bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma fluctuations at critical density in a CO2 laser plasma interaction
Authors: Walsh C. J. , Baldis H. A. , Evans R. G.
Published: March 2003

The nature of large-amplitude ion and electron fluctuations near the critical density in a CO2 laser-plasma interaction has been investigated using time resolved Thomson scattering. Ion fluctuations traveling along ∇ne are shown to be associated with a laser-driven shock wave. Both parametric decay and resonance absorption processes produce the large-amplitude electron plasma fluctuations.
Ion-resonance instability in a modified betatron accelerator
Authors: Uhm Han S. , Davidson Ronald C.
Published: March 2003

The ion resonance stability properties of an intense electron beam in a modified betatron are investigated within the framework of the linearized Vlasov–Maxwell equations. The analysis includes the important influence of equilibrium electric and magnetic self-fields as well as the applied toroidal field Bθ. It is shown that the maximum growth rate is an increasing function of the fractional charge neutralization f.
Design and testing of low-temperature intense electron beam diodes
Authors: Sloan M. L. , Davis H. A.
Published: March 2003

A new design procedure for axisymmetric space-charge-limited diodes has been developed which allows direct specification of the beam current radial profile and the anode geometry, with subsequent solution for the cathode shape required to produce the desired beam. This new procedure has been implemented in a simple design code for diodes immersed in a strong guide magnetic field, resulting in a specific design configuration for generation of a pulsed 2.1 MV, 14 kA, low emittance, constant current density electron beam. Subsequent experimental measurements on such a diode show excellent agreement with theory, with a measured normalized beam emittance of less than 450π mrad-cm over 90 nsec of beam pulse.
A scaling model for plasma columns produced by CO2 laser-induced breakdown in a solenoidal field
Authors: Ahlborn Boye , Vlases G. C. , Pietrzyk Z. A.
Published: March 2003

An analytical model is derived for the plasma cylinder produced by a long pulse (approximately μsec) CO2 laser of power pl (watts) which is incident upon neutral hydrogen imbedded in a strong axial magnetic field. Under certain conditions the leading edge of the plasma propagates away from the laser as an optical detonation where the leading shock front fully ionizes the background gas, and the inverse bremsstrahlung absorption zone immediately behind it is equivalent to the chemical energy release zone in an ordinary detonation. The front velocity is Vod =(3Ei/m)1/2, where Ei is the ionization (and dissociation) energy. This velocity is in agreement with experiments and with certain stability considerations. Radial expansion takes place immediately behind the detonation front and reduces the density to about one-third of the initial filling gas density. Far behind the leading edge the laser-produced plasma acquires an equilibrium radius R and steady pressure, density, and temperature determined by a balance between laser energy absorption and conduction and radiation losses. The density profile maintains a shallow minimum on axis.
Synchrotron emission from the relativistic ring electrons in the ELMO Bumpy Torus
Authors: Uckan T. , Uckan N. A.
Published: March 2003

The energy spectrum of the synchrotron radiation emitted by the relativistic ring electrons in ELMO Bumpy Torus (EBT) has been calculated for various classes of isotropic and anisotropic ring electron distribution functions. Calculations have been carried out for present (EBT-I/S) and planned (EBT-P) experiments. The ring temperatures in EBT-I and EBT-S are approximately 200 and 500 keV, respectively. The projected ring temperature in EBT-P is about 1000–1500 keV. The calculations indicate that the radiation is predominantly in higher harmonics (l≥γ2/2) and the radiation spectrum monotonically decreases and becomes almost flat at high frequencies, as observed in the experiments. With increasing temperature or anisotropy ratio (T⊥/T∥), the total emission increases, the slope of the spectrum at ω≫ωc decreases, and the peak of the spectrum moves to higher frequency. The ratio of the extraordinary wave intensity to ordinary wave intensity decreases with temperature but increases with anisotropy ratio. Calculated spectra and relative intensity levels of EBT-I and EBT-S are found to be in reasonable agreement with the experimental measurements. Both measurements and calculations show that synchrotron losses are low in EBT-I and EBT-S. However, in future experiments (EBT-P, reactor, etc.), radiation losses will play an important role in determining the ring power balance. Correlations of calculated intensity variations with temperature, density, beta, anisotropy ratio, etc., are given that can be used as a useful tool for comparison of theory and experiment, as well as in the determination of ring properties and scaling of the radiation with ring parameters.
Role of the relativistic mass variation in electron cyclotron resonance wave absorption for oblique propagation
Authors: Fidone I. , Granata G. , Meyer R. L.
Published: March 2003

The role of the relativistic mass variation on wave absorption in the electron cyclotron range of frequencies is investigated. It is first shown that the validity of the nonrelastivistic linear dispersion relation for a Maxwellian plasma is restricted by the conditions N2∥≫Te/mc2 and N2∥≫‖1−ω2c/ω2‖. A numerical investigation of wave damping in a plasma slab located in an inhomogeneous tokamak-like magnetic field shows that for most angles of practical interest the latter condition is easily violated and, therefore, the nonrelativistic dispersion relation yields inaccurate results. The problem of the validity of the nonrelativistic quasilinear equation for oblique propagation is also discussed. Using a quasilinear model equation, it is shown that the inclusion of the relativistic mass variation in the diffusion coefficient results in a basic change of the wave–particle selective interaction compared to the nonrelativistic approximation for any value of Te or N∥.
Electrostatic drift modes with stochastic particle diffusion
Authors: Zhang C. F. , Marchand R. , Lee Y. C.
Published: March 2003

