bluuebunny/crossref_metadata_2025 huggingface dataset:
Flute stabilization by a cold line-tied blanket
Authors: Segal David , Wickham Michael , Rynn Nathan
Published: March 2003

The curvature-driven flute instability in an axisymmetric mirror was stabilized by an annular line-tied plasma blanket. A significant temperature difference was maintained between core and blanket. Theoretical calculations support the experimental observations.
Drag measurements for axisymmetric motion of a torus at low Reynolds number
Authors: Amarakoon A. M. D. , Hussey R. G. , Good Bill J. , Grimsal Edward G.
Published: March 2003

Measurements are presented for the drag on a torus moving along its axis of rotational symmetry at low Reynolds number. If D is the outside diameter of the torus, and d is the thickness in the axial direction, then the measurements cover the range s0 = 1 (the closed torus) to s0 = 135, where s0≡(D/d)−1. The effect of a coaxial cylindrical boundary (diameter H) is taken into account by an empirical correlation. The values of drag obtained by extrapolating to a fluid of infinite extent are in good agreement with the exact solution obtained by Majumdar and O’Neill. When πd(s0)1/2≪H, the empirical boundary correlation is consistent with the result of Brenner for a particle of arbitrary shape. Measurements with outer boundaries of square and circular cross section indicate that the relative effect of the two boundary shapes on the drag is the same for the torus as that found by Happel and Bart for a sphere. Empirical results are presented for the case in which the torus is strongly influenced by a coaxial cylindrical boundary.
Nonlinear wave interactions in supersonic wind-generated waves
Authors: Lekoudis Spyridon G. , Nayfeh Ali H. , Saric William S.
Published: March 2003

A nonlinear analysis is presented for the case of second-harmonic resonant interactions on the interface of a liquid film and a supersonic gas stream. The analysis takes into account the effects of the gas viscosity and mean profiles. Moreover, the extent of the gas disturbance layer is not restricted to the region in which the mean profiles are linear. Self-sustained oscillations are calculated for low and intermediate liquid Reynolds numbers. The present solution is compared with available experimental data. The predicted and observed wave amplitudes, wavenumbers, and frequencies are in good agreement for intermediate liquid Reynolds numbers. The calculated amplitudes overpredict the observed values by about a factor of three for low liquid Reynolds numbers.
A statistically-derived subgrid model for the large-eddy simulation of turbulence
Authors: Yoshizawa Akira
Published: March 2003

A subgrid model in the large-eddy simulation of turbulent flows is derived from the statistical viewpoint. The subgrid-scale (SGS) Reynolds stress playing a central role in the derivation is estimated from a shear-turbulence theory which is based on the direct-interaction formalism incorporated with the separation of scales of mean and fluctuating fields. A resulting subgrid model is written in terms of the grid-scale (GS) velocity, the GS pressure, and the SGS turbulent energy. The Smagorinsky model used frequently in the large-eddy simulation is derived under the special situation that production and dissipation terms are dominant in the transport equation for the SGS turbulent energy.
Propagation of hydromagnetic waves in a rotating nonisothermal compressible atmosphere: WKB approximation
Authors: Rudraiah N. , Venkatachalappa M. , Sekar R.
Published: March 2003

The propagation of internal Alfvén-inertio-acoustic-gravity waves in a rotating, stratified, inviscid, perfectly conducting nonisothermal atmosphere in the presence of a nonuniform magnetic field is investigated near the critical levels using the group velocity method based on the WKB approximation. It was found that these waves (approaching the critical level from one side) are neither transmitted nor reflected, but are completely captured in the neighborhood of the critical levels and are constrained thereafter to propagate in the horizontal directions. The same wave, if it approaches the critical level from the other side, is transmitted across the critical level without any absorption or reflection, thus establishing the valve effect, as in the case of Rudraiah and Venkatachalappa, even in the absence of the components of rotation in the plane normal to the direction in which the medium varies.
Ion acoustic double layers in the presence of plasma source
Authors: Okuda H. , Ashour-Abdalla M.
Published: March 2003

Steady-state plasma turbulence and the formation of negative potential spikes and double layers in the presence of ion acoustic instabilities have been studied by means of one-dimensional particle simulations in which the velocities of a small fraction of electrons are replaced by the initial drifting Maxwellian at a constant rate. A steady state is found where negative potential spikes appear randomly in space and time giving rise to an anomalous resistivity much greater than previously found. Comparisons of the simulation results with laboratory and space plasmas are discussed.
Kinetic theory of electrostatic ballooning instabilities
Authors: Tuda Takashi , Itoh Kimitaka , Tokuda Shinji , Itoh Sanae-Inoue
Published: March 2003

The numerical solution of a high-n electrostatic ballooning mode is obtained. Correct forms of electron and ion responses to the wave are retained. Instabilities which arise because of toroidal effects are found. Dependence on the aspect ratio, magnetic shear, wavelength, electron–ion–temperature ratio, and the electron temperature gradient is also studied. The growth rate γ is large and satisfies the relation ω<γ∼ω*. Depending on the parameters, the real frequency ω changes sign. The drift mode, which has been found to be stable in a slab plasma, persists in the toroidal plasma, almost always remaining stable. The density fluctuations in tokamaks, which have been observed when the microwave scattering method is used, can be qualitatively explained by the theory of the electrostatic ballooning mode.
The absorption mechanism of the ordinary mode propagating perpendicularly to the magnetic field at the electron cyclotron frequency
Authors: Cairns R. A. , Lashmore-Davies C. N.
Published: March 2003

