bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbation expansion and weakly nonlinear analysis for two-dimensional vortices in curved or rotating channels
Authors: Finlay W. H.
Published: July 2002

Spectral simulation is used to test the perturbation expansion of Davey [J. Fluid Mech. 14, 336 (1962)] applied to two-dimensional, streamwise-oriented vortices in curved channel flow and rotating channel flow over a wide range of parameters. The power series dependence on Reynolds number implied by the perturbation expansion provides an excellent description of the amplitudes of all ten independent Fourier modes resolved in the numerical simulations. Weakly nonlinear analysis, associated with truncating the perturbation expansion, is developed for rotating channel vortices and compared to the full simulations. For both channel geometries, significant increases in the accuracy of weakly nonlinear theory occurs with increasing vortex wavenumber; this is explained by examining spatial energy spectra.
Effect of diffusion on chaotic advection in Stokes flow
Authors: Dutta P. , Chevray R.
Published: July 2002

The effect of molecular diffusion has been studied both numerically and experimentally for the case of Stokes flow between two eccentric cylinders. The parameters (eccentricity, time periods, and directions of rotation of the cylinders) are so chosen that for the nondiffusive case, we obtain a mixed phase space of regular and chaotic regions. This enables us to study the relative effects of molecular diffusion in the two kinds of regions of the same flow. The Lagrangian description of a diffusing particle has been modeled by the use of Langevin equation (as done by Aref and Jones in Ref. 1) in which the instantaneous velocity of a particle has a deterministic component (as a result of the flow solution) and a stochastic component (arising out of the Brownian motion of the molecules).

The stochastic component is assumed to be a Gaussian process with zero mean and variance proportional to the diffusivity. The numerical investigation is carried out by locating circular blobs of particles in various regions of the given phase space. Each blob is represented by about 1000 evenly distributed points inside a small circle. With a given value of diffusivity, D, our two-cylinder system is stirred by completing three cycles of motion followed by three reverse cycles. For a deterministic case, the particles are expected to return to their initial locations after flow reversal, as has been observed for the case with D=0. For D>0, the mean-square separation of the particles from their initial locations has been evaluated with the idea of finding an effective diffusivity for each blob location. The mean-square separations for the stirred cases are then compared with that of the unstirred case for the same value of D. It has been found that the mean-square separation in a regular region is slightly greater than that of the unstiffed case, whereas in the chaotic region it is greater than that of the corresponding unstirred case by an order of magnitude. The reason for this enhancement of diffusion by stirring, as explained by Aref and Jones,1 is that stirring causes an increase of intermaterial area for diffusion to act over a longer interface. Since stretching is identified to be the cause of enhancement of diffusion, Lyapunov exponents have been evaluated for various regions of the flow. In the regular regions, the Lyapunov exponents have been found to be nearly zero, and it is found that the greater the separation, the greater the exponent in that region. A correlation between the two quantities has been established. However, the Lyapunov exponents in the chaotic regions have been found to be highly positive, indicating a high rate of stretching. It is also found that there is a variation of separation within the chaotic region, indicating a difference in the degree of chaos between various regions within the chaotic sea. Corresponding experimental observations, made by using the same apparatus as in Chaiken et al.,2 show good agreement with the numerical results.
Experimental analysis of stability limits of capillary liquid bridges
Authors: Bezdenejnykh N. A. , Meseguer J. , Perales J. M.
Published: July 2002

This paper deals with the influence of axial microgravity on the stability limits of axisymmetric, cylindrical liquid columns held by capillary forces between two circular, concentric, solid disks. A fair number of experiments have been performed and both the maximum and the minimum volume of liquid that a capillary liquid bridge can withstand have been obtained as a function of the geometry of the liquid bridge and of the value of the axial microgravity acting on it. Experimental results are compared with published theoretical predictions made by other investigators and discrepancies between those results criticized.
Contact angles
Authors: Merchant Gregory J. , Keller Joseph B.
Published: July 2002

Young [Philos. Trans. (1805)] derived an equation for the contact angle between a liquid–gas interface and a solid boundary, but doubts have been raised about its validity. This issue is reexamined on the basis of a new integral equation for the interface [J. B. Keller and G. J. Merchant, J. Stat. Phys. 63, 1039 (1991)]. The equation is solved asymptotically by the method of matched asymptotic expansions for small values of the range of intermolecular forces divided by a typical macroscopic length. The leading term in the outer expansion satisfies the Young–Laplace partial differential equation for the interface. The leading term in the boundary-layer expansion satisfies a simplified integral equation. Matching the solutions of these two equations shows that the slope angle at the solid boundary, of the leading term in the outer expansion, is indeed given by the Young equation. Numerical solutions of the boundary-layer integral equation are presented to show how the interface varies near the solid boundary.
On the spatial stability of tube flows subject to body forces
Authors: Goering D. J. , Humphrey J. A. C.
Published: July 2002

