bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface relaxation in ion-etch nanopatterning
Authors: Stepanova M. , Dew S. K.
Published: February 2004

By the means of an atomistic simulation, we investigate the influence of surface mobility on the morphology of surfaces during ion-beam etching at oblique incidence. Depending on the level of surface mobility, three different kinds of etch ripples self-organize in our simulation. For surface scaling, we observe two different regimes. The average height of ripples increases during etching or saturates depending on the surface mobility. The average interripple distance scales with the expression Deff/|ν|, where ν is the surface tension and Deff is an effective transport coefficient proportional to the frequency of irreversible relaxation jumps at the surface.
Analytical solution of the almost-perfect-lens problem
Authors: Merlin R.
Published: February 2004

The problem of imaging for a slab of a lossless left-handed material with refractive index n=−(1−σ)1/2 is solved analytically for |σ|≪1. The electromagnetic field behavior is determined largely by singularities arising from the excitation of surface polaritons with wave vector q→±∞. Depending on the sign of σ, the near-field is either odd or even with respect to the lens middle plane. Consistent with other nonanalytical studies, the resolution depends logarithmically on |σ|. With minor alterations, these results apply as well to the electrostatic limit.
High quantum efficiency AlGaN solar-blind p-i-n photodiodes
Authors: McClintock R. , Yasan A. , Mayes K. , Shiell D. , Darvish S. R. , Kung P. , Razeghi M.
Published: February 2004

We report AlGaN-based back-illuminated solar-blind ultraviolet p-i-n photodetectors with a peak responsivity of 136 mA/W at 282 nm without bias. This corresponds to a high external quantum efficiency of 60%, which improves to a value as high as 72% under 5 V reverse bias. We attribute the high performance of these devices to the use of a very-high quality AlN and Al0.87Ga0.13N/AlN superlattice material and a highly conductive Si–In co-doped Al0.5Ga0.5N layer.
Effects of remote-surface-roughness scattering on carrier mobility in field-effect-transistors with ultrathin gate dielectrics
Authors: Saito Shin-ichi , Torii Kazuyoshi , Shimamoto Yasuhiro , Tsujikawa Shimpei , Hamamura Hirotaka , Tonomura Osamu , Mine Toshiyuki , Hisamoto Digh , Onai Takahiro , Yugami Jiro , Hiratani Masahiko , Kimura Shin’ichiro
Published: February 2004

We examined effects of the remote surface roughness, which is the roughness between the polycrystalline silicon gate and gate dielectric, on the inversion carrier mobility of metal-insulator-semiconductor field-effect-transistors with ultrathin gate dielectrics. We calculated the effective mobility by the linear response theory and found that the scattering from the remote surface roughness reduces the effective mobility especially at high vertical fields. The effective mobility is severely reduced, if the correlation length of the remote surface roughness is comparable to the inverse of thermal de Broglie wave number. We show that the hole mobility reduction experimentally found for the transistor with the Al2O3 gate dielectric can be explained by this scattering.
Band-gap renormalization in highly excited GaN
Authors: Nagai Takehiko , Inagaki Takeshi J. , Kanemitsu Yoshihiko
Published: February 2004

We have studied the band-gap renormalization in highly excited GaN thin films by means of photoluminescence (PL) spectral measurements from 6 to 300 K. The renormalized band-gap energy is determined from the low-energy edge of the broad PL band due to the high-density electron and hole (e–h) plasmas. The reduction of the band-gap energy depends on the density of e–h plasmas, but is independent of temperature. The renormalized band-gap energy is calculated using two theoretical models. Our results suggest that the e–h pair correlation plays an essential role in highly excited GaN.
Crossover from tunneling to incoherent (bulk) transport in a correlated nanostructure
Authors: Freericks J. K.
Published: February 2004

We calculate the junction resistance for a metal–barrier–metal device with the barrier tuned to lie just on the insulating side of the metal–insulator transition. We find that the crossover from tunneling behavior in thin barriers at low temperature to incoherent transport in thick barriers at higher temperature is governed by a generalized Thouless energy. The crossover temperature can be estimated from the low-temperature resistance of the device and the bulk density of states of the barrier.
Photoluminescence and Raman scattering in three-dimensional Si/Si1−xGex nanostructures
Authors: Kamenev B. V. , Tsybeskov L. , Baribeau J.-M. , Lockwood D. J.
Published: February 2004

We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.
Directed growth of nickel silicide nanowires
Authors: Decker C. A. , Solanki R. , Freeouf J. L. , Carruthers J. R. , Evans D. R.
Published: February 2004

Deposition of nickel silicide nanowires has been achieved in the temperature range of 320 to 420 °C by decomposition of silane on nickel surfaces. The substrates consisted of Ni foils and thin Ni films (∼10–100 nm) evaporated on 1-μm-thick layers of SiO2 predeposited on Si wafers. Nanowire growth between two metal pads was achieved with aid of an electric field. It was found that thinner diameter nanowires were produced at low temperatures and that the density of the nanowires was dependent on the reactor pressure. The current–voltage relationship of these nanowires has also been examined.
Nanometers-thick self-organized Fe stripes: bridging the gap between surfaces and magnetic materials
Authors: Fruchart O. , Eleoui M. , Vogel J. , Jubert P. O. , Locatelli A. , Ballestrazzi A.
Published: February 2004