The nonrelativistic hot plasma resonances, which occur for the O mode propagating at right angles to the equilibrium magnetic field are explained in terms of a mode conversion mechanism. The hot plasma introduces additional wave branches—the transverse cyclotron harmonic waves. At an electron cyclotron resonance, an incident O mode is partially converted to a cyclotron harmonic wave and partially transmitted as an O wave. The energy converted to the cyclotron harmonic wave is shown to be identical to the energy absorbed when relativistic effects are included. A simple argument is given showing why earlier nonrelativistic calculations of the absorption gave the same result as the relativistic theory.
Method for finding minimum energy three-dimensional magnetohydrodynamic equilibria with given constraints
Authors: Chu M. S. , Jensen T. H. , Dy B.
Published: March 2003

A practical numerical method for finding the magnetohydrodynamic equilibria of minimum magnetic energy subject to certain constraints is described. The constraints may be those of ideal magnetohydrodynamics or the constraint of constant local μ suggested by Taylor. No assumptions of symmetries are necessary to use the method. Examples of the method for finding Taylor states are described, and the potential of the method for simulation of plasma dynamics is discussed.
Steady-state planar ablative flow
Authors: Manheimer W. M. , Colombant D. G. , Gardner J. H.
Published: March 2003

Steady-state planar ablative flow in a laser produced plasma is studied. The calculations relate all steady-state fluid quantities to only three parameters, the material, absorbed irradiance, and laser wavelength. The fluid is divided into three regions; the subcritical expanding plasma, the steady-state ablation front, and the accelerated slab. Boundary conditions at the interfaces of these regions are given. If the absorbed irradiance is nonuniform, the nonuniformity in ablation pressure is calculated. Results are compared with experiment and fluid simulation for both uniform and nonuniform illumination.
Ideal magnetohydrodynamic stability of axisymmetric mirrors
Authors: D’Ippolito D. A. , Hafizi B. , Myra J. R.
Published: March 2003

The governing partial differential equation for general mode-number pressure-driven ballooning modes in a long-thin, axisymmetric plasma is derived within the context of ideal magnetohydrodynamics. It is shown that the equation reduces in special limits to the Hain–Lüst equation, the high-m diffuse p(ψ) ballooning equation, and the low-m sharp-boundary equation. A low-β analytic solution of the full partial differential equation is presented for quasiflute modes in an idealized tandem mirror model to elucidate the relationship of the various limiting cases.
Density modification and energetic ion production at relativistic self-focusing of laser beams in plasmas
Authors: Jones D. A. , Kane E. L. , Lalousis P. , Wiles P. , Hora H.
Published: March 2003

A two-dimensional time-dependent laser plasma interaction code is described and used to model the interaction between a 5 psec Nd glass laser pulse of peak power 1013 W and a 35 times ionized tin target. Parameters are chosen so that relativistic self-focusing down to a diameter of the order of the wavelength of the laser light is observed in a distance comparable to the half intensity vacuum beam diameter. At later times strong modification of the plasma density by the nonlinear force is observed. The combination of relativistic self-focusing and axial nonlinear force results in the acceleration of tin ions to a maximum energy of 5 GeV, in agreement with previous approximate calculations.
Filamentation of laser light in flowing plasmas
Authors: Short R. W. , Bingham R. , Williams E. A.
Published: March 2003

A dispersion relation is derived for the filamentation instability in the presence of plasma flow. We find that thresholds and growth rates may be significantly altered by flow transverse to the filaments.
Harmonic launching of ion Bernstein waves via mode transformation
Authors: Ono M. , Wong K. L. , Wurden G. A.
Published: March 2003

Ion Bernstein wave excitation and propagation via finite ion-Larmor-radius mode transformation are investigated theoretically and experimentally. It is shown that in the ion cyclotron range of frequencies ω≲4Ωi, with modest ion temperatures (Ti≲10 eV), the finite-Larmor-radius effect removes the wave singularity at the lower-hybrid resonance layer, enabling an externally initiated electron plasma wave to transform continuously into an ion Bernstein wave. In an ACT-1 hydrogen plasma (Te≂2.5 eV, Ti≲2.0 eV), externally excited ion Bernstein waves have been observed for ω≲2Ωi as well as for ω≲3Ωi. The finite ion-Larmor-radius mode transformation process resulting in strong ion Bernstein wave excitation has been experimentally verified. Detailed measurements of the wave dispersion relation and of the wave-packet trajectory show excellent agreement with theory. The dependence of the excited ion Bernstein wave on the antenna phasing, on the plasma density, and on the neutral pressure (Ti) is also investigated. The present work provides information relevant to the physics of ion Bernstein wave propagation and waveguide coupling as well as to the rf–plasma interaction during ion-cyclotron-range-of-frequency heating in fusion plasmas.
Reply to the comments of F. Winterberg
Authors: Ottinger P. F. , Mosher D. , Goldstein Shyke A.
Published: March 2003

