bluuebunny/crossref_metadata_2025 huggingface dataset:
Experiment on the interaction between a plasma and a neutral gas. II
Authors: Danielsson L. , Brenning N.
Published: March 2003

An interaction between a plasma and a neutral gas in relative motion is demonstrated. If the relative velocity is high enough, the interaction leads to heating of the electrons, partial ionization of the gas, and braking of the motion. The limiting velocity is found to be very close to Alfvén’s critical velocity. Two new means of plasma diagnostics are tried for the first time in this experiment. Measurements of the polarization of emitted spectral lines have yielded information on the velocity distribution of the electrons. An ion beam interaction technique has been employed to determine the density of energetic electrons. New evidence is also presented of electron heating and of the necessity of a magnetic field for the interaction to take place. The previously observed minimum gas density is also discussed.
Wave propagation in a quasi-chemical equilibrium plasma
Authors: Fang Ta-Ming , Baum Howard R.
Published: March 2003

Wave propagation in a quasi-chemical equilibrium plasma is studied. The plasma is infinite and without external fields. The chemical reactions are assumed to result from the ionization and recombination processes. When the gas is near equilibrium, the dominant role describing the evolution of a reacting plasma is played by the global conservation equations. These equations are first derived and then used to study the small amplitude wave motion for a near-equilibrium situation. Nontrivial damping effects have been obtained by including the conduction current terms.
Low frequency resonance cone structure in a warm anisotropic plasma
Authors: Burrell Keith H.
Published: March 2003

Asymptotic analysis of the electrostatic Green’s function for an antenna in a warm, anisotropic, nearly collisionless plasma reveals the presence of new resonance cone structures which exist in addition to the cold plasma resonance cone for frequencies below the lower hybrid frequency. These new cones, the ion acoustic resonance cones, occur for frequencies 0 ⩽ ω ⩽ min(ωpi, ωci) and max(ωpi, ωci) ⩽ ω ⩽ ωlh; they exist only when the electron temperature is nonzero and are important when Ti/Te ≪ 1. The interference structure associated with this cone is quite sensitive to the ion temperature and could be the basis of an ion temperature diagnostic technique for plasmas that meet the conditions of the theoretical derivation: Ti/Te ≪ 1 and ωci ≫ ωpi.
Measurement of fluid velocity inside porous media with a laser anemometer
Authors: Johnston William , Dybbs Alexander , Edwards Robert
Published: March 2003

A successful measurement of fluid velocity inside porous media is presented. A laser anemometer was used to measure velocities in a packed bed of transparent spheres. The refractive index of the spheres was matched with that of the flowing fluid.
Defeat of shear stabilization by peaking of the diamagnetic frequency profile
Authors: Sauthoff N. R. , Okabayashi M. , Schmidt J. A.
Published: March 2003

The radial wave equation describing collisionless drift instabilities in plasma is solved including the effects of a peaked diamagnetic frequency profile. Radially localized fluctuations with no radial energy propagation are found in cases with sufficiently low shear and large peaking.
Effect of rotation on dynamic stabilization of a stratified fluid
Authors: Chakraborty B. B. , Bandyopadhyay Manjari
Published: March 2003

Vertical oscillations, together with a uniform rotation about a vertical axis, are found to have a stabilizing influence on a stratified fluid with density increasing exponentially in a direction (vertically up) opposite to that of gravitational acceleration.
Turbulent mixing in a two-dimensional jet
Authors: Uberoi Mahinder S. , Singh Param Indar
Published: March 2003

An ensemble of instantaneous temperature profiles across a two-dimensional heated jet was obtained by shooting a fine platinum resistance thermometer across the jet at speeds much higher than the local velocity of the jet. Each recording of the ensemble was shifted to have a common center, and ensemble averages were obtained counting only the profiles for which the spatial location in question was inside the jet. This conditional averaging produced mean and mean-square fluctuations which are quite flat across the jet, indicating that the fluid inside the jet is well-mixed. Visual inspection of instantaneous records showed no cold spots in the jet. Ensemble averages were also measured by aligning the edge or beginning of the instantaneous profiles. This led to a sharper rise in the mean temperature profile and an overshoot in the mean-square temperature fluctuations. This indicates that mixing across the turbulent-nonturbulent interface is stronger than in the main body of the jet.
Ionization nonequilibrium effects on a radiating shock layer
Authors: Sabetta F. , Cunsolo D. , Cenedese A.
Published: March 2003

The stagnation region of a shock layer in front of a blunt body re-entering the atmosphere at high altitude and high velocity is considered. At these flight conditions the ionization is out of equilibrium in a considerable fraction of the shock layer thickness. The nonequilibrium phenomenon modifies the distribution of all the thermodynamic, radiative, and transport properties of the gas. To quantitatively evaluate these modifications and their influence upon the radiative heat flux, the governing equations for nonequilibrium conditions are obtained and numerically solved for real air. A simple method of evaluating nonequilibrium absorption and emission coefficients is also proposed. The numerical results show that, while nonequilibrium slightly affects the body heating load, important variations occur in the shock layer structure and the radiative flux emergent at the shock front is considerably reduced with respect to complete equilibrium conditions. This fact must be taken into account in all studies dealing with precursor effects.
Mobilities of K+ ions in hot gases
Authors: Mason E. A. , Viehland L. A. , Ellis H. W. , James D. R. , McDaniel E. W.
Published: March 2003

