bluuebunny/crossref_metadata_2025 huggingface dataset:
End loss from a theta pinch
Authors: Weitzner Harold
Published: March 2003

Based on the approximations of slow time variation and slow axial coordinate variation in an ideal magnetohydrodynamic flow, a simplified set of equations is obtained which applies to the problem of plasma outflow from a theta pinch. The characteristic speed of the flow, the signal speed, is the cusp wave speed. In this model, no high speed Alfvén waves are present in the low density region. An argument is presented indicating the existence of shocks in the low density outflow region. It is possible that such shocks may permit the analogy of hydrodynamic choking of the flow. A numerical scheme is described for computing a solution of the model equations. A sample computation gives plasma line density profiles and mass flux profiles for one particular case. The computation could not be carried out long enough to see pronounced shock effects in the main body of the plasma. The flow did not appear to approach a steady flow, but it did indicate the great effect of the flow in the throat on the remainder of the flow.
Two-dimensional spatial structure of the dissipative trapped-electron mode
Authors: Rewoldt G. , Tang W. M. , Frieman E. A.
Published: March 2003

The complete two-dimensional structure of the dissipative trapped-electron mode over its full width, which may extend over several mode-rational surfaces, is discussed. The complete integrodifferential equation is studied in the limit krρi<1, where ρi is the ion gyroradius, and kr, the radial wavenumber, is regarded as a differential operator. This is converted into a matrix equation which is then solved by standard numerical methods. Solutions obtained are in reasonably good agreement with one-dimensional analytic solutions, in the limits where such results are expected to be valid. More significantly, the present approach can readily treat many physically important cases for which purely analytic solutions are difficult to obtain. The results indicate that the differential equation formulation of the eigenmode equation is valid only for long wavelength modes (kϑρi≲0.3, with kϑ being the poloidal wavenumber). For such cases it is found that shear stabilization estimates obtained from the one-dimensional radial solution are quite inaccurate for modes overlapping only a small number of mode-rational surfaces, but become more accurate for modes overlapping many mode-rational surfaces.
Separable solutions for tokamak plasma transport
Authors: Jensen T. H.
Published: March 2003

The transport problem for tokamak plasmas, and particularly for noncircular cross section tokamaks, is complicated and can generally only be solved numerically. It is shown that under special circumstances time and space coordinates can be separated so that the transport problem is significantly simplified. These circumstances are close to those desired in experiments and the necessary assumptions regarding transport coefficients are in accordance with the best present day knowledge, namely, neo- or pseudo-classical-like heat transport and Spitzer-like resistivity. The time behavior of the solutions is particularly simple; the plasma current is proportional to t1/3 (where t is time) and the plasma temperature is proportional to t2/3
Residual trapped-ion instabilities in tokamaks
Authors: Tang W. M. , Adam J. C. , Ross David W.
Published: March 2003

It is demonstrated that in addition to the usual dissipative trapped-ion instability, which is basically an electron diamagnetic drift mode, ion diamagnetic modes of this type can also be generated. Such modes can be driven unstable by ions which have average unfavorable magnetic drifts and can persist as residual instabilities in tokamak systems with flat or reversed gradient profiles.
Dynamic stabilization of dissipative drift modes by oscillating azimuthal magnetic field—Experiment and theory
Authors: Van Andel H. W. H. , Teichmann J. , Mah S. Q.
Published: March 2003

An experimental study of dynamic stabilization of dissipative drift modes in a weakly ionized plasma is presented. Stabilization is effected by means of the external generation of a small, localized oscillating magnetic field component directed parallel to the wave vector of the instability. Stabilization of a drift mode identified in the plasma is shown to be effective using this method. The phase difference between density and potential perturbation is reduced, while it is shown that under certain conditions the anomalous diffusion associated with the drift mode is also reduced. The experiment is in qualitative agreement with linear theory, which is presented.
Linear response theory of a turbulent magnetized plasma
Authors: Nambu Mitsuhiro
Published: March 2003

It is proposed that the dominant nonlinear effects of high-frequency turbulence on the low-frequency mode are composed of two parts. One is the electron orbit modification due to the ponderomotive force, and the other is the modulation effect of the ponderomotive force. The effective dielectric constant of ion-cyclotron waves in the presence of electron-cyclotron turbulence is obtained by using the linear response theory of a turbulent plasma. The critical size and the Reynolds number of a turbulent plasma are defined.
Relativistic runaway electron beams in the Oak Ridge tokamak
Authors: Knoepfel H. , Spong D. A. , Zweben S. J.
Published: March 2003

The conditions for the generation and containment of relativistic electron beams in a tokamak discharge were studied by deliberately driving Ormak into a strong runaway regime. The experimental results show that beams with typical currents of 60 kA and mean electron energies of 3 MeV could be generated reproducibly and then contained safely over e-folding decay times of 35 msec without damaging the machine. The experiments substantially confirm and extend the results of previous modeling studies on runaway discharges. They also implicitly define the experimental conditions for the establishment of a steady, toroidal beam-plasma system that can be of interest for plasma physical studies.
Radiation from a source in a cold magnetoactive plasma, re-examined. Application to cyclotron and multipole radiation
Authors: Hoyng Peter , Stevens Gerard A.
Published: March 2003