The spatial stability of multiple solutions for fully developed laminar flow through a curved tube and through a straight tube in the presence of buoyant effects was studied using a three-dimensional numerical representation of the fully elliptic Navier–Stokes and energy equations. Several recent numerical studies have reported the existence of multiple solutions for fully developed laminar flow in curved tubes; see, for example, Yang and Keller [Appl. Numer. Math. 2, 257 (1986)] or Daskopoulos and Lenhoff [J. Fluid Mech. 203, 125 (1989)]. A similar solution multiplicity for the case of fully developed laminar flow in a heated straight tube has been discussed by Nandakumar et al. [J. Fluid Mech. 152, 145 (1985)]. Each of these studies has reported solutions of a two- and four-cell type, which are symmetric about the midplane of the tube and invariant in the axial (streamwise) flow direction. In addition, the flow visualization study of Cheng and Yuen [J. Heat Trans. 109, 55 (1987)] indicates that two- and four-cell solutions can be obtained experimentally for the flow developing in a curved tube with a 180° bend. The present study investigates the stability of two- and four-cell solutions with respect to perturbations that are asymmetric about the midplane of the tube. Perturbed two- and four-cell solutions are imposed as entrance boundary conditions for three-dimensional simulations of the full tube cross section and the evolution of the flow is observed as it progresses downstream from the entrance plane. For the conditions explored, present results indicate that the two-cell solutions are stable, and asymmetric perturbations are observed to decay as the flow travels downstream. Four-cell solutions are unstable when asymmetric perturbations are introduced, and growth of the perturbations eventually results in a complex three-dimensional transition from four-cell to two-cell flow. Details of the transition flow are described and transition mechanisms are identified.
Radiation-driven thermocapillary flows in optically thick liquid films
Authors: Hitt Darren L. , Smith Marc K.
Published: July 2002

A thin liquid film on a horizontal solid surface undergoing radiative heat transfer with an external heat source and the surrounding environment is considered. Thermal gradients along the free surface give rise to a thermocapillary flow in the liquid that is opposed by a hydrostatic pressure gradient within the film. Transient and steady-state solutions are obtained for the interfacial shape and temperature and the velocity field. These results are compared with those from another model, in which a temperature distribution is imposed on the free surface of the film. At a critical value of the dynamic Bond number, a cusp in the form of a free-surface slope discontinuity appears in this fixed free-surface temperature model, but not in the radiation model. When the Bond number is less than this critical value, the time required to thin the film by a significant fraction of its original thickness is much larger with the radiation model. It is shown how the thermal boundary conditions used in the models directly cause these differences.
Quenching of external kink modes by localized radio-frequency fields
Authors: D’Ippolito D. A. , Goedbloed J. P.
Published: July 2002

The quenching of external kink modes by a spatially localized, externally applied surface force is studied in a low-beta cylindrical tokamak model. It is shown that m=2 modes may be stabilized by the application of a radio frequency (rf) field in the ion cyclotron range of frequencies (ICRF) excited by either a toroidal or a poloidal antenna structure surrounding the plasma. Whereas linear mode coupling induced by toroidal localization is negligible, that caused by poloidal localization may degrade the stabilization and, thus, require increased power levels for stability. The model is directly applicable to the ponderomotive force exerted by an evanescent slow wave in the ion cyclotron range of frequencies. Quantitative estimates are made for ponderomotive stabilization of kink modes by means of ion-Bernstein-wave (IBW) antennas. This method of stabilization has the advantage that the antenna system does not have to match the spatial or temporal structure of the unstable modes. For typical IBW antenna systems, it is found that the required rf fields for kink stabilization are reasonable if the current profile is not too flat.
Magnetosonic and kinetic Alfvén current drive analysis
Authors: Tsui K. H. , de Assis Altair S.
Published: July 2002

Quasilinear equations are used together with the energy conservation equation to assess the merits of magnetosonic and kinetic Alfvén current drive. It is found that the Alfvén mode has a higher gain in terms of efficiency and current than the magnetosonic mode.
Self-consistent resonance absorption with two-layer profile steepening
Authors: Ahedo E. , Sanmartín J. R.
Published: July 2002

Resonance absorption of p-polarized light, incident at angle θ on a flowing, stratified plasma, is analyzed; profile steepening within (i) a layer around the turning point, and (ii) a thinner, embedded sublayer at the critical surface is taken into account self-consistently. The entire steepened region is taken as collisionless and isothermal. The structure of the main layer shows a variety of regimes, depending on how the flow crosses a sonic point. The structure of the sublayer is also determined; it is entirely subsonic (with no wave breaking) for a well-defined, broad parameter range. Density changes across both layer and sublayer, and fractional absorption, are given in terms of [(wavelength)2×intensity/temperature], θ, and (temperature/mec2). The flow outside the double structure is also analyzed for particular conditions.
Response to Schalit and Bellan
Authors: Miller R. L. , Chan V. S. , Ohkawa T.
Published: July 2002