The zero-field mobilities of K+ ions in He, Ne, Ar, H2, D2, N2, O2, NO, CO, and CO2 at gas temperatures ranging from 300 °K to ≈104 °K have been computed from room temperature mobilities measured as a function of E/N, the ionic energy parameter.
Stability of large aspect ratio diffuse profile tokamaks with circular flux surfaces
Authors: Vahala George
Published: March 2003

It is shown that the stability of axisymmetric diffuse profile tokamaks is always governed by that of the straight cylindrical screw pinch if one assumes small deviations from circular flux surfaces and small inverse aspect ratio ε, regardless of the ordering of β in ε. Unstable modes with very small growth rates or those which are very highly localized are excluded from this treatment. Suydam-type modes are included. The sharp boundary and diffuse profile results are compared and commented on.
Electromagnetic ion cyclotron instability driven by ion energy anisotropy in high-beta plasmas
Authors: Davidson R. C. , Ogden Joan M.
Published: March 2003

Low frequency (ω≈ωci=eiB0/mic) transverse electromagnetic perturbations propagating parallel to a confining magnetic field B0êz are shown to exhibit instability in the presence of ion energy anisotropy with Ti⊥≳Ti∥. The characteristic maximum growth rate for Ti⊥≫Ti∥ is γM≈(βi⊥/2)1/2ωci, where βi⊥=8πniTi⊥/B20, and the wavelengths corresponding to instability are of order c/ωpi, where ωpi is the ion plasma frequency. Within the context of a quasi-linear model, it is shown that the characteristic time scale for energy isotropization through nonlinear response of the ions to the instability is several γ−1M. Since γ−1M≪τii (the ion-ion binary collision time) in typical high-density pinch experiments, this instablity appears to provide a viable collective mechanism for ion energy isotropization during the implosion or post-implosion phases of these experiments. It is also shown that the instability persists in the limit of weakly magnetized ions (‖ω+iγ‖≫‖ωci‖, k2zr2Li≫1) and strongly magnetized electrons (‖ω+iγ‖≪‖ωce‖, k2zr2Le≪1) provided βi⊥≫1. The instability also has applications to astrophysical plasmas with Ti⊥≳Ti∥ as well as laboratory plasmas heated by neutral-particle injection perpendicular to B0.
Stagnation-point heat transfer correlation for ionized gases
Authors: Bade W. L.
Published: March 2003

On the basis of published laminar boundary layer solutions for argon, xenon, nitrogen, and air, the effect of external-flow ionization upon stagnation point heat transfer is correlated by the 0.7 power of the Prandtl number ratio across the boundary layer.
Finite heat conduction effects on ion cyclotron and drift cylotron modes
Authors: Hudson Mary K.
Published: March 2003

Finite heat conduction effects on the ion cyclotron and drift cyclotron instabilities are investigated, and found to be destabilizing in certain regions of parameter space (ν∥≫ω).
Inverse problem in transport theory. II
Authors: Case K. M.
Published: March 2003

Earlier work on a simple inverse transport problem is extended. An infinite set of sum rules are obtained which relate sums of products of the Legendre coefficients of the scattering function to Fourier coefficients of the logarithm of the spectral function. Further, simple formulas are obtained for sensitivity coefficients which describe how the Legendre coefficients are altered by small changes in experimentally measured moments.
Lift-generated instability of the plane Couette flow of a particle-fluid mixture
Authors: Drew Donald A.
Published: March 2003

The linear stability of the plane Couette flow of a dilute particle-fluid mixture with lift forces included is studied. An instability is found for Stokes number greater than about 1.23×10−4. An initial particle velocity component at an angle of about 135° from the direction of the fluid flow will grow as it propagates with the fluid for Stokes numbers greater than the critical value.
Stability of plane Couette flow with respect to finite two-dimensional disturbances
Authors: Lessen Martin , Cheifetz Michael G.
Published: March 2003

The stability of plane Couette flow is studied by applying a finite two-dimensional disturbance to the basic laminar motion and following the timewise evolution of the quasi-steady flow and the disturbance. This is accomplished by combining a Meksyn–Stuart type method with a quasi-steady technique. Results show that plane Couette flow tends to slowly converge to a stable state for the Reynolds numbers and disturbance intensities studied. This is in disagreement with past investigations which were able to obtain nonlinear neutral stability curves.
Nonparallel stability of boundary-layer flows
Authors: Saric William S. , Nayfeh Ali Hasan
Published: March 2003

The spatial stability of two-dimensional incompressible boundary-layer flows is analyzed using the method of multiple scales. The analysis takes into account the streamwise variations of the mean flow, the disturbance amplitude, and the wavenumber. The theory is applied to the Blasius and the Falkner–Skan flows. For the Blasius flow, the nonparallel analytical results are in good agreement with the experimental data. The results show that the nonparallel effects increase as the pressure gradient decreases.
Aspects of ’’multitime initial-value problem’’ originating from boundary layer equations
Authors: Wang K. C.
Published: March 2003

