bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of the eddy damped quasi-normal Markovian spectral transport theory to premixed turbulent flame propagation
Authors: Ulitsky Mark , Collins Lance R.
Published: July 2002

The eddy damped quasi-normal Markovian (EDQNM) turbulence theory was applied to a modified Kuramoto–Sivashinsky field equation to develop a spectral model for investigating the single and two-point scalar statistics associated with a flame front (treated as a passive scalar interface) propagating through isotropic turbulence. As a result of the presence of a uniform mean gradient in the scalar field, all correlations involving the scalar were found to be functions of both the wave number, k, and μ, the cosine of the angle between the k vector and the mean gradient vector. An infinite Legendre expansion separated out the wave number and angle dependencies, where the first term in each series accounted for the isotropic contribution to the correlations and the higher order terms accounted for the anisotropy introduced as a result of the mean gradient. It was found that while strong anisotropy existed in the scalar field at short times, at steady state the scalar field became nearly isotropic. A parameter study was then conducted to ascertain the effect of independently varying u′/sL and Reλ (where u′ is the rms velocity, sL is the laminar burning velocity, and Reλ is the Reynolds number based on the Taylor microscale). The turbulent burning velocity increased with increases in either u′/sL or Reλ; however, the model predicted a finite turbulent burning velocity as u′/sL→∞, even though flame quenching was not accounted for. This finite asymptote for the burning velocity was traced to the constitutive relationship used for the flame thickness and the ratio of the Markstein length to the flame thickness. It was also shown that the dominant wrinkling of the flame surface and subsequent contribution to the turbulent burning velocity occurred at smaller and smaller length scales as the inertial range of the scalar spectrum increased. Single point models will therefore have great difficulty reproducing this significant result. Scalar spectra exhibited changes over all wave numbers as either u′/sL or Reλ was modified. Transfer spectra, which arose in the form of convolution integrals as a result of the advection and propagation processes, were also analyzed and separated into their pairwise spectral interactions to determine which nonlinear terms in each integral were dominant.
Radial segregation of granular mixtures in rotating cylinders
Authors: Khakhar D. V. , McCarthy J. J. , Ottino J. M.
Published: July 2002

Simultaneous mixing and segregation of granular materials is of considerable practical importance; the interplay among both processes is, however, poorly understood from a fundamental viewpoint. The focus of this work is radial segregation—core formation—due to density in a rotating cylinder. The flow regime considered is the cascading or continuous flow regime where a thin layer of solids flows along a nearly flat free surface, while the remaining particles rotate as a fixed bed along with the cylinder. The essence of the formation of a central segregated core of the more dense particles lies in the flow, mixing, and segregation in the cascading layer. The work involves experiments and analysis. A constitutive model for the segregation flux in cascading layers is proposed and validated by particle dynamics and Monte Carlo simulations for steady flow down an inclined plane. The model contains a single parameter, the dimensionless segregation velocity (β), which is treated as a fitting parameter here. Experimental results for the equilibrium segregation of steel balls and glass beads are presented for different fractions and different extents of filling. There is a good match between theoretical predictions and all experimental results when the value of dimensionless segregation velocity is taken to be β=2. The extent of segregation is found to increase with increase in the dimensionless segregation velocity and dimensionless diffusivity but is independent of the level of filling. Lagrangian simulations based on the theory and experiments demonstrate the competition between segregation and mixing. In the case of slow mixing, the intensity of segregation monotonically decreases to an equilibrium value; for fast mixing, however, there exists an optimal mixing time at which the best mixing is obtained.
Calculations of subsonic and supersonic turbulent reacting mixing layers using probability density function methods
Authors: Delarue B. J. , Pope S. B.
Published: July 2002

A particle method applying the probability density function (PDF) approach to turbulent compressible reacting flows is presented. The method is applied to low and high Mach number reacting plane mixing layers. Good agreement is obtained between the model calculations and the available experimental data. The PDF equation is solved using a Lagrangian Monte Carlo method. To represent the effects of compressibility on the flow, the velocity PDF formulation is extended to include thermodynamic variables such as the pressure and the internal energy. Full closure of the joint PDF transport equation is made possible in this way without coupling to a finite-difference-type solver. The stochastic differential equations (SDE) that model the evolution of Lagrangian particle properties are based on existing models for the effects of compressibility on turbulence. The chemistry studied is the fast hydrogen–fluorine reaction. For the low Mach number runs, low heat release calculations are performed with equivalence ratios different from one. Heat release is then increased to study the effect of chemical reaction on the mixing layer growth rate. The subsonic results are compared with experimental data, and good overall agreement is obtained. The calculations are then performed at a higher Mach number, and the results are compared with the subsonic results. Our purpose in this paper is not to assess the performances of existing models for compressible or reacting flows. It is rather to present a new approach extending the domain of applicability of PDF methods to high-speed combustion.
On the effect of nonuniform seeding on particle dispersion in two-dimensional mixing layers
Authors: Wang Qunzhen , Squires Kyle D. , Wang Lian-Ping
Published: July 2002