The effect of stochastic particle diffusion on the collisionless universal mode in slab geometry is considered. Cross-field particle diffusion is assumed to be caused by a model static magnetic field turbulence. The resultant eigenmode equation is an integral equation for the perturbed electrostatic potential, which is solved numerically. As stochastic particle diffusion is gradually turned on, its effect on the usual drift branch is found to be stabilizing. At the same time, however, there appears a new, stochasticity-induced mode which, under certain circumstances, may become unstable.
Expulsion of a plasmoid from a spatially nonuniform magnetic field
Authors: Humanic D. G. , Goede H. , Dawson J. M.
Published: March 2003

The spatial evolution of an isolated plasma released from a region of large magnetic field has been investigated using a two-dimensional electrostatic particle code with a fixed nonuniform magnetic field. In general, the plasmoid is seen to accelerate and cool in accordance with the predictions of magnetohydrodynamic theory for a low-β plasma. However, it is found that two trailing plasma edges with their associated polarization charges are gradually shed leaving wakes of oppositely rotating vortices several Larmor radii in diameter.
Nonlinear theory of the ∇B×B instability in weakly ionized plasmas
Authors: Alfsen Knut H. , Holter O/ivin
Published: March 2003

The instability of a weakly ionized inhomogeneous plasma in a strong nonuniform magnetic field is investigated. By a particular scaling of the harmonics of the linearly unstable modes, an explicit calculation of the finite saturation amplitudes is performed. A nonlinear analysis shows the oscillating equilibrium states to be stable only in a subinterval of the linearly unstable wavenumbers. An alternative scaling of the harmonics establishes the validity of the results well into the unstable regime. There is excellent agreement with a previous numerical analysis by Shiau.
Single mode saturation of the rotational instability in a low-β, finite Larmor radius plasma
Authors: Janssen Peter A. E. M.
Published: March 2003

The nonlinear dynamics of the rotational instability in a low-β, finite Larmor radius plasma is investigated. Due to the effect of finite Larmor radius and line bending it is possible to have only a single, slightly unstable mode. By means of the multiple time scale method the evolution in time of this mode is determined. As a result, recurrence in the amplitude of the unstable mode is found, whereas on the average the particle transport vanishes.
Universal instability in a plasma sheath
Authors: Marchand R. , Gladd N. T. , Lee Y. C.
Published: March 2003

Low-frequency drift waves are considered in a planar sheath of neutral plasma with thickness on the order of the ion gyroradius. An integral eigenmode equation is derived and solved numerically using fast Fourier transforms. The analysis is valid for arbitrary wavenumbers and accounts for shear in the equilibrium magnetic field. It is found that unstable universal drift modes can exist in this strongly localized plasma. In contrast to what has been found previously in connection with weakly inhomogeneous plasmas, it is observed here that shear can have a destabilizing influence. The relevance of this idealized model to experiments is also discussed.
Two-and-a-half-dimensional magnetohydrodynamic turbulence
Authors: Montgomery David , Turner Leaf
Published: March 2003

Incompressible magnetohydrodynamic turbulence is considered in a geometry in which the fields are all independent of the z coordinate, e.g., but have all three components. This is a limit frequently called ’’two-and-a-half-dimensional.’’ It is conjectured that simultaneous inverse cascades of magnetic helicity and mean square z-component of vector potential may exist. At high Reynolds numbers, selective decays to states of finite pressure gradients appear to be possible. Anti-dynamo theorems are difficult to prove for this geometry.
α-particle ripple losses during slow down in a tokamak reactor
Authors: Anderson D. , Hamnén H. , Lisak M.
Published: March 2003

An investigation is made of the effect of toroidal field ripple mirrors on the confinement of α particles. It is found that the diffusion of α particles into the loss regions of the ripple mirrors during slow down gives rise to significant particle and energy losses.
Toroidal and scattering effects on lower-hybrid wave propagation
Authors: Bonoli Paul T. , Ott Edward
Published: March 2003

The effects of toricity and density fluctuations on the propagation of lower-hybrid waves are studied. These effects result mainly from changes in k∥ as the wave propagates (k∥ is the wavenumber parallel to the applied magnetic field). With respect to toroidal effects, k∥ changes due to the nonconstancy of the poloidal mode number in toroidal geometry. Using ray tracing techniques, an assessment of this toroidal effect has been made. It is found that waves, which on the basis of the cylindrical geometry theory would be inaccessible, may actually propagate to the plasma center and damp. The result is related to the onset of ray ergodicity. Further, it is shown that for accessible waves the optimal poloidal launch point is near the top or bottom of the torus. In order to include the effects of density fluctuations, an electromagnetic wave kinetic equation is derived and solved by a Monte Carlo technique. It is found that scattering can further enhance accessibility.
Efficiency of resonant absorption of electromagnetic waves in an inhomogeneous plasma
Authors: Adam J. C. , Serveniere A. Gourdin , Laval G.
Published: March 2003

