bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuations in Monatomic Gases
Authors: Montgomery David
Published: February 2004

The problem of calculating distribution-function autocorrelations of the form 〈f(x1, v1, t1) f(x2, v2, t2)〉 is treated for a dilute monatomic gas. Two-time probability distributions of the type introduced by Rostoker for the plasma case are used. A perturbation expansion in the density is performed on the generalized BBGKY hierarchy which results. It is shown that the problem of determining the fluctuation spectra can be reduced to solving for a function which obeys the linearized kinetic equation for a dilute neutral gas with a particular choice of initial conditions, a result previously obtained by van Leeuwen and Yip, using diagrammatic perturbation theory. In the limit of infinite wavelengths and hard-sphere interactions, this equation reduces to the linearized Boltzmann equation.
Application of Maximum Likelihood Methods to Ion Swarm Experiments
Authors: Barnes William S.
Published: February 2004

A new device for performing ion swarm experiments is analyzed mathematically, and expressions for ion swarm distributions in this apparatus are derived. Means are presented for estimating the values of ionic drift velocities, diffusion coefficients (both parallel and perpendicular to the electric field), and ion-molecule reaction rate constants by the use of maximum likelihood estimators. Formulas for estimating the statistical fluctuations of the above estimates which are due to the statistical fluctuations of the ion populations are also presented. Methods for judging the appropriateness of the model used for both the experimental geometry and the ion transport process are discussed. For the purpose of illustrating the techniques, these methods are applied to several data samples for the ions H+ and H3+.
Harmonic Generation in Ionized Nitrogen
Authors: Varnum William S. , Desloge Edward A.
Published: February 2004

The effect of collisions on the generation of harmonics in ionized molecular nitrogen subjected to an alternating electric field is examined theoretically. The electron distribution function is approximated by a function containing four undetermined parameters. The Boltzmann transport equation is then used to obtain a set of kinetic equations for these parameters. The kinetic equations are used to determine the harmonic generation in a uniform ionized gas subjected to a homogeneous alternating electric field. The nonlinearity which must be present in the kinetic equations in order to produce harmonics is assumed to arise from velocity-dependent electron-molecule collision frequencies. It is shown that the principal harmonic generated is the third harmonic. An expression for the third harmonic current density is obtained. Available elastic and inelastic cross-section data are used to apply the general result to the case of ionized nitrogen, and the results are compared with two related microwave discharge experiments.
Ambipolar Diffusion in Radio-Frequency-Excited Magnetoplasmas
Authors: Robertson Harry S. , Herring John J.
Published: February 2004

A theoretical analysis of ambipolar diffusion in electrodeless, weakly ionized, collision dominated, radio-frequency-excited magnetized plasmas reveals an essential dependence of the steady-state density profile on the electron-density dependence of the net volume-ionization rate. In particular, if and only if the ionization rate is simply proportional to the electron density, the particle fluxes are ambipolar; otherwise, there exist divergenceless current cells for which the axial and radial current densities are not everywhere zero. Experimentally, it was found impossible to produce such a uniform plasma in argon, where the ionization process is two stage, and goes as n2, whereas the same method of excitation easily yielded a uniform cesium vapor plasma, for which the ionization process is proportional to n.
Motion of a Newly Formed Plasma in a Magnetic Field
Authors: Zinn John
Published: February 2004

Analytic expressions are derived for the motion of a hypothetical collisionless plasma which is created with isotropic velocities at time t = 0 within a uniform magnetic field. The electron and ion masses are assumed to be equal. It is found that the tendency of the particle magnetic moments to cancel the original B field is counteracted by macroscopic currents induced in the plasma through the changing field. The equations for the B field variation and for the macroscopic motion resemble the magnetohydrodynamic equations, with superimposed oscillatory terms of frequency ω and (ωp2 + ω2)1/2.
Collisional Damping of Plasma Oscillations
Authors: McBride John B.
Published: February 2004

The linearized kinetic equation with a uniform external magnetic field is solved for the perturbed electron distribution function. The collision term is assumed small, but otherwise is arbitrary. Using this solution, the general dispersion relation for electron waves is derived. One mode, longitudinal plasma oscillations along the magnetic field, is worked out in detail with the Landau (Fokker-Planck) collision term. An equivalent moment equation approach is also used, and it is found that the divergence of the heat flow tensor must be retained in the third-moment equation in order to obtain the correct first-order thermal correction to the damping rate. Both methods yield ωI = −12ωp(2/π)1/2(lnΛ/Λ)[1 − (2 − 4 2/5)k2λD2]. This damping rate decreases as k increases, and disagrees with earlier results based on the Landau equation. It appears that these previous treatments have not included the complete thermal correction.
Collisional Damping of Transverse Waves in a Plasma
Authors: McBride John B.
Published: February 2004