The effect of nonuniform seeding on the dispersion of fluid elements and heavy particles has been investigated in two-dimensional, incompressible mixing layers. The cross-stream dispersion of fluid elements can be enhanced using nonuniform seeding with most particles near the saddle point because of the greater lateral extent of streamlines in this region, though the increase in dispersion compared to a uniform seeding occurs only after a few vortex turnover times. Compared to fluid elements, additional mechanisms—ejection from vortex cores, separatrix crossing, and the effect of the initial particle velocity—must be considered in the analysis of nonuniform seeding on heavy particle dispersion. The influences of these additional mechanisms are first investigated in a Stuart vortex. With increasing response time, vortex ejection and separatrix crossing shift the streamwise position maximizing lateral transport towards the vortex core. While changes in the initial particle velocity increase/decrease displacement, lateral dispersion may still be enhanced by appropriate nonuniform seeding of particles near the saddle point. Numerical simulations of the incompressible Navier–Stokes equations are then used to study cross-stream dispersion in a temporally evolving two-dimensional mixing layer. Stokes numbers St in the calculations were 0.05, 1, 10, and 100 where St is defined as the ratio of the particle response time to the time scale formed using the vorticity thickness of the initial mean flow. Particles were initially distributed nonuniformly at the interface between the two streams or along a line parallel to the interface. Simulation results show that the seeding location maximizing lateral dispersion is both time and Stokes number dependent, with larger increases in dispersion for the interface seeding. For Stokes numbers of order unity cross-stream dispersion exhibits a weak dependence on initial position since particles are efficiently ejected from the vortex core with subsequent motion confined to the nearby region outside the separatrix in one of the freestreams. Simulation results also show that substantial increases in particle dispersion can be obtained using nonuniform seedings relative to that obtained from an initially uniform distribution.
Transient growth in parallel two-phase flow: Analogies and differences with single-phase flow
Authors: van Noorden T. L. , Boomkamp P. A. M. , Knaap M. C. , Verheggen T. M. M.
Published: July 2002

Transient disturbance growth in parallel two-phase flow is studied. When the disturbance growth is measured in terms of the kinetic energy norm, which is commonly used for single-phase flow, the disturbance growth function does not converge as the number of eigenmodes used in the computation increases. A solution to this problem is presented in the form of a norm that also includes the potential energy of the disturbed interface. This solution is used to examine the two-phase flow experiment by Kao and Park.
Interactions of three components and subcritical self-sustained amplification of disturbances in plane Poiseuille flow
Authors: Bergström Lars
Published: July 2002

A low-dimensional nonlinear model for the normal velocity (v) and normal vorticity (η) disturbance development in plane Poiseuille flow is studied. The study is restricted to the interactions of a pair of oblique components of the form ei(αx±βz) and the component of the form ei2βz, where α and β are streamwise and spanwise wave numbers, respectively. The disturbances considered are also assumed to be highly elongated in the streamwise direction. Owing to the non-normal properties of the basic equations, the η disturbance is first transiently amplified. Then, if the Reynolds number (R) and the initial disturbance are sufficiently large, the nonlinear interactions lead to a self-sustained process of disturbance amplification at subcritical R. For large R (R≳5000), the threshold disturbance amplitude scales like R−3. The results also strongly indicate that the nonlinear feedback from η to v is crucial for the establishment of the self-sustained process.
Extremum principles for selection in the Saffman–Taylor finger and Taylor–Saffman bubble problems
Authors: Aldushin A. P. , Matkowsky B. J.
Published: July 2002

We consider the Saffman–Taylor problem describing the displacement of one fluid by another having a smaller viscosity, in a porous medium or in a Hele–Shaw configuration, and the Taylor–Saffman problem of a bubble moving in a channel containing moving fluid. Each problem is known to possess a family of traveling wave solutions, the former corresponding to propagating fingers and the latter to propagating bubbles, with each member characterized by its own velocity and each occupying a different fraction of the channel through which it propagates. To select the “correct” member of the family of solutions we propose two related extremum principles. Employing these principles for a symmetric family with zero surface tension (σ=0) selects the solution (finger or bubble, viscous or inviscid) which happens to be the same as that obtained by taking the limit σ→0 of the nonzero isotropic surface tension problem. For other problems, e.g., perturbation by anisotropic surface tension, the fingers selected by the two approaches are not necessarily the same. We claim that the finger selected by our criteria describes what will be observed on an intermediate time scale in which the effect of perturbations is neglected, whereas the finger selected by taking the limit of vanishingly small perturbations describes what will be observed on the asymptotic time scale O(1/σ) for the problem with the perturbation included. The intermediate time scale is much shorter than the asymptotic time scale. For infinitesimal σ the asymptotic time scale may be well beyond any reasonable observational time scale.
Particle dispersion in variable density and viscosity shear flows
Authors: Soteriou Marios C. , Yang Xu
Published: July 2002

