bluuebunny/crossref_metadata_2025 huggingface dataset:
Gyrosheath near the tokamak edge
Authors: Hazeltine R. D. , Xiao H. , Valanju P. M.
Published: July 2002

A new model for the structure of the radial electric-field profile in the edge during the high confinement (H mode) is proposed. Charge separation caused by the difference between electron and ion gyromotion, or more importantly in the tokamak, the banana motion (halo effect) can self-consistently produce an electric dipole moment that causes the sheared radial electric field. The calculated results based on the model are consistent with DIII-D [Fusion Technol. 8, 441 (1985)] and Tokamak Experiment for Technology Oriented Research (TEXTOR) [J. Nucl. Mater. 122, 123, 1124 (1984)] experimental results.
Nonlinear wave–particle scattering and absorption of electromagnetic and electrostatic waves in a magnetized plasma
Authors: Sugaya R.
Published: July 2002

Nonlinear wave–particle scattering and absorption (nonlinear Landau and cyclotron damping) of electromagnetic and electrostatic waves by high energy or relativistic electrons in a homogeneous magnetized plasma are investigated theoretically by numerical analysis of general and simplified expressions of nonlinear wave–particle coupling coefficients. It is shown that strong nonlinear absorption of the extraordinary wave can occur by nonlinear scattering into the other extraordinary waves or Bernstein waves induced by high-energy stationary or drifting electrons, and they can be accelerated. Nonlinear absorption by Bernstein wave or ordinary wave is weaker than that by extraordinary wave. These nonlinear absorption mechanisms can compete with linear absorption in a fusion plasma, and can exceed it for a sufficiently small k∥ρb. It is verified that nonlinear scattering of extraordinary waves can induce the effective acceleration of relativistic electrons in a fusion plasma.
Electron beam acceleration by nonlinear Landau damping of electromagnetic waves in a plasma
Authors: Sugaya R.
Published: July 2002

Acceleration and heating of an electron beam caused by nonlinear Landau damping of intense electromagnetic waves in a plasma are investigated theoretically based on a velocity-space diffusion equation. The beat wave, which does not satisfy the dispersion relation, is excited by the nonlinear interaction of two electromagnetic waves and interacts nonlinearly with the electron beam. As the frequency of the beat wave approaches the electron plasma frequency, the rates of acceleration and heating of the electron beam and the beat wave energy density become maximum values, being equal to those by stimulated Raman scattering. The beat wave is an electrostatic mode that can have a phase velocity near the light velocity and is able to accelerate the electron beam to a relativistic energy.
Equation-of-state effects on laser-accelerated foils
Authors: Schmalz R. F. , Meyer-ter-Vehn J.
Published: July 2002

One-dimensional numerical calculations are made to simulate the acceleration of thin foils with high-power laser beams. The main objective is to study the effects of a realistic equation-of-state in contrast to an ideal gas approximation. Shock build-up and dynamics show marked differences, compression and entropy production are strongly reduced. The thermodynamic state of the accelerated, dense material is analyzed, and the concept of critical entropy is applied. The possibility of keeping the accelerated foils in a condensed state moving at velocities of more than 100 km/sec is discussed.
Analytic theory of stable resistive magnetohydrodynamic modes
Authors: Pao Young-ping , Kerner Wolfgang
Published: July 2002

The spectrum of stable resistive magnetohydrodynamic modes is shown to be determined by the geometry of anti-Stokes lines. Behavior of the eigenfunctions is also examined.
A review of energy confinement and local transport scaling results in neutral-beam-heated tokamaks
Authors: Kaye Stanley M.
Published: July 2002

Over the past several years, tokamak neutral beam injection experiments have evolved from the brute force study of the effects of global discharge characteristics (Ip, n̄e, Pheat, etc.) on energy confinement to the appreciation that there are effects more subtle, yet controllable, that may influence confinement dramatically. While this evolution from first to second generation experiments is derived from an empirical understanding of ‘‘low’’ and ‘‘high’’ energy confinement modes and how to achieve them operationally, the underlying physics is still unknown. Several theories with different physical bases appear to describe the global scaling of the low confinement mode discharges quite well. On the other hand, little agreement has been found between theoretical and experimentally deduced values of local transport coefficients. While it is known operationally how to achieve any one of several types of high confinement mode discharges, here too, the underlying physics of the transport associated with these modes is poorly understood.
Experimental study of ablation pressures and target velocities obtained in 0.26 μm wavelength laser experiments in planar geometry
Authors: Fabbro R. , Faral B. , Virmont J. , Cottet F. , Romain J. P. , Pépin H.
Published: July 2002

In 0.26 μm wavelength laser experiments that were performed in planar geometry with irradiances between 1013 and 1015 W/cm2, the ablation pressure and the target velocity have been measured using a shock-velocity measurement and the double foil technique, respectively. The conditions are discussed that must be satisfied if the double-foil technique is to give an accurate measurement of the velocity of the dense part of the target. The rocket model has also been improved using a time-dependent applied pressure pulse, in order to accurately describe the relation between ablation pressure, target velocity, and ablated fraction. Pressures up to 50 Mbar have been easily generated since lateral energy transport is rather low with a 0.26 μm wavelength laser.
Self-organization and energy relaxation in a three-dimensional magnetohydrodynamic plasma
Authors: Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

