bluuebunny/crossref_metadata_2025 huggingface dataset:
One- and two-particle Lagrangian acceleration correlations in numerically simulated homogeneous turbulence
Authors: Yeung P. K.
Published: July 2002

Lagrangian statistics of the fluid particle acceleration are studied by direct numerical simulation, in stationary isotropic turbulence (at three different Reynolds numbers) and homogeneous shear flow with uniform mean shear rate. The one-particle acceleration autocorrelation decays rapidly with time, with a zero crossing just over two Kolmogorov time scales. In contrast, two-particle correlations are relatively persistent, especially for particle pairs of small initial separation distance. Results for intermediate times at a Taylor-scale Reynolds number of 140 resemble a t−1 inertial-range scaling suggested in the literature, but even higher Reynolds numbers are needed for more definitive comparisons. Use of a theoretical argument and conditional sampling indicates that the two-particle correlation is determined by a coupling between a correlation localized in space and a particle-pair separation probability density of positive skewness. The scenario which emerges is that whereas some fluid particles accelerate rapidly away from each other, the majority of particle pairs can still be relatively close together and hence help maintain the two-particle correlation at significant levels. Acceleration correlations in homogeneous shear flow are found to display a tendency toward local isotropy.
Comments on “Hopf bifurcation in wakes behind a rotating and translating circular cylinder” [Phys. Fluids 8, 1972 (1996)]
Authors: Van Atta Charles W.
Published: July 2002

Low-order Galerkin computational results [Phys. Fluids 8, 1972 (1996)] predict a decrease of vortex shedding frequency f with increasing cylinder rotation angular velocity ω, in conflict with much earlier experimental results [M.S. thesis, University of California, San Diego, 1968; AIAA J. 7, 1817 (1969)] for which f increases with increasing ω. A possible flaw in the computations is noted, and a correlation of experimental and computational results is presented.
Differential diffusion in low Reynolds number water jets
Authors: Saylor J. R. , Sreenivasan K. R.
Published: July 2002

Experimental data on differential diffusion between two species with large and quite disparate Schmidt numbers were obtained in a turbulent water jet by optically measuring the two species concentrations simultaneously. Experimental conditions were chosen so that the species were dilute and did not affect the water density thereby avoiding inertial effects. Differential diffusion was found to be significant in magnitude, even in the absence of these effects. Schmidt number ratios of 4 and 18 were considered. Differential diffusion was found to be statistically significant and to manifest at scales larger than the computed Batchelor scale. In some instances the concentration signal for the species with larger diffusivity was simply a blurred version of the other, while in other instances structures present in one signal were completely absent from the other. This second observation, presumably a more complex effect due to diffusion across velocity gradients, is discussed.
The effect of nonvertical shear on turbulence in a stably stratified medium
Authors: Jacobitz Frank G. , Sarkar Sutanu
Published: July 2002

Direct numerical simulations were performed in order to investigate the evolution of turbulence in a stably stratified fluid forced by nonvertical shear. Past research has been focused on vertical shear flow, and the present work is the first systematic study with vertical and horizontal components of shear. The primary objective of this work was to study the effects of a variation of the angle θ between the direction of stratification and the gradient of the mean streamwise velocity from θ=0, corresponding to the well-studied case of purely vertical shear, to θ=π/2, corresponding to purely horizontal shear. It was observed that the turbulent kinetic energy K evolves approximately exponentially after an initial phase. The exponential growth rate γ of the turbulent kinetic energy K was found to increase nonlinearly, with a strong increase for small deviations from the vertical, when the inclination angle θ was increased. The increased growth rate is due to a strongly increased turbulence production caused by the horizontal component of the shear. The sensitivity of the flow to the shear inclination angle θ was observed for both low and high values of the gradient Richardson number Ri, which is based on the magnitude of the shear rate. The effect of a variation of the inclination angle θ on the turbulence evolution was compared with the effect of a variation of the gradient Richardson number Ri in the case of purely vertical shear. An effective Richardson number Rieff was introduced in order to parametrize the dependence of the turbulence evolution on the inclination angle θ with a simple model based on mean quantities only. It was observed that the flux Richardson number Rif depends on the gradient Richardson number Ri but not on the inclination angle θ.
Fluctuation growth and instability associated with a singularity of the balance equations
Authors: McWilliams James C. , Yavneh Irad
Published: July 2002

Large-scale flows in the atmosphere and ocean are usually in a state of approximate momentum balance, the simplest form of which is geostrophy. Furthermore, balanced models have often been shown to be quite accurate in this regime, with the quasigeostrophic equations the simplest such model and the balance equations a more accurate one, even though such models exclude the rapidly oscillatory, unbalanced dynamics of acoustic, gravitational, and inertial oscillations. However, this behavior is not universal, and here we investigate the fluid dynamics on one of the margins of this regime. We solve for linearized, inviscid fluctuations about a horizontal shear flow with spatially uniform vorticity and strain rate in a rotating, stratified, incompressible fluid, without making any balanced approximations. In both parallel and elliptical shear flows, we find that a significant increase occurs in the growth of unbalanced fluctuations near the violation of a necessary condition for the time integrability of the balance equations. This condition is that the absolute vertical vorticity everywhere exceeds the modulus of the horizontal strain rate. Thus, we seemingly have found a new boundary to the regime of large-scale dynamics, with its approximate gradient-wind balance, anisotropic velocity field, and mostly “slow-manifold” evolutionary behavior.
On a possible mechanism of anomalous diffusion by Rossby waves
Authors: Dupont F. , McLachlan R. I. , Zeitlin V.
Published: July 2002

