bluuebunny/crossref_metadata_2025 huggingface dataset:
Renormalized dielectric function for collisionless drift wave turbulence
Authors: Dupree Thomas H. , Tetreault David J.
Published: March 2003

Earlier renormalized theories for collisionless drift wave turbulence were deficient in several respects when ω<kzvi and the principal perpendicular motion is the E×B drift. The earlier theories did not preserve the perpendicularity between the E×B ion current and the electric field E and consequently, did not conserve energy. In addition, earlier theories did not correctly treat the case ni∼eφ/T in which the nonlinear convective term -φ×B⋅-ni should be very small. These problems are analyzed and corrected in a new version. The new theory predicts that drift wave energy is converted into parallel ion energy, producing a damping rate γ∥=−(kzvi/ω)2 k⊥2D, where D is a test particle diffusion coefficient. For a fluctuating potential φ due only to drift waves, D is greater than zero only when the threshold eφ/T∼ (k⊥Ln)−1 for ion stochastic motion is exceeded. The nonlinear stabilization will occur slightly above this threshold if γe/ω< (kzvi/ω)2 (γe is the linear growth rate). For this case the density diffusion coefficient Dn (which is less than D) will be Dn∼γe/k⊥2.
Observation of the dissipative trapped electron instability in toroidal geometry
Authors: Prater Ronald , Ejima Seiki , Ohkawa Tihiro , Wong Seung Kai
Published: March 2003

Low frequency plasma density fluctuations have been observed in the common flux volume of the dc octopole. For collision frequencies comparable to the fluctuation frequency (which is close to the diamagnetic drift frequency), the fluctuations are identified as dissipative trapped electron modes. In the high collision frequency range the collisional drift wave is observed. A strong quenching of the trapped electron modes is observed when the local safety factor is a low-order rational number. Very little transport is associated with the observed fluctuations.
Parametric dependence of the ion cyclotron instability in a two-energy-component system
Authors: Brecht S. H. , Hitchcock D. A. , Horton W.
Published: March 2003

The excitation of the ion cyclotron wave and the harmonic ion cyclotron waves by injection of a neutral beam is studied as a function of beam injection energy and angle. The fundamental mode is found to be stable for parameters similar to the current and future neutrally injected tokamaks. The harmonic cyclotron waves are found to have growth rates of a few percent of the ion cyclotron frequency when the thermal ion temperature is below a critical level. The results of these calculations are compared to several current experiments.
Intense relativistic electron beam interaction with a cool theta pinch plasma
Authors: Hammer D. A. , Gerber K. A. , Dove W. F. , Goldenbaum G. C. , Logan B. G. , Papadopoulos K. , Ali A. W.
Published: March 2003

Experimental results are presented for the heating of a 4 m long plasma confined by a uniform magnetic field of 4–5 kG by an intense relativistic electron beam. Beam parameters were 0.5–1 MeV, 25–80 kA, 60–70 nsec pulse duration, and electron density of 2–5×1011/cm3. The initial plasma density ranged from 5×1013/cm3 to 4×1015/cm3 and the electron temperature was 1–3 eV. The lower density cases were partially ionized with Te≫Ti, and the higher density cases were highly ionized with Te≈Ti. In all cases, the energy coupled from the beam to the plasma was greater than can be explained by binary collisions between beam electrons and the plasma particles. Beam energy transferred to the plasma ranged from 2–7%/m, and was uniform over the 4 m length of the plasma. Over most of the density range tested, 5×1013/cm3 to 1.5×1015/cm3, the plasma heating cannot be explained by classical processes. These results are found to be explained quantitatively by the use of a full nonlinear treatment of the electron-electron two-stream instability in the kinetic regime.
Skewness of temperature derivative in a heated grid flow
Authors: Antonia R. A. , Chambers A. J. , Van Atta C. W. , Friehe C. A. , Helland K. N.
Published: March 2003

The magnitude of the skewness of the temperature derivative in a turbulent flow downstream of a heated grid is small and can be reduced to zero by perturbing the flow with a second ambient temperature grid.
Turbulent entrance flow using a two-equation model
Authors: Deshpande M. D. , Giddens D. P.
Published: March 2003

The problem of turbulent entrance flow is calculated using a two-equation model of turbulence and the results are compared with experimental data available in the literature. The main features of the flow are properly predicted; however, important differences between theory and experiment exist.
Phase mixing in the continuous spectrum of Alfvén waves
Authors: Pritchett P. L. , Dawson J. M.
Published: March 2003

The predicted damping of Alfvén waves in a nonuniform plasma is observed in a magnetohydrodynamic computer calculation. The results are interpreted in terms of a simple model of independent oscillators.
Laser scattering measurement of nonthermal density fluctuations in a theta-pinch plasma
Authors: Ng A. K. H. , John P. K.
Published: March 2003

Nonthermal density fluctuations of scale length less than λD are observed in a theta-pinch plasma by laser scattering measurements. Results show the thermal spectrum modulated approximately at harmonics of ωp. For the plasma ne≃1016 cm−3 and Te≃5 eV.
Air transport coefficients from electric arc measurements
Authors: Devoto R. S. , Bauder U. H. , Cailleteau J. , Shires E.
Published: March 2003