A reply to the comment on the propagation of intense ion beams in straight and tapered z−discharge plasma channels is presented.(AIP)
Nonlinear saturation of free electron lasers around gyroresonance
Authors: Lin A. T. , Lin Chih Chien , Taguchi T. , Cheng W. W.
Published: March 2003

A mechanism is proposed to interpret the experimental results which show a reduction in the short wavelength output power when gyroresonance is reached. The presence of an axial magnetic field in a free electron laser around gyroresonance introduces a strong long wavelength Raman backscattering instability which itself gives rise to electron trapping and causes the desired short wavelength radiation to saturate earlier than unmagnetized free electron lasers.
Hamiltonian description of stratified fluid dynamics
Authors: Henyey F. S.
Published: March 2003

A Hamiltonian is constructed for the description of internal motions of a stratified fluid in terms of Eulerian coordinates. The Hamiltonian describes both internal waves and motions with vertical vorticity and the couplings between them. The construction is made by a motivated recipe, based on a motivated Legendre transformation. The role of the Lin constraints in the recipe is clarified. The number of canonical variables (4) exceeds the number of physical degrees of freedom (3), giving rise to a ‘‘gauge’’ invariance. The group of gauge transformations is indentified.
Asymptotic solution of the diatomic Boltzmann equation
Authors: Zappoli Bernard
Published: March 2003

A matched asymptotic expansion technique is used to derive the equations describing the hydrodynamic behavior of a diatomic gas. As an example, for which the complete description is explicitly known, sound dispersion due to vibrational relaxation is shown to correspond to an inner asymptotic description.
Electron cyclotron resonance heating of weakly relativistic plasmas
Authors: Chu K. R. , Hui B.
Published: March 2003

Electron cyclotron resonance heating of magnetically confined plasmas is examined with a new formalism which allows convenient evaluation of wave energy deposition in electron energy and momentum spaces, as well as in real space. The formalism applies to wave propagation in general directions in a weakly relativistic plasma. The special case of wave perpendicular to the magnetic field is examined analytically and illustrated with numerical case studies. In that analysis, a general magnetic field profile is assumed. For the mirror-type devices, it is shown that, in contrast to tokamak heating, efficient wave absorption is obtainable at electron temperatures as low as a few eV. For the tandem mirror which operates at a very high temperature, it is shown that electron cyclotron resonance heating (ECRH) results in either edge heating or formation of suprathermal electrons in the center. In comparison, it is shown that in the linear magnetic field profile of the tokamak, high electron temperature would actually favor heating of thermal electrons provided the wave is injected from the low magnetic field side.
Induced electron-cyclotron emission from inhomogeneous, anisotropic plasmas with electron population inversion
Authors: Ziebell L. F. , Dillenburg D.
Published: March 2003

The amplified emission of electron cyclotron radiation near the fundamental frequency from an inhomogeneous, anisotropic plasma slab with a loss-cone-type electron velocity distribution, is investigated in a linear theory. Plasma polarization effects are consistently included. Expressions are developed in the WKB approximation for emission in the ordinary and the extraordinary modes, for propagation perpendicular to the magnetic field. Numerical results are given for the extraordinary mode, for which effects are strongest. It is shown that the amplification is sensitively dependent on the ratio of parallel-to-perpendicular temperature and on inhomogeneities in the magnetic field.
Profile steepening by resonance absorption in spherically-expanding plasmas
Authors: Gwinn C. R. , Estabrook Kent , Kruer W. L.
Published: March 2003

A 1 1/2 -dimensional electrostatic particle code has been used to investigate density profile steepening due to resonance absorption in a spherically expanding plasma. Three types of profile are found: the widely studied subsonic variety, a supersonic variety with stable overdense bumps, and a transsonic variety in which overdense bumps form behind the critical surface and pass through it. A plateau lies between pairs of bumps, and the wave breaking amplitude increases dramatically when this plateau is near the critical density.
High-energy ion tail formation in the ion-acoustic instability—Three-dimensional quasilinear approach
Authors: Ishihara Osamu , Hirose Akira
Published: March 2003

A three-dimensional study of the quasilinear evolution of the ion distribution function in the current driven ion acoustic instability shows that the high-energy ion tail extends in the direction perpendicular to the electron drift as well as the parallel direction. The parallel and perpendicular tails are characterized by temperatures 20Tc and 10Tc, respectively, where Tc is the bulk ion temperature. The fluctuation level W/nTe reaches 5×10−3, followed by a decay due to the high-energy tail.
End effects on the transition to time-dependent motion in the Taylor experiment
Authors: Lorenzen A. , Pfister G. , Mullin T.
Published: March 2003

Laser-Doppler velocimetry has been used to measure the critical Reynolds number for the onset of time-dependent motion and its characteristic features in Taylor-Couette flow. For moderate values of the aspect ratio Γ the critical Reynolds number is observed to be extremely sensitive to length effects, and the form of oscillation arising at the stability limit is found to change in neighboring ranges of Γ. At even higher Reynolds number the oscillation type may change and hysteresis phenomena are observed. These new features are beyond the reach of any available theory and they highlight the need for caution when transition to turbulence observations are interpreted.
Plasma surface impedance for coupling to the ion-cyclotron and ion-Bernstein waves
Authors: Puri Satish
Published: March 2003