We study the advection of passive tracers by traveling plane Rossby waves of finite amplitude. In distinction with previous studies the nonlinearity of the wave field is taken into account in the first order of perturbation theory by considering the Lagrangian transport by resonant wave triads. Using the waves’ phases as new dynamical variables we reduce the problem to the study of a specific one-and-a-half degree of freedom Hamiltonian system with nonharmonic modulation. By using a symplectic integrator we study this system numerically and find an interesting series of bifurcations of its phase portrait as the nonlinearity increases. As is standard in the systems of this type we commonly see a chaotic sea with elliptic islands in the phase space, which means that in the physical space the resonant triads give rise to chaotic mixing and ballistic transport, respectively. The relevance of these results to the transport properties of β-plane turbulence is discussed.
Spin coating and air-jet blowing of thin viscous drops
Authors: McKinley I. S. , Wilson S. K. , Duffy B. R.
Published: July 2002

Using the lubrication approximation we investigate the spreading of a finite-sized thin drop of incompressible Newtonian fluid on a planar substrate subjected to a jet of air blowing normally to the substrate. Three specific problems are studied in detail: a jet of air acting normally to the substrate when gravity effects are negligible, a jet of air directed vertically downward onto a sessile drop on a horizontal substrate, and a jet of air directed vertically upward onto a pendent drop on a horizontal substrate. The dynamics of the moving contact line are modeled by means of a generalized “Tanner Law” relating the contact angle to the speed of the contact line. Both symmetric two-dimensional and axisymmetric three-dimensional geometries are considered; the latter case is equivalent to that of an axisymmetric drop spreading on a substrate rotating with constant angular velocity (the simplest model for spin coating). Quasistatic solutions for the drop profile are obtained in the limit of small capillary number and the stability of equilibrium solutions to small perturbations is determined. The analysis is then extended to include drops with a dry patch at their center which are found to be unconditionally unstable in all the cases investigated.
Measurement of the nonlinear behavior of acoustical rigid porous materials
Authors: Aurégan Y. , Pachebat M.
Published: July 2002

The measurement of the flow resistivity of porous materials shows that two types of behavior can be observed, depending on the value of a Reynolds number based on the porous material microgeometry. For Reynolds numbers (Re) smaller than a critical Reynolds number, the increase of the resistivity is quadratic in Re. For Re larger than this value the increase is linear in Re (Forchheimer’s law). A comparison between acoustic measurements and an equivalent fluid model shows that the main effect of high sound level on sound propagation through rigid porous materials is the variation of the flow resistivity.
A Lagrangian model for turbulent dispersion with turbulent-like flow structure: Comparison with direct numerical simulation for two-particle statistics
Authors: Malik Nadeem A. , Vassilicos J. C.
Published: July 2002

A wide range of relative two-particle dispersion statistics from the Lagrangian Kinematic Simulation (KS) model, which contains turbulent-like flow structures, compares well with Yeung’s [Phys. Fluids 6, 3416 (1994)] DNS results. In particular, the Lagrangian flatness factor μ4(t) compares excellently (better than Heppe’s [J. Fluid Mech. 357, 167 (1998)] nonlinear stochastic model). For higher Reynolds numbers the results from KS show that μ4(t) is significantly greater than 3 over a wide range of times within the inertial range of time scales.
Structure and dynamics of the wake of bubbles and its relevance for bubble interaction
Authors: Brücker Christoph
Published: July 2002

The flow in the wake of single and two interacting air bubbles freely rising in water is studied experimentally using digital-particle-image-velocimetry in combination with high-speed recording. The experiments focus on ellipsoidal bubbles of diameter of about 0.4–0.8 cm which show spiraling, zigzagging, and rocking motion during their rise in water, which was seeded with small tracer particles for flow visualization. Under counterflow conditions in the vertical channel, the bubbles are retained in the center of the observation region, which allows the wake oscillations and bubble interaction to be observed over several successive periods. By simultaneous diffuse illumination in addition to the light sheet, we were able to record both the path and shape oscillations of the bubble, as well as the wake structure in a horizontal and vertical cross section. The results show that the zigzagging motion is coupled to a regular generation and discharge of alternate oppositely oriented hairpin-like vortex structures. Associated with the wake oscillation, the bubble experiences a strong asymmetric deformation in the equatorial plane at the inversion points of the zigzag path. The zigzag motion is superimposed on a small lateral drift of the bubble, which implies the existence of a net lift force. This is explained by the observed different strength of the hairpin vortices in the zig and zag path; a seemingly familiar phenomenon was found in recent numerical results of the sphere wake flow. For spiraling bubbles the wake is approximately steady to an observer moving with the bubble. It consists of a twisted pair of streamwise vortex filaments which are wound in a helical path and are attached to the bubble base at an asymmetrical position. The minor axis of the bubble is tilted in the tangential plane as well as in the radial plane toward the spiral center. Due to the pressure field induced by the asymmetrically attached wake two components of the lift force exist, one that causes the lateral motion and the other a centripetal force that keeps the bubble on a circular path. A mechanism is proposed to explain the reason for one bubble to spiral or to zigzag. Experiments with two simultaneous released bubbles show that bubble interaction is strongly triggered by the wake dynamics. Once a bubble is captured in the wake of a rocking bubble, it accelerates and rises via successive jumps until they collide. The jumps are explained by the upwards induction effect of the ring-like heads of the hairpin vortices being shed from the leading bubble. The final collision and repulsion thereafter abruptly enlarges the wake for a short moment, which is suggested to be one major contribution to the amplification of turbulence production in bubbly flows.
On singularity formation in three-dimensional vortex sheet evolution
Authors: Brady M. , Pullin D. I.
Published: July 2002

