bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of the plasma sheath in collisional to weakly collisional plasmas
Authors: Zawaideh Emad , Kim N. S. , Najmabadi Farrokh
Published: July 2002

Recently, a set of two fluid equations has been derived that describes the transport of plasmas with anisotropic pressure along the lines of force and is valid for collisional to weakly collisional regimes [Phys. Fluids 29, 463 (1986); 31, 3280 (1988)]. These transport equations coupled with Poisson’s equation are used to study the structure of the nonquasineutral transition region between the plasma and the material wall. This transition region is defined as the electric sheath region. The Bohm sheath criteria has been derived by various authors by examining behavior of the solution to the plasma-sheath equation. These investigations have shown that only for a supersonic flow would an increasing electric potential (in magnitude) exist in the sheath region to accelerate the ions further toward the wall. For subsonic flows, the behavior of the electric potential is purely oscillatory, which is not physical (electrons would trap in these potential wells and wipe them out). This work shows that when a more accurate set of equations is used, the solution is purely oscillatory when the flow speed is below the ion ‘‘thermal speed.’’ For flow velocities in the range between the thermal and sound speed, there exist solutions where the electric field is positive definite and oscillatory but the potential is monotonic. Therefore, solutions to the sheath problem exist for subsonic flow and Bohm’s sheath criteria can be violated.
Spectrum of fast Alfvén eigenmodes in a cylindrical current-carrying plasma
Authors: Ballico M. J. , Cross R. C.
Published: July 2002

In this paper, the spectrum of fast Alfvén eigenmodes is examined for a cylindrical, current-carrying, inhomogeneous plasma, at frequencies above the ion cyclotron frequency. Although the spectrum for m=0 and m=±1 modes in a current-free plasma has been studied extensively, the influence of a steady plasma current and the properties of high m modes have not received as much attention. It is shown that there exists a wide spectrum of positive m modes that propagate in the ion cyclotron range of frequencies, regardless of the plasma density or device size. These modes propagate, as surface waves, in the plasma edge region. Mode splitting, caused by the finite plasma current, is sufficiently large in tokamak devices that high m modes propagate in only one direction parallel to the equilibrium Bz field. High m modes are guided along the steady magnetic field lines in the plasma edge and are therefore potentially very dangerous from an edge heating point of view.
Unstable drift waves in a self-consistent stationary state with saturated magnetic shear
Authors: Katou K.
Published: July 2002

The stability of dissipative drift wave eigenmodes in a stationary state is investigated. Small magnetic shear, generated self-consistently by an electron plasma current through Ampère’s law, is included. This magnetic shear is found to be stabilizing (destabilizing) if the electron drift consistent with this is antiparallel (parallel) to the propagation direction of the drift waves. As a result, it is concluded that the introduction of magnetic shear does not effectively stabilize the drift wave instability.
A solvable self-similar model of the sausage instability in a resistive Z pinch
Authors: Lampe Martin
Published: July 2002

A solvable model is developed for the linearized sausage mode within the context of resistive magnetohydrodynamics. The model is based on the assumption that the fluid motion of the plasma is self-similar, as well as several assumptions pertinent to the limit of wavelength long compared to the pinch radius. The perturbations to the magnetic field are not assumed to be self-similar, but rather are calculated. Effects arising from time dependences of the z-independent perturbed state, e.g., current rising as tα, Ohmic heating, and time variation of the pinch radius, are included in the analysis. The formalism appears to provide a good representation of ‘‘global’’ modes that involve coherent sausage distortion of the entire cross section of the pinch, but excludes modes that are localized radially, and higher radial eigenmodes. For this and other reasons, it is expected that the model underestimates the maximum instability growth rates, but is reasonable for global sausage modes. The net effect of resistivity and time variation of the unperturbed state is to decrease the growth rate if α≲1, but never by more than a factor of about 2. The effect is to increase the growth rate if α≳1.
Breakdown-induced distortion of high-power microwave pulses in air
Authors: Löfgren M. , Anderson D. , Lisak M. , Lundgren L.
Published: July 2002

A simple analytical modeling is made of the main breakdown properties of high-power microwave pulses propagating in air. Particular emphasis is given to breakdown levels, breakdown times, pulse erosion, and optimal pulse energy transfer. Comparison with recent experimental results shows good agreement.
Electron inertia and lower-hybrid fluctuation-driven modified low-m tearing modes
Authors: Siva Rama Prasad P. V.
Published: July 2002

