bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of rotational temperature in nitrogen using electron beam induced fluorescence
Authors: Kassem Ahmed E. , Hickman Roy Scott
Published: February 2004

The determination of the rotational temperature in nitrogen using measurements of electron beam-induced fluorescence, depends upon the formulation employed to calculate the temperature from the spectral data as well as upon the resolution of the spectrometer. The errors introduced in the measurements by the use of the prevalent forced semilogarithmic data analysis and by incomplete resolution of the rotational lines are examined and evaluated. An alternative method of data analysis is presented. When the errors are accounted for, the dependence of the measured temperature on the number of rotational lines used in the analysis is eliminated, and the apparent rotational temperature becomes less dependent upon the gas density and the true gas temperature.
General theory of enhanced induced emission in plasmas
Authors: Oberman C. , Auer G.
Published: February 2004

A simple formalism yields a general expression for the enhanced induced emission in plasma influenced by a strong electromagnetic or electrostatic pump. Applications to subcritical Brillouin, Raman, and other resonant and nonresonant interactions are given.
Self-precession and frequency shift for electromagnetic waves in homogeneous plasmas
Authors: Arons Jonathan , Max Claire Ellen
Published: February 2004

The nonlinear propagation of an arbitrarily polarized electromagnetic wave in a uniform plasma is studied. It is shown that nonlinear effects cause precession of the polarization ellipse as the wave propagates. The ellipticity remains constant, but the orientation of the principal axes is rotated relative to its initial value. A relativistic Vlasov model is used to study nonlinear frequency shifts as well as self-precession, in a plasma of arbitrary temperature. Even when T3 ≫ mec2, the qualitative nature of these two processes remains unchanged, although their dependence on the plasma density is altered in significant ways. Implications of these effects for plasma instabilities driven by strong electromagnetic waves are briefly discussed.
Application of electromagnetic particle simulation to the generation of electromagnetic radiation
Authors: Lin A. T. , Dawson J. M. , Okuda H.
Published: February 2004

A three velocity and one-space dimensional nonrelativistic electromagnetic particle simulation code employing the fast Fourier transform algorithm is described and used to simulate the amplification of electromagnetic radiation by an electron beam passed over a rippled static magnetic field. In the beam frame the rippled magnetic field looks like an intense electromagnetic pump and thus a parametric instability can be produced. In one case, it was observed that 30% of the beam energy was converted to electromagnetic radiation.
Initial heating rate of a droplet in a spherical microwave cavity
Authors: Garelis E.
Published: February 2004

A simple analytical relationship for the Q of a resonant spherical cavity with a central droplet is derived assuming superconducting cavity walls. In this case Q = (ω × stored energy)/( (power loss) where power loss refers to the energy rate delivered to the central droplet. It is assumed that the skin depth δ is small compared with the droplet radius a, and that the wavelength, λ = 2πc/ω, be small compared with the cavity radius b, and large compared with the droplet radius. The equation is Q = (bc2) / (a2δω2). It is shown that in the case of conducting cavity walls a short wavelength minimizes the wall losses.
Stability criteria and the maximum growth rate in magnetohydrodynamics
Authors: Spies G. O.
Published: February 2004

Variation methods, originally developed for deriving stability criteria from the energy principle, are well suited for obtaining bounds for the maximum growth rate from a modified energy principle. An upper bound is derived which corresponds to a sufficient stability criterion, and a lower bound is derived which corresponds to a necessary criterion. Each of these bounds equals the actual maximum growth rate in special cases.
Wave-particle interactions in a beam-plasma system
Authors: Nyack C. A. , Christiansen P. J.
Published: February 2004

An experiment on large amplitude electrostatic waves is described, the results of which illustrate the phase velocity variations during the first bounce period predicted by the single-wave model. The spatial growth of sidebands, after the initial large amplitude wave has reached saturation due to nonlinear trapping effects, is also described. It is found that the amplitude of the phase velocity variation is proportional to φ1/2 (where φ is the wave amplitude) and that the slower phase velocity, higher-frequency sideband (for instability near the plasma frequency) is always the dominant one.
Threshold conditions for electron trapping by nonlinear plasma waves
Authors: Canosa José , Gazdag Jenö
Published: February 2004

Numerical solutions of the nonlinear Vlasov equation show that the long time behavior of an electrostatic plasma wave depends critically on the value of the parameter γτ (γ is Landau's damping coefficient and τ is the electron bounce time), both for large and small initial wave amplitudes. When the initial wave amplitude is large, the initial nonlinear damping is stronger than Landau damping; for smaller initial amplitudes, the behavior is initially described by Landau's theory. For both types of problems if γτ > 0.5, the wave damps monotonically; if γτ < 0.5, the numerical results indicate that the plasma performs amplitude oscillations. When γτ ≈ 0.5, the plasma displays its critical behavior; here, after some initial damping the wave amplitude levels off at a constant value. The critical behavior observed for mild nonlinear problems is in good agreement with the experimental results of Franklin, Hamberger, and Smith. For a moderately strong nonlinear wave displaying the critical behavior, the wave performs periodic oscillations with a frequency smaller than Landau's linear frequency; this frequency shift is explained because the uniform electron distribution develops a plateau centered at Landau's phase velocity.
Stability boundaries for temperature and density gradient flute modes in mirror machines
Authors: Abhyankar V. G. , Simon A.
Published: February 2004