Three-dimensional steady boundary layer equations are considered as a model case of a more general class of problems called ’’multitime initial-value problem’’ for a parabolic system. Ideas developed in recent years for solving three-dimensional steady boundary layers are generalized for application to other similar problems, such as unsteady two-dimensional boundary layers. An unsteady counterpart of the wedge-shaped zones of influence and dependence is defined and then used to rationalize the calculation of reversed flow in two-dimensional unsteady case. Schemes used previously for the latter flow are re-examined in light of this dependence rule, and it is found that these schemes could satisfy such a rule.
Synchrotron radiation spectrum for a relativistic plasma column
Authors: Davidson R. C.
Published: March 2003

The synchrotron radiation spectrum is calculated for a relativistic nonneutral plasma column that rotates about an axis of symmetry aligned parallel to a uniform applied magnetic field B0êz. The applied magnetic field provides radial confinement of the electrons, and the electron density is assumed sufficiently low so that equilibrium self-fields have a negligible effect on the particle trajectories (ω2p/ω2c ≪1, where ωp is the electron plasma frequency and ωc is the electron cyclotron frequency). The analysis is carried out for the general class of radially confined equilibria described by equilibrium distribution functions of the form f0e(x,p) =F (H) G (Pϑ) δ (pz), where H is the energy, Pϑ is the canonical angular momentum, and pz is the axial momentum. The detailed form of the emission spectrum Il(ω) exhibits a sensitive dependence on the choice of distribution functions F (H) and G (Pϑ). The characteristics of the emission spectra for hollow electron layers are quite different from the emission spectra obtained when the electron density is peaked on axis. Since the influence of equilibrium self-fields on the particle trajectories is neglected in the present analysis, the results can also be applied to the synchrotron emission from rotating neutral plasma columns in which there is over-all charge neutrality.
Observation of clumps in simulation plasma
Authors: Dupree Thomas H. , Wagner C. E. , Manheimer Wallace M.
Published: March 2003

Phase space density granulations (or ’’clumps’’) have been observed in a one-dimensional computer simulation plasma. Fluctuations were initially distributed over a wide range of velocities in a Maxwellian plasma. Clumps were observed at the thermal velocity 150 plasma periods later, long after the fluctuations should have decayed down to the thermal equilibrium level according to conventional theory. Within the accuracy of the simulation, the phase space density correlation function obtained from the simulation agrees well with the theoretical expression for clumps.
Observability and controllability of plasma instabilities
Authors: Sen Amiya K.
Published: March 2003

Although the questions of whether all the dynamical states are accessible (’’observability’’) and amenable to control (’’controllability’’) play central roles in control science and engineering, these have been completely ignored in the literature of control of plasma instabilities. These problems are discussed and the general conclusion is that any plasma instability of discrete spectrum is completely observable and controllable with a few, but, most often, a single pair of, suitably located point sensor and suppressor.
Instabilities in toroidal plasmas
Authors: Pao Young-ping
Published: March 2003

Surface-preserving instabilities in axisymmetric toroidal plasmas are studied by a normal-mode analysis. Their growth rates are calculated. Localized instabilities and noncircular diffuse pinches are also examined.
Two waves on a beam-plasma system
Authors: DeNeef Peter
Published: March 2003

The nonlinear behavior of two unstable waves that grow simultaneously at closely spaced frequencies on a low density, initially monoenergetic beam-plasma system is calculated. The exponential spatial growth of both waves stops in the region where the beam is trapped by the waves. When the initial amplitude ratio ‖ε‖ (⩽1) of the two waves exceeds 0.4, both waves play a significant role in the trapping, so the larger wave saturates at a lower potential than when launched alone. The behavior of an intermodulation product is calculated as a contribution to the spectral broadening, and the total energy in this wave plus the two launched waves is shown to evolve spatially in approximately the same way as the energy of a single frequency wave that is launched alone.
Effect of temperature structure on the initial acceleration of a spherical thermal
Authors: Hains F. D.
Published: March 2003

The internal vortex motion observed in nuclear fireballs as they begin to rise is due to differential buoyancy associated with its internal temperature structure, rather than to viscous effects.
Spatially growing waves in an ion beam-plasma system
Authors: Fujita T. , Ohnuma T. , Adachi S.
Published: March 2003

An externally excited ion wave in an ion beam-plasma system was found to grow spatially in the case where the ion beam velocity is nearly equal to an ion acoustic velocity. The phase velocity and growth rate are in good agreement with the results based on the kinetic dispersion equation.
Analysis of a semiclassical model for rotational transition probabilities
Authors: Deiwert George S. , Yoshikawa Kenneth K.
Published: March 2003

A semiclassical model for rotational transition probabilities in dilute gases is used to compute the relaxation of a gas to equilibrium. The governing Boltzmann equation is solved via the direct simulation Monte Carlo method.
Wake field of a charged particle in a conducting medium
Authors: Hirakawa Hiromasa
Published: March 2003

