bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation of the modified Simon–Hoh instability in an electron beam produced plasma
Authors: Sakawa Y. , Joshi C. , Kaw P. K. , Chen F. F. , Jain V. K.
Published: July 2002

An intermediate frequency (fci≤f≤fce) electrostatic instability has been observed in an electron beam produced, cylindrical plasma column. This instability has been identified as a new instability, the modified Simon–Hoh instability (MSHI), which has an instability mechanism similar to the Simon–Hoh instability (SHI). This instability can occur in a cylindrical collisionless plasma if a radial dc electric field exists and if this radial dc electric field and the radial density gradient are in the same direction. The origin of the dc electric field is found to be the difference between the ion and the electron radial density profiles. In such a plasma if the ions are essentially unmagnetized but the electrons are magnetized, a velocity difference in the θ direction can arise because of the finite ion Larmor radius effect. This leads to a space charge separation between the electron and ion density perturbations in the θ direction. The consequent perturbed azimuthal electric field Eθ1 and the enhancement of the density perturbation by the Eθ1×B0 velocity occur in the same manner as in the SHI. The instability frequency is decided by the ion azimuthal drift velocity. This new instability has been investigated through experiments and theory.
Self-focusing and Raman scattering of laser pulses in tenuous plasmas
Authors: Antonsen T. M. , Mora P.
Published: July 2002

The propagation and self-focusing of short, intense laser pulses in a tenuous plasma is studied both analytically and numerically. Specifically, pulses of length of the order of a few plasma wavelengths and of intensity, which is large enough for relativistic self-focusing to occur, are considered. Such pulses are of interest in various laser plasma acceleration schemes. It is found that these pulses are likely to be strongly affected by Raman instabilities. Two different regimes of instability, corresponding to large and small scattering angles, are found to be important. Small-angle scattering is perhaps the most severe since it couples strongly with relativistic self-focusing, leading the pulses to acquire significant axial and transverse structure in a time of the order of the self-focusing time. Thus it will be difficult to propagate smooth self-focused pulses through tenuous plasmas for distances longer than the Rayleigh length, except for pulse duration of the order of the plasma period.
Coherent magnetic field fluctuations and locked modes in a reversed-field pinch
Authors: Brunsell P. R. , Yagi Y. , Hirano Y. , Maejima Y. , Shimada T.
Published: July 2002

Magnetic field fluctuations have been measured in the TPE-1RM20 reversed-field pinch [in Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 14th International Conference, 1992, Würzburg (International Atomic Energy Agency, Vienna, 1992) Paper IAEA-CN-56/C-4-3]. The dominant modes are m=1/n=7–9, resonant near the axis. The fluctuations are coherent, corresponding to nearly continuous rigid body rotation with poloidal and toroidal frequencies of fθ=130–170 kHz and fφ=11–14 kHz, respectively. Discrete dynamo events (in which toroidal magnetic flux is generated) of two different types are observed at high values of pinch parameter (Θ≳1.6). The first type is triggered by a single, wall-locked m=1/n=7 mode. The slowing down of the mode rotation is in rough agreement with a simulation which assumes the effect of induced eddy current in the vacuum liner. In the subsequent relaxation event, the locked mode decays and the m=1 spectrum cascades to higher-n modes. However, in some cases the saturated m=1/n=7 mode exists for periods up to t=0.4 msec, resembling single helicity Ohmic states. The second type of event is characterized by phase locking of several m=1/n=7–9 modes and the appearance of large m=0/n=1–5 modes. Mode rotation continues through the event, similar to the normal machine operating condition.
Self-consistent growth rate of the Rayleigh–Taylor instability in an ablatively accelerating plasma
Authors: Takabe H. , Mima K. , Montierth L. , Morse R. L.
Published: July 2002

The linear stability of an ablating plasma is investigated as an eigenvalue problem by assuming the plasma to be at the stationary state. For various structures of the ablating plasma, the growth rate is found to be expressed well in the form γ=α(kg)1/2 −βkVa, where α=0.9, β≂3–4, and Va is the flow velocity across the ablation front, and is found to agree well with recent two-dimensional simulations in a classical transport regime. Short-wavelength lasers inducing enhanced mass ablation are suggested to be advantageous to stable implosion because of the ablative stabilization.
Nonlinear transverse electron beam dynamics in a modified betatron accelerator
Authors: Kapetanakos C. A. , Marsh S. J.
Published: July 2002

The transverse electron beam dynamics in a modified betatron accelerator is studied using a new approach. This approach is based on the two exact constants of the motion and the potentials at the center of the beam. The main advantage of the present technique is that the ring orbits can be accurately determined over the entire minor cross section of the torus and not only near the minor axis. The results indicate that the electron ring orbits in the plane transverse to the toroidal magnetic field always close inside the vacuum chamber, and their topology is often substantially different than that predicted from the linear theory.
Rayleigh–Taylor instability in a spherically stagnating system
Authors: Hattori F. , Takabe H. , Mima K.
Published: July 2002