The resonant absorption of electromagnetic waves in an inhomogeneous plasma is studied for values of the resonant plasma wave field such that the ponderomotive effects modify wave propagation. The stationary solutions are obtained and found unstable with low dissipation in most cases. The absorption coefficient is then time dependent. Density cavities are emitted at the resonance and they can induce large frequency shifts for the reflected light.
Stimulated Brillouin scattering from an absorbing plasma
Authors: Speziale Thomas
Published: March 2003

Reflection of light by Brillouin scattering from a plasma is shown to be limited by inverse bremsstrahlung.
Nonlinear classical diffusion in a contained plasma
Authors: Low B. C.
Published: March 2003

The one-dimensional Cartesian model for the resistive diffusion of a force-free magnetic field in a plasma of negligible inertia is treated for the case of confinement between rigid walls made of perfect electrical conductors. An exact analysis is given of the general initial-boundary value problem. The field evolution is driven by a competition between Ohmic loss and the induction effects arising from the tenuous plasma moving to adjust with the diffusing magnetic field. The process is nonlinear and nonlocal. It is shown that the field topology determines whether electrical currents will decay away everywhere or else grow at isolated locations to infinite values in a finite period of time.
A finite element analysis of isothermal fiber formation
Authors: Gifford W. A.
Published: March 2003

The formation of isothermal fibers composed of an incompressible Newtonian liquid is investigated. The fully two-dimensional axisymmetric Navier–Stokes equations are solved in the exit region of the spinneret using the Galerkin finite element method and are matched to an asymptotic one-dimensional equation downstream. The importance of boundary conditions and the manner in which they are imposed are discussed. Inertial, surface tension, and gravitational, as well as viscous effects, are retained in the formulation. Numerical results showing the effect of winder speed on fiber profiles are presented for some sample cases.
Long-wave energy trapping
Authors: Camfield Fred E.
Published: March 2003

Low-wave energy trapping by refraction on a curved coastline is considered. The parameters controlling energy trapping are developed and defined. Solutions are developed for defining the periods of trapped waves for a given physical geometry.
Shock wave interactions with the ocean surface
Authors: Flores Jolen , Holt M.
Published: March 2003

The interaction of plane shock waves with the air–ocean surface is investigated in detail. Interaction patterns for incident waves originating in both air and water are considered and the range of angles of incidence extends from normal to critical, and beyond. For shocks originating above the ocean surface, the interaction is irregular when the critical angle of incidence is exceeded and the transmitted shock is then diffracted about the disturbed ocean surface. For shocks arriving from an underwater source, the interaction is always regular and the reflected wave is always an expansion wave.
Development of two-dimensional structure in cavitons
Authors: Eggleston D. L. , Wong A. Y. , Darrow C. B.
Published: March 2003

Experimental observations of the time and space evolution of resonantly enhanced electric fields and plasma density in cylindrical geometry demonstrate the development of caviton structure in the direction perpendicular to the driving electric field. Electrostatic fields in this perpendicular direction are observed to have growth times on the ion time scale and to develop concurrently with two-dimensional density profile modification near the critical surface.
Application of explosive-driven implosions to fusion
Authors: Glass I. I. , Sagie D.
Published: March 2003

Our explosive-driven implosion facility was used to produce hemispherical implosions in a stoichiometric mixture of deuterium-oxygen. A high-resolution scintillator detection system measured neutrons and γ rays resulting most likely from the fusion of deuterium.
Study of driven lower-hybrid waves in the Alcator A tokamak using CO2 laser scattering
Authors: Slusher R. E. , Surko C. M. , Schuss J. J. , Parker R. R. , Hutchinson I. H. , Overskei D. , Scaturro L. S.
Published: March 2003

Measurements are presented of the properties of lower-hybrid waves excited at the edge of plasmas in the Alcator A tokamak by a two-waveguide array operating at microwave powers up to 70 kW. The waves are studied directly in the plasma by measuring the small angle scattering of CO2 laser radiation. A broad N∥ spectrum is observed in the range 2⩽N∥≲8, with more intensity at lower N∥ (here, N∥ is the parallel index of refraction). The waves are distributed throughout the minor cross section and do not appear to be localized in resonance cones. The wave amplitude is linear in net microwave power into the plasma. A power-independent frequency width of several mega-Hertz and a downshift in the frequency of the waves of the order of 1 MHz are observed. Finally, the wave amplitude is observed to be independent of the relative phase of the exciting, two-waveguide array. Most of these results are explained by taking into account the scattering of the lower-hybrid waves from the large-amplitude, low-frequency density fluctuations previously observed near the plasma edge.
Mode coupling and anomalous dissipation in magnetohydrodynamic turbulence
Authors: Tetreault D. J.
Published: March 2003

An energy-conserving model of magnetohydrodynamic turbulence is described that predicts both mode coupling and turbulent dissipation. The dissipation takes the form of anomalous resistivity and viscosity due to turbulent magnetic fields. The model predicts a dual cascade of energy to large wavenumbers and magnetic flux to small wavenumbers. Turbulent rearrangement of equilibrium magnetic shear generates resonant fluctuations via a mixing length process. The effect of the turbulence on the disruptive instability in tokamaks is discussed.
Electrostatic electron cyclotron radiation from a point source
Authors: Thiel J. , Lembège B.
Published: March 2003