The dispersion of small dense particles by an unsteady planar shear layer formed between two streams of different velocity, density, and viscosity is investigated numerically. The two-phase flow is assumed to be in the dilute regime. The Lagrangian transport element method is employed to provide two-dimensional unaveraged simulations of the carrier flow. The evolution of the particle field is captured by computing the trajectories of individual particles using a reduced form of the equation of particle motion. The analysis focuses on the impact of the densities and viscosities of the two streams on the dispersion of particles of different Stokes numbers (St). The results verify the well established behavior between St and dispersion: maximized dispersion for intermediate St due to centrifugation of particles by the vortices into the irrotational streams, reduced dispersion at low St with particles following the flow and minimal dispersion for large St with particles remaining unaffected by the flow. Variation of the viscosities of the irrotational streams does not substantially alter the above trends except for a shift in the value of St where different behaviors are experienced. This shift is linked to the existence of an effective viscosity that controls the dispersion process. This viscosity is the one characterizing the well mixed vortical structures. Variations in the densities of the two streams decreases the dispersion of intermediate St particles as compared to the uniform density case. This behavior is due to the impact of baroclinic vorticity generation that creates asymmetries in the overall vorticity field that diminish the centrifugation of the particles into one of the two streams.
The motion of a falling liquid filament
Authors: Henderson Diane , Segur Harvey , Smolka Linda B. , Wadati Miki
Published: July 2002

When a liquid drop falls from a fluid source with a slow flow rate, it remains attached to the source by an elongating liquid filament until the filament pinches off. For many fluids, this pinch-off occurs first near the end of the filament, where the filament joins to the liquid drop. For other fluids, the filament pinches off at one or more interior points. In this paper, we study the motion of this filament, and we make two points. First, the flow in this filament is not that of a uniform jet. Instead, we show experimentally that a different solution of the Navier–Stokes equations describes the motion of this filament before it pinches off. Second, we propose a criterion for the location of the first pinch-off. In particular, we analyze the linearized stability of the exact solution, both for an inviscid fluid and for a very viscous fluid. Our criterion for pinch-off is based on this stability analysis. It correctly predicts whether a given filament pinches off first near its ends or at points within its interior for all of our experimental data.
Investigations on the vacuum current in the magnetic insulated Karlsruhe light ion facility high-energy linear induction accelerator (KALIF–HELIA)
Authors: Illy S. , Kuntz M. , Westermann T.
Published: July 2002

In order to increase the applied voltage in pulsed power ion diodes, the Karlsruhe light ion facility will be extended by a voltage adder. An important problem with such a device is how the electron loss current can be controlled in the vacuum feed. Based on a static, one-dimensional analytic model and two-dimensional particle-in-cell (PIC) simulations, a detailed knowledge of the electron flow in the voltage adder is obtained. Time-dependent simulations support qualitatively the observation of laminar electron flow. The electrons form a band corresponding to the section on which they originate. It is demonstrated that with the introduction of guard rings, appropriately positioned in the feed, the electron loss current can be reduced by more than 50%.
Model for collisional fast ion diffusion into Tokamak Fusion Test Reactor loss cone
Authors: Chang C. S. , Zweben S. J. , Schivell J. , Budny R. , Scott S.
Published: July 2002

An analytic model is developed to estimate the classical pitch angle scattering loss of energetic fusion product ions into prompt loss orbits in a tokamak geometry. The result is applied to alpha particles produced by deutrium–tritium fusion reactions in a plasma condition relevant to Tokamak Fusion Test Reactor (TFTR) [Proceedings of the 14th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Würzburg, 1992 (International Atomic Energy Agency Vienna, 1992)]. A poloidal angular distribution of collisional fast ion loss at the first wall is obtained and the numerical result from the transp [‘‘A standard D–T supershot simulation’’ (to be published in Nucl. Fusion, 1994)] code is discussed. The present model includes the effect that the prompt loss boundary moves away from the slowing-down path due to reduction in banana thickness, which enables one to understand, for the first time, the dependence of the collisional loss rate on Zeff.
Formation of vortex chains in a nonuniform magnetized electron plasma
Authors: Shukla P. K. , Srinivas J. , Murtaza G. , Saleem H.
Published: July 2002

It is shown that the equations governing the dynamics of weakly interacting medium-frequency electrostatic waves in a nonuniform magnetoplasma with a fixed ion background can have localized vortex chain solutions.
Ideal magnetohydrodynamic stability in the presence of a resistive wall
Authors: Bhattacharyya S. N.
Published: July 2002

Using the Galerkin method, the equations governing the linear stability of an ideal plasma in the presence of a thin resistive wall is reduced to a standard matrix eigenvalue problem. This can be used to compute the entire spectrum of normal modes. Stability is shown to be governed by an energy principle. As an application, the Galerkin formulation is used to study the stability of an elliptical cross-section straight tokamak, surrounded by a thin resistive wall, to rigid vertical displacements. The Galerkin formulation is also generalized to include equilibrium mass flow of the plasma and allow for arbitrary thickness of the resistive wall. Stability is again shown to be governed by an energy principle.
Measurements of plasma loading in the presence of electrostatic waves
Authors: Riccardi C. , Agostini E. , Fontanesi M.
Published: July 2002

