bluuebunny/crossref_metadata_2025 huggingface dataset:
Finite-amplitude ion-acoustic solitons in weakly inhomogeneous plasmas
Authors: Kuehl H. H. , Imen K.
Published: July 2002

The properties of an ion-acoustic soliton in a weakly inhomogeneous plasma are studied. Unlike previous analyses, the soliton amplitude is not required to be small. The ion generation rate is assumed to be either proportional to the electron density or to be uniform. Assuming that the local soliton width is small compared with the scale length of the plasma inhomogeneity, a perturbation theory is developed which gives the local speed and amplitude of the soliton. For a small amplitude soliton, simple expressions for the local soliton speed, peak soliton potential, and peak soliton ion density are derived. It is shown that both the soliton velocity relative to the ion drift velocity and the peak soliton potential do not vary greatly as the soliton moves from the plasma center toward the sheath. The peak ion density, however, varies over a wider range. These results are shown to agree with those from direct numerical integration of the basic equations.
Numerical simulations of current-limited double layers observed in a linear mirror device
Authors: Inuzuka Hiroshi , Torii Yutaka , Tsukishima Takashige
Published: July 2002

Simulations based on the Vlasov–Poisson scheme are carried out to observe the temporal and spatial development of dopuble layers (DLs) in a linear mirror field. Simulation results show that the magnetic mirrors have a substantial effect in reducing the current passing through the DL. Development of a negative potential region on the low potential side of the DL is also observed, in agreement with the experimental results. The observed strong DLs appear to be formed, triggered by the ion-acoustic instability.
Comments on ‘‘Theory of dissipative density-gradient-driven turbulence in the tokamak edge’’ [Phys. Fluids 2
8, 1419 (1985)]
Authors: Krommes John A.
Published: July 2002

An attempt is made to understand the physical basis, consistency, and quantitative accuracy of the ‘‘clump’’ algorithm as applied to the description of fluid fluctuations. Aspects of the scheme appear to be incompatible with well-established facts about the physics of turbulent fluids.
Stability of field-reversed ion rings
Authors: Friedman Alex , Sudan R. N. , Denavit Jacques
Published: July 2002

Studies of the low-frequency stability of field-reversed ion rings in a dense uniform plasma background have been carried out using a linearized 3-D hybrid simulation model. For a moderately thick ring with aspect ratio on the order of 4:1, the azimuthal mode number l=1 radial mode (magnetohydrodynamic precession) is observed to be stable, because of the favorable field gradient arising from image currents in the outer wall. The l=1 axial (tilt) mode is unstable for the parameters chosen, as are the l=2 radial and axial (kink) modes, and the l=3 radial mode. All other modes are observed to be either stable or very weakly unstable; numerical difficulties arising from the local instability of individual orbits in the equilibrium field make it difficult to examine modes with small growth rates. The structures of the various unstable modes range from near rigid to quite complicated in displacement pattern. Some preliminary evidence for the existence of betatron resonance-driven instabilities is noted.
Propagation of an ion-acoustic soliton in an inhomogeneous plasma
Authors: Chang Hong-Young , Raychaudhuri Santwana , Hill Jacqueline , Tsikis Eugene K. , Lonngren Karl E.
Published: July 2002

Experiments on the propagation of an ion-acoustic soliton into a region of inhomogeneous plasma density are described. It is found that the local amplitude and velocity decrease and the width increases as the soliton propagates into a region of lower density. The results are in good agreement with a model based on the KdV equation and with the recent study of Kuehl and Imen [Phys. Fluids 28, 2375 (1985)].
Analytical theory of nonlinear drift-tearing mode stability
Authors: Scott Bruce D. , Hassam A. B.
Published: July 2002

An analytical theory of the nonlinear stability of drift-tearing modes in the semicollisional regime is presented. It is shown that the overall stability depends critically on the initial island width and the plasma beta. Linear and quasilinear treatments demonstrate that islands smaller than the linear tearing layer are stabilized completely by the influence of the electron temperature gradient. A nonlinear theory is developed for larger islands. Standard algebraic growth is obtained unless diamagnetic effects are strong, in which case the time-asymptotic stability is dependent on the density and temperature profiles.
The three-dimensional instability of strained vortices in a viscous fluid
Authors: Landman M. J. , Saffman P. G.
Published: July 2002

The recent theory describing 3-D exact solutions of the Navier–Stokes equations is applied to the problem of stability of 2-D viscous flow with elliptical streamlines. An intrinsically inviscid instability mechanism persists in all such flows provided the length scale of the disturbance is sufficiently large. Evidence is presented that this mechanism may be responsible for 3-D instabilities in high Reynolds number flows whose vortex structures can be locally described by elliptical streamlines.
Starting conditions for relativistic backward wave oscillators at low currents
Authors: Swegle John A.
Published: July 2002

This paper contains a theoretical treatment of the start oscillation condition for backward wave oscillators with rippled wall slow wave structures, driven by relativistic, low current, annular electron beams. In the low current, or Compton, limit of device operation, this condition amounts to setting a minimum value on the product of the beam current and the cube of the structure length; above the minimum value, the device will spontaneously break into oscillations, even in the absence of an input signal. Also computed is the small signal growth rate of the oscillations, which is shown to be less than the single mode growth rate for an infinitely long structure. Approximate closed form expressions are found in the limit of shallow structure ripples, while numerical results are presented for deeper wall ripples.
Mobility functions for two unequal viscous drops in Stokes flow. I. Axisymmetric motions
Authors: Fuentes Yuris O. , Kim Sangtae , Jeffrey David J.
Published: July 2002

