bluuebunny/crossref_metadata_2025 huggingface dataset:
A nonlocal theory for the heat transport in composites containing highly conducting fibrous inclusions
Authors: Shaqfeh Eric S. G.
Published: July 2002

A theory is developed to describe the heat transfer in composites containing highly conducting fibrous inclusions under conditions in which the average temperature field scales on lengths comparable to the length of the included fibers. Thus, in contrast to previous developments, the fiber samples the details of a rapidly varying temperature field rather than simply a local linear field. Using the method of averaged equations and slender body theory, the average ‘‘extra flux’’ created by the presence of the fibers is demonstrated to be an integral of the temperature gradient about any point weighted by a function which is the appropriate nonlocal conductivity. This representation of the thermal transport in the material is derived explicitly for (a) dilute composites in which nf l3≪1, where nf is the number density of the fiber and l is their length; and (b) semidilute composites in which nfl3≫1 but nflb2≪1, where b is the fiber thickness. In both instances the expressions derived are rigorously valid for fibers that are very long and thin. Associated with the derivation of the semidilute nonlocal theory is the first complete derivation of the semidilute local result, which justifies the arguments made by Batchelor [J. Fluid Mech. 46, 813 (1971)] and demonstrates the mechanisms of ‘‘screening’’ in these multiparticle systems.
Kinetic theory of electron drift vortex modes
Authors: Jones Roger D.
Published: July 2002

Magnetic electron drift vortex modes are examined using a kinetic theory. The dispersion is found to be much different than that obtained from the fluid theory. For frequencies much smaller than the electron plasma frequency, the waves are heavily Landau damped. The modes are weakly damped at frequencies just below the plasma frequency and hence have an electromagnetic rather than magnetic character.
Viscous modulation of the Lamb dipole vortex
Authors: Swaters G. E.
Published: July 2002

A simple analytical singular perturbation theory is developed to describe the viscous adiabatic decay of the two-dimensional Lamb dipole vortex. The vortex parameters (translation speed, radius, and wavenumber) evolve so as to satisfy leading-order globally averaged energy and enstrophy balances. These transport equations are shown to be solvability conditions for the asymptotic expansion. Extensions of the asymptotic procedure to other isolated vortex problems are discussed.
Numerical analysis of a rarefied gas flow past a volatile particle using the Boltzmann equation for hard-sphere molecules
Authors: Sone Yoshio , Takata Shigeru , Wakabayashi Masahiko
Published: July 2002

A spherical condensed phase with a uniform surface temperature that is placed in a slow uniform flow of its vapor gas is considered. The steady behavior of the gas accompanied by evaporation and condensation on the sphere is investigated mainly numerically on the basis of the Boltzmann equation for hard-sphere molecules. The numerical method is a combination of the hybrid-difference-scheme method, capable of describing the discontinuity of the velocity distribution function in the gas, and the numerical kernel method [Phys. Fluids A 5, 716 (1993)]. The velocity distribution function of the gas molecules, the macroscopic variables such as the density, velocity, and temperature of the gas, and the force (drag) acting on the sphere are obtained precisely for the whole range of the Knudsen number (the mean free path of the uniform flow divided by the radius of the condensed phase). In particular, the behavior of the discontinuity of the velocity distribution function in the gas is described accurately.
Dynamics of a nonlinear dipole vortex
Authors: Hesthaven J. S. , Lynov J. P. , Nielsen A. H. , Rasmussen J. Juul , Schmidt M. R. , Shapiro E. G. , Turitsyn S. K.
Published: July 2002

A localized stationary dipole solution to the Euler equations with a relationship between the vorticity and streamfunction given as ω=−ψ+ψ3 is presented. By numerical integration of the Euler equations this dipole is shown to be unstable. However, the initially unstable dipole reorganizes itself into a new nonlinear dipole, which is found to be stable. This new structure has a functional relationship given as ω=αψ+βψ3−γψ5. Such dipoles are stable to head-on collisions and they are capable of creating tripolar structures when colliding off axis. The effects of increasing Newtonian viscosity on the nonlinear dipole is studied revealing that even though the nonlinearity is weakening, the dipole does not relax towards a Lamb dipole.
Kármán vortex formation from a cylinder: Role of phase-locked Kelvin–Helmholtz vortices
Authors: Chyu C. , Lin J.-C. , Sheridan J. , Rockwell D.
Published: July 2002

The formation length of Kármán vortices can be drastically reduced by small-amplitude excitation at a frequency much higher than the natural frequency of Kármán vortex formation, namely the Kelvin–Helmholtz (KH) frequency of the separating shear layer. Phase-locked patterns of KH vortices are attainable. These spatially stationary patterns coexist, however, with the spatial and temporal development of the Kármán vortices.
Streamwise vortices in plane Couette flow
Authors: Dauchot O. , Daviaud F.
Published: July 2002

Experimental observations of various flows have led to the conclusion of the existence of streamwise vortices involved in the destabilization process of these flows. In the plane Couette flow, a linear shear flow, such structures have never been observed, because of the linear stability of the flow. The flow was slightly modified by introducing a wire in its central plane, parallel to the spanwise direction. A destabilization then occurs. It generates streamwise structures periodically spaced in the spanwise direction. These structures have been identified as pairs of counter-rotating streamwise vortices. This Letter characterizes the dependence on the Reynolds number of the behavior of these vortices and how their destabilization leads to turbulence.
Tracer diffusion under shear flow for general repulsive interactions
Authors: Garzó V. , López de Haro M.
Published: July 2002