The ac helicity effect discussed by Schalit and Bellan does not impact the current drive efficiency obtained by Ohkawa.
Experiments utilizing ion cyclotron range of frequencies heating on the TFTR tokamak
Authors: Wilson J. R. , Hosea J. C. , Bell M. G. , Bitter M. , Boivin R. , Fredrickson E. D. , Greene G. J. , Hammett G. W. , Hill K. W. , Hoffman D. J. , Hsuan H. , Hughes M. , Janos A. C. , Jassby D. L. , Jobes F. C. , Johnson D. W. , Phillips C. K. , Mansfield D. K. , McGuire K. M. , Medley S. S. , Mueller D. , Nagayama Y. , Ono M. , Owens D. K. , Park H. K. , Phillips M. , Ramsey A. T. , Schmidt G. L. , Scott S. D. , Stevens J. E. , Stratton B. C. , Synakowski E. , Taylor G. , Ulrickson M. , Wong K. L. , Zarnstorff M. C. , Zweben S. J.
Published: July 2002

A variety of experiments have been performed on the TFTR tokamak [Wilson et al., Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 691] utilizing ion cyclotron range of frequencies (ICRF) heating. Of special interest has been the insight into plasma performance gained by utilizing a different heating scheme other than the usual neutral beam injection (NBI). Utilizing ICRF heating allows control over the power deposition profile independent of the plasma fueling profile. In addition, by varying the minority concentration the power split between ion and electron heating can be varied. Confinement has been examined in high recycling gas fueled discharges, low recycling supershot plasmas, and peaked density pellet fueled discharges. Global confinement is found not to be affected by the method or localization of plasma heating, but the calculated local diffusivities vary with the power deposition profile to yield similar global values. In addition, sawtooth stabilization observed with ICRF heating has been investigated and found to occur in qualitative agreement with theory. ICRF sawtooth stabilized discharges exhibit peaked temperature and density profiles and have a safety factor q that appears to fall well below unity on axis.
Observations of frequency upshift in a pulsed free-electron laser amplifier
Authors: Conde M. E. , Taylor C. J. , Bekefi G.
Published: July 2002

A new phenomenon, the frequency upshift in a pulsed free-electron laser (FEL) amplifier is reported. The measurement is made in an FEL that uses a mildly relativistic electron beam (750 keV, 90 A, 30 nsec) and is driven by a copropagating electromagnetic wave generated by a 33.39 GHz magnetron source. The frequency upshift is ascribed to the rapid temporal change in the effective dielectric coefficient that can be associated with the FEL interaction.
Multiple magnetized double layers in the laboratory
Authors: Intrator T. , Menard J. , Hershkowitz N.
Published: July 2002

Even though there have been many experimental observations of various types of double layers (DL), laboratory data thus far have only produced special examples of the more general case of multiple magnetized double layers. A series of stationary, fully magnetized (ions and electrons), two-dimensional (both parallel and transverse to the magnetic field) weak double layers (DL’s) with three, four, or more potential steps has been observed in a laboratory experiment for the first time. They may play important roles in the auroral zone, the physics of solar flares, high-power gas lasers, and laser pellet ablation plasmas. The transverse electric fields are, on average, larger than the parallel ones, and the transverse spatial size scales with ion gyroradius. The total number of DL potential drops depends on the location and what particular path is followed across any given potential profile. Electron beams acquire additional directed energy across these DL’s along with heating of the bulk distribution and large fluctuating energy density.
Extraction and propagation of an intense, rotating electron beam
Authors: Adler R. J. , Kiuttu G. F. , Hughes T. P. , Sabol B.
Published: July 2002

A hollow relativistic electron beam produced in a strong axial magnetic field has been extracted into a neutral gas cell where the field is zero. In the field-free region, equilibrium is dictated by a balance between the vz×Bθ radial pinch force and the centrifugal outward force. Extraction and propagation of these beams has been studied experimentally with a variety of diagnostics. A strong, low-frequency filamentation instability is observed after extraction. In general, 80% of the beam energy has been extracted and propagated 75 cm. Beam equilibrium properties are in good agreement with simple theory with the exception of the azimuthal plasma current.
The inverse bremsstrahlung absorption coefficient in collisional plasmas
Authors: Cauble R. , Rozmus W.
Published: July 2002

A prescription for the inverse bremsstrahlung (IB) absorption coefficient is presented. The coefficient is derived in terms of a collision frequency that is found from a memory function kinetic formalism. The formalism admits limits that allow a rigorous definition of the coefficient when the plasma is weakly coupled and approximations that provide a reliable calculation when the plasma is strongly coupled. In the latter case, use is made of spatial distribution functions computed from the solution to the set of integral equations describing plasma correlations via the hypernetted chain approximation (HNC). A simple analytic result is compared with the HNC calculations. Results show that, as expected, the classical form for the IB absorption coefficient is of little utility when the plasma is strongly coupled. The theory presented here should provide reasonable predictions of the coefficient.
Wave-particle resonance broadening in a Langmuir turbulence
Authors: Ishihara O. , Hirose A.
Published: July 2002

A new resonance broadening theory formulated in the Fokker–Planck framework shows agreement with numerical analyses in which particle diffusion in velocity space is studied against a background of prescribed Langmuir turbulence, much better than the Dupree–Weinstock theory. The deviation from the quasilinear theory because of modification of particle orbits is stronger than previously conjectured.
Evolution of electron hole and electron wave pulses in a single-ended magnetoplasma
Authors: Iizuka S. , Tanaca H.
Published: July 2002