The Landau (Fokker-Planck) equation is used to calculate the temporal damping rate for the small amplitude, transverse electromagnetic mode (linearly polarized) for (a) no external fields, and (b) a uniform magnetic field. In (a) the damping rate is found by using a solution to the kinetic equation to evaluate the electron current. This damping may be recovered from the zero field limit of a previous result of Buti for the right circularly polarized mode after a correction is made in that result. In (b) a moment equation approach is used, and the heat flow tensor components are evaluated using a solution to the kinetic equation. The damping rate is ωI ≃ −12(2π)1/2ωp3ω02ln ΛΛ[1 − λD2k2ωp4ω02(ω02 − Ω02)2(2ω02 − Ω02) + 2λD2k2ωp25(ω02 − Ω02)2(5ω02 + Ω02 + 32 (ω02 + Ω02))], where ω02 = c2k2 + ωp2. The damping in (b) depends on the magnetic field only in the thermal correction, and reduces to that in (a) when magnetic effects are negligible.
Spatial Ion-Wave Echoes
Authors: Ikezi H. , Takahashi N. , Nishikawa K.
Published: February 2004

Theory and experiments of spatial ion-wave echoes are reported. In the collisionless approximation, the calculated line shape of the second-order echo is found to be a universal function of Δx/δ, the distance from the expected position of the echo divided by the Landau damping length of the ion-wave constituting the echo. Higher-order echoes predicted by Gould are observed up to the fourth order with respect to the external fields at the predicted positions with the predicted dependence of their amplitude on the amplitude of the primary waves. In addition, new types of higher-order echoes which result from the electric fields of other echoes are predicted and observed. Measurements of the peak amplitude of the second-order echo as a function of the separation l between the two exciters indicates that the memory of the past excitation retained in the distribution function damps exponentially with the attenuation length proportional to the wavelength of the excited wave. The effect of ion-neutral collisions on the peak amplitude is studied and explained fairly well by a simple relaxation time model, but the predicted effect of ion-ion collisions based on a Fokker-Planck-type equation is not observed within the separation length l of 20 cm and the plasma density of 1010/cm3.
Thomson Scatter Measurements of Magnetic Field Effects on the Landau Damping and Excitation of Plasma Waves
Authors: Fremouw E. J. , Petriceks J. , Perkins F. W.
Published: February 2004

Radar observations of Thomson scatter from the nonthermal, steady-state level of plasma waves in the ionosphere are used to test the predictions of kinetic theory with regard to how the nonthermal level of the waves depends on the angle between the wavevector and the magnetic field. Nonthermal effects are produced by a high-energy, photoelectron tail on the otherwise Maxwellian electron velocity distribution. Observations were made at several angles varying from nearly parallel to nearly perpendicular (where cyclotron resonance effects play a major role) to the magnetic field. The results are in consistent agreement with kinetic theory.
Optical Absorption and Expansion of Laser-Produced Plasmas
Authors: Dawson J. , Kaw P. , Green B.
Published: February 2004

Calculations of the absorption of laser radiation by an inhomogeneous overdense plasma have been carried out. Such a plasma is produced when a high-powered laser irradiates a solid particle. Nearly complete absorption is found as long as 2R is greater than c⊥ here, R is the scale length for density falloff, c is the velocity of light, and ⊥ is the electron-ion collision time for a plasma frequency equal to the incident laser frequency. For hydrogen this gives complete absorption up to temperatures of 5 keV if R is greater than 5 × 10−2 cm; higher Z materials absorb better. The expansion of a laser-produced plasma was also investigated. In the case of a plasma produced from a thin metallic film, it is found that the expansion preferentially takes place normal to the film which is in agreement with observation.
Axial Current Density Measurements in a Wide Unstable Plasma Column
Authors: Ventrice Carl A. , Smith Nicholas K.
Published: February 2004

Axial current density measurements are made in an argon plasma column by means of a multiring composite anode. The existence of an instability in a column which has an L/R of six is confirmed. The critical magnetic field Bc is determined and found to depend on the neutral gas pressure pg in proportion to pg0.45. In all cases, when B ≤ 1.36Bc, a reversion is observed to begin, back to a configuration similar to that which exists when B < Bc. When B < Bc, the axial current density on the tube axis is observed to increase with increasing B, indicating that a “modified Schottky” theory must apply.
Collisionless Tearing Mode Instabilities in a High-β Theta Pinch
Authors: Freidberg J. P. , Morse R. L.
Published: February 2004

The stability of a high-β theta pinch against tearing modes is investigated using the Vlasov equations in both slab and cylindrical geometries. For the slab case it is shown that necessary and sufficient conditions for stability of one-dimensional equilibria is that the magnetic field have no field reversal. For the cylindrical case the stability condition is more stringent; that is, all field configurations with reversals are unstable. In addition, because of the destabilizing effect of rotation, the magnetic field must be sufficiently positive biased in order to achieve stability. When interpreted in terms of the density profile, the stability criterion is equivalent to the condition that profiles with a density maximum on axis are stable while those with a density maximum off axis are unstable.
Macroscopic Quasilinear Theory of High-Frequency Radiation in a Cold Plasma
Authors: Taussig Robert T.
Published: February 2004