An experimental analysis of the plasma impedance with respect to the coupling of ES (electrostatic) waves is described in this paper. The waves are excited through a slow-wave antenna and the experiment performed in a toroidal device [C. Riccardi et al., Plasma Phys. 36, 1791 (1994)]. The measured impedance is compared with a simple theoretical model for magnetized homogeneous plasma, in order to establish the presence of bulk or surface waves and of some nonlinear effects when power is raised.
Wave propagation in an inhomogeneous anisotropic plasma
Authors: Hau L.-N. , Lin C.-A.
Published: July 2002

A model is developed for small-amplitude waves propagating in a magnetically structured plasma with anisotropic pressures. Closure of the magnetohydrodynamic (MHD) equations is provided by a pair of polytropic laws, p⊥ρ−1B1−γ⊥= C⊥ and p∥ρ−γ∥Bγ∥−1 = C∥, such that for γ⊥=2, γ∥=3 the usual Chew–Goldberger–Low double-adiabatic expressions are recovered and for γ⊥=1, γ∥=1 double-isothermal conditions are obtained [L.-N. Hau and B. U. Ö. Sonnerup, Geophys. Res. Lett. 20, 1763 (1993)]. The wave equation represents the counterpart of that obtained for the isotropic plasma using the single energy equation, pρ−γ=C. Two applications are considered: the occurrence of surface waves on a magnetic interface and the field-line resonance.
Anomalous pinch flux in tokamaks driven by the longitudinal adiabatic invariant
Authors: Nycander J. , Yankov V. V.
Published: July 2002

The turbulent particle flux in tokamaks has an inward component, the pinch flux, which is independent of the density gradient. It is proposed that this flux represents a tendency of the particle distribution to approach turbulent equipartition, which means that the phase space density is constant on hypersurfaces defined by those invariants that are not destroyed by the turbulence. For tokamaks the two first adiabatic invariants give the peaked density profile n∼1/q of trapped particles. Sharp gradients are predicted near the separatrix of divertor plasmas. The physical mechanism of the pinch is as follows. When a parcel of trapped particles is displaced inward by the turbulent fluctuations, their parallel velocity must increase in order to keep the longitudinal invariant J constant. This is equivalent to adiabatic compression, and increases the density. The turbulence is assumed to be caused by thermally driven electrostatic modes.
Numerical calculation of axisymmetric electrostatic modes for cold finite-length non-neutral plasmas
Authors: Jennings Johnny K. , Spencer Ross L. , Hansen K. C.
Published: July 2002

A numerical calculation of mode frequencies for cold, non-neutral plasmas is reported. The numerical method can be applied to any axisymmetric plasma shape in a trap. Here, it is used to study axisymmetric electrostatic modes in a long conducting cylinder. These modes were previously studied by Prasad and O’Neil [Phys. Fluids 26, 665 (1983)] and by Dubin [Phys. Rev. Lett. 66, 2076 (1991)]. In contrast to Dubin’s calculation, the effects of a nearby cylindrical wall, including its influence on the shape of the plasma equilibrium, are considered. It is found that for plasmas with aspect ratios (length divided by diameter) near unity the numerical results can be approximately obtained by judiciously combining Dubin’s calculation, and the Trivelpiece–Gould dispersion relation for infinitely-long geometry. For aspect ratios larger than about three, the Trivelpiece–Gould dispersion relation can be used in a simple way to obtain the numerically-computed mode frequencies with an accuracy of 1%, or better. The potential use of this calculation as a plasma diagnostic is also discussed, and it is argued that at the present level of accuracy (1–2%) its usefulness is marginal, but that an improvement by an order of magnitude might make it more interesting.
Parallel forces induced by waves in ion cyclotron range of frequencies
Authors: Wang C. Y. , Batchelor D. B. , Jaeger E. F.
Published: July 2002

A new mechanism is identified for radio frequency (RF) wave induced force and current drive caused by a kinetic flux carried by coherent oscillations of plasma particles. The induced force along a magnetic field line by RF heating of plasmas is calculated with a one-dimensional slab model, and found to be contributed by the plasma power absorption and spatial inhomogeneity. The inhomogeneity term consists of a kinetic flux and a flux related to RF wave helicity injection rate. The effect from the kinetic flux exists only in a warm plasma, whereas, in contrast, the effect from the helicity related flux exists even in a cold plasma. The forces are calculated for both electrons and ions in the ion cyclotron range of frequencies (ICRF), and the effects from the ion cyclotron and the second harmonic resonance are included.
Theory-based transport modeling of the gyro-radius experiments
Authors: Kinsey Jon E. , Bateman Glenn
Published: July 2002