Formation and propagation of the electron phase-space hole and electron wave pulse (Trivelpiece–Gould mode) are experimentally investigated in a single-ended plasma in the very first stage of double layer formation. As soon as the rarefaction pulse excited at a collector terminating the plasma arrives at a plasma source, the electron holes are formed and propagate toward the collector with a speed comparable to the electron thermal speed. The electron energy distribution function dips at the hole and becomes broader in the upper stream region of hole propagation. The compression pulse formed in front of the plasma source just after rarefaction pulse arrival does not have a direct effect on the hole excitation.
The correlation dimension of broadband fluctuations in a tokamak
Authors: Sawley M. L. , Simm W. , Pochelon A.
Published: July 2002

The correlation integral has been studied in detail for broadband magnetic and density fluctuations observed in the TCA tokamak [Proceedings of the 11th Symposium on Fusion Technology (CEC, Luxembourg, 1981), Vol. I, p. 601]. These studies, which provide information on the correlation dimension ν, have shown no evidence for a low-dimensional attractor. It has therefore been concluded that the broadband fluctuations are associated either with a high-dimensional system (ν>10), or with a nonstationary state for which the concept of the correlation dimension has no meaning.
Linear and nonlinear description of drift instabilities in a high-beta plasma
Authors: Aydemir A. Y. , Berk H. L. , Mirnov V. , Pogutse O. P. , Rosenbluth M. N.
Published: July 2002

A nonlinear system of equations is derived for drift waves in a high-beta plasma (β≫1). The magnetic field pressure is taken small compared to the particle pressure. Pressure balance is established by having a uniform particle pressure with the density and temperature gradients in opposite directions. The primary purpose of the magnetic field is to inhibit radial heat flux. This is the principle of such plasma fusion systems as the wall sustained multiple mirror, compressed liner, and magnetic-insulated inertial fusion, where the heat is contained over a relatively short radial scale length and a long axial scale length. The nonlinear equations for the mathematical model contain drift instabilities that give rise to radial heat and particle fluxes that can enhance the losses expected from classical collisional effects. The linear and nonlinear evolution of the model is studied here.
A cyclotron-maser instability associated with a nongyrotropic distribution
Authors: Freund H. P. , Dong J. Q. , Wu C. S. , Lee L. C.
Published: July 2002

A stability analysis for the cyclotron-maser instability in the presence of a nongyrotropic electron distribution is presented. The model configuration describes a uniformly magnetized cold ambient plasma that contains a relatively diffuse suprathermal electron component coherently bunched in gyrophase. The stability of perturbations propagating parallel to the ambient magnetic field is considered, and substantial growth rates are found to occur. The results are contrasted with those found for a comparable gyrotropic loss-cone distribution, and it is found that the nongyrotropic instability is characterized by substantially higher growth rates.
The evolution of cross helicity in driven/dissipative two-dimensional magnetohydrodynamics
Authors: Ghosh S. , Matthaeus W. H. , Montgomery D.
Published: July 2002

A numerical study of the evolution of cross helicity in driven/dissipative magnetohydrodynamics (MHD) is presented. The magnetofluid is incompressible and a two-dimensional (2-D) periodic geometry is considered. Cross helicity, a measure of the correlation between fluctuations in the magnetic field and the velocity field, is injected by use of correlated Gaussian forcing over a finite bandwidth in wavenumber. Numerical experiments include the driving of initially uncorrelated spectra with highly correlated forcing and the driving of correlated spectra with anticorrelated forcing. A recurring and persistent feature of the simulations is the appearance of oppositely signed cross helicity at small scales relative to large scales. A simple argument based on the Elsässer variables and used previously in the context of decaying turbulence explains many of the observed features. The effect of a uniform external magnetic field is considered and the relation to purely decaying 2-D MHD turbulence is discussed.
Magnetic fluctuations can contribute to plasma transport, ‘‘self-consistency constraints’’ notwithstanding
Authors: Krommes John A. , Kim Chang-Bae
Published: July 2002

The recent conclusion of Terry, Diamond, and Hahm (TDH) [Phys. Rev. Lett. 57, 1899 (1986)] that in a turbulent, collisionless plasma ‘‘magnetic transport including quasilinear magnetic flutter transport  . . .  does not contribute to the relaxation of 〈f〉, and thus is not responsible for electron energy or momentum transport’’ is argued to be irrelevant or incorrect for a variety of situations of physical interest, including saturation by quasilinear plateau formation, induced scattering, and, most important, conventional mode coupling. The well-established theory of the mean infinitesimal response function and the spectral balance equation provides a unifying framework for understanding the work of TDH. In particular, the cancellations which lead to the TDH conclusion are special cases of well-known relationships between the response functionparticle propagator, and dielectric function. A more general, concise, and manifestly gauge-invariant algebraic derivation of the cancellations is given. Although the cancellations occur in a certain limit, the conclusions of TDH do not follow in general: The TDH picture of steady-state turbulence as consisting of small-scale ‘‘incoherent’’ ballistic ‘‘clumps’’ shielded by long-wavelength ‘‘coherent’’ dielectric response is misleading physically and incomplete mathematically since it does not describe correctly the often dominant process of renormalized n-wave coupling, particularly important for the ions. Although ion ballistic response is negligible, ions are important nevertheless: Their nonlinear contribution to the saturated potential can drive parallel electron currents, hence magnetic fluctuations, through linear mechanisms. Thus, when ion nonlinearities are considered, formulas for the magnetic contribution to transport emerge which are quite similar to the quasilinear formula.
Contact line stability at edges: Comments on Gibbs’s inequalities
Authors: Dyson D. C.
Published: July 2002