An analysis is presented of the radiation emitted by an arbitrary source embedded in a cold, magnetoactive plasma and physically distinct from the latter. The plasma is supposed to be infinite and homogeneous; its dielectric properties are described by a dielectric tensor ε. Expressions for the radiation fields are derived using the technique of Fourier decomposition. An expression for the vector potential is constructed and elaborated as far as possible for an arbitrary current source. The approach differs from that in previous work on technical points, the main one being the sequence in which the various integrations are carried out. The radiation flux is defined on the basis of Poynting’s vector S; a distinction is made between current sources behaving as a given function of time and randomly fluctuating sources. In the latter case an ensemble average is preferred over a time average. A comparison is made with existing treatments in the literature, and a variety of defects is pointed out. The general result for the radiation flux is then specified for cyclotron radiation from a stationary ensemble of electrons and for multipole radiation. Throughout the paper a compact notation is used based on the work of Bremmer.
Noncollisional losses in an inhomogeneous plasma
Authors: Meyer-Vernet N. , Meyer P. , Perche C.
Published: March 2003

Calculations of the rf field in a sheath, like that one surrounding an antenna in an infinite plasma, show that resonances occur, yielding very high losses.
Supersonic-subsonic flow transition and shock formation and propagation in a dense plasma beam
Authors: Toffolo W. , Siambis John G.
Published: March 2003

The axial flow of a supersonic high density plasma beam into an asymmetric mirror machine has been investigated experimentally. It has been found that in front of the second mirror peak, two supersonic-subsonic flow transitions and shock formations occur. The subsequent motion upstream, of the first and stronger shock front has been measured by the time-of-flight method through diamagnetic loop and electric double probe signals. The shock front speed has been found to be one third the speed of sound in the neighborhood of the sonic transition and decreases to zero ahead of the upstream mirror.
Nonlinear helical perturbations of a tokamak
Authors: Strauss H. R. , Monticello D. A. , Rosenbluth Marshall N. , White Roscoe B.
Published: March 2003

An analytic study of small amplitude perturbations of a cyclindrical tokamak, using the nonlinear, helically symmetric, reduced magnetohydrodynamic equations of Kadomtsev and Rosenbluth is presented. Results are compared with those of a numerical study of these reduced equations.
Electron kinematics in a plasma focus
Authors: Hohl Frank , Gary S. Peter
Published: March 2003

The results of numerical integrations of the three-dimensional relativistic equations of motion of electrons subject to given electric and magnetic fields are presented. Fields due to two different models are studied: (i) a circular distribution of current filaments, and (ii) a uniform current distribution; both the collapse and the current reduction phases are studied in each model. Decreasing current in the uniform current model yields 100 keV electrons accelerated toward the anode and, as for earlier ion computations, provides general agreement with experimental results.
Trajectories of an ideal vortex pair near an orifice
Authors: Sheffield J. Steven
Published: March 2003

The two-dimensional trajectories of an ideal vortex pair near an orifice are calculated. Three geometries are examined, and the criteria for the vortices to travel away from the orifice are determined.
Turbulent plane Couette flow using probability distribution functions
Authors: Srinivasan R. , Giddens D. P. , Bangert L. H. , Wu J. C.
Published: March 2003

Turbulent Couette flow between parallel plates is studied utilizing a model equation, similar to the Boltzmann equation of kinetic theory, which was developed by Lundgren for the velocity distribution of fluid elements. Solutions to this equation are obtained numerically, employing the discrete ordinate method and finite differences. Two types of boundary conditions on the distribution function are considered, and the results of the calculations are compared with available experimental data. This study establishes that Lundgren’s equation provides a very good description of turbulence for the flow situation considered, and that it offers an analytical tool for further study of more complex turbulent flows. Further, the discrete ordinate method is shown to be an effective method for direct solution for the probability distribution, thus avoiding recourse to assuming, a priori, approximate forms for this function.
Electron distribution functions and quasi-static electromagnetic fields in laser-pellet plasmas
Authors: Bernstein Ira B.
Published: March 2003

The theory of the electron distribution functions in a laser-pellet plasma is developed taking advantage of the dominance of collisions with high charge number ions. For the thermal electrons the results is a description in terms of transport coefficients given in closed form. For the superthermal electrons, which are isotropic in velocity, the description is via a reduced kinetic equation. The adaptation of the scheme to numerical computation is discussed briefly.
Free electron laser
Authors: Kwan T. , Dawson J. M. , Lin A. T.
Published: March 2003

The possibility of a new type of laser has been investigated by computer simulation using a fully relativistic electromagnetic particle code which has one spatial and three velocity dimensions. By passing a relativistic electron beam over a rippled static magnetic field, high frequency electromagnetic radiation is generated. If the ripple wavelength is λ0, the lasing wavelength is roughly λ0/2γ2. Thus, such a laser is continuously tunable by varying γ. It has been observed in simulation that as much as 35% of the beam energy can be converted into radiation, of which as much as nearly 90% can be in the most rapidly growing mode. A theory of the coupling between the negative energy plasma wave and the electromagnetic radiation by means of the rippled magnetic field is presented. Good agreements have been obtained between the simulation and the theory. The saturation mechanism is found to be the trapping of the beam by the unstable plasma wave. A theoretical estimate of the amount of energy that can be converted into radiation from the electron beam is also given.
Stability regions and growth rates for a two-ion component plasma, unmagnetized
Authors: Mynick Harry E. , Gerver M. J. , Birdsall C. K.
Published: March 2003

