bluuebunny/crossref_metadata_2025 huggingface dataset:
A statistically derived system of equations for turbulent shear flows
Authors: Yoshizawa Akira
Published: July 2002

A system of equations governing turbulent shear flows is discussed. In this system the mean velocity, the mean pressure, and four statistical quantities related to the fluctuating field constitute the fundamental quantities for shear flows. A closed system of equations for these quantities is derived statistically with the aid of the two-scale direct-interaction approximation, where the Reynolds stress is expressed in the form of the eddy-viscosity representation. On this basis, some turbulence models are discussed from the statistical viewpoint.
A numerical study of nonlinear Alfvén waves and solitons
Authors: Spangler Steven R. , Sheerin James P. , Payne Gerald L.
Published: July 2002

Finite-amplitude Alfvén waves can be modeled by a nonlinear wave equation termed the derivative nonlinear Schrödinger equation. A computer program has been developed that solves the derivative nonlinear Schrödinger equation via the ‘‘split-step’’ Fourier method. This program has been used to investigate a number of topics in the area of nonlinear Alfvén waves. When analytic envelope solitons are used as initial conditions, the wave packets propagate without distortion and with the expected speed–amplitude relation. When an arbitrary, amplitude-modulated wave is used as an initial condition, the results depend strongly on the β of the plasma and the polarization of the wave. For a left circularly polarized wave in a β<1 plasma, or a right circularly polarized wave with β>1, a collapse instability has been observed in which the wave amplitude increases and modulation scale decreases. For other combinations of polarization and value of β, the wave packet tends to broaden, eliminating the initial modulation.
The stability of solitary waves
Authors: Tanaka Mitsuhiro
Published: July 2002

The stability of finite amplitude, two-dimensional solitary waves of permanent form in water of uniform depth with respect to two-dimensional infinitesimal disturbances is investigated. It is numerically confirmed that the recent analytical results obtained by Saffman (submitted to J. Fluid Mech.) for the ‘‘superharmonic’’ instability of periodic waves hold also in the case of solitary waves.
Fundamental ion-cyclotron frequency heating in tokamaks
Authors: Fruchtman A. , Weitzner H.
Published: July 2002

The interaction of ions with waves at the fundamental ion-cyclotron frequency in tokamaks is studied without the assumption of geometrical optics. Instead, two small parameters, ε1=ρ/λ and ε2=BP/BT, are introduced, where ρ is the Larmor radius, λ is the wavelength, and BP and BT are the poloidal and toroidal magnetic fields. The heating at the resonance surface is studied for a given incoming wave without considering the problem of accessibility. The case ε2≫ε1 is studied in detail and a boundary layer analysis is performed at the resonance surface. The cold plasma theory is not valid at the resonance surface and the currents and fields are found by solving the Vlasov–Maxwell equations. The current is of nonlocal form, so that an integrodifferential equation is derived and solved numerically. Unlike the case of mirror geometry, in the tokamak the electric field is not constant across the boundary layer. The profiles of the electric field and energy flux are presented. It is shown that all the incoming flux is absorbed, and for these particular values of parameters (ε2≫ε1), there is strong heating at the fundamental resonance. In addition, the case when ε2 is not much larger than ε1 is discussed and it is shown that there is not much heating.
Formation of shocklike modified Korteweg–de Vries solitons: Application to double layers
Authors: Chanteur G. , Raadu M.
Published: July 2002

In some commonly occurring situations, such as a stratified fluid with two layers or a plasma with a two-temperature electron component, the long waves are adequately described by a Korteweg–de Vries (KdV) equation having positive or negative solitary wave solutions, depending upon the sign of the quadratic term. For critical values of the physical parameters, the quadratic nonlinearity vanishes and the KdV equation is replaced by a modified KdV (or mKdV) equation. This paper concerns the mKdV equation having a cubic nonlinearity with a negative coefficient. The initial value problem with different asymptotic levels, u1 far to the left and u2 far to the right, is investigated both analytically and numerically, for this mKdV− equation. The necessary and sufficient condition u1u2<0 is demonstrated for a shocklike solution to form, also named double layer (dl) in plasma physics.
Local vortex pairing in a free shear layer
Authors: Moon H. T. , Weidman P. D.
Published: July 2002

The evolution of large scale vortices as coherent structures in a free shear layer and the corresponding energetics in Fourier space are investigated numerically. The results confirm that vortex pairing as a result of selective energy transfer directly to the subharmonic is a rare event when other instability modes are present with random phases. For this naturally perturbed shear flow, vortices are observed to coalesce locally and intermittently in a manner that leaves isolated vortices between paired states. The spatial and temporal intermittency results in a gradual energy transfer from the linearly unstable mode to larger length scales.
The sedimentation of nondilute suspensions in inclined settlers
Authors: Borhan A. , Acrivos A.
Published: July 2002

The base-state convective flows, which are set up when a nondilute sedimenting suspension is placed beneath an inclined wall, are analyzed theoretically using a two-fluid model. Their hydrodynamic stability and the corresponding spatial growth of small two-dimensional disturbances at the clear fluid–suspension interface are then determined over the entire range of the governing parameters through numerical solutions of the relevant Orr–Sommerfeld equations. Two mechanisms for the growth of instability waves at the interface are identified. The results demonstrate that the base-state flow becomes more unstable as inertial effects in the base state become more pronounced and thus, contrary to what has been suggested by earlier investigators, there is no restabilization as the base state approaches the inviscid limit. Increasing the concentration of the suspension is found to have a stabilizing effect on the two-phase interface, particularly when inertial effects dominate in the base state. Similarly, increasing the angle of inclination enhances the stability of the interface when viscous forces are dominant in the base flow.
Vortex breakdown incipience: Theoretical considerations
Authors: Berger S. A. , Erlebacher G.
Published: July 2002

