bluuebunny/crossref_metadata_2025 huggingface dataset:
Small-scale features of vorticity and passive scalar fields in homogeneous isotropic turbulence
Authors: Ruetsch G. R. , Maxey M. R.
Published: July 2002

Small-scale structures of the vorticity and passive scalar fields have been examined by means of direct numerical simulations of homogeneous isotropic turbulence with 963 grid points and Rλ≊60. Both statistical and visual techniques have been used to examine the structure of certain quantities from the evolution equations for enstrophy and the scalar gradient. Tubelike regions of intense enstrophy contain large positive and sometimes large negative enstrophy production, and mostly moderate-valued energy dissipation regions surround these tubes. The most intense regions of the scalar gradient are dissociated from the vortex tubes, and occur as large flat sheets. Within these sheets the scalar gradient production is large, the energy dissipation is small, and in their vicinity only moderate-valued sheetlike enstrophy regions exist. The statistical techniques show that although activity in these intense regions is strong, on a volume normalized basis, by far the largest contributions to the terms in the evolution equations, along with the energy dissipation, are from low-level ‘‘background’’ activity.
An analysis of RNG-based turbulence models for homogeneous shear flow
Authors: Speziale Charles G. , Gatski Thomas B. , Fitzmaurice Nessan
Published: July 2002

In a recent paper [Phys. Fluids A 2, 1678 (1990)], the authors compared the performance of a variety of turbulence models including the K-ε model and the second-order closure model derived by Yakhot and Orszag based on renormalization group (RNG) methods. The performance of these RNG models in homogeneous turbulent shear flow was found to be quite poor, apparently due to the value of the constant Cε1 in the modeled dissipation rate equation, which was substantially lower than its traditional value. However, recently a correction has been made in the RNG-based calculation of Cε1. It is shown herein that, with the new value of Cε1, the performance of the RNG K-ε model is substantially improved. On the other hand, while the predictions of the revised RNG second-order closure model are better, some lingering problems still remain that can be remedied by the addition of higher-order terms.
On a kinetic equation for the transport of particles in turbulent flows
Authors: Reeks M. W.
Published: July 2002

The work described is part of a long-term study in pursuit of a kinetic equation for particles suspended in a turbulent flow. This equation represents the transport of the average particle phase space density and can be used to derive the continuum equations and constitutive relations for a two-fluid model of a dispersed particle flow and to establish the correct form of boundary conditions. A key role in this study is played by the equation of state of the dispersed particles, which is derived here for a Langevin equation of motion with a random driving force not limited to white noise. A suitable form of kinetic equation is then sought that reproduces this equation of state and predicts Gaussian spatial diffusion in the long-term limit with the correct form for the diffusion coefficient in homogeneous stationary turbulence. It is shown that both these requirements are automatically satisfied by constructing forms for the phase space diffusion coefficient that preserve invariance to a random Galilean transformation. The most general form is shown to be an expansion in successively higher-order cumulants of the aerodynamic driving force. When this force is a Gaussian process all terms apart from the first term contract to zero, i.e., the phase space diffusion current is of the form −[μ⋅(∂/∂v) +λ⋅(∂/∂x)]〈W〉 , where 〈W〉 is the average phase space density of particles with velocity v and position x at time t and μ and λ are phase space diffusion tensors whose components are memory integrals involving the correlation of the aerodynamic force along a particle trajectory. With both this form and the more general form of phase space diffusion current, the kinetic equation contracts to the classical Fokker–Planck equation in the white noise limit. Finally with the Gaussian form for the diffusion current, the form of the average phase space density is examined for the important case of a point source diffusing in an unbounded homogeneous stationary flow.
Transition to turbulence in systems with eddy promoters: Implication for the question of transition
Authors: Mikic B. B. , Kapat J. , Ratnathicam J.
Published: July 2002

Experimental findings concerning transition to turbulence for flows in a rectangular channel with cylindrical eddy promoters are reported. The presence of the promoters displaced the transition region to lower flow rates: the critical Reynolds number decreased roughly from 1500 (for an unobstructed channel) to 400 (for the least stable configuration). In the same range of geometric parameters, the transition Reynolds number based on the ‘‘friction velocity’’ remained approximately invariant. These findings are consistent with an operational concept which interprets the phenomenon of transition within the framework of the system’s capacity to sustain turbulence.
Models for direct Monte Carlo simulation of coupled vibration–dissociation
Authors: Haas Brian L. , Boyd Iain D.
Published: July 2002

Models are developed to permit direct Monte Carlo techniques to simulate coupled vibration–dissociation (CVD) behavior prevalent in high-temperature gases. This transient thermochemical phenomenon leads to dissociation incubation, reduced quasisteady dissociation rates, and non-Boltzmann distributions of vibrational energy during both dissociation and recombination. Essential for simulation of rarefied gas dynamics, Monte Carlo methods employ discrete particles to simulate molecular interactions directly, but have traditionally incorporated simplistic reaction models which failed to capture CVD behavior. To identify thermochemical collisions within the gas, a new dissociation selection probability is developed as a function of the extent by which the collision energy exceeds the gap between the dissociation threshold and the molecular vibrational energy of bounded anharmonic oscillators. A free parameter φ in the probability function controls the extent of vibrational favoring in dissociation selection. The new model is modified for application to the unbounded simple harmonic oscillator. Simulation of dissociation- and recombination-dominated thermochemical relaxation of O2 reservoirs, as well as the dissociation incubation behavior of N2 behind strong shock waves, demonstrates the ability of the new models to capture CVD behavior. Parameter φ is assessed empirically for O2 and N2 dissociation by comparison to experimental data.
Effect of neutral particles on the energy confinement of spheromaks
Authors: Mayo R. M. , Choi C. K. , Levinton F. M. , Janos A. C. , Yamada M.
Published: July 2002