The Rayleigh–Taylor instability induced on the fuel–pusher interface in the deceleration phase of shell implosion is investigated in spherical geometry. A linearized equation for the perturbation from the background dynamics described by a self-similar motion is solved analytically and numerically. The effective growth rate of the Rayleigh–Taylor instability is not found to be sensitive to the compressibility. In a spherical system where the gravity and wavelength of the perturbation vary in time and space, the growth of the perturbation is found to be approximately expressed in the form ‖ξ‖∝ Rc exp[∫(αAkeffgeff)1/2dt], where keff and geff are the effective wavelength and gravity at the contact surface, with Rc the radius and αA the Atwood number.
Transport processes in fractals. VI. Stokes flow through Sierpinski carpets
Authors: Adler P. M.
Published: July 2002

The Stokes flow of a Newtonian fluid is calculated inside a porous medium that is spatially periodic, the unit cell being a fractal named a Sierpinski carpet. Complete results are given for the longitudinal permeability. A scaling argument and complete numerical calculations provide two exponents of the power law that differ by only 2% when the construction stage is large; in this limit, the scaling argument provides the same result as the classical Carman equation. The agreement between these two results may be fortuitous and thus has to be considered with caution. Various comments and extensions to three-dimensional media such as the Menger sponge are also presented.
Nonlinear electrical conductivity for a strongly coupled plasma
Authors: Schenter Gregory K. , Liboff Richard L.
Published: July 2002

A nonlinear analysis of electrical conductivity in a plasma is given, stemming from the Uehling–Uhlenbeck equation. Anisotropy due to an applied electric field is incorporated through a Legendre polynomial expansion of the distribution function. The plasma is comprised of ions, electrons, and a neutral component. The electron–ion interaction is described by a shielded Debye potential at high energy and a cutoff Coulomb potential at low energy. A nonlinear equation for the distribution function is solved and yields f̄SL(x) =1/(1+BeA(x)) for the symmetric part of the solution. Nondimensional energy is x, B is a normalization constant, and A(x) is an explicit integral dependent on the electric field and specifics of the interaction. Resulting nondimensional conductivity σ̃, is given by σ̃ = (1)/(3) (2/π)3/2[ac(Z+1)1/2/ ΛQΓD] ∫∞0 f̄SL(x)(d/dx) (x/Q̃)dx, where Z is effective ionization, aC is the ratio of charge to total heavy-particle density, Q̃ is the dimensionless, weighted cross section, and ΛQ and ΓD are quantum and plasma parameters, respectively. Application is made to an aluminum plasma and plots of conductivity versus electric field are obtained. These plots exhibit three distinct regions; with an increase in field strength these are the Ohmic, Coulomb-dominated, and neutral-dominated regions.
Solitary internal waves in a rotating channel: A numerical study
Authors: Katsis C. , Akylas T. R.
Published: July 2002

The effect of slow rotation on the propagation of solitary internal waves in shallow fluids is studied. The Kadomtsev–Petviashvili (KP) equation, appropriately modified to account for Coriolis effects, is solved numerically for two-layer flow in a rotating channel of finite width. In the presence of rotation, an initially straight-crested soliton evolves to a three-dimensional disturbance, which, to a reasonable approximation, has a sech2 profile along the channel but decays slowly as it propagates. Furthermore, the wave amplitude varies exponentially across the channel and the wave crest is curved backward. These trends are in agreement with recent laboratory experiments.
Rigid rotor equilibria of multifluid, neutral plasma columns in crossed electric and magnetic fields
Authors: Prasad Rahul R. , Krishnan Mahadevan
Published: July 2002

A multifluid model is considered for rigid rotor equilibrium in an axisymmetric, semi-infinite, isothermal, fully magnetized plasma column with negligible radial diffusion. The plasma potential profile is self-consistently calculated using a perturbation expansion and is found to be parabolic. The radial ion and electron density distributions are then Gaussians.
Pressure of a freely expanding gas upon a rigid wall
Authors: Strobel George L.
Published: July 2002

A study is made of the flow that results when an initially uniform gas with γ= (5)/(3) expands into a vacuum with a one-dimensional rarefaction and then strikes a rigid wall. For short times, the pressure at the wall increases with the time cubed but this short time behavior ceases to apply when t≥0.3. The time scale is set by the time it takes for the initially static fluid to first reach the wall. The short time behavior calculated here is in agreement with the lowest-order theory of Greenspan and Butler [J. Fluid Mech. 13, 101 (1962)].
Orbits in asymmetric toroidal magnetic fields
Authors: Cary John R. , Hedrick C. L. , Tolliver J. S.
Published: July 2002

Orbits in an asymmetric toroidal magnetic field are studied for the case in which the local variation of the field strength due to ripple is rapid compared with that due to toroidicity. In this case, to lowest order the poloidal variables are constant and particles move primarily in the toroidal direction. Invariants and averaged equations of motion for the locally passing and locally trapped particles are derived based on this approximation. The equations imply that transitions between the locally trapped and locally passing states occur. The probabilities for these transitions are calculated.
Kinetic theory for electrostatic waves due to transverse velocity shears
Authors: Ganguli G. , Lee Y. C. , Palmadesso P. J.
Published: July 2002