Analytical results are obtained for mobility functions which describe the hydrodynamic interactions between two unequal viscous drops. It is assumed that the surface tension is sufficiently high so that the drops retain a spherical shape. Exact solutions are introduced for the velocity images for Stokeslets and higher-order Stokes singularities near a viscous drop and then these image solutions are used to generate expressions valid for all two-sphere geometries except those for which the gap is much smaller than the diameter of the smaller drop. For rigid spheres, these results are used to obtain a closed-form expression for the Stokes–Einstein Brownian diffusion coefficient.
Numerical simulation of Rayleigh–Bénard convection in shallow tanks
Authors: Arter Wayne , Newell Alan C.
Published: July 2002

The FLOW3D [Numerical Methods for Fluid Mechanics II, edited by K. W. Morton and M. J. Baines (Clarendon, Oxford, 1986), p. 499] code, originally designed to simulate industrial heat flow problems, is found to be suitable for studying Rayleigh–Bénard convection in carefully controlled laboratory experiments. The structures of both rolls and defects in fully developed, laminar convection in shallow tanks are described in detail. Simulations at parameters near the threshold for the onset of turbulence in water show a transition between two time-dependent patterns, one roll-like and the other cellular, in which short wavelength instabilities of known type (skew varicose) are implicated. Evidence for spatiotemporal intermittency is seen, although the turbulence is clearly not fully developed.
‘‘Moderate-m’’ ballooning modes in quadrapole stabilized tandem mirrors
Authors: Nevins W. M. , Pearlstein L. D.
Published: July 2002

The beta limits on the central cells of nonaxisymmetric tandem mirrors due to moderate-m ballooning modes are studied. Both finite Larmor radius (FLR) effects and corrections associated with the finite extent of the ballooning modes in the plane perpendicular to B are retained. The assumption of short perpendicular wavelength together with the large ellipticity of the flux surfaces near the magnetohydrodynamic (MHD) anchor cells allows a reduction of the three-dimensional problem into a sequence of three one-dimensional problems. The marginal stable boundary for the Mirror Fusion Test Facility (MFTF-B) (National Technical Information Service Document Nos. 82020108 and UCID-19359) is calculated and compared with that obtained from a low mode number calculation.
Oscillatory flows in coiled square ducts
Authors: Ravi Sankar S. , Nandakumar K. , Masliyah J. H.
Published: July 2002

The development of complex velocity fields in curved ducts from an initially parabolic profile is studied using a three-dimensional numerical model of the parabolized Navier–Stokes equations. The velocity profiles are influenced strongly by a geometrical parameter Rc (the radius of curvature) and a dynamic parameter Dn (Dean number, Re/(Rc)1/2). For Rc < 10 and Dn up to 200, the velocity fields develop into the previously observed two- and four-cell solutions that are axially invariant and symmetric about the midplane. For Rc =100 and Dn>125 oscillatory solutions develop which are periodic in the axial direction, but are asymmetric about the midplane. Increasing the Dean number over a narrow range results in a significant increase in the frequency of such oscillations. Grid sensitivity tests indicate that such oscillations are not a numerical artifact. Development of oscillatory solutions is delayed with decreasing radius of curvature. Thus for Rc =10, axially invariant two-dimensional solutions that retain the symmetry about the midplane could be obtained for Dn as high as 300. This trend is consistent with one of the earliest observations by Taylor [Proc. R. Soc. London Ser. A. 124, 243 (1929)] that steady, symmetric laminar flows can be observed over a larger range of Dean number in tightly coiled tubes. However, when an asymmetric perturbation is imposed at the inlet, oscillatory solutions develop even for low Rc, indicating that symmetric two-dimensional solutions are not stable to asymmetric perturbations, as indicated by Winters [K. W. Winters and R. C. G. Brindley (private communication)]. Numerical results are also presented for flow through curved ducts with periodic step changes in curvature.
A subgrid model for equilibrium chemistry in turbulent flows
Authors: Cook Andrew W. , Riley James J.
Published: July 2002

A method is presented whereby the fast chemistry reaction, Fuel+(r)Oxidizer →(1+r) Product, may be modeled in the context of a large eddy simulation (LES). The model is based on a presumed form for the subgrid-scale probability density function (PDF) of a conserved scalar. The nature of the subgrid-scale statistics is discussed and it is shown that a beta function representation of the subgrid-scale PDF is appropriate. Data from both laboratory experiments and direct numerical simulations (DNS) are used to show that the predictions of the model are very accurate, given the exact values for the filtered scalar and its variance. A possible model for this variance is presented based on scale similarity.
Hysteresis effect in stationary reflection of shock waves
Authors: Ivanov M. S. , Gimelshein S. F. , Beylich A. E.
Published: July 2002