The nonlinear fluid equations for a cold electron-proton plasma are averaged over short time and length scales to obtain high-frequency “radiation” terms as well as the usual macroscopic electromagnetic fields. The result is a self-consistent, quasilinear theory in which the radiation terms can be calculated directly from the plasma equations and Maxwell's equations without reference to an independent radiative transfer equation. Mechanisms for the nonlinear saturation of the absorption of resonant modes of the radiation field can be included in the formulation by introducing a small, finite temperature. When the plasma is steady, uniform, and nonstreaming, with no applied fields, it is possible to derive a radiative intensity function which includes the collective effects of the plasma and obeys an integral form of the classical radiative transfer equation. However, in general there are averaged, high-frequency terms which have no counterpart in the usual radiation theory. These new radiation terms come from waves in the presence of applied magnetic fields or macroscopic nonuniformities in the plasma.
One-Dimensional Free Expansion of a Collisionless Plasma
Authors: Hiroshige Yasushi , Yang H. T.
Published: February 2004

The free expansion of a plasma is usually pictured as a combination of free-streaming ions and an oscillating electron fluid. This conjecture is tested for the one-dimensional expansion of a collisionless plasma initially in thermal equilibrium and confined between two perfectly reflecting planes and then with the right one removed. Two methods of solution are used. The first is an approximate method based on Vlasov's equations and the second is a computer experiment employing the sheet model. The two methods show good agreement in the zeroth-order motion to the end of the computer run which is about 4 electron plasma periods or 0.75 ion plasma period for the mass ratio used. The behavior of the ion follows the center of mass of the plasma which is acted on by the sum of electron and ion pressures. The electron follows a damped sine wave at the electron plasma frequency, with the amount of damping dependent upon the decrease in the pressure at the left boundary.
Kovasznay's Spectral Theory of Turbulence
Authors: Panchev S.
Published: February 2004

A generalization of the classical Kovasznay's spectral theory of turbulence is suggested which leads to energy spectrum without cutoff value, decreasing fast enough in the viscous subrange.
Three-Dimensional Laminar Jet Mixing
Authors: Pai S. I. , Hsieh T. Y.
Published: February 2004

The laminar jet mixing of an incompressible and viscous fluid issuing from a nozzle of rectangular cross section into a uniform stream has been studied theoretically for the case where jet velocity deviates slightly from that of the free stream.
Heat Transfer in Plane Couette Flow of a Rarefied Gas using Bhatnagar-Gross-Krook Model
Authors: Bhatnagar P. L. , Srivastava M. P.
Published: February 2004

Using the linearized BGK model and the method of moments of half-range distribution functions the temperature jumps at two plates are determined, and it is found that the results are in fair agreement with those of Gross and Ziering, and Ziering.
Numerical Solution of the Lorentz Force Equation by High-Order Integration
Authors: Faulkner James E.
Published: February 2004

Methods are developed to generate a Taylor series expansion to any desired order for the Lorentz force equation in order to follow a charged particle orbit for long periods of time. These methods are applied to a magnetic well geometry.
Ion Flute Mode with Temperature Gradients
Authors: Malik S. K.
Published: February 2004

The effect of temperature gradients on the ion flute mode is investigated using the slab model with periodic varying gravity to simulate the effect of magnetic curvature and the condition for the instability is obtained.
Observation of Fast Neutrals Projected from a Coaxial Gun
Authors: Gloersen Per
Published: February 2004

Xenon atom and ion densities were measured downstream from a coaxial plasma gun using vacuum ultraviolet emission, absorption spectroscopy, and a Langmuir probe. A pulse of fast neutrals was observed to emerge, in addition to a faster pulse of ions.
Physical Mechanism for the Collisionless Drift Wave Instability
Authors: Meade D. M.
Published: February 2004

The growth rate for the collisionless drift instability is calculated in a manner which elucidates the physical mechanism for the instability.
Stability of a Horizontal Fluid Layer with Zero Shear Boundaries
Authors: Goldstein R. J. , Graham D. J.
Published: February 2004

The onset of thermal convection in a horizontal fluid layer heated from below and bounded, above and below, by zero shear boundaries is studied experimentally. The measured value of the critical Rayleigh number is in reasonable agreement with the closed form solution obtained by Rayleigh and is in good agreement with that expected for the actual thermal boundary conditions experienced in the laboratory.
Experiments on the Stability of Viscous Flow between Eccentric Rotating Cylinders
Authors: Versteegen Peter L. , Jankowski Daniel F.
Published: February 2004

The hydrodynamic stability of viscous flow between eccentric rotating cylinders has been studied experimentally. The ratio of the radii of the cylinders was 1 :2; several eccentricities were investigated. In the concentric configuration good agreement was achieved with previous experimental and theoretical work. Results obtained with the cylinders rotating in the same direction show that the effect of eccentricity can be stabilizing or destabilizing depending upon the speed of the outer cylinder and the amount of eccentricity. In the counterrotating case, a new flow phenomenon different from the usual vortex pattern was observed for large eccentricity.
Experiments on Finite-Disk Generated Gravity Modes
Authors: Kaiser Jack A. C. , Murty T. S.
Published: February 2004

Experimental determination of the frequencies of the fundamental axisymmetric gravity mode in a nonrotaing flat-bottom circular cylinder has been made for a series of forcing disk sizes with the aim of extrapolation to infinitesimal disk size. Two different resonance criteria, either maximum amplitude of the mode at the rim of the basin or minimum phase-lag between the water and the forcing disk at the center have been used. The periods determined in this manner showed systematic differences. A comparison of these experimental values with the theoretical values for an equivalent annulus showed that the water below the disk behaves more or less as an isolated material region.
Reflection of Acoustic Pulses from Stable and Instable Interfaces between Moving Fluids
Authors: Friedland Allan B. , Pierce Allan D.
Published: February 2004