A wall-stabilized arc has been operated at various currents in air at 1 atm pressure. Measurements have been made of the axial electrical field strength, the emitted radiation, and the radial temperature profiles. These data were used to deduce the electrical conductivity, the emission coefficient for optically thin radiation, and the thermal conductivity as a function of temperature. The deduced values are found to be in accord with theoretical computations.
Multi-electrode plasma probe for orbit-limited-current measurements. II. Numerical verification
Authors: Parker L. W. , Laframboise J. G.
Published: March 2003

A flat-ended variant of a multi-electrode electrostatic plasma probe design, previously proposed by Laframboise and Parker, is described. The probe consists of a collector, a guard, and a sleeve electrode. Besides being easier to construct, this modified probe geometry is more amenable to numerical verification of its operating properties. Such a verification is carried out by numerically following selected collisionless orbits of charged particles in the electric field of the probe. By the same means, an orbit-limited operating regime of electrode bias combinations is also found. It is found that if the sleeve is given an attractive bias at least three times as large as that of the collector and guard, relative to space potential, then one achieves the intended probe operation, which simulates that of an orbit-limited spherical Langmuir probe in collisionless conditions but with certain restrictions on probe operation removed or relaxed. It is concluded that experimental tests of the proposed probe configuration are desirable.
Effect of density fluctuations on lower hybrid resonance cone propagation
Authors: Bellan P. M. , Wong K. L.
Published: March 2003

Experimental measurements show that coherent, azimuthal density fluctuations (1) focus lower hybrid resonance cones azimuthally, and (2) modulate the radial location of the resonance cones. A simple theory based on wave refraction is presented; this theory is in good agreement with the experimental observations. The theory is extended to give a physical description of wave propagation through turbulent, isotropic (perpendicular to B) noise such as has been observed in tokamaks; it is found that the turbulence causes the lower hybrid wave vectors to have an angular spread in the plane perpendicular to B.
Current driven weak ion acoustic turbulence in a magnetized plasma
Authors: Bitter M. , Paris P. J.
Published: March 2003

The current driven ion acoustic instability has been studied in a quiescent magnetized argon plasma for electron densities in the range from 1010 to 1011 cm−3, an electron temperature Te of 2.2 eV, an electron-ion temperature ratio Te/Ti of 15, a homogeneous magnetic field of 400 G, and neutral gas pressures in the range from 1×10−3 to 5×10−3 mm Hg. Coulomb collisions are important for these experimental conditions. The instability occurs for dc currents in the range from 100 to 600 mA. The saturation appears to be explained by electron trapping. The nature of the instability is identified from measurements of the cross power density spectrum and the characteristic parameters like electron drift velocity, collisional damping, electron temperature, and electron-ion temperature ratio are determined precisely from an observation of low frequency ion acoustic test waves using an unperturbative method of microwave interometry.
Soluble theory of nonlinear beam-plasma interaction
Authors: Mynick Harry E. , Kaufman Allan N.
Published: March 2003

A soluble theory of the post-saturation portion of a beam-plasma interaction is developed, concentrating on explaining the results of O’Neil, Winfrey, and Malmberg. Analytic progress is made possible by applying a certain constraint procedure, characterized by the ’’rotating-bar’’ approximation, to a Hamiltonian formulation of the problem. The procedure yields, from the original N-particle Hamiltonian H, a new, reduced Hamiltonian H̃, which has only two particle-related degrees of freedom, and which maintains the conservation laws of energy and momentum possessed by H. The equations of motion coming from H̃ still describe the self-consistent interaction of a mode of the plasma with the beam particles, as opposed to previous work, and, because of the great reduction in the number of degrees of freedom, explicit expressions for the nonlinear frequency shift, and growth rate, of the mode can be obtained, which are in very good agreement with the simulation results of O’Neil, Winfrey, and Malmberg.
Experimental study of the axial ion energy distribution in a fast theta pinch
Authors: Chiang T. T. , DeSilva A. W.
Published: March 2003

Ion energy distributions along the axis of the Maryland linear theta pinch were measured by a parallel-plate electrostatic deflection type energy analyzer. Two possible sources of end loss are discussed. The first is acceleration in the axial potential which is a result of the cooling of electrons near the end of the tube. The second is the axial ion temperature that results from relaxation due to different microinstabilities among ion energy components. The particle measurement suggests that the second one is dominant. A two-temperature Maxwellian distribution is observed along the axis. The absence of observable ions from farther than 10–20 cm into the plasma also implies a strong turbulent scattering process is operating.
Whistler wave ducting caused by antenna actions
Authors: Sugai H. , Maruyama M. , Sato M. , Takeda S.
Published: March 2003