The problem of transition between regular and Mach reflection of planar shock waves over straight wedges in steady flows was numerically studied by the DSMC method. It is shown that the transition from regular to Mach reflection takes place, in accordance with the detachment criterion, while the opposite transition occurs at smaller angles. The hysteresis effect was observed at increasing and decreasing shock wave angles.
A comparison of simulation methods for rarefied gas flows
Authors: Struckmeier J. , Steiner K.
Published: July 2002

Simulation methods like DSMC are efficient tools to compute rarefied gas flows. Using supercomputers it is possible to include various real gas effects like vibrational energies or chemical reactions in a gas mixture. Nevertheless it is still necessary to improve the accuracy of the current simulation methods in order to reduce the computational effort. To support this task the paper presents a comparison of the classical DSMC method with the so-called Finite Pointset Method. This new approach was developed during several years in the framework of the European space project HERMES. The comparison given in the paper is based on two different test cases: a spatially homogeneous relaxation problem and a two-dimensional axisymmetric flow problem at high Mach numbers.
Blob formation
Authors: Keller Joseph B. , King Andrew , Ting Lu
Published: July 2002

When a thread or column of liquid breaks, a growing blob of liquid forms on the broken ends of the thread as surface tension pulls the ends back toward the rest of the thread. Asymptotic expansions of the shape of the blob and of the flow in it are constructed. Similar expansions are found for the cylindrical blob at the edge of a broken film or sheet of liquid. These results supplement previous calculations of the mass and velocity of the blob.
Passive scalar conditional statistics in a model of random advection
Authors: Ching Emily S. C. , Tsang Y. K.
Published: July 2002

We study numerically a model of random advection of a passive scalar by an incompressible velocity field of different prescribed statistics. Our focus is on the conditional statistics of the passive scalar and specifically on two conditional averages: the averages of the time derivative squared and the second time derivative of the scalar when its fluctuation is at a given value. We find that these two conditional averages can be quite well approximated by polynomials whose coefficients can be expressed in terms of scalar moments and correlations of the scalar with its time derivatives. With the fitted polynomials for the conditional averages, analytical forms for the probability density function (pdf) of the scalar are obtained. The variation of the coefficients with the parameters of the model result in a change in the pdf. Three different kinds of velocity statistics, (i) Gaussian, (ii) exponential, and (iii) triangular, are studied, and the same qualitative results are found demonstrating that the one-point statistics of the velocity field do not affect the statistical properties of the passive scalar.
Thermocapillary and buoyant flows with low frequency jitter. II. Spanwise jitter
Authors: Grassia P. , Homsy G. M.
Published: July 2002

A temperature gradient is applied along a fluid filled slot. A basic state is considered where the slot is subject to thermocapillary forces and vertical mean gravity, each of which produces a parallel flow and a vertical advected temperature gradient, and is also subject to streamwise mean gravity, which will make the applied temperature stratification either stable or unstable. When this basic state is perturbed by jitter imposed in the spanwise direction, normal to the plane of the basic flow, the resulting fluid motion is three dimensional. The flow and temperature fields are found to have a simple functional dependence on streamwise and spanwise coordinates, but retain a complicated dependence on vertical coordinate. Perturbation equations describing the vertical variation of these fields are derived when the jitter is weak. At first order in the spanwise jitter, there is a time periodic spanwise-streamwise circulation around the slot. As this circulation also advects heat, it produces spanwise temperature gradients, enabling thermocapillarity and vertical gravity to generate subsidiary spanwise flows. At next order in the weak spanwise jitter, parallel streamwise flows are encountered, along with streamwise and vertical temperature gradients. In most parameter regimes these are opposed to the flow and temperature fields in the basic state. A thorough parametric investigation is performed where the weak spanwise jitter equations are solved, assuming for simplicity that streamwise gravity is absent. This leads to comparatively simple polynomial solutions in vertical coordinate for the various fields. A large number of parameters can still affect the solutions, however, and a detailed parametric investigation is performed. Interesting behavior is found at small Biot number, with trapping of heat producing large temperatures in the slot and large subsidiary flows. The spanwise to streamwise aspect ratio is another influential parameter, since geometric constraints encountered at extreme values of this ratio suppress certain velocity components of the flow and enhance others, thereby suppressing or enhancing temperature advection. These advected temperature fields themselves produce subsidiary velocities and subsidiary temperatures, which can exhibit a subtle and often counterintuitive dependence on the spanwise-streamwise aspect ratio.
Lubricating motion of a sphere in a conical vessel
Authors: Masmoudi K. , Lecoq N. , Anthore R. , May S. , Feuillebois F.
Published: July 2002

The final stage of sedimentation of a spherical particle moving along the axis of a conical vessel containing a viscous incompressible fluid is studied both theoretically by lubrication analysis and experimentally by laser interferometry. The particle settling velocity varies like d5/2, where d is the gap. There is an excellent agreement between this result from lubrication theory and experiment, the upper bound being for a gap of about 1/30 radius and the lower practical bound being at the size of the particle roughness.
Dynamic testing of subgrid models in large eddy simulation based on the Germano identity
Authors: Meneveau Charles , Katz Joseph
Published: July 2002