Neutral density profiles are calculated for plasma and edge conditions consistent with those of the S-1 spheromak [Plasma Physics and Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. 2, p. 535]. Multiple generation charge exchange is the dominant transport mechanism for edge neutrals to the plasma center. Central neutral densities may be as high as 1%–5% of the electron density. The high-edge neutral population makes it possible to explain the value and scaling of resistivity in decaying spheromaks. Large neutral components also provide an ion energy-loss mechanism through charge exchange.
Propagation of drift waves in toroidal plasmas
Authors: Yoshikawa S. , Cheng C. Z.
Published: July 2002

Drift wave patterns in toroidal plasmas are studied. The dispersion relation was simplified to retain both the shear and the toroidal coupling effects. Since the dispersion relation does not depend on the toroidal angle φ the dispersion is solved in the two-dimensional space made up with minor radius and poloidal angle. The dispersion relation can be reduced into second-order, partial differential equations of a hyperbolic type. The one-dimensional convective mode analysis, which was originated in the 1960s, was extended into the two-dimensional analysis. Depending on the strengths of the magnetic shear, one can obtain either convective or localized solutions. The results show that the plasma is expected to be unstable for large azimuthal mode number and that the plasma instability tends to be more stabilized for large mass ions.
Existence of quasihelically symmetric stellarators
Authors: Garren D. A. , Boozer A. H.
Published: July 2002

Quasihelically symmetric stellarators are nonaxisymmetric, toroidal configurations in which the strength of the magnetic field depends on only one angular coordinate, instead of two, within the constant pressure surfaces. The confinement properties of these configurations are similar to those of axisymmetric tokamaks. Although stellarators with exact quasihelical symmetry are proven not to exist, good approximations can be found.
Quasilinear diffusion in stochastic magnetic fields: Reconciliation of drift-orbit modification calculations
Authors: Myra J. R. , Catto Peter J. , Mynick H. E. , Duvall R. E.
Published: July 2002

An apparent difference between two calculations of drift-orbit modifications to the spatial diffusion rate of particles in stochastic magnetic fields is addressed [H. E. Mynick and J. A. Krommes, Phys. Rev. Lett. 43, 1506 (1979); J. R. Myra and P. J. Catto, Phys. Fluids B 4, 176 (1992)]. The calculations are reconciled by noting the relevance of an inequality which was not discussed in these studies. It is shown, both analytically and by Monte Carlo simulation, that the diffusion coefficient can be sensitive to the spectral width of the magnetic turbulence relative to a finite Larmor radius parameter.
Theory of resistivity-gradient-driven turbulence
Authors: Garcia L. , Diamond P. H. , Carreras B. A. , Callen J. D.
Published: July 2002

A theory of the nonlinear evolution and saturation of resistivity-driven turbulence, which evolves from linear rippling instabilities, is presented. The nonlinear saturation mechanism is identified both analytically and numerically. Saturation occurs when the turbulent diffusion of the resistivity is large enough so that dissipation caused by parallel electron thermal conduction balances the nonlinearly modified resistivity gradient driving term. The levels of potential, resistivity, and density fluctuations at saturation are calculated. A combination of computational modeling and analytic treatment is used in this investigation.
Evolution of a curved vortex filament into a vortex ring
Authors: Moin Parviz , Leonard Anthony , Kim John
Published: July 2002

The deformation of a hairpin-shaped vortex filament under self-induction and in the presence of shear is studied numerically using the Biot–Savart law. It is shown that the tip region of an elongated hairpin vortex evolves into a vortex ring and that the presence of mean shear impedes the process. In addition, evolution of a finite-thickness vortex sheet under self-induction is investigated using the Navier–Stokes equations. The layer evolves into a hairpin vortex, which in turn produces a vortex ring of high Reynolds stress content. These results indicate a mechanism for the generation of ring vortices in turbulent shear flows, and a link between the experimental and numerical observation of hairpin vortices and the observation of Falco [Phys. Fluids 20, S124 (1977)] of ring vortices in the outer regions of turbulent boundary layers.
Hyper-resistivity produced by tearing mode turbulence
Authors: Strauss H. R.
Published: July 2002

Tearing mode turbulence produces a hyper-resistivity or effective anomalous electron viscosity. The hyper-resistivity is calculated for the mean magnetic field quasilinearly, and for long-wavelength modes using the direct interaction approximation. The hyper-resistivity accounts for current relaxation in reversed-field pinch experiments, and gives a magnetic fluctuation sealing of S−1/3. It causes enhanced tearing mode growth rates in the turbulent phase of tokamak disruptions. In astrophysics, it limits magnetic energy growth due to the dynamo effect, and may explain rapid reconnection phenomena such as solar flares.
Local ion direction of motion and electron flow in a magnetically insulated diode
Authors: Maron Y. , Litwin C.
Published: July 2002