Whistler waves launched from an antenna damp away for small incident power. With increasing power, undamped nondiverging waves (the ducted waves) are observed, together with a field-aligned density trough and electron heating. However, the density trough is found not only in the wave propagation regime (ω/ωc<1), but also in the wave evanescence regime (ω/ωc≳1). This implies that the density depression is mainly created by the effect of the antenna near-zone field rather than by the wave radiation pressure. The intense localized field near the antenna gives rise to electron heating which leads to the density trough. The ducting of antenna-launched whistler waves has been explained as a filamentation instability in terms of nonlinear wave-plasma interactions.
Internal wave solitons
Authors: Meiss J. D. , Pereira N.R.
Published: March 2003

A numerical solution of the Benjamin–Ono equation for internal waves shows soliton behavior. Two and three Lorentzian solitons pass through one another unscathed. An initial Lorentzian with larger than soliton amplitude decays into solitons, with velocities predicted by the five conservation laws.
Magnetic field transport in laser-fusion targets
Authors: Langdon A. Bruce
Published: March 2003

The spatial extent of magnetic field sources in laser-irradiated targets can be small enough for the well-known nonlocal nature of plasma conductivity to strongly affect magnetic field diffusion and saturation.
Effective Ohm’s law for magnetized plasmas with anisotropic inhomogeneities
Authors: Shamma S. E. , Martinez-Sanchez M. , Louis J. F.
Published: March 2003

Reduction formulae for the effective, or macroscopic, Ohm’s law parameters are derived for inhomogeneous plasmas with anisotropic conductivity fluctuations having two general types of geometry: (a) elongated or shortened in the direction of the magnetic field and (b) two-dimensional, with the direction of constant properties lying in the plane perpendicular to the magnetic field. In each case, two approaches are used: (a) a small perturbation method and (b) an approximate method where each region in the plasma is considered separately, and consistency conditions are used to relate the results corresponding to each separate region to the effective properties of the whole plasma. Both methods are found to agree well when the fluctuations are weak, but differences appear at high fluctuation levels and, for nonuniformities very elongated along B↘, when the Hall parameter β is high. Comparison with available exact solutions valid at high β and strong fluctuation levels indicates that the self-consistency method gives accurate results even in these cases. The results of these analyses are used to evaluate the performance reduction in magnetohydrodynamic channels with plasma nonuniformities of several geometries, including axial streamers, perfectly isotropic fluctuations, and fluctuations elongated along B↘; the power density is reduced most strongly when β and the rms of the fluctuations are high, and also when the inhomogeneities are stretched along the magnetic field.
Intensity dependence of inverse bremsstrahlung absorption in an inhomogeneous standing wave
Authors: Faehl R. J. , Roderick N. F.
Published: March 2003

The intensity dependence of inverse bremsstrahlung due to the oscillating velocity of electrons in a laser field can severely degrade collisional absorption efficiency. Previous work has concentrated on the classical and quantum expressions for the absorption rate. Total absorption requires evaluation of such expressions in the inhomogeneous field structure within the plasma, however. Both analytic and numerical calculations to determine the magnitude of the fractional reduction in absorption have been undertaken for the realistic case of an inhomogeneous standing wave. A simple physical model is derived and extrapolated to Airy-like field distributions. Numerical calculations employing fluid simulations with a heuristic intensity dependence were found to be in excellent agreement with an analytic model.
Effect of a Coriolis force on the stability of plane Poiseuille flow
Authors: Flaherty J. E. , DiPrima R. C.
Published: March 2003

The stability of the viscous flow between two parallel horizontal plates due to a constant reduced pressure gradient in a system rotating about a vertical axis is studied. The critical value of the Reynolds number R, based on the reduced pressure gradient, is a function of a dimensionless rotation parameter T, the Taylor number. A numerical solution of the eigenvalue problem shows that (i) the minimum point for the viscous instability mode associated with plane Poiseuille flow at T=0 disappears at a value of T of about T≃0.4, and (ii) for T≠0 a new instability mode appears as a result of the Coriolis effect on the basic flow and in the perturbation equations. This new instability gives the critical value of R for values of T as small as 0.06.
Interfacial stability with mass and heat transfer
Authors: Hsieh D. Y.
Published: March 2003

A simplified formulation is presented to deal with interfacial stability problems with mass and heat transfer. For Rayleigh–Taylor stability problems of a liquid-vapor system, it is found that the effect of mass and heat transfer tends to enhance the stability of the system when the vapor is hotter than the liquid, although the classical stability criterion is still valid. For Kelvin–Helmholtz stability problems, however, the classical stability criterion is found to be substantially modified due to the effect of mass and heat transfer.
Rapid distortion solutions for a stress transport turbulence model in contracting flow
Authors: Naot D.
Published: March 2003

The concept of rapid distortion is used to simplify the Reynolds stress closure equations. A solution for axisymmetric contracting turbulent flow is obtained and compared with the results of the rapid distortion theory of Batchelor and Proudman. The fluctuation ratio ū2̄/v2̄ now predicted compares favorably with the experimental data of Uberoi. The irreversibility of rapid distortion is discussed in view of experimental evidence.
Nonlinear behavior of linearly unstable magnetohydrodynamic modes
Authors: Pao Young-ping
Published: March 2003