Normally the Germano identity is used in large eddy simulation (LES) of turbulent flows to evaluate model coefficients so as to minimize the square error associated with replacing a particular model in the identity. However, one can also explicitly evaluate this error and thus establish the accuracy with which a particular model is capable of reproducing the flow features at scales between the grid and test filters. This idea is tested a priori based on tophat filtered experimental data in the far field of a round jet [Liu et al., J. Fluid Mech. 275, 83 (1994)]. Of the models considered, the mixed nonlinear model yields the smallest error. This approach offers the capability to dynamically choose among available parametrizations, based only on the resolved fields during LES.
Low-order parabolic theory for 2D boundary-layer stability
Authors: Govindarajan Rama , Narasimha R.
Published: July 2002

We formulate here a lowest order parabolic (LOP) theory for investigating the stability of two-dimensional spatially developing boundary layer flows. Adopting a transformation earlier proposed by the authors, and including terms of order R−2/3 where R is the local boundary-layer thickness Reynolds number, we derive a minimal composite equation that contains only those terms necessary to describe the dynamics of the disturbance velocity field in the bulk of the flow as well as in the critical and wall layers. This equation completes a hierarchy of three equations, with an ordinary differential equation correct to R−1/2 (similar to but different from the Orr–Sommerfeld) at one end, and a “full” nonparallel equation nominally correct to R−1 at the other (although the latter can legitimately claim higher accuracy only when the mean flow in the boundary layer is computed using higher order theory). The LOP equation is shown to give results close to the full nonparallel theory, and is the highest-order stability theory that is justifiable with the lowest-order mean velocity profiles for the boundary layer.
On the onset of convective instabilities in cylindrical cavities heated from below. II. Effect of a magnetic field
Authors: Touihri R. , Ben Hadid H. , Henry D.
Published: July 2002

The effect of a constant and uniform magnetic field on electrically conducting liquid-metal flow, in cylindrical cavities heated from below, is numerically analyzed by using a spectral element method to solve the three-dimensional Navier–Stokes and Ohm equations. The cavity is characterized by its aspect ratio defined as A=H/D. The lateral surfaces are adiabatic and all the boundaries are electrically insulating. The flow with a vertical magnetic field has the same symmetries as that without a magnetic field, so that similar convective modes (m=0, m=1, and m=2) occur, but they are not equally stabilized. Here m is the azimuthal wave number. For A=0.5, for sufficiently large values of the Hartmann number Ha, the mode m=2 becomes the critical mode in place of m=0. The horizontal magnetic field breaks some symmetries of the flow. The axisymmetric mode disappears giving an asymmetric mode m=02, i.e., a combination of the m=0 and m=2 modes, whereas the asymmetric modes (m=1 and m=2), which were invariant by azimuthal rotation without a magnetic field, now have two possible orientations, either parallel or perpendicular to the applied magnetic field B. These five modes are differently stabilized, weakly if the axis of the rolls is parallel to B and strongly if the axis is perpendicular. Beyond the primary thresholds, the secondary bifurcation, found in the pure thermal case for A=0.5, becomes an imperfect bifurcation consisting of two disconnected branches.
Spectral nonequilibrium in a turbulent mixing layer
Authors: Wilson Peter , Andrews Malcolm , Harlow Francis
Published: July 2002

The stochastically steady buoyancy-driven turbulent mix of unstably thermally stratified hot and cold water in a channel is examined by means of thermocouples placed at various stations downstream from an entrance splitter plate. Two-time correlations of temperature fluctuations are converted to the approximate two-point correlations of density fluctuations that would occur in a time-evolving mixing layer. The spectral structure exhibits strong nonequilibrium near the splitter plate. Further downstream the spectral structure evolves to self-similarity. Evolution of the structural features is compared with the predictions of a turbulence spectral transport theory for the nonequilibrium behavior of a time-evolving mixing layer between two fluids of different density. Results of the comparison give tentative validation to most aspects of the theoretical description and suggest closure alterations for further enhancement of the transport-equation validity.
Thermocapillary-Coriolis instabilities in liquid bridges
Authors: Le Cunff Cédric , Zebib Abdelfattah
Published: July 2002

Coriolis effects on thermocapillary instabilities in a liquid bridge of infinite length are investigated. The flow is driven by an imposed temperature gradient along the z direction in a (r,θ,z) rotating frame of reference. When the rotation vector is parallel to the z axis, the return flow, obtained as the unperturbed velocity profile with no rotation, is not modified. In this case, rotation is stabilizing and traveling waves are the prefered mode of instability. If the rotation is orthogonal to the z axis, the base flow is modified and is no longer one-dimensional. Asymptotic methods valid for small rotation are used to calculate the base flow which may become multicellular, both in the radial and azimuthal directions, depending on the Prandtl, Marangoni, and Biot numbers. Linear stability analysis of the new base flow indicates that this type of rotation can be destabilizing while traveling azimuthal waves continue to be the prefered form of convection. Finally, the effect of rotation is studied in a finite length cylinder by direct numerical simulation.
Intermittency feature of shear stress fluctuation in high-Reynolds-number turbulence
Authors: Tsuji Yoshiyuki , Dhruva Brindesh
Published: July 2002

