bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of a stabilizing solute gradient on the onset of thermal convection
Authors: Kaviany M.
Published: March 2003

In this study the effectiveness of a stabilizing solute concentration gradient in delaying the onset of convection in a horizontal fluid layer subjected to transient cooling from the top is considered. The linear amplification theory, which has been successful in predicting the onset time in layers with a uniform initial distribution, is used. The initial stable density stratification can cause the magnitude of the introduced disturbances to vanish. Therefore, the disturbances are reintroduced and the process is repeated until growth is obtained. An allowance is made for molecular diffusion of solute concentration, thus making its distribution time dependent. Quantitative results are obtained for the onset time in a system involving double diffusion of heat and salt in water. The effect of the layer thickness on the dimensional onset time is examined. The results show that for a situation where the onset time is significantly delayed, the dimensional onset time depends on the fluid depth.
Analytical solutions of model equations for two phase gas mixtures: Transverse velocity perturbations
Authors: Cavalier J. Frederick , Greenberg William
Published: March 2003

Model equations for a dilute binary gas system are derived, using a linear BGK scheme. Complete analytical solutions for the stationary half-space problem are obtained for transverse velocity perturbations. The method of solution relies on the resolvent integration technique.
Shock waves in superfluid helium
Authors: Moody D. M. , Sturtevant B.
Published: March 2003

Exact solutions of the equations of motion of liquid helium II can be compared to experiments to test Landau’s two-fluid theory. The best flows with which to conduct such tests are those in which amplitudes and gradients are large and in which the calculations and measurements are free from wall effects, e.g., shock waves. The four fundamental conservation equations of superfluid mechanics have been integrated across a one-dimensional discontinuity (shock wave) propagating into undisturbed helium II to yield a set of four algebraic equations (jump conditions) which, when supplemented by thermodynamic state information, establish the equilibrium flow state behind the shock wave for a given wave speed and undisturbed flow state ahead of the shock. These jump conditions have been solved numerically for 19 points on the helium II p-T diagram with upstream Mach number as the independent parameter. Representative results of the calculations are presented for pressure shocks, temperature raising shocks, and temperature lowering shocks. The results are compared to previous analytical approximate solutions to test the validity of those approximations. They are also compared to experimental data for shock waves in helium II as a means of testing the correctness of the full nonlinear two-fluid equations.
Trapping of plasma waves in cavitons
Authors: Tanikawa T. , Wong A. Y. , Eggleston D. L.
Published: March 2003

The observed characteristics of electron plasma waves trapped inside a density cavity, a caviton, created by the electrostatic fields associated with the waves verify quantitatively the predictions of the Zakharov equations. A two-frequency excitation technique was used to demonstrate the existence of eigenmodes for the trapped electron plasma waves in the caviton.
The effects of compressibility of the resistive ballooning mode
Authors: Hender T. C. , Carreras B. A. , Cooper W. A. , Holmes J. A. , Diamond P. H. , Similon P. L.
Published: March 2003

The linear stability of the resistive ballooning mode, as described by the resistive magnetohydrodynamic (MHD) model, is investigated both analytically and numerically. When the pressure evolution is approximated by fluid convection (reduced MHD model), an instability driven by geodesic curvature, with a growth rate γ∼η1/3 β2/3p, is found. For conditions relevant to the Impurity Study Experiment (ISX-B), it is shown that for modest poloidal bets (βp≂1), high current, and relatively high temperatures, compressibility has a significant stabilizing influence, relative to the pressure convection model. However, at high βp (≳2), low current, and lower temperatures, compressibility has much less effect.
Renormalized toroidal magnetohydrodynamic equilibria
Authors: Yamagishi T.
Published: March 2003

The magnetohydrodynamic equilibrium problem in a toroidal–helical configuration is reformulated by a renormalization of the unperturbed cylindrically symmetric equilibrium state by using the average of the helical perturbation. To the first order in toroidal effect, a general differential equation for the toroidal displacement is derived, which reduces to the tokamak result in the weak helical field limit and to the stellerator result in the weak plasma current limit.
Simulation studies on stability of EBT
Authors: Ohsawa Y. , Dawson J. M.
Published: March 2003

Stability and beta limits of the ELMO Bumpy Torus (EBT) are studied by using a 2 1/2 -D fully relativistic, electromagnetic particle code. A slab model is used which is bounded in the x direction (corresponding to the radial direction) and which is periodic in the y direction (corresponding to the poloidal direction). The imposed magnetic field is in the z direction (the ignorable coordinate) and is a function of x so that the plasma flutes in the absence of an EBT ring. The hot electron ring (layer) is shown to be effective in suppressing the Rayleigh–Taylor instability, when kyρh ≳1, where ρh is the Larmor radius of hot electrons. However, in a system where kyρh <1 for long wavelength modes, the plasma exhibits instability. One possible mode which agrees fairly well with our observations is the low-frequency hot electron interchange instability.
A thermal-barrier-potential measurement using the plasma Čerenkov effect
Authors: Feinberg B.
Published: March 2003

