bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic Theory of Time-Dependent Correlation Functions in a Binary Gas Mixture
Authors: Boley C. D. , Yip Sidney
Published: February 2004

Time-dependent correlation functions in a binary mixture of gases are studied on the basis of kinetic theory. The problem is formulated as an initial-value solution to the linearized Boltzmann equation for the mixture. The analysis is made tractable by the use of an appropriate kinetic model which is developed for Maxwell molecules (1/r4 repulsive potential). The resulting description yields density correlation functions which are valid for arbitrary ratios of wavelength to mean free path. In particular, the results can be used to discuss deviations from hydrodynamic behavior. With the present theory a detailed analysis of the spectral distribution of laser light scattered by dilute binary gas mixtures is made. Explicit results for xenon-helium mixtures are shown to be in very good agreement with the recent measurements of Clark and Benedek.
Dynamics of the Inverse Pinch
Authors: Weisbach Michael F. , Ahlstrom Harlow G.
Published: February 2004

Experiments are performed on the plasma produced in a large inverse pinch using argon with no applied magnetic fields or preheating. A direct observation is made of the precursor ionization which initially raises the electron temperature and electrical conductivity. Calculations show that most of the mass is entrained in the current sheet, but that less than complete sweeping occurs at the lower velocities. The shock which is embedded in the current sheet closely resembles an ordinary gasdynamic shock. Consideration of the shock jump condition derived from the induction equation is shown to determine the frequent inability of the shock to separate from the current sheet.
Effects of Collisions on Perpendicular Longitudinal Ion Wave Propagation in a Magnetized Plasma
Authors: Berry L. A. , Nickel J. C.
Published: February 2004

The effect of ion-ion and ion-neutral collisions on longitudinal ion waves propagating perpendicular to a magnetic field has been calculated. Ion-neutral collisions are incorporated into the Vlasov calculation via a BGK collision term while the ion-ion collision portion of the calculation is based on a model Fokker-Planck equation. It is shown that the effect of both types of collisions is to cause the dispersion relations to become more fluid-like. For quite modest values of the ion-ion collision frequency (νii/ωci∼0.1), the dispersion relations are essentially those predicted by the moment equations.
Plasma Electron Velocity Distributions Determined from the Polarization of Free-Free Bremsstrahlung
Authors: Raimbault P. A. , Shohet J. L.
Published: February 2004

The degree of polarization of the free-free bremsstrahlung from a plasma, as measured by Compton scattering, may be used to determine the electron velocity distribution. In this experiment, a hot electron plasma was made by means of electron cyclotron resonance heating in a magnetic mirror. The measured degree of polarization varied as mirror ratio and amount of lower off-resonance heating were changed. A theoretical computation of polarization, utilizing a nonrelativistic quantum mechanical theory, which matched the experimental conditions, showed that either a loss-cone or a Harris distribution could fit the observed data, except when lower off-resonance heating filled the loss cone. Under these conditions, only the Harris distribution was correct.
Solitary Waves in a Two Ion Beam, Magnetized Plasma
Authors: Goldenbaum G. C. , Gerber K. A. , Levine L. S. , Krall N. A.
Published: February 2004

Experimental evidence and a theory is presented for the properties of solitary waves which propagate at right angles to a magnetic field in a plasma which contains two classes of ions with a relative drift velocity between them. Two kinds of nonlinear wave motion are seen to exist. One type of wave reduces to the magnetosonic solitary wave as the density of one of the ion components becomes small. The other type of solitary wave appears to be an ion beam wave. For example, if one of the ion components is at rest in the laboratory, the wave velocity approaches a value less than the moving ion velocity. Unlike the magnetosonic solitary wave these waves can have velocities much greater than twice the Alfvén speed.
Some Analytic Solutions to a Nonlinear Heat Transfer Problem
Authors: Gilpin R. R.
Published: February 2004

A two-state system such as an electric discharge or a combustion process is modeled as a nonlinear heat transfer problem. A model is chosen which lends itself to some simple analytic solutions. These solutions serve to illustrate the behavior observed for such a system.
Simulation of Whistler Instabilities in Anisotropic Plasmas
Authors: Ossakow S. L. , Haber I. , Ott E.
Published: February 2004

Computer simulations are used to study whistler turbulence in plasmas driven by initially anisotropic bi-Maxwellian and loss-cone electron distributions. Linear growth saturates when the growth rate is of the order of the particle trapping frequency.
Magnetic Compression of Astron-Type E Layers
Authors: Schmidt George
Published: February 2004

The compression of a simplified model of an E layer, by a rising confining field is investigated. A formula is obtained for particle energy as a function of the confining field, and it is shown that field reversal decreases with compression.
Boundary Layer Growth in Shock Tube Flow of Partially Ionized Mercury
Authors: Kim Yong Wook
Published: February 2004

A detailed study of the reflected shock bifurcation in partially ionized mercury in a heated shock tube shows that both the bifurcation and the primary shock flow boundary layer grow proportional to the square of the distance each respective shock travels. A general criterion for the occurrence of reflected shock bifurcation in partially ionized monatomic gases is presented.
Spatial Measurement of the Magnetic Field Direction in Plasma
Authors: Cano R. , Chaignot G. , Fidone I. , Schwartz M. J.
Published: February 2004