A closed form solution for the electromagnetic field of a charged particle passing through a medium of finite conductivity σ is presented. The wake field of the particle extends over a region of a characteristic length 1/μ0σv, and is confined behind a plane which contains the particle. The electric lines of force reverse themselves, indicating current neutralization by the changing magnetic field. It is noted that the induced current around the charge has no dc component.
Nonlinear, three-dimensional magnetohydrodynamics of noncircular tokamaks
Authors: Strauss H. R.
Published: March 2003

Rosenbluth’s nonlinear, approximate tokamak equations of motion are generalized to three dimensions. The equations describe magnetohydrodynamics in the low β, incompressible, large aspect ratio limit. Conservation laws are derived and a well-known form of the energy principle is recovered from the linearized equations. The equations are solved numerically to study kink modes in tokamaks with rectangular cross section. Fixed-boundary kink modes, for which the plasma completely fills the conducting chamber, are considered. These modes, which are marginally stable to lowest order in circular tokamaks, become unstable with large growth rates, comparable to the growth rates of free boundary kink modes. The unstable modes are found using linearized, two-dimensional equations. The linear results are used as initial values in the nonlinear, three-dimensional computations. The nonlinear results show that the magnetic field is perturbed only slightly, while a large amount of plasma convection takes place carrying plasma from the center of the chamber to the walls.
Orthogonal conductivity of a toroidal plasma
Authors: Boozer Allen H.
Published: March 2003

The orthogonal conductivity of a toroidal plasma is calculated in the fluid regime. If the damping time for toroidally directed angular momentum is τN, the orthogonal conductivity is shown to be σ⊥= (ρc2/B2p)/τn for large τN. Here ρ is the mass density, c is the speed of light, and Bp is the poloidal component of the magnetic field. For large τN, the flow induced by the orthogonal electric field is almost purely toroidal and of magnitude c (Er−E0r)/Bp where E0r is the electric field required for ambipolar diffusion.
Frictional force exerted on a slowly rotating eccentrically positioned sphere inside a circular cylinder
Authors: Greenstein T. , Som T. J.
Published: March 2003

Approximate expressions are presented and numerical values provided which enable the frictional force to be computed for the case when an eccentrically positioned sphere arbitrarily rotates in a viscous fluid bounded externally by an infinitely long circular cylinder.
Existence and convergence proof in the calculation of axisymmetric magnetohydrostatic equilibria
Authors: Sestero A.
Published: March 2003

A typical interation scheme for the calculation of axisymmetric magnetohydrostatic equilibria is examined, and an existence and convergence proof is derived, in terms of a few classical results of functional analysis.
Drag on an axially symmetric body in the Stokes’ flow of micropolar fluid
Authors: Ramkissoon H. , Majumdar S. R.
Published: March 2003

The Stokes’ flow problem is considered for micropolar fluids in which the obstacle has an axis of symmetry, and the flow at distant points is uniform and parallel to this axis. A general expression for the drag is derived by using the arguments involving an axisymmetric point force and application is illustrated for the flow past a sphere.
Electron transport properties in high-temperature air
Authors: Devoto R. S.
Published: March 2003

The electrical conductivity and electron thermal conductivity and thermal diffusion coefficient have been computed for air in thermal equilibrium at temperatures of 2000–20 000 °K and pressures of 1, 10, and 100 atm. Comparison of the electrical conductivity with experiments in the wall-stabilized electric arc show good agreement with one set of experiments but only fair agreement with another.
Theory of intense ion beam acceleration
Authors: Antonsen Thomas M. , Ott Edward
Published: March 2003

The application of high voltage pulse power techniques to the production of intense ion beams is of great interest for plasma confinement, plasma heating, and pellet implosion. The main problem is that application of a high voltage to a simple anode–cathode gap will draw both an electron current from the cathode and an ion current from the anode, and the electrons will receive most of the input energy due to their smaller mass. Two methods of efficient intense ion beam production are considered: (1) the magnetically insulated diode and (2) the ’’reflex-triode’’. The relativistic equilibria, the ion current dependence on accelerating voltage, its dependence on applied magnetic field (in the first method), and its variation due to a velocity distribution of the electrons (in the second method) are determined. For both methods the ion current can be substantially enhanced with respect to the Langmuir–Child current due to the presence of the negative electron space charge. In the case of magnetic insulation this enhancement increases as the magnetic field is lowered and diverges as it approaches the critical value past which electrons can traverse the gap. For the reflex triode the enhancement is increased by relativistic electron effects and by a population of electrons with energies less than the full voltage across the gap.
Steady wake behind a sphere
Authors: Nakamura Isao
Published: March 2003

The characteristics of the steady wake behind a sphere were studied experimentally using dyed water for visualization. This work infers the possibility of the existence of a closed recirculating eddy behind a sphere even at R=10 or lower and shows that the eddy can preserve its steady state up to the case for R=190 (R represents the Reynolds number based on the diameter of the sphere). In addition, a new profile for the variation of the wake separation angle against the steady state Reynolds number is presented.
Instability as a source for traveling ion waves
Authors: Sato N. , Popa G. , Märk E. , Mravlag E. , Schrittwieser R.
Published: March 2003