A method is proposed to measure the dip in potential in the thermal-barrier region of a tandem mirror. Plasma waves at a frequency near the plasma frequency of the cold electron component of a two-temperature plasma are enhanced by the plasma Cerenkov effect, and can thus be used to scatter a laser beam. The measurements of cold plasma density as a function of position provide a sensitive measurement of the potential depression.
Drag of evaporating or condensing droplets in low Reynolds number flow
Authors: Dukowicz John K.
Published: March 2003

The steady-state drag of evaporating or condensing droplets in low Reynolds number flow is computed. Droplet drag in air is obtained for five representative liquids (water, methanol, benzene, heptane, octane) for a range of ambient temperatures, pressures, and vapor concentrations. The drag is in general increased for a condensing droplet, and decreased for an evaporating droplet. The changes in drag can be quite large and depend in detail on the degree of evaporation or condensation, and on the individual liquid and vapor properties. The present results are used to test the existing experimentally derived correlations of Eisenklam and Yuen and Chen in the low Reynolds number regime. The Yuen and Chen correlation is found to be quite successful, but only in the case of condensation or mild evaporation. An improved correlation is suggested for evaporating droplets.
Asymmetric instabilities in buoyancy-driven flow in a tall vertical annulus
Authors: McFadden G. B. , Coriell S. R. , Boisvert R. F. , Glicksman M. E.
Published: March 2003

Linear stability of the one-dimensional flow between infinite vertical coaxial cylinders induced by heating the inner cylinder is considered for various ratios κ of the inner radius to the outer radius, and for Prandtl numbers P appropriate to air and water. For air with P=0.71 the least stable disturbance is nonaxisymmetric for κ<0.44 and is axisymmetric for κ>0.44, and in either case the instability is due to the action of the shear forces. For P=3.5, the situation is similar, except that the asymmetric shear mode is superceded by an axisymmetric instability driven by buoyancy forces for 0.03<κ<0.16. Wave speeds, wavenumbers, and critical Grashof numbers for these cases and for the case of zero Prandtl number are given.
Statistical analysis of the deviation of the Reynolds stress from its eddy-viscosity representation
Authors: Yoshizawa Akira
Published: March 2003

An improvement of the eddy-viscosity representation for Reynolds stress is made from the statistical viewpoint. The Reynolds stress is calculated with the aid of the two-scale direct-interaction formalism, and its deviation from the eddy-viscosity representation is found under general mean flows. This result theoretically elucidates the noncoincidence of the zeros of Reynolds stress and mean strain, which is frequently observed in asymmetric turbulent shear flows.
A zero-dimensional transport model for field-reversed configurations
Authors: Rej D. J. , Tuszewski M.
Published: March 2003

A zero-dimensional theoretical model is developed to study energy, particle, and magnetic flux confinement during the equilibrium phase in field-reversed configurations. The plasma is heated by adiabatic compression from the external magnetic field and by ohmic dissipation. Energy is lost from lower-hybrid-drift induced particle transport, classical and anomalous thermal conduction, and impurity line radiation. As an example, the model is used to analyze data measured in the FRX-C experiment.
Electric breakdown waves: Exact numerical solutions. Part I
Authors: Fowler Richard G. , Hemmati Mostafa , Scott Robert P. , Parsenajadh Soghra
Published: March 2003

Electrical breakdown is generally a progressive wave phenomenon governed by fluid dynamical equations relating chiefly to the electron gas. Computer solution of these equations is investigated extensively, resulting in improvements in their formulation, and in the understanding of the conditions under which solution is possible. The most significant new discovery is the importance of heat conduction, especially at the shock front of the wave.
Plasma wall sheath contributions to flux retention during the formation of field-reversed configurations
Authors: Milroy R. D. , Slough J. T. , Hoffman A. L.
Published: March 2003

Flux loss during field reversal on the TRX-1 field-reversed θ pinch is found to be much less than predicted by the inertial model of Green and Newton. This can be explained by a pressure bearing, conducting sheath which naturally forms at the wall and limits the flux loss. A one-dimensional (r-t) magnetohydrodynamic (MHD) numerical model has been used to study the formation and effectiveness of the sheath. The calculations are in excellent agreement with experimental measurements over a wide range of operating parameters. The results indicate that good flux trapping can be achieved through the field reversal phase of FRC formation with much slower external field reversal rates than in current experiments.
Tilt stability of arbitrary aspect ratio ion rings
Authors: Ruchti C. B. , Lovelace R. V.
Published: March 2003

A study is made of the low-frequency tilt stability of finite aspect ratio field-reversed ion rings. A new analytic-numerical method is described which overcomes the large-aspect-ratio assumption of previous analytical treatments. The method involves the numerical evaluation of the Vlasov time history integrals for the ring particles using orbits determined numerically in numerically calculated self-consistent equilibria. The method is applied to the tilt stability of fat, noncircular cross-section rings. The condition for tilt instability is found to be of the form ωz>kΩ, where the parameter k varies from 0.7 to 1.0 for the cases studied, and where ωz is the mean axial betatron frequency and Ω is the mean toroidal circulation frequency. For weak rings (field-reversal factor δ=Bself/Bex≤1), k approaches unity, whereas for strong rings (δ≊2), k≊0.7. As a given ring is elongated axially it is found that ωz decreases, while Ω changes relatively little. A saturation mechanism is proposed whereby tilt unstable rings reach a stable equilibrium through axial lengthening.
Nonlinear space-charge wave propagation on thin annular electron beams
Authors: Fenstermacher D. L. , Seyler C. E.
Published: March 2003