Experimental results of the determination of the local value of θ(x) = By(x) / Bz(x) in an externally imposed sheared magnetic field are presented. Two microwave methods are used. The first method employs an incident ordinary wave. Measurement of the polarization of the extraordinary wave generated by shear coupling yields the local direction of B. In the second method a large amplitude incident extraordinary wave is used. Measurements of the polarization of locally generated harmonics yield the direction of B at the point of generation. Typically, we have θ≃4 × 10−2 rad. The precision obtained with both methods is 10−2 rad.
Radiative Transport Analysis of Electromagnetic Propagation in Isotropic Plasma Turbulence
Authors: Feinstein David L. , Butler Frank E. , Piech Kenneth R. , Leonard Anthony
Published: February 2004

The problem of electromagnetic wave propagation in a turbulent plasma is formulated in terms of the radiative transport equation. A singular eigenfunction solution is obtained for the case of isotropic plasma turbulence, and detailed numerical calculations are presented. The intensity distribution is studied as a function of the turbulent spectrum and relative strength of scattering attenuation to total attenuation. For a highly forward peaked scattering law characteristic of many physical situations it is found that the reflected backscatter intensity is relatively insensitive to the angle of incidence, except as grazing incidence is approached. The importance of multiple scatter is studied as a function of the properties of the medium.
Stability of a Combined Current and Vortex Sheet in Compressible, Perfectly Conducting Fluids
Authors: Jindia R. K. , Chakraborty B. B.
Published: February 2004

The stability of a combined current and vortex sheet in compressible and perfectly conducting fluids is studied. In contrast with the incompressible fluid case, a current sheet is found to have, for a given average magnetic energy, a destabilizing influence.
Comparison of the Electromagnetic Reflection Coefficients for the Random Plasma Half-Space and Slab
Authors: Wasneski Richard J. , Jarem John
Published: February 2004

The electromagnetic reflection coefficient for a random plasma half-space is compared to a slab computer experiment. It is demonstrated that the negative values of the change in reflection coefficient due to turbulence arise from the path length traveled by a multiple scattered wave component.
Fluctuations in guiding center plasma in two dimensions
Authors: Taylor J. B. , Thompson W. B.
Published: February 2004

Statistical mechanics is developed for a two-dimensional guiding center plasma. Because there is no kinetic energy associated with guiding center motion, this development is unconventional. Thermal equilibrium is discussed, and an interesting limiting case is noted. Using the random phase assumption, a kinetic equation for the density fluctuations is obtained, which has thermal equilibrium and its limiting form as the only stationary states. However, despite the phase averaging this kinetic equation is reversible and when disturbed the system oscillates about equilibrium. Similar oscillatory behavior appears in the microscopic correlation function of the fluctuations, and the oscillation frequencies are obtained explicitly; however, these oscillations do not significantly change the macroscopic diffusion coefficient derived earlier by McNamara and Taylor.
Privileged equilibria of a collisionless homogeneous or inhomogeneous plasma
Authors: Minardi Ettore
Published: February 2004

A statistical method for describing the collisionless (in general non-Maxwellian) equilibria and the stability of inhomogeneous Vlasov plasmas is proposed and applied to various electrostatic cases. By means of this method the entropy and the other thermodynamic potentials can be calculated. The entropy principle allows the calculation of the electrostatic equilibria which are preferably chosen by the plasma. It is shown that the Bernstein-Greene-Kruskal sinusoidal wave is a maximum of the entropy provided dissipative processes are neglected. Examples are given of the general property of a collisionless system to evolve toward the marginal stability point. In one example the evolution of a bump-in-tail distribution toward a plateau is easily shown. It also follows from the theory that the collisionless dissipative processes always imply the excitation of short wavelength modes.
Non-Maxwellian toroidal equilibria with spheroidal plasma-vacuum interfaces
Authors: Dobrott D. , Morikawa G. K.
Published: February 2004

Axially symmetric toroidal plasma equilibria with spherical or near-spherical plasma-vacuum boundaries are examined when variations in the distribution function produce anisotropic pressures. A separable analytic solution is found when the pressure deviation from isotropy is small; the degree of anisotropy is used as an expansion parameter in the perturbation scheme. In general, the plasma-vacuum boundaries deform slightly and spherical boundaries cannot be maintained by externally applied fields that remain finite at large distances. When p⊥>p‖, the plasma becomes a prolate spheroid and when p‖<p⊥ it becomes oblate.
Relaxation of anisotropic collisionless plasma
Authors: Hamasaki S. , Krall N. A.
Published: February 2004

The relaxation of anisotropic collisionless plasma distributions due to whistler mode instabilities is calculated through quasilinear theory, for various values of background magnetic field strength. The results are compared with previous computer simulation experiments and with the predictions of classical (collision dominated) transport theory.
Simultaneous translational and rotational Brownian movement of particles of arbitrary shape
Authors: Chow T. S.
Published: February 2004