A three-dimensional self-organization process in an incompressible dissipative plasma is investigated in detail by means of a magnetohydrodynamic simulation. In an initial phase the total magnetic energy rapidly decreases via magnetic reconnection driven by a global ideal unstable mode, while the total magnetic helicity decreases very slowly throughout the simulation run. This selective dissipation is found to be caused by the excitation of nonhelical, small spatial scale structures of the magnetic field (‘‘magnetic bubble’’) in the vicinity of the reconnection points. Magnetic reconnection takes place at minimum points of the magnetic helicity density where the magnetic bubbles are predominantly created. The disappearance of the bubbles leads to a rapid dissipation of the total magnetic energy. While the magnetic helicity is globally conserved during this process, its spectral density is transferred to the low wavenumber region (‘‘inverse cascade’’). After the rapid dissipation of the magnetic energy, the magnetic configuration adjusts itself to a stable equilibrium with a minimum energy without any drastic decrease in the magnetic energy and helicity. The final stable state takes either an axially symmetric configuration or a helically symmetric configuration depending on the initial condition, which is in good agreement with the relaxation theory.
The spectrum of resistive viscous magnetohydrodynamics
Authors: Riedel K. S.
Published: July 2002

The spectrum of scalar resistivity εη̃, scalar viscosity εμ̃, incompressible magnetohydrodynamics (MHD) is calculated in a slab geometry. The equilibrium may have a mode rational surface. The Alfvén continuum of ideal MHD is replaced by point eigenvalues on specific curves in the stable ω half-plane. As the dissipation parameter ε decreases, the curves containing these eigenvalues remain fixed and do not approach the ideal MHD continuum. The spatial resonances of these eigenfunctions are calculated. Viscosity removes the accumulation of eigenvalues at ω=0. The position of the curves depends on the ratio of resistivity to viscosity, but the topology of the curves is independent of this ratio.
Inertial range structure of turbulent velocity and scalar fields in a Lagrangian renormalized approximation
Authors: Kaneda Yukio
Published: July 2002

The inertial range structure of turbulence is studied on the basis of an approximation derived by a systematic method of Lagrangian renormalized expansions. This method is also applied to the problem of a passive scalar field convected by turbulence, and some of its consequences are examined. Numerical values are obtained for various dimensionless constants in the inertial range including those in the k−5/3 spectrum law for the turbulent energy and the scalar field.
Resistive Alfvén spectrum of tokamaklike configurations in straight cylindrical geometry
Authors: Kerner W. , Lerbinger K. , Riedel K.
Published: July 2002

The resistive Alfvén spectrum for tokamaklike configurations with one or two singular surfaces is analyzed by combined WKBJ treatment and numerical solution using a spectral code. It is found that the ideal continua disappear; these are approximated only at the end points, at the singular surfaces, and at selected interior points (given by extrema of ωA(r)=‖k ⋅ B0‖/(ρ0)1/2). The normal modes are damped and the eigenvalues lie on specific curves that become independent of resistivity for small η.
The statistics of the organized vortical structure in turbulent mixing layers
Authors: Bernal L. P.
Published: July 2002

The statistics of the large scale vortex structure in turbulent mixing layers have been investigated theoretically. It is shown that similarity in the fully developed flow results in a common description of the Eulerian and Lagrangian statistics. In the Eulerian frame of reference, a conservation equation is derived and solved to show that the distribution of vortex circulation is lognormal. It is also shown that the standard deviation normalized by the mean value of the distribution depends only on the amalgamation mechanism. The value for pairing is in good agreement with experimental measurements. These results are used to calculate the life span and survival probabilities of the vortices in the Lagrangian frame of reference. These distributions are in good agreement with direct measurements of the life span probability and with space-time correlation measurements, respectively. Some implications of these results on the dynamics of the large scale vortices in the fully developed turbulent flow are discussed.
Diocotron instability for intense relativistic non-neutral electron flow in planar diode geometry
Authors: Davidson Ronald C. , Tsang Kang T. , Uhm Han S.
Published: July 2002

The extraordinary-mode eigenvalue equation is used to investigate detailed properties of the diocotron instability for sheared, relativistic electron flow in a planar diode. The theoretical model is based on the cold-fluid-Maxwell equations assuming low-frequency flute perturbations about a tenuous electron layer satisfying ω2pb(x)≪ω2c and ‖ω−kVy(x)‖2≪ω2c. The cathode is located at x=0; the anode is located at x=d; the outer boundary of the electron layer is located at x=x+b<d; and the inner boundary of the layer is located at x=x−b<x+b. The extraordinary-mode

eigenvalue equation is solved exactly for the case where nb(x)/ γb(x) =n̂b/γ̂b =const within the electron layer (x−b<x<x+b). Here, nb(x) =n̂b cosh [κ̂(x−x−b)]/ cosh[κ̂(x+b−x−b)] is the equilibrium density profile, γb (x)=cosh[κ̂(x−x−b)] is the relativistic mass factor, and ω̂D =κ̂c=4πn̂bec/B0 =const is the diocotron frequency. The analysis leads to a transcendental dispersion relation for the complex eigenfrequency ω in terms of the wavenumber k (in the flow direction), the relativistic flow parameter θ=ω̂D(x+b−x−b)/c, and the geometric factors Δi=x−b/d, Δ0=(d−x+b)/d, and Δb=(x+b−x−b)/d. It is found that the diocotron instability is completely stabilized by relativistic and electromagnetic effects whenever 2(ΔiΔ0)1/2/Δb <(sinh θ)/θ, i.e., stabilization occurs whenever the flow is sufficiently relativistic (sufficiently large θ) and/or the vacuum regions are sufficiently narrow (sufficiently small Δ0 or Δi).
An analytical solution of a collisional-radiative model for nonequilibrium argon plasmas
Authors: Braun C. G. , Kunc J. A.
Published: July 2002