We have fabricated 5-nm-high Fe(110) stripes by self-organized (SO) growth on a slightly vicinal R(110)/Al2O3(112̄0) surface (R=Mo,W). Remanence, coercivity, and domain patterns were observed at room temperature. This contrasts with conventional SO epitaxial systems, that are superparamagnetic or even nonmagnetic at RT due to their flatness. Our process should help to overcome superparamagnetism without compromise on the lateral size if SO systems are ever to be used in applications.
Active plasmonics: Controlling signals in Au/Ga waveguide using nanoscale structural transformations
Authors: Krasavin A. V. , Zheludev N. I.
Published: February 2004

We have developed a concept for active plasmonics that exploits nanoscale structural transformations which is supported by rigorous numerical analysis. We show that surface plasmon-polariton signals in a metal-on-dielectric waveguide, containing a gallium section a few microns long, can be effectively controlled by switching the structural phase of gallium. The switching may be achieved by either changing the waveguide temperature or by external optical excitation. The signal modulation depth could exceed 80% and switching times are expected to be in the picosecond–microsecond time scale.
Terahertz quantum-cascade lasers based on an interlaced photon-phonon cascade
Authors: Köhler Rüdeger , Tredicucci Alessandro , Mauro Cosimo , Beltram Fabio , Beere Harvey E. , Linfield Edmund H. , Davies A. Giles , Ritchie David A.
Published: February 2004

A THz (λ∼80 μm) quantum-cascade laser utilizing alternating photon- and phonon-emitting stages has been developed to achieve efficient extraction of electrons from the lower laser level. Thermal backfilling of electrons is drastically reduced leading to an operation up to 95 K and a weak temperature dependence of the power versus current slope efficiency. The threshold current density is 280 A cm−2 at 6 K and increases to 580 A cm−2 at 90 K. Peak output powers of 10 mW at 30 K and 4 mW at 80 K are obtained.
Imaging buried structures with photoelectron emission microscopy
Authors: Siegrist K. , Ballarotto V. W. , Breban M. , Yongsunthon R. , Williams E. D.
Published: February 2004

Imaging of devices buried under oxides up to 0.5 μm thick is demonstrated with the photoelectron emission microscope. Specifically, we have observed contrast reversal of entrenched Ti lines, time-dependent oxide charging effects, and visibility of biasing effects when a SiO2 overlayer is present. An internally consistent explanation for all observations is that transport of electrons photoinjected from the buried structures to accessible states in the near-surface region allows for observation of material contrast. The observed contrast reversal is most likely the result of the formation of a TiO2–SiO2 interface, which increases the threshold for photoinjection into the oxide overlayer. Gradual accumulation of charge in trap states in the oxide under continuous UV exposure provides a consistent explanation for the observed time-dependent effects. Contrast effects that accompany biasing of the buried structure are due to the resulting lateral electric field, consistent with known field-effect contrast mechanisms.
Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films
Authors: Park Jung H. , Kim Min G. , Jang Hyun M. , Ryu Sangwoo , Kim Young M.
Published: February 2004

The origin of ferromagnetism in ZnO-based systems was investigated using Co-doped ZnO thin films as prototypical examples of II–VI-based diluted magnetic semiconductors. In spite of the atomic-scale dissolution of Co ions in wurtzite ZnO, both the magnetization-temperature curve and the magnetization-field curve demonstrated that Zn1−xCoxO thin films were paramagnetic for x⩽0.12. On the other hand, Zn1−xCoxO films with x greater than 0.12 were characterized by the Co-metal clustering and apparently showed room-temperature ferromagnetism. The discrepancy between the zero-field cooling and the field cooling curves further indicates that Co-doped ZnO films (for x>0.12) are superparamagnetic and the observed ferromagnetism originates from the nanometer-sized Co clusters.
High-throughput growth temperature optimization of ferroelectric SrxBa1−xNb2O6 epitaxial thin films using a temperature gradient method
Authors: Ohkubo I. , Christen H. M. , Kalinin Sergei V. , Jellison G. E. , Rouleau C. M. , Lowndes D. H.
Published: February 2004

We have developed a multisample film growth method on a temperature-gradient substrate holder to quickly optimize the film growth temperature in pulsed-laser deposition. A smooth temperature gradient is achieved, covering a range of temperatures from 200 to 830 °C. In a single growth run, the optimal growth temperature for SrxBa1−xNb2O6 thin films on MgO(001) substrates was determined to be 750 °C, based on results from ellipsometry and piezoresponse force microscopy. Variations in optical properties and ferroelectric domains structures were clearly observed as function of growth temperature, and these physical properties can be related to their different crystalline quality. Piezoresponse force microscopy indicated the formation of uniform ferroelectric film for deposition temperatures above 750 °C. At 660 °C, isolated micron-sized ferroelectric islands were observed, while samples deposited below 550 °C did not exhibit clear piezoelectric contrast.
Drift Velocities and Thermal Flux Vectors in a Seeded Plasma with Magnetic Fields
Authors: Hassan H. A.
Published: February 2004