A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit (kyρi ≪1, where ky is the wave vector in the y direction and ρi is the ion gyroradius) the dispersion differential equation is recovered for the transverse Kelvin–Helmholtz modes for arbitrary values of k∥, where k∥ is the component of the wave vector in the direction of the external magnetic field assumed in the z direction. For higher temperatures (kyρi>1) the ion-cyclotron-like modes described earlier in the literature by Ganguli, Lee, and Palmadesso [Phys. Fluids 28, 761 (1985)] are recovered. In this article the integral equation is reduced to a second-order differential equation and a study is made of the kinetic Kelvin–Helmholtz and the ion-cyclotron-like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.
Hydrodynamic transport properties of hard-sphere dispersions. I. Suspensions of freely mobile particles
Authors: Phillips R. J. , Brady J. F. , Bossis G.
Published: July 2002

The hydrodynamic transport properties of hard-sphere dispersions are calculated for volume fractions (φ) spanning the dilute limit up to the fluid–solid transition at φ=0.49. Particle distributions are generated by a Monte Carlo technique and the hydrodynamic interactions are calculated by Stokesian dynamics simulation. The effects of changing the number of particles in the simulation cell are investigated, and the scaling laws for the finite-size effects are derived. The effects of using various levels of approximation in computing both the far- and near-field hydrodynamic interactions are also examined. The transport properties associated with freely mobile suspensions—sedimentation velocities, self-diffusion coefficients, and effective viscosities—are determined here, while the corresponding properties of porous media are determined in a companion paper [Phys. Fluids 31, xxxx (1988)]. Comparison of the simulation results is made with both experiment and theory. In particular, the short-time self-diffusion coefficients and the suspension viscosities are in excellent agreement with experiment.
Contact-line motion of shear-thinning liquids
Authors: Weidner D. E. , Schwartz L. W.
Published: July 2002

It is demonstrated that, for the slow advance of a viscous liquid onto a previously dry substrate, the well-known moving contact line paradox is alleviated for liquids exhibiting power-law shear-thinning behavior. In contrast to previous models that allow contact-line motion, it is no longer necessary to abandon the no-slip condition at the substrate in the vicinity of the contact point. While the stress is still unbounded at the contact point, the equations of motion are shown to be integrable. A three-constant Ellis viscosity model is employed that allows a low-shear Newtonian viscosity, and may thus be used to model essentially Newtonian flows where shear thinning only becomes important in the immediate vicinity of the contact point. Calculations are presented for the model problem of the progression of a uniform coating layer down a vertical substrate using the lubrication approximations. The relationship between viscous heating and shear-thinning rheology is also explored.
Lubrication theory for reactive spreading of a thin drop
Authors: Braun R. J. , Murray B. T. , Boettinger W. J. , McFadden G. B.
Published: July 2002

Solder drops spreading on metallic substrates are a reactive form of the wetting problem. A metallic component may diffuse in the liquid toward a metal substrate, where it is consumed by a reaction that forms a solid intermetallic phase. The resulting spatial variation in the composition of the drop may cause composition gradients along the free surface of the drop. Together with any thermal gradients along the free surface, Marangoni effects may, in turn, modify the bulk transport in the spreading drop. Motivated by this situation, we extend lubrication theory for the spreading of thin drops in the presence of gravity and thermocapillarity to include mass transport and solutocapillarity. We use an approximate solute profile in the drop to derive coupled evolution equations for the free surface shape and concentration field. Numerical solutions for the nonreactive (single component) drop agree well with previous theory. In the reactive case, we are only able to compute results for parameters outside of the range for solder materials. Including reactive effects in the model impacts the flow patterns and spreading rates at relatively early times; but by the end of the spreading, solutal effects have died out in the model.
Heat flow and temperature and density distributions in a rarefied gas between parallel plates with different temperatures. Finite-difference analysis of the nonlinear Boltzmann equation for hard-sphere molecules
Authors: Ohwada Taku
Published: July 2002

Heat flow and temperature and density distributions in a rarefied gas between two parallel plates at rest with different uniform temperatures are analyzed numerically on the basis of the full nonlinear Boltzmann equation for hard-sphere molecules and the Maxwell-type boundary condition by a finite difference method where the collision term is computed direct numerically. The accurate results are presented for the case in the density measurement by Teagan and Springer [Phys. Fluids 11, 497 (1968)], where the temperature ratio is 1.326, the value of the accommodation coefficient is 0.826, and the ratio of mean free path to plate spacing (Knudsen number Kn) is 0.0658≤Kn≤0.7582. It is found that there is a considerable difference between the present density distribution and the experimental data. The reason for this discrepancy is also discussed. The accurate numerical results of the linearized problem are also presented for comparison.
Stability of flow in a channel with a suddenly expanded part
Authors: Mizushima J. , Okamoto H. , Yamaguchi H.
Published: July 2002