Self-consistent predictive transport simulations of temperature and density profiles have been carried out for ten dimensionally similar low (L) mode discharges from the Tokamak Fusion Test Reactor (TFTR) [D. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)], Doublet III-D Tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], and the Joint European Torus [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)], where only the normalized Larmor radius was allowed to vary. It is found that a purely gyro-Bohm transport model predicts temperature and density profiles that match the experimental data from these ρ* scans very well. In particular, a combination of theoretically derived transport models is used in these simulations, including the Weiland model for transport due to drift waves (ion temperature gradient and trapped electron modes) and the Guzdar–Drake model for transport due to resistive ballooning modes. These gyro-Bohm transport models depend very sensitively on the shapes as well as the magnitudes of the profiles. As the magnetic field, density and temperature are changed in each dimensionally similar series of discharges, the penetration length of neutrals from the edge varies considerably. This effect causes the shape of the density profiles to change near the edge of the plasma, which causes the scaling of our transport model diffusivities to differ significantly from their fundamentally gyro-Bohm scaling.
Spectrum of ballooning instabilities in a stellarator
Authors: Cooper W. A. , Singleton D. B. , Dewar R. L.
Published: July 2002

The recent revival of interest in the application of the ‘‘ballooning formalism’’ to low-frequency plasma instabilities has prompted a comparison of the Wentzel–Kramers–Brillouin (WKB) ballooning approximation with an (in principle) exact normal mode calculation for a three-dimensional plasma equilibrium. Semiclassical quantization, using the ideal magnetohydrodynamic (MHD) ballooning eigenvalue to provide a local dispersion relation, is applied to a ten-field period stellarator test case. Excellent qualitative agreement, and good quantitative agreement is found with predictions from the TERPSICHORE code [D. V. Anderson et al., Int. J. Supercomput. Appl. 4, 34 (1990)] for toroidal mode numbers from 1 to 14 and radial mode numbers from 0 to 2. The continuum bands predicted from three-dimensional WKB theory are too narrow to resolve.
Experimental study of spatial distribution of Ar glow discharge plasma
Authors: Guo X. M. , Zhou T. D. , Pai S. T.
Published: July 2002

The characteristics of the spatial distribution of Ar glow discharge plasma were experimentally investigated. By means of direct comparisons between theory and experiment, the effects of the variation of gap separation, gas pressure, and electrode radius on the spatial distributions of electron density and electric field were studied. Results indicate that the maximum electron density moves toward the cathode as the gap separation or gas pressure increases while variation of electrode radius produces little effect. Predictions from a theoretical model have been experimentally verified. General agreements between theory and experiment were found to be reasonably good except in the cathode region, where discrepancy exists.
Pulsed currents carried by whistlers. VI. Nonlinear effects
Authors: Urrutia J. M. , Stenzel R. L.
Published: July 2002

In a large magnetized laboratory plasma (n≂1011 cm−3, kTe≥1 eV, B0≥10 G, 1 m × 2.5 m), current pulses in excess of the Langmuir limit (150 A, 0.2 μs) are drawn to electrodes in a parameter regime characterized by electron magnetohydrodynamics (ωci≪ω≪ωce). The transient plasma current is transported by low-frequency whistlers forming wave packets with topologies of three-dimensional vortices. The generalized vorticity, Ω, is shown to be frozen into the electron fluid drifting with velocity v, satisfying ∂Ω/∂t≂∇×(v×Ω). The nonlinearity in v×Ω is negligible since v and Ω(r,t) are found to be nearly parallel. However, large currents associated with v≥(2kTe/me)1/2 lead to strong electron heating which modifies the damping of whistlers in collisional plasmas. Heating in a flux tube provides a filament of high Spitzer conductivity, which permits a nearly collisionless propagation of whistler pulses. This filamentation effect is not associated with density modifications as in modulational instabilities, but arises from conductivity modifications. The companion paper [Stenzel and Urrutia, Phys. Plasmas 3, 2599 (1996)] shows that, after the decay of the transient wave magnetic field, magnetic helicity remains in the plasma due to temperature-gradient driven currents.
Radiative and three-body recombination in the Alcator C-Mod divertor
Authors: Lumma D. , Terry J. L. , Lipschultz B.
Published: July 2002

Significant recombination of the majority ion species has been observed in the divertor region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under detached conditions. This determination is made by analysis of the visible spectrum from the divertor, in particular the Balmer series line emission and the observed recombination continuum, including an apparent recombination edge at ∼375 nm. The analysis shows that the electron temperature in the recombining plasma is 0.8–1.5 eV. The measured volume recombination rate is comparable to the rate of ion collection at the divertor plates. The dominant recombination mechanism is three-body recombination into excited states (e+e+D+⇒D0+e), although radiative recombination (e+D+⇒D0+hν) contributes ∼5% to the total rate. Analysis of the Balmer series line intensities (from nupper=3 through 10) shows that the upper levels of these transitions are populated primarily by recombination. Thus the brightnesses of the Balmer series (and Lyman series) are directly related to the recombination rate.
Energetic particle orbits in the National Spherical Tokamak Experiment
Authors: Mikkelsen D. R. , White R. B. , Akers R. J. , Kaye S. M. , McCune D. C. , Menard J. E.
Published: July 2002