Ion motion in the acceleration region of a magnetically insulated ion diode and electron flux to the anode are studied locally. Two classes of slowly growing ion deflections are observed, indicating the presence of transverse electric fields in the diode gap. A simple model, which treates the diode as an emitting surface perturbed away from planarity, is offered to infer profiles of the electric field. These profiles are consistent with the observation that one of the ion-deflection classes is associated with a significant fraction of the increases of the electron flux to the anode. The inferred growth rates of the perturbations suggest that the observed ion deflections are caused by a nonuniform expansion of the anode plasma. The transverse electric fields associated with the perturbations constitute a significant (as much as 20%) fraction of the diode accelerating field. Short duration ion deflections accompanied by intense electron bursts to the anode are also observed. The data suggest that these deflections and the electron bursts originate at processes in the cathode plasma.
Simulation of nonlinear viscous fingering in miscible displacement
Authors: Tan C. T. , Homsy G. M.
Published: July 2002

The nonlinear behavior of viscous fingering in miscible displacements is studied. A Fourier spectral method is used as the basic scheme for numerical simulation. In its simplest formulation, the problem can be reduced to two algebraic equations for flow quantities and a first-order ordinary differential equation in time for the concentration. There are two parameters, the Peclet number (Pe) and mobility ratio (M), that determine the stability characteristics. The result shows that at short times, both the growth rate and the wavelength of fingers are in good agreement with predictions from our previous linear stability theory. However, as the time goes on, the nonlinear behavior of fingers becomes important. There are always a few dominant fingers that spread and shield the growth of other fingers. The spreading and shielding effects are caused by a spanwise secondary instability, and are aided by the transverse dispersion. It is shown that once a finger becomes large enough, the concentration gradient of its front becomes steep as a result of stretching caused by the cross-flow, in turn causing the tip of the finger to become unstable and split. The splitting phenomenon in miscible displacement is studied by the authors for the first time. A study of the averaged one-dimensional axial concentration profile is also presented, which indicates that the mixing length grows linearly in time, and that effective one-dimensional models cannot describe the nonlinear fingering.
Injection, trapping, and acceleration of an electron beam in a stellatron accelerator
Authors: Mandelbaum B. , Ishizuka H. , Fisher A. , Rostoker N.
Published: July 2002

The UCI modified betatron [Phys. Rev. Lett. 53, 266 (1984)] was converted into a stellatron accelerator by the addition of helical quadrupole coils to the betatron configuration. An experimental study of the injection, trapping, acceleration, and disruption of a beam in this apparatus has been conducted. The stellarator field applied was of up to 10 kG and with a rotational transform of ι/≃0.1– 0.15. Electrons were injected from a thermionic emitter typically operated by applying 20 kV, 2– 4 μsec pulses to the hot cathode, which emitted ∼3 A. A beam of ∼200 A was trapped in the torus and accelerated by the betatron field applied immediately after the injection. The beam life was extended by increasing the toroidal field. The beam current suffered a partial disruption after it reached its peak value. After that the beam lost electrons gradually, accompanied by generation of x rays, until the entire beam was lost close to the peak of the accelerating betatron field. Electron energies of up to ∼4 MeV were reached. The dependence of the beam current and its lifetime, upon the applied fields and injection conditions, was explored.
Görtler vortices in boundary layers with streamwise pressure gradient: Linear theory
Authors: Goulpié Pascal , Klingmann Barbro G. B. , Bottaro Alessandro
Published: July 2002

Linear theory is used to analyze the stability of two-dimensional boundary layer flows to stationary Görtler vortices. The basic flow profiles in the boundary layer are described by the Falkner–Skan similarity solutions. We approach the problem both with local linear theory (with the streamwise position held fixed) and with a streamwise marching technique (to represent the evolution of the inlet disturbance). Comparisons of solutions obtained by the two methods are presented: The results are consistent in showing that adverse pressure gradients are destabilizing, as in the case of Tollmien–Schlichting waves. This is at odds with recent findings by Otto and Denier and underscores the sensitivity of the results to initial conditions.
A logarithmic 3d Euler inequality
Authors: Gibbon J. D. , Gibbons J. , Heritage M.
Published: July 2002

The magnitude of the vorticity ω=|ω| for the 3d incompressible Euler equations on the domain Ω=[0,L]3 with boundary conditions un|∂Ω=0 is shown to satisfy the inequality ‖logω(t)‖2−‖logω(0)‖2⩽∫0t‖ω(τ)‖2 dτ, for smooth initial data with no zeros in ω. The notation is ‖ω‖22=∫Ωω2dV and t is time. The case when initial data have zeros in ω is also discussed.
The nature of inviscid vortex breakdown
Authors: Goldshtik Michael , Hussain Fazle
Published: July 2002

We show that vortex breakdown appears as a jump bifurcation due to structural instability of swirling flows when their solution locally fails to exist and the flow transits to another stable or metastable state. The flow pattern inside the steady separation zone and, consequently, the vortex breakdown features depend on the flow history. Therefore, we take into account the flow pattern inside the separation zone. The stagnation zone model (without velocity jump) excels the traditional analytic continuation method (leading to a recirculation zone) in that solutions always exist, and, for large enough inflow swirl, exhibit nonuniqueness and folds due to smooth variations of flow parameters, thus predicting the experimentally observed hysteretic jump transitions.
Direct numerical simulation of the turbulent channel flow of a polymer solution
Authors: Sureshkumar R. , Beris Antony N. , Handler Robert A.
Published: July 2002