Electrostatic radiation from a monopole antenna in a magnetoplasma is studied theoretically in the frequency range between the electron gyrofrequency and its second harmonic. The scalar potential created by a point source is calculated numerically using the least-damped-root approximation, which involves considering only two roots of the dispersion equation, corresponding to the oblique mode and to the cyclotron harmonic mode. The potential distribution can be understood as being due both to interference between these two modes and to diffraction of the cyclotron harmonic mode alone. Theoretical spatial distribution fit the experimental data. In particular, the theory confirms that cyclotron harmonic waves should be detectable in all directions from the source, as was observed experimentally. A good agreement with experimental data is shown by theoretical curves of constant phase and of constant amplitude. An approximate analytical model of the potential distribution is presented, and is compared with the numerical model and with experimental data. Finally, it is shown how such data can be interpreted to yield the electron density and temperature.
Filamentation instability of a relativistic hollow electron beam
Authors: Siambis John G. , Uhm H. S.
Published: March 2003

The filamentation instability properties of a relativistic hollow electron beam confined in axial flow by a uniform magnetic field, in the absence of background plasma, in a pipe are investigated via the Vlasov–Maxwell equations. The instability is found to be have two sidebands, one with a spectrum of positive wavenumbers k and the other with a spectrum of negative wavenumbers. The spectral point k = 0, associated with the diokotron instability, is excluded from the two unstable sidebands of the filamentation instability. Only in the limit of zero axial beam flow (γ→1), does the diokotron instability become asymptotically part of the filamentation instability spectrum. In this limit, the two sidebands of the filamentation instability merge asymptotically and symmetrically toward the diokotron instability spectral point, k = 0, in agreement with the basic driving physical mechanisms and geometry configurations for these two distinct and different instabilities.
Alfvén wave generation in a turbulent plasma
Authors: Yu M. Y. , Shukla P. K.
Published: March 2003

A stationary spectrum of high-frequency Langmuir wave turbulence is found to be unstable with respect to Alfvén wave perturbations. This instability seems to explain some magnetic pulsation phenomena in the magnetosphere.
Drift Hamiltonian in magnetic coordinates
Authors: White R. B. , Boozer A. H. , Hay Ralph
Published: March 2003

A Hamiltonian formulation of the guiding-center drift in arbitrary, steady-state, magnetic and electric fields is given. The canonical variables of this formulation are simply related to the magnetic coordinates. The modifications required to treat a large class of ergodic magnetic fields using magnetic coordinates are explicitly given in the Hamiltonian formulation.
Comment on ‘‘A note on Poisson’s equation for pressure in a turbulent flow’’
Authors: Adrian Ronald J.
Published: March 2003

A comment on a note on Poisson’s equation for pressure in a turbulent flow is presented. (AIP)
Effect of long internal waves on the evolution of deep-water surface gravity waves
Authors: Ma Yan-Chow
Published: March 2003

Equations describing the resonant interaction between long internal waves and short surface waves are derived and represented in wavenumber space. Various evolution equations can arise depending on the slopes of the two wave fields and the ratio of their wavelengths. The existence of the internal waves will affect the stability of a short surface plane wave. A coupled set of transport equations is derived for deterministic internal waves and Gaussian random surface waves. The amplification rate of perturbations can be analyzed for a range of spectral bandwidths.
Potential flow signature of a turbulent spot
Authors: Atta C. W. Van , Sokolov M. , Antonia R. A. , Chambers A. J.
Published: March 2003

Hot-wire velocity measurements in the free stream above turbulent spots generated in a laminar boundary layer on a flat plate show that the regions of largest positive and negative longitudinal potential flow disturbance occur at about 1.5 spot heights above the surface. These regions are located above the forward and rear primary foci previously identified as points of accumulation for particle paths and as extremal points for ensemble mean velocity and temperature perturbations. A three-dimensional linearized quasisteady potential flow similarity solution for the flow field around a moving, growing bump on the wall, with a shape given by the measured unsteady boundary layer displacement thickness, exhibits the same main features as the measured potential flow field.
Structure of linearized Navier–Stokes equations for compressible flows
Authors: Ytrehus Tor , Smolderen Jean J.
Published: March 2003

The transition from slightly viscous to viscosity-dominated steady-state, compressible, small-disturbance flow is studied in the framework of the linearized Navier–Stokes equations. The classical picture of independent modes for vorticity, pressure, and temperature disturbances is preserved at all disturbance wavelengths down in the molecular scale. Particular transitions do, however, occur within the pressure mode itself, causing the governing equations to display new and qualitatively different features at the smaller disturbance wavelengths.
A general class of self-similar power-driven expansions
Authors: Caporaso G. J.
Published: March 2003

Self-similar solutions for the nonadiabatic expansion of a gas in planar, cylindrical, and spherical geometry due to heating of a fairly general form are presented. The solution for an isothermal expansion which has previously been studied in the literature is recovered as a special case. Some additional examples of physical interest which reveal the variety of possible gas profiles resulting from the general solution are discussed.
Current-induced instabilities in rotating hydromagnetic flows between concentric cylinders
Authors: Kurzweg U. H. , Khalfaoui A. H.
Published: March 2003