The stability of a two-dimensional flow in a symmetric channel with a suddenly expanded part is investigated numerically and analyzed by using the method of the nonlinear stability theory. From results of the numerical simulation, it is shown that the flow is steady, symmetric and unique at very low Reynolds numbers, while the symmetric flow loses its stability at a critical Reynolds number resulting in an appearance of asymmetric flow. The transition from the steady symmetric flow to the steady asymmetric one is found to occur due to the symmetry breaking pitchfork bifurcation when the aspect ratio, the ratio of the length of the expanded part to its width, is large. It is also found that the bifurcated flow becomes symmetric again when the Reynolds number is increased and the resultant symmetric flow loses its stability becoming periodic in time as the Reynolds number is further increased. On the other hand, when the aspect ratio is small there occurs no pitchfork bifurcation and the direct transition from the steady symmetric flow to a periodic flow occurs due to a Hopf bifurcation. The critical aspect ratio is found to be about 2.3. The critical Reynolds numbers for these bifurcations are evaluated.
Transverse flow and mixing of granular materials in a rotating cylinder
Authors: Khakhar D. V. , McCarthy J. J. , Shinbrot Troy , Ottino J. M.
Published: July 2002

The focus of this work is analysis of mixing in a rotating cylinder—a prototype system for mixing of granular materials—with the objective of understanding and highlighting the role of flow on the dynamics of the process. The analysis is restricted to low speeds of rotation, when the free surface of the granular solids is nearly flat, and when particles are identical so that segregation is unimportant. The flow is divided into two regions: a rapid flow region of the cascading layer at the free surface, and a fixed bed of particles rotating at the angular speed of the cylinder. A continuum model, in which averages are taken across the layer, is used to analyze the flow in the layer. Good agreement is obtained between the predictions of the flow model for the layer thickness profile and experimental results obtained by digital image analysis. The dynamics of the mixing process are studied by advecting tracer particles by the flow and allowing for particle diffusion in the cascading layer. The mixing model predictions for distribution of tracer particles and mixing rates are compared qualitatively and quantitatively to experimental data. Optimal operating conditions, at which mixing rates are maximum, are determined.
Asymmetric salt fingers induced by a nonlinear equation of state
Authors: Özgökmen Tamay M. , Esenkov Oleg E.
Published: July 2002

The impact of the nonlinearity in the equation of state associated with the change in the thermal expansion coefficient with temperature on the structure of fingers growing from an interface between two mixed layers is investigated using a numerical model. It is shown that the nonlinearity acts to enhance the buoyancy force acting on the descending fingers with respect to that acting on the ascending fingers, resulting in narrower and faster-growing descending fingers than ascending fingers. The results are discussed with emphasis on the vertical variability of properties along the fingers.
Direct numerical simulations analysis of flame surface density models for nonpremixed turbulent combustion
Authors: Van Kalmthout Eric , Veynante Denis
Published: July 2002

From phenomenological considerations, Marble and BroadwellReference 1 have introduced the flame surface density concept to describe nonpremixed turbulent combustion. In this approach, the averaged reaction rate is modeled as the product of the local reaction rate per unit of flame area, ṁF, by the flame area per unit volume, Σ̄. This approach is attractive because it decouples the chemical problem (ṁF) from the description of the turbulence combustion interaction (Σ̄). In this paper, a theoretical analysis providing two alternatives exact, but unclosed, balance equations for the flame surface density Σ̄ is first developed. Then, direct numerical simulations of a spatially developing turbulent reacting mixing layer are used to analyze the structure of the reaction zone and to propose and validate closures for the Σ̄-balance equations. The flame surface density concept is found to provide a relevant description for nonpremixed turbulent flames.
Three-dimensional blade–vortex interaction in the strong vortex regime
Authors: Krishnamoorthy S. , Marshall J. S.
Published: July 2002

A study of three-dimensional interaction between a blade and an initially columnar vortex was performed using flow visualization experiments and computer simulations for problems where the blade is translated normal to and impacts upon the vortex core. The reported research focuses on cases where the vortex is sufficiently strong that the blade boundary layer separates before the blade penetrates into the vortex core. The study examines the topology and evolution of the separated secondary vorticity from the blade and its interaction with the primary vortex. In particular, it is observed that the separated vorticity from the blade has the form of a series of vortex loops that wrap around the outer surface of the primary vortex core and spread both upstream and downstream along the vortex axis. Both the initial formation of these vortex loops and their eventual interaction with the primary vortex are investigated. Computations based on a series of idealized models of the secondary vorticity are performed in order to examine the primary vortex response to wrapped vortex loops.
On the three-dimensional Rayleigh–Taylor instability
Authors: He Xiaoyi , Zhang Raoyang , Chen Shiyi , Doolen Gary D.
Published: July 2002

The three-dimensional Rayleigh–Taylor instability is studied using a lattice Boltzmann scheme for multiphase flow in the nearly incompressible limit. This study focuses on the evolution of the three-dimensional structure of the interface. In addition to the bubble and spike fronts, a saddle point is found to be another important landmark on the interface. Two layers of heavy-fluid roll-ups, one at the spike tip and the other at the saddle point, were observed. The secondary instability in the horizontal planes entangles the already complicated structure of the interface. Parallel computations are utilized to accommodate the massive computational requirements of the simulations.
Transport reduction by current profile control in the reversed-field pinch
Authors: Sarff J. S. , Almagri A. F. , Cekic M. , Chaing C.-S. , Craig D. , Den Hartog D. J. , Fiksel G. , Hokin S. A. , Harvey R. W. , Ji H. , Litwin C. , Prager S. C. , Sinitsyn D. , Sovinec C. R. , Sprott J. C. , Uchimoto E.
Published: July 2002