It can be shown that for linearly unstable magnetohydrodynamic modes near the threshold of linear instability, the mode amplitude A (t) evolves according to ∂2A/Λt2 =λ2A+αA3 which can lead to nonlinear stabilization, explosive instability, or eventual decay, depending on the sign of α. This is demonstrated explicitly for the m=1 kink modes in a sharp boundary plasma pinch.
Plasma convection instability in an inhomogeneous magnetic field
Authors: Ottinger P. , Guillory J. , Lashinsky H.
Published: March 2003

A convection instability characteristic of plasmas in an inhomogeneous azimuthal magnetic field is treated in the linear stage and in nonlinear saturation. The analysis is done in such a way that collisional and collisionless limits can be taken, and these limits are displayed along with the more general intermediate result. The instability, known previously in the literature in its collision-dominated form, is shown to be a ’’flute’’ instability with collisional modifications to the growth rate. The nonlinear saturation is analyzed by examining a finite amplitude restoring-force term in the differential equation that describes the instability. This term is due to the fact that the instability convects plasma into striations of the plasma column surface, modifying the density gradient driving force. The effects of finite ion gyroradius are displayed, and applications of this study to convection cells in a thermal plasma and to exploding wire plasmas are discussed.
Completion of the multi-point turbulent equations using an approximation to the mean value theorem
Authors: Fox Ronald L.
Published: March 2003

An approximation to the form of the multi-point interactions is combined with the mean value theorem and coincident properties of the distribution functions to produce a closed set of equations. The solution for isotropic flow is similar to that previously obtained by discarding three-point interactions.
Two-dimensional depletion of a lower hybrid pump by quasi-mode excitations
Authors: Sen A. , Karney C. F. F. , Bers A.
Published: March 2003

The depletion of a finite extent lower hybrid pump wave due to the parametric excitation of quasi-modes is calculated for parameters relevant to tokamak plasmas.
Parametric decay in a finite width pump, including the effects of three-dimensional geometry and inhomogeneity
Authors: Reiman A.
Published: March 2003

The effects of two- and three-dimensional geometry are described for parametric decay of a finite width pump in an inhomogeneous medium. The analysis in three dimensions focuses on the geometry appropriate to decay in a lower hybrid resonance cone. The time asymptotic behavior of two- and three-dimensional interactions (space aysmptotic behavior of steady-state interactions) is described in terms of the growing normal modes of equivalent one-dimensional equations. Numerical solutions are given for growth rates and thresholds in a pump with a Gaussian profile in a one-dimensional inhomogeneous medium.
Slope reversal of a monotonically decreasing electron tail in a strong magnetic field
Authors: Haber I. , Huba J. D. , Palmadesso P. , Papadopoulos K.
Published: March 2003

An electron distribution with a long tail can be driven unstable by the resonant transfer of tail particle energy to electron cyclotron waves. The implications of this instability are examined by following its nonlinear evolution. The growth of wave energy can result in the formation of a positively sloped region of the electron distribution function. This positive slope causes a second mode to go unstable which subsequently results in large pitch angle diffusion of the tail particles.
Particle simulation studies of the lower hybrid drift instability
Authors: Winske D. , Liewer P. C.
Published: March 2003

The nonlinear phase of the lower hybrid drift instability in the high drift velocity regime (relative electron-ion drift velocity greater than ion thermal speed) has been investigated using particle simulation in a two-dimensional sheath geometry. The anomalous resistivity is measured at various times and is shown to be in good agreement with quasi-linear theory during both the linear and nonlinear phase of the instability. Saturation is due to ion trapping.
Nonlinear evolution of drift Alfvén and double tearing instabilities
Authors: Lin A. T.
Published: March 2003

A particle model which neglects the compressional components of the axial magnetic field perturbation has been developed to investigate the time evolution of drift Alfv́en and double tearing instabilities in a finite but low β (plasma pressure/magnetic pressure) plasma. It is observed that in a currentless plasma the shear Alfv́en wave and the drift wave couple together and become unstable. The smoothing out of the density profile limits the growth of the instability. In a plasma with current flowing along the magnetic field line the instability becomes a drift-tearing type which is driven by the magnetic field energy associated with the plasma current. The growth and the interaction of sets of magnetic islands on adjacent rational surfaces results in generation of smaller islands and reconnection of magnetic lines of force between different rational surfaces. This effect enhances local heat and particle transport.
Coupled localized electron-plasma waves and oscillatory ion-acoustic perturbations
Authors: Spatschek Karl H.
Published: March 2003

The equations describing the coupling of high-frequency electrostatic waves with ion fluctuations allow stationary one-dimensional localized solutions which have not been reported previously. It is shown that these solutions follow from scaling laws different from those known for sonic Langmuir solitons, but similar to those used for static ion response. The present treatment generalizes the theory for subsonic Langmuir solitons by consistently including the second harmonic contribution in the low-frequency response. The physical importance of the new solutions is discussed.
Investigation of shock compression in toroidal geometry in the high voltage belt pinch
Authors: Söldner F.
Published: March 2003