In this work, we present from first principles a direct numerical simulation (DNS) of a fully turbulent channel flow of a dilute polymer solution. The polymer chains are modeled as finitely extensible and elastic dumbbells. The simulation algorithm is based on a semi-implicit, time-splitting technique which uses spectral approximations in the spatial coordinates. The computations are carried out on a CRAY T3D parallel computer. The simulations are carried out under fully turbulent conditions albeit, due to computational constraints, not at as high Reynolds number as that usually encountered in polymer-induced drag reduction experiments. In order to compensate for the lower Reynolds number, we simulate more elastic fluids than the ones encountered in drag reduction experiments resulting in Weissenberg numbers (a dimensionless number characterizing the flow elasticity) of similar magnitude. The simulations show that the polymer induces several changes in the turbulent flow characteristics, all of them consistent with available experimental results. In particular, we have observed, associated with drag reduction, a decrease in the streamwise vorticity fluctuations and an increase in the average spacing between the streamwise streaks of low speed fluid within the buffer layer. These findings suggest a partial inhibition of turbulence generating events within the buffer layer by the macromolecules after the onset of drag reduction. This inhibition is further shown to be associated with an enhanced effective viscosity attributed to the extensional thickening properties of polymer solutions, as proposed in the past by Metzner, Lumley and other investigators. Using the simulation results obtained for different sets of parameter values which modify the relaxational and extensional properties of the model, we propose a set of criteria for the onset of drag reduction.
A particle scheme for the numerical solution of the Enskog equation
Authors: Frezzotti Aldo
Published: July 2002

It is shown that the kinetic equation proposed by Enskog for a dense hard sphere gas can be solved numerically by a particle simulation method. The technique can be considered an extension of the well known DSMC method used to solve the Boltzmann equation. Unlike a recently proposed Nanbu-like particle scheme, the present method exactly preserves momentum and energy. The calculation of the density profile in a dense gas in equilibrium near a hard wall is presented as an example.
Velocity gradient distributions in fully developed turbulence: An experimental study
Authors: Belin F. , Maurer J. , Tabeling P. , Willaime H.
Published: July 2002

We report measurements of the probability distribution function of the velocity derivatives, and the corresponding hyperflatness factors, up to order 6, as a function of the microscale Reynolds number Rλ. The measurements are performed in a flow produced between counter-rotating disks, using low-temperature helium gas as the working fluid, in a range of microscale Reynolds numbers lying between 150 and 2300. Consistently with previous studies, a transitional behavior is found around Rλ≈700. We determine a simple scaling law, in terms of Rλ, which allows the collapse of the tails of the pdf of the velocity derivatives onto a single curve, below the transition. We find well-defined relative power laws for the hyperflatness factors Hp and Hp*, throughout the entire range of variation of Rλ: H4=F=(0.99±0.05)H60.376±0.015 and H5*=(0.95±0.05)H60.67±0.022. These results are compared to those of previous investigators and to various theoretical approaches both statistical (multifractal model) and structural (i.e., based on a model of fine scales).
Oscillatory convective motion in deformed liquid bridges
Authors: Shevtsova V. M. , Legros J. C.
Published: July 2002

The transition from two-dimensional thermoconvective steady flow to a time-dependent flow is considered for a liquid with a high Prandtl number (Pr=105) in a liquid bridge with a curved free surface. Both thermocapillary and buoyancy mechanisms of convection are taken into account. The computer program developed for this simulation transforms the original nonrectangular physical domain into a rectangular computational domain. To solve the problem in body-fitted curvilinear coordinates, the time-dependent Navier–Stokes equations were approximated by central differences on a stretched mesh. For liquid bridges with a flat interface, the instability corresponding to an azimuthal wave number of m=0 is not found for the investigated range of Marangoni numbers. The instability corresponding to an m=0 is found for relatively low Marangoni numbers only in liquid bridges with a nonflat, free surface, and nonzero Rayleigh number. The steady state becomes unstable to axially running waves. It is shown that the onset of instability depends strongly upon the volume of the liquid. The stability boundary is reported for the aspect ratio Γ=height/radius=4/3 and for a wide range of liquid bridge volumes. The physical mechanism of the oscillations is based on the temporal interaction of the temperature sensitive free surface with the small local disturbances, created by temperature distribution inside the liquid bridge.
Optimal disturbances and bypass transition in boundary layers
Authors: Andersson Paul , Berggren Martin , Henningson Dan S.
Published: July 2002

Streamwise streaks are ubiquitous in transitional boundary layers, particularly when subjected to high levels of free-stream turbulence. Using the steady boundary-layer approximation, the upstream disturbances experiencing maximum spatial energy growth are numerically calculated. The calculations use techniques commonly employed when solving optimal-control problems for distributed parameter systems. The calculated optimal disturbances consist of streamwise vortices developing into streamwise streaks. The maximum spatial energy growth was found to scale linearly with the distance from the leading edge. Based on these results, a simple model for prediction of transition location is proposed. Available experiments have been used to correlate the single constant appearing in the model.
Temperature anisotropy in a cyclotron resonance heated tokamak plasma and the generation of poloidal electric field
Authors: Choe W. , Chang C. S. , Ono M.
Published: July 2002