Instantaneous shear stress fluctuations are considered for high-Reynolds-number (up to Rλ≈104) turbulent flow field using the concept of maximum norm. The maximum norm is defined as the largest change within a box of a given size. We have applied this technique to a variety of flow fields and a wide range of Reynolds numbers. Results indicated that the maximum norm has a power law behavior with the size of the box. This novel approach is applied to the dissipation field and the scaling behavior is found to agree very well with that obtained using conventional methods. Thus, we obtain the intermittency exponents for both shear stress and the dissipation rate. We find that the shear stress fluctuation is less intermittent than the dissipation field. Implications of these results are discussed briefly.
A note on Rayleigh stability criterion for compressible flows
Authors: Le Duc Anne , Leblanc Stéphane
Published: July 2002

This paper examines a sufficient instability criterion at large wave numbers for inviscid compressible swirling basic flows depending only on the radial coordinate. The growth rate of these instabilities, obtained through a WKB high wave number analysis, is compared with a normal mode analysis. It is proved analytically that both growth rates match exactly in the limit of high wave numbers. Furthermore, the eigenvalues and localized eigenfunctions of the most amplified discrete modes are in agreement analytically and numerically.
Kinetic instabilities of three-layer thermocapillary creeping flows
Authors: Kliakhandler Igor L. , Nepomnyashchy Alexander A.
Published: July 2002

It is shown that plane viscosity stratified system under longitudinal temperature gradient may exhibit long-wavelength instabilities of a purely kinetic nature which persist at arbitrary small Reynolds number. The first instability is so-called alpha effect, and the second one is a new surface-tension-induced instability. The weakly nonlinear equations for the evolving interfaces are derived and simulated.
On the prediction of gas–solid flows with two-way coupling using large eddy simulation
Authors: Boivin Marc , Simonin Olivier , Squires Kyle D.
Published: July 2002

The purpose of this paper is to examine the feasibility of large eddy simulation (LES) for predicting gas–solid flows in which the carrier flow turbulence is modified by momentum exchange with particles. Several a priori tests of subgrid-scale (SGS) turbulence models are conducted utilizing results from direct numerical simulation (DNS) of a forced homogeneous isotropic turbulent flow with the back effect of the particles modeled using the point-force approximation. Properties of the subgrid-scale field are computed by applying Gaussian filters to the DNS database. Similar to the behavior observed in single-phase flows, a priori test results show that, while the local energy flux is inaccurately estimated, the overall SGS dissipation is reasonably predicted using the conventional Smagorinsky model and underestimated using the Bardina scale-similarity model. Very good agreement between model predictions and DNS results are measured using closures whose coefficients are computed using the resolved field, the so-called dynamic subgrid models, with the mixed model yielding more accurate predictions than the dynamic Smagorinsky model. A priori test results are then confirmed in actual LES calculations used to investigate the sensitivity of the predictions to mesh refinement. The LES was performed at infinite turbulent Reynolds number and for a range of particle response times and mass loadings. Grid resolution in the LES was varied from 323 to 963 collocation points, with particle sample sizes of 885 000 for each response time. LES predictions of the flow with two-way coupling are independent of mesh refinement when using the dynamic mixed model and when the particle relaxation time becomes larger than the characteristic time scale of the unresolved fluid turbulent field.
On the nature of rotational shear stabilization in toroidal geometry and its numerical representation
Authors: Miller R. L. , Waltz R. E.
Published: July 2002

In this paper a simple one-dimensional model problem is treated as a paradigm for understanding the nature of rotational shear stabilization in toroidal geometry and its numerical representation. The model is first formulated in a ballooning mode angle θ0 space, where the convective nature of the stabilization is clear. If θ0 is treated as a continuum variable, the slightest rotational shear eventually convects a ballooning mode through both the unstable and stable poloidal angles of a torus, resulting in a stable slab-like time-averaged growth rate and eigenmode growth rate. However, if θ0 is treated as a discrete variable, unstable eigenmodes remain at weak velocity shear. By transforming to real X space, we are able to physically interpret these unstable modes. Continuous θ0 formulations correspond to a radially infinite box and discrete θ0 formulations correspond to a finite box. If care is taken to make the rotational shear constant throughout the whole box, the unstable eigenmodes are found to be localized to the edge and the stable continuum-like modes are localized but distributed throughout the interior. In the case of constant rotational shear, it is found that a critical rotational shear exists for complete toroidal stabilization independent of representation (continuous or discrete θ0) or box size.
Electron cyclotron resonance heating by strong monochromatic wave in nonuniform magnetic field
Authors: Kiwamoto Y. , Saito T. , Tatematsu Y. , Katanuma I. , Yoshimura Y. , Tamano T.
Published: July 2002

Numerical studies are made on dynamics of electrons under electron cyclotron resonance heating (ECRH) by a strong monochromatic wave in a nonuniform magnetic field. The response of the electrons is described in terms of gains in energy and magnetic moment. These expressions are employed to define a response function which connects the velocity distributions of electrons before and after heating in a wide range of conditions. The response function is applied to calculate the velocity distribution of cold electrons which stream up the magnetic slope and are heated at the resonance. This configuration simulates a possible scheme for controlling an inward flux of cold electrons in diverter regions of tori and in mirror ends by mirror reflection enhanced by ECRH. The velocity distribution shows wing-like structures which are not expected from diffusion equations.
Finite beta plasma equilibrium in toroidally linked mirrors
Authors: Ilgisonis V. I. , Berk H. L. , Pastukhov V. P.
Published: July 2002