A uniform plasma, with a thermal core and a warm ring, in velocity space, is examined for instabilities. The parameters are the ratio of ring to core densities, the ratio of core thermal velocity to ring peak velocity, and the ring thermal velocity. The boundary between stable and unstable distributions is given, as dependent on these parameters, as obtained from the Penrose criterion. In addition, the upper and lower bounds on the wavenumbers (kmax, kmin) are found for typical distributions, as well as growth rates, ωi(k). For either dense ring or dense core distributions, the distributions are unstable for (vt/vr) ≲0.1, independent of the thermal spread of the ring, where vt is the core thermal spread, and vr is the ring peak speed. For more equal ring and core densities, the stability boundary moves to larger values, dependent on the thermal spread of the ring. Growth rates of ωi≈0.01 to 0.1 ωp are typical. Application of these results to magnetized plasmas is given.
Finite-Larmor-radius stabilization in a sharp-boundary Vlasov-fluid screw pinch
Authors: Turner Leaf
Published: March 2003

Starting from the basic equations of the Vlasov-fluid model, assuming small ion Larmor radii, and assuming unstable, long-wavelength perturbations in a sharp-boundary screw pinch whose equilibrium distribution of ion velocities is isotropic, a derivation is presented of the dependence upon ξ⊥(r) and its derivatives of the dominant terms in perturbations of the ion distribution, the ion number density, the ion fluid velocity, the pressure tensor, and the heat flux vector. A differential equation governing ξ⊥(r) is derived and solved. A dispersion relation for a perturbation with arbitrary azimuthal mode number is derived by application of boundary conditions appropriate for a sharp-boundary screw pinch. It is found that ideal magnetohydrodynamics and the Vlasov-fluid model yield identical dispersion relations only for an m=1 mode. For m⩾2, the Vlasov-fluid model yields a damping of the ideal magnetohydrodynamic growth rate with increasing ion temperature. The m=2 instability is extinguished at an ion temperature equal to one-fourth of that given by Freidberg’s analysis. This result allows greater leeway for wall stabilization of m=1 modes while maintaining finite-Larmor-radius stabilization of m=2 modes in theta-pinch machines, such as scyllac.
Plane shock interacting with thermal layer
Authors: Gion Edmund J.
Published: March 2003

Some extensions of Griffith’s work on shock/thermal layer interactions have been made. In particular, flow properties in a transition region between the foot of the shock and the thermal layer may be obtained; temperatures are reported.
Sheath inverse bremsstrahlung in laser produced plasmas
Authors: Catto P. J. , More Richard M.
Published: March 2003

During reflection from a sheath, electrons originating in the overdense plasma absorb laser light.
Theory of nonlinear diffusion of plasma across the magnetic field of a toroidal multipole
Authors: Drake J. R. , Berryman James G.
Published: March 2003

A phenomenological derivation of a one-dimensional model of crossfield plasma diffusion in toroidal multipole geometry is presented. Experimental studies have shown previously that this one-dimensional model describes the observed density profile shape and decay in the Wisconsin levitated octupole in certain regimes. Simplified mathematical studies of the model (which did not give adequate attention to geometrical effects) have also shown that a stable profile should evolve from an arbitrary initial particle distribution. Here, a numerical treatment and a discussion of the effects of multipole geometry are given. Comparison is made with experimental results. Finally, analytical studies give additional evidence that the observed rapid evolution toward a stable profile should be expected theoretically. It is concluded that the phenomenological model is generally useful for determining diffusion coefficient magnitude and scaling with density once the density profiles have been determined experimentally.
Adiabatic approximation used in nonlinear theory of backscattering
Authors: Nguyen Duc-Long , Parbhakar K. J.
Published: March 2003

The steady state nonlinear backscattering problem is solved using the matched asymptotic method. The domain of validity of Tang’s adiabatic approximation is established.
Structure of a turbulent swirl
Authors: Uberoi Mahinder S.
Published: March 2003

Velocity and pressure distributions are derived in an infinitely long swirl as functions of space and time by postulating an expression for turbulent stresses and neglecting laminar stresses. Constant angular momentum per unit distance along the swirl axis appears as a parameter.
Perturbation expansion of the Navier–Stokes equations for shock waves
Authors: Powell John D. , Batteh Jad H.
Published: March 2003

A systematic perturbation expansion is developed for the steady-state Navier–Stokes equations describing shock propagation in an ideal gas. The temperature dependence of the viscosity and thermal conductivity is accounted for, although the specific heat and Prandtl number are assumed constant. It is shown that if the first n−1 terms in the expansion are known, the nth term can always be expressed in terms of a quadrature. Explicit calculation of the expansion is carried out to second order in the shock strength, and the results are compared with other calculations. It is shown that the results are in surprisingly good agreement with more exact calculations even for rather strong shock waves. The effect on shock structure of the temperature-dependent transport coefficients is discussed.
Three-wave resonant interactions of gravity-capillary waves
Authors: Case K. M. , Chiu S. C.
Published: March 2003

