bluuebunny/crossref_metadata_2025 huggingface dataset:
The use of magnetohydrodynamic effects to investigate fluid flow in electrically conducting melts
Authors: Baumgartl J. , Hubert A. , Müller G.
Published: July 2002

A measurement method is introduced which can detect steady and unsteady flow in electrically conducting fluids. In this method, the flow region is subjected to an externally applied weak magnetic field B0. The induced magnetic field b generated by fluid flow across the field lines of B0 is used for the flow investigations. It can be measured outside the flow region with the advantage that neither electrical nor mechanical connections with the fluid are necessary. The results obtained by the experiment are confirmed by a time-dependent three-dimensional numerical simulation. It is indicated how fluid flow configurations could be deduced by an appropriate measurement of the induced field b.
Inferring secondary flows from smoke or dye flow visualization: Two case studies
Authors: Finlay W. H. , Guo Y. , Olsen D.
Published: July 2002

Spectral element simulations of the steady, incompressible, parabolized Navier–Stokes equations are used to compare numerically simulated smoke or dye (tracer) patterns with numerically calculated spatially developing flow patterns in the following two geometries: curved channel flow and twisted square duct flow (which consists of three joined 90° square curved ducts with perpendicular planes of curvature). Secondary flows in these two geometries are caused by streamwise-oriented vortices, which have been visualized in previous experiments by viewing smoke or dye patterns in cross-sectional planes perpendicular to the streamwise direction. Simulations of tracer patterns (obtained by tracking weightless particles) show that only when there is little streamwise variation of the secondary flow do tracer patterns provide a correct qualitative indication of the secondary flow patterns. For example, tracer patterns misrepresent merging of curved channel vortices and the locations and shapes of the twisted duct vortices. These results highlight the difficulty in obtaining consistent interpretations of tracer patterns in flows with significant streamwise variation, and in obtaining a priori predictions of the validity of inferring secondary flow patterns from tracer patterns. For the two case studies examined, it is found that unless there is little streamwise variation of the secondary flow structure, inferences of secondary flow patterns from experimental tracer patterns should be made only when well validated by other methods.
Analysis of a class of simplified models for nonlinear gravity wave interactions
Authors: Willemsen Jorge F.
Published: July 2002

Kolmogorov-type cascade processes have long been postulated to explain power-law falloff of the spectrum of gravity wave autocorrelations. In an effort to understand these processes more deeply, simplified models of the nonlinear Hasselmann resonant four-wave interaction have been introduced. These models respect the fundamental structural aspects of that interaction, but are otherwise chosen for computational simplicity. In this paper the concept of quasilocality within the context of such models is extended to its logical limit in one spatial dimension. This results in differential rather than integral equations for the wave fields. One scaling solution to the new equations has a spectral exponent independent of the details of the model interaction, and is stable to small perturbations. In addition, under weak restrictions on the first and second derivatives of the model interactions, a further pair of two-parameter families of scaling solutions exist. The spectral exponents in this case depend on the details of the model, but unstable as well as stable perturbations to these solutions are supported.
Polarization and energy evolution of electromagnetic waves in sheared toroidal plasmas
Authors: Airoldi A. , Orefice A. , Ramponi G.
Published: July 2002

The evolution, through a toroidal magnetoactive plasma of the polarization of an electromagnetic wave and of the energy of the modes into which it splits, is analyzed here under the condition of negligible geometric refraction, also taking into account the possible wave absorption as a result of electron cyclotron resonances. The problem is reduced to the solution of a simple set of first-order differential equations, avoiding both the complexity of the full Maxwell system and the limitations of a WKB treatment. This differential set is solved numerically, and its results are presented and discussed.
Radiation guiding in a free electron laser
Authors: Seo Yoonho , Tripathi V. K. , Liu C. S.
Published: July 2002

Analytic solutions of radially localized eigenmodes of radiation in Compton regime free electron lasers are obtained to demonstrate the radiation guiding property of the electron beam. The radial width of the radiation field Rs is found to be related to the beam radius Rb by Rs∼Rbp1/6, where p=(c/Rb)3/β0ωp0ω20, in a cold beam approximation. Here β0c, ωp0, and ω0 are the electron quivering velocity resulting from the pump, the equilibrium beam plasma frequency on the beam axis, and the pump wave frequency, respectively, in the electron beam rest frame. Finite temperature effects, curvature effects, and transverse motional effects of the beam are also studied.
Modeling transport in toroidal plasmas: Status and issues
Authors: Houlberg W. A. , Ross D. W. , Bateman G. , Cowley S. C. , Efthimion P. C. , Pfeiffer W. W. , Porter G. D. , Shumaker D. E. , Sugiyama L. E. , Wiley J. C.
Published: July 2002

The scope and detail of physics contained in computational models for fluid (density, momentum, energy) transport in toroidal plasmas have steadily increased during the past two decades. There has been considerable success in the development and verification of models for sources and sinks of particles, energy, momentum, and magnetic flux. Transport codes have collectively become very useful tools in interpreting experimental data and in providing guidance for new experiments. However, a more thorough understanding of the fundamental transport processes of magnetically confined plasmas and development of improved computational models are needed to enhance the predictive capabilities of transport codes. It is argued that fluid transport modeling by itself cannot lead to a complete understanding of transport—there must be a very strong collaboration among theory, experiment, and modeling on both the fluid and kinetic levels.
Reflection at the resonance layer of the fast Alfvén wave in ion cyclotron heating
Authors: Chow C. , Fuchs V. , Bers A.
Published: July 2002