The coupling of the translational and rotational Brownian motion of a particle due to its arbitrary geometric shape is studied. The generalized kinetic equation for the Brownian particle distribution function is derived from the Liouville equation. The correlation functions for random force and torque are calculated from hydrodynamic fluctuation theory. It is found that the coupling effect plays an appreciable role in the diffusivities of asymmetric particles. For the special case of a sphere where there is no coupling, the autocorrelation function for the random torque in rotational Brownian motion is given in closed from. Its asymptotic from at large times is proportional to (time)−5/2. As an example of an asymmetric particle, the diffusivity tensor is worked out for two disks connected by a rigid rod.
Neoclassical plasma transport in axisymmetric toroidal systems
Authors: Glasser Alan H. , Thompson William B.
Published: February 2004

The variational technique for calculating all the neoclassical transport processes in the banana regime, developed by Rosenbluth, Hazeltine, and Hinton for the case of tokamak with small rotational transform and large aspect ratio, is generalized to arbitrary axisymmetric toroidal systems with a small fraction of trapped particles. The use of the scalar magnetic potential as a coordinate, as in Rutherford's treatment of the Lorentz model, is eliminated by the use of a coordinate-free solution procedure, allowing the treatment of nonvacuum fields. As an example, results are given for doublet II.
Structure of strong shock waves in xenon. I: Electron temperature measurements
Authors: Golobic Robert A. , Nerem Robert M.
Published: February 2004

Time-resolved electron temperature measurements were taken behind incident normal shock waves in an arc-driven shock tube using xenon as the test gas at an initial driven tube pressure of 0.1 Torr. At shock wave velocities from 6 to 10 km/sec, the two-line intensity ratio technique involving the spectral lines of XeII was used to determine the electron temperature. The data indicate an electron temperature peak almost immediately behind the shock front which is well above the predicted equilibrium temperature. This peak temperature decays rapidly (∼ 1 μsec) to a plateau temperature well below the predicted equilibrium temperature. The importance of both radiative energy loss and ionizing collisions on the electron temperature decay is considered in the interpretation of the data.
Resonant excitation of electrostatic modes with electromagnetic waves
Authors: Schmidt George
Published: February 2004

Two electromagnetic waves, propagating in opposite directions, can resonantly excite electron or ion waves, if the difference frequency Δω and wave number Δk satisfy the linear dispersion relation of such modes. This process damps the higher-frequency electromagnetic wave and amplifies the lower-frequency one (nonlinear Landau damping). The amplitude of the electrostatic mode may be limited by Landau damping, trapping, or by loss of resonance due to the frequency shift of the electromagnetic modes.
Parallel velocity shear instabilities in an inhomogeneous plasma with a sheared magnetic field
Authors: Catto Peter J. , Rosenbluth Marshall N. , Liu C. S.
Published: February 2004

Injection of fast neutrals with a component of the beam velocity along the magnetic field B0 results in a plasma having a sheared mean velocity along B0. If this parallel velocity shear U′ and the density inhomogeneity scale length Ln are large enough so that U ′|Ln |/(τ + 1) 1/2υi > 1 then a parallel Kelvin-Helmholtz instability is excited, where vi is the ion thermal speed and τ is the electron temperature/ion temperature. Magnetic shear stabilization requires a shear length Ls comparable to or less than |Ln |, namely, τL s /3 (τ + 1) < υi /U ′ < |Ln |/(τ + 1) 1/2. For U ′|Ln |/(τ + 1) 1/2υi < 1, however, this parallel Kelvin-Helmholtz instability is stabilized and only drift wave instabilities can be excited. For τ ≈ 1 and (M / m) 1/2 ≈ 43, the first of these drift wave instabilities is unstable for U ′/Ωi > 172| (Ln / Ls)3/2 | provided |Ln /Ls| < ai /30|Ln |, while the second is stable for U ′ /Ωi < 136ai /|Ls | provided |Ls /Ln | > 600 and U ′ /Ωi ≪ 1, where ai and Ωi are the ion gyration radius and frequency.
Nonlinear development of the two ion beam instability
Authors: Baker D. R.
Published: February 2004

The nonlinear development of the two ion beam instability has been experimentally investigated. The ion acoustic turbulence grows and stabilizes in conjunction with a significant change in the average ion distribution function. A quasilinear type theory is developed for ion waves in a boundary value problem which appears to predict most of the observed nonlinear behavior of the instability including excited waves and the growth of the background noise. A brief discussion of trapping and strong turbulence effects is given.
Role of turbulence in the coupling of a magnetic pulse to a collisionless plasma
Authors: Davis William D. , DeSilva Alan W.
Published: February 2004

The coupling of a magnetic pulse to an initially magnetized collisionless plasma is studied experimentally using a high voltage theta pinch. The plasma parameters studied include: the turbulent electric fields, particle heating, magnetic and electric field profiles, and the plasma resistivity. The results indicate that the interaction of the plasma with the magnetic pulse can be understood using a fluid model which includes anomalous resistivity and particle heating. The source of the anomalous resistivity is investigated based on the presently available instability theories.
Turbulent diffusion coefficients of toroidal plasmas
Authors: Yoshikawa S.
Published: February 2004