The equations for the triad resonance interactions of gravity-capillary waves are derived by the Krylov–Bogoliubov–Mitropolski method. Some special two-dimensional solutions are found.
Bäcklund transformation for the resonant three-wave process
Authors: Case K. M. , Chiu S. C.
Published: March 2003

A Bäcklund transformation for the resonant three-wave equations is derived. Both bounded and shock-type solitons are obtained. For the shock-type solitons, special two-soliton solutions reveal that solitons can be created or annihilated.
Anomalous electron-ion energy exchange from the trapped electron mode
Authors: Manheimer Wallace M. , Ott E. , Tang W. M.
Published: March 2003

It is shown that in steady state the dissipative trapped electron mode does not give rise to net plasma heating. However, it does cause anomalous energy exchange between electrons and ions, with the ions heating and the electrons cooling.
Saturation of the parametric-decay instability near the lower-hybrid frequency
Authors: Chen Liu , Berger R. L.
Published: March 2003

The nonlinear evolution and saturation of parametrically excited lower-hybrid waves are studied numerically by solving the corresponding weak-turbulence wave kinetic equation. It is found that nonlinear electron and ion Landau damping provide effective saturation mechanisms only for pump powers close to the threshold value.
Higher order adiabatic invariants and nonadiabatic diffusion for geometric mirrors
Authors: Lee Y. C. , Samec T. K. , Fried Burton D.
Published: March 2003

The adiabatic invariant governing motion of charged particles in gently tapered, high-order, linear multipole magnetic field geometries is derived correct to second order in a small parameter of order ‖dR/dz‖, where R (z) is the average radius of the surface fields at axial position z. The diffusion arising from higher order terms is calculated and compared with numerical studies of particle motion in such fields.
Excitation of ion acoustic waves with probes
Authors: Chen T. , Schott L.
Published: March 2003

A large cylindrical Langmuir probe (length=30 cm, diameter=1.25 mm) and a probe of similar dimensions consisting of a conductor completely enclosed in an insulator are compared as exciters of ion acoustic waves. When the potential drop across the sheath of the Langmuir probe is increased from −143 to +10 V, the ion wave amplitude increases by a factor of 40. This result can be qualitatively explained with a simple model which relates the wave amplitude to the sheath radius and to the ac sheath impedance. When large negative pulses (≳100 kTe/e) are applied to the probes, the plasma response is similar for the biased and the insulated probe. The advantages of the insulated probe as an exciter of ion waves over ordinary probes are discussed. Ion waves with peak-to-peak amplitudes (δn/n) ≃0.44 can be excited with the insulated probe.
Mechanical mixing of polymer additive in turbulent drag reduction
Authors: Stenberg Lars-Göran , Lagerstedt Torgny , Sehlén Ove , Lindgren E. Rune
Published: March 2003

Turbulent drag reduction induced by injection of polymer into pipe flow exhibits major changes with variations of the rotational speed of an upstream mixer. This is shown to unify the concepts of homogeneous and heterogeneous drag reduction.
Calculation of noncylindrically symmetric space-charge flow
Authors: Quintenz J. P. , Poukey J. W.
Published: March 2003

A cylindrical diode is proposed which can be used to generate and direct the flow of charged particle beams. The two-dimensional partial differential equations which model the device are formulated and solved numerically.
Ion-beam-excited, electrostatic, ion cyclotron instability
Authors: Michelsen P. , Pécseli H. L. , Rasmussen J. Juul
Published: March 2003

The stability limits of the ion-beam-excited, electrostatic, ion cyclotron instability were investigated in a Q-machine plasma where the electrons could be heated by microwaves. In agreement with theory, the beam energy necessary for excitation decreased with increasing electron temperature.
Quasi-simple wave solutions for acoustic gravity waves
Authors: Seshadri V. S. , Sachdev P. L.
Published: March 2003

The special class of quasi-simple wave solutions is studied for the system of partial differential equations governing inviscid acoustic gravity waves. It is shown that these traveling wave solutions do not admit shocks. Periodic solutions are found to exist when there is no propagation in the vertical direction. The solutions for some particular cases are depicted graphically.
Velocity shear instability of Alfvén waves in a high β collisionless plasma
Authors: Dobrowolny M.
Published: March 2003

An instability of Alfvén waves driven by parallel velocity shear has been derived for a high β collisionless plasma. The unstable waves propagate in a narrow cone around the magnetic field.
Plasma wave instability in the field of an intense electromagnetic wave
Authors: Amato M. A. , Miranda L. C. M.
Published: March 2003

The plasmon scattering by electrons in the presence of an intense laser field is discussed. It is shown that in the strong field limit the plasmon population may become unstable.
Effect of polymer additives on the small-scale structure of grid-generated turbulence
Authors: McComb W. D. , Allan J. , Greated C. A.
Published: March 2003