The power reflection coefficient for D(H), DT(3He), and D (3He) ion cyclotron heating is found in closed form. The fast wave equation previously solved numerically is solved by a slowly varying amplitude expansion about the Budden equation asymptotics. The coupling between the fast Alfvén wave and the ion Bernstein wave is described by this formalism. The dissipation associated with the ion cyclotron resonance is accounted for by the slow variation. Validity criteria for this method are presented.
Bifurcation of poloidal rotation and suppression of turbulent fluctuations: A model for the L–H transition in tokamaks
Authors: Shaing K. C. , Crume E. C. , Houlberg W. A.
Published: July 2002

The poloidal momentum balance equation in tokamaks is shown to have bifurcated solutions; the poloidal flow velocity Up can suddenly become more positive when the ion collisionality decreases. The corresponding radial electric field Er becomes more negative, suppresses turbulent fluctuations, and improves plasma confinement. A heuristic argument is employed to illustrate the effects of Er on turbulent fluctuations. A more negative value of Er and/or a more positive value of dEr /dr can suppress the fluctuation amplitudes, if dP/dr<0 (with r the local minor raidus and P the plasma pressure). The theory is employed to explain the L–H transition observed in tokamaks.
Poloidal rotation and density asymmetries in a tokamak plasma with strong toroidal rotation
Authors: Stacey W. M.
Published: July 2002

A neoclassical model for calculating poloidal rotation and poloidal density asymmetries in a tokamak plasma with vφ∼vth and E/Bθ∼O(1) is developed. Application is made to the analysis of a deuterium plasma with a dominant carbon impurity. The dependences of the results on collisionality, the anomalous radial deuterium flux and the toroidal rotation speed are evaluated. The implications of the calculated poloidal velocities and density asymmetries for the magnitude of the gyroviscous torque are discussed.
Analytic formula for fully relativistic Thomson scattering spectrum
Authors: Naito O. , Yoshida H. , Matoba T.
Published: July 2002

A compact formula for fully relativistic Thomson scattering spectrum including depolarization term is presented. By rational approximation, an analytic formula with high accuracy (relative error<0.1% at 100 keV) is obtained, which is applicable to a wide range of plasmas.
Test of tokamak low-mode–high-mode transition theory in stellarators
Authors: Shaing K. C.
Published: July 2002

The tokamak L–H (low-mode–high mode) transition theory can be tested in stellarators in a controlled manner by making use of the two or more local maxima in poloidal viscosity in these devices. Depending on the relative magnitudes of the toroidal and helical components of the magnetic-field spectrum, the local maxima, and thus transition, can occur either at a critical poloidal E×B Mach number Mp of the order of unity, similar to that of a tokamak, or at an Mp of the order of ‖m−nq‖/m, similar to that of a helically symmetric torus. Here, E(B) is the electric (magnetic) field strength, m (n) is the poloidal (toroidal) mode number of the helical component of the ‖B‖ spectrum, and q is the safety factor. Possible limitations on the test due to the effects of the charge-exchange momentum loss are discussed.
Effect of fusion reactions on the stopping power of a non-neutral plasma
Authors: Ordonez C. A.
Published: July 2002

Conventional scattering theory based on the binary-collision approximation is used to evaluate the effect of fusion reactions on the energy loss of energetic ions which pass through an ion non-neutral plasma. The effect appears as a term in a generalized expression for the Coulomb logarithm. Using the general form of the Coulomb logarithm, fusion reactions are calculated to change the stopping power of a pure triton plasma by up to 12% for energetic (≤10 MeV) deuterons which pass through the plasma.
New first-order perpendicular drift velocities
Authors: Martín Pablo
Published: July 2002

A treatment in orbit theory has been performed assuming that the parallel electric field E∥ and E⊥ are of zeroth order in m/q, i.e., they are O(1). A proper analysis is developed in such a way that no secularities appear. New first-order drift velocities have been found which depend on E∥.
Neoclassical transport fluxes in the plateau regime in nonaxisymmetric toroidal plasmas
Authors: Shaing K. C. , Hirshman S. P. , Callen J. D.
Published: July 2002

The neoclassical fluxes in the plateau regime in an arbitrary large aspect ratio nonaxisymmetric toroidal system are calculated using fluid equations. The results are evaluated explicitly for a stellarator and for a rippled tokamak. For typical stellarator parameters, the helical magnetic field contribution to the particle and heat fluxes is comparable to the toroidicity contribution, and therefore, cannot be neglected. In addition, the helical magnetic field contribution to the parallel viscosity is capable of reversing the directions of the bootstrap current and Ware pinch flux in the plateau regime. In contrast, the neoclassical particle and heat fluxes in a rippled tokamak are comparable to those in an axisymmetric tokamak after the radial electric field and parallel flow velocity are determined (without external sources).
Current drive with fast phase velocity lower-hybrid waves
Authors: Decyk Viktor K. , Abe Hirotada
Published: July 2002