Low-frequency instability is excited by passing a dc electron current in a collisionless Q-machine plasma. The current is driven by applying a positive bias to a grid situated between the hot plate and the cold end plate. The instability shows a standing-wave pattern between the hot plate and the grid. The frequency can be controlled by varying the hot plate-grid separation. This instability is used as a source for excitation of traveling waves behind the grid. The excited waves have the same dispersion relation as ion waves excited by applying external signals to the grid.
Nonlinear development of absolute and convective instabilities
Authors: Seidl M. , Carr W. , Boyd D. , Jones R.
Published: March 2003

The instability resulting from the interaction of a weak, monoenergetic electron beam with a magnetoplasma (ω≳ωCE) has been studied experimentally. A calculation, using the dispersion relation appropriate to the experiment, shows that the interaction may take the form of convective or absolute instability, depending on plasma temperature. This is in qualitative and quantitative agreement with the experiment. The convective mode has spatially amplified waves which are steady state in time. Saturation is due to beam trapping, and agrees quantitatively with a trapping calculation which includes cyclotron damping. After saturation the wave damps, which is attributed to untrapping of the beam by relatively low-level amplified plasma noise. The absolute instability is a feedback oscillation due to the negative group velocity in the plasma. It rises suddenly to large amplitude near the electron gun and has a short over-all length. There are two nonlinear states: one is steady state in time, and agrees with a beam trapping calculation for absolute instability; the second has temporal quenching, or relaxation oscillations. This is attributed to the interaction of several oscillation modes strongly coupled by the nonlinear beam dynamics. This effect may limit wave amplitudes below the value predicted by trapping theory.
Computer simulation of nonlinear interaction between a monochromatic whistler wave and an electron beam
Authors: Matsumoto Hiroshi , Yasuda Yutaka
Published: March 2003

A computer experiment on the self-consistent nonlinear interaction between a large amplitude whistler mode wave propagating along an external magnetic field and plasma particles is described. The plasma is composed of cold electrons, cold ions, and a hot electron beam with the mean velocity around the resonance velocity of the propagating large amplitude whistler mode wave. In addition to an amplitude oscillation of the whistler mode wave, an interesting evolution of the velocity distribution function of the hot beam electrons is observed. These results are first compared with the theory by Palmadesso and Schmidt and then are explained by a simpler physical picture. The computer code for the present simulation followed nonrelativistic dynamics of 3072 electrons in a self-consistent electromagnetic field. The total energy in the system under consideration was conserved well with a maximum numerical error of 0.3%.
Effect of secondary flow on the decaying torsional oscillations of a sphere and a plane
Authors: DiPrima R. C. , Liron N.
Published: March 2003

The correction to the torque on a torsionally oscillating sphere in a viscous fluid due to the induced secondary flow is calculated when the thickness of the shear layer is small compared with the radius of the sphere. The calculations correct an error in the formulas given by Carrier and DiPrima. Using the method of multiple scales, the effect of finite amplitude on the logarithmic decrement and added moment of inertia of the sphere are obtained for the decaying oscillations of the sphere. The results are in good agreement with the experimental measurements of Folse, Strahan, and Nail. Similar formulae are given for the decaying oscillations of a plane.
Statistical mechanics of two-dimensional vortices in a bounded container
Authors: Pointin Y. B. , Lundgren T. S.
Published: March 2003

The equilibrium statistics of a large number of two-dimensional point vortices of arbitrary sign, evolving in an arbitrary domain closed by a bounded curve, are investigated in the microcanonical formulation. The resulting differential equations for the spatial distribution function of the vortices are numerically integrated in various cases and the associated thermodynamic functions are computed. The case of a globally neutral, spatially uniform distribution is particularly studied for its connections with two-dimensional turbulence and the use of the random phase approximation. Some numerical simulations of vortex motion in a circular domain support the theoretical development.
Kinetic model for the interaction of nonequilibrium radiation with a diatomic gas
Authors: Saraf J. R.
Published: March 2003

A kinetic model for the internal fluid interacting with nonequilibrium radiation is proposed. The rotational and vibrational relaxations are allowed at different internal temperatures. The radiative emission is taken to be corresponding to the nonlocal thermodynamic equilibrium.
Hydrodynamic free expansion of a localized relativistic plasma
Authors: Vitello P. , Salvati M.
Published: March 2003

A hydrodynamical treatment of the free expansion into vacuum by a relativistic plasma with an embedded magnetic field is presented. Both a linear and a spherical geometry are considered. For times when the system has expanded to sizes much larger than the initial size the energy density, number density, velocity, and magnetic field profiles are given. The general features of relativistic free expansion are discussed and compared with those of nonrelativistic free expansion.
Approximate Fokker–Planck collision operator for transport theory applications
Authors: Hirshman S. P. , Sigmar D. J.
Published: March 2003

An analytically tractable approximation is developed for the linearized Fokker–Planck collision operator describing a plasma nearly in thermal equilibrium. This approximate operator preserves the symmetry properties of the exact collision integral which imply the physical conservation laws, self-adjointness, and the H theorem. A renormalization procedure is developed to accurately treat collisions between particles of arbitrary masses. For large or small mass ratios, the approximate operator reduces to the standard expansions of the exact operator. In the case of identical particle collisions, the present approximation provides a significant improvement over the ’’model operator’’ previously given in the literature, yet retains the simplicity of former operators necessary for analytic work. The recalculation of the classical transport coefficients with this operator reduces to the solution of a coupled set of algebraic equations and indicates its reliability for use in complex neoclassical transport situations. The neoclassical electrical conductivity calculation demonstrates the new physical features of the approximate operator.
Exact Vlasov–Maxwell equilibria with sheared magnetic fields
Authors: Channell Paul J.
Published: March 2003