The general analytical solution of a collisional-radiative model, including atom–atom inelastic collisions, for nonequilibrium, partially ionized, steady-state plasmas is presented. This is applied to argon plasmas and the results are compared with previous numerical calculations. The model includes three atomic energy levels; the ground level, one excited level, and continuum, and assumes a Maxwellian electron and atom distribution functions. All rates for collisional and radiative processes are expressed as analytical functions. The model is solved algebraically to obtain analytical expressions for the electron and excited atom densities as a function of the plasma parameters. In the limit of an optically thick plasma the solution agrees with results for complete thermodynamic equilibrium. Solutions are presented for a wide range of conditions (3000≤Te≤24 000 K, 105≤Ne≤NA cm−3, 1014≤NA≤1018 cm−3).
Modeling of reversed-field-pinch magnetic probe measurements
Authors: Miller Guthrie
Published: July 2002

Magnetic probe measurements carried out on the ZT-40M reversed field pinch [Fusion Technol. 8, 1571 (1985)] for low current (50 kA) discharges with and without F-Θ pumping are described. The internal probe measurements are used to determine the evolution of the axisymmetric equilibrium profiles. Of particular interest is the profile response to the large-amplitude, oscillatory, toroidal, and poloidal driving voltages applied during F-Θ pumping. The results are compared with a two-field-line stochastic magnetic field model.
A numerical study of the viscous supersonic flow past a flat plate at large angles of incidence
Authors: Drikakis D. , Durst F.
Published: July 2002

A numerical study of the viscous supersonic flow past a flat plate is presented. The objective is to investigate the supersonic flow at high angles of incidence where large flow gradients occur. The effect of the angle of incidence and the Reynolds number (Re) in the flow structure especially in the formation of the separation region is investigated. The study is based on the solution of the full Navier–Stokes equations by high resolution schemes, and it focuses on the supersonic flow over the plate at Re≤105. Results on fine computational grids are presented for flow angles up to 20°. The calculations reveal that the flow remain attached for angles of incidence less than a=5°. For a=5° and Re=105, separation of the flow at the trailing edge appeared. Increasing the flow angle (a≳5°) moves the separation point upstream while a reverse flow region forms for the entire range of the Reynolds numbers used in this study. The results reveal that for large angles of incidence, the variation of the Reynolds number has significant effects on the variation of the flow variables. The flow behind the trailing edge is also affected from the flow angle as well as from the Reynolds number. Comparisons are also presented between viscous and inviscid solutions. The comparisons show that the viscous effects are dominant on the upper surface of the plate as well as behind the trailing edge. These effects become stronger when the flow angle is a=20°.
Effect of cross-flow on Görtler instability in incompressible boundary layers
Authors: Zurigat Y. H. , Malik M. R.
Published: July 2002

Linear stability theory is used to study the effect of cross-flow on Görtler instability in incompressible boundary layers. The results cover a wide range of sweep angle, pressure gradient, and wall curvature parameters. It is shown that the cross-flow stabilizes Görtler disturbances by reducing the maximum growth rate and shrinking the unstable band of spanwise wave numbers. On the other hand, the effect of concave wall curvature on cross-flow instability is destabilizing. Calculations show that the changeover from Görtler to cross-flow instabilities is a function of Görtler number, pressure gradient, and sweep angle. The results demonstrate that Görtler instability may still be relevant in the transition process on swept wings even at large angles of sweep if the pressure gradient is sufficiently small. The influence of pressure gradient and sweep can be combined by defining a cross-flow Reynolds number. Thus, the changeover from Görtler to cross-flow instability takes place at some critical cross-flow Reynolds number whose value increases with Görtler number.
The effect of shear in selective withdrawal
Authors: Clarke S. R. , Imberger J.
Published: July 2002

The evolution of the withdrawal through a line sink of an initially quiescent, linearly stratified fluid in a semi-infinite, horizontal duct is investigated. It is shown that due to the shear present in the withdrawal layer the previously suggested mechanism for the control of the higher mode shear fronts, which assumes that the velocity of the fronts is balanced by the oncoming flow, cannot occur. An alternative mechanism for the control of this flow is proposed based on solutions for the vertical structure of linear, long, internal waves in horizontal shear. This results in a model for unsteady selective withdrawal in agreement with steady-state solutions.
On the consistency of Reynolds stress turbulence closures with hydrodynamic stability theory
Authors: Speziale Charles G. , Abid Ridha , Blaisdell Gregory A.
Published: July 2002