A particle simulation is used to test the hypothesis that the small, low phase velocity part of the antenna spectrum used in lower-hybrid current drive experiments is important in bridging the gap between the velocity of the bulk of the electron distribution and the phase velocity of the major part of the spectrum. A simulation with the small, low phase velocity part of the spectrum numerically removed showed a strong quenching of the current compared to a simulation with the full spectrum, consistent with the hypothesis.
Corrugation stability for plane relativistic shock waves
Authors: Anile A. M. , Russo G.
Published: July 2002

A definition is given of corrugational stability for plane-fronted shock waves. By using the theory of singular surfaces it is shown how one can obtain necessary conditions for corrugation stability. The theory is then applied to relativistic shock waves and yields a restriction on the state equation of a relativistic fluid in order for it to allow the existence of corrugationally stable shocks.
The influence of side wall heat transfer on convection in a confined saturated porous medium
Authors: Weidman P. D. , Kassoy D. R.
Published: July 2002

The effect of side wall heat transfer on the stability of natural convection in a vertically oriented finite slab of saturated porous material is considered. All six bounding faces are impermeable. A temperature contrast between the top and bottom horizontal surfaces provides the mechanism for destabilization. The narrow vertical end walls are perfectly insulated. The thermal conditions on the broad vertical side walls range from perfectly insulating to fully conducting as determined by the value of B, the Biot number based on slab height. An asymptotic analysis of the general solution is made in the narrow gap limit ε≪1, where ε is the cross-slab width-to-height ratio. In this case the relevant heat transfer variable is B̄=εB, the Biot number based on the narrow slab width. In the limit ε → 0 when B̄=O(1), including the case B̄→∞ that corresponds to a linear side wall temperature profile, tall, vertical, three-dimensional, finger-like cells are found at the critical Rayleigh number Rc=π2/ε2. In the limit B̄ → 0 corresponding to perfect insulation, one obtains two-dimensional, O(1) aspect ratio rolls with axes normal to the side walls at the critical value Rc=4π2. These two-dimensional rolls predominate only for B̄=O(ε2), and transition to tall narrow three-dimensional cells occurs when O(ε2)≪ B ≪ O(1).
Steady state whistler turbulence and stability of thermal barriers in tandem mirrors
Authors: Litwin C. , Sudan R. N.
Published: July 2002

The effect of the whistler turbulence on anisotropic electrons in a thermal barrier is examined. The electron distribution function is derived self-consistently by solving the steady state quasilinear diffusion equation. Saturated amplitudes are computed using the resonance broadening theory or convective stabilization. Estimated power levels necessary for sustaining the steady state of a strongly anisotropic electron population are found to exceed by orders of magnitude the estimates based on Fokker–Planck calculations for the range of parameters of tandem mirror (TMX-U and MFTF-B) experiments [Nucl. Fusion 25, 1205 (1985)]. Upper limits on the allowed degree of anisotropy for existing power densities are calculated.
Current gradient driven flute vortices in magnetized plasmas
Authors: Shukla P. K.
Published: July 2002

It is shown that an external electron current gradient in a nonuniform magnetized plasma can drive a purely growing instability involving coupled convective cells and magnetostatic modes. Accounting for the mode coupling, it is demonstrated that the quasistationary state of the unstable flute modes can lead to a spatially ordered dipolar vortex structure. The latter could alter the transport properties and it may also be responsible for a relaxed state in a force-free magnetized plasma.
A pdf modeling study of self-similar turbulent free shear flows
Authors: Haworth D. C. , Pope S. B.
Published: July 2002

A modeled transport equation for the joint probability density function (pdf) of the velocities and a scalar has been solved numerically for four self-similar turbulent free shear flows: the plane mixing layer, the plane wake, the plane jet, and the axisymmetric jet. In the pdf equation, convection is treated exactly but the effects of viscosity, molecular diffusion, and the fluctuating pressure gradient have to be modeled. Five different models are evaluated; four of these are based on the Langevin equation for the fluid particle velocities, and the fifth is a particle pair interaction model. In each case, a stochastic mixing model represents the effects of molecular diffusion, and conditional modeling is included to account for intermittency. The resulting modeled pdf equation is solved by a Monte Carlo method. Calculated spreading rates and profiles of the mean velocity and Reynolds stresses show generally good agreement with experimental data for three of the five velocity models. However, discrepancies between the calculated and measured intermittency factor profiles, in particular for the plane mixing layer, reveal weaknesses in the conditional modeling.
The effects of electric field fluctuations on bootstrap current and resistivity in toroidal plasmas
Authors: Shaing K. C.
Published: July 2002

The effects of electric field fluctuations on bootstrap current and resistivity in toroidal plasmas are studied in the plateau regime. A tokamak is discussed as an example. The bootstrap current density is more sensitive to the fluctuations than the plasma resistivity because the bootstrap current density depends on the poloidal mode number of the fluctuations, while plasma resistivity depends on the parallel wave vector.
Particle image displacement velocimetry measurements of a three-dimensional jet
Authors: Lourenco L. , Krothapalli A.
Published: July 2002