A warm fluid description of a relativistic thin annular electron beam is used to investigate large amplitude space-charge waves for currents up to the space-charge limit. The beam is centered within a conducting cylindrical boundary and is guided by a strong magnetic field in vacuum. Exact steady-state periodic and solitary wave solutions are found numerically up to the breaking amplitude and are characterized by nonlinear wave velocities which vary significantly from linear phase velocities for currents ≳0.7IL. A time-dependent pseudospectral code was developed and used to understand the dynamics of nonlinear wave interactions. The general character of these waves is to propagate as a stable superposition of solitary waves whose interactions are nearly elastic and preserve the coherence and form of the waves. The advantages of thin annular beams and the detrimental effects of warm beams are discussed. Recommendations are given concerning the amplitude, frequency, wavelength, and beam temperature of nonlinear waves for potential use in collective acceleration experiments.
Collective ion acceleration by a reflexing electron beam: Model and scaling
Authors: Mako F. , Tajima T.
Published: March 2003

Analytical and numerical calculations are presented for a reflexing electron beam type of collective ion accelerator. These results are then compared to those obtained through experiment. By constraining one free parameter to experimental conditions, the self-similar solution of the ion energy distribution agrees closely with the experimental distribution. Hence the reflexing beam model appears to be a valid model for explaining the experimental data. Simulation shows in addition to the agreement with the experimental ion distribution that synchronization between accelerated ions and electric field is phase unstable. This instability seems to further restrict the maximum ion energy to several times the electron energy.
Linear longitudinal oscillations in collisionless plasma diodes with thin sheaths. Part I. Method
Authors: Kuhn S.
Published: March 2003

A method is proposed for treating linear longitudinal perturbations in one-dimensional collisionless plasma diodes with a uniform plasma region and thin electrode sheaths. The method is comprehensive in that it allows for very general equilibrium, initial, boundary, and external-circuit conditions. Upon Laplace-transforming the Vlasov and Poisson equations in both space and time, appropriate evaluation of all pertinent relations leads to a set of 2+2nσ(nσ is the number of particle species) coupled integral equations in x and v for the following quantities (which are the time Laplace transforms of the respective physical perturbations): j̃e(ω) (external-circuit current density), Ẽ(x,ω) (electrostatic field), f̃σl(v>0,ω), and f̃σr(v<0,ω) (velocity distribution functions of the plasma-bound particles at the left- and right-hand plasma boundaries, respectively), where σ is the species index. The formal solution of these integral equations and the inverse Laplace transformation (ω→t) are discussed in general terms. In particular, it is shown that the intrinsic eigenfrequencies are given by the zeros of the coefficient determinant of the integral equations. A comparison with previous treatments is given, and it is concluded that extensions of the method proposed to more general systems should be feasible.
Linear longitudinal oscillations in collisionless plasma diodes with thin sheaths. Part II. Application to an extended Pierce-type problem
Authors: Kuhn S.
Published: March 2003

The integral-equation method developed in Part I is applied to a Pierce-type diode [J. Appl. Phys. 15, 721 (1944)] whose external circuit involves a resistor, an inductor, and a signal generator. The general linear perturbational problem is solved analytically for the small-amplitude quantities j̃e(t) (external-circuit current density) and Ẽ(x, t) (electrostatic field). Each of these quantities can be constructed from a ‘‘spatial’’ Green’s function (accounting for initial perturbations of the plasma), a ‘‘temporal’’ Green’s function (accounting for external-generator signals), and two functions associated with the initial state of the external circuit. The solutions generally exhibit an initial transient and an asymptotic part, the latter being a superposition of eigenmodes only. Systematic numerical results for eigenfrequencies and eigenmode profiles in some typical parameter regions demonstrate that the linear response and stability behavior of the diode system may substantially depend on the properties of the external circuit.
Parametric coupling of a whistler pump with a drift wave
Authors: Antani S. N. , Kaup D. J.
Published: March 2003

Parametric decay of a whistler pump into another whistler plus a drift wave is considered. Calculation of the linear growth rate shows that for the typical parameters of interest, scattering is restricted to the nearly forward direction. Further, comparison of the growth rate magnitude for this process with that for the stimulated Brillouin scattering shows that the drift-wave decay is a stronger process.
Formation of an epithermal ion species in a quiescent negative-potential-regime operated EBT
Authors: Chang C. S.
Published: March 2003

During the T mode of operation, a negative potential well is formed, whose depth is much greater than the background ion species thermal energy. As a result, the cold ions experience rapid inward (radial) diffusion, falling down the electrostatic potential hill and acquiring kinetic energy. The generation of a warm epithermal ion species through this mechanism is examined using simple physical models, and it is compared with the experimental observations.
Experimental and theoretical studies of the effects of nonuniformities in nonequilibrium magnetohydrodynamic flows
Authors: Lytle John K. , Louis Jean F.
Published: March 2003