Tracer diffusion in a steady shear flow state is analyzed. A kinetic model incorporating a temperature dependence in the collision frequencies is used. This allows for the consideration of a general repulsive intermolecular interaction. A perturbative scheme is applied to get the shear rate dependence of the tracer diffusion tensor in terms of the mass ratio, the force constants ratio, and a parameter characterizing the interaction potential considered. In addition, the heat flux arising from the concentration gradient of the tracer species is also evaluated. The results are illustrated for the two extreme cases of Maxwell molecules and hard spheres.
On the nature of direct instabilities in double-component convection with different boundary conditions
Authors: Tsitverblit Naftali A.
Published: July 2002

A destabilizing gradient of one density-affecting component maintained by a linear combination of the flux and fixed-value boundary conditions is considered with stable stratification imposed by the difference in the values of another component. It is emphasized that, even if the diffusivities of the components are equal, there exists a band of directly unstable finite wavelengths in the region of the stable net stratification irrespective of instability of the waves in the vicinity of zero wave number. The steady instability is related to the unequal vertical diffusion gradients formed in the perturbed state due to the different boundary conditions. Implications of this insight for interpretation of previous theories are also discussed.
Numerical investigation of flow structure and bifurcation phenomena of confined plane twin-jet flows
Authors: Soong C. Y. , Tzeng P. Y. , Hsieh C. D.
Published: July 2002

This work numerically investigates laminar plane twin-jet flows with side-wall confinement. Time-dependent computations on the full physical domain are performed in light of the possible emergence of flow unsteadiness and an asymmetric pattern. Of particular concern is how wall-confinement and jet momentum influence the transition of flow patterns. Results presented herein provide further insight into the complexities involving issues such as the variety of flow structure and the related bifurcations, jet–jet and jet–wall interactions, and flow instability involved in the confined twin-jet flow field.
The low-Reynolds number spreading of axisymmetric drops and gravity currents along a free surface
Authors: Dorsey Chad W. , Manga Michael
Published: July 2002

Experimental results are presented for the spreading of buoyant drops and gravity currents along a free surface. The spreading occurs at low Reynolds number, and no interfacial tension exists between the spreading and ambient fluid. Theoretical results suggest that distinct spreading rates occur in three regimes: λ≪[ln(R/a)]−1a/R, a/R≪λ≪R/a, and λ≫R/a, where λ is the ratio of the spreading and ambient fluid viscosities and R/a is the aspect ratio of the drop or current. Experimentally measured spreading rates in these three regimes show agreement with the theoretical solutions for long-term spreading.
Statistics of filtered velocity in grid and wake turbulence
Authors: Cerutti Stefano , Meneveau Charles
Published: July 2002

Data on spatially filtered turbulence are commonly needed for a priori sub-grid model studies and for a posteriori testing of large eddy simulation (LES) codes. In this paper, hot-wire anemometry is used to record very long records of such data, required for good convergence of high-order statistics. An array consisting of four X-wire probes placed transversal to the flow direction is built. Unlike previous single-probe hot-wire measurements, which only allowed stream-wise filtering using Taylor’s hypothesis, the array permits cross-stream filtering as well. Measurements which are spatially filtered at a length-scale Δ pertaining to the inertial-range of turbulence are performed in grid and wake turbulence. The data can be used directly to compare with results from LES. From the data, fundamental differences between filtered and unfiltered velocity fields are examined through probability density functions and the scaling behavior of high-order structure functions. A comparative study of probability density functions of filtered and unfiltered velocity increments shows that the tails of the distributions are affected by the filtering even at scales much larger than the filter scale. Significant differences are also observed in regard to the scaling of structure functions. It is shown that extended self-similarity, a recent technique for measuring inertial range scaling exponents, yields questionable results when applied to structure functions of filtered velocity.
Spontaneous generation of toroidal currents
Authors: Ma Sifeng , Dawson John M.
Published: July 2002

A loss-cone mechanism for self-generation of toroidal currents is examined in toroidal magnetic configurations with a mirror-like vertical (poloidal) field. The loss cone is found to be asymmetric with respect to the toroidal velocity. Particles that carry a negative toroidal current are better confined than those that carry a positive current, giving rise to a net toroidal current that can be sufficiently large to close magnetic surfaces. This mechanism is found to be dependent on the curvature drift of particles, indicating that a low-aspect-ratio toroidal configuration is more favorable for this type of current generation.
Fluid model of collisionless plasma with finite Larmor radius effects
Authors: Smolyakov A. I. , Pogutse I. O. , Hirose A.
Published: July 2002

A closed set of moment equations for the plasma density, velocity and pressure is proposed by imposing linear closure for the heat flux and viscosity. The formulation is valid for arbitrary finite Larmor radius parameter k⊥ρ (k⊥ is the perpendicular wave vector, ρ is the particle Larmor radius) and retains main convective nonlinearities. Reduced moment equations relevant to the drift-wave modes are derived in the low frequency limit (ω≪ωB, ωB is the particle cyclotron frequency).
Role of neutrals in the phase transition model
Authors: Carreras B. A. , Diamond P. H. , Vetoulis G.
Published: July 2002