The sensitivity of the onset and the location of vortex breakdowns in concentrated vortex cores, and the pronounced tendency of the breakdowns to move upstream have been characteristic observations of experimental investigations; they have also been features of numerical simulations and led to questions about the validity of these simulations. This movement upstream may be a migration in time for fixed values of the relevant parameters, or the movement of the breakdown location closer to the entrance to the flow or computational domain with small changes in these parameters. This behavior seems to be inconsistent with the strong time-like axial evolution of the flow, as expressed explicitly, for example, by the quasicylindrical approximate equations for this flow. An order-of-magnitude analysis of the equations of motion near breakdown leads to a modified set of governing equations, analysis of which demonstrates that the interplay between radial inertial, pressure, and viscous forces gives an elliptic character to these concentrated swirling flows. Analytical, asymptotic, and numerical solutions of a simplified nonlinear equation are presented; these qualitatively exhibit the features of vortex onset and location noted above.
The single-particle distribution function for rapid granular shear flows of smooth inelastic disks
Authors: Goldhirsch I. , Tan M-L.
Published: July 2002

The velocity distribution function, f1, for a (linear) shear flow of a system of rigid inelastically colliding disks in a plane is measured by applying a novel algorithm to results of (MD) simulations involving 200 000 particles. The need to consider such a relatively large system is explained. It is found that f1 is well fitted by an exponent of a second-order polynomial in the norm of the fluctuating velocities with angle-dependent coefficients (which also depend on the density and the granular temperature). Other characterizations of the system studied in this paper are presented as background material. A hitherto unnoticed property of systems with Lees–Edwards boundary conditions has been discovered and its origin is briefly explained.
Explosive breakup of a liquid jet by a swirling coaxial gas jet
Authors: Hopfinger E. J. , Lasheras J. C.
Published: July 2002

The breakup of a round water jet by a swirling coaxial annular air jet, issuing from convergent jet nozzles has been studied experimentally. The intensity of the swirl and the water to air mass flux ratio have been varied over a large range. It was found that the liquid jet is little affected by the swirl when the swirl number, here defined by the ratio of tangential to axial air jet nozzle velocities, is below a critical value (Scr). Just above this value the liquid jet undergoes an explosive radial expansion and disintegration. A simple model shows that the physically relevant parameters are the gas to liquid momentum flux ratio M and the ratio of the nozzle’s diameters. For small momentum flux ratios, Scr was found to depend on M−1/2 until an asymptotic constant value is reached at large M. Surface tension has no effect on the breakup when air velocities are large, however, membrane-type breakup is dominant at the lower air velocities when the aerodynamic Weber number is of order 100 or less.
Flow between a stationary and a rotating disk shrouded by a co-rotating cylinder
Authors: Lopez J. M.
Published: July 2002

Boundary layers on stationary and rotating disks have received much attention since von Kármán’s [Z. Angew. Math. Mech. 1, 233 (1921)] and Bödewadt’s [Z. Angew. Math. Mech. 20, 241 (1940)] studies of the cases with disks of infinite radius. Theoretical treatments have focused on similarity treatments leading to conflicting ideas about existence and uniqueness, and where self-similar solutions exist, whether they are physically realizable. The coupling between the boundary layer flows and the interior flow between them, while being of practical importance in a variety of situations such as turbomachinery and ocean circulations, is not well understood. Here, a numerical treatment of the axisymmetric Navier–Stokes equations, together with some experiments for the case of finite stationary and rotating disks bounded by a co-rotating sidewall is presented. We show that in the long time limit, solutions are steady and essentially self-similar. Yet the transients are not. In particular, axisymmetric waves propagate in the stationary disk boundary layer when the vortex lines entering the boundary layer develop inflection points, and there are subsequent eruptions of vortical flow out of the boundary layer deep into the interior at large Reynolds numbers.
Determination of the constant coefficient in scale similarity models of turbulence
Authors: Cook Andrew W.
Published: July 2002

Scale similarity models are discussed for the subgrid-scale stress tensor and subgrid-scale variance of a passive scalar in a turbulent flow. The modeling involves sampling of the resolved fields by applying a spatial filter at a scale greater than the grid spacing of the computational mesh. A method is described for determining the constant of proportionality present in each model. The model constant for the subgrid-scale stress tensor is determined such that the subgrid-scale turbulent kinetic energy is exact on average. The constant is shown to depend on the size of the sampling filter, the turbulence Reynolds number and the degree of resolution of the large eddies.
Wall-function treatment in pdf methods for turbulent flows
Authors: Dreeben Thomas D. , Pope Stephen B.
Published: July 2002

A wall-function boundary condition is developed for the pdf/Monte Carlo method. Like traditional wall functions, this reproduces the logarithmic velocity profile and shear stress in equilibrium flow conditions. A constant-stress analysis for the pdf, and a linear-stress analysis for the first two moments of the pdf are developed as the basis for this wall-function approach. Stable and accurate boundary conditions are derived and demonstrated with fully-developed channel flow.
Taylor–Couette flow with buoyancy: Onset of spiral flow
Authors: Kuo Dah-Chyi , Ball Kenneth S.
Published: July 2002