A study of wake nonuniformities in a nonequilibrium magnetohydrodynamic plasma is described. Experimental results are presented from an electrodeless disk-shaped channel in both radial flow and swirl flow configurations. The experimental conditions are analyzed with a nonequilibrium plasma model that accounts for variations in the wake gas properties and wake width. The results show that the wake nonuniformities in the radial flow channel are weak, but in the swirl flow channel they are strong because of an instability. This instability is a consequence of the wake orientation. Furthermore, the intensity of the nonuniformity in an unstable plasma is dependent upon the wake gas velocity and wake width.
Coherent spontaneous emission from a modulated beam injected in a magnetized plasma
Authors: Lavergnat J. , Lehner T. , Matthieussent G.
Published: March 2003

The linear stage of the coherent emission of a modulated beam injected in a cold plasma is investigated. The feature of the radiation produced is shown to be governed by the ratio ω2p/ω2c of the background plasma. This kind of emission provides an efficient way to convert the kinetic energy of the beam particles into electromagnetic wave energy: The power given to waves averaged over periods of the beam modulation is found to be an appreciable part of the initial beam power. The transfer can be maximized in the electron whistler range (Cerenkov emission) for electron beams whose characteristics are designed to met the ‘‘focalization condition’’ (double pole in the dispersion relation). The most suitable mode is the normal Doppler shift ion cyclotron mode for ion beams. Application to relevant ionospheric plasmas is considered.
Formation of virtual cathodes and microwave generation in relativistic electron beams
Authors: Kwan Thomas J. T. , Thode Lester E.
Published: March 2003

Simulation of the generation of a relativistic electron beam in a foil diode configuration and the subsequent intense microwave generation resulting from the formation of the virtual cathode is presented. The oscillating virtual cathode and the trapped beam electrons between the real and the virtual cathodes were found to generate microwaves at two distinct frequencies. Generation of high-power microwaves with about 10% efficiency might reasonably be expected from such a virtual-cathode configuration.
The dispersive effects of Basset history forces on particle motion in a turbulent flow
Authors: Reeks M. W. , McKee S.
Published: March 2003

A calculation of the asymptotic particle diffusion coefficient in an unbounded stationary homogeneous turbulent flow is performed for a particle subjected to both added mass and Basset history forces, as well as the normal Stokes drag. The original value obtained by Tchen is shown to be in error by an additional term which for a sphere of diameter d is given by (d2/2πνf)σ, νf being the kinematic viscosity, and σ the covariance of the incipient relative velocity between particle and local fluid. Noticeably this term does not depend upon the particle density. This extra term is consistent with values obtained for the particle diffusion coefficient from a numerical simulation.
Periodic motion of two and four vortices in a cylindrical pipe
Authors: Hardin J. C. , Mason J. P.
Published: March 2003

The motion of two and four rectilinear vortices inside a cylindrical pipe is studied under the restriction that the total circulation be zero. In the two-vortex case, it is shown that the motion is always periodic and an expression for the period is derived. In the four-vortex case, the motion is determined not to be periodic in general. However, a class of solutions where the motion is periodic is found. Several sample calculations of the vortex motion are included.
A spectral theory for weakly nonlinear instabilities of slowly divergent shear flows
Authors: Plaschko Peter , Hussain A. K. M. F.
Published: March 2003

A general theory is proposed for the downstream evolution of nonlinearly interacting waves in a nonparallel shear flow. The waves are represented as Fourier integrals over the spectral range. A multiple scales expansion is employed to predict evolutions of the Fourier mode amplitudes A(x,ω)—where x is the streamwise coordinate and ω is the frequency—by assuming that both nonlinear and nonparallel effects can be expressed in terms of the same expansion parameter ε. The theory produces in the leading order the Orr–Sommerfeld equation, and in the next order a Landau-type integrodifferential equation for A. The linear part of the equation contains only the first-order spatial derivative, and the nonlinear part consists of wave–wave interactions integrated over the entire spectral range of A. The approach is very general in that no Reynolds number (Re) scaling is involved as Re is retained independent of ε throughout and that no Landau constant is involved so that each wave can grow or decay in space or time. The Landau-type equation is too intricate for analytic solution but is suitable for numerical solution. An asymptotic analysis for large x via the method of stationary phase reveals that the subharmonic components provide the dominant contribution to the nonlinear terms, consistent with experimental observations.
A reformulation of lower-hybrid wave propagation and absorption
Authors: Grossmann William , Weitzner Harold
Published: March 2003

A full wave solution of the propagation and resonant absorption of lower-hybrid waves in an inhomogeneous plasma using the cold plasma model is carried out and compared with results obtained in an infinite homogeneous plasma medium. It is found that the density gradient coupled to finite poloidal model number allows the lower-hybrid wave to reach the hybrid layer for the case of purely perpendicular propagation. Such a wave is evanescent in the infinite homogeneous plasma. The analysis also shows that, for the cold plasma model, roughly equal amounts of energy are absorbed by the ions and electrons in the resonant layer. The analysis may have some bearing on the experimental efforts to propagate lower-hybrid waves into the interior of tokamak plasmas.
Spinup or spindown of a rotating electrically conducting fluid in a magnetic field
Authors: Mathews J. L. R. , Nath G.
Published: March 2003