A set of fluid equations is used to determine the stability boundaries for flute instability of a plasma in a nonuniform magnetic field, in the presence of simultaneous density (δ′) and temperature (λ′) gradients. The resulting surfaces in δ′, λ′, P, P space (P proportional to plasma density) show the various conditions leading to instability for either magnetic wells or ordinary mirrors. It is demonstrated that a modified form of the fluid equations given by Varma is most appropriate for high-energy injection plasma. A temperature gradient in the same direction and of the same magnitude as the magnetic field gradient is always destabilizing.
Modified Kadomtsev spectrum from renormalized plasma turbulence theory
Authors: Choi Duk-In , Horton Wendell
Published: February 2004

From the collisionless Vlasov-Poisson equations the renormalization of plasma turbulence is derived by a selective summing of secular terms to all orders in perturbation theory. The resulting renormalized theory agrees with earlier renormalizations of quasilinear theory and the mode coupling equation exhibits the same symmetries known in weak turbulence theory. The present formulation differs from that of Rudakov and Tsytovich by containing a separation of the turbulent k scales and by the associated feature of retaining the form of induced wave scattering in the nonresonant mode coupling terms. The theory is applied to the problem of finding the stationary spectrum for current driven ion-acoustic turbulence. Balancing the induced wave scattering off screened ions with a quasilinear growth rate, a modified Kadomtsev spectrum having a low kλD cut off is obtained self-consistently within the collisionless theory. The turbulent energy density and effective collision frequency are computed from the new spectrum.
Computer simulation of pulse trapping and pulse stacking of relativistic electron layers in astron
Authors: Byers J. A. , Holdren J. P. , Killeen J. , Langdon A. B. , Mirin A. A. , Rensink M. E. , Tull C. G.
Published: February 2004

During the past two years computer simulation has been used extensively to obtain a detailed description of pulse trapping and pulse stacking of relativistic E layers in astron. Resistors are essential to good trapping, in agreement with experiment. In the code, pulse trapping can easily be arranged to be 100% efficient—in marked contrast to the experiment. Details of pulse stacking are dependent on resistor configuration, degree of charge neutralization, and external well shape, but the field reversal increase invariably runs into a saturation due to axial expansion of the layer. This process can be described as a phase space exclusion or, alternatively, as nonadiabatic axial heating. The pulse-stacking process involves a tearing and bunching, and it is also nonadiabatic in the radial motion; as a consequence, radial expansion always occurs, and this can also act as a limitation to field reversal unless the resistor locations allow considerable radial room. Very tightly focused (axially) pulses can result from vacuum pulse trapping if system conditions are optimized correctly. This in turn lessens the axial expansion problem for vacuum layers. In contrast, the radial expansion problem is much more severe for vacuum layers, and this effect causes radial loss at some layer strength well short of field reversal. Large-current (2500 A) nuetralized pulses result in field reversal with one pulse.
Astron E-layer equilibria formed by injection
Authors: Lovelace R. V. , Fessenden T. J. , Sudan R. N. , Fleischmann H. H.
Published: February 2004

A study is made of the collisionless E-layer equilibria which are formed by the injection and trapping of a relativistic electron beam in a cylindrical conducting vessel. Assuming axisymmetry, information about the general E-layer equilibria is derived from the conservation of canonical angular momentum. Of particular interest for the design of experiments is the range of radial thicknesses and the average radius of the layers as a function of layer strength ζ. Under certain conditions there is shown to be a maximum radial thickness Δrmax, dependent only on ζ. For a number of practical cases Δrmax/r¯ ≈ ζ1/2, where r¯ is the average radius. The balance of axial pressures and over-all energy conservation are imposed as constraints on the general equilibria. For most of the cases studied these constraints are incompatible with the assumption of a radially thin E layer. In such cases the thin-layer equilibria are inaccessible, but it is expected that the thick-layer equilibria are accessible.
Intense relativistic nonneutral E-layers—Equilibrium theory
Authors: Davidson Ronald C. , Mahajan S. M.
Published: February 2004

Equilibrium properties of intense relativistic nonneutral E layers are studied within the framework of the steady-state Vlasov-Maxwell equations. The analysis is carried out for an infinitely long E layer aligned parallel to a uniform magnetic field B0êz, assuming that the E layer is thin (radial thickness is small in comparison with the mean radius R0) and ν/γ0 ≪ 1 (ν is Budker's parameter, and γ0mc2 is the electron energy). Equilibrium properties are calculated for the class of thin E-layer equilibria in which all electrons have the same canonical angular momentum Pθ, but an arbitrary distribution in energy H, i.e., fe0 (H, Pθ) = R0F (H + eφ¯) δ (Pθ − P0), where R0, φ¯0, and P0 are constants. One of the most important features of the analysis is that the influence of equilibrium self-fields on the radial thickness of the E layer can be large, even though ν/γ0 ≪ 1 is assumed and the self-field corrections to the mean radius of the E layer are small.
Effects of magnetic field gradients on strong relativistic electron rings
Authors: Meger R. A. , Lovelace R. V. , Woodall D. M. , Fleischmann H. H.
Published: February 2004