The problem of finite pressure plasma equilibrium in a system with closed magnetic field lines consisting of quadrupole mirrors linked by simple toroidal cells with elliptical cross sections is analyzed. An appropriate analytical procedure is developed, that uses conformal mapping techniques, which enables one to obtain the magnetic field structure for the free boundary equilibrium problem. This method has general applicability for finding analytic solutions of the two-dimensional Dirichlet problem outside an arbitrary closed contour. Using this method, the deformations of the plasma equilibrium configuration due to finite plasma pressure in the toroidal cell are calculated analytically to the second order in a λ expansion, where λ∼β/εE, β is the ratio of plasma pressure to the magnetic field pressure, ε is the inverse aspect ratio, and E is the ellipticity of the plasma cross section. The outer displacement of the plasma column is shown to depend nonlinearly on the increase of plasma pressure, and does not prevent the achievement of substantial β∼10% in the toroidal cells.
Temporal behavior of the electron density profile during limiter biasing in the HYBTOK-II tokamak
Authors: Sasaki S. , Uesugi Y. , Takamura S. , Sanuki H. , Kadota K.
Published: July 2002

A rapid change in the detailed edge electron density profile during positively biased limiter experiment is obtained with high temporal resolution of ∼20 μs using a laser blowoff lithium beam probe technique in HYBTOK-II tokamak [IEEE Trans. Plasma Sci. PS-19, 885 (1991)]. The transient change in the electron density profile and the movement of the position where a steep density gradient is formed are experimentally observed. The temporal behavior of the electron density profile correlates well with reductions in the biasing current and Hα emission in the scrape-off layer, and the suppression of electrostatic fluctuations near the edge of the core region.
Response to ‘‘Comment on ‘The absorption mechanisms of whistler waves in cool, dense, cylindrically bounded plasmas’ ’’ [Phys. Fluids B 5, 3864 (1993)]
Authors: Harvey B. M. , Lashmore-Davies C. N.
Published: July 2002

In Shoucri’s Comment, he suggests that the off diagonal terms in the dielectric tensor, Σxz, σyz can be as important as the elements included. We provide an answer to this question by considering a whistler wave propagating in an infinite, uniform plasma with wave number, κ=(κ∥, 0, δz). (AIP)
Lower-hybrid poloidal current drive for fluctuation reduction in a reversed field pinch
Authors: Uchimoto E. , Cekic M. , Harvey R. W. , Litwin C. , Prager S. C. , Sarff J. S. , Sovinec C. R.
Published: July 2002

Current drive using the lower-hybrid slow wave is shown to be a promising candidate for improving confinement properties of a reversed field pinch. Ray-tracing calculations indicate that the wave will make a few poloidal turns while spiraling radially into a target zone inside the reversal layer. The poloidal antenna wavelength of the lower hybrid wave can be chosen so that efficient parallel current drive will occur mostly in the poloidal direction in this outer region. Three-dimensional resistive magnetohydrodynamic computation demonstrates that an additive poloidal current in this region will reduce the magnetic fluctuations and magnetic stochasticity.
Reynolds stress of localized toroidal modes
Authors: Zhang Y. Z. , Mahajan S. M.
Published: July 2002

An investigation of the two-dimensional (2-D) toroidal eigenmode problem reveals the possibility of a new consistent 2-D structure, the dissipative ballooning mode of the second kind (BM-II). In contrast to the conventional ballooning mode, the new mode is poloidally localized at π/2 (or−π/2), and possesses significant radial asymmetry. The radial asymmetry, in turn, allows the dissipative BM-II to generate considerably larger Reynolds stress as compared to the standard slab drift-type modes. It is also shown that a wide class of localized dissipative toroidal modes are likely to be of the dissipative BM-II nature, suggesting that at the tokamak edge, the fluctuation generated Reynolds stress (a possible source of poloidal flow) can be significant.
Broadband density fluctuation measurements using a heavy ion beam probe on the Texas Experimental Tokamak
Authors: Heard J. W. , Crowley T. P. , Schoch P. M. , Hickok R. L. , Ross David W. , Zhang B. Z. , Yang X. Z.
Published: July 2002

Broad bandwidth (60 kHz ≤f≤500 kHz), long wavelength (k̄θ≤2  cm−1) density fluctuation measurements were made using a heavy ion beam probe on the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Technol./Fusion 1, 479 (1981)]. Fluctuation amplitudes and correlation length scalings do not correspond to linear or quasilinear drift wave theory. Power-weighted root-mean-square (rms) wave number estimates are much lower (〈k̄θ〉rmsρs≤0.1, ρs=[miTe]1/2/eB) than quasilinear theoretical expectations (k̄⊥ρs∼0.3). Core results indicate strong turbulence with a wave number spread, Δk∼k̄ at fixed frequency f, consistent with strong turbulence predictions, while edge results can be considered consistent with weak turbulence. In between the core and edge (0.6≤r/a≤0.8) is a transition region.
Collisional ballooning mode dispersion relation in the banana regime
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