An auxiliary poloidal inductive electric field applied to a reversed-field pinch (RFP) plasma reduces the current density gradient, slows the growth of m=1 tearing fluctuations, suppresses their associated sawteeth, and doubles the energy confinement time. This experiment attacks the dominant RFP plasma loss mechanism of parallel streaming in a stochastic magnetic field. The auxiliary electric field flattens the current profile and reduces the magnetic fluctuation level. Since a toroidal flux change linking the plasma is required to generate the inductive poloidal electric field, the current drive is transient to avoid excessive perturbation of the equilibrium. To sustain and enhance the improved state, noninductive current drivers are being developed. A novel electrostatic current drive scheme uses a plasma source for electron injection, and the lower-hybrid wave is a good candidate for radio-frequency current drive.
Current sheet formation along three-dimensional magnetic separators
Authors: Longcope D. W. , Cowley S. C.
Published: July 2002

It was shown by Syrovatskii [Sov. Phys. JETP 33, 933 (1971)] that a perfectly conducting two-dimensional magnetic field stressed externally develops a current sheet at an X point. A generalization of this analysis shows that a generic three-dimensional magnetic field develops a closed ribbon of current along separators already existing in the field. A separator is a field line that begins and ends at different magnetic null points. A current ribbon will occur if there are two distinct separators beginning and ending at the same two nulls, thus forming a loop. The magnetic field containing the current ribbon is in force-free equilibrium. An expression is found for the net current induced in the ribbon, and the magnetic energy stored by it, in terms of the amount of external stress. An electric field parallel to the current will facilitate reconnection and will dissipate the current and release the stored energy.
Stimulated Brillouin scattering reflectivity in the case of a spatially smoothed laser beam interacting with an inhomogeneous plasma
Authors: Tikhonchuk V. T. , Mounaix Ph. , Pesme D.
Published: July 2002

The stimulated Brillouin scattering (SBS) instability is investigated theoretically in the case of a spatially smoothed laser beam interacting with an inhomogeneous plasma in the regime of strong ion acoustic damping. The domain of parameters being considered corresponds to most of the present day experiments carried out with nanosecond laser pulses interacting with preformed plasmas: the characteristic length for convective amplification is assumed to be much shorter than the longitudinal correlation length of the laser field. The SBS reflectivity of one individual hot spot is analytically computed taking into account thermal noise emission and pump depletion within the hot spot. The SBS reflectivity of the whole beam is then obtained by summing up the individual hot spot reflectivities in accordance with their statistical distribution.
Fluid models for kinetic effects on coherent nonlinear Alfvén waves. II. Numerical solutions
Authors: Medvedev M. V. , Shevchenko V. I. , Diamond P. H. , Galinsky V. L.
Published: July 2002

The influence of various kinetic effects (e.g., Landau damping, diffusive and collisional dissipation, and finite Larmor radius terms) on the nonlinear evolution of finite amplitude Alfvénic wave trains in a finite-β environment is systematically investigated using a novel, kinetic nonlinear Schrödinger (KNLS) equation. The dynamics of Alfvén waves is sensitive to the sense of polarization as well as the angle of propagation with respect to the ambient magnetic field. Numerical solution for the case with Landau damping reveals the formation of dissipative structures, which are quasi-stationary, S-polarized directional (and rotational) discontinuities which self-organize from parallel propagating, linearly polarized waves. Parallel propagating circularly polarized packets evolve to a few circularly polarized Alfvén harmonics on large scales. Stationary arc-polarized rotational discontinuities form from obliquely propagating waves. Collisional dissipation, even if weak, introduces enhanced wave damping when β is very close to unity. Cyclotron motion effects on resonant particle interactions introduce cyclotron resonance into the nonlinear Alfvén wave dynamics.
Transport and performance in DIII-D discharges with weak or negative central magnetic shear
Authors: Greenfield C. M. , Schissel D. P. , Stallard B. W. , Lazarus E. A. , Navratil G. A. , Burrell K. H. , Casper T. A. , DeBoo J. C. , Doyle E. J. , Fonck R. J. , Forest C. B. , Gohil P. , Groebner R. J. , Jakubowski M. , Lao L. L. , Murakami M. , Petty C. C. , Rettig C. L. , Rhodes T. L. , Rice B. W. , St. John H. E. , Staebler G. M. , Strait E. J. , Taylor T. S. , Turnbull A. D. , Tritz K. L. , Waltz R. E. ,   DIII-D Team
Published: July 2002

Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have been produced by combining the benefits of a hollow or weakly sheared central current profile [Phys. Plasmas 3, 1983 (1996)] with a high confinement (H mode) edge. In these discharges, low-power neutral beam injection heats the electrons during the initial current ramp, and “freezes in” a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L mode) to high (H mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high-performance phase is terminated nondisruptively at much higher βT (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang–Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E×B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium–tritium (DT) fuel mixture indicates that such plasmas could produce a DT fusion gain QDT=0.32.
Damped diocotron quasi-modes of non-neutral plasmas and inviscid fluids
Authors: Spencer Ross L. , Rasband S. Neil
Published: July 2002