Experimental investigations on the RECE-Berta device at Cornell show that weak gradients in the applied magnetic field do not influence the stability of strong relativistic electron rings for field differences across the rings of up to 0.8%. The main effect of such gradients is a radial displacement of the equilibrium position of the rings toward the low-field region of the tank. The magnitude of this displacement depends on the field gradient, and the strength and geometry of the electron ring. A theory for the displaced electron ring equilibria is presented which assumes a balance of the forces due to the external field and the image fields arising from currents in the conducting tank wall. The theory shows good quantitative agreement with the observed ring movement. From these results it appears that the final “dump” of the rings can be explained by the existence of a gradient in the basic mirror field.
Electrical conductance of a confined arc discharge
Authors: Elliott C. James
Published: February 2004

The methodology for the computation of the conductance of a confined arc discharge has been established. A limiting case which describes the capillary argon ion laser at a filling pressure near 0.2 Torr, at a discharge tube radius of 1 mm, and at a discharge current of 7 A has been treated. The theoretical method utilizes a variational principle which resembles the standard approach of Robinson and Bernstein and Nighan. The derivation is carefully explained and applies to the case where the electron mean-free-path for momentum transfer is much larger than the radius of the discharge tube. The variational principle formulation has provided a convenient way of deriving computationally simple upper and lower bounds for the electron conductance of the discharge tube. These bounds are particularly important in light of the computation time now required for an accurate calculation.
Suppression of spatial waves by distortion of jet velocity profile
Authors: Chan Y. Y. , Templin J. T.
Published: February 2004

It is shown experimentally that the turbulent shear layer of a circular jet having a bell-shaped velocity profile cannot support a wave disturbance. The spatial growths of the axisymmetric pressure waves decay rapidly downstream from the nozzle exit for a wide range of forcing frequencies investigated.
Hydrodynamic stability of plane porous slip flow
Authors: Gersting John M.
Published: February 2004

The hydrodynamic stability of the flow between two plane porous plates with velocity slip at the plates is examined using a finite difference scheme. Results in the form of critical Reynolds number as a function of cross flow with velocity slip as a parameter are given.
Limits on the effective viscosities of polar fluids
Authors: Cowin S. C.
Published: February 2004

For a wide class of flows, limits on the effective bulk and shear viscosities of a polar fluid are obtained. It is shown that, under circumstances which cause the effective shear viscosity to increase, the effective bulk viscosity decreases.
Surface coupled interfacial waves for self-gravitating systems
Authors: Zahn Markus
Published: February 2004

Dispersion relations for self-gravitating systems are obtained using the direct duality with space charge coupled systems by taking electrohydrodynamic results and replacing the charge density with the mass density, the electric field with the gravitational field, and the permittivity by −1/4πG.
Improved sufficient conditions for magnetohydrodynamic stability with closed field lines
Authors: Nelson D. B. , Spies G. O.
Published: February 2004

A well-known sufficient condition for magnetohydrodynamic stability of toroidal equilibria with closed field lines is improved by taking into account the compressive energy in the energy principle. As a result, the condition also becomes necessary for stability in special cases.
Einstein relation for heating and acceleration during parametric instabilities
Authors: Weinstock J.
Published: February 2004

Two simple expressions are derived which relate the maximum and mean turbulent diffusion tensor to the anomalous dissipation rate during parametric instabilities. The results apply to conditions in which saturation is due to resonance broadening.
Electron trapping in a growing electrostatic wave
Authors: Hobbs W. E. , DeGroot J. S.
Published: February 2004

The orbits of a distribution of electrons, initially cold and uniformly distributed in space, driven by an exponentially growing electrostatic wave, have been followed numerically. It is found that the average field at trapping is characterized by ωB/ω ≈ π/4 (ωB2 = ekE/m) for growth rates λ/ω ≤ 14.
Beam-plasma instability in an inhomogeneous plasma
Authors: Bollinger L. D. , Carr W. , Liu H. , Seidl M.
Published: February 2004

Unstable waves (ω > ωce) are observed to be localized in the axial gradient of a beam-produced plasma. This results from an increase of phase velocity due to the gradient, which permits Landau damping by the beam.
Turbulent heating of ions in a toroidal experiment
Authors: Hirose A. , Mah S. Q. , Skarsgard H. M. , Strilchuk A. R.
Published: February 2004

Previous ion temperature measurements are interpreted in terms of the stochastic heating resulting from the finite correlation time τc ∼ 2π/ω) of the observed ion acoustic turbulence. The damping of the fluctuations appears to be provided largely by the ions.
Electronic nonequilibrium in a supersonic expansion of ionized gas
Authors: McGregor D. D. , Mitchner M.
Published: February 2004