The evolution of the low-m tearing modes driven by the electron inertia and background lower-hybrid fluctuations is studied in a collisionless tokamak plasma. It is found that the low-m modes driven by the electron inertia are significantly modified by the presence of even a moderate level of fluctuations. The frequencies and growth rates of these modified modes are considerably larger than that of corresponding inertial-driven modes.
An exact nonlinear spherical surface wave solution
Authors: Chakraborty B. B.
Published: July 2002

An exact solution for a strongly nonlinear electrostatic surface wave on the spherical boundary of a cold plasma is obtained. As in the case of a cylindrical plasma boundary discussed earlier by Stenflo and Yu [Phys. Fluids B 1, 2543 (1989)], the nonlinear dielectric constant is found to depend upon the geometry as well as the electric field amplitude.
Observation of ion-cyclotron heating due to mode conversion of fast waves in a simple mirror
Authors: Yasaka Y. , Takeno H. , Fukuyama A. , Toyoda T. , Miyakita M. , Itatani R.
Published: July 2002

In this paper the observation of mode conversion of the m=−1 (right-rotating propagation) fast magnetosonic wave into the slow ion-cyclotron wave via the ion–ion hybrid resonance in a mirror magnetic field is presented. The left-circularly polarized component of the wave magnetic field strongly depends on the minority ion concentration in a helium–hydrogen plasma. The associated ion heating up to 250% of the initial value is in correlation with the enhancement of the left-circularly polarized component of the wave field, showing evidence of ion cyclotron heating due to the mode conversion. The two-dimensional calculation of the wave propagation and mode conversion is performed and compared with the experiment.
Radiation temperature of nonequilibrium plasmas
Authors: Arunasalam V.
Published: July 2002

In fusion devices measurements of the radiation temperature Tr(ω,k) near the electron cyclotron frequency ωc and the second harmonic 2ωc in directions nearly perpendicular to the confining magnetic field B (i.e., k≊k⊥ ) serve to map out the electron temperature profiles Te(r,t). For optically thick plasmas at thermodynamic equilibrium Tr=Te. However, there is increasing experimental evidence for the presence of nonequilibrium electron distributions (such as a drifting Maxwellian with appreciable values of the streaming parameter ξ=vd/vt, a bi-Maxwellian, an anisotropic Maxwellian with T⊥≠T∥, etc.) in tokamak plasmas, especially in the presence of radio-frequency heating. Examined here (both nonrelativistically and relativistically), is the dependence of Tr on ξ, T⊥/T∥, Th/Tb, nh/nb, etc., where nb, nh, Tb, and Th are the densities and temperatures, respectively, of the bulk and the hot components of the bi-Maxwellian plasma. The bi-Maxwellian results predict that the ratio Tr/Te is a very sensitive function of the ratios nh/nb and Th/Tb. Further, these relativistic and nonrelativistic results satisfy the well-known ‘‘limit c→∞ correspondence principle,’’ showing that the intensity of the emission and absorption line is independent of the line broadening mechanism. An alternative derivation of the ‘‘Trubnikov O-mode factor’’ is given in the Appendix. A number of side issues relevant to the existing disagreement between the Trubnikov results and those of some authors (in the published literature) for the O-mode processes are raised, and the plausible physical reasons for this disagreement are also pointed out in the Appendix and footnotes.
A fluid description for the discharge equilibrium of a divergent electron cyclotron resonance plasma source
Authors: Guan G. , Mauel M. E. , Holber W. M. , Caughman J. B. O.
Published: July 2002

A fluid description of the presheath of a magnetized plasma is used to model a divergent electron cyclotron resonance (ECR) plasma source. The fluid equations are moments of the time-independent Boltzman equation when cross-field particle motion occurs only through a static E×B drift. Closure is obtained by neglecting third-order moments. The electrons are assumed to have constant temperature along the magnetic field, to obey a Maxwell–Boltzmann potential-density relationship, and to be warmer than the ions. Interactions between plasma and neutral gas are included by specifying the profile of the gas density along the magnetic field and collision cross sections. A form of the equations is derived that can be used to study ions with anisotropic temperatures. The model is used to estimate the axial profiles of the density, ion flow, and ion temperature of an ECR plasma source. The calculated global relationships between (1) the electron temperature and the equilibrium neutral gas density, and (2) the absorbed microwave power and the equilibrium plasma density are comparable with experimental measurements. Furthermore, the calculated ion temperature is comparable to recently reported measurements [Appl. Phys. Lett. 57, 661 (1990) and Appl. Phys. Lett. 58, 458 (1991)].
Driven random-phase three-wave interactions: Cycles, bursts, and stochasticity
Authors: Robinson P. A.
Published: July 2002