Analytic expressions for the surface impedance for coupling radio-frequency energy into a slab plasma at the ion-cyclotron range of frequencies are obtained. For the case of fast-wave coupling, the modification due to finite ny, corresponding to azimuthal variation in cylindrical geometry, is included. For this case (ny≠0), subtle and important changes in the fast-wave accessibility condition are introduced by the gradient effects. For the slow-wave coupling to the ion-Bernstein modes, the plasma surface impedance exhibits critical dependence upon the electron density and ion temperature at the plasma edge.
Numerical study of drift wave turbulence with simple models for wave–wave nonlinear coupling
Authors: Waltz R. E.
Published: March 2003

Scattering experiments in tokamaks show a broad drift wave frequency spectrum Δω/ω∼O(1) at fixed wavenumbers indicating a strong turbulent state with unstable drift modes strongly decorrelated at a rate Δω by nonlinear wave–wave coupling to stable modes. In this paper a two-dimensional study of homogeneous stationary drift wave turbulence with simple models of the wave–wave coupling is described. It shows that linearly driven-damped stationary states exist and demonstrates that approximate weak coupling theories based on the direct interaction approximation (DIA) can provide a faithful and practical description of the turbulence. In addition, it is found that simple models which include nonlinear E×B as well as nonlinear polarization drift coupling are in rough agreement with the experimental turbulence levels, transport coefficients, and decorrelation rates.
Lower-hybrid drift instability saturation mechanisms in one-dimensional simulations
Authors: Chen Yu-Jiuan , Birdsall C. K.
Published: March 2003

The linear properties and saturation mechanisms of the lower-hybrid drift instability have been investigated using a one-dimensional particle-hybrid simulation. For low drift velocities (vd≪vti), ion trapping and current relaxation (vd→0) are competing processes for stabilization. If the relative electron–ion drift velocity is kept constant in time, ion trapping causes the end of instability growth; if this drift is allowed to decrease consistent with momentum balance, then saturation is due to current relaxation.
Analytical double layers
Authors: Schamel H. , Bujarbarua S.
Published: March 2003

Based on realistic velocity distributions, analytic solutions of monotonic double layers which essentially agree with observations are obtained. A universal constant is found which relates potential drop, free ion energy, and trapped ion temperature in a simple manner. The Bohm criterion and the scaling law for the width of strong double layers are confirmed analytically.
Curvature-driven instabilities in a hot electron plasma: Radial analysis
Authors: Berk H. L. , Van Dam J. W. , Rosenbluth M. N. , Spong D. A.
Published: March 2003

The theory of curvature-driven instabilities is developed for a plasma interacting with a hot electron ring whose drift frequencies are larger than the growth rates predicted from conventional magnetohydrodynamic theory. A z-pinch model is used to emphasize the radial structure of the problem. Stability criteria are obtained for the five possible modes of instability: the conventional hot electron interchange, a high-frequency hot electron interchange (at frequencies larger than the ion cyclotron frequency), a compressional instability, a background pressure-driven interchange, and an interacting pressure-driven interchange. Numerical plots of the marginal stability boundaries are presented for parameter values corresponding to the EBT-S and EBT-P bumpy torus experiments.
Effects of ion bounce resonances on ballooning-interchange modes
Authors: Rosenbluth M. N. , Catto P. J. , Lee X. S.
Published: March 2003

In an axisymmetric device, an ion distribution function sharply peaked in pitch angle may be employed to obtain ballooning-interchange stability by optimizing the time these ‘‘sloshing’’ ions spend in regions of good curvature. However, the resulting magnetohydrodynamically stable modes can be driven unstable by the sloshing ion bounce motion resonating with the background wave. The stability conditions and growth rate for these sloshing ion bounce resonant modes are investigated and an example presented.
General frequency gyrokinetics
Authors: Lee X. S. , Myra J. R. , Catto P. J.
Published: March 2003

A gyrokinetic formalism is developed which enables a treatment of linear waves of arbitrary wave frequency ω for general magnetic field geometries. The treatment is an extension of previous gyrokinetic techniques which were restricted to the low-frequency (ω<Ω) regime. In addition to the expected magnetic drift terms in the wave–particle resonances, additional corrections related to the distinction between particle and guiding center location, and to oscillatory terms in the particle orbits can be important for general frequencies and wavenumbers. Within the limitation of an eikonal ansatz the appropriate linear gyrokinetic equation is derived for an unperturbed distribution function depending only on energy and guiding center location. Specific applications of the formalism in an inhomogeneous magnetic field are presented for both the ion drift-cyclotron instability at finite beta, and the gyroresonant damping of ion Bernstein modes in Maxwellian plasmas.
The equilibrium and stability of rotating plasmas
Authors: Hameiri Eliezer
Published: March 2003