Computations of damped diocotron oscillations (quasi-modes) are described for non-neutral plasmas and inviscid fluids. The numerical method implements a suggestion made by Briggs, Daugherty, and Levy some 25 years ago [Phys. Fluids 13, 421 (1970)] to push the branch line that forms the continuum into the complex ω-plane by solving the mode equation in the complex r-plane. For the special case of power-law density profiles the calculation finds the same quasi-mode frequencies found recently by Corngold [Phys. Plasmas 2, 620 (1995)]. It is found that the feature of the continuum eigenfunctions which indicates the presence of a nearby quasi-mode is continuity of the derivative of the regular part of the eigenfunctions near the singularity. The evolution of Rayleigh modes, found in density profiles with steps, is also studied as the density steps are smoothed.
Effects of ion and electron drifts on large amplitude solitary waves in a relativistic plasma
Authors: Roychoudhury Rajkumar , Venkatesan S. K. , Das Chandra
Published: July 2002

The effect of ion and electron drifts on the existence of arbitrary amplitude solitary waves is studied using Sagdeev’s pseudopotential method. It is found that if the electron drift velocity u0 is finite, solitary waves may exist for relatively large values of v0/c, where v0 is the ion drift velocity and c is the velocity of light.
Low-frequency nonlinear stationary waves and fast shocks: Hydrodynamical description
Authors: Gedalin M.
Published: July 2002

Stationary one-dimensional nonlinear waves in two-fluid hydrodynamics are studied analytically in the assumption of polytropic pressure and massless electrons. Particular attention is paid to the presence of soliton solutions, which exist when the asymptotic plasma velocity is in the range v+2<v2<vF2 or vSL2<v2<v−2, where vSL, vI, vF, and vS are the slow, intermediate, fast and sound speeds, respectively, and v−=min(vS,vI), v+=max(vS,vI). A general nonlinear solution is derived in the parametric representation. Inclusion of weak dissipation changes qualitatively the behavior of solutions allowing for fast shock-like solutions. A generalized expression for the whistler precursor wavelength is derived.
Dynamics of thin current sheets and their disruption by ballooning instabilities: A mechanism for magnetospheric substorms
Authors: Bhattacharjee A. , Ma Z. W. , Wang Xiaogang
Published: July 2002

Multipoint satellite observations indicate the development of thin current sheets and an impulsive intensification of the cross-tail current density in the growth phase at near-earth distances during a short interval (<1 min) just before onset, after a period of sluggish growth (∼0.5–1.5 h). These multiple time scales are accounted for by analysis and two-dimensional magnetohydrodynamic simulation of the magnetotail in the high-Lundquist number regime, including the earth’s dipole field. In the slow growth phase, a thin current sheet develops spanning Y points that stretch from the midtail region (∼30RE) to the near-earth region (∼10RE). This is followed by an impulsive enhancement in the current sheet amplitude due to flux pileup, consistent with observations. The stretched magnetotail with an embedded thin current sheet is found to be unstable to an ideal compressible ballooning instability with rapid spatial variation in the dawn–dusk direction. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two-dimensional magnetotail configurations containing a thin current sheet, realized during the impulsive growth phase. Line-tied boundary conditions are imposed at the ionosphere, and shown to have a strong influence on the linear stability of ballooning modes at near-earth distances. It is suggested that the ideal ballooning instability provides a possible mechanism for disrupting the cross-tail current at substorm onset.
Stability of resistive wall modes in reversed field pinches with longitudinal flow and dissipative effects
Authors: Guo S. C. , Freidberg J. P. , Nachtrieb R.
Published: July 2002

The stability of the nonresonant external kink modes in a reversed field pinch (RFP) under resistive wall boundary conditions is investigated. The effects of toroidal plasma rotation and parallel viscosity, as well as the finite β are included in the analysis. The influence of the equilibrium parameters on the mode instability is carefully studied by employing the conventional α–Θ0 equilibrium models. It is found that the behavior of the resistive wall mode (RWM) is rather sensitive to the reversal parameter F of RFPs. The effects of plasma rotation together with the viscosity and/or ion sound wave continuous spectrum damping can stabilize the RWM for certain positions of the wall relative to the plasma for both the low β (or zero β) and high β plasmas. For typical RFP operating parameters, the rotation velocity required to stabilize the modes is in the range between the ion sound wave and Alfvén wave velocities. This is larger than the natural rotation velocity in current RFPs, hence based on the above linear theory active feedback is needed for a long pulse RFP. A further nonlinear study is necessary.
Statistics of the hot spots of smoothed beams produced by random phase plates revisited
Authors: Garnier J.
Published: July 2002