It is shown that if a doubly-infinite vortex sheet has cylindrical shape and strength distributions at some initial time, then this property is retained in its subsequent evolution. It is also shown that in planes normal to the generator of the cylindrical sheet geometry, the nonlinear evolution of the sheet is the same as that of an equivalent strictly two-dimensional sheet motion. These properties are used to show that when an initially flat vortex sheet is subject to a finite-amplitude, three-dimensional normal mode perturbation, weak singularities develop along lines which are oblique to the undisturbed velocity jump vector at a time that can be inferred from an extension of Moore’s [Proc. R. Soc. A 365, 105 (1979)] result for two-dimensional motion.
Numerical simulation of bubble-type vortex breakdown within a tube-and-vane apparatus
Authors: Snyder Deryl O. , Spall Robert E.
Published: July 2002

Numerical calculations for three-dimensional, unsteady, laminar, bubble-type vortex breakdown within a tube-and-vane-type apparatus at a Reynolds number of 2000 and circulation number Ω=1.41 are presented. This study is unique in that rather than specifying the inlet swirl velocity through a fit to experimental data (or a Burgers profile), the swirl was induced by directing the fluid through an array of 16 turning vanes, the arrangement being similar to that employed in the original experimental works of Sarpkaya [J. Fluid Mech. 45, 545 (1971); AIAA J. 12, 602 (1974)]. The interior of the resulting breakdown bubble consisted of one primary torroidal recirculation cell, which was tilted from, and found to gyrate about, the bubble centerline. The dominant frequency of gyration was identified, and the mechanism of fluid exchange examined. Subsequent calculations were performed using fixed inlet swirl and axial velocity profiles that were obtained from the results computed using the full geometry (including the turning vanes). Results revealed no significant difference in the downstream breakdown location or structure.
Numerical simulation of pattern formation in the Bénard–Marangoni convection
Authors: Tomita Hirofumi , Abe Kanji
Published: July 2002

The Bénard–Marangoni convection is numerically simulated using the full three-dimensional Navier–Stokes equation. It is shown that hexagonal convection appears when the surface-tension effect dominates over the buoyancy effect. When two effects are comparable to each other, a mixed type of roll and hexagonal convections is stably obtained. It is found that the kinetic energy production by buoyancy does not depend on the convection pattern, whereas that by surface-tension strongly depends on the convection pattern. When the surface-tension effect is dominant, the kinetic energy production by surface-tension in the hexagonal convection is larger than that in the roll convection. Hexagonal convection gives a vertical heat transfer larger than that of roll convection.
Collisionless Simon–Hoh instability in a strongly double-sheared electric field
Authors: Tao Y.-Q. , Conn R. W. , Schmitz L. , Tynan G.
Published: July 2002

It has been shown in an earlier paper [Phys. Fluids B 5, 344 (1993)] that the ion flow speed of a plasma can be much smaller than the E×B speed in a strongly double-sheared electric field. In this paper, the stability of the plasma is investigated. It is found that a new instability, driven by the difference between the ion and electron flow speed, occurs and may dominate the Kelvin–Helmholtz (KH) instability in a nonuniform plasma. This new instability has a driving mechanism similar to that of the Simon–Hoh instability and is thus called the collisionless Simon–Hoh (CSH) instability. When the CSH instability dominates, the plasma does not become more unstable as the electric field is increased, in contrast to scenarios where the KH instability is dominant.
Frequency multiplied harmonic gyrotron-traveling-wave-tube amplifier
Authors: Choi J. J. , Ganguly A. K. , Armstrong C. M.
Published: July 2002

Numerical simulations of a W-band two-stage tapered, frequency multiplied gyrotron- traveling-wave-tube amplifier are reported. Unlike conventional harmonic gyrodevices, a drive signal at the fundamental harmonic frequency is injected in the first stage for beam modulation, and amplified output radiation is extracted from the third harmonic cyclotron resonance interaction. Numerical results show that broadband millimeter wave radiation is obtained with an efficiency of 10%–15%, a gain of ∼30 dB, and an instantaneous bandwidth of ∼10% at a center frequency of 95 GHz for Δvz/vz=2%.
A numerical study of the Alfvén continuum damping of toroidal Alfvén eigenmodes
Authors: Chu M. S. , Greene J. M. , Ling W. , Turnbull A. D. , Berk H. L. , Rosenbluth M. N.
Published: July 2002