Numerical simulations of the effects of buoyancy on the stability and morphology of Taylor–Couette flow have been conducted. The three-dimensional equations of motion are discretized using a hybrid Chebyshev collocation/Fourier spectral method. The problem geometry consists of an air-filled vertical annulus with radius ratio, η=ri/ro=0.5 (where ri and ro are the inner and outer radii, respectively), and aspect ratio, Γ=L/(ro−ri)=10 (where L is the height of the annulus). The flow is generated by combined heating and rotation of the inner cylinder. Results for various values of the Reynolds number, Re, and Grashof number, Gr, show several bifurcations of the system. The most notable change in flow structure with increasing rotational effects is the onset of spiral flow in certain parameter ranges. Compared to existing analytical and experimental results, the current numerical results show good agreement.
The generation of two-dimensional vortices by transverse oscillation of a soap film
Authors: Afenchenko V. O. , Ezersky A. B. , Kiyashko S. V. , Rabinovich M. I. , Weidman P. D.
Published: July 2002

An experimental investigation of the dynamics of horizontal soap films stretched over circular or square boundaries undergoing periodic transverse oscillations at frequencies in the range 20–200 Hz is reported. Concomitant with modes of transverse flexural oscillations, it was observed that two-dimensional vortices in the plane of the film are excited. The vortices may be either (i) large, scaling with the size of the cavity or (ii) small, localized at a wavelength or half-wavelength of the membrane modes. In the experiments a stable generation of one, two, …, ten pairs of counter-rotating vortices were observed in finite regions of amplitude-frequency parameter space. The circulation strength of vortices in a given vortex pattern increases with increasing external forcing and with decreasing soap film thickness. A theoretical model based on the wave-boundary interaction of excited Marangoni waves reveals a vorticity generation mechanism active in vibrating soap films. This model shows that vorticity is generated throughout the entire liquid volume by viscous diffusion, and qualitatively reproduces many steady vortex patterns observed in the experiment. However, the model cannot explain the existence of the sometimes intense vortices observed far from the film boundary that do not appear to be generated by diffusive processes.
Lobe dynamics applied to barotropic Rossby-wave breaking
Authors: Koh Tieh-Yong , Plumb R. Alan
Published: July 2002

We applied the methods of lobe dynamics to the problem of transport across the edge of a barotropic vortex patch. The model used captures the essential dynamics of filament shedding in the wintertime stratospheric polar vortex. Two approaches were adopted for the problem: (1) the dominant periodic component of the vortical flow was identified and conventional lobe dynamics methods for periodic dynamical systems were applied to it; (2) the full aperiodic, dynamically consistent flow was retained and a modified brand of lobe dynamics was used to quantify the transport. Our results show that in the periodic case, much reversible transport occurs across the lobe dynamical boundary due to overlapping intruding and extruding lobes. In the aperiodic case, a small amount of intrusion was noted, contrary to the well-established fact that potential vorticity shedding in barotropic vortices is uniquely outwards. In our discussion, we argue that while lobe dynamics provides a rigorous framework for quantifying transport across the lobe dynamical boundary, this boundary may not be appropriate for quantifying transport across internal transport barriers, such as the stratospheric polar vortex edge.
Two-dimensional Navier–Stokes simulations of gaseous mixtures induced by Richtmyer–Meshkov instability
Authors: Mügler Claude , Gauthier Serge
Published: July 2002

Two-dimensional numerical calculations of the fluid instability of shock-accelerated interfaces between a heavy fluid and a light one are carried out in order to simulate experiments performed by Poggi et al. [Phys. Fluids 10, 2698 (1998)]. In these experiments, the laser Doppler anemometry technique gives measurements of the fluctuating velocity. Experimental data show that a turbulent mixing zone is generated by the incident shock wave. This turbulent regime is reproduced by two-dimensional calculations. Before the first reshock, several quantities in the mixing zone, such as bubble and spike fronts, turbulent kinetic energy, enstrophy, adopt a quasi self-similar behavior versus time. In particular, we can see in numerical simulations the decay of the turbulent kinetic energy before the first reflected shock wave–mixing-zone interaction and its strong enhancement by reshocks. Furthermore, spectral analysis of the numerical results exhibit a k−3 energy spectrum. Experimental measurements also show that the turbulent boundary layers which develop on the shock-tube walls accelerate the fluid flow in the middle of the tube. Numerical simulations clearly reproduce both this acceleration and the lambda-shock structure observed in experiments.
Non-neutral ion plasmas and crystals, laser cooling, and atomic clocks*
Authors: Bollinger J. J. , Wineland D. J. , Dubin Daniel H. E.
Published: July 2002

Experimental work which uses Penning and Paul traps to confine non-neutral ion plasmas is discussed. Penning traps use a static uniform magnetic field and a static electric field to confine ions. The Paul trap uses the ponderomotive force from inhomogeneous radio-frequency fields to confine ions to a region of minimum field strength. In many atomic physics experiments, these traps are designed to produce a harmonic restoring force for small numbers of stored ions (<104). Under these conditions and at low temperatures, both traps produce plasmas with simple shapes whose mode properties can be calculated exactly. Laser cooling has been used to reduce the temperature of trapped ions to less than 10 mK with ion spacings less than 20 μm. At such temperatures and interion spacings, the Coulomb potential energy between nearest neighbor ions is greater than the ion thermal energy and the ions exhibit spatial correlations characteristic of a liquid or crystal. Laser beams also apply a torque which, by changing the plasma angular momentum, changes the plasma density. Atomic clocks are an important application of ion trap plasmas. Better control of the plasma dynamics will reduce fluctuations in the relativistic time dilation, yielding better clocks.
Turbulent edge transport in the Princeton Beta Experiment-Modified high confinement mode
Authors: Tynan G. R. , Schmitz L. , Blush L. , Boedo J. A. , Conn R. W. , Doerner R. , Lehmer R. , Moyer R. , Kugel H. , Bell R. , Kaye S. , Okabayashi M. , Sesnic S. , Sun Y. ,   the PBX-M Group
Published: July 2002