The temperature anisotropy generated by cyclotron resonance heating of tokamak plasmas is calculated and the poloidal equilibrium electric field due to the anisotropy is studied. For the calculation of anisotropic temperatures, a bounce-averaged Fokker–Planck equation with a bi-Maxwellian distribution function of heated particles is solved, assuming a moderate wave power and a constant quasilinear cyclotron resonance diffusion coefficient. The poloidal electrostatic potential variation is found to be proportional to the particle density and the degree of temperature anisotropy of warm species.
Transverse two-stream instability of a beam with a Bennett profile
Authors: Whittum David H.
Published: July 2002

A relativistic electron beam magnetically self-focused in a plasma is subject to an electrostatic hose instability. A linear theory of hose growth is formulated, taking into account nonlinear focusing of the beam and collisionless damping due to a plasma gradient. Theory is compared to particle-in-cell simulations of three-dimensional transport of a beam with a Bennett profile, through a broad uniform plasma, as well as a plasma channel matched to the beam.
Kinetic Alfvén waves driven by velocity shear
Authors: Wang X. Y. , Liu Z. X. , Li Z. Y. , Zhang X. B.
Published: July 2002

Considering the effect of the finite ion gyroradius, the kinetic Alfvén wave in an anisotropic medium driven by a parallel velocity flow was investigated and the full dispersion relation was given. In deducing the dispersion relation, the coupling between electrostatic and electromagnetic oscillations, effects of the resonant electrons and the longitudinal motion of ions are considered. Based on the analytical and numerical calculations, two instability boundaries are found. They are 1<(k⊥dV0/dx)/kzωci<νA2/(Ti⊥/mi) and (k⊥dV0/dx)/kzωci>νA2/(Ti/mi). Resonant electrons serve as the main energy source in these two regimes, while the ion longitudinal motion only suppresses the mode in the second regime. In the boundary layer of the magnetopause, the temperature anisotropy also modifies the instability by lowering the frequency and reducing the growth rate. The above results show that the low-frequency Alfvén waves with the scale of perpendicular wavelengths greater than the ion gyroradius in the magnetopause can be efficiently driven by a velocity-sheared flow.
Plasma rotation in toroidal devices with circular cross-sections
Authors: Tsypin V. S. , Mikhailovskii A. B. , Galvão R. M. O. , Nascimento I. C. , Tendler M. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

The plasma rotation is theoretically investigated in toroidal devices. The dependence of magnetic axis curvature and torsion on the longitudinal coordinate and magnetic field ripples are taken into account. The calculations are carried out within the large aspect ratio and circular magnetic surfaces approximation. General equations for the relaxation of poloidal and toroidal velocities are obtained. The analysis of these equations is completed for the strongly collisional regime (the Pfirsch–Schlüter regime). It is shown that, as a result of the relaxation due to the ion parallel viscosity, there is an equilibrium with the ion toroidal velocity equal to zero. The general expression for the ion poloidal velocity in the Pfirsch–Schlüter regime is obtained. As in the tokamak case, this velocity is proportional to the ion temperature gradient. It does not depend on the plasma density gradient and on the radial electric field. The problems considered in the paper are of interest, specifically for toroidal devices of the “Drakon” (“Dragon”) kind [V. M. Glagolev et al., in Proceedings of the X European Conference on Controlled Fusion and Plasma Physics, USSR, Moscow (European Physical Society, Petit-Lancy, 1981), Vol. 1, p. E8].
Ideal magnetohydrodynamic stability of the tokamak high-confinement-mode edge region
Authors: Wilson H. R. , Connor J. W. , Field A. R. , Fielding S. J. , Miller R. L. , Lao L. L. , Ferron J. R. , Turnbull A. D.
Published: July 2002

The ideal magnetohydrodynamic (MHD) stability of the tokamak edge is analyzed, with particular emphasis on radially localized instabilities; it is proposed that these are responsible for edge pressure gradient limits and edge localized modes (ELMS). Data and stability calculations from DIII-D [to appear in Proceedings of the 16th International Conference on Fusion Energy, Yokohama (International Atomic Energy Agency, Vienna, 1998), Paper No. IAEA-F1-CN-69/EX8/1] tokamak equilibria indicate that two types of instability are important: the ballooning mode (driven by pressure gradient) and the peeling mode (driven by current density). The characteristics of these instabilities, and their coupling, are described based on a circular cross-section, large aspect ratio model of the tokamak equilibrium. In addition, preliminary results are presented from an edge MHD stability code which is being developed to analyze general geometry tokamak equilibria; an interpretation of the density threshold to access the high-confinement-mode (H-mode), observed on COMPASS-D [Plasma Phys. Controlled Fusion 38, 1091 (1996)] is provided by these results. Experiments on DIII-D and the stability calculations indicate how to control ELMs by plasma shaping.
Electron kinetic effects in standing shear Alfvén waves in the dipolar magnetosphere
Authors: Tikhonchuk V. T. , Rankin R.
Published: July 2002