The Alfvén continuum damping of the toroidal Alfvén eigenmodes is explicitly formulated for a large aspect ratio force-free tokamak using analytic continuation. A set of jump conditions across the Alfvén singularities are derived which have to be satisfied by the mode amplitudes. The method is then applied to a simple model equilibrium. The characteristics of the modes and their damping are investigated. The method is expected to be generalizable to models with more complete plasma dynamics.
A new class of cold fluid non-neutral plasma equilibrium
Authors: Avinash K.
Published: July 2002

Using a variational principle a new class of cold fluid non-neutral plasma equilibria with uniform density equal to the Brillouin limit density but which are differentially rotating are constructed. These equilibria lie asymptotically close to the usual equilibria with Brilliouin limit density and rigid rotation. The experimental significance of these results is briefly discussed.
Drift surface axis position and transform measurements in the Compact Auburn Torsatron
Authors: Gandy R. F. , Lin H. , Hartwell G. J. , Knowlton S. F. , Morimoto S. , Pritchard D.
Published: July 2002

In toroidal plasma confinement devices, the highly energetic fusion products will have trajectories defining drift surfaces that deviate considerably from the magnetic surfaces. An understanding of these drift surfaces is important for efficient operation of a fusion reactor. By appropriate choice of electron energy on the Compact Auburn Torsatron [Fusion Technol. 18, 281 (1990)], normalized drift surface shifts similar to those for fusion products in a reactor can be studied. As a benchmark, a set of experiments have been conducted to study the axis position and central rotational transform associated with varying vertical fields. Corresponding experiments have been conducted which measure the axis position and central transform associated with the drift surfaces of highly energetic electrons. These experiments were compared and a theoretical model incorporating the spatial nonuniformity of the applied vertical magnetic field was developed. The experimental results agreed well with this theory.
On the conduction of a current in a plasma-filled diode
Authors: Fruchtman A. , Benari M. , Blaugrund A. E.
Published: July 2002

Two issues in plasma-filled diodes are addressed: electrostatic transient effects during the rise time of the current, and current neutralization in the bulk of the plasma. For the first issue an analytical method is used to recover some features of the diode behavior recently demonstrated in simulations [Phys. Fluids B 4, 3608 (1992)]. The potential and the electron flow are shown to be oscillatory at the initial phase of current rise, before ions start to move. The possibility of electron trapping in the potential hill is discussed. For the second issue an equilibrium is constructed that describes a plasma of dimensions larger than the electron skin depth. A beam of charged particles moves ballistically into the plasma and the plasma electrons generate a return current that neutralizes the beam current. When the plasma electrons reach the plasma boundary they bend into a skin layer and conduct the current along the plasma boundary.
Electron thermal effects on the Farley–Buneman fluid dispersion relation
Authors: Kissack R. S. , St-Maurice J.-P. , Moorcroft D. R.
Published: July 2002

The traditional linear fluid dispersion relation of Farley–Buneman waves has been generalized by including, for the electron gas, the effects of collisional energy exchange, as well as thermal force and thermoelectric effects associated with heat flow. The formalism used is that of Schunk [Rev. Geophys. Space Phys. 15, 429 (1977)] for Grad’s 8-moment approximation, to which inelastic energy exchange has been added phenomenologically. The resulting dispersion relation recovers both the traditional isothermal and adiabatic limits, as well as the dispersion relation of Pécseli et al. [J. Geophys. Res. 94, 5337 (1989)] as a special case. Owing to the fact that the electron–neutral interaction is far from being of the Maxwell molecule type, it is found that, contrary to suggestions in the literature, adiabaticity does not hold at the larger wavelengths of the instability. In the small wave-number limit, the linear instability threshold speed of the waves takes the form [(γeTe0+Ti0)/mi]1/2, with the effective γe being a sensitive function of aspect angle. Its value can be as small as 0.28 or as large as 3.4 depending on conditions.
Resonant diocotron instability in a non-neutral plasma with finite mass
Authors: Bhattacharyya S. N.
Published: July 2002

The condition for a resonant diocotron instability in a column of non-neutral plasma is generalized to include the effect of plasma inertia. Near the Brillouin limit, for modes with large azimuthal mode number, instability is shown to occur for a nonzero plasma density at the resonant surface, regardless of the density gradient. In the presence of inertia the existence of another singularity is pointed out. Its effect on stability is shown to be small near the Brillouin limit for modes with large azimuthal mode number. For certain values of the number density below the Brillouin limit this singularity can exist in the region of large plasma density, for a density profile close to a step function profile, and is shown to resonantly drive an instability if the average plasma density decreases radially outward at the resonant surface.
Kinetic theory of low-frequency cross-field instability in a weakly ionized plasma. II
Authors: Dimant Ya. S. , Sudan R. N.
Published: July 2002