The phase transition model for the transition from the low-confinement regime (L mode) to high-confinement regime (H mode) is extended to incorporate the effect of the neutral particles on the transition threshold. For usual edge plasma parameters, the increase of effective poloidal viscosity through charge exchange damping and reduction of the effective fluxes by both ionization and charge exchange increases the threshold power required for the transition. For plasmas with the effective energy flux smaller than the convective energy flux, the transition may be triggered by a neutral influx.
Comparisons of experimental measurements and two-dimensional plasma-fluid numerical simulations of a magnetized plasma column
Authors: Chiu G. S. , Cohen S. A.
Published: July 2002

The density, the electron, ion, and neutral temperatures in dense (ne≳1013 cm−3) and cool (Te<8 eV) magnetized helium plasma columns submerged in a cool neutral gas (n0∼3×1015 cm−3) environment have been measured. These plasmas, of similar collisionality to those expected in the scrape-off layer of ITER (International Thermonuclear Experimental Reactor) [R. Parker, Contrib. Plasma Phys. 34, 422 (1994)], are found to obey the low-recycling behavior of approximate plasma pressure balance along the column axis. Density decreases by a factor of about 2 and Te remains isothermal. Computer simulations using the B2 [R. Schneider et al., J. Nucl. Mater. 196–198, 810 (1992)] plasma-fluid code have been performed and are in good agreement with experimental results. Measured ion temperatures are consistent with classical energy transfer from the electrons. Neutrals within the plasma are heated by elastic and charge-exchange collisions. This set of measurements represents the most detailed comparisons between experimental results and B2 predictions.
Regimes of the magnetized Rayleigh–Taylor instability
Authors: Winske D.
Published: July 2002

Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the linear and nonlinear behavior of the magnetized Rayleigh–Taylor instability in slab geometry with the plasma subject to a constant gravity. Three regimes are found, which are determined by the magnitude of the complex frequency ω=ωr+iγ. For |ω|≪Ωi(Ωi= ion gyrofrequency), one finds the typical behavior of the usual fluid regime, namely the development of ‘‘mushroom-head’’ spikes and bubbles in the density and a strongly convoluted boundary between the plasma and magnetic field, where the initial gradient is not relaxed much. A second regime, where |ω|∼0.1Ωi, is characterized by the importance of the Hall term. Linearly, the developing flute modes are more finger-like and tilted along the interface; nonlinearly, clump-like structures form, leading to a significant broadening of the interface. The third regime is characterized by unmagnetized ion behavior, with |ω|∼Ωi. Density clumps, rather than flutes, form in the linear stage, while nonlinearly, longer-wavelength modes that resemble those in fluid regime dominate. Finite Larmor radius stabilization of short-wavelength modes is observed in each regime.
Two-dimensional modeling of the x-radiation output from perturbed Z pinches
Authors: Matuska W. , Bowers R. L. , Brownell J. H. , Lee H. , Lund C. M. , Peterson D. L. , Roderick N. F.
Published: July 2002

Two-dimensional radiation magnetohydrodynamic simulations are presented that demonstrate the effects of magnetically driven Rayleigh–Taylor instabilities on the soft x-ray output from Z pinches. Instability models, which reproduce the current drive wave form and match visible framing camera data for instability wavelength and amplitude for implosions on capacitively driven inductive store systems, are used to study the structure of the x-ray output and the spectrum of radiation emitted from the pinch. The results indicate that standard magnetohydrodynamics is capable of reproducing much of the observed data when two-dimensional effects associated with Rayleigh–Taylor instabilities are included.
Suppression of the Rayleigh–Taylor instability through accretion
Authors: Book David L.
Published: July 2002

The nonlinear growth of the Rayleigh–Taylor instability in an accelerated shell can be reduced by arranging for the shell to sweep up material as it moves. Momentum conservation preferentially slows the thinner ‘‘bubble’’ portions of the perturbed shell, which lead the ‘‘spikes.’’ Results are presented from calculations based on the spherical thin-shell model.
Study of the conditions for spontaneous H(high)-mode transitions in DIII-D
Authors: Carlstrom T. N. , Groebner R. J.
Published: July 2002

A series of scaling studies attempting to correlate the H(high)-mode power threshold (PTH) with global parameters have been conducted. Data from these discharges is also being used to look for dependence of PTH on local edge parameters and to test theories of the transition. Boronization and better operational techniques have resulted in lower-power thresholds and weaker density scaling. Neon impurity injection experiments show that radiation also plays a role in determining PTH. A low-density threshold for the L(low)–H(high) transition has been linked with the locked mode low-density limit, and can be reduced with the use of an error field correcting coil. Highly developed edge diagnostics, with spatial resolution as low as 5 mm, are used to evaluate how the power threshold depends on local edge conditions. Preliminary analysis of local edge conditions for parameter scans of ne, BT, and Ip in single-null discharges, and the X-point imbalance in double-null discharges show that, just before the transition to the H mode, the edge temperatures near the separatrix are approximately constant at 100<Ti<220 eV and 35<Te<130 eV, even though the threshold power varied from 1.5 to 14 MW. During a density scan, the edge ion collisionality, ν*i, varied from 2 to 17, demonstrating that a transition condition as simple as ν*i=const is inconsistent with the data. The local edge parameters of ne, Te, and Ti do not always follow the same global scaling as PTH. Therefore, theories of the L–H transition need not be constrained by these scalings.
Dynamic plasma screening effects on semiclassical inelastic electron–ion collisions in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