The difussion coefficient of a plasma with density gradient-produced fluctuations was calculated. Two different methods of deriving the diffusion coefficients were used. Then, the condition that two methods yield the same result is used to determine the phase angle a between the density and the potential fluctuations. The result shows the pseudo-classical diffusion coefficients. Agreement with experimentally determined plasma confinement times is satisfactory.
Axial structure in a theta-pinch plasma
Authors: Dixon R. H. , Düchs D. F. , Elton R. C.
Published: February 2004

Axial irregularities in a θ-pinch plasma are observed during the initial breakdown and main discharge phases. The structure is associated with a gas-breakdown effect. Wavelengths and growth times are consistent with the development of instabilities associated with thermal waves.
Analysis of the diffusion equation approach in stochastic systems
Authors: Cruty M. R. , So K. C.
Published: February 2004

The phase space density function is obtained both from a diffusion equation and in an exact form, for a specific system. The results are compared.
Analogy between the velocity distribution in a stable atmosphere and in divergent channels
Authors: Engelund Frank
Published: February 2004

An analogy to the Monin-Obukov similiarity hypothesis for flow in thermally stratified surface boundary layers has been developed for the flow of homogeneous fluids in diverging channels and in equilibrium boundary layers.
Bounds of some gas kinetic operators
Authors: Ronen Yigal
Published: February 2004

A test for integral operator bounds in the L1 space with an example from the kinetic theory of gases is presented.
Study of ion acoustic wave damping through Green's functions
Authors: Hsuan Hulbert C. S. , Jensen V. O.
Published: February 2004

Green's function analyses of ion acoustic waves in streaming plasmas show that, in general, the waves damp algebraically rather than exponentially with distance from exciter.
Self-generated magnetic fields in laser plasmas
Authors: Widner M. M.
Published: February 2004

Numerical simulation and analytic models are used to predict spontaneous magnetic field generation in laser-produced plasmas. Calculations indicate magnetic fields are important for plasmas of controlled fusion interest.
Nonlinear resonant inertial wave interactions
Authors: Hughes B. A.
Published: February 2004

The resonant interaction of a triad of inertial waves is investigated theoretically. An additional condition for resonance is found to be necessary compared to short internal wave interactions—namely, that the polarization must be specified. One of the quantities conserved is ω· u calculated for each wave separately and summed over the triad. Here, ω is the vorticity field and u is the velocity field of the inertial wave. Resonance diagrams are given and two explicit examples of amplitude variations are calculated.
Impact of a freely expanding gas on a wall
Authors: Bird G. A.
Published: February 2004

A study is made of the flow which results when an initially uniform gas expands into a vacuum through a complete one-dimensional rarefaction wave, and then strikes either a solid wall or a similar opposite facing rarefaction. A Knudsen number K n is defined by the ratio of the undisturbed mean free path to the distance between the center of the rarefaction and the wall. The direct simulation Monte Carlo method has been used to obtain numerical solutions over a range of K n from 10−4 to 103. Results for the wall pressure profile were also obtained from experiment and from approximate continuum calculations. The pressure profile on the wall or the line of symmetry was found to be insensitive to assumptions about the initial highly rarefied stage of the reflection. On the other hand, when considering the temperature history of the gas adjacent to the wall, a proper molecular analysis of the initial stage of reflection was required for all values of the Knudsen number.
Generalization of Suydam's criterion
Authors: Goedbloed J. P.
Published: February 2004

For low-shear systems Suydam's criterion is generalized to include stability against localized quasikink instabilities. In the obtained criterion, containing pressure gradient, shear, and a term responsible for the quasikink, the shear enters in a completely different way than in an earlier derived criterion by Shafranov. The reason for the discrepancy is pointed out. The present criterion is more restrictive than Suydam's criterion if m is below a certain critical value depending on the shear. The special case of a force-free field having a pitch extremum is separately treated. Several limitations of the use of local criteria are indicated.
Potential well structure in an inertial electrostatic plasma confinement device
Authors: Swanson D. A. , Cherrington B. E. , Verdeyen J. T.
Published: February 2004

Electron beam probing has been used to infer the behavior of potential wells formed by electron injection in a spherical inertial electrostatic plasma containment device. At very low pressures, the electron injection clearly results in the production of a steady state deep negative potential well. When deuterium gas at pressures of up to 10−5 Torr was admitted, the pronounced focusing of the probing electron beam strongly indicated that a steady state positive potential well, due to background ionization, was formed within the negative potential well.
Quasiresonant mode coupling of electron plasma waves
Authors: Kaw P. K. , Lin A. T. , Dawson J. M.
Published: February 2004