The stability of hydromagnetic flow produced in a thin annular region between concentric cylinders by the interaction of a superimposed radial current and an axial magnetic field is examined both when the bounding cylinders are stationary and when rotating. After deriving the small gap approximation equations for the steady-state one-dimensional azimuthal flow and for the corresponding induced electric potential, the equations governing the growth of infinitesimal axisymmetric disturbances are obtained and solved at neutral stability by use of the Galerkin method. Stability results for the nonrotating cylinder case are shown to be expressible in terms of existing results for the stability of hydromagnetic flow produced by an azimuthal pressure gradient. It is found, in our thin film approximation, that the radial current must generally exceed a certain critical value before the one-dimensional flow breaks down into a three-dimensional pattern. Rotation of the outer cylinder enhances stability while rotation of the inner cylinder produces a greater tendency toward instability as reflected by lower Dean numbers at the onset of secondary flow.
An analytical model of a two-phase liquid metal magnetohydrodynamic generator
Authors: Yakhot Alexander , Branover Herman
Published: March 2003

An analytical model of a two-phase liquid-metal magnetohydrodynamic generator is developed. The model is based on the averaging equations governing the two-phase flow in the channel of a liquid-metal magnetohydrodynamic generator. The inhomogeneity of the distribution of the gas phase over the channel cross section is taken into account by introducing the correlation coefficients β1, β2, and β3: 〈αu〉 = β1〈α〉〈u〉, 〈σu〉 = β2〈σ〉〈u〉, 〈ρu〉 = β3〈ρ〉〈u〉. Expressions for these coefficients in terms of the correlation coefficient β1 are obtained. The calculated characteristics of the generator were compared with experimental data obtained at the Argonne National Laboratory. The comparison shows that the calculated and measured characteristics are in fairly good (±15%) agreement.
Existence of a negative potential solitary-wave structure and formation of a double layer
Authors: Hasegawa Akira , Sato Tetsuya
Published: March 2003

A stationary negative potential with a solitary-wave structure can exist in a nonequilibrium plasma with Te≫Ti. The peak value of the potential can exceed the electron temperature, Te/e. The existence of such a structure is responsible for the formation of a double layer in the presence of current.
Whistler instability in the Elmo Bumpy Torus
Authors: Guzdar P. N. , Marchand R.
Published: March 2003

A governing equation for whistler modes in the Elmo Bumpy Torus is presented and solved perturbatively. To lowest order, a discrete spectrum of real frequencies is determined. Finite growth rates are found to first order. Attention is focused on trapped electron resonances. In the frequency range of interest, the destabilizing terms involve summations over a large number of resonances. A threshold condition is obtained, and typical growth rates, corresponding to plausible machine parameters, are presented.
Thermal spectrum of collisionless tearing modes
Authors: Kleva Robert G.
Published: March 2003

The principle of superposition of dressed test particles is used to calculate the thermal spectrum of collisionless tearing modes. The analysis is restricted to modes satisfying the ’’constant -ψ (or A∥)’’ approximation. The ratio of fluctuating to equilibrium magnetic energy contained in the wavenumber range ‖kyd‖<1 is found to be 〈(B̃/B)2〉 = β/(4π)2nLsd2, where β≡8πnTe/B2 is the ratio of the plasma pressure to the magnetic field pressure, Ls is the shear length of the magnetic field, and d is the collisionless skin depth. For typical tokamak discharges 〈(B̃/B2)〉 is of order 10−19.
Adiabatic compression of ion rings
Authors: Larrabee D. A. , Lovelace R. V.
Published: March 2003

A study has been made of the compression of collisionless ion rings in an increasing external magnetic field, Be = ẑBe(t), by numerically implementing a previously developed kinetic theory of ring compression. The theory is general in that there is no limitation on the ring geometry or the compression ratio, λ≡Be (final)/Be (initial)⩾1. However, the motion of a single particle in an equilibrium is assumed to be completely characterized by its energy H and canonical angular momentum Pϑ with the absence of a third constant of the motion. The present computational work assumes that plasma currents are negligible, as is appropriate for a low-temperature collisional plasma. For a variety of initial ring geometries and initial distribution functions (having a single value of Pϑ), it is found that the parameters for ’’fat’’, small aspect ratio rings follow general scaling laws over a large range of compression ratios, 1⩽λ≲103: The ring radius varies as λ−1/2; the average single particle energy as λ0.72; the root mean square energy spread as λ1.1; and the total current as λ0.79. The field reversal parameter is found to saturate at values typically between 2 and 3. For large compression ratios the current density is found to ’’hollow out’’. This hollowing tends to improve the interchange stability of an embedded low β plasma. The implications of these scaling laws for fusion reactor systems are discussed.
On viscous dissipation rates of velocity component kinetic energies
Authors: Corrsin S. , Fournier J.-L.
Published: March 2003

It is shown by an example that in the balance equation for the kinetic energy of a velocity component in a Newtonian viscous fluid, the term usually interpreted as viscous dissipation rate can have negative regions, although the second law of thermodynamics requires that total kinetic energy dissipation rate be positive at each point in space-time. After exploration of an alternate form, it is concluded that the conventional form is correct, and that the thermodynamic requirement is simply not relevant for kinetic energies of individual velocity components.
Mixing by a turbulent wake of a uniform temperature gradient in the approach flow
Authors: Durbin P. A. , Hunt J. C. R. , Firth D.
Published: March 2003