Cagniard's method is used to find the transient solution for the reflection of line-source generated waves from an interface between two fluid half-spaces in relative motion, but with the same density and sound speed. The resulting solution for the reflected wave in the (normally motionless) lower half-space may be conveniently considered as the sum of a specularly reflected wave, a refracted arrival wave, a neutral stability wave comprising one or more resonance waves, and an instability wave. Critical angles limiting the appropriate domains of reception are derived for each wave type as a function of the Mach number M of the fluid motion in the upper half-space. The instability wave appears in the solution when M < 2 2. This feature may possibly represent the prediction of the spatial extent of a turbulent region arising from an inherent instability of the ambient medium which has been triggered by the arival of the incident pulse at the interface.
Electrohydrodynamic Rayleigh-Taylor Bulk Instability
Authors: Turnbull Robert J. , Melcher James R.
Published: February 2004

A stability criterion is found for an initially static, stratified fluid subject to an electric stress. The equilibrium fluid density, permittivity, viscosity, and space charge distributions are functions of the vertical spatial coordinate, with gradients directed parallel to the imposed electric field intensity and gravitational acceleration. In the limit where the fluid is perfectly insulating, sufficient conditions for stability are found; the principle of exchange of stabilities is shown, and variational principles derived for the eigenfrequencies and, in the case of no space charge, for the critical field strength. An experiment demonstrates instability in the dielectrophoretic limit of no bulk free charge. The property gradients are induced thermally, and incipience of instability, as measured by the Schmidt-Milverton heat-transfer technique, is successfully predicted by the theoretical criterion.
Oblique Hydromagnetic Waves in a Cold Plasma
Authors: Mizutani Akira , Taniuti Tosiya
Published: February 2004

Steady, one-dimensional, oblique, hydromagnetic waves of small but finite amplitudes in a cold plasma are considered. Analysis is based on an equation derived by Saffman and is restricted to a class of the waves with a large critical Mach number and high-frequency oscillations. Solutions are given in an asymptotic expansion. In the lowest order of perturbation the equation is integrated to give the solutions in closed form, which exhibit compressional solitary waves.
Electron Beam from a Magnetoplasmadynamic Arc
Authors: Ducati Adriano C. , Jahn Robert G.
Published: February 2004

A long, tenuous, but well-collimated beam of luminosity is observed to emerge from the plume of a magnetoplasmadynamic arc operated in a fiberglass vacuum tank. Examination of the beam trajectory in the magnetic field of the earth, and in other externally imposed fields indicates that the beam consists of a stream of 100-V electrons emitted from the core of the arc, which excite background gas particles to luminosity along their path to the tank wall. Although this beam carries a negligible portion of the arc current, the particle energy corresponds to some two-thirds of the arc voltage, and the beam is capable of damaging material surfaces which it encounters. Formation and emission of such a well-collimated stream of electrons of this energy seems incompatible with present concepts of magnetoplasmadynamic arc structure, and may have implications for space thruster applications.
Effect of Radiation upon the Post-Shock State
Authors: Koch Paul , Gross Robert A.
Published: February 2004

The effect of radiation upon the state of the gas behind a strong shock wave is studied. The equations of radiation gas dynamics are used, and it is shown that the two characteristic times, τ the time for radiative cooling, and λ/c, the mean time of photon flight, determine whether a strong shock wave is optically thin. The steady-state shock jump conditions for hydrogen are determined for plasmas which are optically thin, and they are compared with the Saha equilibrium solutions. The temperature decay, plasma velocity, and density behind strong shock waves are determined for the case of an optically thin bremsstrahlung radiating plasma. The time-dependent equations of the motion of a fluid interacting with radiation are examined and the radiative terms in these equations are presented as part of a general solution of the time-dependent radiative transfer equation.
Precursors Ahead of Shock Waves: II. Photoionization
Authors: Holmes Lawrence B. , Weymann Helmut D.
Published: February 2004

Experimental investigations of precursors ahead of pressure driven shock waves in argon show that for the existing conditions photoionization is the dominant cause for the precursor. A theoretical analysis of the electron temperature and density is presented.
Precursor Vacuum Ultraviolet Radiation in a T-Type Shock Tube
Authors: Greig J. R. , Palumbo G.
Published: February 2004

Using a small bore (1.5 cm) T tube, measurements have been made of the flux of vacuum ultraviolet radiation along the axis of the expansion tube. These measurements coupled with measurements of the precursor emission in the line He i 5876 Å indicate that ionizing radiation does not contribute significantly to precursor ionization and excitation.
Precursor Observations in Electrical Shock Tubes
Authors: Russell Geoffrey
Published: February 2004

An experiment is discussed which demonstrates the separation of the photon induced precursor and electron produced wave from an electrically driven shock tube. The velocity of the fluid-dynamical electron wave is of the order of 108 cm/sec and a plot of its velocity as a function of driven voltage is given.
Shock and Current-Sheet Separation in Magnetic Shock Tubes
Authors: Sorrell F. Y.
Published: February 2004