The consistency of second-order closure models with results from hydrodynamic stability theory is analyzed for the simplified case of homogeneous turbulence. In a recent study, Speziale, Gatski, and Mac Giolla Mhuiris [Phys. Fluids A 2, 1678 (1990)] showed that second-order closures are capable of yielding results that are consistent with linear stability theory for the case of homogeneous shear flow in a rotating frame. It is demonstrated in this paper that this success is due to the fact that the stability boundaries for rotating homogeneous shear flow are not dependent on the details of the spatial structure of the disturbances. For those instances where they are—such as in the case of elliptical flows where the instability mechanism is more subtle—the results are not so favorable. The origins and extent of this modeling problem are examined in detail along with a possible resolution based on Rapid Distortion Theory (RDT) and its implications for turbulence modeling.
Galilean invariance and filtering dependence of near-wall grid-scale/subgrid-scale interactions in large-eddy simulation
Authors: Härtel Carlos , Kleiser Leonhard
Published: July 2002

A numerical study is presented which was conducted to examine how the average transfer of kinetic energy between large and small turbulent scales in the near-wall flow depends on the filter function. In contrast to some previous findings of other authors, the present analysis reveals that these interactions are qualitatively almost independent of the type of filter employed. It is shown that conclusive results for the individual subgrid-stress components (Leonard stresses, cross stresses, and Reynolds stresses) can only be obtained if a Galilean-invariant form of these components is used.
On the linear growth rates of the long-wave modes in Bénard–Marangoni convection
Authors: Wilson S. K. , Thess A.
Published: July 2002

Explicit analytical expressions for the linear growth (and decay) rates of long-wave modes in Bénard–Marangoni convection are derived and discussed. These analytical predictions are shown to be in good agreement with experimental observations and are used to estimate the minimum experimental time necessary in order to observe the long-wave instability under microgravity conditions.
On the small scale structure of simple shear flow
Authors: Garg Sandeep , Warhaft Z.
Published: July 2002

The structure of the small scale velocity field is studied in an approximately homogeneous shear flow (constant mean shear) over the Reynolds number range 156⩽Rλ⩽390. The shear was generated in a wind tunnel using screens of various solidity and a series of straightening channels in the manner of Tavoularis and Corrsin [J. Fluid Mech. 104, 311 (1981)]. We show there is significant skewness (of order 1) of the derivative of the longitudinal velocity in the direction of the mean gradient, and thus that for these Reynolds numbers the flow is anisotropic at the small scales. The skewness slowly decreases with Rλ and is described by the empirical fit: S∂u/∂y=15.4Rλ−0.6. Thus, even if this downward trend continues, our results imply that anisotropy at the third moment continues to very high Rλ. We also show that, over the Rλ range investigated, the kurtosis of ∂u/∂y decreases (due to the diminishing effect of the structures that cause the skewness), implying that there will be a transition in this quantity, since it must increase as intermittency becomes more pronounced at higher Rλ. Transverse (as well as longitudinal) structure functions of the longitudinal velocity are studied up to the fifth moment. It is shown that the third order transverse structure function has a scaling range. Thus, the anisotropy exists at inertial as well as dissipation scales. The results are compared and contrasted with those of a passive scalar (for which it is known that persistent anisotropy exists at the third moment and above).
Dynamics of velocity gradient invariants in turbulence: Restricted Euler and linear diffusion models
Authors: Martı́n Jesús , Dopazo César , Valiño Luis
Published: July 2002

A complete system of dynamical equations for the invariants of the velocity gradient, the strain rate, and the rate-of-rotation tensors is deduced for an incompressible flow. The equations for the velocity gradient invariants R and Q were first deduced by Cantwell [Phys. Fluids A 4, 782 (1992)] in terms of Hij, the tensor containing the anisotropic part of the pressure Hessian and the viscous diffusion term in the velocity gradient equation. These equations are extended here for the strain rate tensor invariants, RS and QS, and for the rate-of-rotation tensor invariant, QW, using HijS and HijW, the symmetric and the skew-symmetric parts of Hij, respectively. In order to obtain a complete system, an equation for the square of the vortex stretching vector, Vi≡Sijωj, is required. The resulting dynamical system of invariants is closed using a simple model for the velocity gradient evolution: an isotropic approximation for the pressure term and a linear model for the viscous diffusion term. The local topology and the resulting statistics implied by this model reproduce a number of trends similar to known results from numerical experiments for the small scales of turbulence.
Fully nonlinear solitary waves in continuously stratified incompressible Boussinesq fluids
Authors: Brown David J. , Christie Douglas R.
Published: July 2002