The trajectories of neutral beam injected energetic ions in spherical tokamaks are examined. The large poloidal magnetic field in the outboard region of spherical tokamaks causes neutral beam injected ions to be born on trapped orbits even with cotangential injection. Numerical solutions to the equations for particle motion and for guiding center drifts are compared in several magnetic equilibria for a range of particle initial conditions. Even when rL/a∼1/4 the guiding center orbits closely resemble the path of the instantaneous center of gyration of the particle motion; exceptions occur primarily for orbits near the trapped/passing boundary. Finite Larmor radius effects are included in guiding center simulations of prompt orbit loss in the National Spherical Tokamak Experiment (NSTX) [J. Spitzer et al., Fusion Technol. 30, 1337 (1996)]. Orbit loss in the NSTX is caused primarily by collisions with the close fitting conducting shell, and severe losses would be expected with counter directed injection. While most orbits are similar to those found in conventional tokamaks, additional orbit types are possible in spherical tokamaks.
The role of the neutral beam fueling profile in the performance of the Tokamak Fusion Test Reactor and other tokamak plasmas
Authors: Park H. K. , Sabbagh S. A. , Batha S. , Bell M. , Budny R. V. , Bush C. , Chang Z. , Johnson D. , Mansfield D. K. , McCune D. , McGuire K. M. , Nazikian R. , Skinner C. , Wieland R. , Yamada M. , Young K. M.
Published: July 2002

Scalings for the stored energy and neutron yield, determined from experimental data, are applied to both deuterium-only and deuterium–tritium plasmas in different neutral-beam-heated operational domains in the Tokamak Fusion Test Reactor [Nucl. Fusion 25, 1167 (1985)]. The domain of the data considered includes the Supershot, high poloidal beta, low-mode, and limiter high-mode operational regimes, as well as discharges with a reversed magnetic shear configuration. The new important parameter in the present scaling is the peakedness of the heating beam fueling profile shape. Ion energy confinement and neutron production are relatively insensitive to other plasma parameters compared to the beam fueling peakedness parameter and the heating beam power when considering plasmas that are stable to magnetohydrodynamic modes. However, the stored energy of the electrons is independent of the beam fueling peakedness. The implication of the scalings based on this parameter is related to theoretical transport models such as radial electric field shear and ion temperature gradient marginality models. Similar physics interpretation is provided for beam heated discharges on other major tokamaks.
Radiation of Langmuir solitons
Authors: Karpman V. I. , Schamel H.
Published: July 2002

It is shown that a Langmuir soliton radiates a resonant wave with the phase velocity equal to the soliton velocity. The group velocity of the radiated wave is less than the soliton velocity and, therefore, the soliton radiates backward. The amplitude of the radiated wave is calculated analytically with asymptotic accuracy in the limiting case of small soliton amplitudes. Soliton attenuation and slowing down, caused by the radiation, are also calculated.
Stability of ideal and resistive modes in cylindrical plasmas with resistive walls and plasma rotation
Authors: Bondeson A. , Xie H. X.
Published: July 2002

The stabilization of cylindrical plasmas by resistive walls combined with plasma rotation is analyzed. Perturbations with a single mode rational surface q=m/n in a finitely conducting plasma are treated by the “resistive kink” dispersion relation of Coppi et al. [Sov. J. Plasma Phys. 2, 533 (1976)]. The possibilities for stabilization of ideal and resistive instabilities are explored systematically in different regions of parameter space. The study confirms that an ideal instability can be stabilized by a close-fitting wall and a rotation velocity on the order of resistive growth rates [J. M. Finn, Phys. Plasmas 2, 3782 (1995)]. However, the region in parameter space where such stabilization occurs is very small and appears to be difficult to exploit in experiments. The overall conclusion from the cylindrical plasma model is that resistive modes can readily be wall stabilized, whereas complete wall stabilization is hard to achieve for plasmas that are ideally unstable with the wall at infinity.
Analytic solutions of the magnetic annihilation and reconnection problems. I. Planar flow profiles
Authors: Watson P. G. , Craig I. J. D.
Published: July 2002

The phenomena of steady-state magnetic annihilation and reconnection in the vicinity of magnetic nulls are considered. It is shown that reconnective solutions can be derived by superposing the velocity and magnetic fields of simple magnetic annihilation models. These solutions contain most of the previous models for magnetic merging and reconnection, as well as introducing several new solutions. The various magnetic dissipation mechanisms are classified by examining the scaling of the Ohmic diffusion rate with plasma resistivity. Reconnection solutions generally allow more favorable “fast” dissipation scalings than annihilation models. In particular, reconnection models involving the advection of planar field components have the potential to satisfy the severe energy release requirements of the solar flare. The present paper is mainly concerned with magnetic fields embedded in strictly planar flows—a discussion of the more complicated three-dimensional flow patterns is presented in Part II [Phys. Plasmas 4, 110 (1997)].
Alpha effect of Alfvén waves and current drive in reversed-field pinches
Authors: Litwin C. , Prager S. C.
Published: July 2002