In dense plasmas, dynamic plasma screening effects are investigated on 1s→2p dipole transition probabilities for electron-impact excitation of hydrogenic ions. The electron–ion interaction potential is considered by introduction of the plasma dielectric function. A semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to visualize the semiclassical transition probability as a function of the impact parameter, projectile energy, and plasma parameters. The transition probability including the dynamic plasma screening effect is always greater than that including the static plasma screening effect. When the projectile velocity is smaller than the electron thermal velocity, the dynamic polarization screening effect becomes the static plasma screening effect. When the projectile velocity is greater than the electron thermal velocity, then the interaction potential is almost unshielded. The difference between the dynamic and static plasma screening effects is more significant for low-energy projectiles. It is also found that the static plasma screening formula obtained by the Debye–Hückel model overestimates the plasma screening effects on the atomic excitation processes in dense plasmas.
Modification of the internal electric field by biasing of the divertor plates in the Tokamak de Varennes (TdeV)
Authors: Lafrance D. , Huang R. , Stansfield B. L. , Haddad E. , Lachambre J.-L.
Published: July 2002

The radial electric field inside the separatrix has been deduced from spectroscopic measurements of impurities on TdeV (Tokamak de Varennes), using the reduced radial momentum balance and two neoclassical models [R. D. Hazeltine, Phys. Fluids 17, 961 (1974) and Y. B. Kim, P. H. Diamond, and R. J. Groebner, Phys. Fluids B 3, 2050 (1991)]. The results from all three models are in fair agreement. Furthermore, the electric field has been deduced using the same models both with and without biasing the divertor plates relative to the machine wall, showing an inward propagation of the effect of the biasing created in the scrape-off layer (SOL). Undeniably, the electric field has been modified well inside the separatrix (0.6≲r/a≲0.9), revealing the possibility of modifying the internal electric field by external means.
Sawtooth oscillations in a damped/driven cryogenic electron plasma: Experiment and theory
Authors: Cluggish B. P. , Driscoll C. F. , Avinash K. , Helffrich J. A.
Published: July 2002

Measurements have been made of nonlinear sawtooth oscillations of the displacement of a magnetized electron column in a cryogenic, cylindrical trap. First reported 7 years ago, these oscillations occur when the displacement is destabilized by a resistive wall and damped by a temperature-dependent collisional viscosity. A typical evolution can last for thousands of seconds. Measurements show that oscillations of the plasma displacement are accompanied by oscillations in the plasma temperature. A simple predator-and-prey model of the temperature and displacement gives rise to a limit cycle solution due to the nonmonotonic dependence of the viscosity on temperature. These limit cycles are in good quantitative agreement with the measured sawtooth oscillations.
Negative-mass instability in magnetron-injection guns
Authors: Savilov Andrei V.
Published: July 2002

The negative-mass instability developing in a weakly-relativistic electron beam during its drift along an inhomogeneous magnetostatic field under the effect of space-charge fields is considered. Using the assumption of the convective character of the instability, simple spatial equations of the electron motion in magnetostatic and space-charge fields are obtained. The effect of the magnetic field inhomogeneity on the increase of the spread in electron energy, caused by the instability, as well as on the evolution of the spatial structure of electron density perturbations, is studied for both linear and saturation stages of the instability. It is shown that the spread in electron energy, caused by the negative-mass instability in magnetron-injection guns, can be up to several percents.
Gaussian short-time propagators and electron kinetics: Numerical evaluation of path-sum solutions to Fokker–Planck equations for rf heating and current drive
Authors: Bizarro João P. S. , Belo Jorge H. , Figueiredo António C.
Published: July 2002

Knowing that short-time propagators for Fokker–Planck equations are Gaussian, and based on a path-sum formulation, an efficient and simple numerical method is presented to solve the initial-value problem for electron kinetics during rf heating and current drive. The formulation is thoroughly presented and discussed, its advantages are stressed, and general, practical criteria for its implementation are derived regarding the time step and grid spacing. The new approach is illustrated and validated by solving the one-dimensional model for lower-hybrid current drive, which has a well-known steady-state analytical solution.
Low-frequency modes in the dust–plasma crystal
Authors: Vladimirov S. V. , Shevchenko P. V. , Cramer N. F.
Published: July 2002

It is shown that there are two low-frequency modes associated with vertical oscillations in the Coulomb crystal of dust grains arranged in two horizontal chains in a sheath region of a low-temperature gas discharge plasma. The dispersion relations and characteristic frequencies of the modes are found.
Hydromagnetic Rayleigh–Taylor instability in high-velocity gas-puff implosions
Authors: Roderick N. F. , Peterkin R. E. , Hussey T. W. , Spielman R. B. , Douglas M. R. , Deeney C.
Published: July 2002

Experiments using the Saturn pulsed power generator have produced high-velocity z-pinch plasma implosions with velocities over 100 cm/μs using both annular and uniform-fill gas injection initial conditions. Both types of implosion show evidence of the hydromagnetic Rayleigh–Taylor instability with the uniform-fill plasmas producing a more spatially uniform pinch. Two-dimensional magnetohydrodynamic simulations including unsteady flow of gas from a nozzle into the diode region have been used to investigate these implosions. The instability develops from the nonuniform gas flow field that forms as the gas expands from the injection nozzle. Instability growth is limited to the narrow unstable region of the current sheath. For the annular puff the unstable region breaks through the inner edge of the annulus increasing nonlinear growth as mass ejected from the bubble regions is not replenished by accretion. This higher growth leads to bubble thinning and disruption producing greater nonuniformity at pinch for the annular puff. The uniform puff provides gas to replenish bubble mass loss until just before pinch resulting in less bubble thinning and a more uniform pinch.
Mitigation of the Rayleigh–Taylor instability by sheared axial flows
Authors: Shumlak U. , Roderick N. F.
Published: July 2002