Using the thirteen-moment method of Grad, explicit expressions for the drift velocities and thermal flux vectors in a five-component nonequilibrium plasma in the presence of electric and magnetic fields, and pressure, temperature (or concentration) gradients are presented. The results are valid over a wide range of the degree of ionization and in situations where charge neutrality does not hold. When terms of the order of the square root of the ratio of the mass of the electron to the mass of a heavy particle can be ignored, a direct and simple derivation of the generalized Ohm's law in a multicomponent plasma can be obtained. The influence of charge nonneutrality on the electrical conductivity is discussed.
Temporal and Spatial Plasma Wave Echoes
Authors: O'Neil T. M. , Gould R. W.
Published: February 2004

It is shown that, if a longitudinal wave is excited in a collision-free plasma and Landau-damps away, and later a second wave is excited and also damps away, then a third wave will spontaneously appear in the plasma. This wave appears long after the first two waves have damped away at a time proportional to the interval between the first two waves, and is in that sense an echo. It is also shown that, if a wave is continuously excited at one point in a plasma and a second wave is continuously excited many Landau damping lengths from the first point, then a third wave will spontaneously appear many Landau damping lengths from the second point. Fundamentally, plasma wave echoes are possible because of the reversible nature of Landau damping. However, small-angle Coulomb collisions are very effective in destroying the echo.
Collisionless Heating and Wave Amplification through Interaction with a Density Gradient
Authors: Lindman E. L.
Published: February 2004

Collisionless heating occurs when a magnetoacoustic wave travels at right angles to a magnetic field in a plasma whose density is increasing in the direction of travel. Inclusion of pressure terms in a fluid description leads to an interaction of the wave with the density gradient which allows a net transfer of energy from the wave to the plasma. The mechanism is, however, reversible. Consequently a wave traveling out of the plasma is amplified, and the plasma loses energy to it. A method for obtaining a net transfer of energy to the plasma from a wave originating externally by varying the magnetic field strength is discussed along with the effect of the density gradient on other heating mechanisms.
Radiative Modes of Oscillations in a Magnetoplasma Slab
Authors: Suh Kiu S.
Published: February 2004

The radiative modes of electromagnetic wave oscillations in a cold magnetoplasma slab are investigated. The dispersion relations are examined analytically by resorting to various specific cases where simple solutions are obtainable. Numerical solutions are also obtained for the transverse magnetic mode of oscillations where the dispersion relation has a tractable form. For several other cases, the dispersion relation is reduced to forms that are identifiable to the transverse magnetic mode case. Numerous radiative modes that yield large radiative effects are shown to exist.
Resistive Instabilities in a Low-Energy Theta Pinch
Authors: Benford James , Lovberg R. H. , Niblett G. B. F.
Published: February 2004

Schlieren and Kerr cell photography is used to study the effect of reversed trapped field on a low-energy theta pinch. At low plasma temperatures the rapid diffusion of magnetic field is observed to inhibit the growth of resistive instabilities of the tearing-mode form, and this result is in accord with theoretical predictions.
Expansion of a Resistive Plasmoid in a Magnetic Field
Authors: Bhadra D. K.
Published: February 2004

Calculations are presented which show the effect of resistivity in the expansion of a laser-produced hot plasma against a magnetic field. Without resistivity, the expanding plasma has periodic behavior due to bouncing back from the field; but with resistivity, there is superimposed a slow diffusion (across the field) on the periodic oscillations. This is illustrated by considering a spherically symmetric plasma produced by a giant laser pulse (delivering power of the order of 1010W) illuminating a particle of dimension 10-2cm and subsequently following the time development of its radius, skin-depth, and temperature by integrating the nonlinear equations of motion and Ohm's law.
Laminar Diffusion Flame in the Core of a Vortex
Authors: Whang Y. C.
Published: February 2004

A steady laminar diffusion flame burning in the core of a vortex is studied. Solutions of the full Navier-Stokes equation and the mass-transfer equation are obtained to study the effect of the swirling motion on the flame and its flow field for different values of the Schmidt number. The flow conditions depend on two parameters: Γ∞, the circulation of the vortex and Re, the Reynolds number based on the velocity at the axis of symmetry. The velocity distribution is strongly affected by the circulation. As Γ∞ increases when Re remains constant the width of the jet flow in the core region also increases. However, the mass concentration distribution and the position of the flame depend principally on Re, and on Γ∞ only slightly.
Molecular Velocity Distribution-Function Measurements in a Flowing Gas
Authors: Muntz E. P.
Published: February 2004

A technique is described for making accurate, localized measurements of the molecular velocity distribution function in rarefied flows of helium. The method is based on the observation of the Doppler profile of a helium emission line that is excited by a beam of energetic electrons. It has been demonstrated experimentally that distribution-function measurements can be obtained. Applicability of this technique to rarefied gas flows and its accuracy are discussed.
Nonequilibrium Structure of Hydromagnetic Gas-Ionizing Shock Fronts in Argon
Authors: Hoffert Martin I.
Published: February 2004