The consistent kinetic approach developed in Paper I [Ya. S. Dimant and R. N. Sudan, Phys. Plasmas 2, 1157 (1995)] is applied to obtain the general dispersion relation of the two-stream E×B instability in collisionally dominated weakly ionized plasmas for wave frequencies small compared to the ion–neutral collision frequency. This dispersion relation covers the whole low-frequency band from the asymptotic short-wave limit studied in Paper I to the long-wave limit. Previous theories employing simplified kinetic theory or fluid equations for electron behavior are only correct in the long-wave limit. The principal new results are that the threshold conditions for this instability and the growth rates are altered from those predicted by earlier simplified theories.
Theory and calculation of current profile control near rational surfaces in a tokamak by electron cyclotron waves
Authors: Lazzaro E. , Ramponi G.
Published: July 2002

The effect of electron cyclotron current drive (ECCD) on nonlinear resistive magnetohydrodynamic (MHD) perturbations in a tokamak is analyzed. Along the ray trajectories of electron cyclotron wave (ECW) beams injected from the low-field side in the ordinary mode (O-mode) polarization, the power deposition, and current drive efficiencies are calculated using fully relativistic analytical expressions. The driven current is inserted in a consistent theory of nonlinear magnetic island evolution, which provides the scaling of the radio-frequency (RF) driven current required to control magnetic reconnection in terms of the plasma equilibrium parameters and optical properties. It is shown that the threshold for error field driven islands can be increased by ECCD and that the current profile around the q=2 rational surface can be shaped to control the evolution of rotating magnetic islands by modulation of the gyrotron power source.
Modulational excitation of drift waves by a beam of lower-hybrid waves
Authors: Benkadda S. , Popel S. I. , Tsytovich V. N. , Devynck P. , Laviron C.
Published: July 2002

The results of theoretical and experimental studies on excitation of drift waves in a tokamak plasma under the conditions of experiments on lower-hybrid (LH) plasma heating and current drive are presented. It is shown that for sufficiently strong LH pump levels the main effect resulting in the drift wave excitation is the LH wave modulational instability. It is found that the modulational excitation of long-wavelength drift oscillations (with the wavelengths exceeding the length of the LH pump wave) is described by a dispersion equation similar to the equation for the usual hydrodynamical beam instability. This allows us to treat the excitation of the long-wavelength drift waves as modulational excitation by a beam of the LH waves. The theoretical results obtained are compared with the data of the tokamak Tore Supra [Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), p. 9] experiment. Qualitative accordance of the theoretical and experimental results is demonstrated.
Simulations of electrostatic modes of non-neutral plasmas with small aspect ratio in a Penning trap
Authors: Mason Grant W. , Spencer Ross L. , Bennett Jonathan A.
Published: July 2002

The dependence on induced charge, experimental geometry, and temperature of electrostatic modes in very low aspect ratio non-neutral plasmas in a Penning trap is considered. The modes are of interest as non-destructive diagnostics of the shape of the plasmas. These investigations include equilibrium calculations of plasma shapes and profiles at finite temperature and particle-in-cell simulations of axisymmetric modes. The results of the simulations are compared to the zero-temperature theory by Dubin [Phys. Rev. Lett. 66, 2076 (1991)] taken to first-order in the aspect ratio and to experimental measurements by Weimer et al. [Phys. Rev. A 49, 3842 (1994)]. In general, it is concluded that the Dubin theory provides a means to obtain reasonable estimates of plasma parameters, including density, radius, and axial length, for plasmas in the very important regime for which the axial length is comparable to the Debye length. In addition, dependence on induced charge, equilibrium shape, and plasma temperature are identified which can likely be used to improve agreement between theory and experiment.
Partial reconnection in the nonlinear internal kink mode
Authors: Biskamp D. , Sato T.
Published: July 2002

The effect of the electron pressure gradient in Ohm’s law on the nonlinear development of the internal kink mode is investigated. While pressure fluctuations have a destabilizing effect, the average pressure gradient giving rise to diamagnetic flows is stabilizing. If the latter is strong enough, it leads to saturation at finite island size. The relevance of the results for the sawtooth phenomenon in tokamak plasmas is discussed.
Microwave-plasma in a simple magnetized torus
Authors: Rypdal K. , Fredriksen Å. , Olsen O. M. , Hellblom K. G.
Published: July 2002

In a magnetized torus with no poloidal field component, a weakly ionized plasma is produced by microwaves at 2.45 GHz in the O-mode as well as the X-mode. The neutral gas pressure pg ranges from 5×10−5 to 1×10−3 mbar, ne∼1−5×1016 m−3, and Te∼2−6 eV. The O-mode is only weakly absorbed at the electron cyclotron resonance (ECR), but is partly converted to the X-mode by wall reflections. The X-mode is absorbed via the upper hybrid resonance (UHR), presumably through conversion to and absorption of electron Bernstein waves (EBW). For pg>1×10−3 mbar the EBW absorption is collisional, but for lower pg a collisionless transit particle mechanism could be responsible. Typically the spatial plasma distribution depends mainly on the major radius R, and the measured ne(R) increases monotonically with R from the ECR to an UHR near the outer wall. Te is determined by the particle balance, and is proportional to the ionization energy. The average ne is determined by power balance, and increases with wave power.
Dynamics of plasma containing a population of negatively charged heavy vortex elements
Authors: Das Chandra , Bhattacharya Rajarshi , Chakraborty B.
Published: July 2002