In his discussion of contact line equilibrium for a system comprising two liquids and a solid, Gibbs [Scientific Papers (Dover, New York, 1961), Vol. 1, p. 326] used an argument that resulted in two inequalities he claimed to be applicable when the three-phase contact line coincides with an edge on the solid surface. A simple counterexample is given that shows Gibbs’s inequalities lack universal applicability. A serious objection to Gibbs’s argument is noted and his discussion is altered to remove the objectionable feature. This leads to modified inequalities, which surprisingly are known and have been attributed to Gibbs by recent authors.
Gyrophase-coherent electron cyclotron maser
Authors: Kho T. H. , Lin A. T. , Chen Liu
Published: July 2002

The basic physics of an electron cyclotron maser driven by a coherently gyrophased, helical electron beam in a uniform magnetic field is analyzed. Theoretical predictions of gain enhancement and reduction in different regimes of the cyclotron instability, compared to a gyrotropic cyclotron maser, are verified with computer simulations. A physical explanation is presented for these findings. Beam-to-radiation energy conversion efficiency in the autoresonance regime is studied with computer simulations and compared with that for a gyrotropic beam.
Surface strain modulation of insoluble surface film properties
Authors: Barter J. D.
Published: July 2002

The complex dispersion relation of capillary waves on water has been measured in a wavelength regime (λ∼1–2 cm) of interest to radar imaging. Two noninvasive sensors: a capacitive wave height antenna and a one-dimensional scanning laser slope gauge, are used to study the alteration of surface tension and surface elastic modulus in the presence of monomolecular surface films. On a static water surface, these sensors are compared to accepted standard techniques. The surface tension results are compared to a Wilhelmy plate while the elastic modulus results are compared to equations of state obtained by Langmuir trough techniques. The sensors are then employed to measure the response of surface film properties to surface areal strains for film pressures and strain rates, which are comparable to natural films in the presence of ship-generated surface currents. In the experiments reported in this paper, only insoluble surfactants are used. The monolayer surface films considered in this study are found to exhibit higher elastic modulus (∼8 mN/m) at small surface pressure (π≤1 mN/m) than expected from previous work reported in the literature. The response of these films to surface strain is consistent with our static surface measurements. The modulation of film parameters is found to be in phase with internal wave-generated oscillatory surface strain with periods on the order of 9 s. This implies that the film conforms to the strained water surface with negligible slippage in this regime. These measurements demonstrate the capability of this approach to continuously monitor surface films in situ with detector system response time of the order of 1 s. The technique is suited to large or deep containers and is applicable to sheltered ocean environments.
On subgrid scale modeling in large eddy simulations of compressible fluid flow
Authors: Fureby C.
Published: July 2002

The purpose of this study is to theoretically and numerically investigate the behavior of different subgrid scale (SGS) models for large eddy simulations (LES) of compressible fluids in engineering applications. Various models have been analyzed including eddy viscosity, scale similarity, mixed and dynamic models and finally the concept of monotone integrated large eddy simulation (MILES). The theoretical investigation concerns primarily the demonstration and motivation of the frame indifference constraint and the application of this physical requirement on the various SGS models. LES’ utilizing the presented models have been carried out in a configuration corresponding to a bluff body flow in a rectilinear duct. Experimental data, obtained by LDV, are used for investigating the simulation results. It is found that the results from the numerical simulations are fairly resilient to the SGS models selected when the computational grid is fine enough. However, the different SGS models reproduce the energy dissipation or the interscale energy transfer −B⋅D̃ and the SGS fluxes div B in highly dissimilar ways.
On the large eddy simulation of a turbulent channel flow with significant heat transfer
Authors: Wang Wen-Ping , Pletcher Richard H.
Published: July 2002

A large eddy simulation of a planar channel flow with significant heat transfer at a low Mach number was performed to study effects of fluid property variations on the near-wall turbulence structure. A compressible dynamic subgrid scale model was used to model the residual-scale turbulence. Two low Reynolds number channel flows with one wall heated and one wall cooled at temperature ratios of 1.02 and 3.0 were simulated to study the effects of property variations at low Mach number. Several features of the flow were observed to vary with the heat transfer level including velocity and temperature rms values. Specifically, the temperature-velocity correlations were found to exhibit stronger dependency on heat transfer rate. At the higher heat transfer rate, density fluctuations at levels characteristic of flows at much higher Mach numbers were observed. Heating appeared to enhance velocity fluctuations whereas density and temperature percentage fluctuations were greatest near the cooled wall when scaled by their local mean values.
The effect of wall compliance on inflexion point instability in boundary layers
Authors: Cooper Alison J. , Carpenter Peter W.
Published: July 2002