An experimental and analytical study of nonequilibrium among the populations of the excited levels of potassium vapor passing through a supersonic nozzle is presented. In the experiment a small amount of potassium vapor was injected into a flow of a mixture of argon and helium, heated by an arc jet electric discharge to a temperature of approximately 2500°K at a pressure of 3 atm. The resulting plasma was passed through a supersonic nozzle (attaining Mach 2.2 at nozzle exit); the gas temperature dropped from 2500° to 1000°K in a time scale of 3 × 10−5sec. The electron temperature (though somewhat independent of the gas temperature) also decreased rapidly, causing the free electrons to recombine with ions of potassium. The recombined electrons passing through the energy level structure of potassium formed the basis of the electronic nonequilibrium. Spectrometric measurements (line intensity and line reversal) of the gas were performed upstream of the nozzle and at the nozzle exit. The electronic populations of expanded potassium vapor were observed to depart from equilibrium in qualitative accordance with theoretical predictions. Larger electron excitation cross sections (than the present theoretical ones) in the energy region near threshold would bring theoretical calculations of excited level populations and electron density at the nozzle exit into better agreement with measured values for these quantities.
Temperature measurements behind shock waves using a line radiation technique
Authors: Semerjian H. , Dobbins R. A.
Published: February 2004

A method of determining the temperature of hot gases is described that is based on the measurement of the intensity of radiation on the center of an emission line. The line is selected on the basis that it possesses blackbody intensity at the central frequency and that its width is large compared with the available spectral resolution of the optical equipment. With these conditions fulfilled, no information on the specific properties of the spectral line is required to recover temperature. The method is applied to determine the temperature behind a shock-wave propagating into xenon gas. The temperature of the shock-heated gas is calculated by Rankine-Hugoniot conditions based on the measured shock velocity. A comparison between measured and theoretical temperatures is presented, and it shows improved agreement at higher temperatures and pressures. The most serious limitation on the accuracy of our implementation of this experiment is considered to be the uncertainty of the shock velocity in the presence of moderate shock attenuation.
Stagnation-point flow in a rotating cylinder
Authors: Davis R. W. , Ludford G. S. S.
Published: February 2004

Linear theory is used to study the pure stagnation-point flow created inside a rotating cylinder by introducing fluid uniformly through the top surface and removing it uniformly through the side wall. Of main interest is the completely new role played by the E1/2 layer at the side wall, although the (unsteady) E1/4 and E1/3 layers also have novel features.
Spatial stability of some Falkner—Skan profiles with reversed flow
Authors: Taghavi H. , Wazzan A. R.
Published: February 2004

The Orr-Sommerfeld equation is solved numerically for the boundary layer profiles which are the solutions to Stewartson's branch of the Falkner-Skan equation. These profiles are of significance in describing post-separation flows with negative skin friction. The cases considered are for the Hartree pressure gradient parameter β = −0.18, −0.15, −0.10, and −0.05. The profiles become more stable as β decreases from -0.05 to -0.18 in contrast to the results of the Falkner-Skan similarity profiles with positive skin friction.
Experimental study of an aligned magneto-fluid dynamic flow
Authors: Pincosy P. A. , Ahlstrom Harlow G.
Published: February 2004

A magneto-fluid dynamic flow interaction in the shock tube flow of an electrically conducting test gas over a cone aligned with an applied magnetic field was experimentally observed, where RM = 0.25. For the variable applied field up to 0.9 W/m2, the maximum Ia = 0.50, and A varied from greater than to less than unity. Gas dynamic flow changes were not observed, but the measured unsteady induced magnetic field data effectively illustrated the role of diffusion in the propagation of the fast and slow magnetoacoustic waves. In addition, the direct observation of the fast magnetoacoustic wave was a very significant result.
Collisional parameters of a test particle and a streaming plasma
Authors: Siambis J. G. , Stitzer S. N.
Published: February 2004

The collisional interaction of a test-charged particle with a streaming Maxwellian distribution of field particles is considered analytically. Values are found for the collisional parameters of dynamical friction force, diffusion tensor, and rate of energy change for a wide range of field particle densities, temperatures, and streaming velocities. Retention of the velocity dependence of the Coulomb logarithm leads to substantial differences in the values of the parallel component of the diffusion tensor and the rate of energy change for the test particle as compared with the Spitzer values. Collision frequencies for momentum transfer, π/2 angular deflection, and energy change for the test particle and the test plasma are defined, simply related to each other, compared with the Spitzer definitions and evaluated for a simple case. The new definitions for the collision frequencies place in evidence the enhancement of collisional momentum and energy transfer in the absence of stationary thermal equilibrium.
Magnetic field dependence of plasma relaxation times
Authors: Montgomery David , Joyce Glenn , Turner Leaf
Published: February 2004

A previously derived Fokker-Planck collision integral for an electron plasma in a dc magnetic field is examined in the limit in which the Debye length is greater than the thermal gyroradius, which is in turn greater than the mean distance of closest approach. It is demonstrated that the collision integral can be satisfactorily approximated by the classical Landau value (which ignores the presence of a dc magnetic field) if the following replacement is made: In the Coulomb logarithm, the Debye length is replaced by the gyroradius. This induces a fundamental logarithmic dependence on magnetic field in the relaxation times. Numerical comparison of the asymptotic approximations with the previously derived exact results is made, and good agreement is found. The simplification this introduces into the description of collision processes in magnetized plasmas is considerable.
Nonlinear excitation of ion acoustic waves
Authors: Michelsen P. , Hirshfield J. L.
Published: February 2004