The stabilizing effect of sheared axial flows on the Rayleigh–Taylor instability in Z-pinch implosions is investigated. Dispersion relations are derived that include the effects of a density discontinuity and a sheared flow which produce the Rayleigh–Taylor and Kelvin–Helmholtz instabilities. The dispersion relations are examined in two limiting cases of practical interest, a uniform plasma implosion and an annular plasma implosion. The analytical results show a region of parameter space which provides complete stability. The analytical model is verified with nonlinear calculations using experimental implosion parameters and geometries. The results indicate that mitigation of the Rayleigh–Taylor instability is possible with a sufficient sheared axial flow.
Current-vortex filament model of nonlinear Alfvén perturbations in a finite-pressure plasma
Authors: Lakhin V. P. , Schep T. J. , Westerhof E.
Published: July 2002

A low-β, two-fluid model is shown to possess solutions in the form of current-vortex filaments. The model can be viewed as that of reduced magnetohydrodynamics, extended with electron inertia, the Hall term and parallel electron pressure. These drift-Alfvén filaments are the plasma analogs of point vortices in the two-dimensional Euler and the Charney–Hasegawa–Mima equations. The discrete system has the same global and local invariants as the original, continuous system. In an unbounded plasma, systems of two and three filaments are integrable. When the global linear momenta vanish, the four filament problem is also integrable. Stationary equilibria of a dipole, tripole, and of von Kármán streets are presented. The phase-space of two interacting, balanced pairs of filaments is analyzed in detail. New periodic four filament configurations are identified in plasma cases that do not exist in Euler systems. It is also shown that a collapse of the four vortices can occur in a finite time.
A new mathematical approach for finding the solitary waves in dusty plasma
Authors: Das G. C. , Sarma Jnanjyoti
Published: July 2002

In a sequel to a recent work [Das, Sarma, and Talukdar, Phys. Plasmas 5, 63 (1998)], the different nonlinear plasma-acoustic waves, based on the fluid approximation, have been derived showing the coexistences of dust-acoustic waves in plasmas contaminated by dust-charged grains. The features of the nonlinear waves, depending on the plasma composition, describe various natures of solitary waves. A new formalism, known as the tanh method and stemming from the modified simple wave solution technique, has been developed for finding the soliton propagation in the nonlinear plasma wave dynamics. The method is straightforward, with minimal mathematical manipulation, finding the heuristic formation and propagation of ion-acoustic solitary waves in the dusty plasma. The main aim is, based on the tanh method, to revisit the results in a simpler case and extending them to explain the behavior of higher-order nonlinear waves derived in generalized multicomponent plasmas. The theoretical observations highlight the salient features of nonlinear waves coexisting with the dust-acoustic wave. The new findings might expect the effect, because of the dust-charged grains, to be the common feature of nonlinear waves in the dusty plasmas, and could be of interest for future experiments in laboratory as well as in space plasmas.
Collisionless nonideal ballooning modes
Authors: Kleva Robert G. , Guzdar Parvez N.
Published: July 2002

The nonzero inertia of the electron is shown to destabilize pressure-driven ballooning modes in collisionless tokamak plasmas that are stable in the ideal magnetohydrodynamic (MHD) approximation. The effect of the electron mass is characterized by the collisionless electron skin depth de=c/ωpe, where ωpe=4πne2/me is the electron plasma frequency. The growth rate of electron inertia ballooning modes increases with the magnitude of de, and also with the magnitude of the ratio β of the plasma thermal pressure to the pressure in the confining magnetic field.
Experimental and theoretical investigation of local synergy between ion Bernstein and lower hybrid waves in the Princeton Beta Experiment—Modified
Authors: Paoletti F. , Cardinali A. , Bernabei S. , Post-Zwicker A. , Tighe W. , Von Goeler S.
Published: July 2002

The local synergistic behavior of lower hybrid and ion Bernstein waves on the Princeton Beta Experiment-Modified tokamak [Phys. Fluids B 2, 1271 (1990)] is experimentally studied using a two-dimensional hard x-ray camera. The hard x-ray bremsstrahlung emission from suprathermal electrons, generated with lower hybrid current drive, is enhanced during ion Bernstein wave power injection. This enhancement is observed in limited regions of space, suggesting the formation of localized current channels. The effects on plasma electrons during combined application of these two types of waves are theoretically investigated using a quasilinear model. The numerical code simultaneously solves the three-dimensional (R,Z,Φ) torbidal wave equation for the electric field, in the Wentzel–Kramers–Brillouin–Jeffrey approximation, and the Fokker–Planck equation for the distribution function in two dimensions (v∥,v⊥) with an added quasilinear diffusion coefficient. The radial profile of the noninductively generated current density, the transmitted power traces, and the total power damping curve are calculated. The beneficial effects of a combined utilization of ion Bernstein and lower hybrid waves on the current drive are emphasized. The numerical results are compared with the experimental observations.
The isentropic exponent in plasmas
Authors: Burm K. T. A. L. , Goedheer W. J. , Schram D. C.
Published: July 2002