Steadily driven, undriven, and stochastically driven three-wave decay processes involving groups of random-phase waves are investigated analytically and numerically. Steadily driven systems in which one product wave is suppressed exhibit neutrally stable Lotka–Volterra cycles, as for the true two-component case, whereas three-component systems are stable below a critical driver strength and unstable beyond that point. Initially unstable, but undriven, systems produce bursts of product waves, after which the parent waves fall to a final level that is an exponentially decreasing function of their initial level. Three-component systems where the product waves have near-equal dissipation rates are an exception to the latter behavior; in such systems the final parent-wave level is almost independent of the initial one. Stochastic driving gives rise to bursts of product waves in a cycle of fluctuating period, whereas a low-level noise source tends to stabilize the system.
Equilibration rate of spin temperature in a strongly magnetized pure electron plasma
Authors: Chen Shi-Jie , Dubin Daniel H. E.
Published: July 2002

The equilibration of spin temperature Tspin with kinetic temperature T is examined in a weakly correlated pure electron plasma in the strongly magnetized limit, where the distance of closest approach is large compared to the Larmor radius. In this limit, the spin precession frequency Ωp=gΩc/2 is large so the component of spin along the magnetic field is an adiabatic invariant that is broken only by resonant magnetic fluctuations of frequency Ωp. (Here Ωc is the electron cyclotron frequency and g≂2.002.) In this case, the most important spin flip mechanism stems from electron–electron collisions in a spatially inhomogeneous magnetic field. Such collisions cause an exchange of spin and cyclotron quanta, and consequently the conventional many-electron adiabatic invariant (i.e., the total number of cyclotron quanta) is broken and is replaced by a new adiabatic invariant, equal to the sum of the spin and cyclotron actions. A quantum Boltzmann equation is derived to describe the equilibration of Tspin toward T.
Collective behavior of vortices in a cylindrical magnetized plasma
Authors: Yabuki K. , Kono M.
Published: July 2002

The modulated vortex model introduced by Zabusky and McWilliams [Phys. Fluids 25, 2175 (1982)] is applied to a cylindrical plasma to study dynamical behavior of a many-vortex system. Anticyclones are shown to be long lived while cyclones are dissolved into the core and outer dresses which are left behind the core. This observation is in good agreement with the experimental results by Antipov et al. [Sov. J. Plasma Phys. 14, 648 (1988)].
Comments on ‘‘Relativistic effects on microwave reflectometry’’ [Phys. Fluids B 4, 3460 (1992)]
Authors: Bindslev H.
Published: July 2002

New simple approximate relativistic expressions for the Hermitian part of the dielectric tensor, the real part of the refractive index and the locations of cutoffs have recently been proposed [Phys. Fluids B 4, 3460 (1992)]. It was suggested that this approximation would be of use to reflectometry as well as to a number of other applications. There are, however, significant limitations in the accuracy of this approximation, which are pointed out here.
Reply to ‘‘Comments on ‘Relativistic effects on microwave reflectometry’ [Phys. Fluids B 4, 3460 (1992)]’’
Authors: Mazzucato E.
Published: July 2002

An approximation is discussed for interprelation of X-mode reflectometry in tokamak experiments. The nonapplicability of this approximation to wave propagation at large oblique angles to the magnetic field is also discussed. (AIP)
Trapped ion absorption of non-Maxwellian plasmas in ion cyclotron radio-frequency heating
Authors: Chen Yan Ping , Tsai Shih Tung
Published: July 2002