The mean temperature distribution is analyzed in the turbulent wake of a circular cylinder when there is a linear temperature gradient flow upstream of the cylinder. Using the same variable eddy diffusivity derived from published temperature measurements in the wakes of heated cylinders, it is shown that the mean temperature distribution has a self-similar form which agrees with experimental results of Alexopoulos and Keffer. But, the location and magnitude of the maximum temperature perturbations are in error by 25% and 50%, respectively. The reasons for the limitations of the eddy diffusivity model in wake flows are discussed. The variance of temperature fluctuations (θ′̄)2̄ in a linear temperature gradient mixed by homogeneous grid turbulence are then analyzed on the basis of Durbin's theory. It is shown that the measurements of (θ′̄)2̄ are more plausibly fitted by the curve (θ′̄)2̄∝x1/2 than by Sullivan’s analysis of the same data.
Injection of a relativistic electron beam into neutral hydrogen gas
Authors: de Haan P. H. , Janssen G. C. A. M. , Hopman H. J. , Granneman E. H. A.
Published: March 2003

The injection of a relativistic electron beam (0.8 MeV, 6 kA, 150 nsec) into hydrogen gas of 190 Pa pressure results in a plasma with density ne≃1020 m−3 and temperature kTe≲kTi≃3.5 eV. The results of the measurements show good agreement with computations based on a model combining gas ionization and turbulent plasma heating. It is found that a quasistationary state exists in which the energy lost by the beam (about 6% of the total kinetic energy of the beam) is partly used to further ionize and dissociate the gas and for the other part is lost as line radiation.
An alternative derivation of the parallel ion viscosity
Authors: Bravenec R. V. , Berk H. L. , Hammer J. H.
Published: March 2003

A set of double-adiabatic fluid equations with additional collisional relaxation between the ion temperatures parallel and perpendicular to a magnetic field are shown to reduce to a set involving a single temperature and a parallel viscosity. This result is applied to a recently published paper [R. V. Bravenec, A. J. Lichtenberg, M. A. Leiberman, and H. L. Berk, Phys. Fluids 24, 1320 (1981)] on viscous flow in a multiple-mirror configuration.
Plasma oscillations and sound waves in collision-dominated two-component plasmas
Authors: Hansen J. P. , Sjögren L.
Published: March 2003

Charge, mass, and electron density fluctuation spectra of strongly correlated, fully ionized two-component plasmas within the framework of the Mori–Zwanzig memory function formalism are analyzed. All dynamical correlation functions are expressed in terms of the memory functions of the ion and electron velocity autocorrelation functions by a generalized effective field approximation which preserves the exact initial values (i.e., static correlations). The theory reduces correctly to the mean field (or collisionless Vlasov) results in the weak coupling limit, and yields charge density fluctuation spectra in good agreement with available computer simulation data, as well as reasonable estimates of the transport coefficients. The collisional damping and frequency shift of the plasma oscillation mode are sizeable, even in the long wavelength limit. The theory also predicts the propagation of well-defined sound waves in dense plasmas in thermal equilibrium.
Theory of nonlinear ion-electron instability
Authors: Boutros-Ghali Teymour , Dupree Thomas H.
Published: March 2003

A nonlinear theory describing the interaction of ion and electron ’’clumps’’ in a turbulent Vlasov plasma is presented. Numerical and analytical solutions predict that, in the presence of a drift vd of the average electron distribution, a spectrum driven by such fluctuations will be unstable to drifts which are appreciably below those required for the onset of a linear instability. The instability exists over a large range of the parameter space vd and Te/Ti (the electron to ion temperature ratio); moreover, the growth rate is amplitude dependent being approximately proportional to the trapping time. These results are in good agreement with recent computer simulations.
Scaling laws for single-shell DT gas-filled spherical targets for heavy-ion fusion
Authors: Magelssen G. R. , Gula W. P.
Published: March 2003

Scaling relationships for high-z shell DT gas-filled spherical targets irradiated by a constant power heavy-ion beam pulse are derived and compared with numerical calculations. Target performance as a function of incoming heavy-ion particle energy is numerically determined and limitations on the analytical scaling relationships identified.
A new class of fluid instabilities: Vorticity-induced waveforms on falling parabolic jets
Authors: Snyder B.
Published: March 2003

Steady streams issuing from horizontal circular ducts form stationary surface waves whose angular symmetry depends on the vorticity distribution within the duct. For Poiseuille flow, the observed sequences of wavelengths and frequencies are shown to be consistent with Crocco’s theorem: With fall the stream narrows, inducing secondary flow patterns dependent on the rate of narrowing. These findings suggest a method for obtaining scaling estimates of prevalent vorticity patterns in ducts.
Comments on ‘‘General analysis of the stability of superposed fluids’’
Authors: Shivamoggi B. K.
Published: March 2003

Comments on ’’General analysis of the stability of superposed fluids’’ are present. (AIP)
Reply to comments on ‘‘General analysis of the stability of superposed fluids’’
Authors: Plesset Milton S. , Prosperetti Andrea
Published: March 2003

Previous results by Plesset and Hsieh on the effects of compressibility for Rayleigh–Taylor instability are shown to be valid, and an alternative brief deduction is given.
Hydromagnetic Rayleigh–Taylor instability of a rotating stratified fluid
Authors: Chakraborty B. B.
Published: March 2003