A theoretical study has been made of the structure of so-called gas-ionizing hydromagnetic shock waves propagating into “cold” argon in the presence of realistic nonequilibrium effects. To satisfy the definition of these waves, it must be postulated that electrons are created solely by thermal ionization within the front. Comparison of the magnitudes of viscous and magnetic Reynolds numbers indicates that if such structures exist, they will consist of a narrow hydrodynamic (viscous)shock, followed by a much larger region of ionization, relaxation and magnetohydrodynamic interaction. For fronts with this architecture, a modified Zeldovich-von Neumann-Döring approximation applies. Moreover, in this approximation it is impossible to construct nontrivial steady-state structures for “fast” gas-ionizing shocks. Solutions are found, however, for “slow” shocks and these are presented for a family of hydromagnetically oblique gas-ionizing fronts at a Mach number of 20 and Alfvén number of 2−12 for parametrically varied values of the upstream electric field.
Vibrational Relaxation of Diatomic Gases behind Shock Waves
Authors: Hsu C. T. , McMillen L. D.
Published: February 2004

Herman and Rubin's work on vibrational relaxation with variable heat bath temperature is extended to the constant enthalpy and constant total enthalpy conditions. Further discussions of this relaxation process and its relation with vibrational relaxation times are made.
Experimentally Determined Structure of the Shock Reflection Process in Ionizing Xenon
Authors: Smith Jerome A.
Published: February 2004

The results of an experimental investigation of the reflection of strong shocks in xenon from the end wall of a shock tube are presented. The reflection of the incident shock structure, consisting of a frozen shock front, a region of relatively uniform frozen flow, and an ionization front, was observed with a fast-rise (0.3 μsec) pressure gauge mounted in the shock-tube end wall. The incident shock Mach number was varied from 11-20, and the initial pressure was varied from 0.1-1.5 mm Hg. The interaction between the reflecting shock and the ionizing gas in the incident shock structure produces a complicated series of shock and rarefaction waves; those waves that propagate back to the end wall were observed with the pressure gauge. A simple model which includes the gross features of the shock reflection process is used to calculate end wall pressures. The calculated pressures agree well with the experimental observations. In addition, ionization relaxation times for xenon behind the incident and reflected shocks are presented. The relaxation time data yield a better understanding of the ionization relaxation process in monatomic gases and provide an estimate for the electron-atom, inelastic cross section for xenon.
Growth of Waves on an Accelerated Jet
Authors: Crowley Joseph M.
Published: February 2004

An electric field normal to a conducting liquid jet causes spatial instability of the kink mode. The growth rate of this instability can be calculated from a dispersion relation only if the jet velocity is constant. In this paper, the growth of waves on a falling jet is studied under the assumption that the fractional change in the velocity is small, and several approximate dispersion relations for the limit of very fast and very slow growth are given.
Hydromagnetic Duct Flow: Theory and Experiment
Authors: Glaberson William I. , Donnelly Russell J. , Roberts Paul H.
Published: February 2004

The theory of hydromagnetic flow through a straight channel in the presence of a uniform applied external magnetic field transverse to the direction of flow is discussed. Examples are worked for the cases of circular, elliptic, and rectangular cross sections. Experiments were performed with such ducts. In the circular and elliptical cases the potential induced across the duct was measured as a function of angle with respect to the applied field. Rectangular ducts were investigated by means of pressure gradient measurements. All results were in good accord with theory except for an aluminum rectangular duct which acted as if it were actually nonconducting, presumably due to an insulating surface layer.
Hydrodynamic Model of a Spherical Plasma Produced by Q-Spoiled Laser Irradiation of a Solid Particle
Authors: Fader Walter J.
Published: February 2004

The motion of a plasma produced by Q-spoiled laser irradiation of a LiH particle has been analyzed as a spherical hydrodynamic expansion of a quasineutral ideal gas of ions and electrons in local thermal equilibrium. Using Dawson's form of the plasma optical absorption coefficient, digital computer calculations were made for laser pulse and particle parameters approximating those of the experiments of Haught and Polk. Calculated distributions of plasma pressure, density, temperature, and velocity are in agreement with solutions obtained for the system of equations by the method of separation of variables. For given laser pulse energy, the calculated plasma energy is maximized for rise times and pulse widths of the order of the time required for expansion of the plasma to the size of the laser focal spot. The calculated energy of the plasma divided by its total number of ions and electrons was found to be approximately independent of the degree of ionization assumed.
Response of a One-Dimensional Vlasov Plasma to External Electric Fields
Authors: Harding Rollin C.
Published: February 2004

The nonlinear response of a Vlasov plasma has been calculated numerically for several externally applied electric field configurations. In the presence of a standing wave of frequency ω0 ≳ ωpe, the energy of the particle distribution and the envelope of the particle electric field were found to oscillate slowly with approximately the period of the trapped particles. Analogous spatial oscillations have been reported by Malmberg and Wharton. Bernstein, Greene, and Kruskal equilibria were calculated for a constant, sinusoidal inhomogeneity in the background ion charge density and found to be numerically stable against small perturbations. Landau-damping rates for the perturbations are compared with the calculations of Jackson and Raether. In the presence of a slowly varying (ω0 ≪ ωpe) inhomogeneous background (simulating an ion acoustic wave) the electrons were found to obey a locally isothermal equation of state to within a few percent for the times that could be calculated. The response of an initially uniform plasma to a suddenly applied external field was also calculated.
Nonlinear and Collisional Effects on Landau Damping
Authors: Denavit J. , Doyle B. W. , Hirsch R. H.
Published: February 2004