The excitation of ion acoustic waves by nonlinear coupling of two transverse magnetic waves generated in a microwave cavity was investigated. Measurements of the wave amplitude showed good agreement with calculations based on the Vlasov equation.
Absolute and convective ion beam instability studied through Green's function
Authors: Jensen V. O. , Michelsen P. , Hsuan Hulbert C. S.
Published: February 2004

A Vlasov plasma with a double-humped, unstable ion velocity distribution function is considered. A δ function in space is assumed as the initial perturbation and the plasma response to this perturbation is calculated, i.e., the Green's function for the problem is found. The response can be divided into two parts: a self-similar, damped part of the form t−1h(x/t), and an unstable, exponentially growing part. The conditions for absolute and convective growth of the latter are discussed.
Kinetic theory of plasma scrape-off in a divertor tokamak
Authors: Hinton F. L. , Hazeltine R. D.
Published: February 2004

The structure of the plasma boundary, in a tokamak with a poloidal divertor, is considered. The assumption, of hot, collisionless ions, and cold, collisional electrons is shown to be self-consistent. The density gradient length is shown to be of the order of ρpi, the ion poloidal gyroradius, at typical poloidal angles. Plasma diffusion across the separatrix, and particle motion along the field lines to the collector plates, are described by a kinetic equation. From the solution of the boundary-layer problem, the ion flux and ion energy flux, to the plates, are calculated. The ratio of these fluxes, and the corresponding ratio for the electrons, complete the specification of divertor boundary conditions, to be used in tokamak transport codes.
Ion cyclotron waves and fast waves in a toroidal cavity
Authors: Swanson D. G.
Published: February 2004

The propagation of ion cyclotron waves and fast waves in a toroidal cavity with a spatially varying static magnetic field as in a tokamak or stellarator is examined. Using a cold plasma model in a square cross section conducting toroidal cavity and neglecting effects of electron inertia, the wave equation is reduced to an ordinary differential equation which is numerically integrated subject to boundary conditions to find eigenmodes which represent cavity resonances. For the fast wave, only very small perturbations from the corresponding uniform plasma guide problem are found, and it is concluded that effects of toroidicity on the fast wave are not significant. For the ion cyclotron wave it is found that the region of propagation is generally confined to the high magnetic field side of the cyclotron resonance and for low order modes vertically and azimuthally, the region of propagation does not extend to the resonance location. A new type of wave behavior appears due to toroidicity which is similar to the ion cyclotron wave but depends differently on wavelength and may be important for fusion plasma heating because of its possibly greater accessibility. On the basis of the wave profiles, optimum antenna designs will be profoundly affected by toroidicity.
Probability distribution for Fourier components of the electric field in weak plasma turbulence theory
Authors: O'Neil T. M.
Published: February 2004

Probability distributions for Fourier components of the electric field, including joint distributions for various Fourier components and multiple time distributions for the same component, are determined using the central limit theorem of probability theory and two assumptions within the spirit of weak turbulence theory. The distributions are all Gaussians or simple integrals of Gaussians. This statistical framework is applied to the special case where the turbulence is dominated by the wave-particle interaction. In this case, quantities appearing in the distributions as parameters, such as the mean and standard deviation, are determined by quasilinear theory and Dupree's recent theory of phase space granulation, or clumps.
Numerical simulation studies of a turbulent heating experiment
Authors: Whitfield D. W. A. , Skarsgard H. M.
Published: February 2004

Electrostatic, one-dimensional, numerical plasma simulations have been used to study results from the University of Saskatchewan turbulent heating experiment. In a simulation plasma with an initial sinusoidal density perturbation (approximating the effect of a bumpy magnetic field parallel to the plasma current in the experiment), the application of a constant external-electric field can cause the electrons to split into two groups, one freely accelerating and the other trapped near zero velocity in a potential well resulting from the inhomogeneous ion density. The ensuing electron-electron instability can cause reduced acceleration and heating of the electrons, even when the ions are infinitely massive. This process appears to have a role in the experiment although close quantitative agreement cannot be claimed. A common and important feature of the simulations was the tendency of all observed instabilities to heat the electrons at a rate which maintained the ratio of electron drift to thermal velocity near unity. This heating rate has been observed in the experiment.
Observation of anomalous soft x-ray emission from a turbulently heated high voltage θ-pinch
Authors: Oktay Erol
Published: February 2004

Soft x-ray emission from the Maryland high voltage θ-pinch is measured as a function of two experimental parameters which in effect determines the electron density in the initial plasma. The x-ray intensity increases rapidly as the electron density is lowered; however, the electron temperature deduced from thin foil absorption of these x rays does not vary significantly. The absolute intensity of these about 3-keV x rays is orders of magnitude larger than the calculated intensity of bremsstrahlung radiation. To enhance x-ray emission, we put a small Cu target in the plasma, and measure absorption characteristics and intensity of these target x rays also. The time of x-ray emission correlates with those of neutron emission and implosion of magnetic field on the axis.
Synchrotron radiation spectrum for an intense relativistic E layer
Authors: Davidson Ronald C. , Mahajan S. M.
Published: February 2004