The Raleigh–Taylor instability of an infinitely conducting, rotating, stratified fluid in the presence of a horizontal magnetic field is studied. The fluid density and the magnetic field strength are arbitrary functions of the vertical coordinate. An estimate of the upper bound for the growth rate of the instability of any unstable mode of disturbances of a given wavelength, which cannot occur unless the fluid density increases with height at some points, is obtained. This estimate is found to give a satisfactory result in a Rayleigh–Taylor instability model for which the dispersion relation is known exactly. A sufficient condition for stability of a disturbance is also obtained.
Resonant interaction of ion-acoustic solitons in three-dimensions
Authors: Khazei Mehyar , Bulson Jeffry M. , Lonngren Karl E.
Published: March 2003

The two- and three-dimensional collision of spherical ion acoustic solitons is experimentally observed. In each case, a new soliton is created which approximately satisfies the resonance conditions K̄α = Σni = 1K̄i; Ω(K̄α) = Σni = 1 Ω(K̄i), n = number of colliding solitons. In three dimensions the soliton is localized along the axis of collision.
Numerical studies of a field-reversed theta-pinch plasma
Authors: Milroy R. D. , Brackbill J. U.
Published: March 2003

A two-dimensional magnetohydrodynamic computer code has been used to model the formation of a field-reversed theta-pinch plasma. The simulations have been performed in the parameter range of the recent FRX-B experiments. It is found that an anomalous resistivity is required to explain the observed decay of reversed magnetic flux. The reconnection process is found to be sensitive to the density and amount of trapped reverse bias flux but insensitive to the assumed magnitude of anomalous resistivity. The reconnection process is more rapid and the axial plasma motion induced by reconnection is more pronounced for larger bias flux. If a straight coil is used instead of the usual shaped coil (end mirrors), full reconnection does not occur for low initial bias fields. These results are in general agreement with FRX-B observations.
Dynamic magnetic x points
Authors: Leboeuf J. N. , Tajima T. , Dawson J. M.
Published: March 2003

Two-and-one-half dimensional magnetostatic and electromagnetic particle simulations of time-varying magnetic x points and the associated plasma response are reported. The stability and topology depend on the geometry, irrespective of the plasma β. The electrostatic field and finite Larmor radius effects play an important role in current penetration and shaping of the plasma flow. The snapping of the field lines, and dragging of the plasma into, and confinement of the plasma at, an o point (magnetic island) is observed. Magnetic island coalescence with explosive growth of the coalescence mode occurs and is accompanied by a large increase in kinetic energy and temperature as well as the formation of hot tails on the distribution functions.
Wave enhancement of electron runaway rate in a collisional plasma
Authors: An Z. G. , Liu C. S. , Lee Y. C. , Boyd D. A.
Published: March 2003

The effects of plasma waves on the electron runaway production rate is studied. For a wave packet with a one-dimensional spectrum directed along the electric field and with a phase velocity range containing the critical velocity vc for runaway, the runaway production rate is found to be enhanced by many orders of magnitude. For an isotropic wave spectrum, however, the runaway production rate is reduced because of the wave-enhanced pitch angle scattering.
Nonlinear interaction of a powerful laser with an electron plasma
Authors: Yu M. Y. , Shukla P. K. , Tsintsadze N. L.
Published: March 2003

Interaction of a high-power circularly polarized electromagnetic wave with forced Langmuir oscillations is shown to result in solitary wave structure whose electron density profile has a depression at the center and humps on the sides.
Dynamic collisionless absorption of a laser pulse in a homogeneous plasma by nonlinear forces
Authors: Dragila R. , Hora H.
Published: March 2003

At very high laser intensities, the response of plasma is decreased due to the nonlinear effects of the Coulomb collisions and due to a (nonlinear force-induced) decrease in the parametric decay and a relativistic decrease in resonance absorption. The remaining process is the dynamic transfer of optical energy to a homogeneous plasma by nonlinear forces. The process due to the intensity profile of a pulse of finite duration τ0 is studied using time-dependent nonlinear forces. The motion of the initially resting plasma behind the pulse is derived and the highly nonlinear decay length formulated. The absorption is strong at intensities close to or above the relativistic threshold and at plasma densities near the cutoff density.
Investigation of gain kinetics in a CO gas dynamic laser
Authors: Stricker J. , Tilleman M.
Published: March 2003

A detailed experimental investigation of small-signal gain in a CO gasdynamic laser, produced in a shock tunnel is presented. Gains of the P7(3) transition were measured at a distance 44 cm downstream from the nozzle throat for various CO–Ar and CO–N2–Ar mixtures at stagnation temperatures of 1800°–3800 °K and stagnation pressures of 18–40 atm. Optimum gas temperatures and compositions were observed. Laser oscillations were observed simultaneously in two cavities located at 44 and 56 cm downstream from the nozzle throat. The experimental data on gain have been compared with a numerical model. A quantitatively good argeement between calculations and experiments was found which permits the utilization of the mathematical model for a better understanding of the processes involved in the laser operation. Small signal gains and multi-line laser power outputs were calculated along the nozzle for various initial conditions. Calculations indicate that maximum gain and maximum power occur for P3(4)-P5(4) lines at a stagnation temperature of about 3000 °K and a gas composition of CO:Ar = 20:80.
Spectral analysis of noisy nonlinear maps
Authors: Hirshman S. P. , Whitson J. C.
Published: March 2003