Circularly polarized Alfvén waves give rise to an α-dynamo effect that can be exploited to drive parallel current. In a “laminar” magnetic the effect is weak and does not give rise to significant currents for realistic parameters (e.g., in tokamaks). However, in reversed-field pinches (RFPs) in which magnetic field in the plasma core is stochastic, a significant enhancement of the α effect occurs. Estimates of this effect show that it may be a realistic method of current generation in the present-day RFP experiments and possibly also in future RFP-based fusion reactors.
On resonance absorption and continuum damping
Authors: Jaun A. , Appert K. , Hellsten T. , Vaclavik J. , Villard L.
Published: July 2002

The absorption of power is studied with fluid and gyrokinetic plasma models, when Alfvén resonances provide for a weak damping in a partially standing wave field. Examples chosen in slab and toroidal geometry show that the fluid predictions based on resonance absorption are generally very different from the Landau damping of mode-converted slow waves. They, in particular, suggest that the continuum damping of toroidal Alfvén eigenmodes (TAE) and the power deposition profiles obtained in the ion-cyclotron range of frequencies (ICRF) using fluid plasma models can be very misleading.
Nonlinear propagation of inertial Alfvén waves in auroral plasmas
Authors: Shukla P. K. , Stenflo L. , Bingham R.
Published: July 2002

The nonlinear interaction between large amplitude dispersive inertial Alfvén waves (DIAWs) and electrostatic density perturbations is considered. For this purpose, a pair of coupled equations is derived, accounting for the ponderomotive force driven density perturbations. The modulational instability of a constant amplitude DIAWs is studied, and the possibility of DIAW induced quasistationary (supersonic) density channels (cavities) is pointed out. Specifically, it is found that ponderomotive force driven density cavities cannot be formed within the quasistationary model. The relevance of our investigation to the nonlinear structures and the ion energization in space and laboratory plasmas is discussed.
Antiscreening versus screening excitations for low-energy ion-ion collisions in dense plasmas using the screened hyperbolic-orbit trajectory method
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Plasma screening effects are investigated on antiscreening and screening channels for excitation of a one-electron target ion by a one-electron projectile ion in dense plasmas. In weakly coupled plasmas, the ion-ion interaction potential is given by the nonspherical Debye-Hückel model. The semiclassical screened hyperbolic-orbit trajectory method is applied to the path of the projectile ion in order to investigate the variation of the antiscreening and screening transition probabilities as functions of the impact parameter and collision energy. The results show that the antiscreening transition probability is always greater than the screening transition probability. It is found that the plasma screening effect on the antiscreening transition probability is more effective than that on the screening transition probability. The maximum position of the transition probability is getting closer to the target core with increasing projectile energy. The plasma screening effect on the antiscreening excitation cross section is also found to be stronger than that on the screening excitation cross section.
Image charge effects on bremsstrahlung radiation from electron–dust grain scatterings in dusty plasmas
Authors: Jung Young-Dae
Published: July 2002

Low energy bremsstrahlung process in electron–dust grain Coulomb scatterings in dusty plasmas is investigated using the classical trajectory method. Attractive interaction between the projectile electron and induced image charge inside the dust grain by the projectile electron is included to obtain the total interaction potential between the projectile electron and the dust particle. The classical straight-line trajectory method is applied to represent the differential bremsstrahlung radiation cross section as a function of the scaled impact parameter and projectile energy. The results show that there exists a nonradiation position in the bremsstrahlung radiation due to the balance between the repulsive and attraction interactions. The image charge effects on the bremsstrahlung radiation is dominant for small impact parameters. For large impact parameters, the image charge effect on the bremsstrahlung cross section for the hard photon radiation is found to be more significant than that for the soft photon radiation.
On the physics of Landau damping
Authors: Stubbe P. , Sukhorukov A. I.
Published: July 2002

The quantitative explanation of neutral sound absorption at extremely low pressures within the framework of Landau’s theory (Sukhorukov and Stubbe [Phys. Plasmas 2, 4059 (1995)]) has revived the question about the physical nature of Landau damping. In the present paper it is shown in several ways that the physical mechanism behind linear Landau damping does not depend on the presence of charges. It is shown, in particular, that in the collisionless limit the damping rate of a given spatial and temporal structure is entirely determined by its spectrum, independent of whether the particles of the system are charged or uncharged. The role of charges lies in their influence on the shape of the spectrum. The conventional picture of Landau damping, involving the resonant conversion of wave energy into particle kinetic energy by electrostatic interaction, is critically investigated, and the shortcomings of this picture are unveiled. Landau damping is then described as thermal spread, which applies to both charged and uncharged gases. A correct estimate of the damping rate, based on this picture, is given in the low-damping limit.
First experiments in the Dusty Plasma Experiment device
Authors: Thomas Edward , Watson Michael
Published: July 2002