A whole field experimental technique, commonly referred to as particle image displacement velocimetry (PIDV), is used for the measurement of the instantaneous two-dimensional velocity fields in the transition region of a three-dimensional jet issuing from a rectangular nozzle with aspect ratio 4. The experiments were performed using an air jet at a Reynolds number based on the hydraulic diameter of 3600. The rollup of the laminar shear layer into vortices and their subsequent interactions are examined.
Beam–plasma dispersion relations by Lorentz transformation
Authors: McKinstrie C. J.
Published: July 2002

The dispersion tensor for linear wave propagation in an arbitrary number of moving plasma species is derived using an approach that is based on Lorentz transformations. This approach requires less computational effort than the direct approach, in which the equation of motion for each plasma species is linearized about an equilibrium state with nonzero velocity vs. As a specific example, the effects of an axial magnetic field and transverse beam temperature on the collisionless Weibel instability are studied.
A direct interaction approximation treatment of high Rayleigh number convective turbulence and comparison with experiment
Authors: Hartke Gregory J. , Canuto V. M. , Dannevik William P.
Published: July 2002

Recently published experimental results [Int. J. Heat Mass Transfer, 23, 738 (1980)] on thermally driven high Rayleigh number turbulent convection have shown that the N∝R2/7 relation (where N is the Nusselt number and R is the Rayleigh number), which is valid up to R≊5×108, is superceded at this point by the relation N=AσR1/3 that holds at least up to R≊1011. For water (Prandtl number σ=6.6), the experimental value for Aσ was found to be Aσ=0.0556±0.001. In the present work, the equations for a turbulent fluid driven by thermal convection are solved using the two-point closure prescription of the direct interaction approximation. The theoretical N vs R relation at high R is found to be of the form N=AσR1/3 and for σ=6.6, and the predicted value of the coefficient Aσ is computed to be Aσ≲0.08, in good agreement with the experimental value. Extension of the model to situations other than convection is discussed.
The dynamics of periodically driven bubble clouds
Authors: Smereka P. , Banerjee S.
Published: July 2002

An averaged two-fluid model is used to study the motion of a cloud of bubbles. The linearized equations of motion are shown to be a wave equation with both dissipation and dispersion. The fully nonlinear equations are also examined and it is demonstrated that the cutoff frequency of the cloud is equal to the natural frequency of a single bubble. The steady linear response of a periodically driven bubble cloud is then derived. Resonances are seen to arise when the driving frequency is below the cutoff frequency. The inner core of the cloud is shielded by an outer layer when the driving is above the cutoff frequency. The nonlinear dynamics of periodically driven bubble clouds is studied numerically. It is found that the cutoff frequency is crucial in determining whether or not the cloud will behave like a single bubble. Also, under some conditions the cloud is seen to behave like a damped and driven single-degree-of-freedom Hamiltonian system.
Turbulent mixing of a passive scalar
Authors: Holzer Mark , Siggia Eric D.
Published: July 2002

The statistically stationary state of a turbulently advected passive scalar is studied, with an imposed linear mean gradient in two dimensions, via a number of numerical experiments. For a synthetic Gaussian velocity field, which is generated by a linear stochastic process, and whose spectra and Eulerian correlation time follow Kolmogorov scaling on all scales, the exponents of the scalar spectra are consistent with 5/3 or 17/3 depending on the diffusivity. For large Péclet numbers (Pe), the probability density function (PDF) of the scalar gradients perpendicular to the mean is well fit, from about 0.1–10 times the root-mean-square value, by a stretched exponential with exponent ∼0.6. The PDF for gradients parallel to the mean has similar tails and a 𝒪(1) skewness for all Pe studied. The scalar has a ramp-and-cliff structure similar to that first seen in shear-flow experiments with scalars. A physical picture of the mechanism by which the ramp-and-cliff features form is given. A second model with the velocity evolving under the Euler equations restricted to a band of wave numbers produces the k−1 Batchelor spectrum when the scalar is dissipated with a hyperdiffusivity (∝k4). For physical dissipation (∝k2), the PDF of the scalar has exponential tails, and for gradients less than the cutoff set by the maximum strain, the PDF of the gradients is similar to that obtained with the stochastic velocity model. The PDF of the dissipation is approximately stretched exponential like the gradient PDFs and not lognormal. The skewness of the gradients parallel to the mean decreases with decreasing autocorrelation time of the velocity, and the gradient PDFs assume a limiting form in the white-noise limit.
Instability and breakdown of internal gravity waves. I. Linear stability analysis
Authors: Lombard Peter N. , Riley James J.
Published: July 2002

We have performed three-dimensional linear stability analysis, based on Floquet theory, to study the stability of finite amplitude internal gravity waves. This analysis has been used to compute instability growth rates over a range of wave amplitudes and propagation angles, especially waves above and below overturning amplitude, and identifies several new characteristics of wave instability. Computation of instability eigenfunctions has allowed us to analyze the energetics of the instability and to clarify the paths of energy transfer from the base wave to the instability. We find that the presence of wave overturning has no qualitative effect on the wave instability, except for the limiting case when the wavenumber vector is vertical. Instabilities which are nearly two-dimensional are closely related to second-order wave–wave interactions. But the three-dimensional instabilities, more prominent at higher wave amplitudes, may be caused by higher order resonance interactions. The energetics of the instabilities range from being shear driven to being driven by ‘‘density gradient’’ production (the potential energy analog of ‘‘shear’’ production); this characteristic is strongly dependent on wave propagation angle and the three-dimensionality of the instability.
On the stability of the Jeffery–Hamel flow
Authors: Uribe F. J. , Díaz-Herrera Enrique , Bravo A. , Peralta-Fabi R.
Published: July 2002

