bluuebunny/crossref_metadata_2025 huggingface dataset:
Conversion between cold plasma modes in an inhomogeneous plasma
Authors: Weitzner Harold , Batchelor D. B.
Published: March 2003

For a particular angle of incidence, it is possible for an ordinary mode electromagnetic wave, incident on an inhomogeneous plasma slab, to be totally converted into an extraordinary mode. A previous analysis of this process has been extended to permit an arbitrary angle of incidence and to include magnetic field gradients. Although this process has recently been proposed as a method by which toroidal plasmas might be electron cyclotron heated from the outside (low magnetic field region), it was found that for plasma parameters characteristic of modern tokamaks, significant mode conversion occurs only for waves incident within a very narrow cone (δϑ∼1°) about the critical angle. Also addressed is the question of the location at which ordinary/extraordinary mode conversion occurs.
Interaction of a low density runaway beam with cavity modes
Authors: Morales G. J.
Published: March 2003

An idealized problem is investigated which illustrates the role of wave-particle interactions in the evolution of runaway beams. The model considers the interaction between a weak cold beam, driven by an external static electric field E, and waves quantized by the geometry. The waves may correspond to Gould–Trivelpiece modes fixed by the length of the experimental device, or to the finite length of the experimental device, or to the finite Fourier modes encountered in computer simulations. The physics consists of the sweeping of the accelerated beam through the resonance provided by each cavity mode. This process is formulated in analogy with the O’Neil–Winfrey–Malmberg problem but uses a spatially averaged description based on the exact energy and momentum conservation laws with the dynamics simplified through a WKB representation of the dispersion relation. This model shows that the beam can be clamped in velocity with the momentum push being transferred to the waves. The model has been extended to the relativistic and multi-mode cases. For E=0 the nonrelativistic results are in good agreement with the work of O’Neil, et al., while in the relativistic case the model reproduces the nontrivial features of the computer study by Lampe and Sprangle.
Adiabatic equations of state for intense relativistic particle beams
Authors: Siambis John G.
Published: March 2003

Adiabatic equations of state for scalar pressure and anisotropic diagonal pressure have been obtained for intense, relativistic, unneutralized electron beams. These adiabatic equations of state complete the set of fluid equations needed to study beam dynamics and interactions from the fluid point of view. In particular, they facilitate the inclusion of thermal effects of momentum spread effects in stability analyses.
Collisionless plasma expansion into a vacuum
Authors: Denavit J.
Published: March 2003

Particle simulations of the expansion of a collisionless plasma into vacuum are presented. The cases of a single-electron-temperature plasma and of a two-electron-temperature plasma are considered. The results confirm the existence of an ion front and verify the general features of self-similar solutions behind this front. A cold electron front is clearly observed in the two-electron-temperatures case. The computations also show that for a finite electron-to-ion mass ratio, me/mi, the electron thermal velocity in the expansion region is not constant, but decreases approximately linearly with ξ=x/t, where x is distance and t is time. A self-similar solution, derived from the relation Ten1−γe=const, where Te is the electron temperature, ne is the electron density, and γ is a constant (instead of the isothermal assumption made in earlier theories), yields a linearly decreasing ion acoustic speed, c≃c0−(γ−1) ξ/2, and comparison with computer simulation results show that the constant γ−1 is proportional to (Zme/mi)1/2, where Z is the ion charge number.
Exploding pusher performance − A theoretical model
Authors: Rosen M. D. , Nuckolls J. H.
Published: March 2003

A simple model of ’’exploding-pusher’’ laser fusion targets is presented. The peak ion temperature is calculated by assuming that the ion heating mechanism is a shock followed by isentropic compression. The compression is calculated by conservation of mass and reasonable assumptions on the density profile at peak compression time. The neutron yield predictions of this model agree over a broad range of parameter space with those of complex one-dimensional numerical simulation, which in two dimensions have tracked experimentally measured yields quite accurately.
Realistic Vlasov slab equilibria with magnetic shear
Authors: Mynick Harry E. , Sharp William M. , Kaufman A. N.
Published: March 2003

A method is described for generating exact Vlasov slab equilibria to model high-beta plasmas with strong magnetic shear. Physically reasonable distribution functions that account for such effects as end losses and density gradients are constructed from constants of the motion and used to calculate the self-consistent electrostatic and magnetic fields. As an application, numerically calculated equilibria modeling a tormac sheath are presented.
Enhanced interaction between electrons and large amplitude plasma waves by a dc electric field
Authors: Leboeuf J. N. , Tajima T.
Published: March 2003

A simulation study of the interaction between relativistic electrons and large amplitude plasma waves driven by a dc electric field is performed using a one-dimensional electrostatic particle code. Two distinctly different initial conditions, one with beam electrons and the other with a pump plasma wave, yield qualitatively similar results. Runaway electrons accelerated by the dc field act as efficient converters of the external dc field energy to ac (wave) energy. The momentum distribution develops an elongated tail which sustains electroacoustic modes with frequency less than the plasma frequency. Multiple harmonic branches of the plasma waves are clearly observed. A collective threshold dc field is found beyond which the runaway electrons free fall in the dc field and no two-stream instability occurs. These simulation results are discussed in the light of a recent tokamak experiment where momentum clamping of the runaways is observed.
Analysis of the stabilization of the drift-cone mode by the injection of rf power
Authors: Hershcovitch Ady , Politzer Peter A.
Published: March 2003