A theoretical formalism which allows the generation of a large class of exact Vlasov–Maxwell equilibria with sheared magnetic fields is presented. All quantities are assumed to vary in only one spatial direction, x, and the magnetic field has components only in the y and z directions. The Vlasov equations are solved by making the distribution functions depend only on constants of the motion. The Maxwell equations are then reduced to finding the motion of a pseudo-particle in a two-dimensional potential. Three examples corresponding to sheet-like, sheath-like, and wave-like equilibria are presented.
Stabilization of drift cyclotron loss cone instability with additions of small amounts of cool plasma
Authors: Gerver M. J.
Published: March 2003

The effect of the addition of a cooler ion component on the drift cyclotron loss cone instability was investigated using the local approximation linear dispersion relation. The parameters considered were mirror ratio, density gradient, density (including finite β), ratio of cool to hot ion temperature, and type of cool component (Maxwellian or loss cone). Several different regimes of the parameter space were defined and mapped, according to the characteristics of the instability (e.g., source of free energy, critical density gradient, range of unstable wavenumbers, maximum growth rate). The most effective stabilization was found to occur for addition of a small Maxwellian cool component with density ratio (nc/nh) ≳3.4[(ah/n)(dn/dx)]3/2 (R−1/2−R−1)1/2 and with temperature ratio satisfying 1≳ (Tc/Th)3/2(nh/nc), (R−1/2−R−1)−1≳0.43; ah is the hot ion Larmor radius, R is the mirror ratio, nc and nh are the cool and hot ion densities, and Tc and Th are the cool and hot ion temperatures. The stabilization allows a larger critical gradient, (ah/n) (dn/dx) ≈R1/4 (ωci2/ωpi2+me/mi)1/2 , and a larger minimum unstable wavenumber kah≈R1/4(ωci2/ωpi2 +me/mi)−1/2, than that with no cool component. The instability was also studied numerically for (ah/n) (dn/dx) ∼1, and the minimum nc/nh required for this stabilization was consistent with the density ratio used in the 2XIIB experiment at Livermore. For β≳0, even smaller amount of cool plasma is needed for this stabilization.
High conductivity magnetic tearing instability
Authors: Cross Mark A. , Van Hoven Gerard
Published: March 2003

The field-reconnection instability of a magnetic neutral sheet is studied using magnetohydrodynamic theory, including the electron-inertia term in Ohm’s law. The behavior is described for the full range of the Coulomb-resistivity vs collisionless skin-depth parameter ζ=1020.3B T1.5/lnΛ) a2.5 n2.3 (cgs), describing the field reversal. Such long-wavelength tearing can occur over a wide span of physical conditions from those of solar active regions (ζ≪1) to those in the geomagnetic tail (ζ≫1). In particular, the growth rate of the infinite-conductivity instability may be expressed as ω≈1020.9B/a2.5n1.3, for a hydrogen plasma. In this limit, the time scale of auroral development is found to be 103.2 sec, a value close to that observed. The regime of applicability of this formulation is also discussed, and the results are compared with those obtained by other methods, including the Vlasov equation.
Resonant electron effects on trapped electron instabilities
Authors: Catto P. J. , Tsang K. T. , Callen J. D. , Tang W. M.
Published: March 2003

The necessity of distinguishing between perpendicular and parallel components in electron velocity space, rather than simply considering energy, is pointed out and employed to estimate the effects of finite collisionality on the trapped electron instability.
Stability of a high-β, l=3 stellarator
Authors: Freidberg J. P. , Grossmann W. , Haas F. A.
Published: March 2003

The stability of an infinitely long, high β, l=3 stellarator is investiated. The calculation is carried out by using the new scyllac expansion in the sharp boundary ideal magnetohydrodynamic model. It is found that for any given size l=3 field allowed by equilibrium considerations and mode number m, an infinite but discrete set of wavenumbers k exist for which the plasma is unstable to all β; that is, the critical β equals zero. These modes can be described as long wavelength interchanges. Thus, with regard to sharp boundary stability, l=3 is less desirable than l=1 for the basic scyllac magnetic field.
Vlasov equilibrium and electrostatic stability properties of a nonrelativistic nonneutral E layer
Authors: Davidson R. C. , Uhm Hwan-sup , Mahajan S. M.
Published: March 2003