Due to the presence of a population of heavy, negatively charged vortex elements, plasma becomes birefringent to a low frequency circularly polarized wave of infinitesimally small amplitude. The right circular polarization part remains dispersive whereas the left circular polarization part decays if the rotation associated with the vorticity is anticlockwise as seen by an observer looking antiparallel to the direction of the axis of rotation.
Role of finite parallel wave number in low frequency fluctuations in toroidal current less plasma
Authors: Kaur Rajwinder , Singh A. K. , Mattoo S. K.
Published: July 2002

In this paper we present results related to studies of low frequency electrostatic fluctuations in a toroidal plasma of varying length and without a toroidal current. The toroidal length of plasma is varied from fully toroidal (major radius R=45 cm to 278, 212 and 142 cm. The characteristics of fluctuations changed, showing the effect of finite system length on the nature of the fluctuations. The major results are 1 a drastic reduction in the amplitude of fluctuations, 2 this reduction is mainly due to the disappearance of coherent peaks at 3 kHz and its harmonics observed in unbounded system, 3 the presence of a broad coherent feature around 10 kHz, only in bad curvature region, at low magnetic field (200 G) and 4 the appearance of a large number of coherent peaks, in one particular system length, at a higher magnetic field (400 G). These results indicate that the Rayleigh–Taylor instability with finite k‖ may be excitable in the device.
Steady-state magnetohydrodynamic plasma flow past conducting sphere
Authors: Chacko Z. , Hassam A. B.
Published: July 2002

An analytic solution to the problem of strongly magnetized plasma flow past a smooth, conducting sphere is considered. The magnetic field is taken to be uniform at very large distances and the sphere is assumed to be unmagnetized. In addition, the flow speed is assumed to be subsonic and super-Alfvénic. It is shown that a steady state solution is possible only if the frozen-in condition can be relaxed near the surface of the sphere. By inclusion of a small resistivity, the presence of two, nested boundary layers near the surface is demonstrated. The magnetic field is shown to drape about the sphere with a scale size of the order of the square root of the resistivity.
Theory of drift waves in the presence of parallel and perpendicular flow curvature. II. Toroidal model
Authors: Sen S. , Storer R. G.
Published: July 2002

The combined effect of the perpendicular and the parallel flow curvatures on the toroidal branch of the collisionless drift waves is investigated. The full analytical stability analysis shows that the ratio of the stabilising influences of the perpendicular to the parallel flow curvature scales as α2Lv2/Ln2, where α is a numerical factor whose value is usually around 3 and Lv, Ln are respectively the velocity shear and the density variation scale length. Thus at the plasma edge (since Lv∼Ln∼ρiθ, where ρiθ is the ion poloidal gyroradius) the perpendicular flow is expected to play the major role in stabilising microinstabilities and turbulence in the improved regimes of confinement (like the high confinement-modes). However, at the plasma core (since Ln∼a≫Lv∼ρiθ, where a is the minor radius of the cross section) it is the parallel component of the flow which plays the dominant role. Furthermore, since the confinement improvement in the core is usually related to the toroidal velocity and V‖ coming from the toroidal flow is much more that V⊥ coming from the toroidal flow, our result clearly indicates that it is the parallel component of the toroidal flow and not the perpendicular component, as is usually thought, that is responsible for the core confinement improvement. This conclusion is consistent with our earlier result calculated in a sheared slab configuration [S. Sen and A. Sen, Phys. Plasmas 3, 2224 (1996)].
Local analysis and integral constraints of kinematic-dynamo fields in chaotic Beltrami flows
Authors: Yao Demin
Published: July 2002

The kinematic-dynamo problem with steady Beltrami flows at high magnetic Reynolds number is studied by a local analysis along unstable Lagrangian trajectories in the chaotic region and from some global considerations. The chaos properties of the flow, especially the local characteristic directions, are related to the time evolution and to the spatial distribution of the field solutions derived from the local analysis. Global considerations show that there are some integral constraints that a growing eigenmode of the magnetic field should satisfy, and an antidynamo result is given for the understanding of why some chaotic flows are not capable of fast dynamo action.
Frequency response of multipactor discharge
Authors: Valfells Agust , Kishek R. A. , Lau Y. Y.
Published: July 2002

This paper analyzes the frequency response of a two-surface multipactor in a rf circuit. An equation for the frequency band in which steady state multipactor can occur is derived in terms of the secondary emission properties of the surface, the quality factor Q of the rf circuit, and the operating voltage. The steady-state multipactor current is also derived, and is shown to be in excellent agreement with numerical computations that follow the temporal evolution of the multipactor discharge.
Particle simulation and electron heating effects in plasmas produced by laser pulse
Authors: Cao Lihua , Chang Wenwei , Yue Zongwu
Published: July 2002