The stabilization of the drift-cone mode in the PR-6 mirror device by injection of a burst of microwaves at the electron cyclotron frequency is analyzed using Dupree’s model of strong turbulence. The results of this calculation are in good agreement with the observations and may provide the first adequate interpretation of this experiment.
Marginal convection in a large rigid box
Authors: Stewartson K. , Weinstein M.
Published: March 2003

The Nusselt number defining the heat transfer across the bottom of a large rectangular box containing fluid whose lower horizontal face is uniformly heated from below, when the Rayleigh number is approximately equal to its critical value for the onset of convection, is calculated. The general method follows that used by Daniels in his study of the same problem, except that he assumed that the horizontal boundaries are stress-free. The simplicity of his approach is lost when the more realistic conditions of rigid boundaries are taken into account and a number of extensive computations are required before the Nusselt number variation can be determined.
Steady stratified flows over a long obstacle
Authors: Stevens W. K. , Su C. H.
Published: March 2003

The nonlinear problem of the flow of a stratified fluid of finite depth over an obstacle is investigated. The flow is assumed to be steady and two dimensional. The effects of viscosity are neglected and the pressure field is assumed to be hydrostatic. Analytic solutions are obtained for compressible flows with constant entropy profiles far upstream. Solutions for large Froude number flows are also obtained. A numerical procedure is introduced which permits the calculation of all possible steady flow fields with or without closed streamlines for any given upstream velocity and entropy (or density) profiles. Calculations are carried out for upstream data obtained from actual measurements in the Rocky Mountains and comparison is made between the solutions corresponding to the compressible and incompressible models.
Inertial range bispectra in turbulence
Authors: Van Atta C. W.
Published: March 2003

The inertial range bispectrum of turbulence in the atmospheric boundary layer is consistent with the functional form expected from dimensional similarity arguments. The angular wavenumber dependence is in substantial agreement with theoretical results obtained by Herring using the test field model.
Rayleigh–Bénard instability in nonreactive binary fluids. I. Theory
Authors: Gutkowicz–Krusin Dina , Collins Michael A. , Ross John
Published: March 2003

The hydrodynamic stability (to both stationary and oscillatory convection) of binary, Boussinesq fluids in Bénard geometry, with both the Soret and Dufour effects included is studied. A variational principle for the Rayleigh number Rcr at the onset of stationary convection is found; the complete exact solutions for Rcr are also given. Apart from trivial multiplicative factors, Rcr depends only on a single dimensionless parameter ψ which depends on the transport and thermodynamic coefficients. For ψ⩾0 (’’normal’’ Soret effect) the system becomes stable only if heated from below (R≳0), and for ψ<−1 instability may set in only if the fluid is heated from above (R<0). For intermediate values of ψ there exist both negative and positive solutions for Rcr. No exact solutions are known for Rcr at the onset of oscillatory convection, but two integral expressions for Rcr are derived in this case. Expressions for the amplitude of convective motion and the Nusselt number as a function of ‖R-Rcr‖ for both stationary and oscillatory convection are also derived. These expressions may be used to determine whether heat flux measurements are a useful tool for the study of the onset of instability in binary fluids.
Transverse plasma oscillations
Authors: Arthur M. D. , Greenberg William , Zweifel P. F.
Published: March 2003

An operator theoretic approach is used to solve the linearized Vlasov–Maxwell equations for transverse plasma oscillations. In particular, the special cases of simple and second-order real zeros of the plasma dispersion function are treated and formulae for the amplitude of the plasma waves are presented. An existence and uniqueness theorem for the solution to the Vlasov–Maxwell transverse mode plasma equation is proved in an appendix. In a second appendix, a general characterization for the zeros of the plasma distribution function is presented for the case of any double humped equilibrium distribution.
Electrostatic temperature gradient drift instabilities
Authors: Gary S. Peter , Sanderson J. J.
Published: March 2003

The linear Vlasov dispersion relation for electrostatic instabilities driven by a cross-field current due to density and temperature gradients in a unidirectional magnetic field is examined in the local approximation. Both electrons and ions are taken as magnetized, and propagation is in the plane defined by the drift velocities and the magnetic field. The effects of temperature gradients on three different instabilities are studied and compared. Electron (ion) temperature gradients parallel (antiparallel) to the density gradient enhance the maximum growth rate of the universal drift instability; but, an antiparallel ∇T for either species is necessary to enhance the growth of the lower hybrid drift or to lower the threshold of the ion cyclotron ion drift instability. Oblique propagation of the ion cyclotron ion drift instability corresponds to lower thresholds than at perpendicular propagation.
High-frequency convective loss-cone instability in short mirror machines
Authors: Gerver M. J.
Published: March 2003

