bluuebunny/crossref_metadata_2025 huggingface dataset:
Delayed onset of ribbing instability due to finite-size effects
Authors: de Bruyn John R. , Pan Lihong
Published: July 2002

A fingering pattern forms at the interface between air and a thin layer of oil, confined between two diverging cylindrical surfaces, when one of the cylinders rotates about its axis. We present results of measurements on this pattern near its onset, and show that the onset values of both the control parameter (the cylinder rotation speed) and the pattern wavelength are strongly influenced by effects due to the finite length of the experimental system. In a short system, the onset of the fingering instability occurs at substantially higher values of the rotation speed than predicted by calculations performed for the infinitely long system.
Resonant generation of nonlinear capillary–gravity waves
Authors: Zhu Yong
Published: July 2002

The model considered in this paper consists of an incompressible, inviscid fluid flow over bottom topography with the surface tension acting on it. A forced KdV equation with negative dispersion for capillary–gravity waves is derived with the inclusion of the effect of surface tension (for the case of Bond number being larger than 1/3) when the flow is near resonant. The most important result is that a typical solution of the present forced KdV equation consists of a train of concave solitary-like waves which propagate downward, a convex near the topography and a train of upstream dispersive waves.
Experiments on streamwise vortices in curved wall jet flow
Authors: Matsson O. John E.
Published: July 2002

Hot-wire anemometry and smoke visualizations are used to study the jet on a concave wall. The measurements were performed in the region 91<Go<198, where Go is the Görtler number. It is found that streamwise vortices are amplified on the concave wall. The growth of vortices is dependent on the initial amplitude, i.e. the highest initial amplitude gives the maximum strength of vortices. The amplification occurs in the laminar region while the time-averaged strength of vortices decreases further downstream in the transition region. No merging or splitting of vortex pairs was found. Regular oscillations of the time signal occur at higher Görtler numbers before breakdown. Smoke visualization shows that the horseshoe mode appears with a streamwise wave length of the same order as the triggered spanwise wave length. The streamwise velocity fluctuations indicating the secondary instability were found to coincide better with the total shear distribution rather than streamwise velocity gradients in the spanwise or wall normal directions. Power spectra show distinct peaks at different frequencies. At streamwise positions near the start of curvature, the filtered root-mean-square (rms) levels of the streamwise velocity component around 95 Hz, corresponding to the horseshoe mode, show maximum amplitude between different vortex pairs. The filtered frequency around 22 Hz, which is the dominating frequency further downstream, shows maximum amplitude further away from the wall, where the velocity profile has an inflectional character. However, as the flow proceeds downstream the maximum at 22 Hz moves against the wall.
Dynamics of concentration and vorticity modification in a cellular flow laden with solid heavy particles
Authors: Druzhinin O. A.
Published: July 2002

The processes of the accumulation of solid heavy particles settling under gravity and modification of the carrier flow are studied analytically and numerically. The dilute limit is considered when the effects of particle–particle interactions are neglected. The flow Reynolds number is assumed to be large enough to disregard effects related to the viscous dissipation. Particles with a small Stokes (St) number are considered, when approximate solution for the particulate velocity in the form of a series in St can be derived.

Analytical solutions are obtained for initially uniform distribution of particles in a cellular flow (formed by a lattice of two-dimensional vortices with different circulations) describing dynamics of the particulate concentration and corresponding modification of the fluid flow. Two different regimes of large and moderate flow Froude number (Fr) are considered. Solutions obtained show that in the first case (Fr≫1), when the gravitational settling of the particles is insignificant, the concentration of particles drops at the centers and grows at the periphery of the vortices, so that sheets of the increased concentration are formed in the vicinity of the flow separatrices. Due to the coupling between the particulate and fluid dynamics, the flow vorticity is reduced at the cells centers. The flow gradients grow due to the drag forces between the two phases and sheets of increased vorticity are generated in the vicinity of the concentration sheets. In the case Fr∼1 the concentration sheets correspond to the particle settling paths. Vorticity is effectively generated in the vicinity of these paths, where the drag force based on the local slip velocity is enhanced by the settling. As a result, vortex centers are shifted toward the settling paths, so that the whole symmetry of the flow is changed by the coupling. Results of direct numerical simulations performed for St∼1 show a good agreement with analytical solutions.

It is observed also that the average value of particle settling velocity may exceed that in the still fluid (similar to the earlier result obtained by Maxey [Philos. Trans. R. Soc. London Ser. A 333, 289 (1990)] under assumption that the influence of the particulate on the carrier flow remains negligible).
Marangoni instability of bi-component droplet gasification
Authors: Aharon I. , Shaw B. D.
Published: July 2002