The hydromagnetic spinup or spindown of an incompressible, rotating, electrically conducting fluid over an infinite insulated disk with an applied magnetic field is studied when the impulsive motion is imparted either to the fluid or to the disk. The nonlinear partial differential equations governing the flow are solved numerically using an implicit finite-difference scheme. It is found that the spinup (or spindown) time due to impulsive motion of the disk is much shorter than the spinup (or spindown) time due to the impulsive motion of the distant fluid. The spinup (or spindown) time for the hydromagnetic case is comparatively smaller than the corresponding nonmagnetic case. Spindown is not merely a mirror reflection of spinup.
Equilibrium modeling of ISX-B tokamak discharges
Authors: Charlton L. A. , Wieland R. M. , Neilson G. H.
Published: March 2003

Poloidal magnetic measurements of the multipole moments of the toroidal current density and of the external multipole fields provide information on the current profile in ISX-B. This can be used to prescribe a form for the current profile which produces an equilibrium consistent with profile measurements, magnetic data, and the q=1 surface. This procedure has been applied satisfactorily in a number of cases, several of which are presented here.
Ion skin depth scaling and cusp distortion in picket fence geometry
Authors: Ratliff S. T. , Dawson J. M.
Published: March 2003

Computer simulations show that when a low-frequency (ω=0 in the simulations) external electric field is applied to a cusp-confined plasma, some of the electrons are trapped by the cusp fields, and the ions do a significant amount of shielding of the external E field (skin effect). The effect of varying the cusp spacing relative to the ion collisionless skin depth on cusp distortion, electron current, and ion current was also studied.
Electron ring startup model for ELMO Bumpy Torus
Authors: Hamasaki S. , Krall N. A. , Sperling J. L.
Published: March 2003

The microwave power requirements for EBT electron ring startup are calculated based on the assumption that the ring electrons originate as runaways accelerated by the first harmonic resonance of the extraordinary mode. The calculated microwave power threshold for startup is consistent with the experimental data for the microwave power requirements deduced from the C–T transition points. The ring model is also compatible with the experimentally observed ring location if the subsequent heating and toroidal drift motion of the energetic electron after the initial startup is considered in determining the ring spatial structure.
Neoclassical transport in a bumpy closed field line device in the collisional, small electric field regime
Authors: Hastings D. E.
Published: March 2003

Neoclassical diffusion coefficients are obtained in the collisional, small electric field regime for any bumpy closed field line device. A variational principle is used and energy scattering is included in the collision operator.
Alfvén ion-cyclotron instability in tandem-mirror plasmas. II
Authors: Smith Gary R. , Nevins William McCay , Sharp William M.
Published: March 2003

The linear theory of Alfvén ion-cyclotron instability is extended by a rigorous treatment of plasma nonuniformity along the equilibrium magnetic field. A standard phase-integral dispersion relation provides stability boundaries. Two ion distribution functions allow modeling of plasmas formed in various ways, including neutral-beam injection both perpendicular and oblique to magnetic field lines. Stability results are quantitatively consistent with past and present experiments. The results further indicate that two important features of stable mirror cells in future experiments are large mirror ratios and pitch-angle distributions that are broad or are peaked far from perpendicular to the magnetic field.
Three-dimensional, nonlinear evolution of the Rayleigh–Taylor instability of a thin layer
Authors: Manheimer Wallace , Colombant Denis , Ott Edward
Published: March 2003

A numerical simulation scheme is developed to examine the nonlinear evolution of the Rayleigh–Taylor instability of a thin sheet in three dimensions. It is shown that the erosion of mass at the top of the bubble is approximately as described by two-dimensional simulations. However, mass is lost into spikes more slowly in three-dimensional than in two-dimensional simulations.
Reflection of a planar ion-acoustic soliton from a finite plane boundary
Authors: Nishida Y.
Published: March 2003

Experimental observations on the partial reflection of a planar ion-acoustic soliton from a plane rigid boundary with sharp density gradients are presented. The reflected wave attenuates rapidly, forming a spherical wave front apart from a small reflector. The reflection coefficient of about 25% at the boundary has no apparent dependence on the incident wave amplitude and the sheath thickness in front of the reflector in the case of the disk reflector, while it has a dependence in the case of the mesh reflector, having even larger values of up to about 50%.
Convective stimulated Raman scattering instability in UV laser plasmas
Authors: Seka W. , Williams E. A. , Craxton R. S. , Goldman L. M. , Short R. W. , Tanaka K.
Published: March 2003

Time-resolved stimulated Raman scattering (SRS) spectra from UV laser-produced plasmas are reported. In analyzing these spectra the theory for SRS in inhomogeneous plasma is extended. This theory is applied to obtain temporally resolved electron temperatures in the plasma corona. Typical coronal temperatures for plastic targets irradiated at 1015 W/cm2 range up to 1.8 keV. A comparison is made with the predictions of hydrodynamic simulations.
Physical model of current drive by ac helicity injection
Authors: Bellan P. M.
Published: March 2003