In a rotating equilibrium state, the velocity and magnetic fields are shown to share the same flux surfaces. A simplified derivation is given of a second-order (not necessarily elliptic) partial differential equation which determines axisymmetric equilibrium states. For general configurations, equations on flux surfaces which determine the Alfvén and cusp continuous spectrum are derived and the stability investigated. These equations are written without the use of any particular coordinate system. Similar equations yield a sufficient condition for global stability of axisymmetric equilibria if the flow is parallel to the magnetic field up to a rigid rotation of the plasma. This condition is also necessary for stability in a mirror configuration with no toroidal field and a pure rigid rotation.
Perturbed-trajectory derivation of quasilinear diffusion and application to mirror plasmas
Authors: Smith Gary R. , Cohen Bruce I.
Published: March 2003

The quasilinear diffusion equation in action space is derived by analyzing particle trajectories perturbed by a fluctuating electromagnetic field. Averaging over angle variables replaces the usual ensemble averaging. A relation between the Fokker–Planck friction and diffusion coefficients is proven generally and demonstrated explicitly when the fluctuating field is an electrostatic ion-cyclotron wave. Discussion of this case, which is important in mirror confinement, unifies theories of quasilinear transport in mirror plasmas and of stochastic ion motion.
Plasma heating in field-reversed theta pinches
Authors: Steinhauer Loren C.
Published: March 2003

A method is devised to simulate plasma formation and heating in a field-reversed theta pinch which accounts for implosion kinetics, resistive dissipation, and compression (both radial and axial). Although containing several simplifying features, e.g., assumed radial profiles, distinct steps in the formation process, and a single adjustable parameter, the model nevertheless conserves energy and momentum in a global sense. Dissipation of reversed flux is governed by imposing an empirical ‘‘relaxation condition’’ on the drift parameter. Despite the simplicity of the method, predicted temperatures and radii agree well with experiments in most cases. The most important trend observed is the significant increase in temperature at higher reverse bias field. This arises from resistive dissipation and axial compression. In most cases, the predicted temperature exceeds (sometimes significantly) the temperature derived for a simple bounce and compression.
Long wave dispersion relations for surface waves in a magnetically structured atmosphere
Authors: Rae I. C. , Roberts B.
Published: March 2003

A means of obtaining approximate dispersion relations for long wavelength magnetoacoustic surface waves propagating in a magnetically structured atmosphere is presented. A general dispersion relation applying to a wide range of magnetic profiles is obtained, and illustrated for the special cases of a single interface and a magnetic slab. In the slab geometry, for example, the dispersion relation contains both the even (sausage) and odd (kink) modes in one formalism.
The effects of Ohmic heating and stable radiation on magnetic tearing
Authors: Tachi T. , Steinolfson R. S. , Van Hoven G.
Published: March 2003

A study is made of the effect of a temperature-dependent Coulomb-like resistivity on the planar tearing mode. The local evolution of the temperature is described by an energy equation which includes Joule heating and optically thin radiation. The resulting system of coupled linear magnetohydrodynamic equations is solved numerically, and eigenfunctions and growth rates are obtained. In the absence of radiation, there are two distinct solutions above a critical value of the magnetic Reynolds number S, a tearing-like mode and a Joule-heating mode. Below this point, the growth rates coalesce into a conjugate-complex pair. When stable radiation (dR/dT>0) is added, the heating mode disappears and a modified tearing excitation exists to much lower values of S before its growth is cut off by Ohmic heating. Examples are given for solar coronal parameters, and for those characteristic of fusion-research devices. The introduction of an effective value for the resistivity, in the presence of energy transport, allows a simple qualitative discussion of the different modes.
Nonlinear excitation of convective cells by magnetic drift waves
Authors: Yu M. Y. , Shukla P. K.
Published: March 2003

In a nonuniform plasma, the convective cell mode has a density variation. It is shown that this variation can be important in the parametric coupling of magnetic drift waves and convective cells.
Electrostatic Kelvin–Helmholtz instability in a radially injected plasma cloud
Authors: Sydora R. D. , Wagner J. S. , Lee L. C. , Wescott E. M. , Tajima T.
Published: March 2003

An electrostatic finite-sized particle simulation model is used to study the early time-scale phenomena associated with an impulsively injected plasma, expanding radially, normal to a strong ambient magnetic field. Results of the simulation show the early formation of a radial polarization electric field due to ion–electron charge separation which causes electrons to drift in the azimuthal direction. Velocity shear within this motion gives rise to a Kelvin–Helmholtz (diocotron) instability, creating a radial fluted pattern as the plasma expands. In the nonlinear stage of instability, azimuthal variations in the electric field are observed which cause electrons to drift across the magnetic field, thereby reducing the space charge created by energetic ion expansion. The results of a linear stability analysis reveal that for a decreasing amount of charge separation in the plasma, the number of unstable azimuthal modes at maximum growth rate increases. Also, as the thickness of the electron ring increases, a fewer number of unstable modes develops for a given amount of charge separation. It is concluded that electrons become unstable at the very early time scale, within an ion gyroperiod, and that unstable electrons lead to modification of the ion dynamics.
Weak cubic Langmuir turbulence
Authors: Hansen P. J. , Nicholson D. R.
Published: March 2003