Fast magnetic compression is investigated in the high voltage belt pinch at two initial densities ne0=3×1013 cm−3 and ne0=7×1013 cm−3. At the lower density the electrons are heated to 3 keV in the piston region, the ions trapped in the piston obtaining an energy of 0.7 keV. A third of the ions are reflected off the piston attaining an energy of 2.5 keV. At the higher initial density a central β=1 plasma is formed. Electrons are heated to 1 keV in the sheath, the piston ions obtaining an energy of 0.4 keV. The degree of ion reflection is 60%. Ion acoustic turbulence in both cases dominates the implosion phase and gives rise to anomalous field diffusion and electron heating. Steep electron density and temperature gradients decisively enhance the effective drift velocity for wave growth. The experimental results are compared with numerical calculations using a hybrid code which includes anomalous transport. Scaling studies with the hybrid code in a wider density range show, in agreement with experiment, increasing efficiency of shock heating for higher initial densities due to increasing ion reflection.
Drag force on a slowly moving sphere in a medium with variable viscosity
Authors: Mathews J.
Published: March 2003

The drag force on a sphere moving slowly through an incompressible fluid with viscosity depending on distance from the sphere as η=η∞[1+(R0/r)s] is calculated. Boundary conditions at the sphere appropriate to a rigid sphere (no-slip) and to a bubble are considered. The general results are given, as well as specializations to (i) large spheres, (ii) small spheres, and (iii) the case s=4, which originally motivated the calculation.
Detailed similarity in the turbulent wake of a heated cylinder
Authors: LaRue John C. , Libby Paul A.
Published: March 2003

One of the well-known, fundamental features of some free turbulent shear flows, such as the wake downstream of a cylinder, is similarity of the profiles of unconditioned quantities of interest in fluid mechanics such as the mean and higher moments of the temperature. Recent work has indicated that profiles of zone-averaged, conditioned statistics are also similar. Results are presented which are based on conditioned sampling and show that similarity within the turbulent fluid exists in much greater detail than has heretofore been thought; in particular, the statistical properties of structures having nominally the same characteristic lengths are similar. Additionally, it is shown that these turbulent structures consist of two interface regions having relatively large gradients and a core region with relatively smaller gradients of the temperature moments. These latter gradients indicate preferential entrainment on the downstream edges of the turbulent structures.
Characteristics of flow cavitation in dilute solutions of polyethylene oxide and polyacrylamide
Authors: Ting Robert Y.
Published: March 2003

The effect of high molecular weight drag-reducing polymers on flow-induced cavitation was studied. Cavitation was generated on the surface of a disk rotating in a test chamber. Cavitation inception was determined as a function of flow velocity, temperature, and pressure. Polymeric additives used included polyethylene oxide and polyacrylamide at various concentrations. The results showed that the inception of flow-induced cavitation was suppressed by the high molecular weight polymers. This suppression was represented by a substantial reduction in the values of the cavitation inception parameter in polymer solutions. Polymeric additives were also found to have a strong effect in reducing the overall size of fully developed cavities. These effects were briefly discussed in terms of polymer viscoelasticity.
Equilibrium form of a steady plasma
Authors: Demidov A. S.
Published: March 2003

A free boundary problem concerning equilibrium plasma configurations is investigated. It is shown that under some conditions a plasma is only one-component, i.e., can have only one simply connected component of connectivity. There are cases where a plasma must be two-component, i.e., have two components of connectivity (each component is simply connected). The conditions under which a plasma can be both one-component and two-component are also described. A complete description is given of all (symmetric) plasma configurations for a magnetic field flux κ in a shell with a special cross section Γα. It is shown that the manifold of all such configurations is homeomorphic to the two-parameter manifold of the real zeros of some third-order polynomial with real coefficients dependent on α and κ.
Magnetic field effects on the plasma bremsstrahlung spectrum
Authors: Pangborn R. J.
Published: March 2003

The effect of a constant, external magnetic field on the bremsstrahlung spectrum of a Maxwellian plasma is investigated. The collisional damping, assumed small, is obtained and by means of Kirchoff’s law, an expression for the bremsstrahlung emission coefficient is written. It is found that the emission is greatly enhanced for certain frequencies and directions.
Transport of a nonneutral electron plasma due to electron collisions with neutral atoms
Authors: Douglas M. H. , O’Neil T. M.
Published: March 2003

Transport of a nonneutral electron plasma across a magnetic field is caused by electron scattering from ambient neutral atoms. A theoretical model of such transport is presented, assuming the plasma is quiescent and the scattering is elastic scattering from infinite mass scattering centers of constant momentum transfer cross section. This model is motivated by recent experiments. A reduced transport equation is obtained by expanding the Boltzmann equation for the electron distribution in inverse powers of the magnetic field. The equation together with Poisson’s equation for the radial electric field, which must exist in a nonneutral column, determine the evolution of the system. When these two equations are properly scaled, they contain only a single parameter: the ratio of initial Debye length to initial column radius. For cases where this parameter is either large or small, analytical solutions, or at least partial solutions, are obtained. For intermediate values of the parameter, numerical solutions are obtained.
Cyclotron harmonic absorption via the Green’s function for the mode conversion-tunneling equation
Authors: Swanson D. G.
Published: March 2003