The effect of polyethyleneoxide (Polyox grade WSR 301) on grid-generated turbulence was investigated using laser anemometry and flow-visualization techniques. It was found that the polymer additive reduced both the turbulent intensity and the rate of decay behind the grid. At typical drag-reducing concentrations, turbulent energy spectra were qualitatively the same as those in water, in agreement with the results of other investigations. However, at higher additive concentrations, the dissipation-range spectra showed noticeable attenuation. This seemed to be a threshold effect with onset at a polymer concentration between 100 and 250 ppm. This result was supported by photographs of dye-injection tracer but in this case the onset concentration for small-eddy suppression was between 50 and 100 ppm.
Nonlinear saturation of the collisionless drift instability
Authors: Simon Albert , Gross Leonard S.
Published: March 2003

A collisionless spatially nonuniform plasma is considered in a finite conducting cylinder in a uniform magnetic field. The magnetic field is increased by a small fractional amount over the critical value for onset of instability. A single mode grows and is saturated by nonlinear effects. Analytic expressions are obtained for the time-asymptotic saturation amplitude and frequency shift. Indeterminate equations and singular functions are resolved by methods similar to those used earlier in the one-dimensional bump-on-tail analysis, and the form of the modified equilibrium distribution and potential is obtained. A specific numerical example demonstrates that saturation is due to increased ion Landau damping, and that this saturated amplitude is unstable to small deviations. Hence, the onset of the instability is predicted to be of the ’’hard’’ variety. This result holds both in the ’’quasi-linear’’ approximation and in the full nonlinear case.
Electron stabilization of drift-cone modes
Authors: Aamodt R. E.
Published: March 2003

The mechanism of electron trapping in E×B guiding center motion is proposed as the mechanism for absolute stabilization of drift-cone modes in mirror geometry. The calculated amplitudes are shown to be in reasonable agreement with experimental results in present mirror machines.
Plasma effects on the spontaneous emission of synchrotron radiation from weakly relativistic electrons
Authors: Freund H. P. , Wu C. S.
Published: March 2003

A method for computing the spectral emissivity of spontaneous synchrotron radiation is discussed. The Klimontovich formalism in plasma kinetic theory is adopted in which an ensemble average of the microscopically emitted power is considered. The present method clarifies the meaning of the random phase approximation which is imposed in several existing theories of synchrotron radiation. Both the effects of dielectric polarization and two-particle correlations are included in the present discussion. The theory is applied to the case of a plasma in thermal equilibrium, for which it is shown that the effect of pair correlations on the emissivity vanishes. On the other hand, the effect of dielectric polarization is studied numerically for a wide range of parameters.
Correlation of intense electron beam dynamics with the operation of the relativistic cyclotron maser
Authors: Talmadge S. , Marshall T. C. , Schlesinger S. P.
Published: March 2003

The relation between the particle dynamics of an intense relativistic electron beam and its accompanying microwave radiation has been explored. A diagnostic has been developed which makes use of the strong forward directed x-ray radiation pattern of relativistic electrons colliding with a low Z thin target foil, to determine the transverse electron momentum of the beam as it moves along a magnetic field. The transverse momentum of the beam electrons is controlled both by a magnetic undulator, consisting of spaced iron rings similar to those used in previous cyclotron maser experiments, and by a carefully wound electromagnet consisting of alternating dipoles. The effect of relative undulator strength on the beam momentum distribution is determined, together with total microwave power as measured by a calorimeter. The results, which are interpreted with the aid of a numerical model, indicate a positive correlation between perpendicular momentum and radiated power. The spread in perpendicular velocity of the beam electrons was found to depend on the type of undulator used, the relative undulator strength and also influences the amount of radiated power.
Nonlinear saturation of drift waves
Authors: Hasselberg Günter , Rogister André , El-Nadi Adel
Published: March 2003

Equations have been derived which describe the nonlinear evolution of standard drift waves (driven by inverse Landau damping) and of dissipative trapped electron modes. Assuming that all but the n=0 modes are shear stabilized, it is shown that the nonlinear equations admit eigenfunctions of the same form as the linear equations with, however, the shear length Ls=q/(ε dlnq/dr) replaced by another length ls<Ls. ls decreases with increasing turbulence thereby stabilizing the mode. The turbulence level and the diffusion coefficient are self-consistently calculated. In the case of the standard drift wave, good agreement is found with experimental results in linear devices. In the case of the trapped electron mode, estimates obtained with parameters derived from published profiles of ST tokamak discharges are quite reasonable. Stabilization arises through nonlinear diamagnetic drifts.
Linear wave conversion in an unmagnetized, collisionless plasma
Authors: Speziale Thomas , Catto P. J.
Published: March 2003

Analytic solutions to the problem of linear wave conversion in a cold, unmagnetized, collisionless plasma are obtained for nearly all angles of incidence. The resulting absorption coefficients are in excellent agreement with numerical work in the appropriate limits and differences in previous solutions are reconciled. The warm plasma equations are also solved, and the independence of the absorption coefficient to thermal effects verified. The singularities neglected by Piliya are properly treated by viewing the frequency dependence as arising from a Laplace transform.
Structure of temperature fluctuations in a turbulent boundary layer
Authors: Antonia R. A. , Danh H. Q.
Published: March 2003