The cubically nonlinear Schrödinger equation model of Langmuir turbulence is solved in the weak turbulence limit. Steady-state power-law solutions for the energy spectra are found in arbitrary dimensionality. In one spatial dimension, the theory incorrectly predicts that no spectrum evolves in time. In three spatial dimensions, numerical solutions are obtained for the undriven, undamped, initial value problem and for the driven, damped, initial value problem.
Emission, absorption, and polarization of gyrosynchrotron radiation of mildly relativistic particles
Authors: Petrosian Vahé , McTiernan James M.
Published: March 2003

In this paper are presented approximate analytic expressions for the emissivity and absorption coefficient of synchrotron radiation of mildly relativistic particles with an arbitrary energy spectrum and pitch angle distribution. From these, an expression for the degree of polarization is derived. The analytic results are compared with numerical results for both thermal and nonthermal (power law) distributions of particles.
Analytical approximations for the electron cyclotron absorption coefficient at the fundamental frequency in the Doppler regime
Authors: Celata C. M.
Published: March 2003

The index of refraction and the electron cyclotron absorption coefficient at ω≊ωc in the Doppler regime are examined for a plasma in thermal equilibrium. Regimes where the known analytical approximations fail are quantitatively delineated. The exact numerical solution to the dispersion relation is given in the regimes where known analytical theory fails.
Ballooning mode spectrum in general toroidal systems
Authors: Dewar R. L. , Glasser A. H.
Published: March 2003

A WKB formalism for constructing normal modes of short-wavelength ideal hydromagnetic, pressure-driven instabilities (ballooning modes) in general toroidal magnetic containment devices with sheared magnetic fields is developed. No incompressibility approximation is made. A dispersion relation is obtained from the eigenvalues of a fourth-order system of ordinary differential equations to be solved by integrating along a line of force. Higher-order calculations are performed to find the amplitude equation and the phase change at a caustic. These conform to typical WKB results. In axisymmetric systems, the ray equations are integrable, and semiclassical quantization leads to a growth rate spectrum consisting of an infinity of discrete eigenvalues, bounded above by an accumulation point. However, each eigenvalue is infinitely degenerate. In the nonaxisymmetric case, the rays are unbounded in a four-dimensional phase space, and semiclassical quantization breaks down, leading to broadening of the discrete eigenvalues and the accumulation point of the axisymmetric unstable spectrum into continuum bands. Analysis of a model problem indicates that the broadening of the discrete eigenvalues is numerically very small, the dominant effect being broadening of the accumulation point.
Application of the stellarator expansion for plasma stability studies in stellarators
Authors: Anania G. , Johnson J. L.
Published: March 2003

A numerical code, which utilizes the stellarator expansion, is developed and tested. It is used to investigate the magnetohydrodynamic stability properties of several stellarator configurations, including Heliotron E, Wendelstein VII-A, a modular-coil device, and ATF-1.
A survey of free-boundary ion ring equilibria
Authors: Ruchti C. B.
Published: March 2003

A systematic survey is made of free-boundary, large-orbit ion ring equilibria for the case where the distribution function is narrowly peaked at a given value of the single-particle energy (ℋ) and the canonical angular momentum (𝒫θ). Suitably normalized, such equilibria depend on only two free parameters. The survey covers essentially all of the parameter space where field-reversed equilibria exist. Results are presented as contour plots in the (P̄, H̄) plane of various physical quantities. The results may be reinterpreted so as to apply to relativistic electron rings on which there are experimental observations.
Theory of the filamentation instability in an intense electron beam propagating through a collisional plasma
Authors: Uhm Han S.
Published: March 2003

The filamentation instability of an intense electron beam propagating through a collisional plasma is examined. The effects of the transverse energy and of the pinch force (self-pinch as well as an applied magnetic field) are included. It is assumed that the characteristic perpendicular wavelength of the perturbations is much less than the beam radius. The stability analysis is carried out for the beam electron distribution in which all beam electrons have the same energy in rotating frames and a Lorentzian distribution in axial canonical momentum. From the stability analysis, it is found that the transverse energy and the pinch force strongly stabilize the instability. Moreover, the axial momentum spread plays a major stabilizing role.
Vlasov simulation of stimulated Raman scattering in one dimension
Authors: Hiob Eric , Barnard A. J.
Published: March 2003

Vlasov simulations of stimulated Raman scattering (SRS) are presented, both for low laser irradiance when the plasma waves remain linear and for high laser irradiance when particle trapping occurs. At low irradiance the energy in scattered radiation and in plasma kinetic energy approach values given by the Manley–Rowe relations. At high irradiance the effective damping rate of the plasma waves is greatly increased over the Landau damping rate while electron trapping is occurring. A relativistic model for electron trapping has been developed. It yields expressions for the number and maximum energy of the trapped electrons and the threshold for their production. Forward SRS dominates at high plasma temperatures and produces faster trapped electrons compared to backward SRS.
Visual observations on the amplification of artificial disturbances in turbulent shear flows
Authors: Taneda Sadatoshi
Published: March 2003