A study into the effects of wall compliance on boundary layer flow in an adverse pressure gradient has been undertaken. The two-dimensional flow given by a Falkner–Skan profile was considered where a powerful inviscid instability mechanism occurs as a result of a point of inflexion in the boundary layer profile. Results are presented for two cases: (a) a purely inviscid calculation where the compliant wall is modelled as a thin plate supported by a spring foundation, (b) a double-layer viscoelastic wall model with viscous effects retained. Wall parameters which ensure the absence of hydroelastic instability were used and found to produce growth rate reductions in excess of 50% of the corresponding rigid wall values.
Some spin-up effects on the geostrophic and quasi-geostrophic drag on a slowly rising particle or drop in a rotating fluid
Authors: Ungarish M.
Published: July 2002

In this paper the initial “spin-up” stage of the flow field generated by the slow axial motion of a symmetric particle or drop in a rotating fluid in a “short cylinder” of length 2H is considered for small Rossby and Ekman numbers, Ro and E. The motion starts from solid body rotation. Attention is focused on time-dependent effects in the extended core (outside the E1/2 Ekman and E1/3 Stewartson layers) and their influence on the behaviour of the drag force. When the particle velocity is established by an impulsive start, the flow-field and the drag build up on the spin-up time scale; the particle advances ∼2RoE−1/2H during the transient stage. Results for the time-dependent geostrophic case, and also for various values of ε=(12H)1/2E1/4, for both sphere and disk particles, are presented. It is shown that when the particle velocity is established by release under a constant axial (say, buoyant) force, during the spin-up some rapid inertial oscillations may appear which are inconsistent with some of the assumption of the present analysis and require a separate investigation. The present theory is in qualitative agreement with available experiments, but a quantitative comparison requires new experiments.
Unsteady nature of leading edge vortices
Authors: Menke Matthew , Gursul Ismet
Published: July 2002

Flow visualization and velocity measurements were carried out in leading edge vortices over a delta wing. Very large velocity fluctuations were observed well upstream of breakdown and also in the absence of breakdown. The maximum rms swirl velocity can exceed the free stream velocity depending on the angle of attack. Examination of the probability density functions and spectra suggest that the core of the vortex is characterized by large amplitude, broad band random velocity fluctuations. It was suggested that these large velocity fluctuations are due to apparently random displacements of the vortex core. A simple model of the flow that produced the essential features of the experimental results was presented.
An anelastic, scale-separated model for mixing, with application to atmospheric transport phenomena
Authors: McLaughlin Richard M. , Forest M. Gregory
Published: July 2002

We present and analyze a simplified, scale-separated, anelastic fluid model which is designed to assess the influence of weak compressibility in the diffusive transport of a passive scalar. Our model incorporates a slowly varying, density induced, anisotropy into a fixed, rapidly varying (small scale) fluid flow. This anisotropy is physically motivated through the anelastic mass balance which retains vertical density variations occurring over the atmospheric scale height (∼8 km). Consequently, these steady flows are nonsolenoidal over large scales and approximately incompressible on small scales. We apply homogenization methods to calculate the large scale, effective mixing experienced by a passive scalar diffusing in the presence of this small scale flow. Over large scales, the evolution of the scalar field is governed by an effective, variable coefficient, anelastic mixing equation. The variable coefficients entering this equation are shown to depend nontrivially upon both the large scale anisotropy as well as the structure of the small scale fluid flow. We establish that anelastic effects produce anisotropic mixing properties not shared by the analogous incompressible closure. Specifically, the mixing equation possesses exact nontrivial bounded steady states, whereas the incompressible regime has only constant (i.e., spatially homogeneous) steady states. Furthermore, the anelastic mixing equation is shown numerically to possess local regions of trapped (Fig. 3) and focused (Figs. 6 and 7) contaminants, behavior not possible in the analogous incompressible model. These results imply that anelastic effects, which occur naturally in the atmosphere, provide mechanisms which locally reduce mixing and generate inhomogeneities in large scale concentration fields.
Evolution of the Weibel instability in relativistically hot electron–positron plasmas
Authors: Yang T.-Y. Brian , Arons Jonathan , Langdon A. Bruce
Published: July 2002

Analytical and numerical studies of the evolution of the Weibel instability in relativistically hot electron–positron plasmas are presented. Appropriate perturbations on the electromagnetic fields and the particle orbits, corresponding to a single unstable mode, are determined analytically and used as initial conditions in the numerical simulations to excite a single unstable mode. A simple estimate of the saturation amplitude is also obtained analytically. Numerical simulations are carried out when a single unstable mode is favorably excited. Comparisons of the simulation results with the analytical ones show very good agreement. Also observed in the simulations are mode competition, mode suppression, and the difference in the long-term evolution between the magnetized and unmagnetized plasmas. For relativistic unmagnetized plasmas, energy-like global constraints, which are conservation laws in addition to the conservation of energy and momentum, are derived. Numerical simulations of the multimode evolution are described. Simulation results show growth in electromagnetic energy in the early stage, a narrowing in the bandwidth and a shift in the peak of the spectrum to longer wavelength in the subsequent evolution, and a decrease in the temperature anisotropy. In a simulation for an unmagnetized plasma, it is observed that the system reaches a steady state halfway through the simulation. In contrast, the peak of the spectrum continues to shift to lower wave number k, and the temperature anisotropy continues to decrease during the entire simulation for a magnetized plasma.
Experimental observation of Alfvén wave cones
Authors: Gekelman Walter , Leneman David , Maggs James , Vincena Stephen
Published: July 2002