This study is concerned with a fully nonlinear theoretical treatment of internal solitary waves in continuously stratified, incompressible, inviscid, shear-free Boussinesq fluids. Results are presented for wave propagation in both deep and shallow fluids with four different ambient stability profiles. Only the dominant mode with the greatest wave speed is considered. The morphology of finite-amplitude internal solitary waves in Boussinesq fluids is shown to be very sensitive to the precise form of the stability profile. The calculations indicate that a wave of maximum amplitude, which is less than the total fluid depth, exists for all internal solitary waves in continuously stratified Boussinesq fluids of finite depth. There is apparently no upper limit on the amplitude of internal solitary waves in many physically realistic unbounded fluids. Large amplitude waves of this type are mutually similar in form and the morphology of these waves appears to be independent of the ambient stability profile in the waveguide layer. It is shown that the properties of highly nonlinear waves with recirculating flow depend on the density distribution and vorticity of the trapped fluid inside the closed circulation cell. Fluid velocity components associated with the wave motion are evaluated and used to calculate the surface perturbation pressure. The surface perturbation pressure signature for internal solitary waves is found to change with the onset of recirculation from a single-crested profile at small wave amplitudes to a bimodal profile at large wave amplitudes. Results for solitary waves in finite-depth fluids differ from those found for deep fluids in that the surface perturbation pressure at the center of the wave eventually changes sign as wave amplitude increases.
A dynamical model for turbulence. IX. Reynolds stresses for shear-driven flows
Authors: Canuto V. M. , Dubovikov M. S. , Yu G.
Published: July 2002

We present a new expression for the one-point Reynolds stress τij in terms of the strain and vorticity of the large scales. The τij are expressed in terms of only five basic orthogonal tensors rather than the traditional ten tensors. The expression for τij, Eq. (24), contains no adjustable parameters. The derivation of τij is based on the two-point closure dynamic equations for the spectral Reynolds stresses Rij(k) that were developed earlier and the results of which were validated on a wide variety of data comprising shear, buoyancy, two-dimensional (2-D) turbulence, rotation, etc. For the case of homogeneous turbulence, we also derive an expression for the empirical coefficients of the ε equation that depend on the invariants of the flow, the turbulent kinetic energy K and the production P. Examples for special flows are given. The new expressions for τij are shown to reproduce well data from Tavoularis and Corrsin, DNS data, stationary data (pipe flow, channel flow, and homogeneous flow), and the Smagorinsky–Lilly constant, which is shown to be a dynamical variable since it depends on the ratio P/ε and on the invariant {S3}S−3.
Stability characteristics of wavy walled channel flows
Authors: Selvarajan S. , Tulapurkara E. G. , Ram V. Vasanta
Published: July 2002

The linear temporal stability characteristics of converging–diverging, symmetric wavy walled channel flows are numerically investigated in this paper. The basic flow in the problem is a superposition of plane channel flow and periodic flow components arising due to the small amplitude sinusoidal waviness of the channel walls. The disturbance equations are derived within the frame work of Floquet theory and solved using the spectral collocation method. Two-dimensional stability calculations indicate the presence of fast growing unstable modes that arise due to the waviness of the walls. Neutral stability calculations are performed in the disturbance wavenumber–Reynolds number (αs−R) plane, for the wavy channel with wavenumber λ1=0.2 and the wall amplitude to semi-channel height ratio, εw, up to 0.1. It is also shown that the two-dimensional wavy channel flows can be modulated by a suitable frequency of wall excitation ωg, thereby stabilizing the flow.
Nonlinear equilibrium solutions in stratified plane Poiseuille flow
Authors: Li Hsi-Shang , Fujimura Kaoru
Published: July 2002

Bifurcation characteristics of the primary and secondary instabilities of thermally stratified plane Poiseuille flow are discussed based on nonlinear equilibrium solutions. For low-Prandtl number fluid under strongly stable stratification, the bifurcation diagram of the primary instability around the turning point of the linear neutral curve exhibits nonlinear degeneracy. As a result, the early transition process can be accurately predicted by the linear critical Reynolds number and the associated supercritical bifurcation surface. Secondary instability of the steady basic flow under the influence of thermal stratification is further analyzed using Floquet theory. The stratification effect on the three-dimensional breakdown of K-type has been established by studying the amplification rates and threshold amplitudes. In unstably stratified flow, K-type breakdown leads to nonlinear equilibrium solution identical to the mixed mode solution reported by Fujimura and Kelly [Phys. Fluid 7, 68–79 (1995)] in a weakly nonlinear study of mode interaction. As a result, deriving amplitude equations for the physical mechanism of secondary instability can be formulated rigorously under unstable stratification.
The subgrid-scale estimation model for high Reynolds number turbulence
Authors: Domaradzki J. A. , Yee P. P.
Published: July 2002

We propose a formulation of the subgrid-scale estimation model in which the effects of the estimated subgrid scales on the resolved scales are obtained through the truncated Navier–Stokes dynamics and the calculation of the subgrid-scale stress tensor is not required. For high Reynolds number isotropic turbulence the model predicts the k−5/3 spectrum with the correct value of the Kolmogoroff constant.
Nonlinear geostrophic adjustment, cyclone/anticyclone asymmetry, and potential vorticity rearrangement
Authors: Kuo Allen C. , Polvani Lorenzo M.
Published: July 2002