The high-frequency convective loss-cone mode is found to be absolutely unstable, due to (exponentially small) reflection of waves at the mirror throats; the critical length for stability is only a few ion Larmor radii. For β≫me/mi, the following methods are shown not to substantially increase the critical length: (1) gradual tapering of the mirror throats (to reduce reflection); (2) partial filling of the loss cone with small amounts of warm plasma; (3) considering only low k⊥ modes (since high k⊥ modes may saturate at a low level). This result places serious limitations on the design of mirror fusion reactors.
Excitation of spherical ion acoustic solitons with a conducting probe
Authors: Ze Frederic , Hershkowitz Noah , Chan Chung , Lonngren Karl E.
Published: March 2003

Spherically expanding ion acoustic solitons which satisfied the predicted properties are launched from small hemispherical probes. Both an anodized (insulated) probe and a conducting probe mounted in series with a capacitor are used. A comparison of the solitons shows that the launching techniques are experimentally equivalent.
Effect of scrape-off on tokamak radial transport
Authors: El-Nadi A.
Published: March 2003

A general procedure is presented to evaluate the tokamak particle and energy losses in the presence of a poloidal divertor or a toroidal limiter, assuming cold collisional electrons and hot collisionless ions in the scrape-off layer. The case of the limiter is treated in detail and approximate expressions for the losses, to be incorporated in the tokamak transport equations, are obtained. The importance of the barely trapped particles in determining the radial profiles is emphasized.
Stark broadening induced by the intense electric field of a pulsed relativistic magnetron
Authors: Shefer R. E. , Bekefi G.
Published: March 2003

The Stark broadened spectrum of the Hβ line of hydrogen has been observed in a pulsed microwave field of 20 nsec duration, a frequency of 4.6 GHz, and a field strength of 100 kV/cm, generated by a relativistic electron beam magnetron.
Reply to comments by R. Van Dooren
Authors: Berryman James G.
Published: March 2003

In reply to comments by Van Dooren, it is shown that invariance properties of the Korteweg-de Vries equation may be used to obtain a linear stability analysis of any particular solution of the said equation, not only the solitary wave solution.
Reply to comments by J. H. Misguich
Authors: Catto Peter J.
Published: March 2003

In answer to Misguich, Catto asserts that improved trajectories were employed only for the adiabatic response. Further, Cato’s purpose was to attempt a simple resolution of certain earlier contradictory results, not a complete self-consistent nonlinear description. (AIP)
Decay instability of laser radiation in a magnetoplasma
Authors: Tripathi V. K. , Sharma R. R.
Published: March 2003

The self-generated mega-Gauss magnetic fields in laser pellet fusion plasmas open some potential channels of parametric decay of laser radiation. In the vicinity of the cutoff, the pump decays efficiently into (i) lower hybrid and upper hybrid modes, (ii) ion acoustic and upper hybrid modes, (iii) fast ion wave and plasma wave, and (iv) Bernstein mode and plasma wave. At higher pump powers, nonlinear Landau damping and cyclotron damping by electrons possess large growth rates. These instabilities are also relevant to tokamak-type plasmas where nonlinear effects are important in the vicinity of the upper hybrid resonance.
Quasi-linear theory of cyclotron maser instability
Authors: Uhm Han S. , Davidson R. C.
Published: March 2003

The nonlinear development of the cyclotron maser instability is investigated within the framework of a quasi-linear model that makes use of total energy balance for the system. The analysis shows that the instability saturates by the development of an energy spread. The corresponding efficiency of microwave generation is calculated. Moreover, the saturation time of the instability is obtained in terms of the initial perturbed field intensity.
Observation of ultralow frequency oscillations in the trapped electron regime
Authors: Grubb D. P. , Emmert G. A.
Published: March 2003

Large scale near zero frequency, oscillations are observed in the linear multimirror experiment when the plasma is unstable to two dissipative trapped electron modes simultaneously. The oscillations are flutelike and may be either coherent or turbulent.
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o
c’’ liquid spray vacuum leak detection method
Authors: Fiszdon W.
Published: March 2003

The common method of spreading volatile liquid on vacuum systems to check for leaks is reexamined. Simple experiments are made to determine the parameter that characterizes the effectiveness of candidate liquids. It is found that the most important properties are density and viscosity, not vapor pressure.
Emissivity near the electron cyclotron frequency from a thermal plasma of finite density
Authors: Bornatici M. , Engelmann F.
Published: March 2003

The emission around the electron cyclotron frequency from a thermal plasma of finite density in both the extraordinary and the ordinary mode is evaluated and shown to fulfill Kirchhoff’s law. A recent treatment of the extraordinary mode is found to be incorrect.
ExB effects on nonlinear mode-converted lower-hybrid waves
Authors: Kuehl H. H.
Published: March 2003

The effect of the nonlinear ExB motion of electrons is included in the derivation of the differential equation for mode-converted lower-hybrid waves.
Stability properties of collisionless universal drift modes in sheared slab geometry
Authors: Antonsen Thomas M. , Mahajan Swadesh M.
Published: March 2003

Several properties of collisionless drift modes are discussed. The effects of electron temperature gradients, non-Maxwellian electron velocity distributions, model dissipative effects, and the existence of marginally stable eigenmodes in the absence of dissipation are examined.
Self-trapped cylindrical laser beams
Authors: Anderson D. , Bonnedal M. , Lisak M.
Published: March 2003