The spatial evolution of the radial profile of the magnetic field of a shear Alfvén wave launched by a disk exciter with radius on the order of the electron skin depth has been measured. The waves are launched using wire mesh disk exciters of 4 mm and 8 mm radius into a helium plasma of density about 1.0×1012 cm−3 and magnetic field 1.1 kG. The electron skin depth δ=c/ωpe is about 5 mm. The current channel associated with the shear Alfvén wave is observed to spread with distance away from the exciter. The spreading follows a cone-like pattern whose angle is given by tan θ=kAδ, where kA is the Alfvén wave number. The dependence of the magnetic profiles on wave frequency and disk size are presented. The effects of dissipation by electron–neutral collisions and Landau damping are observed. The observations are in excellent agreement with theoretical predictions [Morales et al., Phys. Plasmas 1, 3765 (1994)].
Nonlinear dynamics of twisted magnetic flux tubes
Authors: Lau Yun-Tung
Published: July 2002

The nonlinear response of an axisymmetric magnetic flux tube under the influence of an azimuthal twist force is studied. For a constant twist force, the tube approaches an inertial collapse phase in finite time. The competing pinch and magnetic pressure forces cause radial oscillation during the collapse. The plasma pressure is negligible in the process. When the tube is subject to a random twist, each end of the tube absorbs angular momentum preferentially in the direction of the initial field line winding. Thus again, the tube is twisted up and collapses in finite time. For typical solar parameters, the collapse time is a few tens of the coronal Alfvén time. Both magnetic and kinetic energy increase explosively near the collapse as a result of the twist. The tube contracts to about one-tenth of its original size before reaching the kink threshold twist angle.
Theory of relativistic gyro-traveling wave devices
Authors: Latham P. E. , Nusinovich G. S.
Published: July 2002

Using a Hamiltonian formalism, nonlinear, fully relativistic, multimode, multifrequency equations are derived which describe gyro-traveling wave devices. Nonuniform waveguides and nonlinearly tapered magnetic fields are incorporated into the analysis. The formalism is used to analyze the effect of velocity spread on Doppler upshifted operation. It is shown that with present technology, gyro-traveling wave devices cannot operate far from cutoff if high efficiency is desired. As an example, the analysis is applied to a 10 GHz, 430 kV, 240 A gyrotwistron operating at the fundamental cyclotron harmonic with a tapered wall radius and magnetic field. A realistic design that achieves over 30% efficiency is produced. The issue of stability of this device with respect to competition from parasitic modes is taken up in a companion paper [Phys. Plasmas 2, 3511 (1995)].
Observation and analysis of particle drift islands in a torsatron
Authors: Gandy R. F. , Lin H. , Hartwell G. J. , Knowlton S. F. , Pritchard D. , Hanson J.
Published: July 2002

Magnetic islands in toroidal plasma confinement devices are generated by resonant magnetic error fields. Energetic particle drift surfaces can differ substantially from magnetic flux surfaces. In this Letter the first direct measurement of islands in energetic particle drift surfaces are presented. The understanding of these drift islands is important for the optimization of plasma confinement. These drift islands are demonstrated to form in energetic electron drift surfaces as a result of resonant magnetic field perturbations and resonant, applied electric field perturbations. The results, once corrected for inherent magnetic error fields, agree well with theoretical predictions.
Finite banana width effect on magnetoacoustic cyclotron instability
Authors: Chen Yan Ping , Tsai Shih Tung
Published: July 2002

The finite banana width (FBW) effect on the coupling between magnetoacoustic waves and the near harmonic gyro-oscillations of the energetic ions/α particles in tokamaks are studied. The gyrokinetic equation with FBW effect is rederived for the energetic trapped ions. The dispersion relation and growth rate of the magnetoacoustic cyclotron instability (MACI) are obtained. It is found that the coherence interaction between the energetic ion trajectory and mode field has a significant effect when the Larmor radius of energetic ions is larger than the wavelength of MACI. Near the low field side the FBW effect destabilizes the mode, while away from it the FBW gives a stabilizing effect.
Spectra of developed Langmuir turbulence in a nonisothermal magnetized plasma
Authors: Vyacheslavov L. N. , Burmasov V. S. , Kandaurov I. V. , Kruglyakov E. P. , Meshkov O. I. , Sanin A. L.
Published: July 2002