The processes of resonant absorption, vacuum heating, and anomalous skin effect in plasmas produced by a laser pulse are studied using the two-dimensional multi-time-scale, fully electromagnetic relativistic particle simulation code with mobile ions. The mechanisms of electron heating are expounded and compared.
Observation of particle transport barriers in reverse shear plasmas on the Tokamak Fusion Test Reactor
Authors: Efthimion P. C. , von Goeler S. , Houlberg W. A. , Synakowski E. J. , Zarnstorff M. C. , Batha S. H. , Bell R. E. , Bitter M. , Bush C. E. , Levinton F. M. , Mazzucato E. , McCune D. , Mueller D. , Park H. , Ramsey A. T. , Roquemore A. L. , Taylor G.
Published: July 2002

Perturbative experiments on the Tokamak Fusion Test Reactor [Phys. Plasmas 4, 1736 (1997)] (TFTR) have investigated transport in reverse shear plasmas. On TFTR, reverse magnetic shear plasmas bifurcate into two states with different transport properties: reverse shear (RS) and enhanced reverse shear (ERS) with improved core confinement. Measurements of the 14 MeV t(d,n)α neutrons and charge-exchange recombination radiation spectra are used to infer the trace tritium and helium profiles, respectively. The profile evolution indicate the formation of core particle transport barriers in ERS plasmas. The transport barrier is manifested by an order-of-magnitude reduction in the particle diffusivity (DT,DHe) and a smaller reduction in the pinch within the reverse shear region. The low diffusivities are consistent with neoclassical predictions. Furthermore, DT and DHe≈χeff, the effective thermal diffusivity. Although the measured coefficients imply no helium ash accumulation, the situation is uncertain in a reactor due to unknown χeff scaling.
Modeling of the nonlinear growth of neoclassical tearing modes
Authors: Yu Qingquan , Günter Sibylle
Published: July 2002

The nonlinear growth of neoclassical tearing modes is studied numerically. In the present computational model the perturbed bootstrap current, as well as the nonlinear coupling between different harmonics of the same helicity, are included, and the large aspect-ratio tokamak approximation is used. This work is focused on the nonlinear growth time and the saturated width of the magnetic island. The perturbed bootstrap current is found to play a destabilizing role in the nonlinear island as in previous analytical theories. For sufficiently large islands, the island width at saturation is not linearly proportional to the fraction of the bootstrap current. The formation of the island leads to a near constant decrease in plasma pressure in the region between the magnetic axis and the island. The simulated island growth time, as well as the saturated island width, agree very well with experimental observations.
Vertical asymmetry of the helical magnetic-field ripple in torsatrons and heliotrons
Authors: Smirnova M. S.
Published: July 2002

Possible reasons are considered for the vertically asymmetric helical field ripple to occur in torsatrons and heliotrons [K. Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)]. Vacuum magnetic configurations of present day l=2 torsatrons/heliotrons are analyzed using a harmonic expansion of the magnetic-field strength along a field line. It is shown that a typical feature of such configurations is a vertical asymmetry of their helical field ripple. This kind of vertical asymmetry of the ripple in torsatrons and heliotrons is caused by the presence of the helical satellite harmonics in a Fourier spectrum of the magnetic field. In configurations that are far from the quasihelically symmetric, due to these harmonics effects, the depth of the secondary magnetic wells along field lines can differ not only inside and outside the torus, but also in the upper and lower parts of the torus.
Relation between double layers and flux for a collisionless discharge with two negative components
Authors: Sheridan T. E. , Braithwaite N. St. J. , Boswell R. W.
Published: July 2002

The ion flux exiting a steady-state planar discharge with collisionless, kinetic positive ions and two negative Boltzmann components (i.e., a modified Tonks–Langmuir model) is calculated. For the plasma approximation, when the negative component temperature ratio exceeds 10.78, there are two solutions for the potential at the plasma edge over some range of the negative component density ratio. The existence of collisionless double layer solutions is confirmed by solving the plasma–sheath equation numerically for finite charge non-neutrality. A simple physical criterion for the existence of a double layer is given—a double layer is formed when the positive ion flux associated with the smaller (in absolute value) plasma edge potential is less than that associated with the larger edge potential. The flux calculated for finite nonneutrality is shown to agree with the flux calculated for the plasma approximation when the larger value of the flux is taken in the double-solution regime. Simple approximate expressions for the flux are derived.
Collisional delta-f scheme with evolving background for transport time scale simulations
Authors: Brunner S. , Valeo E. , Krommes J. A.
Published: July 2002

The δf approach is extended for simulating the transport time-scale evolution of near-Maxwellian distributions in collisional plasmas. This involves simultaneously advancing weighted marker particles for representing the intrinsically kinetic component δf, and fluid equations for the parameters of the shifted Maxwellian background fSM. The issue of increasing numerical noise in a collisional δf algorithm, due to marker particle weight spreading, is addressed in detail, and a solution to this problem is proposed. To obtain higher resolution in critical regions of phase space, a practical procedure for implementing sources and sinks of marker particles is developed. As a proof of principal, this set of methods is applied for computing electrical Spitzer conductivity as well as collisional absorption in a homogeneous plasma.
Impact of
E
×
B
drifts on impurity distribution in the scrape-off layer of a tokamak
Authors: Morozov D. Kh. , Rozhansky V. A. , Herrera J. J. E. , Soboleva T. K.
Published: July 2002