The synchrotron radiation spectrum is calculated for an intense relativistic nonneutral E layer including self-field effects. The analysis is carried out for the class of thin E-layer equilibria described self-consistently by equilibrium distribution functions of the form fe0 (x, p) = R0F(H)δ(Pθ − P0), where H is the energy, is the canonical angular momentum, and R0 and p0 are constants. Since individual electrons execute radial betatron oscillations about a helix with mean radius R0, the single-particle emission spectrum is modified from the conventional Schott-Trubnikov result. To determine the spectrum of radiation emitted by all electrons in the E layer, the single-particle emission spectrum is averaged over the distribution function fe0 (x, p) that self-consistently generates the E layer equilibrium.
Electromagnetic wavefunctions for parabolic plasma density profiles
Authors: Baũos Alfredo , Kelly Daniel L.
Published: February 2004

The one-dimensional Helmholtz equation w″ + k02g(z)w(z) = , k0 = 2π/λ0 , is solved making use of an extension of Langer's method. The function g(z) is restricted to the class of Epstein profiles, g(z) = g(0) + [1 − g(0)] tanh2(z/2λ) , where λ is the scale length, corresponding to no turning points, a quadratic turning point at z = 0, or a pair of turning points, depending on whether g(0) is positive, zero, or negative. This method yields uniformly valid solutions in terms of confluent hypergeometric functions. First, a solution w+(z) is obtained for 0 ≤ z which, as z → ∞, becomes the transmitted plane wave. Then, applying the method of analytic continuation, a linear combination is obtained w−(z) = wi(z) + wr(z) valid for z ≤ 0 which, as z → − ∞, yields the superposition of the incident and reflected plane waves. As an illustration of this method and as an application to parabolic density profiles, the mean energy density of the electric field is obtained numerically, in the vicinity of z = 0, when the parameter ratio λ/λ0 ≥ 100, and for the case of a single quadratic turning point and also for the case in which there are two neighboring turning points. With this choice of parameters, the results yield useful information on the tuning criterion for parametric excitation of the ionosphere.
Wavenumber surfaces of magnetoatmospheric waves
Authors: Yeh Tyan
Published: February 2004

Small-amplitude wave motion in a gravitationally stratified atmosphere permeated by a magnetic field consists, for each Fourier component with a certain frequency and horizontal wavenumber, of six quasiplane waves. Their mixture varies with height due to the continual interaction among these quasiplane waves. The simultaneous effects of compressibility, gravity, and a horizontal or a vertical magnetic field are shown by various wavenumber surfaces for decoupled plane waves.
Microscopic plasma Hamiltonian
Authors: Peng Y-K. M.
Published: February 2004

A Hamiltonian for the microscopic plasma model is derived from the Low Lagrangian after the dual roles of the generalized variables are taken into account. The resulting Hamilton equations are shown to agree with the Euler-Lagrange equations of the Low Lagrangian.
Asymptotic theory of nonlinear surface waves on a viscous fluid in an inclined channel of arbitrary cross section
Authors: Shen M. C. , Shih S. M.
Published: February 2004

An asymptotic theory is developed for the study of three-dimensional nonlinear surface waves on an incompressible, viscous fluid with surface tension in an inclined channel of arbitrary cross section. The method used here is based upon a multiparameter singular perturbation scheme within the framework of long-wave approximation. The nonlinear problem is reduced to a sequence of linear elliptic mixed boundary-value problems, which may be solved by means of known methods. Their solutions are then used to determine the wave speed and evolution equations governing the nonlinear wave motion. The results obtained give a quantitative description of a three-dimensinnal bore structure in an inclined channel of arbitrary cross section, and a critical Reynolds number is also defined as a criterion for the instability of the wave motion.
Longitudinal flow of a lenticular liquid filament down an inclined plane
Authors: Allen R. F. , Biggin C. M.
Published: February 2004

The theoretical analysis of the velocity distribution within a lenticular liquid filament flowing down an inclined plane is presented. The solution based on a series expansion, whose validity depends on the smallness of the filament's height to width ratio, is compared graphically with an exact numerical solution. The gas-liquid lenticular interface is determined in such a way that all constants are known directly in terms of measurable physical parameters.
Diffusion experiments with numerically integrated isotropic turbulence
Authors: Riley James J. , Patterson G. S.
Published: February 2004

Results are presented of computer simulations of particle diffusion on a numerically integrated, decaying isotropic turbulent flow field. The diffusion of both fluid particles and “small” spherical particles is studied. Comparisons are made of the various temporal autocorrelation coefficients. It is found that for “short” times, the Lagrangian (fluid point) velocity correlation is larger than the Eulerian correlation, while for longer times the opposite is true. It is also found in the “small” spherical particle case that as the response time of the particle is increased, the velocity autocorrelation coefficient increases.
Similarity laws for turbulent flow of dilute solutions of drag-reducing polymers
Authors: Huang T. T.
Published: February 2004