The isentropic exponent for gases is a physical quantity that can ease significantly the hydrodynamic modeling effort. In gas dynamics the isentropic exponent depends only on the number of degrees of freedom of the considered gas. The isentropic exponent for a plasma is lower due to an extra degree of freedom caused by ionization. In this paper it will be shown that, like for gases, the isentropic exponent for atomic plasmas is also constant, as long as the ionization degree is between 5%–80%. Only a very weak dependence on the electron temperature and the two nonequilibrium parameters remain. An argon plasma is used to demonstrate the behavior of the isentropic exponent on the plasma conditions, and to make an estimation of the value of the isentropic exponent of a customary plasma. For atmospheric plasmas, which usually have an electron temperature of about 1 eV, a sufficiently accurate estimate for the isentropic exponent of plasmas is 1.16.
Flow driven resistive wall instability
Authors: Veeresha B. M. , Bhattacharyya S. N. , Avinash K.
Published: July 2002

The stability of a perfectly conducting fluid layer flowing along a magnetic field, parallel to a finitely conducting thin wall is examined. Finite layer width and compressibility of the fluid are shown to significantly lower the flow velocity required for instability to set in. The effect of axial flow on the stability of a cylindrical pinch surrounded by a resistive wall is examined. Flow is shown to have a destabilizing effect.
Alfvén wave current drive in tokamak rotating plasma with negative magnetic shear
Authors: Ding N. , Huang L. , Qiu X. M.
Published: July 2002

The current drive due to Alfvén waves in a tokamak rotating plasma with negative magnetic shear is studied in this paper. In cylindrical geometry, an expression for driving current density Jz is given by means of the single-fluid magnetohydrodynamic (MHD) model taking plasma rotation and magnetic shear into account. Furthermore, a set of differential equations in r for the perturbed fields Er, Eθ, and Ez is derived. The current drive due to the compressional Alfvén wave and the shear Alfvén wave is considered, respectively. It is found that the efficiency of the Alfvén wave current drive without the magnetic shear is independent of rotating plasma density. Moreover, it is shown that a higher efficiency can be obtained in the presence of rotation. For the shear Alfvén wave, the magnetic shear has a more distinct effect on the current drive than the one for the compressional Alfvén wave. The effect of the negative magnetic shear on the Alfvén wave current drive is opposite to the effect of the positive, in that the negative shear enhances the driven current density Jz. These results show that the Alfvén waves may be an excellent current drive candidate for tokamak fusion reactors.
Influence of nonuniform plasma rotation on reactively unstable ion temperature gradient modes
Authors: Davydova T. A. , Weiland J.
Published: July 2002

The influence of flow shear and curvature on growth rate and space structure of reactively unstable Ion-temperature-gradient-(ITG) driven modes has been studied. For the case of strongly inhomogeneous rotation, a further developed version of the direct method of J. B. Taylor and H. R. Wilson [Plasma Phys. Controlled Fusion 38, 1999 (1996)] has been used where the specific effects of toroidal flows have also been included. It is found that whereas velocity curvature usually has a dominant influence on dissipative modes, velocity shear is more important for reactive modes.
Enhanced acceleration of energetic ions by oblique shock waves
Authors: Masaki Tomohiro , Hasegawa Hiroki , Ohsawa Yukiharu
Published: July 2002

Interactions of energetic ions with a shock wave propagating obliquely to a magnetic field are studied by means of a one-dimensional, relativistic, electromagnetic, particle simulation code with full ion and electron dynamics. It is found that energetic ions that barely enter the shock region can be accelerated by the transverse electric field. At the same time, their parallel momenta are increased by the magnetic field. Because of this, after the encounter with the shock wave, some of the energetic ions can move with it for long periods of time during which they undergo the acceleration repeatedly. These particles eventually go away from the shock wave to the upstream region, and their acceleration ceases.
On a bootstrap-like mechanism of radio frequency wave current drive in tokamaks
Authors: Tsypin V. S. , Nascimento I. C. , Galvão R. M. O. , Kuznetsov Yu. K.
Published: July 2002

A new type of radio-frequency (rf) current drive in tokamaks is considered in this paper. The basic physical mechanism is connected with the dependence of the ion and electron viscosities on the time-averaged rf wave intensity. The new terms appear in the surface-averaged Ohm’s law via the electron viscosity. Therefore, it is a bootstrap-like mechanism of rf current drive, not yet considered for tokamaks. At some special conditions, this type of the current drive can overcome the conventional one.
Long time implosion experiments with double gas puffs
Authors: Shishlov A. V. , Baksht R. B. , Fedunin A. V. , Fursov F. I. , Kovalchuk B. M. , Kokshenev V. A. , Kurmaev N. E. , Labetsky A. Yu. , Oreshkin V. I. , Russkikh A. G. , Fisher A. , Moosman B. , Weber B. V.
Published: July 2002