Cyclotron harmonic absorption is included in the analysis of the mode conversion-tunneling process by solving for the Green’s function of the fourth-order mode conversion-tunneling equation. For the first cyclotron harmonic where ω≈2ωci and the wave propagates exactly across the magnetic field, the solution is given in closed form in terms of the basic homogeneous solutions. For finite k∥, the solution is given by solving an integral equation where the kernel is not self-adjoint since the basic mode conversion-tunneling equation is not self-adjoint. For cavity modes, it appears that mode conversion losses are generally greater than harmonic cyclotron absorption losses although both are of the same order, and the result implies that electron heating may dominate ion heating for these processes.
Condensation of water via rarefaction wave
Authors: Courtney Welby G. , Forshey David R.
Published: March 2003

Condensation of water vapor via the rarefaction wave in a shock tube was investigated by measuring pressure as a function of time. At high temperature (260°K), condensation could be adequately explained by the classical nucleation theory combined with a small accommodation coefficient for particle growth. However, at low temperature (230°K), condensation was considerably faster than permitted by classical nucleation, and the classical theory of nucleation is therefore considered to be invalid. Condensation shocks due to the coupling of gasdynamics with condensation kinetics (heat release rate) were also observed.
Focusing resonance cones
Authors: Levine B. , Greene G. J. , Gould R. W.
Published: March 2003

The potential for an oscillating ring source immersed in cold, magnetized, collisionless plasma in the resonance cone regime (K∥/K⊥<0) is evaluated exactly and asymptotically, giving insight into the gross spatial behavior of the focusing resonance cones. The nature of the singularity in potential is clarified by the introduction of a noninfinitesimal collision frequency. Thermal effects are considered numerically, revealing an interesting interference structure in potential as well as a density depression near the focus of the cone due to the ponderomotive force.
Simulation study of Bernstein modes
Authors: Kamimura T. , Wagner T. , Dawson J. M.
Published: March 2003

The properties of Bernstein modes were investigated through computer simulations using two-dimensional and two-and-one-half-dimensional (i.e., two spatial and three velocity coordinates) electrostatic models with fixed magnetic field. The measured discrete spectrum was found to agree with the linear dispersion relation for these modes. The quasi-periodic phenomenon of early phase-mixing damping and later recurrence, predicted by Baldwin and Rowlands, was observed. For large wavenumber k⊥, the initial damping rate is the same as that for Landau damping in an unmagnetized plasma; for small k⊥, however, it is much stronger. The recurrence peaks slowly damp in time at a rate proportional to k2⊥D, where D is the measured cross-field particle diffusion coefficient which is dominated by convective transport. Finally, splitting of the main spectral peaks and the appearance of subpeaks at half-integral multiples of the cyclotron frequency are observed and may be explained by nonlinear mode coupling.
Nonlinear behavior of a monochromatic wave in a one-dimensional Vlasov plasma
Authors: Shoucri Magdi M. , Gagné Réal R. J.
Published: March 2003

The nonlinear evolution of a monochromatic wave in a one-dimensional Vlasov plasma is studied numerically. The numerical results are carried out far enough in time for phase mixing to dominate the asymptotic state of the system. A qualitative comparison with previously reported simulations is given.
Turbulence theory for the dissipative trapped electron instability
Authors: Tsang K. T. , Callen J. D. , Vahala George
Published: March 2003

Drift orbit diffusion induced by turbulence acting on trapped electrons is shown to reduce and broaden the magnetic drift resonance and produce the dominant nonlinear saturation mechanism for the dissipative trapped electron instability. The fluctuation level obtained from such a theory is found to be consistent with present experimental observations.
Density gradient drift instabilities: Oblique propagation at zero beta
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 a density gradient 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 velocity and the magnetic field. Three instabilities are studied and compared: the lower hybrid drift, ion cyclotron drift, and the universal drift instability. Thresholds and properties at oblique propagation are studied for a wide range of drift speeds and Te/Ti ratios. Each of these modes propagates at sufficiently oblique angles to prevent stabilization due to finite length in typical linear theta pinches greater than several hundred ion gyroradii in length.
Destruction of trapping oscillations
Authors: Dimonte G. , Malmberg J. H.
Published: March 2003

A somewhat unconventional traveling wave tube was built to investigate the nonlinear behavior of the beam-plasma instability beyond the first trapping oscillation of the wave amplitude. In the small cold beam limit, the equations governing the evolution of the beam-plasma instability are mathematically identical to those describing the traveling wave tube. The traveling wave tube has the advantage that the slow wave structure will remain linear for the wave amplitudes reached in the experiments; furthermore, it does not introduce noise. Five trapped particle oscillations are observed following the saturation of a single launched wave. Two mechanisms for destroying these oscillations have been found. The first involves wave damping and can occur for decrements smaller than 0.01 k0. The second is a result of the modulation of the main wave by unstable sidebands. In addition to the experiments, the equations which describe the interaction are solved numerically. The experimental observations are in excellent agreement with the results of the numerical calculations.
Rotational instabilities in a theta pinch
Authors: Freidberg J. P. , Pearlstein L. D.
Published: March 2003