The first probe measurements of edge turbulence and transport in a neutral beam induced high confinement mode (H-mode) are reported. A strong negative radial electric field is directly observed in H-mode. A transient suppression of normalized ion saturation and floating potential fluctuation levels occurs at the low confinement mode to high confinement mode (L–H) transition, followed by a recovery to near low mode (L-mode) levels. The average poloidal wave number and the poloidal wave-number spectral width are decreased, and the correlation between fluctuating density and potential is reduced. A large-amplitude coherent oscillation, localized to the strong radial electric field region, is observed in H-mode but does not cause transport. In H-mode the effective turbulent diffusion coefficient is reduced by an order of magnitude inside the last closed flux surface and in the scrape-off layer. The results are compared with a heuristic model of turbulence suppression by velocity-shear stabilization.
Two-harmonic prebunching of electrons in multicavity gyrodevices
Authors: Nusinovich G. S. , Dumbrajs O.
Published: July 2002

The theory of multicavity gyrodevices has been developed for the case when two prebunching cavities may operate at different harmonics of cyclotron frequency. It is shown that when the first cavity operates at the fundamental and the second cavity at the second harmonic, the harmonic content in an electron beam current and the efficiency of operation in the third, output cavity are higher than these when two first cavities operate at the fundamental. The efficiency of operation of the output cavity at the first four cyclotron harmonics was studied with the account for electron velocity spread for both, single- and two-harmonic prebunching cases. The results obtained permit one to estimate the efficiency of operation of multicavity gyrodevices at cyclotron harmonics and its sensitivity to electron velocity spread.
Steady-state improved confinement studies in the JT-60U tokamak
Authors: Fukuda T. ,   the JT-60 Team
Published: July 2002

As a result of dynamic control of the heat deposition and current profile, notably a large fusion product 4×1020 m−3 s keV (equivalent QDT=0.25) was sustained for 1.5 s in the high-confinement mode (H-mode) discharge accompanied with the edge localized modes (ELM), while the highest performance achieved transiently was 1.2×1021 m−3 s keV (equivalent QDT=0.46). A doubly formed transport barrier at the q=3 surface and near the edge is essential for the improvement in confinement. It has also been found that the active shear formation in the toroidal flow velocity, as well as the magnetic shear, can increase the critical edge pressure at the onset of ELMs, which constrain the maximum fusion performance. Further extension to the high-βN (=〈βt〉apBT/Ip) regime yielded the values of βN=2.9, βp=2.6 and the H-factor (=τE/τEITER-89P) of 2.5 for 0.7 s under the full current-drive condition. In these high-βN discharges, the bootstrap component was 74% of the total plasma current before the beam injection, while the neutral beam current drive (NBCD) fraction was 37%. It is stressed that effective current drive in the central region, which can sustain the peaked current profile against the bootstrap component, is indispensable for the efficient steady-state operation. Major results from the confinement physics studies, such as the characteristics of the primary barrier and the turbulence transport across the secondary barrier, are also addressed in this paper.
Study of the power exhaust and the role of impurities in the Torus Experiment for Technological Oriented Research (TEXTOR)
Authors: Pospieszczyk A. , Samm U. , Bertschinger G. , Bogen P. , Claassen H. A. , Esser G. , Gerhauser H. , Hey J. D. , Hintz E. , Könen L. , Lie Y. T. , Rusbüldt D. , Schorn R. P. , Schweer B. , Tokar M. , Winter J. ,   the TEXTOR Teamc), Durodie F. , Koch R. , Messiaen A. M. , Ongena J. , Telesca G. , Vanderplas R. E. , van Nieuwenhove R. , van Oost G. , van Wassenhove G. , Weynants R. R.
Published: July 2002

Controlled application of radiating impurities in the boundary layer can help to solve the problem of power exhaust in a fusion reactor. Experiments in the Torus Experiment for Technological Oriented Research (TEXTOR) [J. Nucl. Mater. 145–147, 3 (1987)] are presented, which show that impurities with sufficiently high atomic number (≥10) are well suited for this purpose. Injection of neon, a gas recycled at the wall, enabled the establishment of a quasistationary radiating boundary layer, from which more than 90% of the input power was emitted. The required neon density was established by means of a feedback control for the neon influx, which was made possible by the toroidal pump limiter Advanced Limiter Technology (ALT-II) [J. Nucl. Mater. 162–164, 115 (1989)]. Alternatively, or in addition silicon was introduced as a condensing element—either by surface reactions from siliconized walls or by silane [SiH(D)4] injection—which revealed self-controlling mechanisms effective with changing plasma parameters. In neither case was a significant increase in central impurity concentration observed and good energy confinement time was maintained up to the highest plasma densities. Based on the information from various refined edge diagnostics, the underlying mechanisms for the buildup of a radiating plasma mantle and the interdependences of neon and silicon on other impurities are discussed.
Helium transport and exhaust studies in enhanced confinement regimes in DIII-D
Authors: Wade M. R. , Hillis D. L. , Hogan J. T. , Maingi R. , Menon M. M. , Mahdavi M. A. , West W. P. , Burrell K. H. , Gohil P. , Groebner R. J. , Hong R.-M. , Kellman D. H. , Phillips J. C. , Seraydarian R. P. ,   the DIII-D Team, Finkenthal D. F.
Published: July 2002