The problem of separation of a shock wave from the current driving it, is experimentally investigated over a wide range of Mach numbers in a single gas. In order to do this, a parallel plate shock tube was developed which produces repeatable shocks in hydrogen from Mach 20-180 at initial pressures from 75-460 μHg. In the present work, no preheating or external magnetic fields were used and the shock is completely collision dominated. The shock-wave and current-sheet trajectories were determined by piezoelectric pressure probes and magnetic pickup loops, respectively, and separation of the shock wave from the driving current was measured directly at speeds above 8.0 cm/μsec and initial pressures above 250 μHg. Experiments were performed at speeds up to 22 cm/μsec at lower pressures, but separation was not observed at these conditions. The electron temperature behind the shock and within the current layer was determined by using the ratio of continuum intensities across the Balmer series limit, and these temperature measurements were used in connection with the other data to explain most of the observed results. The data are also used to examine the applicability of the skindepth model and shock-formation theory in predicting separation in magnetic shock tubes.
Acoustic Wave Propagation in a Gas Discharge
Authors: Schulz Michael , Ingard Uno
Published: February 2004

When ordinary sound propagates in a weakly ionized gas, the wave motion of neutral particles controls that of charge carriers. The ion-density, electric-field, and electron-temperature responses to a given gas-density perturbation are calculated by means of linearized hydrodynamic equations for the case of wave propagation along the axis of a discharge. The equations include ionizing collisions, ambipolar diffusion, wall recombination, and collisions leading to intercomponent momentum and energy transfer. Because of competing effects of ions and electrons, no observable wave amplification can result from intercomponent momentum transfer. In addition to previously reported consequences of elastic collisions, the calculated dispersion relation contains an amplification term characterized by a time constant τν which is approximately the reciprocal ionization frequency (per electron) times the gas-to-plasma pressure ratio.
Electrostatic Instability in Plasma Due to an Anisotropic Ion Velocity Distribution
Authors: Grewal Mahesh S.
Published: February 2004

An electrostatic instability in an infinite uniform homogeneous plasma imbedded in a uniform magnetic field is discussed. The instability is jointly driven by (a) the difference in θ⊥−i and θ‖−i, the ion temperatures perpendicular and parallel to the applied magnetic field, and (b) the non-Maxwellian nature of the distribution of the ion velocity perpendicular to the magnetic field. The results show that for the instability to occur θ⊥−i must be greater than about seven times θ‖−i and the square of the ion plasma frequency must be greater than about 0.35 times the square of the ion gyrofrequency.
Dynamic Stabilization of the Theta Pinch
Authors: Wesson J. A. , Haas F. A.
Published: February 2004

Previous work has shown the β = 1 theta pinch to be stable for the mode m = 1 and unstable for m > 1. It is shown that a periodic theta pinch having a radius R(z) = R0 [1 + δf(z)], where δ ≪ 1, can be dynamically stabilized for m ≪ 1/δ.
Penetration of a High-Frequency Field in a Low-Pressure Plasma
Authors: Kofoid M. J. , Cleva H. V.
Published: February 2004

Comparisons between calculated and directly measured values of radial penetration of axially directed magnetic fields in cylindrical plasmas have been effected. Using elementary theory the curve of field intensity vs radius was calculated which matched the measured intensity. The level of conductivity demanded by theory was then compared with that deduced directly from the measured ne and Te. When the experimental conditions corresponded closely to those assumed in the elementary theory development, the conductivities agreed well. The investigation was made in 5-cm-diam noble gas discharges at 5-500 μ with steady 500 kc/sec fields of 50-80 G at the plasma surface. Te and ne were measured vs radius; the ranges of values were Te = 2-12 eV and ne = 1011-1014 per cm3. The penetrations measured were those of the fields producing the plasmas.
Anomalous Skin Effect in a Gaseous Plasma
Authors: Kofoid M. J.
Published: February 2004

Experimental data are presented for the skin effect in a plasma in which the electron thermal motions are sufficient to increase the field penetration. The results are consistent with the Weibel criteria for the existence of anomalous penetration and with the theoretical anomalous penetration depth.
Structure of and Particle Orbits in Neutral Plasma Sheaths
Authors: Enoch Jacob
Published: February 2004

Self-consistent solutions of the Vlasov equations for a locally neutral plasma sheath are obtained for three cases: (a) a field-reversed confined sheath, (b) a sinusoidal magnetic field solution, and (c) a non-field-reversed confined sheath solution. In the first two cases exact analytic solutions are obtained; in the third an approximation is obtained for the interior structure of the sheath. Particle trajectories are calculated by two different methods for the interior region of the non-field-reversed sheath.
Ion Cyclotron Instability of Energetic Plasma Confined in Magnetic Mirrors in the Model C Stellarator
Authors: Brown I. G. , Rothman M. A. , Sinclair R. M.
Published: February 2004