A finite amplitude ion fluctuations (wave number ki) couples a primary electron plasma wave (wave number k0) to a whole spectrum of plasma oscillations (wave numbers k0 ± n k i). Simple fluid equations are used to analyze the effect and computer simulation is used to verify the theory, and show the limits of the analysis. If the variation in plasma frequency Δωp associated with the ion fluctuation exceeds the frequency mismatch between the primary wave and the nth mode, then a large fraction of the energy can be transferred to that mode, while if the mismatch in frequency is greater than Δωp only a relatively small energy transfer occurs. Because the frequency of plasma oscillations varies only weakly with wavelength, very small ion fluctuations are sufficient to couple long-wavelength waves to short-wavelength waves which are Landau damped. This phenomenon may also account for some experimental observations of fluctuations in which the frequency and wave number do not appear to satisfy the normal plasma dispersion relation.
Transverse effects in the inhomogeneous beam-plasma interaction
Authors: Aghevli R. , Rowe J. E.
Published: February 2004

The interaction of a periodically inhomogeneous electron beam of finite transverse extent with its surrounding plasma is investigated. The hydrodynamic model is adopted for both the beam, which is of finite transverse extent, and the plasma, which can assume either an infinite or a finite transverse extent. The results indicate that for zero external magnetic field no stabilizing effects occur. This is in contrast to the results of the one-dimensional analysis previously reported. The dispersion relation for the finite magnetic field case is also presented and discussed.
Emission of coherent microwave radiation from a relativistic electron beam propagating in a spatially modulated field
Authors: Friedman M. , Herndon M.
Published: February 2004

Intense electromagnetic radiation has been observed from the interaction of an intense relativistic electron beam and a spatially modulated magnetic field. An efficiency of conversion of electron kinetic energy into microwave radiation of the order of 1% has been achieved, with total rf power radiated greater than 30 MW. A parametric study has shown that the device has not reached saturation over the range of parameters tested. The experimental results show that the new device differs from conventional generators in several respects. For example, the frequency and the bandwidth of the rf radiation can be tuned over a wide range and the power at the harmonics of the fundamental frequency (∼ 1010 Hz) is the same order of magnitude as the power in the fundamental. Infrared radiation has also been observed; more than 1 MW has been found even for wavelengths under 10 μm.
Fluid flow with variable properties in a two-dimensional channel
Authors: Bhat M. K. , Bose T. K.
Published: February 2004

Asymmetric velocity distribution for a fluid with a highly temperature sensitive viscosity coefficient in a two-dimensional channel is obtained as a function of the temperature difference between channel plates. Numerical results for three liquids have been obtained.
Measurement of the mean period between bursts
Authors: Lu S. S. , Willmarth W. W.
Published: February 2004

The effect of a high pass filter on the measurements of the mean period between bursts is studied. The second mild maximum of the autocorrelation curve of the fluctuating streamwise velocity is not necessarily produced by bursting.
Magnetic field generation due to radiation pressure in a laser-produced plasma
Authors: Stamper J. A. , Tidman D. A.
Published: February 2004

The role of radiation pressure gradients in directly generating magnetic fields in laser-produced plasmas is discussed. The mechanism is related to the thermal sources of magnetic field discussed in earlier work in that a solenoidal electric field is produced by the radiation pressure gradient.
Entrainment by a rising thermal
Authors: Simons Girard A.
Published: February 2004

A theoretical model is proposed to describe the entrainment of ambient air by a rising thermal. The model is appropriate for the first few revolutions of a strongly heated thermal whose motion is dominated by its initial conditions. At sufficiently early times, only the outer edge of the thermal is turbulent. The ambient gas mixes with the thermal gas along the turbulent interface and is convected into the thermal along streamlines. The gas densities and species concentrations on each streamline are obtained by superimposing a turbulent mixing layer on the edge of Hill's spherical vortex. Solutions are obtained in terms of the dimensionless ratio of the mixing length to the thermal radius (l/a ) and the entrainment coefficient is shown to be directly proportional to l/a.
Mean velocity and shear stress distributions in turbulent boundary layers
Authors: Perry A. E. , Schofield W. H.
Published: February 2004

A universal velocity defect law for turbulent boundary layers developing in adverse pressure gradients is proposed. The velocity scale for this law is independent of the wall shear but related instead to the local maximum in the Reynolds stress profile. The proposal and its implications are checked against a very large body of experimental results and a partial connection between the mean velocity field and the shear stress field is established. A comparison between the present theory and the mixing length theory is presented. The experimental data do not appear to support many of the assumptions nor the predictions of the mixing length theory. The distinction between equilibrium and nonequilibrium layers made by Clauser is shown to be unnecessary as all classes of layers appear to conform to the new universal velocity defect law proposed, provided that the maximum shear stress is sufficiently larger than the wall shear stress.
Nonlinear stability of a liquid adjacent to a supersonic stream
Authors: Nayfeh Ali Hasan , Mook D. T.
Published: February 2004

A nonlinear second-order analysis shows that all small disturbances of the interface between a semi-infinite, inviscid liquid and a uniform supersonic gas stream are unstable, as according to linear theory. Moreover, the growth rates increase with the amplitude of the disturbance.
K
-dependent Brownian diffusion constant in a critical mixture
Authors: Lyons Kenneth B. , Mockler Richard C. , O'Sullivan William J.
Published: February 2004