Collisional ballooning mode theory in the banana regime is developed for tokamak configurations from the gyrokinetic formalism. A general dispersion relation is obtained, which in principle can deal with a collision operator of any type. However, investigation of an approximate Fokker–Planck collision operator developed in recent neoclassical transport theory is detailed. The most significant feature of the present theory as compared to the customary treatment lies in that the distinction between particle and fluid velocities is made in the ordering analyses. This reveals that the eigenfrequency of modes is determined by balancing the small-parallel-ion-velocity (SPIV) effect [L.-J. Zheng and M. Tessarotto, Phys. Plasmas 1, 3928 (1994)], instead of the fluid inertia one, with the instability drives. Since the parallel-electric-field effect is found to be negligible as compared to the SPIV effect, in contrast to the customary resistive ballooning mode picture, the leading collisional effect is demonstrated to be the modification of the SPIV effect instead of the relaxation of the frozen-in-law. The ion–ion collisions are the cause for this modification, while the electron collisional effect is shown to be negligible.
Enhanced shear damping and ion transit resonance damping of electron drift waves in toroidal plasmas
Authors: Rogister André
Published: July 2002

Shear stabilization of low mode number, toroidally uncoupled electron drift waves is shown to be larger in axisymmetric tori than in cylinders, in sharp contrast to theoretical predictions obtained at higher mode numbers. When the wave frequency is larger than the thermal ion transit frequency, the enhancement factor, which is related to the toroidicity induced ∇B and curvature drifts of passing ions across the flux surfaces, is naturally reminiscent of the results of neoclassical diffusion theory in the Pfirsch–Schlüter regime. Even larger ion transit resonance damping arises in the opposite limit. The ratio of the sum of the revisited shear and Landau damping rates (resulting from interaction with passing ions and passing electrons, respectively) to the trapped electron excitation rate of these low frequency modes turns out to be approximately constant throughout the confinement zone of tokamaks with parabolic density profiles. An attempt is made to justify this remarkable result, as well as the transition from global gyro-Bohm to global Bohm scaling at very low collisionality.
Active core profile and transport modification by application of ion Bernstein wave power in the Princeton Beta Experiment-Modification
Authors: LeBlanc B. , Batha S. , Bell R. , Bernabei S. , Blush L. , de la Luna E. , Doerner R. , Dunlap J. , England A. , Garcia I. , Ignat D. , Isler R. , Jones S. , Kaita R. , Kaye S. , Kugel H. , Levinton F. , Luckhardt S. , Mutoh T. , Okabayashi M. , Ono M. , Paoletti F. , Paul S. , Petravich G. , Post-Zwicker A. , Sauthoff N. , Schmitz L. , Sesnic S. , Takahashi H. , Talvard M. , Tighe W. , Tynan G. , von Goeler S. , Woskov P. , Zolfaghari A.
Published: July 2002

Application of Ion Bernstein Wave Heating (IBWH) into the Princeton Beta Experiment-Modification (PBX-M) [Phys. Fluids B 2, 1271 (1990)] tokamak stabilizes sawtooth oscillations and generates peaked density profiles. A transport barrier, spatially correlated with the IBWH power deposition profile, is observed in the core of IBWH-assisted neutral beam injection (NBI) discharges. A precursor to the fully developed barrier is seen in the soft x-ray data during edge localized mode (ELM) activity. Sustained IBWH operation is conducive to a regime where the barrier supports large ∇ne, ∇Te, ∇νφ, and ∇Ti, delimiting the confinement zone. This regime is reminiscent of the H(high) mode, but with a confinement zone moved inward. The core region has better than H-mode confinement while the peripheral region is L(low)-mode-like. The peaked profile enhances NBI core deposition and increases nuclear reactivity. An increase in central Ti results from χi reduction (compared to the H mode) and better beam penetration. Bootstrap current fractions of up to 0.32–0.35 locally and 0.28 overall were obtained when an additional NBI burst is applied to this plasma.
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P simulations of International Thermonuclear Experimental Reactor plasmas
Authors: Budny R. V. , McCune D. C. , Redi M. H. , Schivell J. , Wieland R. M.
Published: July 2002

The TRANSP code [R. V. Budny et al., Nucl. Fusion 35, 1497 (1995)] is used to construct comprehensive, self-consistent models for plasmas within the separatrix surface in the International Thermonuclear Experimental Reactor (ITER) [Technical Basis for the ITER Interim Design Report, Cost Review and Safety Analysis (International Atomic Energy Agency, Vienna, 1996)]. Steady state profiles of two plasmas from the ITER ‘‘Interim Design’’ database are used. Effects of 1 MeV neutral beam injection, sawteeth mixing, toroidal field ripple, and helium ash transport are included. Results are given for the fusion rate profiles, and parameters describing effects such as the alpha particle heating of electrons and thermal ions, and the thermalization rates. The modeling indicates that the deposition of the neutral beam ions will peak in the plasma center, and the average beam ion energy will be half the injected energy. Sawtooth mixing will broaden the fast alpha profile. The toroidal ripple loss rate of alpha energy will be 3% before sawtooth crashes and will increase by a factor of 3 immediately following sawtooth crashes. Various assumptions for the thermal He transport and the He recycling coefficient at the separatrix Rrec are used. If the ratio of helium and energy confinement times, τ*He/τE is less than 15, the steady state fusion power is predicted to be 1.5 GW or greater. The values of the transport coefficients required for this fusion power depend on Rrec. If this is larger than about 0.5, and if the inward pinch is small the required He diffusivity must be much larger than that measured in tokamaks.
Variational structure for dissipationless linear drift-wave equations
Authors: Brizard Alain J.
Published: July 2002