Vlasov equilibrium and stability properties of a nonrelativistic nonneutral E layer are investigated for the class of thin (a ≪R0) E-layer equilibria in which all electrons have the same value of the energy H and the same value of canonical angular momentum Pϑ, i.e., fe0(H, Pϑ) = (n0R0/2πm) δ (H+eφ̄0−mV02/2) δ (Pϑ−P0), where n0, R0, φ̄0, V0, and P0 are constants. No a priori restriction is made that the density is low (ωp02≪ωc2), and equilibrium and stability behavior is studied for both fast (P0≳0) and slow (P0<0) rotational equilibria. Equilibrium space-charge effects are shown to have a large (defocusing) effect on the radial thickness of the E layer when ωp02≈ωc2. The electrostatic stability analysis includes both negative-mass (orbit variations with Pϑ) and diocotron (angular velocity shear) effects. The system is shown to be stable whenever the equilibrium flow is space-charge neutralized, i.e., whenever f=1, where f=ni0/ne0 is the fractional charge neutralization. For f≪1, a necessary and sufficient condition for instability is given by (l−2) a/R0≳f, where l is the azimuthal mode number of the perturbation. Moreover, depending on system parameters, the growth rate can be a substantial fraction of the electron plasma frequency ωp0.
Nonlinear wavenumber shift of an ion acoustic wave
Authors: Sugai H. , Hatakeyama R. , Saeki K. , Inutake M.
Published: March 2003

The phase velocity of an ion acoustic wave propagating in a collisionless plasma is observed to decrease with wave amplitude at the same frequency. The nonlinear shift in the wavenumber is found to be proportional to the square root of the initial wave amplitude, showing the effects of trapped ions. The observed results are in good agreement with nonlinear theory provided by analogy with an electron plasma wave.
Mechanism generating turbulence in weakly collisional volume created plasmas
Authors: Escande D. F. , Schmitt J. P. M.
Published: March 2003

The problem of the stability of a plasma created by volume ionization is considered in the low collisional range. A mechanism where the trapping of light slow particles in the plasma by the sheaths drives the plasma unstable is investigated. The problem is simulated by a one-dimensional continuum numerical code. The instability is observed, and the proposed mechanism is demonstrated to be responsible for it. The resulting turbulence randomizes the light particle velocities. Collisions and a current driven into the plasma are shown to have stabilizing effects.
Intense relativistic electron beam equilibria in vacuum without a magnetic guide field
Authors: Chodorow Alan M. , Erteza Ahmed
Published: March 2003

Vacuum transport of dense, charged particle beams is investigated as an alternative to plasma neutralization of the beam, which may produce relatively large beam-plasma interaction losses. A solution of the vacuum transport equations, with beam properties invariant in the direction of propagation and without the aid of a magnetic guide field is found and interpreted. The energy required to provide favorable boundary conditions for eliminating beam divergence is shown to be acceptable.
Autoacceleration of intense electron beams by coaxial cavity structures
Authors: Siambis John G.
Published: March 2003

Tonks’ theorem and Faraday’s law are applied to study the autoacceleration interaction of an intense hollow beam with a coaxial transmission line cavity, assuming the constant-current approximation. The autoacceleration voltage is found proportional to the beam current and the inductance per unit length of the coaxial transmission line. Comparison with experimental data indicates that the coupling coefficient α is of the order 2/3. The creation of a virtual cathode near the autoacceleration region under certain beam and geometry conditions gives rise to a virtual cathode accelerating voltage. This voltage is found to be proportional to the inductance per unit length of the annular region between the hollow beam and the drift tube as well as the amount of charge per unit length reflected from the virtual cathode. The ratio of the virtual cathode voltage to the autoacceleration voltage is a small number and for optimum operating conditions it should be minimized.
Lagrangian–Eulerian relation and the independence approximation
Authors: Weinstock J.
Published: March 2003

The relation between Lagrangian and Eulerian functions is basic to the understanding of turbulent diffusion. To explore this relation, the independence approximation was suggested by Corrsin and used by Saffman. Here, a theoretical investigation is made of the Lagrangian–Eulerian relation and the validity of the independence approximation. A renormalized expansion is introduced for this purpose. It is shown that the independence approximation is the first term in a systematic expansion of the Lagrangian function, and is valid when an explicit condition on the turbulence spectrum is satisfied. Correction terms to the independence approximation are derived. The Lagrangian function is then related to the Eulerian function by an integral equation. This equation justifies a result previously derived by Saffman, and is a little more general. In addition, a calculation is made of the probability distribution of a particle displacement, including non-Gaussian corrections. These corrections are consistent with the non-Gaussian form found in numerical simulations by Peskin, and may explain the observation of a curious behavior concerning the independence approximation. Within the context of the independence approximation, it is seen that the Lagrangian correlation time is less than or equal to the Eulerian correlation time.
Kinetic analysis of the problem of evaporation and condensation
Authors: Matsushita Tadashi
Published: March 2003

The mass transfer between a vapor and its condensed phase (liquid or solid) is considered on the basis of the reduced, linearized BGK equation as the governing equation and the finite element method as a tool. The results obtained for the vapor flow between two interfaces show that the parameter related to the latent heat of phase transition plays an important role in determing the flow characteristics.
Effect of the lift force on the stability of uniform fluidization
Authors: Drew Donald A.
Published: March 2003

The state of uniform fluidization, where the uniform upflow of a fluid balances the weight of a bed of particles, it shown to be unstable to small disturbances when the effect of the lift force on the particles is included. The lift force is dominant during the initial growth of the disturbances. The lift force on the particles is assumed to be proportional to the slip and to the one-half power of the shear rate. Perturbations from the state of uniform fluidization are studied. The resulting perturbation problem is such that the evolution of the smallest disturbances is governed by a nonlinear equation. Particular initial conditions corresponding to growing solutions are found.
Induction and transient times in chemical reactions: Application to O2 dissociation
Authors: Carbonell R. G. , McCoy B. J.
Published: March 2003