A better understanding of helium transport in the plasma core and edge in enhanced confinement regimes is now emerging from recent experimental studies on DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. Overall, the results are encouraging. Significant helium exhaust (τHe*/τE∼11) has been obtained in a diverted, H-mode plasma with edge-localized modes (ELM’s) simultaneous with a central source of helium. There is no evidence of central peaking of the helium density profile even in the presence of this central source. Detailed analysis of the helium profile evolution indicates that the exhaust rate is limited by the exhaust efficiency of the pump (∼5%) and not by the intrinsic helium transport properties of the plasma. Perturbative helium transport studies using gas puffing have shown that DHe/χeff ∼1 in all confinement regimes studied to date (including H mode and VH mode). Furthermore, there is no evidence of preferential accumulation of helium in any of these regimes.
Modeling of parasitic current collection by solar arrays in low-earth orbit
Authors: Davis V. A. , Gardner B. M. , Guidice D. A.
Published: July 2002

In this paper we describe the development of a model of the electron current collected by solar arrays from the ionospheric plasma. This model will assist spacecraft designers in minimizing the impact of plasma interactions on spacecraft operations as they move to higher-voltage solar arrays. The model was developed by first examining in detail the physical processes of importance and then finding an analytic fit to the results over the parameter range of interest. The analytic model is validated by comparison with flight data from the Photovoltaic Array for Space Power Plus diagnostics (PASP Plus) flight experiment [D. A. Guidice, 34th Aerospace Sciences Meeting and Exhibit, Reno, NV, 1996, AIAA 96-0926 (American Institute of Aeronautics and Astronautics, Washington, DC, 1996)].
Discrete spectrum of Alfvén ion–ion hybrid waves
Authors: Intrator T. , Vukovic M. , Elfimov A. , Probert P. H. , Winz G.
Published: July 2002

In the Phaedrus-T tokamak [R. Majeski et al., Phys Fluids B 5, 2506 (1993)], Alfvén waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions can couple Alfvén and ion–ion hybrid waves and have a large effect on the wave numbers accessible for a given launched frequency. A discrete spectrum and toroidal damping for these modes has been identified by measuring dispersion properties at the edge. Landau damping is predicted to be large and spatially localized and to be responsible for the experimentally observed electron heating (T. Intrator et al., ‘‘Alfvén ion–ion hybrid wave heating in the Phaedrus-T tokamak,’’ to appear in Phys. Plasmas) and current drive near the core of the tokamak plasmas.
Analysis of noble gas recycling at a fusion plasma divertor
Authors: Brooks Jeffrey N.
Published: July 2002

Near-surface recycling of neon and argon atoms and ions at a divertor has been studied using impurity transport and surface interaction codes. A fixed background deuterium–tritium plasma model is used corresponding to the International Thermonuclear Experimental Reactor (ITER) [ITER EDA Agreement and Protocol 2, ITER EDA Documentation Series No. 5 (International Atomic Energy Agency, Vienna, 1994)] radiative plasma conditions (Te≤10 eV). The noble gas transport depends critically on the divertor surface material. For low-Z materials (Be and C) both neon and argon recycle many (e.g., ∼100) times before leaving the near-surface region. This is also true for an argon on tungsten combination. For neon on tungsten, however, there is low recycling. These variations are due to differences in particle and energy reflection coefficients, mass, and ionization rates. In some cases a high flux of recycling atoms is ionized within the magnetic sheath and this can change local sheath parameters. Due to inhibited backflow, high recycling, and possibly high sputtering, noble gas seeding (for purposes of enhancing radiation) may be incompatible with Be or C surfaces, for fusion reactor conditions. On the other hand, neon use appears compatible with tungsten.
Recent results from high performance and steady-state researches in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade
Authors: Kimura H. ,   the JT-60 Team
Published: July 2002

High IP hot ion high confinement (H) mode at IP up to 4.5 MA has been exploited. Sawtooth stabilization by ion cyclotron range of frequencies (ICRF) heating is effective to improve performance in this regime. The performance is limited by the onset of giant edge localized modes (ELMs). It was found that the edge pressure gradient at the onset of ELMs can be increased with increasing triangularity δ up to 0.4 at IP<1.2 MA. The normalized beta (βN) value at the ELM onset also increases from ∼1 to ∼2.8 when δ is increased from ∼0.1 to ∼0.33, respectively. In the reversed-shear operation, an internal transport barrier (ITB) appears, not only for the ions but also for the electrons. The improved confinement region is quite large (within r/a∼0.65). The highest confinement enhancement factor (H factor) and βN achieved so far are, respectively, 2.6 and 2.4 for reversed-shear plasmas. A scaling law of the H-mode threshold power, which is consistent with the International Thermonuclear Experimental Reactor (ITER) [Y. Shimomura, Phys. Plasmas 1, 1612 (1994)] scaling, is derived. The power threshold for ITB formation in the high βp mode depends on the electron density, but not on the toroidal field. It is validated that the control of the toroidal rotation is effective to control toroidicity-induced Alfvén eigenmodes (TAE modes).
Three-dimensional simulations of the parallel velocity shear instability
Authors: McCarthy D. R. , Booth A. E. , Drake J. F. , Guzdar P. N.
Published: July 2002