The electron kinetic response to an electric current driven by a standing shear Alfvén wave (SAW) is considered for the case of a dipolar geometry. The parallel electric field is found from the electron gyrokinetic equation along with the SAW dispersion coefficient. Electron trapping in the dipolar magnetic field significantly reduces the parallel electric conductivity and in this way increases the amplitude of the parallel electric field and SAW dispersion. It is demonstrated that the two-fluid hydrodynamic equations used by many authors significantly underestimate the electron response and, consequently, the magnitude and location of the parallel electric field under conditions where the electron bounce frequency is larger than the SAW frequency. This is especially important in a plasma with density depressions near the Earth’s polar magnetosphere.
Linear rate oxidation of silicon for oxidation effect investigation
Authors: Mizuo Shoichi , Higuchi Hisayuki
Published: July 2002

Oxidation of Si was performed by varying oxygen partial pressure during oxidation. A linear relation between oxide thickness and oxidation time was obtained under a linear increase of oxygen partial pressure. The effect of linear oxidation was examined on B and P diffusion in Si. Linear rate oxidation can be used for investigation of oxidation enhanced or retarded diffusion of impurities and growth kinetics of stacking faults in Si.
Accurate phase capacitance spectroscopy of transition metal silicon diodes
Authors: Evans Howard L. , Wu Xu , Yang Edward S. , Ho Paul S.
Published: July 2002

A complete understanding of Schottky barrier devices requires a knowledge of the electronic states at the metal-semiconductor interface. For this reason, a novel and accurate technique for measuring the capacitance of forward biased Schottky diodes has been developed. It is found that the measurement is extremely sensitive to the phase error and that the lock-in amplifier behaves in a nonlinear fashion at high signal current. These difficulties have been resolved to realize the interface state spectrum of a Pd/silicon Schottky barrier.
Measurement of mobility-lifetime product in hydrogenated amorphous silicon p-i-n type diodes
Authors: Könenkamp R. , Hermann A. M. , Madan A.
Published: July 2002

We have applied the junction recovery technique to different configurations of hydrogenated amorphous silicon type diodes and show that the recovered charge consists predominantly of holes. The technique is used for the measurement of the mobility-lifetime product for recombination, which was found to be dependent upon the level and type of doping.
Photothermal deflection spectroscopy of a powder-layer photoelectrochemical structure
Authors: Tamor M. A. , Hetrick R. E.
Published: July 2002

Photothermal deflection spectroscopy has been used to characterize a new photoelectrochemical (PEC) structure consisting of a thin porous layer of microscopic TiO2 or WO3 powder above a metallized substrate. For TiO2, factors affecting PEC activity such as crystal structure and degree of electronic conductivity can be correlated with their corresponding optical absorption spectra. For WO3, electronic charge transfer between powder and substrate is revealed by electrochromism in the powder under cathodic bias in acid solution.
Broadband Y-branch electro-optic GaAs waveguide interferometer for 1.3 μm
Authors: Buchmann P. , Kaufmann H. , Melchior H. , Guekos G.
Published: July 2002

Electro-optic Mach–Zehnder rib waveguide interferometers with low loss Y branches and Schottky barrier electrodes have been fabricated in n−/n+-GaAs by reactive ion etching. With antireflection coating and optimized Y branches device losses of the interferometers at 1.3 μm were as low as 4 dB. In a push-pull configuration drive voltages of only 13 V suffice to attain a modulation depth of 12.7 dB. The optical signal bandwidth of these interferometers was measured to exceed 4.5 GHz.
Defects in ultrafast quenched aluminum-doped silicon
Authors: Chantre A.
Published: July 2002

Deep level transient spectroscopy has been applied to the study of defects in ultrafast quenched (cw laser irradiated) aluminum-doped silicon. Iron-aluminum pairs, with energy levels H(0.13) and H(0.20), are observed. A third hole trap, H(0.52), is identified as associated with the aluminum-vacancy center. No quenched-in interstitial aluminum is detected. The results support the view that dopant diffusion in silicon takes place primarily via vacancies at high temperature.
Deep Fe and intrinsic defect levels in Ga0.47In0.53As/InP
Authors: Goetz K.-H. , Bimberg D. , Brauchle K.-A. , Jürgensen H. , Selders J. , Razeghi M. , Kuphal E.
Published: July 2002

Two deep traps in Ga0.47In0.53As/InP:Fe at a depth of 110 meV and 150 meV, respectively, are observed for the first time using low-temperature photoluminescence and deep level transient spectroscopy. The dependence of luminescence intensity on the growth process itself (liquid phase epitaxy, vapor phase epitaxy, and metalorganic chemical vapor deposition) and its parameters (growth temperature, layer thickness) and the substrate doping is reported and leads to the unambigous identification of the 150-meV acceptorlike trap as being caused by Fe impurities. Fe diffuses from the substrate to the epitaxial layer during the growth process. This outdiffusion is less pronounced for layers grown at lower temperature. The level at 110 meV which is also observed in layers grown on InP:S substrate is tentatively assigned to an intrinsic defect of Ga0.47In0.53As.
New piezoelectric Ta2O5 thin films
Authors: Nakagawa Yasuhiko , Gomi Yasuo
Published: July 2002

Crystallized Ta2O5 thin films have been deposited on fused quartz substrates by reactive dc diode sputtering. Crystalline structures and piezoelectric properties of the films were investigated. The electromechanical coupling coefficient K2 of the surface acoustic waves was 0.5% for hk=1.0. This value is comparable to that for ZnO thin films.
Low-energy optical generation and detection of acoustic pulses in metals and nonmetals
Authors: Bourkoff E. , Palmer C. H.
Published: July 2002