Within the context of the rotating shallow water equations, it is shown how initially unbalanced states possessing certain symmetries dynamically evolve to lose those symmetries during nonlinear geostrophic adjustment. Using conservation law methods, it is demonstrated that the adjustment of equal and opposite (circular) mass imbalances results in a balanced end state where cyclones are stronger than anticyclones; the reverse holds true for momentum imbalances. In both cases, the degree of this asymmetry is shown to be directly proportional to the amount of initial imbalance (a measure of the nonlinearity occurring during time-dependent adjustment). On the other hand, the degree of asymmetry is maximal for imbalances of Rossby deformation scale. As for the potential vorticity, it is shown that its final profile can be noticeably different from its initial one; from an Eulerian perspective, this rearrangement is not confined to uniform shifts of potential vorticity fronts. Direct 2D numerical initial value problems confirm the asymmetry in the predicted final states and establish a relatively fast time scale for adjustment to complete. The robustness of these results is confirmed by studying, in addition, the adjustment of elliptical mass imbalances. The numerical integrations reveal that, during geostrophic adjustment, potential vorticity rearrangement occurs irreversibly on a fast wave time scale.
Lagrangian stability of fluids and multifluid plasmas
Authors: Elsässer Klaus
Published: July 2002

Weber’s transformation is used to show how Lin’s constraint should be replaced if fluid equations are derived from Hamilton’s principle. The same technique is used to derive a three-circulation theorem and a generalization of Ertel’s theorem for perfect multifluid plasmas. The Hamiltonian and Lagrangian formulation of the equations for fluid and electromagnetic potentials is given, with a discussion of their multivaluedness and their gauge and time dependence for static magnetohydrodynamic equilibria. The linear stability of these equilibria is shown to depend on the weight of a single negative eigenvalue of the internal energy variation, compared with all other (positive) contributions to the ‘‘energy’’ functional.
Transport suppression by shear flow generation in multihelicity resistive-g turbulence
Authors: Sugama H. , Horton W.
Published: July 2002

Turbulent momentum transport given by the Reynolds stress is considered as a candidate for explaining the production and sustainment of the mean shear flow in the high confinement ‘‘(H)’’ mode. The fluctuation mechanism for the shear flow generation and transport reduction in the three-dimensional (3-D) multihelicity system is given. The profiles of the Reynolds stress, shear flow, and thermal flux in the 3-D case are compared with those in the two-dimensional (2-D) case. The Beklemishev–Horton theory for the anomalous transport which multiplies the 2-D transport by the density of distinct mode rational surfaces is found to overestimate the observed flux due to the disappearance of a subset of modes on certain rational surfaces. The mixing-length theory, in which the anomalous transport is independent of the density of mode rational surfaces, underestimates the thermal flux.
The role of the Rayleigh–Taylor instability in laser-driven burnthrough experiments
Authors: Delettrez J. , Bradley D. K. , Verdon C. P.
Published: July 2002

Experiments were conducted to confirm that the Rayleigh–Taylor instability is the main process controlling the burnthrough time in imploding spherical experiments. In these experiments the laser irradiates targets overcoated with a parylene layer, in which one or more thin signature layers of moderate- to high-Z material are embedded to signal the penetration of the heat front. Target parameters were varied to study the effect on the burnthrough time of changes to target acceleration, Atwood number, and ablation velocity. The effects of improved laser uniformity through the introduction of smoothing by spectral dispersion are also presented. The results agree well with those obtained from a multimode mix model. This suggests that burnthrough experiments can be used to measure improvements in laser-irradiation or target-fabrication uniformity and to test methods to mitigate the growth of the Rayleigh–Taylor instability.
Comment on ‘‘Physics of the magnetotail current sheet’’ [Phys. Fluids B 5, 2663 (1993)]
Authors: Burkhart G. R. , Dusenbery P. B. , Speiser T. W.
Published: July 2002

The discussion on the current sheet catastrophe2 is carried further in this comment. In particular, the choice of boundary conditions where the incoming portion of the distribution function is a drifting Maxwellian is discussed. (AIP)
Effect of toroidal flow on the stability of a diffuse high-beta tokamak
Authors: Bhattacharyya S. N.
Published: July 2002

The effect of toroidal flow on the stability of a diffuse current model tokamak to external kink modes is examined. For a high-β tokamak it is shown that, to leading order in the inverse aspect ratio, the effect of toroidal flow is simply to modify the kinetic pressure by adding to it the flow pressure. Then the limit on the plasma beta in the presence of toroidal flow can be obtained from the results for a static plasma. It is shown that, for a given safety factor profile and shape of the plasma boundary, the maximum plasma beta for stability to external kink modes is always lower in the presence of flow compared to a static configuration. This is true for any arbitrary profile of the toroidal flow velocity.
Effects of a poloidal divertor separatrix on resistive magnetohydrodynamic instabilities in a tokamak
Authors: Uchimoto E. , Callen J. D. , Chang Zuoyang , Prager S. C.
Published: July 2002

A computer code package has been developed to simulate the linear and nonlinear evolution of long-wavelength resistive magnetohydrodynamic (MHD) instabilities in a four-node poloidal divertor tokamak (e.g., Wisconsin Tokapole II [Nucl. Fusion 19, 1509 (1979)]). Distinguishing features of this package include the use of a full set of three-dimensional (3-D) nonlinear resistive MHD equations and the inclusion of the divertor separatrix and the plasma outside the divertor separatrix in the computational domain. The present numerical results suggest that the plasma current outside the divertor separatrix tends to linearly stabilize the resistive MHD instability dominated by the m=2, n=1 mode, and, to a lesser extent, that dominated by the m=1, n=1 mode. (Here, m and n are poloidal and toroidal mode numbers, respectively.) However, the nonlinear evolution of the m=1, n=1 dominant instability is not significantly affected by the divertor configuration; the m=1, n=1 island is shown to reconnect totally by developing a large region of magnetic stochasticity. Hence, the cause of the partial reconnection observed in Tokapole II seems to lie beyond the scope of the classical resistive MHD model.
Nonlinear ion-mixing-mode particle transport in the dissipative trapped electron regime
Authors: Ware A. S. , Terry P. W.
Published: July 2002