The nonlinear mode structure and anomalous momentum transport of the electrostatic parallel velocity shear instability in a uniformly magnetized slab were studied using three-dimensional (3-D) nonlinear fluid simulations. Within this simplified model, the system depends upon two dimensionless parameters α=2πqRρs/Lv2, which is a ratio of the sound transit time to the linear growth time of the mode and ρ̂=ρs/LV, which is a ratio of the ion Larmor radius and the radial scale length of the parallel velocity profile. In the limit of ρ̂=0, the mode structure evolved nonlinearly to a state with m=2/α and n=1 for α<1, and m=2 and n=α for α>1 where m is the poloidal mode number and n is the toroidal mode number. The transport had a value of D∼ρscs/2 for all α. For ρ̂≠0 the system displayed a different saturated state, evolving to m=1 and n=α/2 for all α. The transport remained D∼ρscs/2 for all α. The effect of the Larmor radius is this 3-D system is similar to the well known inverse cascade in a two-dimensional incompressible fluid.
Nonlinear evolution of neoclassical double tearing mode
Authors: Yu Qingquan
Published: July 2002

A theory for the nonlinear neoclassical double tearing mode is presented. The equations governing the nonlinear evolution of the mode are obtained for both the locked islands and the islands with different rotation frequencies. The coupling of the two islands plays a destabilizing role in the island growth, especially when the phase between the two islands is locked. The perturbed bootstrap current has a strong stabilizing effect on the inner island but a weak destabilizing effect on the outer island. The inner island width is decreased when there is a relative rotation between the two islands, while the outer island is less affected by plasma rotation. The outer island width is found be an important parameter in determining the inner island width and the stability of the nonmonotonic q-profile.
Stability of negative central magnetic shear discharges in the DIII-D tokamak
Authors: Strait E. J. , Casper T. A. , Chu M. S. , Ferron J. R. , Garofalo A. , Greenfield C. M. , La Haye R. J. , Lao L. L. , Lazarus E. A. , Miller R. L. , Navratil G. A. , Ren C. , Rice B. W. , Semenov I. , Sips A. C. C. , Söldner F. X. , Stallard B. W. , Taylor T. S. , Turnbull A. D. ,   DIII-D Team
Published: July 2002

Discharges with negative central magnetic shear (NCS) hold the promise of enhanced fusion performance in advanced tokamaks. However, stability to long wavelength magnetohydrodynamic modes is needed to take advantage of the improved confinement found in NCS discharges. The stability limits seen in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] experiments depend on the pressure and current density profiles and are in good agreement with stability calculations. Discharges with a strongly peaked pressure profile reach a disruptive limit at low beta, βN=β(I/aB)−1⩽2.5 (% m T/MA), caused by an n=1 ideal internal kink mode or a global resistive instability close to the ideal stability limit. Discharges with a broad pressure profile reach a soft beta limit at significantly higher beta, βN=4 to 5, usually caused by instabilities with n>1 and usually driven near the edge of the plasma. With broad pressure profiles, the experimental stability limit is independent of the magnitude of negative shear but improves with the internal inductance, corresponding to lower current density near the edge of the plasma. Understanding of the stability limits in NCS discharges has led to record DIII-D fusion performance in discharges with a broad pressure profile and low edge current density.
Turbulent Richtmyer–Meshkov instability experiments with strong radiatively driven shocks
Authors: Dimonte Guy , Schneider Marilyn
Published: July 2002

The Richtmyer–Meshkov instability is investigated in the turbulent regime with strong radiatively driven shocks (Mach >10) on the Nova laser [Phys. Rev. Lett. 70, 1806 (1993)]. The targets are planar with near solid density and Atwood number ∼−0.9. The interfacial perturbations are three dimensional (3-D), random, and nonlinear with a broad scale distribution such that they develop into a turbulent mixing zone (TMZ). The TMZ is diagnosed radiographically using x-ray dopants localized to the center of the target to avoid edge effects. Two different diagnostic configurations yield comparable results. The total width of the TMZ is found to increase in time as H∼tΘ, with Θ∼0.5±0.1. When compared to shock tube data, this result supports the suggestion [Phys. Fluids 8, 614 (1996)] that Θ decreases with Mach number. The data are used to test turbulence models and to determine the effective drag coefficient Cd∼2.6±1.2 for potential flow mix models in the high compression regime.
Observation of vortex-like coherent structures in the edge plasma of the ADITYA tokamak
Authors: Joseph B. K. , Jha R. , Kaw P. K. , Mattoo S. K. , Rao C. V. S. , Saxena Y. C. ,   Aditya Team
Published: July 2002

Vortex-like coherent structures are observed in the edge plasma of ohmically heated ADITYA tokamak [ Phys. Rev. Lett. 69, 1375 (1992)]. The structures are observed on statistical basis when the floating potential fluctuations are analyzed using conditional averaging technique. The structures, which have dipole nature, experience stretching until their radial isolation across the limiter is destroyed. The potential fluctuation also shows non-Gaussian statistics indicating intermittency in broadband turbulence of the edge plasma.
Nonlinear kinetic simulations of stimulated Brillouin scattering
Authors: Giacone R. E. , Vu H. X.
Published: July 2002