Statistics of temperature fluctuations are measured in a slightly heated boundary layer well downstream of a step change in wall heat flux. Measurements include probability density functions and spectra of temperature ϑ and of its time derivative ∂ϑ/∂t, and distributions of the skewness and flatness factors of ϑ and ∂ϑ/∂t. These results are compared with existing data for the velocity and its time derivative. The Reynolds number dependence of high order moments of ∂ϑ/∂t is in qualitative agreement with that obtained by Frenkiel and Klebanoff for velocity derivative data. Spectra of (∂ϑ/∂t)2 exhibit a power law behavior both in the viscous sublayer and in the outer region of the layer, but the exponent is different in the two regions.
Electron acceleration by a localized electric field
Authors: DeNeef C. P. , DeGroot J. S.
Published: March 2003

Using single particle orbit computations the time-averaged distribution funciton of the electrons accelerated by a localized electric field in a plasma is calculated. This model is applied to the resonant absorption of an electromagnetic wave obliquely incident on a steep plasma density gradient. The calculated distribution function of the electrons accelerated parallel to the density gradient agrees with the results of an electrostatic simulation computer code.
Stationary density variation produced by a standing plasma wave
Authors: Michelsen P. , Pécseli H. L. , Rasmussen J. Juul , Sato N.
Published: March 2003

Measurements are presented of a stationary density modulation produced by a standing electron plasma wave. The experimental results are well explained by taking into account the ponderomotive forces on the electrons exerted by the high frequency field.
Finite amplitude envelope solitons
Authors: Schamel H. , Yu M. Y. , Shukla P. K.
Published: March 2003

A general theory of finite amplitude envelope Langmuir solitons is presented. Comparison with recent beam-plasma experiments suggests identification of the observed localized structures with these solitons.
Vlasov theory of plasma oscillations: Linear approximation
Authors: Arthur M. D. , Greenberg William , Zweifel P. F.
Published: March 2003

A functional analytic approach to the linearized collisionless Vlasov equation is presented utilizing a resolvent integration technique on the resolvent of the transport operator evaluated at a particular point. Formulae for the eigenfunction expansion are found for cases in which the plasma disperion function Λ has first and second order zeroes. Special care is taken in the study of real zeroes of Λ culminating in new results for this case. For a simple zero of Λ with nonvanishing imaginary part the van Kampen–Case discrete modes are reproduced. The results are used to obtain the solution to the initial value problem.
Temperature dissipation fluctuations in a turbulent boundary layer
Authors: Sreenivasan K. R. , Antonia R. A. , Danh H. Q.
Published: March 2003

All three components of the dissipation rate of the fluctuating temperature ϑ are measured simultaneously in the inner region of a fully developed turbulent boundary layer at a moderate Reynolds number. Measurements are made with a probe of four cold wires consisting of two closely spaced parallel vertical wires mounted a small distance upstream of two closely spaced parallel horizontal wires. In the inner region of the layer, local isotropy is not closely approximated (∂ϑ/∂z)2〉 (∂ϑ/∂y)2〉 (∂ϑ/∂x)2. The spectral density of the sum χ[= (∂ϑ/∂x)2+(∂ϑ/∂y)2+ (∂ϑ/∂z)2] is similar in shape to that of (∂ϑ/∂y)2 or (∂ϑ/∂z)2 but not as rich in high frequency content as that of (∂ϑ/∂x)2. The probability density of χ has a lower skewness and flatness factor and is more closely log-normal than those of the individual components. This is true regardless of whether χ and its components are unaveraged or locally averaged over a linear dimension r. When averaging is applied, departures from log-normality are diminished but do not disappear entirely. The variance σ2 of the logarithm of the locally averaged χ is proportional to ln r over a wide range of r (rmax/rmin≃30), in contrast to the individual components where this ratio may be as small as 2. The value of the Kolmogoroff constant μϑ determined from the slope of σ vs ln r is about 0.35. This is consistent with the slope of the spectral density of χ and is also in agreement with previous best estimates of μϑ (and μ) obtained at high Reynolds numbers.
Variable property effects on the onset of convection in an elastic porous matrix
Authors: Morland L. W. , Zebib A. , Kassoy D. R.
Published: March 2003

Equations for steady convection in a porous linear elastic matrix are formulated. When porosity changes are of the order of the pressure induced compressions in matrix and fluid, and these in turn can be neglected in comparison with thermal expansions in the fluid, the fluid balance laws uncouple from the equilibrium balance of the matrix. Plane flow in a horizontal layer is considered for temperature increases with depth up to 250°K, and the significant variations of viscosity and thermal expansion coefficients with temperature are included. The critical Rayleigh number for small disturbances harmonic in the horizontal direction is found to be substantially reduced from the constant expansion coefficient value, but the associated wavenumber and incipient flow patterns are little changed. However, if a Rayleigh number based on an appropriate mean value of the expansion coefficient is used, the critical values for constant and variable expansion coefficient solutions are in close agreement.
Heat transfer from cylindrical anemometer probes in CO2–air mixtures
Authors: Chassaing P.
Published: March 2003