Strong Langmuir turbulence driven by a relativistic electron beam has been investigated by the laser scattering technique. The special features of the experiments are as follows: (i) in the regime under study, Langmuir turbulence is well developed, and the temporal and spatial scales of the turbulent region far exceed those of a single caviton; (ii) the dispersion of the electron plasma waves is governed by the magnetic field, although ωpe≫ωBe; and (iii) the temperature of plasma electrons is much higher than that of ions, i.e., the damping of ion-acoustic waves is small. k-Spectra of electron plasma waves are measured in a broad spectral region by means of CO2-laser scattering. Criterion of modulational instability of observed spectra is estimated. The role of conversion and scattering of Langmuir waves by ion-acoustic waves in nonlinear energy transfer is discussed.
Small-scale magnetic field-aligned density irregularities excited by a powerful electromagnetic wave
Authors: Istomin Ya. N. , Leyser T. B.
Published: July 2002

A set of equations describing the evolution of diffusive plasma density and temperature perturbations, and upper hybrid waves, coupled by a dipole electromagnetic pump field is derived. For the first time, instability from infinitesimal perturbations (soft excitation) and instability from a finite level of perturbations (hard excitation) are contained in the same model. Conditions for the formation of density irregularities and the irregularity spectrum in the nonlinearly stabilized instability state in the different excitation regimes are discussed. The results are of relevance for the small-scale magnetic field-aligned irregularities and a number of associated phenomena observed to be excited by a powerful high frequency electromagnetic wave transmitted into the ionospheric plasma.
The effect of Er on motional-Stark effect measurements of q, a new technique for measuring Er, and a test of the neoclassical Er
Authors: Zarnstorff M. C. , Levinton F. M. , Batha S. H. , Synakowski E. J.
Published: July 2002

Previous analysis of motional-Stark Effect (MSE) data to measure the q-profile ignored contributions from the plasma electric field. The MSE measurements are shown to be sensitive to the electric field and require significant corrections for plasmas with large rotation velocities or pressure gradients. MSE measurements from rotating plasmas on the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 2, 2176 (1975)] confirm the significance of these corrections and verify their magnitude. Several attractive configurations are considered for future MSE-based diagnostics for measuring the plasma radial electric field. MSE data from TFTR are analyzed to determine the change in the radial electric field between two plasmas. The measured electric field quantitatively agrees with the predictions of neoclassical theory. These results confirm the utility of a MSE electric field measurement.
Nonlinear simulation of power enhancement of an electromagnetic-wave-wiggler free-electron laser by employing a tapered axial guide magnetic field
Authors: Liang Xiao-Ping , Zhang Shi-Chang , Liu Yaowu , Jin Jian-Bo
Published: July 2002

Three-dimensional (3-D) simulations are presented to demonstrate the power enhancement in an electromagnetic-wave-wiggler free-electron laser by employing a tapered axial guide magnetic field. Results show that the power of 93 kW, which was obtained in a recent experiment, could be increased up to 350 kW by using a proper taper of the axial guide magnetic field. In contrast to a linear conclusion, the efficiency enhancement is not as remarkable as expected, if the taper of the axial guide magnetic field begins close to or after the position where the system has run to saturation. The power enhanced by properly tapering the guide magnetic field is stronger than that which occurs when the guide magnetic field is at resonance but untapered. The device could not operate normally if the guide magnetic field was too weak to focus the electron beam.
Stability properties of coherently gyrating relativistic electron beams
Authors: Davies J. A. , Chen C.
Published: July 2002

An investigation is made of the effects of nongyrotropic equilibrium distributions in the phase angle of p⊥ on the stability properties of a relativistic electron beam propagating along and gyrating about an applied uniform magnetic field. Perturbations are assumed to vary spatially only in the direction of the applied magnetic field, so that generated electromagnetic and longitudinal electric fields propagate parallel to the applied field. The two equilibrium distributions considered are the time-dependent distribution f0(p⊥,pz,ξ) with ξ=φ−Ωct/γ and the axial-dependent distribution f0(p⊥,pz,ζ) with ζ=φ−mΩcz/pz. A Vlasov-Maxwell analysis leads to integral equations relating the field Fourier components. These equations reduce to algebraic equations when no spread in γ is present in the time-dependent equilibrium distribution and when no spread in pz is present in the axial-dependent distribution. Numerical computations for these special cases show that a rich variety of stability properties are obtained by changing the distributions in ξ and ζ.
Multidimensional time-dependent free boundary model of ion extraction
Authors: Gosset Jérôme
Published: July 2002

For ion extraction problems such as plasma ion implantation (PII), ion sources design, and isotope separation (AVLIS), large systems need to be simulated and thus the cells used in particle-mesh simulations need to be large in relation to the Debye length. A multidimensional time-dependent free boundary (FB) model of plasma adapted to such situations has been developed. The equations of the model when the electronic temperature and then the Debye length tend towards zero are established as limits of the nonlinear Vlasov–Poisson system. The plasma then becomes a neutral region limited by a boundary emitting ions in an electron-free sheath. The model does not suppose this emission is space-charge dominated: ions are described continuously by the Vlasov equation. The FB is used only to solve the Poisson equation and acts on the ions only through the resulting potential. Numerical examples based on this model are given.