The radial intensity profile and nonlinear wavenumber shift of an electromagnetic self-trapped cylindrical beam of radiation propagating in a nonlinear medium is investigated analytically using a variational technique involving trial functions. Good agreement is obtained with previous numerical results.
Pseudo-classical transport I: The particle and energy flux
Authors: Nevins William McCay
Published: March 2003

The transport of particles and energy that accompanies the trapping of electrons by a finite amplitude drift wave is calculated. Starting from the drift kinetic equation, it is shown that, in the limit of small collision frequency, the electron entropy source is stationary with respect to variations in the electron distribution function. This variational principal is employed, together with the full Fokker–Planck collision operators, to evaluate the electron transport coefficients and, hence, the flux of particles and energy across the magnetic field. Explicit expressions for the particle and energy flux are obtained in terms of the parameters of the plasma-wave system. These expressions should be used in place of the usual quasi-linear expressions for the particle and energy flux when the autocorrelation time of the wave spectrum is sufficient to permit the trapping of electrons by individual waves. These pseudo-classical transport rates are found to be smaller than the quasi-linear expressions that they replace.
Pseudo-classical transport II: A nonlinear theory of the ’’collisionless’’ drift instability
Authors: Nevins William McCay
Published: March 2003

A self-consistent theory of the evolution of a plasma slab supporting a finite amplitude drift wave that has trapped the resonant electrons is presented. Previous work is extended to include the effect of particle trapping on the evolution of the finite amplitude wave. The connection between this theory and other works, using quasi-linear theory, on the anomalous transport associated with low frequency drift waves is considered. It is shown that, for parameters typical of tokamak plasmas, particle trapping may result in the nonlinear stabilization of the wave at amplitudes, (eΦ0/T) ≃10−2, that are of the same order as those observed in experiments. The application of this work to experiments is discussed, and it is found to be potentially useful in understanding the drift wave spectrum and transport rates observed in computer simulations, stellarators, and future tokamak experiments.
Numerical study of drift-Alfvén waves in a sheared magnetic field
Authors: Tsang K. T. , Whitson J. C. , Smith Julius
Published: March 2003

The coupled radial eigenmode equations for electromagnetic drift waves in a sheared magnetic field are solved numerically. In addition to the familiar drift eigenmodes, a different set of eigenmodes was found which is the analog of the sheared Alfvén waves of local theory in a sheared magnetic field. The drift mode is mainly electrostatic, and the shear Alfvén mode is mainly electromagnetic and closely related to the high-order tearing modes.
Unstable continuous spectrum in magnetohydrodynamics
Authors: Hameiri Eliezer , Hammer James H.
Published: March 2003

The continuous spectrum need not be stable for an equilibrium with flow. The case of a noncircular, rotating ϑ pinch is shown to have an unstable continuum with the instability interpreted as a parametric instability arising from coupling between the rotation frequency and the plasma wave frequencies. On the other hand, equilibria with sub-Alfvénic flow parallel to the magnetic field in incompressible plasmas, as well as static equilibria, have a stable continuum. A formulation to determine the continuous spectrum is given in terms of characteristic surfaces for the equations and does not involve the use of any particular system of coordinates.
Converging nonlinear resonance cones
Authors: Wang W. S. , Kuehl H. H.
Published: March 2003

The theory of nonlinear plasma resonance cones due to a circular ring exciter is presented. The ring source excites a converging resonance cone which produces a maximum electric field at its focus where the strongest nonlinear effects occur. The nonlinearity is assumed to be due to the ponderomotive force. From a numerical integration of the fourth-order nonlinear partial differential equation, it is shown that the resonance cone trajectory is modified by the nonlinearities in a direction opposite to that caused by thermal effects, especially near and after the focus. Steepening of the electric field occurs near the cones and focal region, and the peak electric field on the cones and at the focus is enhanced due to the nonlinearity. The location of the focal point is shifted toward the exciter due to nonlinearities in contrast to the shift due to the thermal effects which is away from the exciter.
Lower-hybrid wave absorption in the presence of simultaneous density and magnetic field gradients
Authors: Puri Satish
Published: March 2003

It is shown that for the lower-hybrid wave (the slower of the two cold-plasma modes propagating at an angle to the static magnetic field), the WKB requirements are satisfied in almost the entire region from the antenna up to and including the ion-cyclotron-harmonic resonance. Expressions for the electric field and the energy density in the WKB approximation are derived. Both the collisionless and collisional damping become significant near the resonance, and the wave attenuation due to the two processes is estimated. From the differential equation for the electric field valid near the resonance, it is found that the wave energy would be completely assimilated even in the absence of a damping mechanism. The effect of impurities, although negligible near the plasma edge, becomes important following wave conversions and a significant fraction of the wave energy may end up heating the impurity ions in the plasma interior. The implications of using the ’’local’’ dielectric tensor approximation in the above context are examined. An alternative scheme for heating at the ion-cyclotron frequency using the lower-hybrid (quasi-shear) wave instead of the usual compressional wave is pointed out.
Wave-plasma coupling at the lower hybrid frequency
Authors: Chan V. S. , Chiu S. C.
Published: March 2003