The Jeffery–Hamel flow is analyzed by making use of a mechanical analogy. The solutions are generated by a finite element method, for the corresponding least action principle and are valid for any width of the channel. Then, the Galerkin method is used to study the linear and temporal stability of some flows for small widths of the channel. An argument is given to show that pure inflow should be unstable for small widths of the channel, a result which is corroborated by our numerical calculations. We find that the flows IOI and IO are unstable for Reynolds numbers near zero. A stability window for some asymmetric flows and certain widths is also found.
Solitary waves with a vortex core in a shallow layer of stratified fluid
Authors: Derzho Oleg G. , Grimshaw Roger
Published: July 2002

This paper is concerned with a theoretical model for large amplitude solitary waves with a vortex core in a shallow layer of stratified fluid with a nearly uniform stratification. Previous work has shown that solitary waves can be calculated up to a critical amplitude for which the horizontal velocity, in a frame for which the wave is at rest, approaches zero at the boundary point beneath the wave crest for a wave of elevation (or above the wave crest for a wave of depression). Here we calculate waves with amplitudes slightly greater than the critical amplitude by incorporating a vortex core located near the aforementioned boundary point. The effect of the vortex core is to introduce into the governing equation for the wave amplitude an extra nonlinear term proportional to the 3/2 power of the difference between the wave amplitude and the critical amplitude. We find that as the wave amplitude increases above the critical amplitude, the wave broadens, which is in marked contrast to the case of small amplitude waves. Further, the wave speed is found to depend nonlinearly on the wave amplitude, again in marked contrast to the linear dependence for small amplitude waves. The specific case of linearly stratified fluid is examined in detail.
Cell size dependence of transport coefficients in stochastic particle algorithms
Authors: Alexander Francis J. , Garcia Alejandro L. , Alder Berni J.
Published: July 2002

Using the Green–Kubo theory, the dependence of the viscosity and thermal conductivity on cell size is obtained explicitly for stochastic particle methods such as direct simulation Monte Carlo (DSMC) and its generalization, the consistent Boltzmann algorithm (CBA). These analytical results confirm empirical observations that significant errors occur when the cell dimensions are larger than a mean free path.
Large amplitude Langmuir and ion-acoustic waves in a relativistic two-fluid plasma
Authors: Nejoh Yasunori , Sanuki Heiji
Published: July 2002

Large amplitude Langmuir and ion-acoustic waves in a weakly relativistic two-fluid plasma are analyzed by the pseudopotential method. The existence conditions for relativistic nonlinear Langmuir waves depend on the relativistic effect, the particular energy, and the ion mass to electron mass ratio. The allowable range of the normalized potential depends on the relativistic effect. It is shown that the Mach number has the significant effect for the formation of relativistic nonlinear ion-acoustic waves rather than the ratio of the ion-acoustic velocity to the velocity of light. The allowable range of the normalized potential depends on the Mach number. The present investigation predicts new findings of relativistic nonlinear Langmuir and ion-acoustic waves in plasmas in which high-speed electrons and ions coexist.
A short mean-free path, coupled neutral-ion transport description of a tokamak edge plasma
Authors: Catto Peter J.
Published: July 2002

The ion and neutral densities and temperatures in the edge region of a diverted plasma can vary over two or more orders of magnitude causing atomic physics processes to modify the usual short mean-free path transport descriptions of magnetized plasmas. The short-mean-free path ordering is particularly relevant to the rather cold, moderately dense plasmas of interest in gas target divertor operation. Under such conditions, charge-exchange, ionization, and recombination couple the ions to the neutrals and alter the transport coefficients of both species. The coupling modifications to the ions are evaluated in the short mean-free path limit by a minor adjustment in standard procedures when neutral–neutral collisions are negligible. The short-mean-free path coupling modifications to the neutrals are evaluated by assuming the charge-exchange rate coefficient is velocity independent. A closed neutral-ion fluid description is obtained by assuming temperatures sufficiently high that ion energy and momentum exchange with the electrons is unaffected by atomic processes and densities sufficiently low that electron–neutral collisions may be neglected.
The analog of Blanc’s law for drift velocities of electrons in gas mixtures in weakly ionized plasma
Authors: Chiflikian R. V.
Published: July 2002

The analog of Blanc’s law for drift velocities of electrons in multicomponent gas mixtures in weakly ionized spatially homogeneous low-temperature plasma is derived. The obtained approximate-analytical expressions are valid for average electron energy in the 1–5 eV range typical for plasma conditions of low-pressure direct current (DC) discharges. The accuracy of these formulas is ±5%. The analytical criterion of the negative differential conductivity (NDC) of electrons in binary mixtures of gases is obtained. NDC of electrons is predicted in He:Kr and He:Xe rare gas mixtures.
Shear-flow generation by drift/Rossby waves
Authors: Guzdar P. N.
Published: July 2002