Velocity similarity laws based on a four-layer mean velocity profile model are deduced for turbulent boundary layers with dilute polymer solutions by means of pipe-flow experiments. Measured drag reduction is found to have three domains: Undersaturated, optimal, and oversaturated. The drag reduction does not increase with increasing concentration in the oversaturated domain where a strong interactive layer dominates the entire logarthmic region of the boundary layer. Drag reduction increases with increasing concentration in the undersaturated domain where the four-layer profile exists in the boundary layer. The boundary between the two domains gives optimal drag reduction, and is determined by the polymer type and concentration, and by a Reynolds number based on shear velocity and boundary-layer thickness. Pipe-flow experiments were made to study the drag-reduction characteristics in the undersaturated domain. The effects of solvent temperature, pipe diameter, polymer type and concentration, and wall shear stress on the measured drag reduction were investigated.
Supersonic flight in a stratified sheared atmosphere
Authors: Sirovich L. , Chong T. H.
Published: February 2004

The problem of steady supersonic level flight in a stratified and sheared atmosphere is considered. The flow away from the body is shown to be governed by a Burgers-type equation. Under the inviscid limit exact analytical solutions are obtained showing the effects of shearing, the temperature, and the pressure variation. The solutions are in agreement with all known results under appropriate conditions.
Gaussian model for fluctuation of a Brownian particle
Authors: Dufty James W.
Published: February 2004

The t−3/2 decay of the velocity autocorrelation function has recently been obtained from a non-Markovian Langevin equation by a number of authors. This approach is extended here to yield the probability distributions for both position and velocity of the Brownian particle. The terms leading to the t−3/2 decay in the velocity autocorrelation function are discussed with regard to spatial diffusion.
Structure of electron-fluid dynamical waves: Proforce waves
Authors: Fowler R. G. , Shelton G. A.
Published: February 2004

A general procedure has been developed for solving the equations governing the electron gas in a breakdown wave. The procedure has been carried out in detail for shock fronted waves. Profiles of the controlling variables are given. A comparison of the calculation of the wave velocity from first principles with the most recent experimental data show excellent agreement even without arbitrary normalization.
Propagation of laser supported deflagration waves
Authors: Boni A. A. , Su F. Y.
Published: February 2004

The propagation of subsonic waves supported by laser energy addition is considered. The propagation velocity is modified to account for radiation diffusion by vacuum ultraviolet photons. A simplified two-band radiation transport model is formulated.
Space charge coupled interfacial waves
Authors: Zahn Markus
Published: February 2004

The propagation and instability characteristics of small signal interfacial waves are examined for charged fluids in planar, cylindrical, and spherical equilibrium. The configurations discussed are electrohydrodynamic versions of the classic fluid mechanical problems of the Rayleigh-Taylor instability with superposed charged layers, of interfacial capillary oscillations of charged liquid cylinders, and extensions of Rayleigh's and Bohr's analysis of charged spherical drops under surface tension. In these geometries, a liquid which is either electrically perfectly conducting with surface charge or perfectly insulating and polarizable with volume charge is immersed within a uniformly charged perfectly insulating region. Using a general set of relations for perturbation field and flow variables defined for a “prototype” layer, the dispersion characteristics for these two limits are easily derived using interfacial boundary conditions and are compared in each of the geometries. For all cases, the system tends to be stabilized if the charges in each region are of opposite sign.
Nonlinear self-similar boundary-value problem for a divergent plasma flow
Authors: Wilhelm H. E.
Published: February 2004

A similarity transformation is given, which reduces the partial, nonlinear differential equations describing a compressible, polytropic plasma flow across an azimuthal magnetic field in a duct with plane inclined walls to an ordinary nonlinear differential equation of second order. The latter is solved rigorously in terms of a hyperelliptic integral. The form of the plasma flow fields in pure outflows (diffuser) is discussed analytically in dependence of the Reynolds (R) and Hartmann (H) numbers and the polytropic coefficient (γ) for given duct angles θ0. The realizable Mach numbers are shown to be eigenvalues of the nonlinear boundary-value problem, M = M(R, H, γ, θ0).
Image effects in bounded plasmas
Authors: Turner Leaf
Published: February 2004

Assuming thermodynamic equilibrium and using the BBGKY hierarchy, the configuration space and number density distributions of electrons and ions in a plasma bounded by a charged, perfectly conducting surface are derived. Modifying the conventional Debye-Hückel approach, the method of images is combined with the use of the Gibbs distribution to obtain new results. It is also shown that the addition of a magnetic field cannot produce any net current if thermodynamic equilibrium is assumed. A drift current is exactly canceled by the current produced by the variation in the number density of particles gyrating about their guiding centers.
Linear plasma oscillation described by superposition of normal modes
Authors: Pécseli H. L.
Published: February 2004