The problem of steady-state nonlinear wave-plasma coupling at the lower hybrid frequency is considered. A nonlinear equation for the electric field in the low-density region at the plasma surface is derived self-consistently. Analytical and numerical solutions for different plasma models show a decrease in coupling efficiency with increasing power level. This result is interpreted using a simple physical picture. Its relevance to experiments is discussed.
Lower hybrid wave scattering by density fluctuations
Authors: Ott Edward
Published: March 2003

A wave kinetic equation is formulated for lower hybrid scattering by low frequency density fluctuations (such as those recently observed in tokamaks). Implications for heating of tokamak plasmas are discussed.
Stability of Bernstein–Greene–Kruskal equilibria
Authors: Schwarzmeier J. L. , Lewis H. R. , Abraham-Shrauner B. , Symon K. R.
Published: March 2003

The recent linear stability analysis of Lewis and Symon for spatially inhomogeneous Vlasov equilibria is illustrated with the case of an unstable periodic wavetrain of strongly inhomogeneous Bernstein–Greene–Kruskal equilibria. The stability formalism involves expanding an auxiliary function related to the perturbation distribution function in terms of the equilibrium Liouville eigenfunctions, and expanding the perturbation potential in terms of the eigenfunctions of an appropriately chosen field operator. The infinite-dimensional dispersion matrix is truncated to M×M by assuming that the normal mode of interest of the perturbation potential can be adequately represented by M eigenfunctions of the field operator; the eigenfrequencies ω are the zeroes of the determinant of the dispersion matrix. A particular Bernstein–Greene–Kruskal equilibrium was chosen as a numerical example, and the growth rate and normal mode of the instability were determined by numerical simulation. The agreement of the theory with the simulation for the growth rate and normal mode of the instability was excellent, and it was possible to choose a field operator a priori such that a 1×1 dispersion matrix was sufficient for obtaining accurate results.
Electrostatic ion cyclotron electron drift instability
Authors: Gary S. Peter , Gerwin R. A.
Published: March 2003

The ion cyclotron electron drift instability is studied using the linear Vlasov dispersion relation for electrostatic instabilities. The sources of free energy are density, temperature and magnetic field gradients across and a current parallel to a unidirectional magnetic field. Propagation is in the plane defined by the drift velocity and the magnetic field. This instability, which is driven by the electron drifts and undergoes ion cyclotron damping, propagates at the ion cyclotron frequency, but at wavelengths much shorter than the ion gyroradius. Although its growth rates are small compared with an ion gyrofrequency, the instability is favored by increasing Te/Ti, and it becomes competetive with the ion cyclotron ion drift instability at Te⩾4Ti.
Kink instabilities of a field reversed ion ring with a toroidal magnetic field
Authors: Finn John M.
Published: March 2003

The low frequency magnetohydrodynamic stability of axisymmetric ion rings in a current-carrying background plasma with a toroidal magnetic field is studied. An energy principle which treats the background plasma by fluid equations and the beam by kinetic theory is employed. The major effects upon stability are the magnetohydrodynamic response of the plasma and beam, a collective reaction of the beam, and betatron resonances. It is found that, if the beam and plasma currents are roughly equal, and if the exterior region contains a cold but highly conducting plasma, there is a window of stability for kinks with safety factor q as low as 1/2. The results also explain the anomalous trapped current losses in the RECE–Christa electron ring experiment.
Modulational instability produced by Langmuir turbulence in a magnetic field
Authors: Pozzoli Roberto , Ryutov Dmitri D.
Published: March 2003

The modulational instability produced by broad spectra of Langmuir oscillations in a plasma in a magnetic field is investigated. The influence of Langmuir turbulence on low-frequency waves is adequately described by introducing an ’’effective temperature’’ of the oscillations. Criteria of modulational instability are given explicitly.
Effect of the nonuniformity of the magnetic field on the beam-plasma instability
Authors: Idehara T. , Tanaka M. , Ishida Y.
Published: March 2003

The experimental investigation of the space charge wave instability of the spiral beam-plasma system in a nonuniform magnetic field is reported. The velocity distribution of the spiral electron beam is varied by the beam acceleration effect resulting from the parallel force due to the negative gradient of field intensity, which affects the trapping of beam electrons on the potential of the unstable wave. As a result, it is observed that the saturation length and the saturation level of the wave become extremely large. The brief theoretical analysis can explain the experimental results, consistently.
Brownian motion in a flowing fluid
Authors: Ramshaw John D.
Published: March 2003

A phenomenological theory is developed for Brownian motion in a flowing incompressible fluid. The Brownian particles are regarded as an ideal gas subject to a position- and time-dependent force field that represents interactions with the host fluid. Ths approach immediately leads to deterministic partial differential equations of motion for the Brownian particles. These equations are then examined in the limit of large friction, in which they imply an expression for the diffusional mass flux of Brownian particles. This expression is a sum of terms representing concentration, forced, thermal, and pressure diffusion. Comparisons are made with earlier work, and with the corresponding expression for the molecular diffusion flux of one component in a binary ideal-gas mixture. The Brownian and molecular diffusion fluxes are found to be identical in form, with the Brownian-particle volume fraction corresponding to the molecular mole fraction.
Effect of a physical inhomogeneity on steady-state detonation velocity
Authors: Engelke Ray
Published: March 2003