The results of a straightforward investigation of heat transfer from hot wires in CO2–air mixtures, taking into consideration three parameters: velocity, temperature, and concentration are presented. Considering forced convection transfer, the final conclusion is expressed in the form of Nu=NA+NBRen. NA(Pr), NB(Pr), and n, are given and the observation of an overheat ratio, for which the composition sensitivity vanishes, is specified.
Collisional theory of electrical resistivity in trapped electron regimes
Authors: Hui B. H. , Winsor N. K. , Coppi B.
Published: March 2003

The distribution function is determined numerically for electrons subject to magnetic trapping, electron-electron, and electron-ion collisions, and an electric field large enough to invalidate the Spitzer–Härm (classical) approximation. The electrical resistivity is determined. It is compared with analytical theory for tokamaks, in the limit of large inverse aspect ratio ε, and small drift velocity Vd/Vth. The resistivity is plotted for values of ε and Vd/Vth for which no analytic theory is available.
Electromagnetic parametric instabilities in an ultra-relativistic plasma
Authors: Arons Jonathan , Norman Colin A. , Max Claire Ellen
Published: March 2003

The parametric decay of an intense, coherent electromagnetic wave into subluminous electrostatic waves and scattered electromagnetic waves in a homogeneous plasma is studied, with particular attention to the effects of relativistic electron temperature. A mode coupling formalism is developed, from which growth rates for stimulated Raman and Brillouin backscattering in one dimension, and for modulational Raman forward scattering, are derived.
Lower hybrid drift instability at low drift velocities
Authors: Freidberg J. P. , Gerwin R. A.
Published: March 2003

The behavior of the lower hybrid drift instability is investigated in the low drift velocity regime by including the effects of the magnetic field on the ions. It is found that the mode is transformed from the nonresonant lower hybrid drift instability to the resonant ion cyclotron drift instability as the drift velocity Vd decreases and is finally stabilized when Vd/Vti∼ (me/mi)1/2.
Vlasov equilibria for a warm finite radius rotating relativistic electron beam in a magnetized plasma
Authors: Ottinger P. F. , Guillory J.
Published: March 2003

A Vlasov equilibrium model is developed which can describe a broad class of beam-plasma systems in an applied field, B0z. The model treats arbitrary density profiles and azimuthal self-magnetic fields as well as a thermal spread in the momentum of the beam. It is assumed that the electron gyroradius is small but not negligible compared with the beam radius, and that the beam is completely charge neutralized but only fractionally current neutralized. The electron orbit equations are solved and are used to simplify the form of the distribution function and the calculation of its velocity moments. The rotation frequency of the beam and the diamagnetic or paramagnetic properties of such warm beam-plasma systems are also studied.
Kinetic theory of tearing instabilities
Authors: Drake J. F. , Lee Y. C.
Published: March 2003

The transition of the tearing instability from the collisional to the collisionless regime is investigated kinetically using a Fokker–Planck collision operator to represent electron-ion collisions. As a function of the collisionality of the plasma, the tearing instability falls into three regions, which are referred to as collisionless, semi-collisional, and collisional. The width Δ of the singular layer around k⋅B0=0 is limited by electron thermal motion along B0 in the collisional and semi-collisional regimes and is typically smaller than ρi, the ion Larmor radius. Previously accepted theories, which are based on the assumption Δ≫ρi, are found to be valid only in the collisional regime. The effects of density and temperature gradients on the instabilities are also studied. The tearing instability is only driven by the temperature gradient in the collisional and semi-collisional regimes. Numerical calculations indicate that the semi-collisional tearing instability is particularly relevant to present day high temperature tokamak discharges.
Gauss–Hertz principle applied to continuous fluid or gas motion
Authors: Moore Richard L.
Published: March 2003

The Gauss–Hertz principle when coupled with an accessory condition based on the assumption that fluid pressure is always positive is used to derive both well known flows such as Couette and Poiseuille, and the velocity and force fields of a magnetically confined plasma.
Two-phase flow analysis of hindered settling
Authors: Thacker W. C. , Lavelle J. W.
Published: March 2003

Kinematical effects which hinder settling are due to the upward return flow of the fluid and to the influence of the sediment on the hydrostatic pressure. Dynamical effects of increased drag must be accounted for with a nonlinear drag force law.
One-dimensional nature of diffuse energy depositions
Authors: Simons Girard A.
Published: March 2003

It is shown that atmospheric energy depositions may be sufficiently diffuse so that the fluid elements rise in a manner which is similar to the one-dimensional flow through a diverging nozzle. A unique solution is obtained for the equilibrium horizontal and vertical position of a heated fluid element.
Radial boundary layers in diffusing toroidal equilibria
Authors: Rosen M. D. , Greene J. M.
Published: March 2003