Simplified analyses using a linear stability approach are developed to predict influences of thermal and solutal Marangoni effects on hydrodynamic stability of bi-component droplets evaporating in a spherically-symmetrical manner in hot environments. It is predicted that with zeotropic mixtures and for ∂σ/∂T<0 and ∂σ/∂y<0 (where σ is surface tension, T temperature, and y the surface mass fraction of the more volatile droplet component), the thermal and solutal Marangoni effects oppose each other in that the thermal effect is stabilizing and the concentration effect is destabilizing. The model is applied to alkane/alkane and alcohol/water mixture droplets. The alkane mixture droplets were predicted to be hydrodynamically stable. For alcohol/water mixtures, the results suggest that critical radii for marginal stability exist; when a droplet is initially pure methanol which subsequently absorbs water from the ambient, the critical radius is predicted to depend upon the relative humidity of the environment.
Burst–interface interactions in free surface turbulent flows
Authors: Rashidi Mehdi
Published: July 2002

Interactions of bursting events (i.e., ejections and inflows) with the free surface in turbulent channel flows have been studied using oxygen bubble visualization and image processing techniques. Experiments indicate that the flow is dominated by the generation of wall ejections, formation of spanwise “upsurging vortices”, and interaction of such structures with the free surface. The spanwise upsurging vortices are seen to evolve near the wall, reach the free surface, form surface patches, roll back in form of spanwise “downswinging vortices”, and mix into the bulk flow. Furthermore, there are evidence of horseshoe and hockeystick type vortices in relation to the bursting events. Measurements of surface characteristics show that the ejection–inflow events are associated with deformation of the free surface and a redistribution of near surface vorticity and velocity fields. It is seen that as upsurging vortices reach the free surface, the surface goes through an elevation or a rise, whereas the surface falls when downswinging vortices of the inflowing fluid return toward the wall. These effects are enhanced as the flow Reynolds number is increased. Conditional averaging of the velocity fields shows that while the surface rise is associated with an increase in the streamwise component of turbulence intensities throughout the liquid layer, surface falls are associated with a slight decrease in streamwise intensities near the free surface. Near the wall, all components of intensities are higher for the cases with surface rise than they are for the ones with surface fall. Contour color plots of the velocities and intensities near the interface region depicts these results and show an increase in the number of spanwise vortices rolling in the direction of flow during the ejection–inflow events. Based on these results and observation of video sequences, a conceptual illustration of burst–interface interactions has been provided.
Chaotic mixing of immiscible impurities in a two-dimensional flow
Authors: Solomon T. H. , Tomas S. , Warner J. L.
Published: July 2002

Experiments compare the chaotic mixing of miscible and immiscible impurities in a two-dimensional flow composed of a chain of alternating vortices. Periodic time dependence is imposed on the system by sloshing the fluid slowly across the stationary vortices, mimicking the even oscillatory instability of Rayleigh–Bénard convection. The transport of a miscible impurity is diffusive with an enhanced diffusion coefficient D* that depends on the size of “lobes” which are, in turn, dependent on the oscillation amplitude. The lobes play an important role in the transport of immiscible impurities as well. In this case, the impurity is broken into a distribution of droplets, whose areas determine the nature of the transport. If the characteristic long-term droplet areas are appreciably smaller than the lobe areas, then there is long-range transport with D* equal to that for the miscible case with the same flow conditions. If the droplet areas remain larger than the lobe areas, then there is no long-range transport.
Time-dependent equations governing the shape of a three-dimensional liquid curtain
Authors: Weinstein Steven J. , Hoff Joseph W. , Ross David S.
Published: July 2002

In a previous paper, Weinstein et al. derive time-dependent equations that govern the response of a planar liquid curtain falling under the influence of gravity and subjected to ambient pressure disturbances. In the previous study, disturbances to the curtain are assumed to be two-dimensional, and thus, the curtain response is independent of widthwise location in the curtain. In this paper, we generalize the previous equations to incorporate the widthwise dimension. The validity of these equations is demonstrated by their ability to predict standing wave shapes in agreement with those studied by Lin and Roberts.
The balancing of thermocapillary flow in a floating zone by ripple-driven streaming
Authors: Lee C. P.
Published: July 2002

It is known that streaming can be generated in a liquid bridge by vibrating an end wall. This phenomenon has been used in an attempt to minimize thermocapillary flow during crystal growth in a floating zone, by inducing such a streaming running in the opposite direction to it [Grugel et al., J. Cryst. Growth 142, 209 (1994)]. In the present theoretical study, the nature of this streaming and its effects on the average flow and temperature fields in a floating zone are investigated. It is noticed that in the experiment, the applied frequencies were high enough such that the corresponding wavelengths of the capillary ripples were much smaller than the dimensions of the zone. It is believed that the ripples were a traveling wave that generated the streaming in the direction of the wave propagation as a result of Stokes drift. For such a wave to be traveling, it must be dissipated sufficiently by viscosity upon reaching the other wall to guarantee negligible reflection there. Accordingly, a model is formulated to study the reduction of thermocapillary flow in a floating zone by means of streaming. It is found that for a half zone, streaming can minimize the thermocapillary flow near the vibrating wall and make the temperature uniform across the zone there. For a full zone, streaming can minimize the flow but cannot make the temperature uniform near the wall. For a long full zone, streaming can similarly minimize the flow. But in addition, the temperature near each wall is uniform with or without streaming.
The investigation of an unstable convective flow using optical methods
Authors: Miozzi M. , Querzoli G. , Romano G. P.
Published: July 2002