Acoustic pulses have been generated in both metals and nonmetals using a low-energy tunable dye laser emitting 6-ns pulses with 20–210-μJ energies. The acoustic waveforms have been detected optically with a calibrated laser interferometer having improved signal-to-noise ratio. In this fashion, noncontact characterization of nonmetallic, as well as metallic samples of small size, has been shown to be feasible, using a potentially portable system. In addition between the relationship acoustic signal level and incident laser energy (and power density) has been measured for a polyimide-glass-fiber composite material.
Space-charge-like scattering in epitaxial GaAs from low temperature and high pressure Hall measurements
Authors: Saxena A. K. , Sinha A. K. , Adams A. R.
Published: July 2002

The Hall electron mobility in GaAs grown by vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD) has been studied both as a function of temperature (77<T<300 K) and hydrostatic pressure (0–8 kbar). The analysis of the data shows that in LPE and VPE crystals, the mobilities are lowered due to space-charge scattering (μSC∝m*−1/2T−1/2) while in MOCVD and MBE samples, a space-charge-like limited mobility (μUN∝m*−nT−1/2) must be included. The value of the exponent n has been found to be 1 for MBE and 2 for MOCVD grown samples. It has been concluded that increasing impurity gradation in these layers is responsible for higher values of n. The pressure coefficient of electron mass in GaAs is determined to be 5.2×10m−4 m*/kbar . The model of central cell scattering due to carbon acceptors fails to account for the experimental data reported in this paper.
Density measurements, calorimetry, and transmission electron microscopy of icosahedral Mn14Al86
Authors: Kelton K. F. , Wu T. W.
Published: July 2002

The density and heat of crystallization of rapidly quenched Mn14Al86 were measured and found to be 3.287±0.038 g/cm3 and 16.2 cal/g, respectively. Transmission electron microscopy revealed nodules of the icosahedral phase with lattice fringes. No Kikuchi lines were observed in either selected area or convergent beam diffraction.
Inorganic ion beam resist for additive plating of metallic interconnects
Authors: Eskildsen Svend Stensig , So/rensen Gunnar
Published: July 2002

A new approach to produce metallic conductors on insulating materials is presented. The conductor is made by electroless plating of a catalytic pattern, which is generated by ion beam decompositon of platinum-metal compounds. On polyimide, metallic lines are produced with good adhesion, and it is demonstrated that the lines can be made between two levels. The plating process on polyimide has a surprisingly small inhibition period of 20 s and a plating speed of 30 nm/min which is about twice the normal.
Sintered n-CuInSe2/Au Schottky diode
Authors: Leccabue F. , Seuret D. , Vigil O.
Published: July 2002

Rectifying CuInSe2/Au contacts were prepared by a vacuum evaporation of Au on n-type sintered CuInSe2 samples in order to evaluate the electro-optical properties of these devices. The low photovoltaic efficiency of the diode is due to the presence of the insulating layer, surface states, and high series resistance. A good quantum efficiency of about 60% in the 0.72–1.24-μm range has been obtained.
Transit time studies of junction location in thin-film solar cells
Authors: Ahrenkiel R. K. , Matson R. J.
Published: July 2002

The active junction in Cd(Zn)S/CuInSe2 thin-film solar cells was investigated by a pulse photoconductivity technique. We observed the diffusion transit time of minority carriers to a homojunction internal to the CuInSe2. No electrical activity at the heterojunction was indicated.
High intensity submillimeter photoresponse of a Si inversion layer
Authors: Verma I. B. , Leung M. , Drew H. D. , Doezema R. E. , Furneaux J. E. , Wagner R. J.
Published: July 2002

The submillimeter wave (496, 385, and 66 μm) photoresponse has been measured in an n-channel Si metal-oxide-semiconductor field-effect transistor at 4.2 K. A fast (≤10 ns) response is observed only in the low carrier density (ns) regime where the dc conductance is activated. Nonlinear photo response is found for I≥10 kW/cm2 and the signal follows a simple saturation behavior. The peak illuminated effective mobility is ∼13 000 cm2/Vs (at ns∼0.7×1011 cm−2) which is more than a factor of 2 larger than the peak 4.2-K mobility of the device (at ns∼2.4×1012 cm−2). At carrier densities greater than 5×1011 cm−2 the response is bolometric with a response time ∼1 μs.
Visible-laser photodeposition of chromium oxide films and single crystals
Authors: Arnone C. , Rothschild M. , Black J. G. , Ehrlich D. J.
Published: July 2002

Visible-laser deposition reactions of the chromyl chloride vapor, CrO2Cl2, have been developed for direct writing of chromium oxides. Deposition at low laser power is by photolysis of adsorbed CrO2Cl2 molecules. Higher powers initiate deposition photochemically and continue it with a combined photolytic/pyrolytic reaction, simultaneously inducing a solid phase conversion of the deposited film. Mixed Cr2O3/CrO2 thin films of 1 nm to several micrometer thickness, as well as 1-mm-long single crystals of Cr2O3, were grown, the latter at rates up to 3 μm/s. The thin films are strongly ferromagnetic. Mass spectrometer and optical transmission measurements show that surface kinetics dominate the nucleation and growth rates.
X-ray refractive index: A tool to determine the average composition in multilayer structures
Authors: Miceli P. F. , Neumann D. A. , Zabel H.
Published: July 2002