In the collisionless case, nonlinear effects considerably reduce Landau damping after times of the order of the period of oscillations of trapped electrons. A perturbation analysis of the nonlinear Vlasov equation, modified by the addition of a simplified Fokker-Planck collision term, is carried out following Montgomery's expansion technique. The modified Landau poles for Maxwellian plasmas are determined. The first-order number density and the second-order spatially homogeneous distribution function are evaluated. A numerical solution of the same equation, using the method of velocity Fourier transforms, is also presented and the results of the two methods are compared. Several values of the collision parameter β, wavelength λ, and initial amplitude ε are considered, showing the competing effects of nonlinearities and collisions on the formation of a plateau in the spatially homogeneous distribution function. Values of β which prevent the formation of the plateau and maintain Landau damping close to the value prescribed by the linear theory are determined.
Plasma Confinement in the General Atomic Toroidal Octopole
Authors: Yoshikawa Masaji , Ohkawa Tihiro , Schupp Arthur A.
Published: February 2004

Plasma confinement in the toroidal octopole device is described. The octopole field is produced by four current-carrying rings which are held in place by supporting rods. A toroidal field may also be applied. The plasma is injected from a coaxial gun. The density is variable from 108cm−3 to 1011cm−3 and the ion and electron temperature is approximately 100 and 6 eV, respectively. Measurements are performed by neutral and ion analyzers and by electrostatic and magnetic probes. The ion confinement time depends on the plasma density. At densities higher than 1011cm−3, the high-energy ions are lost in tens of microseconds by charge exchange interactions with the gas released by plasma bombardment near the injection window. At plasma densities below 1011cm−3 the ion confinement time is several hundreds of microseconds at 100-eV ion energies. The decay is consistent with the loss to the supporting rods. The decay rate is at least an order of magnitude less than predicted from the Bohm diffusion coefficient.
Measurements of Flow Uniformity in Shock Tubes Operating at Low Initial Pressure
Authors: Christiansen Walter H.
Published: February 2004

The rate of heat transfer to fine wire probes was used to measure the flow nonuniformity in a shock tube. Some systematic differences were observed between the measured quantities and those calculated using the theory of Mirels.
Conditions for the Steady One-Dimensional Motion of a Current Sheet
Authors: Burton Rodney L.
Published: February 2004

Current sheets generated in pulsed plasma accelerators propagate into a cold gas with zero initial conductivity. These sheets can possess a steady, one-dimensional structure only if the conductivity of the fluid behind the sheet is also zero.
Numerical Analysis of a Coaxial Accelerator
Authors: Butler T. D. , Cook J. L.
Published: February 2004

The plasma distribution in a coaxial accelerator has been studied using a modified fluid dynamics computing technique. The findings lead to an alternative interpretation of the distribution than that deduced by Lovberg from schlieren photographs.
Fine Structure of Scalar Fields Mixed by Turbulence. I. Zero-Gradient Points and Minimal Gradient Surfaces
Authors: Gibson Carl H.
Published: February 2004

Turbulent mixing of passive scalar properties such as temperature or concentration is discussed. From physical and geometrical considerations it is concluded that the smallest scale features of a steady-state turbulent scalar distribution are primarily determined by the number and distribution of points in the fluid for which the scalar gradient vector is zero, and surfaces for which the gradient magnitude is minimal. Mechanisms for the production, destruction, and motion of such “zero gradient points” and “minimal gradient surfaces” are examined. Initially, zero gradient points must be produced from regions of uniform scalar gradient, but the vast majority result from secondary splitting due to strain induced eccentricities of the closed isoscalar surfaces surrounding maximum or minimum points. An expression for the velocity of surfaces of constant scalar value is derived and used to interpret the Obukhov-Corrsin length scale (D3/ε)14 as the minimum size eddy capable of generating a zero gradient point from a region of uniform gradient, where D is the molecular diffusivity of the scalar and ε is the mean viscous dissipation rate. A steady-state separation distance between zero gradient points of order (D/γ)12 is inferred for scalar fields of arbitrary diffusivity where γ is the root mean square rate of strain of the fluid. Previous theories such as that of Batchelor, Howells, and Townsend have assumed the local strain rate has no effect on scalar structure for large Prandtl number fields.
Role of Diffusion in Conduction in Dielectric Fluids
Authors: Muckenfuss Charles
Published: February 2004

The role of diffusion in the steady conduction process in dielectric fluids is developed. The relation of initial conditions to the mode of conduction is discussed and necessary conditions for the space-charge-limited mode are established. A generalized voltage-current characteristic for the space-charge-limited mode and for a diffusion controlled mode is obtained and properties of the charge density, field, body force, and potential profiles calculated. Certain constraints on integration constants are obtained together with their implications for material and process parameters. The constraints enable us to derive an equation for the volume charge density at a surface of the fluid. For the space-charge-limited mode, aside from material parameters, this is found to be proportional to the two-thirds power of the current density and independent of the thickness of the fluid.
Effect of Coulomb Collisions and Microturbulence on the Plasma Wave Echo
Authors: O'Neil Thomas M.
Published: February 2004