The nonlinear particle transport arising from the convection of nonadiabatic electron density by ion-temperature-gradient-driven turbulence (i.e., ion-mixing mode particle transport) is examined for trapped electron collisionality regimes. The renormalized dissipative nonadiabatic trapped electron phase-space density response is derived and used, along with an ansatz for the turbulently broadened frequency spectrum, to calculate the nonlinear particle flux. In the lower-temperature end of this regime, trapped electrons are collisional and all components of the quasilinear particle flux are outward (i.e., in the direction of the gradients). Nonlinear effects can alter the phase between the nonadiabatic trapped electron phase-space density and the electrostatic potential, producing inward components in the particle flux. Specifically, both turbulent shifting of the peak of the frequency spectrum and nonlinear source terms in the trapped electron response can give rise to inward components. However, in the dissipative regime these terms are small, and the trapped electron response remains dominantly laminar. When the trapped electrons are collisionless, there is a temperature threshold above which the electron-temperature-gradient-driven component of the quasilinear particle flux changes sign and becomes inward. For finite-amplitude turbulence, however, turbulent broadening of both the electron collisional resonance and the frequency spectrum removes this threshold, and the temperature-gradient-driven component remains outward.
Spheromak instability
Authors: Lortz D. , Spies G. O.
Published: July 2002

The ‘‘classical spheromak’’ (current density proportional to the magnetic field, spherical plasma–vacuum interface, homogeneous magnetic field at infinity) is unstable to global magnetohydrodynamic tilting modes, even if an arbitrary, perfectly conducting, rigid wall bounds the vacuum.
Comment on ‘‘The absorption mechanisms of whistler waves in cool, dense, cylindrically bounded plasmas’’ [Phys. Fluids B 5, 3864 (1993)]
Authors: Shoucri Magdi
Published: July 2002

In a recent publication [Phys. Fluids B 5, 3864 (1993)], a study has been presented on the absorption mechanisms of whistler waves in cool, dense, cylindrically bounded plasmas. The dispersion relation obtained was used to explore the transition from Landau dominated damping to cyclotron dominated damping. Although the frequencies of these waves are well below the electron cyclotron frequency, the cyclotron resonance terms were retained in full since for high density plasmas, the density dependent parallel wave number can be large enough to bring thermal electrons into cyclotron resonance. It is the purpose of the present Comment to show that, within the approximations used in the article above, off-diagonal terms, which were neglected, can give an important contribution to the dispersion and damping of the waves for the parameters considered, and therefore should be retained in the calculation.
Comments on ‘‘Electron acoustic surface waves in a two-electron component plasma’’ [Phys. Fluids B 5, 4502 (1993)]
Authors: Shoucri M.
Published: July 2002

The transverse attenuation coefficient for the transverse surface electromagnetic wave is derived. This Comment shows that the result derived in Ref. [Phys. Fluids B 5, 4502 (1993)] for this transverse attenuation coefficient is incorrect.
A one-dimensional model for radiative thermal equilibrium and stability of the tokamak edge plasma
Authors: Deshpande Shishir
Published: July 2002

The multifaceted asymmetric radiation from the edge (MARFE) instability observed in the peripheral radiative layer in the high-density tokamaks is considered for a spatially smooth profile of radiative-loss function in equilibrium. The instability which is driven by the impurity radiation and opposed by the finite thermal conductivity, was considered previously by Drake [J. F. Drake, Phys. Fluids 30, 2429 (1987)] as a surface wave on a sharp radiative layer (step-function-like) at the edge. A novel exponential model is presented for the radiative loss function and it is shown that the two crucial features of MARFE: (i) stability to poloidally symmetric perturbations and (ii) a threshold value of radiative loss for the growth of asymmetric perturbations, explained earlier by Drake’s theory, are contained in this model of radiative equilibrium, also. Since, from the earlier theory, these features appear to be equilibrium specific and related to the form of radiative-loss function, the present work broadens the class of radiation profiles which can explain the MARFE, by recovering all the old results by an entirely different mathematical treatment.
Single-particle motion in nonaxisymmetric static electric and magnetic fields
Authors: Weitzner Harold
Published: July 2002

The technique of iteration of canonical transformations for a Hamiltonian system is applied to study the motion of a single particle of small Larmor radius in static, nonaxisymmetric electric and magnetic fields. For use in applications in kinetic theory, it is asked when is μ(r,v), where μ is the magnetic moment adiabatic invariant, single valued in r, with v held fixed? Even when μ(r,v) is a good adiabatic invariant, it may fail this additional condition, so that f[1/2v2+Φ(r), μ(r,v)] is not an acceptable particle distribution function, where Φ(r) is a single-valued electrostatic potential. It is shown that in a toroidal domain the condition is satisfied only for axisymmetric fields, or for magnetic fields near a zero shear field with nested flux surfaces that fill a toroidal domain. The appropriate transformations to guiding center coordinates and Hamiltonians are given for these cases.
Stability analysis of relativistic gyro-traveling wave devices
Authors: Latham P. E. , Nusinovich G. S.
Published: July 2002