The convective flow field in a vessel is investigated by laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV, namely Particle Tracking Velocimetry—PTV). The vessel is heated from below along a linear element at a temperature higher than that of the fluid. Hot fluid raises up and generates two counterrotating vortices. For a given aspect ratio, the two vortices become unstable and start to oscillate on a vertical plane (orthogonal to the heating element). This regime is investigated for increasing Rayleigh numbers to analyze the transition from regular to irregular conditions. The main transition mechanism is observed to be mostly connected to type II intermittency, a mechanism not frequently observed in experiments. However, at some Rayleigh numbers the present data does not definitely rule out type III intermittency. The phenomenon is analyzed by looking at the main frequencies in the spectrum of the horizontal velocity component and their changes with the Rayleigh number at a point above the heating element. Modifications in the local energy spectrum are analyzed by using the Wavelet Transform (WT) tool. Data obtained by PTV measurements make it possible to point out the spatial configuration of the flow and to determine the two velocity components on the measurement plane. These data are used to clarify the fundamental mechanisms of the transition. Instabilities are observed as sudden changes between two regimes of oscillations of the two counterrotating vortices: the first is characterized by oscillations centered on the vertical axis and the second by nonsymmetrical oscillations.
Buoyancy-driven viscous interaction of a rising drop with a smaller trailing drop
Authors: Davis Robert H.
Published: July 2002

An axisymmetric boundary-integral method was developed and used to study the interaction of two deformable drops (or bubbles) rising (or settling) due to gravity in a viscous medium under conditions of small Reynolds number. The focus is on cases where the smaller drop trails behind the larger drop. When the Bond number is small, interfacial tension keeps the drops nearly spherical, and they separate with time. At higher Bond numbers, however, deformation is significant and the trailing drop is stretched due to the flow created by the leading drop; it may form one or more necks and break when one of these pinches off. The leading drop is flattened due to the flow created by the trailing drop; it may form a depression on its underside which evolves into a plume that rises through its center. Moreover, at sufficiently high Bond numbers, the larger leading drop does not leave the trailing drop behind, but instead may entrain and engulf it within the depression or plume. Systematic results for the parameter ranges which demarcate impending breakup and coalescence are presented.
Vibrational–translational energy exchange models for the direct simulation Monte Carlo method
Authors: Vijayakumar Pradeep , Sun Quanhua , Boyd Iain D.
Published: July 2002

The model which controls the distribution of energy among the different molecular modes is a crucial component of accurate simulation of nonequilibrium rarefied flows. Two new models for the direct simulation Monte Carlo method that govern energy redistribution between the translational and vibrational modes are presented here. The first model is a modified form of the phenomenological Borgnakke–Larsen model. The probability of inelastic collision is evaluated using the relative velocity of collision. The second energy exchange model considered in this study is the multiple quantum-step transition model. The process of vibrational relaxation occurs through transitions between the different energy levels, allowing jumps of more than one level. Probabilities of activation and deactivation which depend on the relative velocity are used here. The new models are compared with existing schemes for several conditions. Significant differences are found for the vibrational energy distribution function computed in a hypersonic bow-shock wave.
Numerical study of Hele-Shaw flow with suction
Authors: Ceniceros Hector D. , Hou Thomas Y. , Si Helen
Published: July 2002

We investigate numerically the effects of surface tension on the evolution of an initially circular blob of viscous fluid in a Hele-Shaw cell. The blob is surrounded by less viscous fluid and is drawn into an eccentric point sink. In the absence of surface tension, these flows are known to form cusp singularities in finite time. Our study focuses on identifying how these cusped flows are regularized by the presence of small surface tension, and what the limiting form of the regularization is as surface tension tends to zero. The two-phase Hele-Shaw flow, known as the Muskat problem, is considered. We find that, for nonzero surface tension, the motion continues beyond the zero-surface-tension cusp time, and generically breaks down only when the interface touches the sink. When the viscosity of the surrounding fluid is small or negligible, the interface develops a finger that bulges and later evolves into a wedge as it approaches the sink. A neck is formed at the top of the finger. Our computations reveal an asymptotic shape of the wedge in the limit as surface tension tends to zero. Moreover, we find evidence that, for a fixed time past the zero-surface-tension cusp time, the vanishing surface tension solution is singular at the finger neck. The zero-surface-tension cusp splits into two corner singularities in the limiting solution. Larger viscosity in the exterior fluid prevents the formation of the neck and leads to the development of thinner fingers. It is observed that the asymptotic wedge angle of the fingers decreases as the viscosity ratio is reduced, apparently towards the zero angle (cusp) of the zero-viscosity-ratio solution.
On the evolution of near-singular modes of the Bickley jet
Authors: Swaters Gordon E.
Published: July 2002