A set of experiments has been performed which demonstrate that ’’hot spots’’ in an explosive control some aspects of the steady-state detonation process. It is known, from experiments, that the relationship between explosive charge size and steady-state detonation velocity (the ’’diameter-effect curve’’) is qualitatively different for homogeneous and heterogeneous explosives. The experiments to be described strongly indicate that this qualitative difference is due to localized hot regions arising from flow modifications due to the presence of physical inhomogeneities. These local hot regions apparently produce an effective chemical reaction rate which allows a stable steady-state flow at a much smaller charge size than is possible in an analogous homogeneous material. Detonation-velocity measurements were carried out on cylindrical rate sticks at a number of radii thus generating diameter-effect curves. The explosives studied were composed primarily of commercial-grade nitromethane confined in Pyrex tubes. Silica impurities, with a known particle-size distribution, were added to the nitromethane to produce a heterogeneous material. The silica impurities were held in suspension with a guar-gum gelling agent. An attempt is made to interpret the new results by use of earlier numerical and experimental results concerning initiation of detonation at hot-spots.
Hypersonic stagnation-point boundary layers with massive blowing in the presence of a magnetic field
Authors: Krishnaswamy R. , Nath G.
Published: March 2003

The effect of massive blowing rates on the steady laminar hypersonic boundary-layer flow of an electrically conducting fluid in the stagnation region of an axisymmetric body with an applied magnetic field has been studied. The governing equations have been solved numerically by combining the implicit finite-difference scheme with the quasi-linearization technique. It is observed that the effect of massive blowing rates is to remove the viscous layer away from the boundary, whereas the effect of the magnetic field is just the opposite. It is also found that the velocity overshoot increases with blowing rates and also with magnetic field. The effect of the variation of the density-viscosity product across the boundary layer is strong only when the blowing rate is small, but for the massive blowing rate the effect is negligible.
Nonlinear evolution of a monochromatic whistler wave in a nonuniform magnetic field
Authors: Vomvoridis J. L. , Denavit J.
Published: March 2003

Computer simulations and theoretical considerations of the growth (or damping) and frequency shift of a monochromatic whistler wave propagating along a nonuniform external magnetic field are presented. The evolution is found to be adequately described by three growth rates, representing (1) the effects of resonant electrons similar to the uniform field case, (2) the effect of the nonuniformity on the untrapped resonant electrons, and (3) the effect of the nonuniformity on the trapped electrons. The applicability of these results to magnetospheric whistler propagation experiments is discussed.
Computer simulation of a Cerenkov interaction between obliquely propagating whistler mode waves and an electron beam
Authors: Kumagai H. , Hashimoto K. , Kimura I. , Matsumoto H.
Published: March 2003

A Cerenkov type wave-particle interaction between an obliquely propagating whistler mode wave and an electron beam has been investigated by a computer simulation experiment. A one-dimensional electromagnetic simulation code which is especially designed to deal with the obliquely propagating wave is used. Two cases of such a wave-particle interaction have been investigated. In the first case no initial wave is assumed to exist in the simulation model space. In the second case, an obliquely propagating whistler mode wave with a finite amplitude is initially assumed to exist. In both cases after linear wave growth, nonlinear saturation and amplitude oscillation are observed. This nonlinear behavior of the interaction can be interpreted by particle trapping in the wave potential well just as in the case of longitudinal electrostatic waves. Waves excited by Cerenkov radiation from the beam electrons develop as a coherent wave due to the instability and propagate in the magnetized plasma. By the oblique beam-plasma interaction, electromagnetic waves can be amplified. Amplification of the waves observed by this computer simulation may be closely related to VLF hiss noise occurring in the earth’s magnetosphere.
Energetics of vortex rollup and pairing
Authors: Ferziger Joel H.
Published: March 2003

The change in kinetic energy between the initial and final states of the rollup of an infinite vortex sheet into an array of uniform vortices is calculated. From the result, an upper limit on the pitch-to-diameter ratio of the vortex array is found. This contrasts with a lower limit found from stability considerations by Moore and Saffman. By extending the calculation to the case of elliptical vortices, it is found that a combination of energy and stability considerations is able to predict, qualitatively, a number of the observed features of the free shear layer.
Effect of density plateau on inverse bremsstrahlung
Authors: Mitrovich Dushan
Published: March 2003

In a spherically symmetric, steady-state, isothermal expanding plasma, the near-critical density plateau that can arise from the ponderomotive force of incident radiation can affect the various absorption mechamisms. The inverse bremsstrahlung process is calculated here and it is shown that, for representative cases, the net absorptivity is little influenced by the existence of the plateau.
Some observations in the wake of a small vertical cylinder attached to a flat plate
Authors: Gupta A. K.
Published: March 2003

Hot wire measurements were made at a wind speed of 4.1 m/sec in the wake of a vertical cylinder of diameter and height 3.1 mm, immersed in a laminar boundary layer. Data show that the appearance of turbulence a short distance downstream is preceded by laminar vortex shedding.
Radiation-induced shock-tube flow nonuniformities in ionizing argon
Authors: Glass I. I. , Liu W. S. , Tang F. C.
Published: March 2003