Long time implosion experiments with argon double gas puffs have been conducted on the GIT-12 [S. P. Bugaev et al., Izv. Vyssh. Uchebn. Zaved., Fiz. 40, 38 (1997)] generator at the current level of 2.2–2.4 MA. A double gas puff was used as one of the alternative ways to improve implosion stability at implosion times from 230 to 340 ns. The results of these experiments were compared with two-dimensional snowplow simulations. The experiments and the simulations show that the final pinch is sufficiently stable when the inner-to-outer shell mass ratio is greater than 1. The maximum argon K-shell yield obtained in the experiments is 740 J/cm with 220 GW/cm radiation power. At the long implosion times, the K-shell yield obtained in the double gas puff implosion is twice the K-shell yield of a 4-cm-radius single gas puff, with more than an order of magnitude increase in radiation power.
Electromagnetic acoustic transducers as wideband velocity sensors
Authors: Hutchins D. A. , Wilkins D. E. , Luke G.
Published: July 2002

The performance of electromagnetic acoustic transducers as wideband velocity sensors has been investigated, using a pulsed laser as a standard acoustic source.
Hydrogen passivation of C acceptors in high-purity GaAs
Authors: Pan N. , Bose S. S. , Kim M. H. , Stillman G. E. , Chambers F. , Devane G. , Ito C. R. , Feng M.
Published: July 2002

The effects of hydrogenation in high-purity p-type GaAs grown by molecular beam epitaxy and metalorganic chemical vapor deposition have been investigated by low-temperature photoluminescence and Hall-effect measurements. Before hydrogenation, photoluminescence measurements showed the dominant acceptor in the original samples was C, while after hydrogenation, the concentration of electrically active C acceptors was significantly reduced and the samples were highly resistive. These electrical and spectroscopic results show that C acceptors in GaAs can be passivated by hydrogenation.
Femtosecond hot-carrier energy relaxation in GaAs
Authors: Schoenlein R. W. , Lin W. Z. , Ippen E. P. , Fujimoto J. G.
Published: July 2002

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split-off band as well as the heavy- and light-hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.
Thermal resistance at interfaces
Authors: Swartz E. T. , Pohl R. O.
Published: July 2002

We report measurements of the solid-solid thermal boundary resistance Rbd, spanning the temperature range from 1 to 300 K. Below 30 K, Rbd is found to be in agreement with the prediction of the acoustic mismatch model. The influence of diffuse scattering at the interface is found to have a very minor influence on Rbd. Above 30 K, Rbd decreases less rapidly with increasing temperature than predicted by the theory. Phonon scattering in thin (∼30 Å) disordered layers near the interface is shown to be a possible explanation. Implications for heat removal from integrated circuits are discussed.
Epitaxial GexSi1−x/Si (100) structures produced by pulsed laser mixing of evaporated Ge on Si (100) substrates
Authors: Abelson John R. , Sigmon Thomas W. , Kim Ki Bum , Weiner Kurt H.
Published: July 2002

Heteroepitaxial GexSi1−x alloy layers have been formed by pulsed laser induced mixing of pure germanium films and Si (100) substrates. Ge films 50–200 Å thick are electron beam evaporated onto Si (100) under ≤1×10−7 Torr vacuum. The near surface of the sample then undergoes a rapid melt and regrowth process using 2–10 pulses from a XeCl excimer laser. The laser has a 37-ns pulse width at 308 nm and its energy density of 0.5–1.5 J/cm2 is precisely homogenized into a 4×4 mm square area. The alloy layers are 250–1600 Å thick, have a Ge fraction x=2.5–19%, and exhibit excellent crystallinity as evaluated by MeV ion channeling and lattice resolution cross-sectional transmission electron microscopy. Unlike layer growth by molecular beam epitaxy, this approach is insensitive to minor levels of contamination because the original Ge/Si interface is melted through during the laser processing.
Insect NMDA receptors mediate juvenile hormone biosynthesis
Authors: Chiang Ann-Shyn , Lin Wei-Yong , Liu Hsin-Ping , Pszczolkowski Maciej A. , Fu Tsai-Feng , Chiu Shu-Ling , Holbrook Glenn L.
Published: July 2002

In vertebrates, the
N
-methyl-
d
-aspartate
subtype of glutamate receptors (NMDAR) appears to play a role in
neuronal development, synaptic plasticity, memory formation, and
pituitary activity. However, functional NMDAR have not yet been
characterized in insects. We have now demonstrated
immunohistochemically glutamatergic nerve terminals in the corpora
allata of an adult female cockroach,
Diploptera
punctata.
Cockroach corpus allatum (CA) cells, exposed to NMDA
in vitro
, exhibited elevated cytosolic
[Ca
2+
], but not in culture medium nominally free of
calcium or containing NMDAR-specific channel blockers: MK-801 and
Mg
2+
. Sensitivity of cockroach corpora allata to NMDA
changed cyclically during the ovarian cycle. Highly active glands of
4-day-old mated females, exposed to 3 μM NMDA, produced 70% more
juvenile hormone (JH)
in vitro
, but the relatively
inactive glands of 8-day-old mated females showed little response to
the agonist. The stimulatory effect of NMDA was eliminated by
augmenting the culture medium with MK-801, conantokin, or high
Mg
2+
. Having obtained substantive evidence of functioning
NMDAR in insect corpora allata, we used reverse transcription PCR to
demonstrate two mRNA transcripts, DNMDAR1 and DNMDAR2, in the ring
gland and brain of last-instar
Drosophila melanogaster
.
Immunohistochemical labeling, using mouse monoclonal antibody against
rat NMDAR1, showed that only one of the three types of endocrine cells
in the ring gland, CA cells, expressed rat NMDAR1-like immunoreactive
protein. This antibody also labeled two brain neurons in the lateral
protocerebrum, one neuron per brain hemisphere. Finally, we used the
same primers for DNMDAR1 to demonstrate a fragment of putative NMDA
receptor in the corpora allata of
Diploptera
punctata
. Our results suggest that the NMDAR has a role in
regulating JH synthesis and that ionotropic-subtype glutamate receptors
became specialized early in animal evolution.
Implications of CD94 deficiency and monoallelic NKG2A expression for natural killer cell development and repertoire formation
Authors: Vance Russell E. , Jamieson Amanda M. , Cado Dragana , Raulet David H.
Published: July 2002