The linear stability spectrum of the Bickley jet has neutral modes which have a phase velocity equal to the maximum jet velocity. Unlike critical levels in monotonic shear flows, the stream function associated with these modes is algebraically singular at the jet maximum. Until recently almost nothing was known about the role these modes played in the stability spectrum of the Bickley jet and that which had been conjectured was, in fact, incorrect. Here, we investigate numerically the nonlinear evolution of “near-singular” perturbations in which the phase velocity of the initial perturbation is asymptotically near but not equal to the maximum jet velocity. We show that these modes are surprisingly stable over time. We also show that there is a clearly defined slow time oscillation in the wave number power spectrum of the perturbation stream function which is the result of a slow time oscillation in the underlying modal amplitude. For an initial near-singular mode with a nonzero phase shift across the critical levels, we show that there is a slow time oscillation in the transverse transport of perturbation energy in which the energy flux goes from one critical level to the other and then reverses and so on all the while satisfying no net energy transfer from the mean flow to the perturbation field.
A unified asymptotic derivation of two-layer, frontal geostrophic models including planetary sphericity and variable topography
Authors: Karsten Richard H. , Swaters Gordon E.
Published: July 2002

A general asymptotic theory for two-layer, frontal geostrophic (FG) models including the effects of planetary sphericity and variable bottom topography is developed. In addition to the standard β-plane approximation, an additional baroclinic correction associated with planetary sphericity, the Veronis effect, enters into the leading-order dynamics of FG models. The Veronis effect depends on the variation of the longitudinal metric as the transformation to Cartesian coordinates is made. The Veronis effect becomes significant at mid to high latitudes for the long length scales associated with FG models which are larger than the internal Rossby radius of deformation. The inclusion of variable bottom topography results in an asymmetry between the dynamics of surface and bottom-trapped currents. Variable bottom topography enters the equations in a similar, but not identical, manner to the β effect. The asymmetry between the dynamics of surface-intensified and bottom-intensified FG currents over sloping topography occurs due to the fact that topography stabilizes surface flows while it destabilizes bottom flows. Physically, the asymmetry arises because sloping topography provides a stabilizing background vorticity gradient for surface-intensified flows. However, for the bottom-intensified flow of a relatively dense water mass, the presence of a sloping bottom allows the continual release of gravitational potential energy as the center of mass of the dense water “slides” down the sloping bottom and is thus a destabilizing rather than a stabilizing effect.
Hydrodynamic interactions between two equal spheres in a highly rarefied gas
Authors: Gopinath Arvind , Koch Donald L.
Published: July 2002

This paper describes hydrodynamic interactions between two spherical particles having equal radii, a, and translating with velocities U1 and U2 in a highly rarefied gas. The center-to-center distance between the two spheres is aχ. The gas is at rest far from the two particles. The spheres move with speeds that are much smaller than the mean thermal speed of the gas molecules so that the Mach number, M≡max(U1,U2)/c̄, characterizing the deviation from equilibrium is much less than one. Here c̄ is the mean thermal speed of the gas molecules. Gas molecules are assumed to be diffusively reflected from the particle surfaces. Our analysis is confined to the case where the particle Knudsen number is very large, i.e., Kno≡λo/a→∞, λo being the mean free path of the gas far from the two particles. We first study the free-molecular drag on the two sphere configuration for arbitrary translations of the spheres. For small Mach number, the general time-dependent, nonlinear problem may be approximated by a quasisteady, linear problem in which the spheres are held fixed and molecules reflected from each sphere have a modified Maxwell–Boltzmann distribution of velocities. A standard integral equation formulation based on flux balances at the particle surfaces is then employed to calculate the drag force acting on the spheres. The results obtained can be used as leading estimates for the forces acting on the spheres when Kno≫1 and 2⩽χ≪Kno. We then consider the case where the flow in the vicinity of each sphere is nearly free-molecular, but the flow in the O(aχ) space between the spheres is nearly continuum in nature. In this limit, the flow in the gap between the spheres is studied using the method of reflections. This approach can be used for arbitrary Kno provided Kno≪χ≪Kno M−1. The leading correction to the drag force due to the hydrodynamic interactions between the spheres when Kno≫1 is obtained. In all cases studied, the temperature of the two spheres is assumed to be the same as that of the surrounding gas.
Ion-temperature-gradient-driven instability and anomalous ion heating in reversed-field pinches
Authors: Guo S. C. , Paccagnella R. , Romanelli F.
Published: July 2002

A possible mechanism is suggested for direct ion heating in reversed-field pinches (RFP). The ion temperature gradient mode induces collisionless ion perpendicular viscosity which damps the velocity fluctuations associated with resistive magnetohydrodynamic (MHD) mode activity. It is found that the growth rate of the ηi instability is much larger than that in a tokamak, because the effects of both bad magnetic curvature and negative compressibility are enhanced in a RFP configuration. The anomalous perpendicular viscosity coefficient is estimated from the mixing length argument and the corresponding ion heating power due to viscous dissipation is found sufficient to balance the ion conductive losses.
Beam generation, acceleration, and extraction on a stellatron accelerator
Authors: Song Y. , Fisher A. , Rostoker N.
Published: July 2002