A path integral equation formalism is developed to obtain the frequency spectrum of nonlinear mappings exhibiting chaotic behavior. The one-dimensional map, xn+1 = f(xn), where f is nonlinear and n is a discrete time variable, is analyzed in detail. This map is introduced as a paradigm of systems whose exact behavior is exceedingly complex, and therefore irretrievable, but which nevertheless possess smooth, well-behaved solutions in the presence of small sources of external noise. A Boltzmann integral equation is derived for the probability distriburtion function p(x,n). This equation is linear and is therefore amenable to spectral analysis. The nonlinear dynamics in f(x) appear as transition probability matrix elements, and the presence of noise appears simply as an overall multiplicative scattering amplitude. This formalism is used to investigate the band structure of the logistic equation and to analyze the effects of external noise on both the invariant measure and the frequency spectrum of xn for several values of λ∈[0,1].
Solitary surface waves
Authors: Gradov O. M. , Stenflo L.
Published: March 2003

By means of the cold electron plasma equations, we have shown that surface soliton solutions can exist in semi-infinite plasmas.
Nonlinear saturation spectra of electric fields and density fluctuations in drift wave turbulence
Authors: Kelley M. C.
Published: March 2003

Electric field and plasma density fluctuations have been observed in the range 0.2⩽k⊥ ρ⩽15 in association with a long wavelength naturally occurring plasma instability. The observed spectral form and amplitude of the waves is in excellent agreement with predictions for drift waves.
Boundary layer corrections to neoclassical ripple transport in tokamaks
Authors: Shaing K. C. , Callen J. D.
Published: March 2003

The discrete nature of the toroidal magnetic field coils spoils the symmetry of the tokamak and creates small modulations of the toroidal field called ripples. Particles trapped in the ripple well drift off the flux surface and enhance both particle and heat fluxes. In the usual ripple transport calculations, the derivative of the perturbed particle distribution function in velocity space is discontinuous at the boundary between the ripple trapped and untrapped regions. To smooth out the particle distribution across the boundary, the particle distribution in the boundary layer is calculated by using the Wiener–Hopf technique. The correction to the ripple-trapped particle distribution function away from the boundary layer and the ripple transport associated with it are obtained. It is found that the particle diffusion and heat conduction coefficients are increased by about a factor of 2 and scale like ν−1/2 as (νeff/ωbδ)1/2→1. Even for very collisionless plasmas, νeff/ωbδ = 10−2, the corrections are still non-negligible and are about 15%.
Confinement scaling of core plasma electrons in the ELMO Bumpy Torus experiment
Authors: Uckan T. , Berry L. A. , Hillis D. L. , Richards R. K.
Published: March 2003

Recent high microwave experiments in the ELMO Bumpy Torus-Scale (EBT-S) at power levels through 200 kW cw at 28 GHz cover a broad parameter range (density, temperature, potential well, etc.), the operating window (T-mode) increases with power, and the plasma characteristics change significantly with operating parameters (power and pressure). Because the density profiles are observed to change with these parameters, compasions with zero-dimensional theoretical expectations for the complete data set are not possible. However, it is shown that for controlled power and pressure combinations, scale lengths and eφ/Te are constant and the electron transport in EBT-S scales neoclassically; the power balance (Pμ∼n2/T1/2e ) and particle balance (τE/τp ∼constant) are consistent with neoclassical predictions.
Stability of low-frequency modes in tandem mirrors and bumpy tori
Authors: Tsang Kang T. , Catto Peter J.
Published: March 2003

The system of equations appropriate of describing low-frequency modes (ω<Ωi) in general geometry for a three-species plasma is derived. The derivation employs an eikonal ansatz and for simplicity assumes Maxwellian unperturbed distribution functions, but permits arbitrary beta and asymmetry. The full system of equations includes ballooning, interchange and compressional Alfvén modes, and drift waves. As an example of the general forms, the stability of electron rings in bumpy tori and/or tandem mirrors is considered. Some simple estimates of the conditions necessary for stable operation are presented.
Magnetic fluctuations, ambipolarity, charge filamentation, and plasma rotation in tokamaks
Authors: Waltz R. E.
Published: March 2003

Using the quasilinear kinetic theory of transport, it is shown that, contrary to the conventional description, the particle flow from magnetic fluctuations due to local normal modes is intrinsically ambipolar on spatial average. This follows essentially from the local frame rotational invariance in the direction perpendicular to the magnetic field. The local field perturbations carry no rotational momentum from the plasma, and rotation can be lost only by slow viscous diffusion. While the plasma avoids rapid global charge-up, local deviations from ambipolarity cause both inward and outward radial electric currents to flow, resulting in charge filamentation of the plasma. The filamentation process can be saturated when balanced against shear viscosity. The charge filaments and corresponding shear in the transverse E×B velocity could be large enough to cause drift- and shear-Alfvén modes to be Kelvin–Helmholtz unstable. These effects are illustrated with numerical solutions of the drift-Alfvén mode equations. A discussion of tokamak experimental data on plasma potentials and rotational momentum loss, and a theoretical estimate of anomalous ion viscosity are given.