The effect of Coulomb collisions and microturbulence on the recently discovered plasma wave echo is considered. The echo depends on perturbations in the particle distribution which are rapidly oscillating functions of velocity. Coulomb collisions and microturbulence, which consist of many small angle collisions, are particularly effective at smoothing out such perturbations and therefore, at destroying the plasma wave echo. It is shown that in the limit of heavy damping, Coulomb collisions make a second-order electron wave echo sharper, damping the wings of the echo more than the peak. It is also shown that a third-order echo may be used as a tool to experimentally evaluate the velocity dependence of the coefficient of the second velocity derivative in the Fokker-Planck operator. For the case where this coefficient is due to microturbulence one could check the well-known statement of quasilinear theory that this coefficient is just the spectral density of the turbulence.
Simultaneous Measurements of Ion-Acoustic-Wave Potential and Plasma Density Perturbation to Yield γe
Authors: Doucet H. , Alexeff I. , Jones W. D.
Published: February 2004

Simultaneous measurements of ion-acoustic-wave potential and accompanying plasma density perturbation show that the two ratios, eV1/kTe and n1/n0, which are related theoretically by γe, are ≪1 and are equal within the limits of experimental error. Thus, these waves are highly linear and γe is experimentally equal to 1.
Dipole and Quadrupole Bremsstrahlung and the Damping of Electron Plasma Oscillations for any Degree of Ionization
Authors: Shkarofsky I. P.
Published: February 2004

Electron-ion, electron-neutral dipole, and k2 radiations and electron-electron quadrupole k2 radiation are obtained from a calculation of current with quadratic gradient terms and agree with other derivations. The collisional time damping of electron plasma oscillations is also evaluated, and agrees with parts of other papers in the literature. A negative k2 collisional electron-ion contribution is found which disagrees with other analyses and computer experiments.
Ion-Ion Velocity Space Instability
Authors: Grewal Mahesh S.
Published: February 2004

An electrostatic instability caused by the difference between θ⊥-i and θ‖-i, the ion temperatures perpendicular and parallel to magnetic field, is discussed. Results show that the instability disappears as the ratio θ⊥-i/θ‖-i approaches roughly a value of 16 and also when for a given value of θ⊥-i/θ‖-i the ratio θ⊥-i/θ‖-e becomes large, θ‖-e being the electron temperature parallel to the magnetic field. From the results it is also concluded that this instability disappears, irrespective of the values of θ⊥-i/θ‖-i, and θ⊥-i/θ‖-e, whenever the plasma density is low enough so that the square of ion gyrofrequency is greater than about 2.8 times the square of ion plasma frequency.
Lack of Equilibrium due to Magnetic Guarding
Authors: Meade D. M.
Published: February 2004

A calculation of the initial acceleration of a plasma slab along the null of a magnetic dipole guard indicates that magnetic dipole guarding should not appreciably decrease plasma losses.
Decay of Neutral Wave into Ion Waves
Authors: Ohnuma Toshiro , Hatta Yoshisuke
Published: February 2004

Neutral wave with large amplitude is treated against the decay into two ion-acoustic waves in a weakly ionized plasma.
Conservation of Circulation Theorem for Superfluids
Authors: Saffman P. G.
Published: February 2004

The reversible two-fluid equations of helium II are shown to possess two integrals of circulation. The first, which expresses conservation of circulation in the superfluid, is well known, but the second, which expresses conservation of the line integral of ρn(un − us)/(ρs) around a circuit moving with the normal fluid, appears to be new.
Jets and Wakes in Tailored Pressure Gradient
Authors: Wygnanski I. , Fiedler H. E.
Published: February 2004

Similarity solutions of the boundary-layer equations representing the flow of jets in an external stream and tailored pressure gradients were obtained. These solutions apply to jets in coflowing and counterflowing streams. A number of analytical solutions not previously published were obtained. Of particular interest are the solutions for small-increment jets which imply that a Gaussian velocity profile exists far downstream from the origin of the jet, even in arbitrary pressure gradient. The solutions were extended to turbulent flow using the hypothesis of a constant eddy viscosity, which in turn, was modified to account for intermittency. The theoretical predictions were compared with the measurements of Gartshore in self-preserving wakes and the agreement between theory and experiment is good.
Electroconvective Instability with a Stabilizing Temperature Gradient. I. Theory
Authors: Turnbull Robert J.
Published: February 2004

A uniform vertical electric field produces an instability in a poorly conducting liquid subject to a vertical temperature gradient. A gradient in conductivity resulting from the temperature gradient causes free charge to accumulate in the fluid when an electric field is applied. For the cases considered, the gradient in dielectric constant can be neglected with the significant electric force being that due to the free charge. The threshold conditions for the instability are predicted using linear perturbation theory. Approximations are made which allow the equations with space-varying coefficients to be solved. The analysis shows that, for liquids with short or moderate electrical relaxation times, the electric field causes the internal gravity wave propagating downward to become unstable.
Electroconvective Instability with a Stabilizing Temperature Gradient. II. Experimental Results
Authors: Turnbull Robert J.
Published: February 2004