The existence of steady-state solutions to the linearized ion and electron Vlasov equation is demonstrated for longitudinal waves in an initially stable plasma. The evolution of an arbitrary initial perturbation can be described by superposition of these solutions. Some common approximations to the full set of equations can be solved in the same way. In some special cases, relevant, for instance, for single-ended Q machine experiments, a problem with given boundary conditions can be solved by superposition of normal modes.
Nonlinear wave-particle interaction in electrostatic waves
Authors: Martin Pablo
Published: February 2004

The unidimensional nonlinear Vlasov equation for electrostatic waves is consistently treated by a perturbation analysis in x-t space, without the usual approximation of small growth or damping rates. This analysis is correct through third-order terms in the field wave amplitude, and it includes the free streaming terms. Assuming the usual quasilinear approximations, a consistent second order analysis leads to a nonlinear damping decrement γ, which is the usual Landau solution γk of the time-dependent distribution function F0(v, t) plus an additional term (k∂ε∂ω)−1  ∫  ωp2k2 1υ − u ∂∂t (1υ − u ∂f0(υ, t)∂υ)dv. This term in the “bump-in-tail” instability increases with the wave amplitude or bounce frequency, it introduces a nonlinear frequency shift and it tends to stabilize the wave at lower saturation levels.
Investigation of ion acoustic waves in collisionless plasmas
Authors: Christoffersen G. B. , Jensen V. O. , Michelsen P.
Published: February 2004

The Green's functions for the linearized ion Vlasov equation with a given boundary value are derived. The propagation properties of ion acoustic waves are calculated by performing convolution integrals over the Green's functions. For Te/Ti less than about 3 it is concluded that the collective interaction is very weak and that the propagation properties are determined almost completely by freely streaming ions. The wave damping, being due to phase mixing, is determined by the width of the perturbed distribution function rather than by the slope of the undisturbed distribution function at the phase velocity as concluded from normal mode calculations.
Magnetohydrodynamic stability theory with closed magnetic field lines
Authors: Spies G.O.
Published: February 2004

Localized (i.e., Suydam-type) displacements are considered in the energy principle for arbitrary closed-line equilibria (constant rational rotational number). The resulting necessary stability condition is more pessimistic than any previously known necessary conditions for closed-line equilibria, but more optimistic than the low-shear limit of the corresponding condition for equilibria with shear. A discussion is given of the presently available stability conditions for general toroidal equilibria with or without shear and of their interrelations.
Nonlinear theory of absolute whistler instabilities in finite β plasmas
Authors: Jamin Eric , Parkinson Dennis , Rogister André , Bornatici Marino
Published: February 2004

The nonlinear evolution and stabilization of absolute whistler instabilities driven by electron pressure anisotropy and propagating obliquely to the external magnetic field in finite β plasmas are investigated without introducing the assumption of random phases. Both in the firehose and the mirror instability regimes the pressure anisotropy relaxes quasilinearly toward a more stable state. The nonlinear wave equation has also been obtained. By choosing the parameters in such a way that the plasma is sufficiently close to linear marginal stability, it has been possible to show both numerically and analytically that the (firehose) instability stabilizes nonlinearly at sufficiently low levels of excitation. The fluctuation level does not asymptotically approach a constant value, but oscillates in time, the maximum amplitude being proportional to the linear growth rate. Also, close to marginal stability, finite Larmor radius effects can play a significant role even in the limit kR ≪ 1. It is found that finite Larmor radius effects are stabilizing if tan2α > 4β‖/β⊥, where α is the angle between the direction of wave propagation and the direction of the background magnetic field and β‖(⊥) is the ratio of parallel (perpendicular) kinetic to magnetic pressure.
Two-dimensional study of electron-ion streaming instabilities
Authors: Lampe Martin , Haber Irving , Orens Joseph H. , Boris Jay P.
Published: February 2004

A two-dimensional study is presented, based on computer simulation, of strong electron-ion streaming instabilities induced by counterstreaming ion beams, for initially cold electrons (ve ≪ vd). Wave growth initially agrees well with linear theory. Higher harmonics appear later, in good agreement with mode coupling theory. The wave spectrum, initially very broad perpendicular to the streaming direction, narrows as the electrons heat and the waves saturate, in accordance with quasilinear theory. Electron heating is explained by nonresonant quasilinear diffusion at early times, and by resonant turbulent diffusion at later times. In contrast to previous one-dimensional studies, it is found that electron heating continues well after wave saturation and after ve > vd. However, the heating rate at late times is somewhat reduced by partial plateau formation in the electron velocity distribution. Coherent electron trapping is not seen, as it is in the one-dimensional situation, but coherent ion trapping does occur and dominates the wave dynamics at late times, terminating wave growth on the time scale studied.
Kink instabilities in a high-β tokamak with elliptic cross section
Authors: Freidberg J. P. , Haas F. A.
Published: February 2004

A toroidal, elliptic cross-section, sharp-boundary model of a high-pressure tokamak with currents confined to the surface, is tested for stability against kink modes. It is shown that the optimum configuration corresponds to a vertical ellipse in which the ratio of the major to minor axes is 2.2. For this case, the maximum critical β for stability against kink modes is β = 0.37a/R. For β > 0.37a/R the model is unstable for all values of the safety factor above and below the Kruskal-Shafranov limit.