of agglutinating antibodies and occurred despite the expression of variant copies of the gene during recurrent waves of parasitemia. A second challenge with a different
P. falciparum
strain, to which there was no agglutinating activity, showed no protection but boosted the immune response to this region during the infection. The level of protection and the evidence of boosting during infection encourage further exploration of this concept for malaria vaccine development.
Dynamic association of transcriptional activation domains and regulatory regions in
Saccharomyces cerevisiae
heat shock factor
Authors: Chen Tianxin , Parker Carl S.
Published: July 2002

In
Saccharomyces cerevisiae
, the heat shock transcription factor (HSF) is thought to be a homotypic trimer that is bound to the promoters of heat shock protein (HSP) genes at both normal and heat shock temperatures. Exposure to heat shock greatly and rapidly induces HSF transcriptional activity without further increasing DNA-binding affinity. It is believed that HSF is under negative regulation at normal growth temperatures, but the detailed mechanism by which HSF is activated is still not clear. We report the analysis of mutations in a conserved arginine (residue 274) at the C-terminal end of the DNA-binding domain (DBD). Two mutations significantly increase both basal activity of HSF at normal temperatures and induced activity on heat shock. We demonstrate by coimmunoprecipitation experiments that the mutations reduce the association between the DNA-binding domain/oligomerization domain and the transcription activation domains. Our studies suggest that the DNA-binding domain of HSF can interact with activation domains directly, and this interaction is important for the repression of HSF activity under normal growth conditions. Destabilizing this interaction by heat or by mutations results in HSF transcriptional activation. We propose that Arg-274 is critical for intramolecular repression of HSF activity in normally growing cells.
On the acidity and reactivity of HNO in aqueous solution and biological systems
Authors: Bartberger Michael D. , Fukuto Jon M. , Houk K. N.
Published: July 2002

The gas phase and aqueous thermochemistry and reactivity of
nitroxyl (nitrosyl hydride, HNO) were elucidated with
multiconfigurational self-consistent field and hybrid density
functional theory calculations and continuum solvation methods. The
pK
a
of HNO is predicted to be 7.2 ± 1.0, considerably
different from the value of 4.7 reported from pulse radiolysis
experiments. The ground-state triplet nature of NO
−
affects the rates of acid-base chemistry of the HNO/NO
−
couple. HNO is highly reactive toward dimerization and addition of soft
nucleophiles but is predicted to undergo negligible hydration
(
K
eq
= 6.9 × 10
−5
).
HNO is predicted to exist as a discrete species in solution and is a
viable participant in the chemical biology of nitric oxide and
derivatives.