It is shown that the ac helicity injection current drive described by Jensen and Chu and by Bevir and Gray can alternately be deduced from the pseudoelectric field ∼〈Ũ×B̃〉/c produced by oscillating velocity and magnetic field vectors Ũ and B̃ (here 〈 〉 denote time average).
Relation of fluid drift to oscillation center drift
Authors: Cary John R.
Published: March 2003

The fluid theory and oscillation center theory of the ponderomotive drift give apparently different results. It is shown that this difference is caused by a ponderomotive diamagnetic effect.
Kelvin–Helmholtz stability with mass and heat transfer
Authors: Nayak A. R. , Chakraborty B. B.
Published: March 2003

The Kelvin–Helmholtz stability of the cylindrical interface between the vapor and liquid phases of a fluid is studied when the vapor is hotter than the liquid and the two phases are enclosed between two cylindrical surfaces coaxial with the interface, and when there is a mass and heat transfer across the interface. Both axisymmetric and asymmetric disturbances are considered. A comparison of the results obtained here with those for the corresponding problem with plane geometry reveals that the plane geometry configuration is more stable than the cylindrical one, and, contrary to the case of Rayleigh–Taylor instability problem, the heat and mass transfer has a destabilizing influence on the system.
Numerical experiments on one-dimensional model of turbulence
Authors: Qian J.
Published: March 2003

The initial-value problem of a forced Burgers equation is numerically solved by the Fourier expansion method. It is found that its solutions finally reach a steady state of ‘‘laminar flow’’ which has no randomness and is stable to disturbances. Hence, strictly speaking, the so-called Burgers turbulence is not a turbulence. A new one-dimensional model is proposed to simulate the Navier–Stokes turbulence. A series of numerical experiments on this one-dimensional turbulence is made and is successful in obtaining Kolmogorov’s k−5/3 inertial-range spectrum. The (one-dimensional) Kolmogorov constant ranges from 0.5 to 0.65.
Conditional averages associated with the fine structure in a turbulent boundary layer
Authors: Rajagopalan S. , Antonia R. A.
Published: March 2003

The periods of strong activity in the high-frequency components of velocity and temperature fluctuations in a turbulent boundary layer are detected using digital data acquisition and processing by a method similar to that of Rao et al. [J. Fluid Mech. 48, 339 (1971)]. These periods are identified with the fine structure of the flow. Conditional averages of temperature and especially velocity associated with the fine structure differ sufficiently from those determined using the variable interval time averaging (VITA) technique to question whether these two techniques trigger on the same events. The characteristic frequency of occurrence, duration and ensemble averages of the velocity and temperature fields associated with the fine structure scale on outer variables when the momentum thickness Reynolds number exceeds a value of about 2000.
Instabilities of shock and detonation waves
Authors: Fowles G. R. , Houwing A. F. P.
Published: March 2003

It is shown that the compatibility conditions for spontaneous splitting of a single shock into two shocks plus a contact discontinuity are satisfied whenever the instability limits derived by D’yakov, Kontorovich, and Erpenbeck are violated. They can be stated as −1<j2 (dV/dP)H <(1−M2−M2V0/V)(1−M2+M2V0/V)−1 <1+2M, where j2=( P−P0)/(v0−v), M=‖(D−u)/c‖, and the derivative is along the Hugoniot curve. One-dimensional splitting can occur when the outer limits are exceeded. Violation of the intermediate upper limit (Kontorovich condition) admits spontaneous generation of steady transverse waves similar to those often observed in detonations.
The occurrence of isothermal discontinuities in relativistic fluids
Authors: Chapline G. F. , Granik A.
Published: March 2003

The effect of radiation on the appearance of an isothermal discontinuity in a relativistic shock wave is considered. When the postshock speed of sound is small compared to the speed of light, algebraic relations are found for the allowed ratios of radiation pressure to matter pressure and for the amplitude of the isothermal discontinuity.
Nonlinear kinetic theory of a single helicity tearing instability
Authors: Swartz K. , Hazeltine R. D.
Published: March 2003

The evolution of a single helicity tearing mode is analyzed by kinetic theory, including modification of the electron orbits by the islands themselves. Using flux coordinates appropriate to island geometry, analytic solutions for the time-dependent island width are obtained. These describe evolution from linear growth to nonlinear saturation in the collisionless limit, and transition from such saturation to slow Rutherford-type growth for increasing finite collisionality. Previous work is thus unified and generalized.
Magnetic island growth
Authors: Boozer Allen H.
Published: March 2003

The response of a finite conductivity plasma to resonant magnetic perturbations is studied. The equations, which are derived for the time development of magnetic islands, help one to interpret the singular currents which occur under the assumption of perfect plasma conductivity. The relation to the Rutherford regime of resistive instabilities is given.
Nonlinear computations of a solar flare model
Authors: Strauss H. , Van Hoven G.
Published: March 2003

A cylindrical axisymmetric tearing mode model for solar flares is investigated numerically. Large magnetic energy release only occurs when there are at least two mode rational surfaces in the current-carrying plasma.
Drift waves in rotating plasmas
Authors: Horton Wendell , Liu Jixing
Published: March 2003