The stability of parasitic modes in gyro-traveling wave tubes is studied, both with and without the operating mode present. Using a Hamiltonian formalism, linear equations are derived which describe the spatial evolution of parasitic modes in the absence of the operating mode. Complications such as a nonuniform waveguide and tapered magnetic field are taken into account. As an example, the linearized equations are applied to a 10 GHz, 430 kV, gyro-traveling wave tube to determine the start current in the absence of the operating mode. Included in the analysis is the effect of frequency-dependent reflection from the output window. A relatively low start current is found, well below the nominal operating current of 240 A. Multi-mode equations are then derived which can be used to analyze the suppression of parasitic modes by the operating mode. An algorithm is developed that allows efficient computation of the nonlinear start current in this regime. The algorithm is applied to the 10 GHz, 430 kV device mentioned above, and a stable configuration is found.
Ship waves in rotating plasma in tokamaks
Authors: Pavlenko Vladimir P. , Yankov Vladimir V. , Bondeson Anders
Published: July 2002

Plasma rotation can lead to excitation of waves in the plasma by perturbations that have zero frequency in the laboratory frame. Drift waves have minimum velocity threshold for excitation. Potential drift waves with kρi≤0.1 (k is the wave number, ρi is the ion cyclotron radius) excited at the edge and propagating toward the plasma interior are used to explain some nonlocal properties of plasma transport observed experimentally in transitions from the low (L) to high (H) confinement regime (L–H transitions).
Correlation between internal tearing modes and edge electrostatic fluctuations in a reversed-field pinch
Authors: Li Guoxiang , Drake J. R. , Bergsåker H. , Brzozowski J. H. , Hellblom G. , Mazur S. , Möller A. , Nordlund P.
Published: July 2002

Edge electrostatic fluctuations, in the Extrap T1 reversed-field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing-mode-related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed-field pinch configuration.
Relativistic electron beam acceleration by cascading nonlinear Landau damping of electromagnetic waves in a plasma
Authors: Sugaya R. , Ue A. , Maehara T. , Sugawa M.
Published: July 2002

Acceleration and heating of a relativistic electron beam by cascading nonlinear Landau damping involving three or four intense electromagnetic waves in a plasma are studied theoretically based on kinetic wave equations and transport equations derived from relativistic Vlasov–Maxwell equations. Three or four electromagnetic waves excite successively two or three nonresonant beat-wave-driven relativistic electron plasma waves with a phase velocity near the speed of light [vp=c(1−γ−2p)1/2, γp=ω/ωpe]. Three beat waves interact nonlinearly with the electron beam and accelerate it to a highly relativistic energy γpmec2 more effectively than by the usual nonlinear Landau damping of two electromagnetic waves. It is proved that the electron beam can be accelerated to more highly relativistic energy in the plasma whose electron density decreases temporally with an appropriate rate because of the temporal increase of γp.
Unified kinetic singular layer equation for converting the magnetohydrodynamic stability condition to a fully kinetic one
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

Conversion of the magnetohydrodynamic (MHD) stability conditions to fully kinetic ones is required, since the primary assumption for MHD description that particles move collectively as fluid cells is violated for nearly collisionless fusion plasmas. A unified kinetic singular layer equation is derived in this paper for implementing this conversion. The scheme outlined in the paper makes the conversion able to be realized in a rather simple way for both low and high n modes (n the mode number). Moreover, the equation covers a wide parameter domain. The frequency ranges from the ion transit frequency to that to be much lower than the ion bounce frequency. In particular, the case with mode frequency comparable to the magnetic drift one can also be treated; and the large aspect ratio assumption is not required. The application to investigate the energetic particle effect on the MHD modes is also outlined in the paper. The results are discussed, with the importance of the q (safety factor) value on the stability reiterated.
Model equations from gyrokinetic theory for a non-neutral plasma to include temperature effects and applications to a plasma of infinite length
Authors: Rasband S. Neil
Published: July 2002

Gyrokinetic equations are derived for applications to non-neutral plasmas in constant, straight magnetic fields wherein E×B drift velocities are of the same order as thermal velocities. The ratio of the E×B rotation frequency to the cyclotron frequency and the ratio of the gyroradius to a plasma scale length are assumed to be of order ε and terms are retained in the gyrokinetic expansion to second order to include finite-Larmor-radius (FLR) effects. A mode equation is obtained for a non-neutral plasma in the infinite-length approximation. The singularities of this equation are compared and contrasted with the familiar mode equation from the cold-fluid approximation. A numerical investigation of m=1 perturbations for a pure electron plasma with parameters chosen to closely approximate those in the report by Driscoll [Phys. Rev. Lett. 64, 645 (1990)] reveals no exponentially unstable modes with significant growth rates and strongly suggests that finite temperature is not the source of the exponential growth seen in the experiments.