An analysis of relaxation phenomena in diatomic molecules, based on a harmonic oscillator master equation formalism, can accurately predict induction times for O2 dissociation in shock tubes, in the temperature range 2850° to 18 000°K, using measured vibrational relaxation times. In this case, the induction times are the direct result of relaxation of lower vibrational states prior to chemical reaction. Transient times for chemical reactions in temperature jump or shock tube experiments are shown to depend on the magnitude of the difference between initial and final temperatures. The phenomenological rate expression is valid for times greater than the transient times for large temperature jumps, and for practically all times for small temperature jumps.
Computer model of a fast toroidal plasma compression, with application to the topolotron
Authors: Nelson H. Mark , Brown Keith H. , Hart Charles A.
Published: March 2003

The method is developed for a comuputer sequence which models a fast toroidal theta or screw pinch for a highly conducting axially symmetric plasma. The computer sequence takes into account the self-inductances of the plasma and the external conductors which drive the compression as well as the mutual inductance which electromagnetically couples the two. The computer sequence is divided into three phases: a snowplow compression phase, an adiabatic compression phase, and a crowbarred circuit phase. The computer sequence is applied to a topolotron and an example is given of a magnetohydrodynamic equilibrium geometry for which the plasma surface possesses an asymptotic magnetic field with limit cycles.
Secondary flows in a slowly oscillating torus
Authors: Munson B. R.
Published: March 2003

Theoretical and experimental results describing the secondary flow in a slowly oscillating torus are presented. In contrast to the outward centrifuging in a stationary curved pipe, the secondary flow in a slowly oscillating torus is from outside to the inside of the bend.
Magnetic field generation due to resonance absorption
Authors: Nishihara Katsunobu , Ohsawa Yukiharu , Mima Kunioki , Tange Toshio
Published: March 2003

A detailed study of the mechanism of mega-Gauss magnetic field generation in the resonant absorption of laser light is presented which takes into account the effects of both the dissipation and thermal motion of electrons.
Sonine polynomial solution of the Fokker–Planck equation
Authors: Abe Kanji , Ushimi Yasunori
Published: March 2003

Using the Sonine polynomial expansion method, the Fokker–Planck equation is accurately solved. The overall relaxation time is considerably shorter than the Spitzer self-collision time.
Two-dimensional magnetohydrodynamic simulation of toroidal pinches II: Belt pinches
Authors: Lui H. C. , Chu C. K.
Published: March 2003

The equations and method of solution used in a previous paper [Phys. Fluids 18, 1277 (1975)] for the simulation of toroidal screw pinches have been modified to simulate toroidal belt pinches and high-beta tokamaks of rectangular cross sections. In addition to accounting for the geometric difference between a belt pinch and a screw pinch, this work permits the inclusion of vertical fields, while the previous work dealt only with conducting shells. Calculated results have been compared with the experimental results of the Belt Pinch I of Garching, and the agreement is again very good. The present code has been used extensively in the design and prediction of the Torus I experiment at Columbia.
Physical origins of the sideband instability
Authors: Canosa José , Wray Alan
Published: March 2003

Over a large range of amplitudes of a main plasma wave, the growth rates of its lower sidebands are found to increase with amplitude significantly more strongly than previously reported. For special initial conditions of experimental interest, the Vlasov computations show that, as the main wave amplitude increases, the sideband growth rates first increase, then reach a peak, and finally start to decrease. These computational results, as well as those obtained for the sideband frequency separation, are explained quantiatively by simple theoretical calculations based on the model that sideband growth is due to a nonlinear resonance between the trapped electrons bouncing in the main wave and a test wave of appropriate frequency.
Magnetic curvature and ion distribution function effects on lower hybrid drift instabilities
Authors: Krall N. A. , McBride J. B.
Published: March 2003

Magnetic curvature is shown to have a relatively weak effect on lower hybrid drift instabilities, but ion temperature gradients can significantly alter stability in the broad sheath limit, ai≪Ln.
Two-point turbulence measurements downstream of a heated grid
Authors: Sepri Paavo
Published: March 2003

Two-point turbulence data from hot and cold wire anemometry are presented which describe the temperature and velocity fields downstream of a heated grid in a low speed wind tunnel. The velocity and temperature statistics are found to differ in that (a) the velocity energy spectrum has no inertial subrange while the temperature spectrum displays a significant region of −5/3 slope and (b) for lateral probe displacements the temperature coherence is larger than that of velocity. This result is surprising since both fields were generated by the same grid, and since the temperature field appears to have been passive. Phase measurements indicate that at least the lower turbulence wavenumbers move with equal convection velocities, thereby supporting Taylor’s hypothesis of quasi-frozen pattern, but this convection velocity appears to be slightly greater than the mean fluid velocity. Cross correlations from the signals of two spatially separated probes are presented for both the velocity and temperature fields. In general, they agree with expectations derived from the theory of isotropic turbulence.