The behavior of two-dimensional periodic disturbances introduced artificially into turbulent shear flows is examined in a wind tunnel using flow visualization methods. It is found that when the wavelength of the disturbances is about 20 δ* in the case of the boundary layer on a flat plate and about 2.4 δ* in the case of the wake behind a circular cylinder, the disturbances are strongly amplified in the downstream direction, where δ* is the displacement thickness of the shear layer. The present experiments suggest that the large-scale coherent motions in natural turbulent shear flows originate from the instability of the mean velocity distributions.
Coherent structures—reality and myth
Authors: Hussain A. K. M. F.
Published: March 2003

The nature and significance of large-scale coherent structures in turblent shear flows are addressed. A definition for the coherent structure is proposed and its implications discussed. The characteristic coherent structure properties are identified and the analytical and experimental constraints in the eduction of coherent structures are examined. Following a few comments on coherent motions in wall layers, the accumulated knowledge from a number of recent and ongoing coherent structure investigations in excited and unexcited free shear flows in the author’s laboratory is reviewed. Also briefly addressed are effects of initial conditions, the role of coherent structures in jet noise production and broadband noise amplification, the feedback effect of coherent structures, the use of the Taylor hypothesis in coherent structure description, negative production, turbulence suppression via excitation, validity of the Reynolds number similarity hypothesis, etc. From the detailed quantitative results, a picture of the state of the art in coherent structure studies emerges. While coherent structures are highly interesting characteristic features of (perhaps all) turbulent shear flows, it is argued that their dynamical significance has been overemphasized. These are predominant only in their early stages of formation following instability, or in resonant situations and excited flows, or in regions adjacent to a wall of a turbulent boundary layer. The coherent Reynolds stress, vorticity, and production are comparable to (and not an order of magnitude larger than) the time-average Reynolds stress, vorticity, and production, respectively, in fully developed states of turbulent shear flows, where incoherent turbulence is also important and cannot be ignored. The concept and importance of coherent structures are here to stay; understanding and modeling of turbulent shear flows will be incomplete without them; but they are not all that matter in turbulent shear flows.
Liquid bridges with thermocapillarity
Authors: Xu Jian-Jun , Davis Stephen H.
Published: March 2003

An axisymmetric liquid bridge is subjected to an axial temperature difference. Thermal variations in surface tension drive a motion that is described using lubrication theory for slender bridges. At leading order in aspect ratio, a class of similarity solutions is obtained valid in the core region away from the endwalls of the bridge. These solutions describe the flows, temperature fields, and interfacial shapes of bridges that may sustain substantial interfacial deflections.
Stability of finite-amplitude convection
Authors: Zippelius Annette , Siggia Eric D.
Published: March 2003

Internally generated vertical vorticity enters the lowest-order amplitude equations for free-slip boundaries in an essential way when the Prandtl number P is finite and the flow three dimensional. For parallel rolls the band of stable wavenumbers is substantially modified from what was previously believed to be correct. In particular there are no stable states for P<0.301. Numerical simulations for free boundaries and larger P suggest a mechanism through which the box size determines the critical Rayleigh number for noisy time dependent convection. A stability analysis of model amplitude equations for rigid boundaries agrees qualitatively with the numerical results of Cleaver or Busse for P≲O(1). There is now considerable continuity between the stability diagrams for rigid and free boundaries.
Coherent structures within the plane turbulent jet
Authors: Moum J. N. , Kawall J. G. , Keffer J. F.
Published: March 2003

A fully developed plane turbulent jet was investigated experimentally by means of hot-wire anemometry and digital data analysis. Pairs of normal hot-wire signals were digitized simultaneously and computer processed to produce autospectra, auto- and cross-correlations, coherence functions, and phase spectra. The results establish that this flow does not contain regularly occurring structures. But, distinct frequency-centered activity near 20 Hz was detected via the coherence functions, and this implies that randomly occurring large-scale coherent structures exist within the flow. Moreover, convection velocities estimated via the corresponding phase spectra suggest that these structures move more slowly than the smaller turbulent eddies.
Theory of mode-induced beam particle loss in tokamaks
Authors: White R. B. , Goldston R. J. , McGuire K. , Boozer Allen H. , Monticello D. A. , Park W.
Published: March 2003

Large-amplitude rotating magnetohydrodynamic modes are observed to induce significant high-energy beam particle loss during high-power perpendicular netural beam injection on the poloidal divertor experiment (PDX). A Hamiltonian formalism for drift orbit trajectories in the presence of such modes is used to study induced particle loss analytically and numerically. Results are in good agreement with experiment.
Influence of equilibrium shear flow along the magnetic field on the resistive tearing instability
Authors: Paris R. B. , Sy W. N.-C.
Published: March 2003

The influence of shear plasma flow along the magnetic field on the resistive tearing instability has been investigated in plane geometry. It is shown that such flows, even at significantly sub-Alfvénic speeds, have a destabilizing effect. Moreover, in nonsymmetric cases, the well-known threshold for tearing mode instability Δ′ext =0 is removed. Thus, in magnetic configurations which would otherwise be stable in the absence of flow, a shear plasma flow can induce instability. Approximate growth rates have been obtained for tokamaks with significant equilibrium plasma rotation and an application to the ISX-B experiments is briefly discussed.
Calibration of the k-ε turbulence model for the diffusion of turbulence
Authors: Sonin Ain A.
Published: March 2003