The stability of the electron drift wave is investigated in the presence of E×B plasma rotation typical of the central cell plasma in tandem mirrors. It is shown that a rotationally driven drift wave may occur at low azimuthal mode numbers. Conditions for rotational instabilities are derived. Quasilinear formulas are given for the anomalous transport associated with the unstable fluctuations.
The effects of neutral beam injection on impurity transport in tokamaks
Authors: Stacey W. M. , Sigmar D. J.
Published: March 2003

A self-consistent theory is presented for particle flows in the flux surface, radial particle fluxes, and the radial electric field in a tokamak. The effects of beam-impurity momentum exchange, radial transfer of toroidal momentum, and the poloidal variation in density and potential which ensue when vφ≂vth are treated—the interaction among these effects is nonlinear. Model problem predictions based on the theory indicate enhanced inward impurity transport with strong counterinjection and outward impurity transport with strong coinjection of neutral beams.
An investigation of nonlinear interactions in a two-dimensional recirculating flow
Authors: Sommeria J. , Verron J.
Published: March 2003

A novel experiment is presented on two-dimensional flows of mercury in a closed domain, in which three-dimensional perturbations are suppressed by the effect of a uniform magnetic field. An elementary process of an inverse energy cascade is observed, which is approximately described by interaction of two triads in Fourier space. The experimental results are in good agreement with a finite difference numerical method.
Numerical simulation of the inverse cascade in two-dimensional turbulence
Authors: Frisch U. , Sulem P.-L.
Published: March 2003

A numerical simulation of large-scale two-dimensional turbulence fed in a narrow band of wavenumbers is presented. The development of the inverse energy cascade predicted by Kraichnan [Phys. Fluids 10, 1417 (1967)] is observed.
Noise-induced transition of the radial electric field in a nonaxisymmetric torus
Authors: Shaing K. C.
Published: March 2003

The effect of external noise on the transition of the radial electric field in a nonaxisymmetric torus is studied. Depending on the size of the fluctuations, the transition of the radial electric field can either exhibit hysteresis or choose the path for which the thermodynamic potential (generalized heat production rate) Ṡl is an absolute minimum. Fluctuations other than the radial electric field are also studied.
Resonant interaction of internal waves in a stratified shear flow
Authors: Tsutahara Michihisa
Published: March 2003

The resonant interactions among internal wave trains in a flow consisting of a middle layer of homogeneous fluid in linear shear flow and continguous upper and lower stratified layers of constant velocities are investigated. Three wave trains satisfying the dispersion relations and the so-called resonant conditions, k1=k2+k3 and σ1=σ2+σ3, are considered graphically, and the resonant triads of the same mode are found to exist. Some multiple resonances, i.e., the resonant interactions among waves which are the members of different triads, are also found to happen. The explosive and decay instabilities of multiple resonances are considered qualitatively, and energy exchange is expected to occur among the resonant triads.
Effects of flow contraction on evolution of turbulence
Authors: Tsugé Shunichi
Published: March 2003

The effect of wind-tunnel contraction on turbulence intensity is discussed. It is shown that only small eddies decay through contraction in agreement with the classical prediction, whereas large eddies are found to be amplified with an increase in the mean flow velocity. The latter effect becomes appreciable for a flow contraction ratio greater than 4, which is in good agreement with observations.
Electron heating caused by the ion-acoustic decay instability in a finite-length system
Authors: Rambo P. W. , Woo Wee , DeGroot J. S. , Mizuno K.
Published: March 2003

The ion-acoustic decay instability is investigated for a finite-length plasma with density somewhat below the cutoff density of the electromagnetic driver (n∼0.7nc). For this regime, the heating in a very long system can overpopulate the electron tail and cause linear saturation of the low phase velocity electron plasma waves. For a short system, the instability is nonlinearly saturated at larger amplitude by ion trapping. Absorption can be significantly increased by the large-amplitude ion waves. These results compare favorably with microwave experiments.
Simulation of bumpy torus with hot-ion rings
Authors: Ohsawa Y. , Dawson J. M.
Published: March 2003

Stability of a bumpy torus which makes use of hot-ion rings instead of hot-electron rings is studied by using a two and one-half-dimensional relativistic, electromagnetic particle code. This model is periodic in the y direction and is bounded in the x direction, with the z direction being an ignorable coordinate. The external magnetic field points in the z direction and decreases with x. A hot-ion layer is introduced parallel to the y-z plane. Two types of instabilities are observed: a high-frequency (ω∼ωci), short-wavelength mode and the low-frequency hot-ion interchange mode. The high-frequency instability is found to be stabilized if the hot-ion density scale length Δ is large enough compared to the hot-ion Larmor radius  ρh;  ρh/Δ≲ 1/2 .
Radial structure of curvature-driven instabilities in a hot-electron plasma
Authors: Spong D. A. , Berk H. L. , Van Dam J. W.
Published: March 2003