A derivation from first principles of the Lagrangian density for the dissipationless linear drift- wave equation recently introduced by Mattor and Diamond [Phys. Plasmas 1, 4002 (1994)] is presented. An exact wave-action conservation law for linear drift waves propagating in a rotating magnetized plasma is then derived from the Lagrangian density without requiring the use of the eikonal representation for the wave fields.
Anisotropic plasma with flows in tokamak: Steady state and stability
Authors: Ilgisonis V. I.
Published: July 2002

An adequate description of equilibrium and stability of anisotropic plasma with macroscopic flows in tokamaks is presented. The Chew-Goldberger-Low (CGL) approximation is consistently used to analyze anisotropic plasma dynamics. The admissible structure of a stationary flow is found to be the same as in the ideal magnetohydrodynamics with isotropic pressure (MHD), which means an allowance for the same relabeling symmetry as in ideal MHD systems with toroidally nested magnetic surfaces. A generalization of the Grad-Shafranov equation for the case of anisotropic plasma with flows confined in the axisymmetric magnetic field is derived. A variational principle was obtained, which allows for a stability analysis of anisotropic pressure plasma with flows, and takes into account the conservation laws resulting from the relabeling symmetry. This principle covers the previous stability criteria for static CGL plasma and for ideal MHD flows in isotropic plasma as well.
Optimization of soft x-ray emission from stagnating compact toroidal plasmas: A computational study
Authors: Dietz D. , Hussey T. W. , Roderick N. F. , Douglas M. R. , Degnan J. H.
Published: July 2002

Computational simulations aimed at optimizing the high-energy, high-power, multikilovolt electromagnetic radiation emitted by a rapidly moving compact toroidal (CT) plasma which stagnates against a stationary “wall” are performed for argon, krypton, and xenon plasmas over a range of CT parameters. CT kinetic energies vary from 2–10 MJ, impact speeds vary from 50–200 cm/μs, and CT masses vary from 5–11 mg. It is found that a 2 MJ Ar CT optimally emits 1–1.5 MJ of essentially K-line radiation (>3 keV) for impact speeds of about 60–90 cm/μs; a 10 MJ Kr CT optimally emits about 1 MJ of essentially K-line radiation (>12.5 keV) for impact speed of about 135 cm/μs; and a 10 MJ Xe CT optimally emits about 3 MJ of essentially L-line radiation (>5 keV), about 0.5 MJ of continuum radiation above 10 keV, and about 0.1 MJ of continuum radiation above 20 keV, all also for impact speed of about 135 cm/μs. Pulse widths vary for the above optima from 7 ns at 135 cm/μs to 30 ns at 60 cm/μs.
Turbulent particle transport in tokamak plasmas with impurities
Authors: Fu X. Y. , Dong J. Q. , Horton W. , Ying C. T. , Liu G. J.
Published: July 2002

The turbulence driven by the ion temperature gradient, the mass shear flow parallel to the magnetic field, and the impurity ion density gradient in confined plasmas is studied in a sheared slab magnetic configuration. The turbulence drive from the temperature gradient and parallel shear flow of the majority ion component is shown to be enhanced by the shear flow and negative density gradient of the impurity ions. The particle diffusion induced by the turbulence is obtained within the framework of quasilinear fluid theory. Optimal transport parameters for an inward “pinch’’ of the majority ions and the outward flow of the impurity ions are determined. The corresponding effective diffusion coefficients that include the pinch effects are computed. Correlations with tokamak experimental observations such as an isotope scaling of plasma confinement time are discussed.
Minimal model for transport barrier dynamics based on ion-temperature-gradient turbulence
Authors: Hu G. , Horton W.
Published: July 2002

Low-order mode coupling equations are derived to describe recent computer simulations of the toroidal ion-temperature-gradient turbulent convection with steady and pulsating sheared mass flows in the transport barrier zone. The three convective transport states are identified with the tokamak confinement regimes called low mode (L-mode), high mode (H-mode), and barrier localized modes (BLMs) when the transport barrier is in the core plasma. The L-mode limit cycle is analytically derived and a bifurcation diagram showing L to H and H to BLM transitions in confinement is constructed numerically. Markovian closure procedures are sought to further reduce the dimensionality of the nonlinear system. First an exact expression is given for the energy transfer rate from the fluctuations to the sheared mass flow through the triplet velocity correlation function. Then the time scale expansion required to derive the Markovian closure formula is given. Markovian closure formulas form the basis for the thermodynamic-like L–H bifurcation models.
Evaluation of the polarimetric transition matrix for a magnetized plasma
Authors: Dattoli G. , Segre S. E.
Published: July 2002

With reference to the change of polarization of an electromagnetic wave traversing a plasma, two alternative numerical schemes are discussed for computing the plasma transition matrix for given profiles of plasma parameters. Such a computation is required for the reconstruction of magnetohydrodynamic equilibria, when the elements of the matrix are included among the measured quantities used in the best fit procedure. Both schemes, which are especially useful when the Cotton–Mouton effects are not small, ensure that the matrix remains unitary during the numerical integration.