This paper revisits and corrects the statistical theory of hot spots of speckle patterns such as those produced by a random phase plate. Analytical expressions are derived which are sensitively different from the previous results of Rose and DuBois [Phys. Fluids B 5, 590 (1993)]. The departure essentially originates from a careful approach which takes into account the fact that the fields are complex-valued, while the standard mathematical theory deals with the maxima of real-valued Gaussian fields. This gives rise to an enhancement of the number of the most intense hot spots. Excellent agreements between the theoretical formulas and numerical simulations are shown.
Filamentation and forward Brillouin scatter of entire smoothed and aberrated laser beams
Authors: Still C. H. , Berger R. L. , Langdon A. B. , Hinkel D. E. , Suter L. J. , Williams E. A.
Published: July 2002

Laser–plasma interactions are sensitive to both the fine-scale speckle and the larger scale envelope intensity of the beam. For some time, simulations have been done on volumes taken from part of the laser beam cross-section, and the results from multiple simulations extrapolated to predict the behavior of the entire beam. However, such extrapolation could very well miss effects of the larger scale structure on the fine-scale. The only definitive method is to simulate the entire beam. These very large calculations have not been feasible until recently, but they are now possible on massively parallel computers. Whole beam simulations show the dramatic difference in the propagation and break up of smoothed and aberrated beams.
High-precision torsional magnetometer: Application to two-dimensional electron systems
Authors: Eisenstein J. P.
Published: July 2002

A dc torsional magnetometer for use in high magnetic fields is described. With a resolution of 10−12 J/T at 5 T and excellent rejection of background moments, this device has been used to study the de Haas–van Alphen effect in two-dimensional electron systems. This resolution is about 100 times that obtained with a commercially available superconducting quantum interference device magnetometer. The device is useful over a wide temperature range including that below 1 K.
Hydrogen localization near boron in silicon
Authors: Pankove J. I. , Zanzucchi P. J. , Magee C. W. , Lucovsky G.
Published: July 2002

Several models of boron neutralized by atomic hydrogen in silicon were tested by secondary ion mass spectrometry and infrared spectrometry. The hydrogen concentration is comparable to that of boron. Boron neutralization appears as a drop in free-carrier absorption and as an increase in resistivity. A new infrared vibrational mode attributed to 〈111〉 vibrations of H tied to Si appears at 1875 cm−1.
Measurement and reduction of interface states at the recrystallized silicon-underlying insulator interface
Authors: Sturm James C. , Plummer James D. , Gibbons James F.
Published: July 2002

Deep level transient spectroscopy and capacitance-voltage measurements have been performed on an inverted metal-oxide-semiconductor (MOS) capacitor structure to measure the interface state density at the recrystallized silicon-underlying insulator interface. The effects of different recrystallization caps, annealing steps, and different underlying oxides have been investigated. An interface state density in the mid 1010 cm−2 eV−1 range can be consistently obtained, enabling well-behaved MOS transistor channels on the bottom of the recrystallized films.
Hydrogen absorption and desorption in Nd2Fe14B
Authors: Cadogan J. M. , Coey J. M. D.
Published: July 2002

Absorption and desorption of hydrogen by the permanent magnet material Nd2Fe14B have been studied using a microcomputer-controlled, thermomanometric analyzer with an initial hydrogen pressure of approximately 1 bar. Primary absorption occurs in the range 200–300 °C and results in the formation of a stable hydride with increased lattice parameters and Curie temperature. Subsequent hydrogen desorption, which is virtually complete by 650 °C, leaves the sample in an intermediate state. A second absorption step at about 720 °C corresponds to disproportionation of the Nd2Fe14B phase into α-Fe, Fe2B, and NdH2. Desorption of hydrogen from a hydrogenated sample heated in vacuum is maximum at 300 °C.
Unpinned (100) GaAs surfaces in air using photochemistry
Authors: Offsey S. D. , Woodall J. M. , Warren A. C. , Kirchner P. D. , Chappell T. I. , Pettit G. D.
Published: July 2002

We have unpinned the Fermi level at the surface of both n- and p-type (100) GaAs in air. Light-induced photochemistry between GaAs and water unpins the surface Fermi level by reducing the surface state density. Excitation photoluminescence spectroscopy shows a substantial decrease in both surface band bending and surface recombination velocity in treated samples, consistent with a greatly reduced surface state density (≂1011 cm−2). Capacitance-voltage measurements on metal-insulator-semiconductor structures corroborate this reduction in surface state density and show that the band bending may be controlled externally, indicating an unpinned Fermi level at the insulator/GaAs interface. We discuss a possible unpinning mechanism.
Ballistic avalanche photodiodes: Ultralow noise avalanche diodes with nearly equal ionization probabilities
Authors: Hollenhorst J. N.
Published: July 2002

Avalanche photodiode structures are proposed which theoretically exhibit very low excess noise. In these designs, the hole and electron ionization probabilities are made very close to unity for a single pass of the multiplication region. This is accomplished by ballistic injection across the gain region. The structures are less complex than other proposed low noise avalanche photodiode structures.
Molecular beam epitaxial growth of high structural perfection CdTe on Si using a (Ca,Ba)F2 buffer layer
Authors: Zogg H. , Blunier S.
Published: July 2002