The stellatron accelerator of the University of California at Irvine (UCI) [Phys. Fluids B 2, 3149 (1990)] was modified to accommodate beam extraction. The magnetic field coils were reconstructed to adapt an extraction port. A fiberglass–graphite–epoxy chamber replaced the glass chamber that cracked frequently during the previous experiment. Improvement in the magnetic field configuration has significantly reduced the current loss during acceleration. A 1.1 kA, 12 MeV electron beam was generated by initially forming a 1.2 kA beam with plasma start-up. The beam radius was 5–7 mm. Beam extraction was performed with two auxiliary pulsed current coils and up to 15% of the beam was extracted from the chamber. The extraction process was analyzed by computational orbit simulations.
Ni-like collisional lasers using moderate power laser drivers
Authors: Norreys P. A. , Djaoui A.
Published: July 2002

The hydrodynamic behavior of a Ta exploding foil using a double pulse in order to achieve favorable conditions for a Ni-like collisional x-ray laser with moderate driver energy have been examined in detail. It is shown that plasma conditions required for high gain lasing can be produced with ×4 reduced laser power as compared with Maxon’s scheme [Phys. Rev. Lett. 70, 2285 (1993)], provided that the second pulse is correctly timed and is delivered before transparency of the target to the incident laser light. This makes water window x-ray lasers within the reach of many moderately sized laser installations.
Comment on ‘‘Critique of atomic physics instability mechanisms: Ionization-driven and radiative microinstabilities in the tokamak edge plasma’’ [Phys. Plasmas 1, 2630 (1994)]
Authors: Ware A. S. , Diamond P. H. , Carreras B. A.
Published: July 2002

The equations of ionization for the tokamak edge plasma are described and their influence on the stability mechanisms and fluctuation levels are analyzed. (AIP)
Large current negative hydrogen ion beam production
Authors: Ando A. , Tsumori K. , Oka Y. , Kaneko O. , Takeiri Y. , Asano E. , Kawamoto T. , Akiyama R. , Kuroda T.
Published: July 2002

Large current negative hydrogen ions are extracted from a multicusp ion source with an extraction area of 25×44 cm2. By seeding a small amount of cesium vapor into a source chamber, the H− current is enhanced to several times larger than that in a pure hydrogen discharge, accompanied by a reduction of the electron current and the filling gas pressure. A H− current up to 16 A is obtained with a beam energy of 40 keV from 560 extraction holes of 9 mm in diameter each, where the current density is 45 mA/cm2. The H− current increases linearly with the input arc power.
Pierce-type dispersion relation for an intense relativistic electron beam interacting with a slow-wave structure
Authors: Chen Chiping
Published: July 2002

A Pierce-type dispersion relation is derived for the interaction of an intense relativistic electron beam with a cylindrical slow-wave structure of arbitrary corrugation depth. It is shown that near a resonance, the Pierce parameter can be expressed in terms of the vacuum dispersion function and the beam current. The dispersion relation is valid in both the low-current (Compton) regime and the high-current (Raman) regime. The dispersion characteristics of the interaction, such as the linear instability growth rate and bandwidth, are analyzed for both regimes.
Diffraction effects for focused electromagnetic waves in a plasma
Authors: Aamodt R. E.
Published: July 2002

The geometric optics approximation is the commonly used description for wave propagation in media where the usual local Wentzel–Kramers–Brillouin (WKB) condition is satisfied. However, diffraction phenomena can give important corrections to a ray trajectory and scattering description when focused waves are launched. The general mathematical description for such electromagnetic beams is formulated for a general magnetized plasma and shown to be of a simple functional form. For propagation perpendicular to the magnetic field in a cold plasma the equations are shown to be particularly simple and specifically demonstrate the importance of retaining anisotropic plasma effects for focused beam experiments. In an equilibrium plasma with a particular class of RF boundary conditions the general functional form is shown to be solvable analytically and the solutions expressed in terms of solutions of the ray approximation.
Action principle and the Hamiltonian formulation for the Maxwell–Vlasov equations on a symplectic leaf
Authors: Flå Tor
Published: July 2002

An action principle for the Maxwell–Vlasov (MV) equation is formulated in terms of the Maxwell fields and the generating function, w(z,t) for deviations from a reference distribution function, f0(z), which labels a symplectic leaf. New formal fields suitable for variations are defined. These fields give rise to a symplectic and Poisson structure. The Hamiltonian formulation of the equations is found in terms of the new formal fields, and it is found how to derive Larsson’s action principle [J. Plasma Phys. 48, 13 (1992); ibid. 49, 255 (1993)] and generalized versions of it on a Lagrangian constraint manifold in a double symplectic space. It is also shown how the relativistic Maxwell–Vlasov system and the Maxwell–Vlasov system with a time-dependent reference state can be formulated as an action principle and Hamiltonian system in terms of eight-dimensional particle phase space coordinates.
Multiple double layers in a glow discharge
Authors: Conde L. , León L.
Published: July 2002

The formation of visible layered structures that develop at the anodic zone of a constricted glow discharge is reported. They are composed of several tridimensional double layers with axial symmetry enclosing different plasma coronas. The dependence of the number and size of plasma shells with the conditions of the discharge is discussed, as well as the nature of these double layers. The experimental results evidence the effect of the distribution of neutrals in the volume of the discharge for the development of double layers.
Fast decay of plasma return currents due to whistler waves
Authors: Gomberoff K. , Fruchtman A.
Published: July 2002