A portion of the plasma in the Model C stellarator is heated by ion cyclotron resonance heating and contained within magnetic mirrors (1.02 ≤ R ≤ 1.3, B0 ≈ 35 kG). The plasma so formed (ne ≈ 1012-1013 cm−3) has little interaction with the main body of toroidal plasma, and has perpendicular ion temperature up to 4 keV while the electron temperature remains at less than 100 eV. New results show that the cooling of this energetic plasma, lasting several milliseconds, is accompanied by emission of radiation at frequencies close to the local ion cyclotron frequency and its harmonics. Experimental results are presented of observations of this rf activity, and related to theoretically predicted instabilities of the ion cyclotron type. It is concluded that the observed behavior can be satisfactorily explained by the presence of an electrostatic, temperature-anisotropy-driven instability. There is no correlation between the rf activity and the cooling time of the mirror-contained plasma; the containment time still seems to be limited by charge exchange.
Temporal Echoes in Collision-Free Plasmas in a Magnetic Field
Authors: Porkolab Miklos
Published: February 2004

It is shown that in a magnetic field the frequency spectrum of temporal echoes is shifted by harmonics of the cyclotron frequency even though the echo times are unaffected.
Numerical Study of the Effect of Surface Tension on Interface Instability
Authors: Daly Bart J.
Published: February 2004

A new technique for including the effect of surface tension in time-dependent, incompressible flow calculations is used to examine the linear and nonlinear phases of Rayleigh-Taylor instability. The variation of the linear growth rate with the surface tension coefficient and (for a fixed coefficient) with the wavenumber of the perturbation is shown to be in good agreement with Chandrasekhar's analytic prediction. In the nonlinear regime, it is shown how surface tension affects the growth of the Rayleigh-Taylor spike and provides the mechanism for drop separation from the spike.
Spectra of Grid Turbulence
Authors: Uberoi Mahinder S. , Wallis Stanley
Published: February 2004

Energy spectra of the longitudinal and lateral turbulent velocities are measured behind grids of different geometries; the isotropic relation is not satisfied. Energy-dissipating eddies are more nearly isotropic than energy-containing eddies. Spectra of the energy-containing eddies and their deviation from isotropy depend on the grid geometry. Various microscales are measured to determine the anisotropy of the energy-dissipating eddies. The error committed in using the isotropic relation for energy decay is determined from the measurements.
Spectra of Turbulence in Wakes behind Circular Cylinders
Authors: Uberoi Mahinder S. , Freymuth Peter
Published: February 2004

One-dimensional spectra of turbulent wakes behind circular cylinders are measured in the range of 50-800 diam downstream of the cylinders and for a Reynolds number range of 320-95 000. The spectra of large-scale (low-wavenumber) turbulence are anisotropic and dynamically similar. The small-scale turbulence is locally isotropic and an inertial subrange was found for large Reynolds numbers. A proposed formula describes the spectra over the entire wavenumber range and for all Reynolds numbers.
Average Velocity Distribution of Turbulent Pipe Flow with Emphasis on the Viscous Sublayer
Authors: Lindgren E. Rune , Chao Junnling
Published: February 2004

A comprehensive experimental investigation is presented of the average velocity distribution of fully developed, smooth turbulent flows through straight pipes of circular cross section with special regard to the distribution within the viscous sublayer, including data at zero distance from the wall. The measurements seem to remove uncertainties and discrepancies inherent from earlier investigations. Current concepts of the universal velocity distribution are confirmed and verified.
Boundary-Layer Analysis of Polarization in Electrodialysis in a Two-Dimensional Laminar Flow
Authors: Solan A. , Winograd Y.
Published: February 2004

The problem of ion transfer in the fluid phase of an electrodialysis stack with parallel flow is considered for two extreme cases: (a) undisturbed laminar flow with purely diffusive mixing; and (b) perfectly mixed flow. Case (a) leads to a thin concentration boundary layer imbedded in the flow near the membrane surface. It is shown that for practically relevant parameters the solution may be approximated by a closed-form similarity solution, and this is compared with the more exact numerical solution. Case (b), which represents the ideal situation, is presented for comparison.
Heat Transfer to Cylinders in Nitrogen-Helium and Nitrogen-Neon Mixtures
Authors: Baccaglini G. , Kassoy D. R. , Libby Paul A.
Published: February 2004

The results of an experimental and theoretical investigation of the heat loss from cylinders at low Reynolds numbers in binary mixtures of nitrogen-helium and nitrogen-neon are presented. The experimental data confirm the previous results for nitrogen-helium mixtures. Because neon exhibits poor thermal accommodation on tungsten, the heat loss in nitrogen-neon mixtures qualitatively involves the same slip effects as does helium. It is shown that an extension to binary gases of the calculation of Clarke and McChesney for the slip condition in pure gases properly accounts for the nitrogen-neon results, but does not do so for the nitrogen-helium results. An explanation is suggested.
Desorption Rates of CO2-Ar Mixtures from Silica Gel at Various Temperatures
Authors: Penner S. S. , McElmury S. S.
Published: February 2004

Experimental measurements have been performed on equilibrium adsorption and desorption rates of CO2-Ar mixtures from silica gel at various temperatures by using a gas-dynamic technique for varying the pressure above the adsorbent bed. Simultaneous measurements of transmission of radiation (in the 4.3-μ region) and of chamber pressure, as functions of time, yielded complete histories of the time-dependent desorption processes. Reduction of observed results by the use of a simplified theoretical analysis shows that changes in surface diffusion coefficients are associated with time-dependent species compositions above the silica gel. The results of our dynamic measurements on the dependence of adsorption on vapor pressure suggest that the surface diffusion coefficient first increases and then decreases with pressure at constant temperature. An estimate of the activation energy for surface diffusion of CO2 in binary mixtures is somewhat larger (4000 cal/mole instead of 3000 cal/mole) than Damköhler's result derived from data of Berl and Weingaertner on pure CO2.
New Results in the Theory of the Classical Electron Gas
Authors: Cohen E. G. D. , Murphy T. J.
Published: February 2004