Epitaxial CdTe has been grown onto Si(111) by molecular beam epitaxy (MBE) with the aid of a graded CaF2-BaF2 buffer layer. The buffer of ∼2000 Å thickness was used to overcome the large lattice mismatch of 19%; it was deposited by MBE in a separate system. The ∼10-μm-thick CdTe films exhibited specular surfaces and showed strong photoluminescence. The width of the near-band-edge peak at 77 K was 12 meV, and (333) x-ray lines were about 80 arc s wide. These values indicate a high structural quality, comparable to well-known CdTe layers on alternate substrates like GaAs, InSb, or sapphire.
Excitation mechanism in thin-film electroluminescent devices
Authors: Okamoto Kenji , Miura Shoshin
Published: July 2002

Time-resolved emission spectra of ZnS:TbF thin films in photoluminescence and electroluminescence were measured and compared within a relatively short delay. It was concluded that the excitation mechanism of the Tb center involves energy transfer from the ZnS host in which the Tb-related center acts as an efficient energy source for the excitation of the 4f8 electronic system in the Tb3+ ions. In addition to the ZnS:TbF thin film, a similar excitation mechanism was found to exist in the ZnS:Mn thin film. It is proposed that the TbF complex center and Mn act as an isoelectronic center or deep electron trap in ZnS thin films.
Laser induced disordering of GaAs-AlGaAs superlattice and incorporation of Si impurity
Authors: Epler J. E. , Burnham R. D. , Thornton R. L. , Paoli T. L. , Bashaw M. C.
Published: July 2002

A scanned Ar+ laser beam is demonstrated to be an effective way to selectively disorder very localized regions of an AlGaAs-GaAs superlattice. As the focused laser beam scans across the sample, the superlattice rapidly melts and then regrows. Scanning electron microscope images of the scan cross section show a micron-sized region of nearly homogeneous composition with a sharp (<400 Å) transition to as-grown superlattice material. In addition, dopants such as Si deposited on the surface in an encapsulation layer are incorporated into the melt in high concentrations during the scanning process. In this way, the recrystallized material can serve as a patterned source for impurity induced disordering during a standard thermal anneal. The depth and width of the disordered region are given as a function of laser power measured before and after the thermal anneal (850 °C, 6 h).
Growth kinetics of molecular beam epitaxially grown GaAs/Al0.3Ga0.7As (100) normal and inverted interfaces in thin single quantum well structures examined via photoluminescence studies
Authors: Voillot F. , Madhukar A. , Tang W. C. , Thomsen M. , Kim J. Y. , Chen P.
Published: July 2002

The role of the relative surface kinetics of Ga and Al in determining the nature of normal and inverted interfaces defining GaAs/Al0.3Ga0.7As thin single quantum well (SQW) structures is examined via photoluminescence and excitation spectra studies on SQW structures grown under conditions determined by reflection high-energy electron diffraction intensity dynamics to shed specific light on this issue. Results are found to be in conformity with the role of surface kinetics exemplified by computer simulations of growth and underscore the critical importance of controlling both the structural and chemical nature of interfaces via choice of optimized growth conditions.
High-corrosion-resistant magneto-optical recording media using TbFeCoTi films
Authors: Kobayashi M. , Asano M. , Maeno Y. , Oishi K. , Kawamura K.
Published: July 2002

The effects of adding Ti to TbFe and TbFeCo films with and without protective layers were studied. The addition of Ti improved the corrosion resistance of the films. The transmittance of TbFeCo films was reduced by a factor of 10 by the substitution of 5% Ti for Fe, and by a factor of 100 by the substitution of 10% Ti for Fe. Pit formations in TbFeCo during aging in a corrosive condition and immersion in a NaCl solution were almost completely eliminated by the substitution of 5% Ti for Fe. The Kerr rotation angle (θk) decreased slightly in TbFeCo by the substitution of 10% Ti for Fe, while improving the corrosion resistance.
Activation efficiency improvement in Si-implanted GaAs by P co-implantation
Authors: Hyuga Fumiaki , Yamazaki Hajime , Watanabe Kazuo , Osaka Jiro
Published: July 2002

Co-implantation effects of N, P, and As are studied for Si-implanted GaAs by the Hall effect and photoluminescence measurements. The P co-implantation enhances and homogenizes the activation efficiency of the implanted Si, and decreases photoluminescence peak intensities of the SiAs and the GaAs acceptors. It also suppresses the variation of the Si activation efficiency among the crystal ingots by half. These results indicate that P co-implantation is a promising method for fabricating active layers of high-performance GaAs large scale integrated circuits.
Stable, self-aligned TiNxOy/TiSi2 contact formation for submicron device applications
Authors: Ku Y. H. , Louis E. , Shih D. K. , Lee S. K. , Kwong D. L. , Alvi N. S.
Published: July 2002

The formation of the TiNxOy/TiSi2 bilayer on Si by rapid thermal nitridation of titanium silicide in NH3 has been studied. The chemical stability in dilute HF and the effectiveness of TiNxOy on TiSi2 as a diffusion barrier for Al are discussed. The results show that this bilayer has good chemical stability in dilute HF at least for 60 s and Al/TiNxOy/TiSi2/Si is thermally stable up to 500 °C for 30 min sintering.