The K -dependent diffusion constant of 0.3μ Teflon particles in the critical mixture isooctane-nitroethane has been measured for 0.001 < T − T c < 2 °C. The results are presented along with a simple theory that gives good agreement by considering the surface energy force on the particle due to concentration fluctuations.
Nonequilibrium structure of argon shock waves
Authors: Nelson H. F.
Published: February 2004

This theoretical study investigates the influence of (1) the nonequilibrium excited state population in the relaxation region and (2) line radiation in the precursor on shock wave structure by considering strong shock waves propagating at Mach 18 and 24 into a theoretical argon-like gas at a pressure of 1 cm Hg and a temperature of 300 °K. The argon atom is modeled as having two bound states plus a continuum, and the calculations include finite atom-atom and electron-atom collisional ionization and excitation rates as well as continuum and line radiation. The electron gas is allowed to be at a different temperature from the atom gas; consequently, three types of of nonequilibrium exist: ionization, excitation, and thermal. In the collisional relaxation region, which serves as a source of radiation to create the precursor, the degrees of ionization and excitation generally lag behind their respective local equilibrium values. By doing so they greatly influence the emission of radiation. The line wing radiation increases the extent of the precursor compared with former studies which considered only continuum radiation. However, elimination of the assumption that the excited states are in equilibrium with the electrons at the electron temperature decreases the magnitude of the precursor compared with results where the assumption has been used because the radiative flux is reduced.
Effect of electrical conductivity and a viscous flow on the stability of a current-carrying hollow cylinder of fluid
Authors: Date T. H. , Gokhale M. H.
Published: February 2004

The stability of the axisymmetric modes of a viscous flow of an annular cylindrical layer of fluid carrying an electric current is considered in the presence of an independent parallel electric current along the axis. Even if the electrical conductivity of the fluid is small, sufficiently strong steady-state flow with a radial velocity gradient can reduce the growth rates so much that the e -folding times of the instabilities is larger than the time of flow over a typical length.
Electromagnetic detonation in a magnetic annular shock tube
Authors: Mar J. Y. S. , Makios V. , Plett E. G.
Published: February 2004

The results of an experimental demonstration of electromagnetic detonation waves are presented. These waves are obtained by discharging a capacitor bank across the annulus of a magnetic annular shock tube filled with hydrogen-oxygen mixtures. Wave speeds of more than twice the usual steady Chapman-Jouguet detonation speeds are observed. Experiments with argon in place of the combustible mixture were also conducted to illustrate the basic difference between electromagnetic shock waves and electromagnetic detonations. Theoretically calculated results are included to provide a better understanding of the observed experimental results. It is suggested that techniques similar to those used in these experiments may be used to produce high-speed detonations for gasdynamic laser applications or to stabilize combustion waves in a supersonic stream.
Instantaneous electron energy distribution function in ion waves
Authors: Grésillon D. , Galison P. L.
Published: February 2004

The assumptions involved in the use of Boltzmann's law to describe the electron density variations in ion acoustic waves are discussed. The need for electron collisions to fill the regions of maximum potential with low-energy electrons is pointed out. A direct method for measuring the electron energy distribution at any time during the period of an ion wave is presented. Experimental results show the existence of electrons with energy small enough for their motion to be constrained to the wave potential well. For higher energies, the distribution stays approximately the same. The method provides a direct measure of plasma potential, which is related to density. The Boltzmann law is still seen to be accurate for relative fluctuations of the order of up to 20%.
Collision terms and density correlations for strongly correlated plasmas
Authors: Nambu Mitsuhiro
Published: February 2004

New collision terms for stable plasmas are formally derived within the framework that the ternary correlation function be expressed in terms of the products of the single-particle and pair-correlation functions. The collision terms obtained are nonlinear integral equations involving both single particle and pair correlation functions. It is investigated as to how the conventional Balescu-Lenard collision terms are modified through pair-correlation functions. As a consequence of such a modification of effective interactions, the decay of pair correlations becomes oscillatory. The effective Debye length for strongly correlated plasmas is introduced, and the sum rule for strongly correlated plasma is also investigated.
Pressure gradient focusing of intense beams
Authors: Olson Craig L.
Published: February 2004

A theory is presented which predicts that a pressure gradient in the neutral gas filling a metallic guide tube or cone will cause an injected intense relativistic electron beam to focus. The pressure variation controls the breakdown time which determines the focusing frozen magnetic field structure. The approximation that the avalanching time goes as E −s is employed [E is the induced electric field, s = s (E , p) is a parameter determined from neutral gas breakdown data, and p is the pressure]. For positive (negative) pressure gradients, it is shown that focus occurs only if s > 1 (s < 1) in the limit that the charge neutralization time can be neglected. When a pressure gradient is used, a cylindrical guide tube should show a focusing effect and the efficiency of cone focusing should be increased.
Nonlinear wave optics of parametric pump radiation in an inhomogeneous plasma
Authors: DuBois Donald F. , Goldman Martin V. , McKinnis Dean
Published: February 2004