The evolution of the return current induced by a charged particle beam in a magnetized plasma is studied. The beam current is perpendicular to the background magnetic field. The return current is shown to depart from the beam along the background magnetic field with a whistler rather than a diffusion or an Alfvén velocity. In a plasma bounded by two conductors the return current oscillates with the whistler period. Analytical expressions for the evolution of the magnetic field and of the plasma return current are derived for a beam with a finite width and with various rise time dependences. When the whistler time is shorter than the rise time of the beam current, the plasma return current does not grow beyond the whistler time.
On three-dimensional toroidal surface current equilibria with rational rotational transform
Authors: Kaiser R.
Published: July 2002

The inverse problem of the existence of magnetohydrostatic equilibria in toroidal geometry with surface currents and vanishing magnetic field inside is considered. The inverse formulation (plasma–vacuum interface given, external wall or conductors to be determined) allows a purely geometric characterization of the problem: Toroidal surfaces with analytic parametrization are admissible plasma–vacuum interfaces if they allow a simple analytic covering by geodesics (field lines) or, equivalently, by equidistant lines (current lines). On the basis of this approach it was shown in a recent publication [Phys. Plasmas 1, 281 (1994)] that equilibria with sufficiently irrational rotational transform exist for configurations with arbitrary but weak deviations from axisymmetry. The present paper completes this work by demonstrating that this result, essentially, cannot be sharpened. More precisely, it is shown that for every rational rotational transform ι there exists a (arbitrarily weak) deformation of the axisymmetric circular torus (with sufficiently large aspect ratio) such that an equilibrium with that toroidal surface as plasma–vacuum interface and that ι does not exist. This does not mean that there are no three-dimensional equilibria with rational ι at all. In fact, in the special case of infinite rotational transform, i.e., field lines are simply closed in the poloidal direction, it is demonstrated that, depending on the type of perturbation, equilibria may survive arbitrarily strong deviations from axisymmetry or may immediately be destroyed. Toroidal surfaces of the former type are the so-called canal surfaces, which are generated by the nonrotating transport of a poloidal section along an arbitrary closed space curve, whereas examples of the latter type are ‘‘bumpy’’ tori, where the poloidal section may vary in magnitude but not in shape along the generating curve. If the poloidal section rotates along the generating curve (‘‘helical’’ torus) so that the rotation velocity vanishes somewhere, the surface is also of the latter type.
Experimental study of low-frequency turbulence in a toroidal plasma
Authors: Prasad G. , Bora D. , Saxena Y. C. , Verma S. D.
Published: July 2002

Low-frequency turbulence has been studied in a plasma embedded in a pure toroidal magnetic field. A study of the variation of the spectral characteristics of the low-frequency turbulence with the variation in νin/Ωi has been carried out with two different sets of density and potential profiles. The spectral relation between density and potential fluctuations indicates that the Rayleigh–Taylor instability excites fluctuations in the long wavelength region, while the Rayleigh–Taylor driven drift instability is responsible for the short wavelength regime for νin/Ωi≪1. Spectral characteristics observed in regions where density gradients are parallel to gravity are similar to the case where density gradients are antiparallel to the gravity and support the view that the fluctuations generated in the bad curvature region are transported to the good curvature region. The results are in agreement with theoretical predictions, numerical simulations, in situ measurements, and other laboratory experiments on drift and Rayleigh–Taylor instabilities. For νin/Ωi≥1, the spectral characteristics change and may correspond to the collisional gradient drift instability or the cross-field instability.
Rapid frequency step-switching of a submillimeter wave gyrotron by modulation of the electron beam
Authors: Idehara T. , Shimizu Y. , Makino S. , Ichikawa K. , Tatsukawa T. , Ogawa I. , Brand G. F.
Published: July 2002

An experimental study of the rapid step-switching of the frequency of a submillimeter wave gyrotron is described. Modulation of the energy of the electron beam in turn modulates the electron mass and hence the cyclotron resonance frequency fc or its second harmonic 2fc, while the magnetic field B0 remains the same. This opens up the possibility of switching from one cavity mode to another. The first experimental results for frequency switching between two fundamental modes and between a fundamental and a second harmonic mode are demonstrated.
Coupling between electron and ion waves in Nd-laser beat-wave experiments
Authors: Moulin F. , Amiranoff F. , Laberge M. , Marquès J. R. , Cros B. , Matthieussent G. , Bernard D. , Jacquet F. , Miné Ph. , Specka A. , Stenz C. , Mora P.
Published: July 2002

The beating between two colinear Nd-YLF and Nd-YAG lasers in a homogeneous plasma generates intense relativistic plasma waves associated with a high longitudinal electric field of the order of 1 GV/m. It is shown that these electron waves couple with ion waves in the regime of modulational instability. Electric field amplitude and saturation time obtained by Thomson scattering are in agreement with theoretical predictions taking this mechanism into account.
Relativistic particle acceleration in an electron–positron plasma with a relativistic electron beam
Authors: Zhao J. , Sakai J. I. , Nishikawa K.-I. , Neubert T.
Published: July 2002