The Dusty Plasma Experiment (DPX) is a new experimental plasma device at Fisk University. DPX is constructed from two, 4 in. diam ISO 100 stainless steel six-way crosses. Initial experiments have successfully produced argon dc glow discharge plasmas. Measurements of the plasma parameters using Langmuir probes, in the absence of the dust particles, give plasma densities n∼1015 m−3 and electron temperatures ranging from Te=2 to 5 eV. Dusty plasmas are made by suspending 40 μm diam silica (SiO2) particles in the argon plasma. Dust clouds in the DPX device have densities nd∼109–1010 m−3. This paper describes the hardware and initial operation of the DPX device. Measurements of the three dimensional spatial structure of dust clouds are discussed. Dust acoustic modes with frequencies in the range f=10–20 Hz in the dust clouds are also identified. Particle image velocimetry techniques are used to characterize the spatial distribution and the temporal evolution of the velocities of the dust particles in the plasma.
Relativistic motion of a charged particle in a superposition of circularly polarized plane electromagnetic waves and a uniform magnetic field
Authors: Qian Bao-Liang
Published: July 2002

The relativistic motion of a charged particle in a superposition of circularly polarized plane electromagnetic waves and a uniform magnetic field is studied by deriving an exact solution to the Lorentz force equation of the charged particle. All of the circularly polarized plane electromagnetic waves propagate parallel to the uniform magnetic field. The explicit expressions of the charged particle position, velocity, and energy are obtained for arbitrary initial conditions, and the behavior of the particle motion is studied both analytically and numerically. It is found that for certain initial conditions, the particle gains energy from the waves and the energy gain could reach a maximum value during the time evolution of the charged particle motion in the nonresonance case. In addition, the particle could be accelerated to a much higher energy at the cyclotron resonance, and, in comparison with the situation of a single wave, the use of the superposition wave that consists of a group of circularly polarized plane electromagnetic waves with different frequencies increases the chance of the occurrence of cyclotron resonance for a charged particle, making it easier for the particle to be accelerated. It has also been observed that the interaction of the charged particle with the superposition of electromagnetic waves can be improved significantly at cyclotron resonance when the frequency and phase differences of the waves remain so small that the phases of the waves are in the same quadrant. The results of the present paper are of interest to particle acceleration and heating applications, as well as to basic plasma processes.
Charged particle distributions and heat transfer in a discharge between geometrically dissimilar electrodes: From breakdown to steady state
Authors: Qin Wei (Ivy) , Cohen Ira M. , Ayyaswamy P. S.
Published: July 2002

The low-current electric discharge from a fine wire anode to a planar cathode in atmospheric pressure air is numerically simulated from high-voltage prebreakdown through electron temperature growth, then ionization and consequent current growth to steady state, limited by a ballast resistor in the external circuit. Conservation of number (mass) for ions and electrons, Gauss’ law for the self-consistent electric field, and energy conservation for electrons have been solved from breakdown to steady state in a body fitted coordinate system generated specifically for these two geometrically dissimilar electrodes. To facilitate the discussion of the results, the discharge has been categorized under (a) electron acceleration period, (b) charged particle generation period, (c) current increase and voltage drop period, and (d) current and voltage stabilization period. Results are given for transient electron, ion, and temperature distributions in the gap as well as current growth and voltage drop across the gap. Heat flux from the discharge to the wire is calculated. The numerical simulations were compared with experiments performed under the same conditions on a wire bonding machine with very close correspondence.
Generation of localized density perturbations by shear Alfvén waves
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

It is shown that the ponderomotive force of finite amplitude shear (or kinetic) Alfvén waves can generate magnetic field-aligned quasistationary density humps as well as dips. The results can have relevance to the shear Alfvén wave induced localized density perturbations that have been recently found in the Large Plasma Device at the University of California, Los Angeles, as well as to those observed by spacecrafts in the Earth’s auroral and magnetospheric plasmas.
Ionization, stopping, and thermalization of hydrogen and boron beams injected in fusion plasmas
Authors: Lifschitz Agustı́n F. , Farengo Ricardo , Arista Nestor R.
Published: July 2002

The ionization, stopping, and thermalization of hydrogen and boron beams, injected, respectively, in boron and hydrogen plasmas, is studied. The evolution of the charge state populations of the neutral beams is described considering the various ionization, excitation, and charge exchange channels. The interaction of the beam with the plasma is described in terms of the Fokker–Planck equation, which is numerically solved to show in detail the evolution of the beam until final thermalization is reached. Beam energies of 640 keV/u (maximum of the cross section for the p-B11 fusion reaction) and 200 keV/u, and various plasma temperatures are considered. It is seen that, due to an important perpendicular-diffusion effect, high energy beams reach effective peak temperatures which are much higher than the plasma temperature, before equilibrium is established. The fraction of fusioned particles is also calculated. Some implications of interest for recently proposed p-B11 fusion reactor systems are drawn out.
Unstable resonator cavity semiconductor lasers
Authors: Salzman J. , Venkatesan T. , Lang R. , Mittelstein M. , Yariv A.
Published: July 2002

GaAs heterostructure lasers with unstable resonator cavities were demonstrated for the first time with both curved mirrors fabricated by etching. Typical output powers of 0.35 W were observed in a stable, highly coherent lateral mode. The laser operated stably in a single longitudinal mode over a large range of injection currents. The external quantum efficiency was 70% of that of a similar laser with both mirror facets cleaved implying good output coupling of the energy from the entire region.