The nonlinear propagation of an intense (pump) beam of electromagnetic radiation that excites a parametric decay instability in an inhomogeneous plasma is calculated self-consistently. A generalization to an inhomogeneous plasma of the nonlinear absorptive conductivity calculated according to several recent theories for the saturated electron-ion decay instability in a homogeneous plasma is added to the linear absorptive conductivity in the wave equation. A model is introduced for the qualitative change in the pump dependence of the nonlinear conductivity in the strong pumping limit. The resulting nonlinear wave equation for radiation propagating into a stratified plasma is integrated numerically. Application is made to the anomalous absorption of the modified radiation in ionosphere modification experiments and to absorption of laser radiation in laser-produced plasmas. The reflection coefficient is computed as a function of incident power. Also computed are waveforms, the Poynting flux, and the ponderomotive force. These are displayed as functions of position in the plasma. A distinct standing-wave pattern is shown to persist very near the reflection point even in cases of nearly total absorption.
Stimulated backward Brillouin scattering in laser heated plasmas
Authors: Brueckner Keith A. , Jorna Siebe
Published: February 2004

The dispersion relation for backward-stimulated Brillouin scattering shows that near threshold the most unstable reflected wave rapidly outruns a laser driven underdense plasma. Laser energy absorption may, however, be affected at power flux densities above 1015 Ws/cm2 for 1.06 μ radiation.
Temperature fluctuations in the turbulent wake behind an optically heated sphere
Authors: Freymuth Peter , Uberoi Mahinder S.
Published: February 2004

Convection, production, diffusion, and dissipation of temperature fluctuations have been measured in the dynamically similar turbulent wake behind an optically heated sphere 136 sphere diameters downstream and at a Reynolds number 4300. The measurements show strong convection and dissipation and moderate diffusion and production. In addition, the dependence of one-dimensional spectra of temperature fluctuations on wake parameters, intermittency, and the skewness of temperature and velocity derivatives have been measured in a Reynolds number range 1000-38 000. The skewness of temperature derivative exhibits locally nonisotropic behavior especially at low Reynolds numbers.
Hot-wire measurements of hypersonic boundary-layer turbulence
Authors: Laderman A. J. , Demetriades Anthony
Published: February 2004

Turbulent fluctuation data in a Mach 9.4 boundary layer are presented. Velocity and temperature fluctuations computed from this experiment are not much different from those in the Mach 2-5 range, but are found to be smaller than the pressure fluctuations.
Stability of a laminar viscous jet—The influence of an ambient gas
Authors: Phinney R. E.
Published: February 2004

Computations of the breakup length of a liquid jet in a gaseous atmosphere are simplified and accelerated. No approximations need to be made, and the range of the parameters is left unrestricted. Comparison between theory and experiment is made.
Effect of microwave radiation on a shock-produced electron precursor
Authors: Dawson Edward F. , Lederman Samuel
Published: February 2004

Results of experiments in which a small pressure driven shock tube was used to produce shocks in argon and air above Mach 10 are presented. This was strong enough to produce low level precursor ionization ahead of the shock. At the same time, the shock tube was used as a waveguide for pulsed X-band microwave radiation with a peak power of a few kilowatts. This further ionized the gas in the precursor region, resulting in an ionization wave which propagated very rapidly toward the microwave source. The velocity of this wave was measured as a function of the microwave power and an interpretation and comparison with other data is given.
Strong shocks, snowplows, and magnetic pistons
Authors: Taussig R. T. , Chen Y. G. , Gross R. A.
Published: February 2004

A comparison is made between experimentally-observed high-speed transverse shock wave data and theoretical snowplow predictions. When high voltages are employed, the snowplow model is found to be inadequate during the early time of propagation. Particle dynamics corresponding to high voltage initial conditions are examined and when an initial E × B drift is incorporated in the snowplow model, good agreement with experimental observations is achieved. Some additional observations of the experiments are briefly described.
Propagation of density perturbations in a collisionless Q-machine plasma
Authors: Michelsen P. , Pécseli H. L.
Published: February 2004

The evolution of the ion velocity distribution function is calculated by means of the linearized Vlasov equation for the case where the initial perturbation consists of a steplike density discontinuity. Some characteristic features in the solution are caused by wave-particle interaction. These features are found experimentally by perturbing the plasma in a single-ended Q machine by means of a grid immersed in the plasma column. The perturbed ion distribution function is measured with an electrostatic energy analyzer.
Growth rates and spectra for a particular axially symmetric equilibrium
Authors: Weitzner Harold
Published: February 2004

An axially symmetric ideal magnetohydrodynamic equilibrium akin to a mirror machine with currents only in the ignorable direction is considered. In order to study the spectrum of modes and growth rates of unstable modes, an expansion is performed in the small parameter, the ratio of plasma radius to axial periodicity length. Many stable continuous spectra and both stable and unstable point spectra are found. Growth rates are computed after a second expansion is made in the small parameter, the fractional distortion of the flux surfaces from cylinders. The high beta wall stabilization effect first noted by Haas and Wesson in their free boundary model is not found in this distributed pressure model which is not a good approximation to theirs. Aside from this one difference, the growth rates here are quite comparable to the sharp boundary model and give further indication of the utility of that model.
Unified positive column theory of gas discharges
Authors: Cohen Ira M. , Whitman Alan M.
Published: February 2004