The stability of a rotating ϑ pinch is investigated in the context of the finite Larmor radius fluid equations. The main result is the discovery of an unstable m=1, n=0 mode (m is the azimuthal mode number and n is the number of radial modes) whose threshold is the lowest of any unstable mode. A comparison with experiment indicates that this mode may be the one responsible for the m=1 ’’wobble’’ instability observed in high temperature ϑ pinches.
Stability of a plane soliton to infinitesimal two-dimensional perturbations
Authors: Saffman P. G. , Yuen Henry C.
Published: March 2003

The stability of two-dimensional infinitesimal disturbances of soliton solutions of the two-dimensional nonlinear Schrödinger equation is considered. Previous results for small transverse wavenumber are extended numerically to arbitrary values.
Generalized Bennett equilibria and particle orbit analysis of plasma columns carrying ultra-high currents
Authors: Gratreau P.
Published: March 2003

The Bennett equilibrium is shown to be a particular case of a class of Vlasov equilibria characterized by a constant axial macroscopic velocity for the ions and the electrons, and by a constant ratio of the densities. It is established that any collisionless plasma column in radial equilibrium, carrying a very high current, may be considered as the superposition, on the thermal background of ions, of two counterstreaming electron beams, the more intense of which carries most energetic electrons in the vicinity of the axis. This property could be interpreted as a partial ’’magnetic neutralization,’’ and thus explain the absence of any current limitation.
Resonance-like structure for soliton characteristics in an electron beam-plasma system
Authors: Gell Y. , Nakach R.
Published: March 2003

The characteristics of ion acoustic solitons in an electron beam-plasma system are considered. The dependence of the amplitude of the soliton on the density of the beam electrons is found to exhibit a pronounced resonance-like structure. A numerical analysis of the analytic expressions for the soliton characteristics (amplitude and width) is performed for different values of the relevant parameters of the system. The existence and origin of the resonance structure is discussed.
Excitation of low frequency waves by streaming ions via anomalous cyclotron resonance
Authors: Wu C. S. , Dillenburg D. , Gaffey J. D. , Goedert J. , Ziebell L. F. , Freund H. P.
Published: March 2003

The effect of streaming ions on the low frequency Alfvén, magnetosonic, and ion-cyclotron waves is considered. It is found that excitations of these waves can occur due to the anomalous cyclotron resonance (i.e., ω+Ωi−k∥v∥=0) process. Analytic expressions for the growth rates are derived for the case of a small population of streaming ions, ns/n0≪1 (where ns and n0 denote the number densities of the streaming and background ions, respectively). The effects of the Landau and cyclotron damping due to the background plasma are included. Numerical examples are also given for the case of ion-cyclotron waves.
Resistive hose instability of a beam with the Bennett profile
Authors: Lee Edward P.
Published: March 2003

The resistive hose instability of a self-pinched relativistic beam is examined with emphasis placed on the important case of the Bennett current profile JB(r) ∝ (1+r2/a2)−2. Previously known results applicable to a general profile are recovered and extended in several directions. An essential new feature in this study is the use of distributed particle mass to model orbital phase-mixing effects produced by the anharmonic pinch field. Resonant growth is considerably reduced, and the instability when viewed in the beam reference frame is shown to be convective rather than absolute. The peak amplitude of a disturbance wave packet moves from the point of its inception in the beam pulse toward the pulse tail. The disturbance subsequently damps if the pulse length is finite; thus, propagation over distances that are long compared with the particle betatron wavelength is possible. The predicted growth rate and group velocity of the mode are shown to be in fair agreement with the results of numerical simulation.
Stationary magnetic field line reconnection in a tearing-unstable plasma slab
Authors: Galeev A.A.
Published: March 2003

A nonlinear theory of a collisionless tearing mode is developed for the case of a plasma slab immersed in a sheared magnetic field and applied to the problem of magnetic field line reconnection. The main nonlinear effect comes from the diffusion of the magnetic field lines due to overlapping of magnetic islands created by the growth of the tearing mode. This effect is described by the effective collision integral in the kinetic equation for the particle distribution function which conserves both momentum and energy in the system. Nonlinear effects reduce the growth rate, but the instability saturation achieved is only due to quasi-linear relaxation. The reconnection rate of the magnetic field lines is calculated on the basis of the nonlinear theory of the tearing mode.
Toroidal mode coupling effects on drift wave stability
Authors: Horton Wendell , Estes R. , Kwak H. , Choi Duk-In
Published: March 2003

Modified drift wave dispersion relations and shear stabilization criteria are derived from an analysis of the radial drift wave equations coupled in azimuthal mode numbers through the toroidal effects of ion magnetic drifts and trapped electrons.
Laser scattering from droplets: A theory of multiplicative and additive stochastic processes
Authors: Szu Hwaling Harold
Published: March 2003