As shown in recent years, a strong electrostatic field exists in the tokamak Scrape-Off-Layer (SOL) and divertor zone. In the present paper it is demonstrated that the E×B drifts resulting from it, compete with the impurity velocities caused by friction with the main ions, as well as with thermal forces. This effect can redistribute impurity flows significantly. Particularly, it may lead to impurity flow reversal. Fluid equations for the test impurities are derived using the two most representative ionization states model. As shown to first approximation, impurity motion does not depend on the ionization state. For a favorable magnetic-field direction, when the ion ∇B drift is directed towards the X-point, in the private region the impurities flow from the outer to the inner divertor plate. On the contrary, in the SOL the impurity flow is directed from the inner to the outer plate. Significant impurity velocity shear exists near the separatrix, and creates the effect of flow reversal. Diffusion across the flux surfaces transports impurities flowing away from the divertor plates to regions with the opposite flow direction. A reduced visual model is presented.
The effect of kinetic processes on Langmuir turbulence
Authors: Sanbonmatsu K. Y. , Vu H. X. , Bezzerides B. , DuBois D. F.
Published: July 2002

Kinetic processes are shown to be crucial in determining the saturation level of stimulated Raman scattering for regimes relevant to NOVA [Campbell et al., Fusion Technol. 21, 1344 (1992)] and the National Ignition Facility [Lindl, Phys. Plasmas 2, 3933 (1995)]. To investigate these kinetic effects, the Zakharov, quasilinear-Zakharov, and reduced-description particle-in-cell simulation models are compared in the test case of a uniformly driven plasma. Good agreement is observed between all three simulation methods for relatively low primary Langmuir wave numbers (k1λDe∼0.1) in weakly driven regimes. In the strongly driven case, quasilinear diffusion provides an important correction to the Landau damping rate, producing saturation levels in agreement with reduced-description particle-in-cell simulations, in contrast to pure Zakharov simulations, which overestimate the saturation significantly. At higher k1λDe∼0.25, both the quasilinear-Zakharov and pure Zakharov models fail. In this regime, the autocorrelation time of the Langmuir wave spectrum is much larger than the quasilinear diffusion time, causing the quasilinear diffusion approximation to break down.
Charge separation at a plasma edge in the presence of a density gradient
Authors: Shoucri M. , Pohn E. , Knorr G. , Bertrand P. , Kamelander G. , Manfredi G. , Ghizzo A.
Published: July 2002

A fully kinetic code for ions [one dimensional (1-D) in space, and using the three velocity dimensions in velocity space] is used to study the problem of the formation of a charge separation with the self-consistent electric field in a plasma in the presence of a density gradient. Electrons are treated using an adiabatic law. Graphical results are presented which follow the formation of a 1-D steady state showing the formation of an oscillating positive potential bump toward the edge of the plasma. These oscillations are closely associated with the gyration of the ions. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect on the rapid buildup of the potential at the edge, and in increasing the charge separation and the associated electric field at the edge, in comparison to the case when no impurity ions are included. The present results show the importance of a kinetic solution to the problem of the equilibrium electric field and charge separation in the presence of a density gradient, and point to the important role played by the finite ions’ gyroradius and the important contribution of impurity ions in this case.
Motion of extended vortices in an inhomogeneous pure electron plasma
Authors: Romé M. , Brunetti M. , Califano F. , Pegoraro F. , Pozzoli R.
Published: July 2002

The motion of extended vortices in a pure electron plasma with an inhomogeneous, centrally peaked, density in a Penning–Malmberg trap is studied by means of a two-dimensional electrostatic Eulerian code that solves the evolution equation for the electron distribution function in the guiding center approximation, coupled to the Poisson equation for the electrostatic potential. Vortices corresponding to electron density clumps propagate inward, as discussed in a recently proposed model for the case of point vortices, and carry inward both high and low density plasma. New, long-lived, structures consisting of a higher and of a lower density vortex pair are formed in the presence of a small amount of vorticity reconnection.
On the possibility of electromagnetically induced transparency in a plasma. I. Infinite plasma
Authors: Gordon D. F. , Mori W. B. , Joshi C.
Published: July 2002

The theory of electromagnetically induced transparency (EIT) in a plasma [S. E. Harris, Phys. Rev. Lett. 77, 5357 (1996)] is examined in the context of an infinite system. A new dispersion relation is derived which accounts for relativistic effects in an overdense plasma. Several branches of the dispersion relation are plotted and discussed. Particle simulations are used to confirm the findings.
Subnanosecond x-ray framing camera
Authors: Finn N. , Hall T. A. , McGoldrick E.
Published: July 2002

A subnanosecond x-ray framing camera is described. Experiments conducted at the Rutherford Appleton Laboratory in which the camera was used to observe six beam laser implosions of microballoons with an interframe time of 500 ps are also described.