Numerical simulation results of stimulated Brillouin scattering (SBS) using HERCULES [H. X. Vu, J. Comput. Phys. 124, 417 (1996)], an adiabatic fluid-electron particle-in-cell (PIC) code, are presented. The results of these PIC simulations are compared against fluid simulations, and good agreement is obtained for sufficiently weak laser intensities. When the laser intensity is sufficiently strong for ion trapping to be significant, PIC and fluid simulations differ substantially. The trapping time and nonlinear frequency shift obtained in the PIC simulations are in good agreement with analytical predictions. The SBS reflectivity is shown to be very sensitive to frequency mismatch between the light wave used to seed the instability and the incident laser.
Fusion plasma experiments on TFTR: A 20 year retrospective
Authors: Hawryluk R. J. , Batha S. , Blanchard W. , Beer M. , Bell M. G. , Bell R. E. , Berk H. , Bernabei S. , Bitter M. , Breizman B. , Bretz N. L. , Budny R. , Bush C. E. , Callen J. , Camp R. , Cauffman S. , Chang Z. , Cheng C. Z. , Darrow D. S. , Dendy R. O. , Dorland W. , Duong H. , Efthimion P. C. , Ernst D. , Fisch N. J. , Fisher R. , Fonck R. J. , Fredrickson E. D. , Fu G. Y. , Furth H. P. , Gorelenkov N. N. , Grek B. , Grisham L. R. , Hammett G. W. , Hanson G. R. , Herrmann H. W. , Herrmann M. C. , Hill K. W. , Hogan J. , Hosea J. C. , Houlberg W. A. , Hughes M. , Hulse R. A. , Jassby D. L. , Jobes F. C. , Johnson D. W. , Kaita R. , Kaye S. , Kim J. S. , Kissick M. , Krasilnikov A. V. , Kugel H. , Kumar A. , Leblanc B. , Levinton F. M. , Ludescher C. , Majeski R. P. , Manickam J. , Mansfield D. K. , Mazzucato E. , McChesney J. , McCune D. C. , McGuire K. M. , Meade D. M. , Medley S. S. , Mika R. , Mikkelsen D. R. , Mirnov S. V. , Mueller D. , Nagy A. , Navratil G. A. , Nazikian R. , Okabayashi M. , Park H. K. , Park W. , Paul S. F. , Pearson G. , Petrov M. P. , Phillips C. K. , Phillips M. , Ramsey A. T. , Redi M. H. , Rewoldt G. , Reznik S. , Roquemore A. L. , Rogers J. , Ruskov E. , Sabbagh S. A. , Sasao M. , Schilling G. , Schivell J. , Schmidt G. L. , Scott S. D. , Semenov I. , Skinner C. H. , Stevenson T. , Stratton B. C. , Strachan J. D. , Stodiek W. , Synakowski E. , Takahashi H. , Tang W. , Taylor G. , Thompson M. E. , Von Goeler S. , Von Halle A. , Walters R. T. , White R. , Wieland R. M. , Williams M. , Wilson J. R. , Wong K. L. , Wurden G. A. , Yamada M. , Yavorski V. , Young K. M. , Zakharov L. , Zarnstorff M. C. , Zweben S. J.
Published: July 2002

The Tokamak Fusion Test Reactor (TFTR) (R. J. Hawryluk, to be published in Rev. Mod. Phys.) experiments on high-temperature plasmas, that culminated in the study of deuterium–tritium D–T plasmas containing significant populations of energetic alpha particles, spanned over two decades from conception to completion. During the design of TFTR, the key physics issues were magnetohydrodynamic (MHD) equilibrium and stability, plasma energy transport, impurity effects, and plasma reactivity. Energetic particle physics was given less attention during this phase because, in part, of the necessity to address the issues that would create the conditions for the study of energetic particles and also the lack of diagnostics to study the energetic particles in detail. The worldwide tokamak program including the contributions from TFTR made substantial progress during the past two decades in addressing the fundamental issues affecting the performance of high-temperature plasmas and the behavior of energetic particles. The progress has been the result of the construction of new facilities, which enabled the production of high-temperature well-confined plasmas, development of sophisticated diagnostic techniques to study both the background plasma and the resulting energetic fusion products, and computational techniques to both interpret the experimental results and to predict the outcome of experiments.
A hybrid semi-Lagrangian Vlasov code. II. Numerical simulation of chirped beatwave experiment
Authors: Ghizzo A. , Bertrand P. , Lebas J. , Johnston T. W. , Shoucri M.
Published: July 2002

In this paper the results are reported of a set of numerical simulations applicable to the problem of particle acceleration by the beating of very high-frequency laser waves (the ratio of driver frequency to plasma frequency close to ω0/ωp=30), including for the first time beat frequency chirping (i.e., pump frequency linearly decreasing with time) at various rates in order to counteract relativistic electron detuning of the plasma oscillation frequency and the resulting Rosenbluth–Liu amplitude limit. This amplitude limit can indeed be evaded, giving extremely high acceleration gradients of more than 25 GeV/m, for the case of numerical parameters relevant to the UCLA experiment (University of California at Los Angeles). A chirp rate that is too fast leads to a sudden loss of frequency lock and a subsequent decay of the driven electric field, in agreement with the behavior of a similarly driven relativistic Lagrangian Rosenbluth–Liu oscillator. Even at chirp rates somewhat below this limiting value a remarkably localized and extraordinarily intense (but as yet ill-understood) electric field structure is formed, one that is unlikely to be compatible with the general requirement for smooth and coherent acceleration.
Dust-acoustic solitary waves and double layers in dusty plasma with variable dust charge and two-temperature ions
Authors: Xie Baisong , He Kaifen , Huang Zuqia
Published: July 2002