Three new coefficients are calculated for the expansion of the potential of average force of the classical electron gas in powers of the plasma parameter ε and its logarithm, and it is shown that there are divergences in the coefficients of some of the higher-order terms in this expansion. The asymptotic long-range behavior of the potential of average force is found to be −c1 ε exp (−c2 ξ)/ξ, with ξ the distance between two electrons in units of the Debye length, c1 = 1 + (38 ln 3 + 16) ε + ..., and c2 = 1 + (ln 3) ε/8 + .... Finally, the internal potential energy of the electron gas is calculated through order ε3. The theoretical results agree well with the Monte Carlo computations of Brush, Sahlin, and Teller up to about ε = 1.
Incompressible Magnetohydrodynamic Entrance Flow in a Plane Channel
Authors: Saric William S. , Touryan Kenell J.
Published: February 2004

The hydromagnetic flow of a conducting, incompressible fluid in the entrance section of a plane channel is analyzed. The problem is generalized to include a small cross-flow, small wall injection, Hall currents, and ion slip. The solution is carried out using two methods. The first is the generalized Kármán-Pohlhausen momentum integral method, and the second utilizes asymptotic power series in the small parameter ξ = (x/LRe)1/2. The equations are solved numerically, and the two results compared with one another, as well as with similar studies in the literature.
Experiments and Calculations in an Inverse Pinch
Authors: Hoffman Alan L.
Published: February 2004

Experiments on the production of collisional shocks in an inverse pinch magnetohydrodynamic shock producing device are described. These experiments are related to the numerical techniques in an accompanying paper in order to explain the observed phenomena. The position of a shock in the rear of a current sheet driven through argon is accounted for by the calculation of relevant plasma parameters and their comparison to previous similarity solutions. The forward position of a shock in helium and hydrogen is similarily explained. The effect of shock produced dissociation in hydrogen is observed by its effect on the gas conductivity and current sheet diffusion. The detailed influence of Mach number and magnetic Reynolds number on shock position is examined experimentally, and methods are proposed for successfully producing high-speed separated shocks.
Experimental Study of the Structure of Plasma Shock Waves in a Fast θ Pinch
Authors: Stamper J. A. , DeSilva A. W.
Published: February 2004

The density, temperature, and magnetic field structures of plasma shock fronts have been observed by means of scattered light, a magnetic probe, and emission spectroscopy. The shocks and initial hydrogen plasma were generated by capacitor discharges through a single-turn coil placed around a cylindrical quartz tube containing hydrogen. Electron velocity distributions (density and temperature) were obtained by analyzing the light scattered at 90° to the incident beam from a Q-switched ruby laser. The observed temperature structure was compared with that calculated from observed particle and current density by assuming local resistive dissipation due to binary electron-ion collisions. Agreement was good in two cases, but, in a case characterized by lower initial ωceτ(0.6), the velocity distribution developed enhanced wings and enhanced heating occured. These effects indicate a diffusion of high-energy electrons from the hot, downstream region. Ion temperatures, obtained from Doppler broadening of a carbon line, are consistent with compressional heating and thermal conduction across field lines.
Stationary Electric Arc in a Cross-Flow and Transverse Magnetic Field
Authors: Hodnett P. F.
Published: February 2004

An electric are is held stationary against a gas flowing transverse to the arc axis by applying a magnetic field perpendicular both to the arc axis and the direction of the gas flow. A theoretical analysis of the problem is undertaken for the case where the arc is sufficiently long for a central region of the arc (called the uniform column) to exist where a two-dimensional treatment, in the plane of the gas flow, is adequate. The Reynolds number of the flow is taken to be small compared with one. The technique of matched asymptotic expansions is used to calculate the two-dimensional distributions of the quantities of interest, e.g., the temperature and velocity of the gas.
Expansion of a Plasma Shell into a Vacuum Magnetic Field
Authors: Poukey J. W.
Published: February 2004

The problem of an initially spherical, superconducting plasma shell expanding into an ambient uniform magnetic field is solved by approximate analytic and numerical methods. The solution is found to depend on only one parameter, the ratio of the initial kinetic energy of the plasma to the magnetic energy initially displaced by the plasma. The dependence of the solutions on this parameter is studied. A criterion for the stability of the solutions is given.
Hydromagnetic Coupling Oscillations and Drift Instabilities in Nonuniform, Collisionless Plasmas
Authors: Tamao Tsutomu
Published: February 2004