The generation of shear flow driven by a large amplitude drift wave, represented by the Hasegawa–Mima–Charney (HMC) equation, has been investigated. It is shown that, besides a finite amplitude threshold for the shear flow instability to occur, there is also a necessary condition on the aspect ratio of the large amplitude drift wave, due to conservation of the average potential vorticity, that needs to be satisfied. A comprehensive comparison of the shear-flow instability criterion for the HMC equation and the incompressible, invisicid, hydrodynamic equation has been undertaken.
Self-similar electron distribution, inverse bremsstrahlung, and heat flux inhibition in high-Z nonuniform plasmas
Authors: Uryupin S. A. , Kato S. , Mima K.
Published: July 2002

The self-similar distribution of electrons is found for a nonuniform underdense plasma that is heated by an intensive laser field. The distribution function is flat-topped for the low-energy electrons. And in the high-energy region, it has a well-pronounced high-energy tail. It is also found how the electron heat flux and the absorption coefficient depend upon both the ratios of electron mean-free path to the inhomogeneity scale of effective temperature and of the oscillation velocity to the thermal velocity. The actual shapes of electron energy distribution, the heat flux limitation, and the electromagnetic radiation absorption rate are given, both for a currentless plasma and for a plasma with a finite electric current.
Ion temperature gradient-driven ballooning mode in tokamaks
Authors: Hirose A. , Zhang L. , Elia M.
Published: July 2002

It is shown that an ion temperature gradient (ηi) induces a collisionless ballooning mode in the MHD (magnetohydrodynamic) second stability regime. Both two-fluid and fully kinetic analyses predict the instability, in qualitative agreement, indicating that the ion magnetic drift resonance plays a key role in destabilization. The instability is characterized by broad eigenfunctions in the ballooning space and at a small magnetic shear has a growth rate comparable with that of the MHD mode. Trapped electrons, the ion transit effect, and magnetosonic perturbation have stabilizing influences, but are unable to suppress the mode.
Flush mounted Langmuir probes in an oblique magnetic field
Authors: Weinlich M. , Carlson A.
Published: July 2002

Due to the high heat flux to the target plates of present day divertor tokamaks, the use of thermally robust flush mounted Langmuir probes becomes more and more important. The surface normal of the probe defines a direction in space which is generally neither parallel nor perpendicular to the magnetic field, and the angle between these two vectors plays an important role for the physics in front of the probe. An analytic description of the sheath physics in front of a target plate is presented, and a model for the analysis of the I–V characteristics of such probes is derived therefrom. The model includes, on the one hand, considerably more physics than previous descriptions, and is, on the other hand, much simpler and more practical than numerical solutions and simulations. Subsequently the application of this model to triple probes is discussed. It will become evident that flush mounted probes can be used as reliably as domed probes to determine the plasma parameters in front of the target plates.
SBS reflectivity from spatially smoothed laser beams: Random phase plates versus polarization smoothing
Authors: Hüller S. , Mounaix Ph. , Tikhonchuk V. T.
Published: July 2002

The reflectivity due to stimulated Brillouin backscattering (SBS) from an ensemble of independent laser speckles is investigated for different speckle statistics. Calculations are based on numerical simulations with a multidimensional code and a compact model describing the main features of speckle self-focusing. In particular, the simulations and the model are applied to speckle ensembles due to the random phase plate (RPP) and polarization smoothing (PS) techniques. A stronger SBS inhibition for PS with respect to the RPP technique is demonstrated.
Self-consistent mean-field forces in turbulent plasmas: Current and momentum relaxation
Authors: Hegna C. C.
Published: July 2002

The properties of turbulent plasmas are described using the two-fluid equations. Global constraints for the fluctuation-induced mean-field forces that act on the ion and electron fluids are derived. These constraints imply functional forms for the parallel mean-field forces in the Ohm’s law and the momentum balance equation. These forms suggest that the fluctuations attempt to relax the plasma to a state where both the current and the bulk plasma momentum are aligned along the mean magnetic field with proportionality constants that are global constants. Observations of flow profile evolution during discrete dynamo activity in reversed field pinch experiments are interpreted.
Neoclassical conductivity and bootstrap current formulas for general axisymmetric equilibria and arbitrary collisionality regime
Authors: Sauter O. , Angioni C. , Lin-Liu Y. R.
Published: July 2002

Expressions for the neoclassical resistivity and the bootstrap current coefficients in terms of aspect ratio and collisionality are widely used in simulating toroidal axisymmetric equilibria and transport evolution. The formulas used are in most cases based on works done 15–20 years ago, where the results have been obtained for large aspect ratio, small or very large collisionality, or with a reduced collision operator. The best expressions to date and to our knowledge are due to Hirshman [S. P. Hirshman, Phys. Fluids 31, 3150 (1988)] for arbitrary aspect ratio in the banana regime and Hinton–Hazeltine [F. L. Hinton and R. D. Hazeltine, Rev. Mod. Phys. 48, 239 (1976)] for large aspect ratio and arbitrary collisionality regime. A code solving the Fokker–Planck equation with the full collision operator and including the variation along the magnetic field line, coupled with the adjoint function formalism, has been used to calculate these coefficients in arbitrary equilibrium and collisionality regimes. The coefficients have been obtained for a wide variety of plasma and equilibrium parameters and a comprehensive set of formulas, which have been fitted to the code results within 5%, is proposed for evaluating the neoclassical conductivity and the bootstrap current coefficients. This extends previous works and also highlights inaccuracies in the previous formulas in this wide plasma parameter space.
Effects of an external magnetic field, and of oblique radio-frequency electric fields on multipactor discharge on a dielectric
Authors: Valfells A. , Ang L. K. , Lau Y. Y. , Gilgenbach R. M.
Published: July 2002