Analytic results in straight cylindrical geometry imply sharp density gradients near the boundary of a plasma decaying by classical diffusion. Utilizing an isothermal one-fluid magnetohydrodynamic model, these results are applied to toroidal configurations and a set of nonlinear equations for a radial boundary layer are obtained. A dominant effect in this region is convective plasma flow along magnetic lines of force, with velocity in the sonic range. This flow pattern matches onto the interior solution of Pfirsch–Schlüter convective flow. Solutions separable in the radial and poloidal directions are found that fulfill both boundary and periodicity conditions, and that result in both smooth subsonic poloidal flow, and weak shock transonic flows. Effects of the flow patterns on diffusion are discussed.
Disrupted states of vortex flow and vortex breakdown
Authors: Faler J. H. , Leibovich S.
Published: March 2003

Flow visualization studies and laser Doppler anemometer measurements on swirling water flows reveal six distinct types of very large amplitude disturbance modes of the vortex core. Three, ’’axisymmetric’’ and spiral vortex breakdowns, and the ’’double helix,’’ have been described by others. A definite order of evolution in parameter space (Reynolds number and circulations) occurs, and is described. Puzzling responses of the axisymmetric and spiral vortex breakdowns to imposed flow transients reported previously are confirmed here, and are traced to the shedding of starting and stopping vortices from swirl vanes. Conclusions bearing upon the validity of some theories of vortex breakdown are possible from the data.
Motion of near-neutrally buoyant tracers in vortical flows
Authors: Charwat A. F.
Published: March 2003

An analysis of the motion of particles of arbitrary density and size in solid body and potential vortex flows is presented. The study provides a basis for precise, quantitative reduction of tracer-particle visualization of such flows in water tunnels.
Spectra of the Orr–Sommerfeld equation
Authors: Murdock John W. , Stewartson Keith
Published: March 2003

It is shown that the continuous spectra of both the temporal and spatial eigenvalues of the Orr–Sommerfeld equation for a Blasius flow can be re-interpreted as discrete spectra of the heat-conduction and diffraction types, respectively. Most of the general properties of the discrete spectrum of the Orr–Sommerfeld equation discovered by Mack can be understood on the basis of a simple model equation.
Upstream influence on the near field of a plane turbulent jet
Authors: Hussain A. K. M. F. , Clark A. Ray
Published: March 2003

The effects of the mean and turbulence characteristics of the upstream (initial) boundary layer on the evolution of the flow in the near field of a plane jet have been experimentally investigated for four initial conditions. Rates of jet widening and centerline mean velocity decay as well as the kinematic and geometric virtual origins show evidence of systematic dependence on initial conditions. The growth rate of longitudinal turbulence intensity, and the mass flux are higher when the initial boundary layer is laminar than when turbulent. Immediately downstream of the exit, the nondimensional entrainment rates for the laminar initial boundary layer cases reach peak values which are about twice the delayed peak values for the fully turbulent initial boundary layer cases. Within the first 40 slit widths, increases in total average streamwise momentum flux range from 20% to 56%, the larger increases occurring for the laminar initial boundary layers; about 10% of each increase is due to the turbulence field. While such increases violate the traditionally accepted momentum flux invariance, they are consistent with the negative mean static pressure data.
Supersonic, nonequilibrium corner expansion flow of ionized argon
Authors: Igra O. , Barcessat M.
Published: March 2003

The nonequilibrium corner-expansion flow field of partially ionized argon was solved numerically using the method of characteristics. The solution assumed a steady, two-dimensional, and inviscid field. The main feature of the present solution, as compared with previous numerical solutions for the corner-expansion flows, is in the relaxation of the requirement that thermal equilibrium be maintained throughout the expansion. As a result, the electron temperature was found to be higher than that of the heavy particles. This is expected, since most of the energy released during the recombination process is absorbed by the free electrons. The specific solution reported here, solved the expansion field for two different corners (−5° and −15°) expansion angle), when both had the same pre-corner flow conditions. Fair agreement was found between the present numerical solution and the experimental results regarding the density and degree of ionization changes along streamlines. Consequently, the present technique can be used with confidence for the flow of various plasmas.
Propagation of nonlinear waves along a magneto-plasma column
Authors: Kunhardt E. E. , Cheo B. Ru-shao
Published: March 2003

Nonlinear waves propagating along a magneto-plasma column are investigated on the basis of the cold fluid model. Three regimes are studied in detail: (1) column in free space, zero magnetic field (surface waves), (2) column surrounded by conductor, weak axial dc magnetic field, i.e., ωce<ωpe, (3) column surrounded by conductor, large axial magnetic field. For each case, the evolution in space of the modal amplitude, which is shown to act as a potential from which the longitudinal component of the electric field may be obtained, is described by a Korteweg–de Vries equation. For a given initial condition, each case is shown to behave quantitatively differently due to differences in the magnitude and sign of the nonlinear and dispersive terms. Correlation with experimental results are discussed.
Nonlinear propagation of ion-cyclotron modes
Authors: Spatschek K. H. , Tagare S. G.
Published: March 2003

It is shown that the nonlinear interaction of large amplitude electrostatic ion-cyclotron oscillations with low-frequency perturbations causes a modulational instability. The most important cases of adiabatic density perturbations and low-frequency changes in the magnetic field strength are considered. The nonlinear equation which governs the asymptotic state of the modulationally unstable ion-cyclotron mode admits a soliton or soliton-like solution for the electric potential.