We present a novel and simple method to determine the average composition of multilayers and superlattices by measuring the x-ray refractive index. Since these modulated structures exhibit Bragg reflections at small angles, by using a triple axis x-ray spectrometer we have accurately determined the peak shifts due to refraction in GaAs/AlxGa1−xAs and Nb/Ta superlattices. Knowledge of the refractive index provides the average fractional composition of the periodic structure since the refractive index is a superposition of the refractive indices of the atomic constituents. We also present a critical discussion of the method and compare the values of the average fractional composition obtained in this manner to the values obtained from the lattice parameter change in the GaAs/AlxGa1−xAs superlattices due to the Al.
Micromachining of integrated optical structures
Authors: Harriott L. R. , Scotti R. E. , Cummings K. D. , Ambrose A. F.
Published: July 2002

Three-dimensional features have been milled into optical materials by scanning a submicron focused gallium ion beam. Different shapes are obtained using computer controlled beam placement and dwell time during sputtering. We have used this technique to create micron-sized facets and reflectors in the active areas of semiconductor lasers. Light output and quantum efficiency measurements indicate that these features are of sufficient quality to fabricate monolithic integrated optical devices. Some of the applications currently being investigated are laser-detector pairs, coupled cavity lasers, lasers with integral lenses, distributed feedback lasers, confocal cavities, and laser cavity length tuning.
Electron spin resonance observations of paramagnetic defects produced in fused silica by high-energy oxygen and krypton ions
Authors: Langevin Y. , Dooryhee E. , Borg J. , Duraud J-P. , Lecomte C. , Balanzat E.
Published: July 2002

Pure fused low-OH silica (Tetrasil SE) has been studied by electron spin resonance (ESR) after it had been irradiated by 50 MeV/amu oxygen and 43 MeV/amu krypton ions at the GANIL accelerator. Paramagnetic defects have been observed which are closely related to intrinsic defects formed by γ-ray irradiation in amorphous SiO2, such as the E′ center and the peroxy radical. ESR signatures of defects formed by heavy ions exhibit, however, specific features in comparison with those formed by other types of irradiation (γ/x rays, electrons, or neutrons). The density of defects increases faster than the energy loss, so that the production of defects by heavy ion irradiation cannot be described by the total energy deposit alone. This is consistent with previous studies of latent ion tracks in insulating minerals using small angle x-ray scattering and track etching techniques.
Self-aligned enhancement-mode and depletion-mode GaAs field-effect transistors employing the δ-doping technique
Authors: Schubert E. F. , Cunningham J. E. , Tsang W. T.
Published: July 2002

This work describes a self-aligned Schottky-gate field-effect transistor (FET) which uses the δ-doping technique during crystal growth by molecular beam epitaxy. In this new FET the δ-doping concepts are employed in three ways: (i) for the highly doped surface to obtain nonalloyed ohmic contacts, (ii) to decrease the parasitic resistances, and (iii) for the electron channel below the gate. A two-dimensional electron gas in a V-shaped quantum well is formed by the δ-doped electron channel. The advantage of nonalloyed ohmic contacts allows us to use a new two-mask, self-aligned FET process to further reduce the parasitic resistances. Both enhancement-mode and depletion-mode δ-doped GaAs field-effect transistors are fabricated. The measured transconductance of the δ-doped field-effect transistor is 240 mS/mm and is comparable to values obtained from selectively doped heterostructure transistors of the same geometry.
Graded-index separate-confinement InGaAs/GaAs strained-layer quantum well laser grown by metalorganic chemical vapor deposition
Authors: Feketa D. , Chan K. T. , Ballantyne J. M. , Eastman L. F.
Published: July 2002

A graded-index separate-confinement strained-layer quantum well laser with pseudomorphic Ga0.63In0.37As quantum well was grown by metalorganic chemical vapor deposition. The lasing wavelength is 0.99 μm at 300 K and the average threshold current density of broad area 146×363 μm devices is 195 A/cm2.
Room-temperature negative differential resistance in strained-layer GaAs-AlGaAs-InGaAs quantum well heterostructures
Authors: Lee G. S. , Hsieh K. Y. , Kolbas R. M.
Published: July 2002

We report the first room-temperature experimental observation of negative differential resistance in a strained-layer double barrier resonant tunneling structure. Room-temperature negative differential resistance (peak-to-valley ratio=1.4) was observed in GaAs-AlGaAs-InxGa1−xAs-AlGaAs-GaAs structure for indium compositions as high as x=0.09. We show that the onset of negative differential resistance can be adjusted by varying only the indium composition while holding all the other design parameters constant.
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u contacts to GaAs based on InAs
Authors: Wright S. L. , Marks R. F. , Tiwari S. , Jackson T. N. , Baratte H.
Published: July 2002

We report preliminary electrical results on n+-InAs/n-GaAs contact structures grown by molecular beam epitaxy. The data indicate that the conduction-band discontinuity is sufficiently small to allow the formation of an ohmic contact to n-type GaAs for very heavily doped InAs layers. The structures require a short-term anneal to obtain a low resistance contact. An InAs layer which is only 200 Å thick is sufficient to provide a specific contact resistance of 10−6 Ω cm2. The contacts appear to be thermally stable for short-term anneals up to 900 °C.