This paper analyzes, separately, the effects of an external magnetic field, the rf magnetic field, and of an oblique rf electric field, on multipactor discharge on a dielectric. Using Monte Carlo simulation, we obtain the susceptibility diagram in terms of the magnetic field, the rf electric field, and the dc charging field for various dielectric materials. We find that a magnetic field parallel to either the rf electric field or the dc electric field does not qualitatively change the susceptibility diagram. However, an external magnetic field perpendicular to both the rf electric field and the dc electric field can significantly affect the susceptibility diagram. Thus oriented magnetic fields lower the upper susceptibility bound when the magnetic field strength is approximately equal to Bres[T]=0.036f(GHz), where f is the rf frequency. Both the lower and upper susceptibility boundary may be raised significantly by a large external magnetic field, B≫Bres. Susceptibility to single surface multipactor is greatest when the rf electric field is nearly parallel to the dielectric, but is dramatically decreased for angles of obliqueness greater than approximately 5°–10°. The rf magnetic field does not affect the lower boundary, but may extend the upper boundary greatly.
Understanding and control of transport in Advanced Tokamak regimes in DIII-D
Authors: Greenfield C. M. , DeBoo J. C. , Luce T. C. , Stallard B. W. , Synakowski E. J. , Baylor L. R. , Burrell K. H. , Casper T. A. , Doyle E. J. , Ernst D. R. , Ferron J. R. , Gohil P. , Groebner R. J. , Lao L. L. , Makowski M. , McKee G. R. , Murakami M. , Petty C. C. , Pinsker R. I. , Politzer P. A. , Prater R. , Rettig C. L. , Rhodes T. L. , Rice B. W. , Schmidt G. L. , Staebler G. M. , Strait E. J. , Thomas D. M. , Wade M. R. ,   DIII-D Team
Published: July 2002

Transport phenomena are studied in Advanced Tokamak (AT) regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomics Energy Agency, Vienna, 1987), Vol. I, p. 159], with the goal of developing understanding and control during each of three phases: Formation of the internal transport barrier (ITB) with counter neutral beam injection taking place when the heating power exceeds a threshold value of about 9 MW, contrasting to co-NBI injection, where Pthreshold<2.5 MW. Expansion of the ITB is enhanced compared to similar co-injected discharges. Both differences are believed to arise from modification of the E×B shear dynamics when the sign of the rotation contribution is reversed. Sustainment of an AT regime with βNH89=9 for 16 confinement times has been accomplished in a discharge combining an ELMing H-mode (edge localized, high confinement mode) edge and an ITB, and exhibiting ion thermal transport down to 2–3 times neoclassical. The microinstabilities usually associated with ion thermal transport are predicted stable, implying that another mechanism limits performance. High frequency magnetohydrodynamic (MHD) activity is identified as the probable cause.
Resonant tunneling of two-dimensional electrons through a quantum wire: A negative transconductance device
Authors: Luryi Serge , Capasso Federico
Published: July 2002

A novel three-terminal resonant-tunneling structure is proposed in which the double potential barrier is defined within the plane of a two-dimensional (2-D) electron gas. The resonant tunneling of 2-D electrons into a 1-D ‘‘quantum wire’’ is controlled not only by a source-to-drain voltage but also by a gate potential. In addition to the negative differential resistance found in conventional resonant-tunneling diodes, our device offers a negative transconductance. This feature is potentially useful for low-power logic circuits.
Picosecond carrier recombination dynamics of semiconductor-doped glasses
Authors: Hsu S. C. , Kwok H. S.
Published: July 2002

The carrier recombination dynamics of CdSxSe1−x-doped color glasses under strong pumping conditions were studied with 20 ps dye laser pulses and a white light continuum. The initial blue shift of the absorption spectrum was followed by a fast coherent transient and a slower decay. The results yield an electron-hole recombination time of ∼400 ps. An interesting wavelength dependence of the decay curves was also noted.
Thermionic emission model for the initial regime of silicon oxidation
Authors: Irene E. A. , Lewis E. A.
Published: July 2002

The very early stage of the thermal oxidation of single-crystal Si has been the subject of continual study for the last two decades. In the light of very recent experimental oxidation data on the initial regime, we report that a simple thermionic electron flux from Si into SiO2 closely agrees with the SiO2 film growth rate. The importance of electrons for the oxidation kinetics has also been attested to in several recent experimental studies. Thus a consistent model is presented for the initial oxidation regime based on the electron flux as the rate limiting step.
Heteromultimers of DEG/ENaC subunits form H
+
-gated channels in mouse sensory neurons
Authors: Benson Christopher J. , Xie Jinghui , Wemmie John A. , Price Margaret P. , Henss Jillian M. , Welsh Michael J. , Snyder Peter M.
Published: July 2002