A uniform vertical electric field produces an instability in a poorly conducting fluid subject to a vertical temperature gradient. A previous paper presents the theory for this instability. Here measurements are made of the voltage at the threshold for instability. Since the instability occurs as an over-stability, the frequency at the threshold was also measured. These experimental results support the theory. Heat-transfer measurements show a significant increase in heat flow under the influence of electric fields.
Time Harmonic, Spherical Harmonic, and Power Series Expansion of the Boltzmann Equation
Authors: Morrone Terry
Published: February 2004

The time dependence of the expansion coefficients of the spherical harmonic expansion of the electron velocity distribution are expanded in a Fourier series. The new coefficients are in turn expanded in a power series in the small parameter δ, the fraction of energy and electron loses per collision. Relationships between the coefficients are derived from the Boltzmann equation. The expansion is applied to calculating harmonic currents in weakly ionized gases in alternating electric and constant magnetic fields.
Wave Coupling across a Density Discontinuity in a Magnetized Plasma
Authors: Morin Stephen , Kahn David
Published: February 2004

The plane-wave solutions of the linearized hydrodynamic equations and Maxwell's equations for a fully ionized plasma are used to investigate the properties of plasma oscillation modes which propagate perpendicular to an external magnetic field. Energy conversion among these modes at a density discontinuity separating two semi-infinite plasmas is investigated. Numerical results illustrate the variations of various energy coupling efficiencies with angle of incidence, propagation frequency, and magnetic field intensity. In contrast to the field-free case previously considered, it is found that energy conversion occurs among all of the plasma modes for normally incident waves. Very high efficiencies of energy conversion between field-modified electron and transverse waves for propagation frequencies below the hybrid frequency (ωe2 + ωB2)12 where ωe and ωB are, respectively, the electron plasma frequency and the electron cyclotron frequency are also found.
Longitudinal Wave Propagation in Viscous Fully Ionized Plasmas
Authors: Kahn David
Published: February 2004

A collisional theory of wave propagation in two fluid fully ionized gases is formulated from the coupled set of Maxwell and Boltzmann equations for an investigation of the collisional damping and dispersion of electron and ion waves. The electron and ion velocity distribution functions of the Boltzmann equations are expanded in Sonine polynomials to an order that gives the collision integrals for the electron and ion self-interactions as well as for the electron-ion interactions. This approximation permits the determination of viscous as well as friction-caused electron and ion wave attenuation and dispersion. It is found that the inclusion of the self-interaction integrals (viscous terms) lead to values of the attenuation and dispersion functions which are substantially different quantitatively (and in some frequency regimes, also qualitatively) from the values obtained previously without these terms. (Major ion wave damping, for example, occurs when the wave and ion-ion collision frequencies are comparable, a result not obtainable from the inviscid theory.) The attenuation and dispersion functions are numerically (and in several limiting cases of interest, analytically) evaluated as functions of the self- and interparticle collision to wave frequency ratios and as functions of the plasma to wave frequency ratios for several different plasmas characterized by different collision to plasma frequency ratios.
Microwave Scattering from Plasma Waves
Authors: Wharton C. B. , Malmberg J. H.
Published: February 2004

Experimental studies of the scattering of electromagnetic waves (34 GHz) from longitudinal plasma waves (250-500 MHz) are described. The plasma waves propagate in a 230-cm-long collisionless plasma column, having an electron density on axis of ne0 = 2.7 × 109cm−3 and an electron temperature Te ≈ 13 eV. Scattering was observed both from plasma waves launched by a probe antenna and from waves grown up to a nonlinear saturation out of the white noise background by resonant interaction with an electron beam. The properties of these waves are accurately determined by auxiliary measurement. By using movable antennas we are able to measure the spatial growth and damping coefficients of plasma waves and to measure their absolute amplitudes. The microwave scattering is observed at a frequency displaced from the microwave frequency by the plasma wave frequency, and at an angle related to the Bragg angle. The measured frequency spectrum, angular dependence, and scattering amplitudes are compared with the theory and with rf probe measurements, giving excellent agreement in all cases.
Nonlinear Theory of Low-Frequency Instabilities
Authors: Dupree Thomas H.
Published: February 2004

It is proposed that the dominant nonlinear effect of low-frequency instabilities is an incoherent scattering of particle orbits by waves. The scattering is considered to be incoherent even when only a single mode is present. Because it is incoherent, one can include wave scattering in the theory just as collisional scattering is included in the linear wave theory. A nonlinear dispersion relation is obtained from the linear dispersion relation simply by adding wave scattering terms to the collisional scattering terms. The wave scattering causes particle diffusion over the transverse wavelength and in the theory appears as an enhanced viscosity. This wave induced viscosity can be computed from the amplitude of the wave. Waves which are linearly unstable grow until the enhanced scattering causes their nonlinear growth rates to vanish. Theoretically computed values of density fluctuations, phase shifts, and flux are in good agreement with experimental values in the cases considered.
Weak Turbulence Theory of Velocity Space Diffusion and the Nonlinear Landau Damping of Waves
Authors: Manheimer Wallace M. , Dupree Thomas H.
Published: February 2004