Plasma-density and electron-number-density profiles of ionizing-argon flows behind moving shock waves were measured using a dual-wavelength Mach–Zehnder interferometer with the hypervelocity shock tube at shock Mach numbers M∼13 and 16 at an initial pressure of 5 Torr and temperature of 300 °K.
Heat flux limitation by magnetic field curvature in laser-plasma interaction
Authors: Yu M. Y. , Shukla P. K. , Spatschek K. H.
Published: March 2003

It is shown that the heat flux due to the Righi–Leduc effect in laser-plasma interaction is significantly reduced when magnetic field curvature is taken into consideration.
Stability of temperature gradient drift-wave eigenmodes in sheared magnetic field
Authors: Chang C. L. , Liu C. S.
Published: March 2003

It is shown that roots of an eigenvalue equation for the electron temperature gradient drift mode in a collisionless plasma in a sheared magnetic field are stable.
Modulational instability of strongly dispersive drift waves and formation of convective cells
Authors: Mima K. , Lee Y. C.
Published: March 2003

The two-dimensional self-modulation instability of strongly dispersive drift waves is investigated in order to explain convective cell excitation in drift wave turbulence. It is shown that the nonlinear evolution of the convective cells can lead to anomalous particle losses.
Free and forced m=0 oscillations of a sharp-boundary Vlasov-fluid screw pinch
Authors: Cayton Thomas E. , Lewis H. Ralph
Published: March 2003

A dispersion differential equation is derived from the linearized equations of the Vlasov-fluid model for a sharp-boundary screw pinch using a very general fast-gyration, small-gyroradius ordering for the ions. Analytic solutions of the dispersion differential equation are obtained. The influence of finite ion gyroradius effects on both free and forced, azimuthally symmetric, small-amplitude oscillations of the plasma column is studied, and the results are compared and contrasted with the asymptotic guiding center plasma description. The forced oscillations are considered for frequencies at and below the first magnetoacoustic resonance. Small, but finite, ion gyroradii can influence the absorption of resonant magnetoacoustic oscillations strongly.
Emission of nonlinear ion acoustic waves from a magnetized plasma boundary
Authors: Schott L.
Published: March 2003

Nonlinear ion acoustic waves with relative amplitudes δn/n≲0.33 are emitted from the plasma boundary layer at the wall of a plasma confinement device using surface magnetic fields, when the voltage applied between the wall which forms the anode and the cathode is pulsed. The voltage pulse modulates the flow of primary electrons from the cathode. This leads to a transient change of plasma potential which induces ion wave emission from the sheath at the wall.
Nonlinear kink instabilities in tokamaks surrounded by force-free fields
Authors: Lui H. C.
Published: March 2003

The theory of Pao for the nonlinear behavior of linearly unstable kink modes in a sharp boundary plasma surrounded by a vacuum is extended to include force-free and distributed plasma currents. Nonlinear stabilization of the external kink (m=1) instability by force-free fields is enhanced by increasing the force-free current up to an optimum value. For fixed force-free currents, nonlinear kink stability is decreased by increasing the distributed plasma current. In general, the optimum force-free current increases as the plasma current increases. A condition to determine the nonlinear kink stability is derived by a perturbational method for long wavelength perturbations.
An investigation of the structure of plasma produced by reflected shock waves
Authors: Phillips M. G. R. , Pugatschew A. A.
Published: March 2003

Space and time resolved measurements of electron density and temperature have been made in the reflected shock plasma produced by a Mach 20 incident shock wave propagating in argon at an initial pressure of 1.5 Torr. The peak electron density was found to decrease away from the reflecting wall in such a way that the plasma was fairly uniform at all times. Close to the reflecting wall (0.2 cm away) the measured peak electron density was close to (i.e., about 20% lower than) the predicted equilibrium value but farther away (1.0 cm) it was lower by a factor of 4. Possible reasons for this discrepancy are discussed. Calculations of reflected shock plasma structure based on incident shock structure are only partially supported by available experimental evidence.
Liapunov stability of generalized Langmuir solitons
Authors: Laedke E. W. , Spatschek K. H.
Published: March 2003

The stability investigations for small amplitude Langmuir solitons are generalized to finite amplitude solitary waves. Previous stability considerations for Langmuir envelope solitons are based on the cubic nonlinear Schrödinger equation or the so-called Zakharov equations. Thus, they are only valid in the weakly nonlinear regime. To discuss the longitudinal stability of finite amplitude solitary waves a more general low-frequency response has to be allowed for. Taking the full nonlinear ion equations, the stability behavior of finite amplitude solitary waves is investigated. The method is completely nonlinear and makes use of Liapunov theory. A stability criterion is derived which proves the longitudinal stability for the known stationary wave solutions. The role of transverse instabilities is also discussed.
Ponderomotive laser effects in collisional plasma transport
Authors: Shkarofsky I. P.
Published: March 2003