The effects of electron impact ionization, three-body recombination, and a thermal energy balance are considered on the positive column of a gas discharge confined between plane parallel walls. The diffusion flux equations (with temperature-dependent ionization and recombination) for conservation of ions and electrons, Poisson's equation for the self-consistent electric field, and the Elenbaas-Heller energy equation for the temperature distribution, are used. Thermal and electrical conductivities are given by mixture rules that are linear interpolations of collision cross sections in fraction ionized. This formulation allows the description of the entire range of gas discharges from dark discharges through arcs. The emphasis here is devoted to the range from the normal glow through the transition to an arc. The end result is the calculation of semi-internal characteristics, ionization collision rate versus current (per unit depth), and electric field versus, current (per unit depth). It is found that the ionization rate has a maximum and the electric field a minimum in the arc domain.
Experimental study of shock-wave reflection from a thermally accommodating wall
Authors: Hanson Ronald K.
Published: February 2004

Shock-tube experiments have been conducted to study the nonequilibrium gas-surface interaction which occurs when a thick shock wave in argon reflects from a coplanar, heat-conducting wall. Fast-response instrumentation (rise time ≲ 0.1 μsec) was used to monitor variations in temperature and normal stress on the surface of the shock-tube end wall during and immediately following reflection of the incident shock wave. The laboratory observations are compared with computer predictions obtained by Deiwert using the direct-simulation Monte Carlo method, and excellent agreement is obtained when a suitable average thermal accommodation coefficient is chosen for the wall surface. In the present case, with the wall surface probably covered with adsorbed gases, best agreement between laboratory and simulation results is found for an accommodation coefficient of 0.75.
Theory and numerical simulation on plasma diffusion across a magnetic field
Authors: Okuda Hideo , Dawson John M.
Published: February 2004

The diffusion of two and two and a half-dimensional plasmas across magnetic fields have been studied theoretically and by numerical simulation. Only diffusion at thermal equilibrium is studied. It is found that there are three regions: for sufficiently weak magnetic fields the diffusion coefficient is the classical one with D⊥ going like B−2; for moderate magnetic fields (ωce≈ωpe) the diffusion rate is enhanced and B−1 is almost independent of B; finally, for very large fields (ωci>ωpi) the diffusion coefficient goes like B−1. The enhanced diffusion at moderate and high magnetic fields is dominated by collective modes; i.e., by thermally excited convective modes. Theory and simulation are in good agreement. It is also shown that the diffusion coefficient behaves essentially the same way for a three-dimensional plasma when the magnetic field lines are closed.
Polarization of nonlinearly generated harmonic for oblique propagation
Authors: Boyd T. J. M. , Fidone I.
Published: February 2004

Measurement of the polarization of harmonic radiation generated in a plasma by an incident extraordinary wave has recently been proposed as a means of directly determining the direction of magnetic field at the point of generation. The method demands a precise knowledge of the direction of the electric field of the harmonic at the region of generation. The effect of oblique propagation of the primary wave on the direction of the electric field of the harmonic is examined.
Shielding of a slow test charge in a collisional plasma
Authors: Stenflo L. , Yu M. Y. , Shukla P. K.
Published: February 2004

The potential of a slowly moving test charge is considered. Collisional effects are described by a simple BGK model as well as a Lorentz model. It is found that the far-field potential may fall off as the inverse square of distance when the collision frequency is larger than the plasma frequency.
Test of statistical models for gases with and without internal energy states
Authors: Huang A. B. , Hwang P. F.
Published: February 2004

The problem of nonlinear rarefied Couette flow with heat transfer has been studied for both monatomic and diatomic gases using the Boltzmann equation with the Bhatnagar-Gross-Krook type models as the governing equation and the method of discrete ordinates as a tool. The calculated results have been compared with the existing experimental data in order to test the accuracy and the applicability of the statistical models for this one-dimensional problem. The calculated density results are found to be in good agreement with available experimental data; the calculated heat flux solution for the linear case are found to always be lower than the experimental data of Teagan and Springer. There seems to be insufficient published experimental data available to draw rigid conclusions. However, the comparisons made here indicate that the statistical models are indeed reasonably accurate so that their use is justified in the type of problems investigated.
Free molecule flow in a right circular cylinder
Authors: Garelis E. , Wainwright T. E.
Published: February 2004

An approximate analytical solution for free molecule flow in a right circular cylinder has been derived, and the results are compared with those of the exact numerical solution for the free molecule flow problem. The agreement is well within 1% of the exact value. The results of this work are compared with those of earlier work.
Synchrotron radiation from a magnetically confined nonneutral hot-electron plasma
Authors: Nee S. F. , Trivelpiece A. W. , Pechacek R. E.
Published: February 2004

The spectrum of the synchrotron radiation emitted by the hot-electron plasma produced in the University of Maryland pulsed magnetic mirror compression experiment has been measured in the 3.0-cm to 0.3 mm wavelength region. The measured spectra are compared with those calculated using a theoretical model for the electron distribution function that conserves the magnetic moments of the trapped electrons during magnetic compression of the hot electron plasma.