The k-ε model admits an analytical solution for purely diffusive turbulence. A comparison with available experiments provides an improved measure of the Prandtl number for k, and suggests that turbulence models can profit from more definitive experimental studies of purely diffusive turbulence.
Numerical solution of the electron distribution function in tandem-mirror thermal barriers
Authors: Matsuda Y. , Rognlien T. D.
Published: March 2003

The relativistic Fokker–Planck equation is solved with electron cyclotron resonance heating (ECRH) in a magnetic mirror for an assumed thermal barrier electrostatic potential profile. Two methods—a finite element technique and a Monte Carlo technique—give nearly the same results. Favorable comparison is made with recently obtained analytical expressions for the relation between axial density ratios and potential profile using parameters expected in the TMX-U and MFTF-B experiments.
Magnetic reconnection driven by the coalescence instability
Authors: Bhattacharjee A. , Brunel F. , Tajima T.
Published: March 2003

A detailed study of magnetic reconnection at an X point driven by the coalescence instability is presented. In particular, the effects of plasma compressibility and the magnitude of the toroidal field on the rate of reconnection are explored. For large toroidal fields, the plasma is almost incompressible and the destruction of magnetic flux proceeds linearly with time. However, when the attractive force between two neighboring islands is strong, and when the poloidal and the toroidal fields are comparable in magnitude, the reconnection rate is found to be faster. Implications for the steady-state models of magnetic reconnection are discussed.
Stabilizing effect of gas conductivity evolution on the resistive sausage mode of a propagating beam
Authors: Lampe Martin , Joyce Glenn
Published: March 2003

Previous theoretical work has shown that a highly current-neutralized charged particle beam propagating in a preionized plasma channel of fixed conductivity is subject to a resistive sausage instability. It is shown that the instability is stabilized, for the case of beam propagation into an initially un-ionized gas, when the effect of beam-collisional ionization on the gas conductivity is modeled fully self-consistently.
Numerical simulation of the axisymmetric hollowing instability of a propagating beam
Authors: Joyce Glenn , Lampe Martin
Published: March 2003

Recent simulations have indicated that highly current-neutralized charged particle beams propagating in an ionizable gas are subject to violent axisymmetric instabilities. These instabilities are shown to be excitations of the m=0, n=2 hollowing mode, for which there is no adequate analytical theory. A wide-ranging simulation study of the instability mechanism and the parameter range of the instability are presented.
The axisymmetric instability of a self-pinched beam with rounded radial density profile
Authors: Chen H. C. , Uhm Han S.
Published: March 2003

The axisymmetric perturbations (sausage and hollowing modes) of an intense relativistic self-pinched electron beam propagating in a resistive plasma background are studied, especially for a beam with rounded radial density profile. The Bennett profiles are assumed for both the equilibrium beam current Jb(r)=Jb(0) (1+r2/R2b)−2 and plasma return current Jp(r)=−fJb(0) (1+r2/R2p)−2, where Rb and Rp are the characteristic radii of the beam and plasma return currents, respectively. It is further assumed that the electric conductivity σ(r) of the plasma channel is proportional to the return current. For a paraxial electron beam with complete space-charge neutralization by the ambient plasma, the axisymmetric modes can be destabilized by the phase lag between the magnetic field and beam current, even without the plasma return current. The plasma return current significantly modifies the growth rate of the instability such that the ratio of plasma current to beam current (−Ip/Ib=fR2p/ iR2b) largely determines the stability character of the beam. Furthermore, for the same fractional current neutralization f, the modes are highly unstable for a smaller ratio of plasma to beam radius Rp/Rb. As compared to the resistive hose instability, the growth rates for the hollowing mode can be larger than those of the hose mode, while the sausage mode is much stabler than the hose mode. Stability properties are illustrated in detail for various system parameters.
Gyrotron amplifier in a helix loaded waveguide
Authors: Uhm Han S. , Choe Joon Y.
Published: March 2003

Stability properties of the cyclotron maser instability in a relativistic annular electron beam propagating through a cylindrical waveguide loaded with a sheath helix are investigated, in connection with application to the gyrotron amplifier. The stability analysis is carried out, assuming that the beam is thin and tenuous. A closed form of the dispersion relation for the cyclotron maser instability is obtained, including the influence of the sheath helix on stability behavior. One of the most important features in this analysis is that the bandwidth of the gyrotron amplifier for a helix mode is very broad. Moreover, it is also shown that for an appropriate choice of the helix radius and pitch angle, the maximum gain of the gyrotron amplifier for a hybrid wave is twice that for an ordinary waveguide.
Time-resolved Brillouin spectra in a CO2 laser plasma interaction
Authors: Baldis H. A. , Walsh C. J.
Published: March 2003

Kerr cell techniques have been used to obtain spectra at 10.6 μm of the radiation backscattered from a CO2 laser plasma interaction. The spectrum is strongly red-shifted at high energies, with spectral modulation. These aspects of the backscattering suggest that the scattering is occurring from ion quasimodes produced by the strong coupling of the laser to the plasma.