Results from three-dimensional electromagnetic particle simulations of an electron–positron plasma with a relativistic electron beam (γ=2) are presented. As part of the initial conditions, a poloidal magnetic field is specified, consistent with the current carried by the beam electrons. The beam undergoes pinching oscillations due to the pressure imbalance. A transverse two-stream instability is excited with large helical perturbations. In the process, background electrons and positrons are heated and accelerated up to relativistic energy levels. Only background electrons are accelerated farther along the z direction due the synergetic effects by both the damped transverse mode and the accompanying electrostatic waves caused by the breakdown of the helical perturbations.
Plasma–maser instability of the electromagnetic radiation in the presence of the lower hybrid turbulence
Authors: Sarma S. N. , Nambu M. , Vladimirov S. V.
Published: July 2002

The generation mechanism of the electromagnetic radiation is studied based on the plasma–maser interaction among the electrostatic lower hybrid turbulence, accelerated electrons and extraordinary mode radiation. The theory agrees with the most striking new features of the auroral kilometric radiation (AKR) in that the AKR bursts are observed at frequencies well above the local electron gyrofrequency. The theory also explains the close correlation between the lower hybrid turbulence and the microwave emission which is reported by recent computer simulations.
The role of the perturbed plasma potential in the dynamic plasma response to photodetachment
Authors: Skinner D. A. , Courteille C.
Published: July 2002

The destruction of negative ions by laser photodetachment suddenly modifies the electron and negative ion densities in a localized region. The subsequent recovery of negative ion density n−(t) may be measured with a second photodetaching laser pulse. These measurements, compared with a ballistic model for n−(t), are the basis of a negative ion temperature diagnostic, which is being developed and employed in H− and D− sources. In this paper the accompanying plasma potential perturbation and its effect on the negative ion response is discussed. The perturbation can introduce an error in negative ion temperature measurements. Late in time, the negative ion density is observed to recover more slowly than indicated by the ballistic theory. An intriguing mechanism is proposed for the late slow recovery of negative ion density, based on a small lingering potential perturbation and a hole in negative ion velocity space. These different theoretical approaches are illustrated with a detailed analysis of an example case from a hydrogen discharge.
Fast nonlinear magnetic reconnection
Authors: Ottaviani M. , Porcelli F.
Published: July 2002

The nonlinear evolution of magnetic reconnection in collisionless and weakly collisional regimes is analyzed on the basis of a two-dimensional incompressible fluid model. The initial equilibria are unstable to tearing modes. In the limit where the stability parameter Δ′ is relatively large, the mode structure is characterized by global convective cells. It is found that the system exhibits a quasiexplosive time behavior in the early nonlinear stage, where the fluid displacement is larger than the inertial skin depth but smaller than the typical size of the convective cells. The reconnection time is an order of magnitude shorter than the Sweet–Parker time for values of the inertial skin depth, of the ion Larmor radius, and of the magnetic Reynolds number typical of the core of magnetic fusion experiments. The reconnection process is accompanied by the formation of a current density sublayer narrower than the skin depth. In the strict dissipationless limit, this sublayer shrinks indefinitely in time. Physical mechanisms limiting this tendency to a singular current density profile are also discussed.
Ballooning modes in the magnetohydrodynamic second stability region
Authors: Nordman H. , Jarmén A. , Malinov P. , Persson M.
Published: July 2002

The ion temperature gradient driven ballooning mode is investigated using two-fluid, gyrofluid, and gyrokinetic descriptions. The linear eigenmode equation is solved numerically in a model equilibrium with shifted circular magnetic surfaces. The localization of the eigenmodes, which persist in the magnetohydrodynamic (MHD) second stability region, and the mode structure, are displayed. The role of finite-Larmor radius (FLR) and magnetic drift resonance effects on the growth rate are elucidated. Negative magnetic shear is found to have a stabilizing effect on the mode.
Two-dimensional magnetohydrodynamic simulation of a flowing plasma interacting with an externally imposed magnetic field
Authors: Hurricane O. A. , Jensen T. H. , Hassam A. B.
Published: July 2002

The problem of plasma flow relative to a modulated magnetic field has been the subject of several studies. One motivation for studying this problem is the possibility of using a deliberately imposed surface of magnetic islands as a means of velocity profile control. This subject is also of importance for the study of stability against ideal and resistive magnetohydrodynamic (MHD) modes and the topic of locked modes. A two-dimensional (2-D) MHD simulation code is used to examine the behavior of a plasma flowing, in steady state, past a modulated magnetic field in ‘‘slab geometry.’’ It is shown that at ‘‘low’’ velocities the stress is dominated by the Maxwell and the viscosity terms and that forces are exchanged between the plasma and the magnetic field in a narrow boundary surrounding the island. It is found that the island is suppressed when the viscous force at the separatrix exceeds the maximum force that can be supported by an island. For ‘‘high’’ velocities (velocities beyond the critical velocity for island suppression), the stress is dominated by the Maxwell and the Reynolds terms, and the exchange of forces is taking place in a narrow region around the point where the plasma flow velocity matches the Alfvén speed.
Conservation relations and the generalized Manley–Rowe relation for the plasma–maser in magnetized plasma
Authors: Saikia Bipuljyoti , Nambu M. , Bujarbarua S.
Published: July 2002