A nonlocal analysis of curvature-driven instabilities for a hot-electron ring interacting with a warm background plasma has been made. Four different instability modes characteristic of hot-electron plasmas have been examined: the high-frequency hot-electron interchange (at frequencies larger than the ion-cyclotron frequency), the compressional Alfvén instability, the interacting background pressure-driven interchange, and the conventional hot-electron interchange (at frequencies below the ion-cyclotron frequency). The decoupling condition between core and hot-electron plasmas has also been examined, and its influence on the background and hot-electron interchange stability boundaries has been studied. The assumed equilibrium plasma profiles and resulting radial mode structure differ somewhat from those used in previous local analytic estimates; however, when the analysis is calibrated to the appropriate effective radial wavelength of the nonlocal calculation, reasonable agreement is obtained. Comparison with recent experimental measurements indicates that certain of these modes may play a role in establishing operating boundaries for the ELMO Bumpy Torus-Scale (EBT-S) experiment. The calculations given here indicate the necessity of having core plasma outside the ring to prevent the destabilizing wave resonance of the precessional mode with a cold plasma.
General equilibrium and stability properties of high-β tokamaks
Authors: Ramos J. J.
Published: March 2003

An analytic approach to the ideal magnetohydrodynamic equilibrium and stability of large-aspect-ratio tokamak configurations in which the fluid and magnetic pressures are comparable, is developed. It is shown that these configurations are unstable against a pressure gradient driven internal mode whose threshold sets an upper limit to the ‘‘second’’ domain of stability.
Absolute growth of coupled forward and backward Raman scattering in inhomogeneous plasma
Authors: Koch P. , Williams E. A.
Published: March 2003

The boundary-value problem describing stimulated Raman scattering induced by a plane coherent electromagnetic wave normally incident upon plasma with a monotonic density profile is solved on a global basis. Propagation of amplified electrostatic waves between the resonant backward- and forward-scattering points and reflection of forward-scattered electromagnetic waves into the resonant backward-scattering region result in an absolute instability. The threshold, growth rate and line spacing for this instability are derived.
Laser-induced profile modification: Effect of electron response
Authors: Aldrich C. H. , Jones R. D. , Lee K.
Published: March 2003

Physical processes which occur at the critical surface such as resonance absorption and scattering of the incident laser light are very sensitive to the exact form of the laser induced density profile. The presence of ‘‘overdense bumps’’ in density profiles has been seen experimentally and calculated theoretically using a simple isothermal model. These structures are important, in that they could, in an experimental situation, reflect a significant amount of light before it reaches the critical surface and considerably affect the physics occuring within a laser pellet target. A generalization of a previous calculation by using an arbitrary equation of state (density response of the electrons to the electrostatic potential φ) of the form ρ=ρ(φ) is obtained. In particular, two models for the electronic response (one including the effects of the presence of trapped electrons within the overdense shock structures) calculating the resulting effect on the produced density profiles are investigated. Qualitatively, it is found that the presence of overdense structures and their one-dimensional stability within an underdense plasma is unaffected by the more general electronic response. Conditions for the existence of these structures are discussed and a detailed comparison with full particle in cell (PIC) code simulations is made, emphasizing the trapped electron model where excellent agreement is obtained.
Cross-correlation measurements in a turbulent boundary layer above a rough wall
Authors: Bessem J. M. , Stevens L. J.
Published: March 2003

Cross correlation between signals from a wall probe and a hot-wire probe indicates that large structures, inclined to the wall at approximately 20 °, exist in a rough-wall boundary layer as reported earlier by Brown and Thomas and by Head and Bandyopadhyay for a smooth wall.
Visual measurement of streamwise vorticity in the mixing layer
Authors: Alvarez Carlos , Martínez-Val Rodrigo
Published: March 2003

The analysis of the film of a turbulent mixing layer shows evidence of highly bent spanwise vorticity filaments within the large coherent structures (bending that produces streamwise vorticity and thence three-dimensionality), likely becaue of a certain type of columnar instability of those structures.
Free surface creeping motion related to a buckling phenomenon
Authors: Tyvand Peder A.
Published: March 2003

The viscous buckling phenomenon studied experimentally by Suleiman and Munson is discussed theoretically. In the linear theory used, no true instability is found, but the steepness of a wavy disturbance may grow in time when the dimensionless shear stress exceeds (3/2)(2)1/2 . Maximal kinematic contraction horizontally determines a critical angle equal to 35.27° between the wave crests of the preferred disturbance and the channel walls.
Nonlinear thermal surface waves
Authors: Gradov O. M. , Stenflo L.
Published: March 2003

It is shown that density profile modifications near a plasma surface can survive at moving localized spots because of the radiation pressure of leaking wave field fluctuations. The properties of these luminous surface cavitons are studied.
Nonlinear mode conversion of intense ordinary waves into electrostatic upper-hybrid waves
Authors: Lin A. T. , Lin Chih-Chien
Published: March 2003

A high-power ordinary wave launched from the low field side is observed to undergo nonlinear mode conversion process and is partially transformed into a short-wavelength electrostatic upper-hybrid wave at the upper-hybrid layer. The nonlinear conversion process can be explained as due to a periodic modulation of upper-hybrid frequency at twice its value, while the frequency modulation comes from the combination of electron mass oscillation and oscillating density modulation induced by the ordinary wave.