The particle-wave interaction to arbitrary order in perturbation theory is investigated. One finds that to every order the ensemble average distribution function obeys a diffusion equation. The diffusion constant specifies how the energy and momentum of particles change in time. By conservation of energy and momentum between waves and particles, the growth or damping rates of waves to any order can be determined. The growth rates for nonlinear instabilities and the damping rates for nonlinear Landau damping are explicitly calculated.
Experiment in Stochastic Heating
Authors: Puri S. , Dunn D. A. , Thomassen K. I.
Published: February 2004

A collisionless plasma in a magnetic mirror is heated by a random electric field, acting transverse to the magnetic field. Heating occurs when the noise spectrum spans the band of electron cyclotron frequencies existing in the mirror. The electron heating rate, 1.0 × 105 〈E2(V/cm)2〉 eV/sec, is in reasonable agreement with the value predicted by a simplified theoretical analysis.
Numerical Studies of Turbulent Heating
Authors: Gary S. Peter , Montgomery David
Published: February 2004

The equations of motion of electrons subject to different combinations of electrostatic waves were integrated numerically. Magnetic fields were ignored throughout. For particular choices of wave amplitude, wavenumber, frequency, and phase, and for given initial velocities, there is calculated the mean-square velocity spreading of a collection of particles subject to three different kinds of wave spectra: (a) a single monochromatic wave; (b) several (up to 10) monochromatic waves with differing phase velocities; and (c) a single wave with a random phase change at regular intervals. Heating occurs due to two basic effects: the “trapping” of particles, operative in all three spectra, and the spreading of the spectral density due to the random phase interruptions in (c). For the parameters chosen, numerical results are that the most efficient heating occurs in case (c).
Guiding Center Drifts in Time-Dependent Meridional Magnetic Fields
Authors: Birmingham Thomas J.
Published: February 2004

The problem of a particle trapped in a time-dependent meridional magnetic mirror field and at the same time subjected to a time-dependent perpendicular electric field (E·B = 0) is considered. Bounce-averaged guiding-center theory is used to derive expressions for the drift velocity conponents. These drift expressions depend in a straightforward fashion on the structures of the magnetic and electric fields and on the particle charge, kinetic energy, longitudinal invariant, and bounce period. In the special case of a static magnetic field, no electric field, and J = 0 particles, the drift equations are integrated and the bounce-averaged guiding-center trajectory obtained.
Kinetic Model for the Shock Structure in a Binary Gas Mixture
Authors: Beylich Alfred E.
Published: February 2004

An extension of the two-fluid model to the problem of plane stationary shock waves in binary mixtures of monatomic inert gases is made. No overshoot exists in the velocity profiles; however, there is an overshoot in the temperature profile.
Form of Inelastic Transport Cross Sections
Authors: Fox R. L.
Published: February 2004

The form of the inelastic transport cross sections is developed from the macroscopic transport equations. The diffusion cross sections for the Lyman α transitions in hydrogen are discussed.
Stability of Electromagnetic Waves Propagating Perpendicular to the Magnetic Field in a Mirror Configuration
Authors: Ossakow Sidney L.
Published: February 2004

In an infinite plasma with an applied unform magnetic field B0 the loss cones of an anisotropic Maxwellian velocity distribution for electrons has a stabilizing effect on linearly polarized electromagnetic waves propagating perpendicular to B0.
Lagrangian-History Statistical Theory for Burgers' Equation
Authors: Kraichnan Robert H.
Published: February 2004

The Lagrangian-history direct-interaction approximation for Burgers' equation yields a k−2 inertialrange spectrum and an infinite-Reynolds-number similarity solution which describes stationary, decaying sawtooth shock waves. This solution differs by a numerical factor from an exact statistical solution of Burgers' equation into which almost all spatially periodic, zero-mean, infinite-Reynolds-number initial ensembles are expected to evolve. The different inertial-range predictions of the approximation for Burgers' equation and Navier-Stokes dynamics (where k−5/3 results) are directly associated with the effects of pressure-induced accelerations on Lagrangian correlation times.
Magnetic Annular Arc-Driven Shock Tube
Authors: Schneiderman Arthur M. , Pugh Evan R. , Patrick Richard M.
Published: February 2004

The production of high-speed shocks in shock tubes has been limited to levels below 1.3 cm/μsec by excessive radiation from the initially stagnant arc-heated driver gas. This limitation can be removed by adding the required energy to a flowing driver gas in a device in which the static temperature is maintained at a sufficiently low level as to keep the radiation losses small. The magnetic annular arc has demonstrated such capabilities in applications for space propulsion and solar wind simulation. Based on this past experience, a magnetic annular arc driver was constructed for a 38.1-mm (1.5 in.) shock tube and was operated at an average power level of 200 MW for 50 μsec at a voltage of 1kV. Helium was used as the driver gas and shock speeds in excess of 2 cm/μsec with a test time of approximately 5 μsec were obtained in air. It was also determined that the empirical scaling relations for the magnetic annular arc operating voltage and anode heat transfer observed in earlier applications are still applicable at the higher-power densities and mass flows appropriate to this experiment.