The plasma–maser interaction among Langmuir waves, Bernstein waves, and the electron distribution function on an equal footing is studied. The total momentum and energy conservation relations between electron kinetic energy and wave energy is satisfied for magnetized plasma. The Manley–Rowe relation in its original form is violated because the resonant electrons carry nonlinear momentum and energy. The plasma–maser process satisfies the generalized Manley–Rowe relation which includes the effective action from electrons in addition to the original one from waves.
Electron–ion collisional excitations in strongly coupled plasmas using the ion-sphere model
Authors: Jung Young-Dae
Published: July 2002

In strongly coupled plasmas, screening effects are investigated on 1s→2p dipole transition probabilities for electron-impact excitation of hydrogenic ions. Electron–ion interaction is described by the ion-sphere model potential. Behavior of the projectile electron is represented by the semiclassical straight-line trajectory method. Variation of the transition probabilities is determined as a function of the plasma-screening strength through the size of the ion sphere. The transition probabilities are increased with an increase of the ion-sphere radius and the projectile energy. These results, in strongly coupled plasmas, are similar to the screening effects on 1s→2p dipole transition probabilities for electron–ion excitations in weakly coupled plasmas described by the classical Debye–Hückel model.
Fluctuation measurements at c/ωpe spatial scales in a tokamak
Authors: Haines E. J. , Tan I. H. , Prager S. C.
Published: July 2002

Magnetic and electrostatic fluctuations have been measured at the short scale length of the collisionless skin depth (c/ωpe) using small magnetic and electrostatic probes in the Tokapole II tokamak [Nucl. Fusion 9, 1509 (1979)]. For certain conditions and at high frequency (the MHZ range), fluctuations are present at the c/ωpe scale, with significant amplitude variation in the range between c/ωpe and the ion gyroradius scale. Wavelength dependence is inferred from measurements with probes of varying sizes. The amplitude of the turbulence at the c/ωpe scale is smaller than the dominant low-frequency turbulence, and is thus not relevant to transport in Tokapole II. Comparison with theoretical treatments of c/ωpe turbulence is discussed.
Microinstability analysis of DIII-D high-performance discharges
Authors: Rewoldt G. , Lao L. L. , Tang W. M.
Published: July 2002

The kinetic stability properties in a number of high performance discharges from the DIII-D tokamak [R. D. Stambaugh for the DIII-D Team, Plasma Physics and Controlled Nuclear Fusion Research, 1994 (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 83] have been analyzed utilizing a comprehensive kinetic eigenvalue code. The instability considered is the toroidal drift mode [trapped-electron-ion temperature gradient (ηi) mode]. This code has been interfaced with equilibria specific to DIII-D plasmas. Experimentally measured kinetic profile data, along with motional stark effect data and external magnetic data, was used, and the corresponding magnetohydrodynamic (MHD) equilibria were computed numerically. In particular, a low confinement mode (L-mode) case, a high-li high confinement mode (H-mode) case, a very high confinement mode (VH-mode) case, and a high plasma pressure/poloidal magnetic pressure (βp) case have been analyzed. For the L-mode case, a wide region of instability was found, while for the H-mode and VH-mode and high-βp cases, only relatively narrow regions of instability were found. An assessment of the influence of velocity-shear flow on these instabilities has also been made, as well as of changes in the electron and ion temperature gradients and density gradients. While the experimental values of the sheared toroidal flow velocity are not sufficient to stabilize the instability, an increase by a factor of two to four in the flow velocity could completely stabilize this mode.
Self-diffraction effects of electron cyclotron Gaussian beams on noninductively driven current in the International Thermonuclear Experimental Reactor tokamak
Authors: Nowak S. , Lazzaro E. , Ramponi G.
Published: July 2002

The role of diffractive effects not included in standard ray tracing calculations on electron cyclotron current drive (ECCD) is analyzed for an International Thermonuclear Experimental Reactor (ITER) [P. H. Rebut, Fusion Eng. Design 30, 85 (1995)] tokamak configuration, using a quasioptical approach. The power absorbed and the current generated by highly collimated Gaussian beams of electron cyclotron waves (ECW) is calculated using realistic ITER equilibria, relativistic treatment of the wave propagation, and of the driven current, expressed in the formalism of 13 moments transport equations. Effects of beam diffraction on the generated current profile are found to be non-negligible, particularly for off-axis power deposition.
Quasicoherent structures in ion temperature gradient turbulence
Authors: Tinkle M. D. , Sen A. K.
Published: July 2002

The origin of the spectral width of the ion temperature gradient (ITG) mode in a cylindrical plasma is studied. Density fluctuation spectra taken at various radial positions suggest the presence of two modes with different radial dependences. Bispectral analysis shows two modes with weak couplings to broadband turbulence. These data may indicate that the spectral width is composed of radial harmonics.