Natural killer (NK) cells are believed to achieve self-tolerance through the expression of self-MHC-specific inhibitory receptors, such as members of the Ly49 and CD94/NKG2 families. IndividualLy49genes are stochastically expressed by NK subsets and are expressed in a monoallelic fashion, but little is known about the mechanisms underlying CD94/NKG2A expression. We show here that, likeLy49genes, mouseNkg2ais stochastically and monoallelically expressed. Thus, a single general mechanism controls expression of all known MHC-specific receptors by mouse NK cells. In addition, we find that DBA/2J mice are naturally CD94-deficient and do not express cell-surface CD94/NKG2A receptors, even on neonatal NK cells. Thus, self-tolerance of neonatal NK cells cannot be attributed to CD94/NKG2A expression. Taken together, the results lead to a reconsideration of current models of NK cell development and self-tolerance.
Spontaneous corticospinal axonal plasticity and functional recovery after adult central nervous system injury
Authors: Weidner Norbert , Ner Arvin , Salimi Nima , Tuszynski Mark H.
Published: July 2002

Although it is believed that little recovery occurs after
adult mammalian spinal cord injury, in fact significant spontaneous
functional improvement commonly occurs after spinal cord injury in
humans. To investigate potential mechanisms underlying spontaneous
recovery, lesions of defined components of the corticospinal motor
pathway were made in adult rats in the rostral cervical spinal cord or
caudal medulla. Following complete lesions of the dorsal corticospinal
motor pathway, which contains more than 95% of all corticospinal
axons, spontaneous sprouting from the ventral corticospinal tract
occurred onto medial motoneuron pools in the cervical spinal cord; this
sprouting was paralleled by functional recovery. Combined lesions of
both dorsal and ventral corticospinal tract components eliminated
sprouting and functional recovery. In addition, functional recovery was
also abolished if dorsal corticospinal tract lesions were followed 5
weeks later by ventral corticospinal tract lesions. We found extensive
spontaneous structural plasticity as a mechanism correlating with
functional recovery in motor systems in the adult central nervous
system. Experimental enhancement of spontaneous plasticity may be
useful to promote further recovery after adult central nervous system
injury.
Crystal structure of the
Mus musculus
cholesterol-regulated START protein 4 (StarD4) containing a StAR-related lipid transfer domain
Authors: Romanowski Michael J. , Soccio Raymond E. , Breslow Jan L. , Burley Stephen K.
Published: July 2002

The x-ray structure of the mouse cholesterol-regulated START protein 4 (StarD4) has been determined at 2.2-Å resolution, revealing a compact α/β structure related to the START domain present in the cytoplasmic C-terminal portion of human MLN64. The volume of the putative lipid-binding tunnel was estimated at 847 Å
3
, which is consistent with the binding of one cholesterol-size lipid molecule. Comparison of the tunnel-lining residues in StarD4 and MLN64-START permitted identification of possible lipid specificity determinants in both molecular tunnels. Homology modeling of related proteins, and comparison of the StarD4 and MLN64-START structures, showed that StarD4 is a member of a large START domain superfamily characterized by the helix-grip fold. Additional mechanistic and evolutionary studies should be facilitated by the availability of a second START domain structure from a distant relative of MLN64.
Overexpression of a Na
+
/H
+
antiporter confers salt tolerance on a freshwater cyanobacterium, making it capable of growth in sea water
Authors: Waditee Rungaroon , Hibino Takashi , Nakamura Tatsunosuke , Incharoensakdi Aran , Takabe Teruhiro
Published: July 2002

The salt tolerance of a freshwater cyanobacterium,
Synechococcus
sp. PCC 7942, transformed with genes involved in the synthesis of a Na
+
/H
+
antiporter, betaine, catalase, and a chaperone was examined. Compared with the expression of betaine, catalase, and the chaperone, the expression of the Na
+
/H
+
antiporter gene from a halotolerant cyanobacterium (ApNhaP) drastically improved the salt tolerance of the freshwater cyanobacterium. The
Synechococcus
cells expressing ApNhaP could grow in BG11 medium containing 0.5 M NaCl as well as in sea water, whereas those expressing betaine, catalase, and the chaperone could not grow under those conditions. The coexpression of ApNhaP with catalase or ApNhaP with catalase and betaine did not further enhance the salt tolerance of
Synechococcus
cells expressing ApNhaP alone when grown in BG11 medium containing 0.5 M NaCl. Interestingly, the coexpression of ApNhaP with catalase resulted in enhanced salt tolerance of cells grown in sea water. These results demonstrate a key role of sodium ion exclusion by the Na
+
/H
+
antiporter for the salt tolerance of photosynhetic organisms.