The thermal effects of resonant coupling hydromagnetic oscillations in inhomogeneous, finite-β plasmas, are studied. There are two thermal effects for such oscillations, the thermal phase mixing due to wave-particle resonance interactions and the drift effect arising from plasma inhomogeneities. Applying the drift kinetic approximation, the basic equations for the coupling modes between the Alfvén and magnetosonic waves in a nonuniform, collisionless plasma are obtained. For the plasma with β ≃ me/mi, ratio of electron to ion mass, it is shown that the electron thermal dissipation of hydromagnetic oscillations with ω/k‖ ≃ VA is most effective within the resonance coupling regions which appear in the cold plasma limit. This dissipation is very large as compared with the same dissipation process in the homogeneous plasma. On the other hand, if the hot plasma with β ∼ 1 is considered, attenuation of hydromagnetic waves due to ion-wave interaction becomes important. It will be an important process for ion heating, and the relevant equations are also derived. Next, drift instabilities in the long-wavelength limit are discussed for the most general case, including the density, temperature, and magnetic field gradients, as well as the ion Larmor radius effect. In particular, the slow mode of drift waves is unstable when Te′B0′ > 0  or Te′N0′ < 0, where prime means the spatial derivative in the radial direction. Finally, discussions of application to the magnetospheric plasma phenomena, which will be associated with inhomogeneities of the thermal constituents, are qualitatively given.
Nonlinear Dynamics of a Single Harmonic Loss-Cone Flute Mode
Authors: Aamodt R. E. , Bodner S. E.
Published: February 2004

The nonlinear evolution of a single-wavelength, longitudinal flute mode with frequency near a harmonic of the cyclotron frequency is determined. It is shown that only a small-amplitude wave is needed to destroy the linearly coherent contributions of different gyrating particles to the perturbed charge density. This nonlinear smearing of the cyclotron resonance is accomplished by vortex filament motion of the hot particles in phase space, and leads to rapid stabilization and subsequent damping of the linearly unstable resonant modes that occur in hot-cold multicomponent loss-cone systems. In such a system with a typical hot-particle Larmor radius R, a mode with wave vector k gains energy from the low-velocity, inverted population hot particles until an energy is attained of the order (kR)−2 times the hot-particle kinetic energy. This field energy then decays, being absorbed by the high-velocity, normally populated hot particles.
Instability of Low-β Magnetic Flute Modes Caused by Transverse Anisotropy
Authors: Morse Richard L.
Published: February 2004

A marginal stability analysis of a symmetric, low-β sheet plasma shows that the shorting of electrostatic potential between magnetic field lines allows the existence of marginally stable magnetic flutes with finite wavelength in the direction perpendiuclar to B¯ and parallel to the plane of the plasma. Two cases are considered. In the small gyroradius limit, the system is shown to be neutrally stable for all wavelengths. In a large gyroradius limit where transverse anisotropy is large, an expression is obtained for the finite wavelength of the marginally stable mode.
Upper Bound on Plasma Containment
Authors: Hurley James
Published: February 2004

Since a plasma cannot be self-contained, it may be supposed that a given external magnetic field is capable of confining only a finite amount of plasma. Two general classes of confined plasmas are considered, and upper bounds are obtained for the total particle energy as simple functionals of the given confining field. As a particular example, the upper bound is evaluated for the Van Allen belts.
Collisionless Thermalization of Electrons in a Beam-Plasma System
Authors: Wada J. Y. , Knechtli R. C.
Published: February 2004

Collisionless thermalization of a stream of energetic electrons injected into a plasma has been observed by means of an electron energy spectrometer. Accurate measurements of the energy distribution of the injected electrons have shown that a mean energy loss as well as an energy spread of the injected electrons occur when the distance traveled by these electrons through the plasma exceeds 10 to 15 e-folding lengths of the fastest growing plasma wave mode. The effects of electron-neutral collisional damping on collisionless thermalization have also been established. It has been shown that collisionless thermalization can be inhibited, if the electron collision mean free path is made small enough, or if the effective plasma thickness is kept smaller than about 20 plasma wavelengths for the injected electrons. These experimental results have been found to be consistent with the theory of double stream interaction between injected electrons and stationary plasma electrons.
Effect of Magnetic Field and Collisions on the Ion Current to a Cylindrical Probe
Authors: Shaw E. K. , Kino G. S.
Published: February 2004

Using a modified pressure theory the parameters of a dc discharge between concentric coaxial cylinders have been calculated with the plasma approximation. Both ion-neutral collisions and a longitudinal magnetic field have been taken into account in this low-and intermediate-pressure plasma. In the limit of large outer to inner radii and electron Debye length much smaller than the probe radius, the results are applicable to a cylindrical Langmuir-type probe in an infinite plasma. In the free-fall limit, the results for ion current and density are shown to agree closely with previous collisionless and no magnetic field theories. Curves are presented which show quantitatively, for the first time, the effect of collisions and a “low” magnetic field on the ion current at the Tonks-Langmuir boundary. Contrary to what has been assumed in the past, the magnetic field is shown to have a considerable effect on the ion current even though the ion gyroradius is assumed to be much larger than the probe dimensions. At the Tonks-Langmuir boundary the Bohm criteria comes as a natural consequence of the theory.
Isotropy in Initial Period Grid Turbulence
Authors: Portfors E. A. , Keffer J. F.
Published: February 2004

Careful corrections of measured turbulent intensities following the method of Champagne, have been applied to the flow downstream of a grid of parallel rods. Results show the turbulence to have become isotropic by a distance of approximately 30 mesh lengths.