The scattering function S (Q,ω) for droplets of superfluid helium near absolute zero is calculated. Assuming that the surface tension τ=τ0+τ̃ (t) is fluctuating with correlation strength α, a wave diffusion decay mechanism for a nondissipative system is found. For each lth multipole of droplet shape oscillation, S (Q,ω) contains two Lorentzian peaks at frequency shifts ± (Clτ0)1/2, with a width αCl/2τ0, where Cl is the Rayleigh form factor. Applying Bose statistics, it is found that the magnitude of τ̃ is not greatly different from that of τ0. Estimates of the scattering intensity show that with present day techniques measurements are possible.
Relationship between Benjamin–Feir instability and recurrence in the nonlinear Schrödinger equation
Authors: Yuen Henry C. , Ferguson Warren E.
Published: March 2003

A simple relationship between the Benjamin–Feir instability associated with uniform solutions of the nonlinear Schrödinger equation and the long time evolution of the unstable solution is reported. The number of modes which actively participate in the energy sharing process associated with the instability is governed by the number of harmonics of the initial disturbance which lie within the unstable region as predicted by the Benjamin–Feir analysis. Generalization of this observation implies that equations which possess high wavenumber cutoffs in the instability characteristics should not thermalize in the conventional sense when undergoing such an instability, since active modes are confined to a finite range of wavenumbers.
Current collection by spherical Langmuir probes drifting in a collisionless plasma
Authors: Makita Hideharu , Kuriki Kyoichi
Published: March 2003

Current-voltage characteristics of drifting spherical Langmuir probes have been studied experimentally in collisionless plasmas. In order to single out the effect of probe speed on the ion current collection, the experiment was carried out by a rotating arm facility. With this device the probe speed as well as the properties of the undisturbed plasma were accurately determined. As predicted by the theories of Kanal, Godard, and Laframboise, the ion current at small speed ratios was found to be either increased or decreased as the probe speed increased depending on the ratio of probe radius to Debye length R/λD. As for the absolute magnitude of ion current change by the probe speed, however, discrepancies were found between these theories and the present experiment. The difference is considered to originate from asymmetry in the potential distribution, which is not included in these theories.
Particle kinetics of selective excitation spectroscopy
Authors: Stern R. A.
Published: March 2003

The role of kinetic velocity distributions on the interaction between resonant laser beams and atoms or ions is shown to affect, and under some conditions dominate, the intensity, line shape, and spatial distribution of diagnostic fluorescence in hot plasmas and gases.
Cyclotron harmonic waves and resonance cone
Authors: Lembège B. , Gonfalone A.
Published: March 2003

Experimental results which show that cyclotron harmonic waves have quasi-cylindrical wave fronts around a point source antenna in a magnetoplasma, are used to demonstrate that these cyclotron waves produce the ’’fine structure,’’ of the resonance cone predicted theoretically for the upper branch.
Linear stability of a growing or collapsing bubble in a slightly viscous liquid
Authors: Prosperetti Andrea , Seminara Giovanni
Published: March 2003

A simplified form of the stability equation appropriate for liquids of small viscosity undergoing nearly spherically symmetric flow is derived on the basis of earlier results. This equation is then applied to the analysis of the stability characteristics of the spherical shape for growing and collapsing cavitation bubbles. It is found that viscosity does not remove the well-known instability associated wihth the collapse process, although it does delay the growth of higher order modes. This feature explains the relatively small number of microbubbles to which a cavitation bubble gives rise upon fragmentation.
Effects of the initial condition on the axisymmetric free shear layer: Effect of the initial fluctuation level
Authors: Hussain A. K. M. F. , Zedan M. F.
Published: March 2003

An axisymmetric air free shear layer was experimentally investigated in order to determine the effect of the initial peak fluctuation level u′pe/Ue on the characteristic measures of the free shear layer. The shear layer width and the similarity parameter depend noticeably on u′pe/Ue. The asymptotic peak turbulence intensity u′p∞ increases with u′pe/Ue but appears to have a limiting value of about 0.18. The location for achievement of self-preservation progressively moves upstream with increasing u′pe/Ue, suggesting that higher initial fluctuations hasten the spectral evolution of the velocity fluctuations and development of the shear layer. The results show that the initial fluctuation level has a much more dramatic effect on the evolution and the average characteristic measures of the free shear layer than the initial momentum thickness has, and may have been the primary reason for discrepancies among results from different previous inverstigations.
Spontaneous synchrotron emission from a plasma with an energetic runaway electron tail
Authors: Freund H. P. , Wu C. S. , Lee L. C. , Dillenburg D.
Published: March 2003

The emissivity of spontaneous synchrotron radiation is computed for a plasma consisting of a background thermal plasma in addition to an energetic runaway electron component. The analysis is performed for both the ordinary and extraordinary modes, for frequencies in the vicinity of the electron plasma frequency and the higher harmonics of the electron gyrofrequency, and for ωe≲Ωe (where ωe and Ωe are the electron plasma and cyclotron frequencies, respectively). The relativistic gyroresonance with the runaway electrons is found to result in a level of spontaneous emission which, for frequencies in the neighborhood of ωe, is significantly enhanced over the thermal radiation.