Acidic extracellular solution activates transient H
+
-gated currents in dorsal root ganglion (DRG) neurons. The biophysical properties of three degenerin/epithelial sodium (DEG/ENaC) channel subunits (BNC1, ASIC, and DRASIC), and their expression in DRG, suggest that they might underlie these H
+
-gated currents and function as sensory transducers. However, it is uncertain which of these DEG/ENaC subunits generate the currents, and whether they function as homomultimers or heteromultimers. We found that the biophysical properties of transient H
+
-gated currents from medium to large mouse DRG neurons differed from BNC1, ASIC, or DRASIC expressed individually, but were reproduced by coexpression of the subunits together. To test the contribution of each subunit, we studied DRG from three strains of mice, each bearing a targeted disruption of BNC1, ASIC, or DRASIC. Deletion of any one subunit did not abolish H
+
-gated currents, but altered currents in a manner consistent with heteromultimerization of the two remaining subunits. These data indicate that combinations of two or more DEG/ENaC subunits coassemble as heteromultimers to generate transient H
+
-gated currents in mouse DRG neurons.
Thyroid hormone is a critical determinant for the regulation of the cochlear motor protein prestin
Authors: Weber Thomas , Zimmermann Ulrike , Winter Harald , Mack Andreas , Köpschall Iris , Rohbock Karin , Zenner Hans-Peter , Knipper Marlies
Published: July 2002

The most impressive property of outer hair cells (OHCs) is their ability to change their length at high acoustic frequencies, thus providing the exquisite sensitivity and frequency-resolving capacity of the mammalian hearing organ. Prestin, a protein related to a sulfate/anion transport protein, recently has been identified and proposed as the OHC motor molecule. Homology searches of 1.5 kb of genomic DNA 5′ of the coding region of the prestin gene allowed the identification of a thyroid hormone (TH) response element (TRE) in the first intron upstream of the prestin ATG codon. Prestin
TRE
bound TH receptors as a monomer or presumptive heterodimer and mediated a triiodothyronine-dependent transactivation of a heterologous promotor in response to triiodothyronine receptors α and β. Retinoid X receptor-α had an additive effect. Expression of prestin mRNA and prestin protein was reduced strongly in the absence of TH. Although prestin protein typically was redistributed to the lateral membrane before the onset of hearing, an immature pattern of prestin protein distribution across the entire OHC membrane was noted in hypothyroid rats. The data suggest TH as a first transcriptional regulator of the motor protein prestin and as a direct or indirect modulator of subcellular prestin distribution.
Inhibition of calcium/calmodulin kinase II alpha subunit expression results in epileptiform activity in cultured hippocampal neurons
Authors: Churn Severn B. , Sombati Sompong , Jakoi Emma R. , Sievert Lawrence , DeLorenzo Robert J.
Published: July 2002

Several models that develop epileptiform discharges and epilepsy have been associated with a decrease in the activity of calmodulin-dependent kinase II. However, none of these studies has demonstrated a causal relationship between a decrease in calcium/calmodulin kinase II activity and the development of seizure activity. The present study was conducted to determine the effect of directly reducing calcium/calmodulin-dependent kinase activity on the development of epileptiform discharges in hippocampal neurons in culture. Complimentary oligonucleotides specific for the α subunit of the calcium/calmodulin kinase were used to decrease the expression of the enzyme. Reduction in kinase expression was confirmed by Western analysis, immunocytochemistry, and exogenous substrate phosphorylation. Increased neuronal excitability and frank epileptiform discharges were observed after a significant reduction in calmodulin kinase II expression. The epileptiform activity was a synchronous event and was not caused by random neuronal firing. Furthermore, the magnitude of decreased kinase expression correlated with the increased neuronal excitability. The data suggest that decreased calmodulin kinase II activity may play a role in epileptogenesis and the long-term plasticity changes associated with the development of pathological seizure activity and epilepsy.
CLOUDS, a protocol for deriving a molecular proton density via NMR
Authors: Grishaev Alexander , Llinás Miguel
Published: July 2002

We demonstrate the feasibility of computing realistic spatial proton distributions for proteins in solution from experimental NMR nuclear Overhauser effect data only and with minimal assignments. The method, CLOUDS, relies on precise and abundant interproton distance restraints calculated via a relaxation matrix analysis of sets of experimental nuclear Overhauser effect spectroscopy crosspeaks. The MIDGE protocol was adapted for this purpose. A gas of unassigned, unconnected H atoms is condensed into a structured proton distribution (cloud) via a molecular dynamics simulated-annealing scheme in which the internuclear distances and van der Waals repulsive terms are the only active restraints. Proton densities are generated by combining a large number of such clouds, each computed from a different trajectory. After filtering by reference to the cloud closest to the mean, a minimal dispersion proton density (foc) is identified. The latter affords a quasi-continuous hydrogen-only probability distribution that conveys immediate information on the protein surface topology (grooves, protrusions, potential binding site cavities, etc.), directly related to the molecular structure. Feasibility of the method was tested on NMR data measured on two globular protein domains of low regular secondary structure content, the col 2 domain of human matrix metalloproteinase-2 and the kringle 2 domain of human plasminogen, of 60 and 83 amino acid residues, respectively.