With two-temperature ions in dusty plasma, the effects of variable dust charge on small-amplitude dust-acoustic solitons and large-amplitude dust-acoustic solitary waves are investigated. It is found that both compressive and rarefactive solitons (and solitary waves), as well as double layers, exist. The introduced small, but finite, number density of the lower temperature ions provides the possibility of the coexistence of large-amplitude rarefactive with compressive dust-density solitary waves. The amplitudes of those dust-density solitary waves become smaller and the regime of Mach number is extended wider for the variable dust charge situation compared to the constant dust charge situation.
Stimulated Raman and Brillouin scattering of polarization-smoothed and temporally smoothed laser beams
Authors: Berger R. L. , Lefebvre E. , Langdon A. B. , Rothenberg J. E. , Still C. H. , Williams E. A.
Published: July 2002

Control of filamentation and stimulated Raman and Brillouin scattering is shown to be possible by use of both spatial and temporal smoothing schemes. The spatial smoothing is accomplished by the use of phase plates [Y. Kato and K. Mima, Appl. Phys. 329, 186 (1982)] and polarization smoothing [Lefebvre et al., Phys. Plasmas 5, 2701 (1998)] in which the plasma is irradiated with two orthogonally polarized, uncorrelated speckle patterns. The temporal smoothing considered here is smoothing by spectral dispersion [Skupsky et al., J. Appl. Phys. 66, 3456 (1989)] in which the speckle pattern changes on the laser coherence time scale. At the high instability gains relevant to laser fusion experiments, the effect of smoothing must include the competition among all three instabilities.
A Paul trap configuration to simulate intense non-neutral beam propagation over large distances through a periodic focusing quadrupole magnetic field
Authors: Davidson Ronald C. , Qin Hong , Shvets Gennady
Published: July 2002

This paper considers an intense non-neutral charged particle beam propagating in the z-direction through a periodic focusing quadrupole magnetic field with transverse focusing force, −κq(s)[xêx−yêy], on the beam particles. Here, s=βbct is the axial coordinate, (γb−1)mbc2 is the directed axial kinetic energy of the beam particles, qb and mb are the charge and rest mass, respectively, of a beam particle, and the oscillatory lattice coefficient satisfies κq(s+S)=κq(s), where S is the axial periodicity length of the focusing field. The particle motion in the beam frame is assumed to be nonrelativistic, and the Vlasov-Maxwell equations are employed to describe the nonlinear evolution of the distribution function fb(x,y,x′,y′,s) and the (normalized) self-field potential ψ(x,y,s)=qbφ(x,y,s)/γb3mbβb2c2 in the transverse laboratory-frame phase space (x,y,x′,y′), assuming a thin beam with characteristic radius rb≪S. It is shown that collective processes and the nonlinear transverse beam dynamics can be simulated in a compact Paul trap configuration in which a long non-neutral plasma column (L≫rp) is confined axially by applied dc voltages V̂=const on end cylinders at z=±L, and transverse confinement in the x−y plane is provided by segmented cylindrical electrodes (at radius rw) with applied oscillatory voltages ±V0(t) over 90° segments. Here, V0(t+T)=V0(t), where T=const is the oscillation period, and the oscillatory quadrupole focusing force on a particle with charge q and mass m near the cylinder axis is −mκq(t)[xêx−yêy], where κq(t)≡8qV0(t)/πmrw2.
Electron confinement in an annular Penning trap
Authors: Robertson Scott , Walch Bob
Published: July 2002

Electron confinement in an annular version of a Malmberg–Penning trap [S. Robertson and B. Walch, Rev. Sci. Instrum. 70, 2993 (1999)] has been investigated for conditions in which mobility is the dominant source of transport. A non-neutral plasma of electrons is contained in the annular region between coaxial cylinders. An axial magnetic field provides radial confinement and electric bias potentials provide axial confinement. The electric field that drives transport is determined primarily by the potential difference applied to the cylinders. The measured density decay rates have the expected dependence upon electric field, collision frequency and magnetic field and are within a factor of 2 of calculated values. Experiments are performed with an axial field of 5–18 mT, a radial electric field of 11–38 V/cm, a helium pressure of 0.01–0.13 mTorr, an initial electron density of ∼106 cm−3, and the density decay times are 1–10 ms.
Growth by molecular beam epitaxy and characterization of InAs/GaAs strained-layer superlattices
Authors: Goldstein L. , Glas F. , Marzin J. Y. , Charasse M. N. , Le Roux G.
Published: July 2002

InAs/GaAs superlattices with ultra-thin InAs (few monolayer) were grown on GaAs substrates. Nucleation of InAs occurs in a two-dimensional or a three-dimensional way depending on the growth conditions. The physical properties: x ray, transmission electron microscopy, and photoluminescence were used to characterize the different growth processes.
A new method for solving the ground-state problem in arbitrary quantum wells: Application to electron-hole quasi-bound levels in quantum wells under high electric field
Authors: Singh Jasprit
Published: July 2002

A method based on the Monte Carlo technique and variational principle is developed to study the ground-state problem in arbitrary quantum wells. A technique is described to use this method to study quasi-bound states in systems. The method is applied to AlGaAs/GaAs quantum wells subjected to high electric fields. Advantages of this approach over the conventional variational approach are identified.