A kinetic approach is used to derive the numerical coefficients multiplying ponderomotive terms in the relations of current flow, energy flow, heat flow, and electron pressure tensor, and in the source terms of the self-generated magnetic field. Laser interaction with a collision-dominated plasma having Z≳≳1 is considered. In the moment equations, the ponderomotive contributions to the momentum transfer and energy transfer are also evaluated. General expressions are obtained, valid for an arbitrary form for the time-averaged isotropic part of the electron velocity distribution function. For a Maxwellian distribution, the results on transport and magnetic field generation are compared with others, and the differences are explained.
Waves and transport in the pure electron plasma
Authors: deGrassie J. S. , Malmberg J. H.
Published: March 2003

Investigations of low frequency waves and transport in a plasma consisting almost purely of electrons are presented here. This plasma is trapped in a cylindrical system with radial confinement supplied by a strong axial magnetic field and axial confinement supplied by electrostatic fields. Very long containment times are possible. Classical transport due to electron-neutral collisions has been investigated and good agreement with the theory of Douglas and O’Neil is obtained. Externally launched diocotron waves are investigated. The modal frequencies agree well with linear theory, but the damping is governed by nonlinear effects. Experimental scaling laws for the damping rates are given. Measurements of spatial transport due to these modes are also presented. A signature of this process is that the transport is strongly localized spatially.
Guiding center plasma with gravitational or gradient drifts
Authors: Joyce Glenn , Liu C. S. , Montgomery David
Published: March 2003

Theory and simulation are presented for the two-dimensional electrostatic guiding center plasma in a uniform gravitational field. It is shown that the gravitational field leads to large electrostatic energies at long wavelengths and associated vortex motion in the plasma.
Effect of convective loss on the parametric decay of cold electron plasma waves
Authors: Wong K. L. , Wilson J. R. , Porkolab M.
Published: March 2003

The resonant decay of cold electron plasma waves was investigated in the Princeton L-3 device in an argon plasma with n∼1010–1011 cm−3, B∼700 G–2 kG, f0∼50–80 MHz (ω0⩾10 ωlh). The decay waves were identified to be ion-acoustic waves and cold electron plasma waves. Because of convective losses in the finite extent pump field, the threshold for parametric decay is considerably higher than the collisional threshold for the dominant E0⊥×B coupling. Coupling due to E0∥ can be important in low density, high magnetic field plasmas. At high densities (n≳3×1010 cm−3), E0⊥×B coupling becomes dominant. The E0⊥×B decay convective threshold was measured at various antenna lengths, magnetic field strengths, and plasma densities. The experimental results are in good agreement with theory. Indication of pump depletion and enchanced Landau heating of electrons were observed in the parametric decay process.
A new method for atom temperature measurements in plasmas
Authors: Gouesbet G. , Valentin P.
Published: March 2003

A new method for atom temperature measurements in plasmas is described. Its principle is to use the thermal diffusion phenomenon acting on a light kind added to the plasma as tracer.
Turbulent mixing with stabilizing surface buoyancy flux
Authors: Kantha Lakshmi H. , Long Robert R.
Published: March 2003

A laboratory experiment is studied with turbulence in salt water produced by an oscillating grid. A stabilizing buoyancy flux leads to a mixed layer depth that is proportional to the properly defined Monin–Obukhov length scale.
Unstable dissipative drift modes in a sheared magnetic field
Authors: Chang C. L. , Drake J. F. , Gladd N. T. , Liu C. S.
Published: March 2003

Collisional, electrostatic drift waves are shown, within the context of a slab geometry with magnetic shear, to be destabilized by a positive electron temperature gradient ηe=dlnTe/dlnn. The temperature gradient produces a well which localizes the drift eigenmode near the mode rational surfaces and for reasonable magnetic shear, e.g., Ln/Ls=0.05, these dissipative drift waves are linearly unstable when the temperature gradient exceeds a threshold of νe≃3. The crucial feature of this analysis is the inclusion of an energy dependent collision model (Lorentz model). Generally, the mode is most unstable for νc/ω*≃20 and is stable for νc/ω* ≳≳<<20. We believe this is the first drift mode, driven solely by diamagnetic currents, which is absolutely unstable within the context of a slab model with magnetic shear. Other collisionless and collisional drift modes are found to be stable when the full electron dynamics are included. Finite β effects are found to have a strong stabilizing influence on this instability.
Response of plane shear layers and wakes to strong three-dimensional disturbances
Authors: Breidenthal R.
Published: March 2003

Shear layers and wakes formed downstream of a splitter plate whose trailing edge has spanwise variations were briefly explored using the reacting flow visualization technique. A fundamental difference in behavior was observed between the two types of flows. The shear layer (which has only one sign of mean vorticity) rapidly forms its characteristic two-dimensional vortex structures, whereas, in contrast, the wake (which has both signs of mean vorticity) forms closed vortex loops. These loops do not grow appreciably in the spanwise direction over the test section length, even though the transverse growth is comparable to the two-dimensional wake behavior. The global structure of the shear layer quickly forgets the initial perturbations, while the wake remembers them. The results strongly suggest that the presence of only one or both signs of mean vorticity in these plane turbulent flows has a fundamental bearing on the spanwise structure and stability of the vorticity field.