The nonlocal power absorption of trapped ions in ion cyclotron radio-frequency heating is calculated for several non-Maxwellian distribution functions by keeping the full particle orbit and wave coupling. For slowing down distribution the finite Larmor radius effect enhances the power absorption in higher harmonic heating with n≥2. In bi-Maxwellian plasma the absorption of trapped ions increases with the power of the ratio of the thermal perpendicular energy to the thermal parallel energy in the infinite aspect ratio limit and is slower in the small inverse aspect ratio case.
Bootstrapped tokamak with oscillating field current drive
Authors: Weening R. H.
Published: July 2002

A magnetic helicity conserving mean-field Ohm’s law is used to study bootstrapped tokamaks with oscillating field current drive. The Ohm’s law leads to the conclusion that the tokamak bootstrap effect can convert the largely alternating current of oscillating field current drive into a direct toroidal plasma current. This plasma current rectification is due to the intrinsically nonlinear nature of the tokamak bootstrap effect, and suggests that it may be possible to maintain the toroidal current of a tokamak reactor by supplementing the bootstrap current with oscillating field current drive. Steady-state tokamak fusion reactors operating with oscillating field current drive could provide an alternative to tokamak reactors operating with external current drive.
Neoclassical tearing dynamo and self-sustainment of a bootstrapped tokamak
Authors: Yuan Y. , Bhattacharjee A.
Published: July 2002

The dynamo effect due to neoclassical tearing modes is considered. It is found that the global constraints on energy and helicity balance require a redefinition of the dynamo field in neoclassical magnetohydrodynamics. The parallel dynamo field is calculated from the boundary-layer equations governing tearing modes. This dynamo effect can sustain a completely bootstrapped tokamak. A typical steady-state current profile has a core region of weak pressure gradients surrounded by a region of strong pressure gradients, where the current profile is entirely due to the bootstrap effect.
Formation of plasma configurations with negative confined magnetic field in different gases of a theta-pinch discharge
Authors: Campos D. O. , Kunze H.-J.
Published: July 2002

A 105 kJ theta-pinch device with a negative bias magnetic field is used to produce plasmas in hydrogen, deuterium, helium, and neon gases. The radial distribution of electron density and temperature is determined as a function of time in the midplane by Thomson scattering. An array of magnetic probes and one flux loop make it possible to measure the plasma diamagnetism and some other important parameters during the first phase of the discharge. The flux retention and other equilibrium properties are derived by fitting a rigid-rotor profile to the experimental density data. These results are compared with theoretical and semiempirical predictions.
The effect of alpha particles on the stability of a bumpy torus reactor
Authors: Wojtowicz D. , Kammash T.
Published: July 2002

The macroscopic stability of an ignited ELMO Bumpy Torus (EBT) reactor is investigated by studying the effects of the alpha particles generated by the deuterium–tritium (D–T) fusion reaction on the background interchange mode, the interacting interchange mode, and the high-frequency compressional Alfvén and coupled modes. A fluid description is used for the background plasma while a kinetic treatment is utilized for the hot-electron species and the alpha particles. It is shown that the alphas tend to mildly destabilize the interacting interchange while stabilizing the background interchange because of their sizable Larmor radii. The destabilization is most pronounced when the beta of the alpha particles is highest, i.e., at birth, and recovery of stabilization takes place as these particles slow down toward thermalization. It is also shown that the alphas completely stabilize the high-frequency modes, so that it can safely be concluded that fusion alphas present no detrimental effects on the stability of an EBT reactor that possesses an appropriate hot-electron ring for macroscopic stability.
Density fluctuations and current penetration on a magnetized plasma column
Authors: Décoste R. , Lachambre J.-L. , Shoucri Magdi
Published: July 2002

An experiment is described where an anomalously rapid current penetration is correlated with the appearance of strong fluctuations measured with a forward laser scattering technique. In this experiment, the current of a magnetized plasma column (5 eV, 1021 m−3) is reversed linearly on a time scale (μsec) much shorter than the classical time for current penetration toward the center of the column. Strong skin currents are observed to develop followed by a rapid current transport phase. The anomaly level of the current penetration is inferred from a comparison with numerical simulations. Simultaneously with the appearance of strong skin currents, we observe the onset of a large fluctuation level (ñe/ne∼1%) which persists for the duration of the anomalous current transport phase. The main contribution is from broadband fluctuations at low frequencies (a few MHz) and long wavelengths (a few mm). The nature of the fluctuations is not clear although a low level of ion-acoustic turbulence appears to be involved.
The interaction of plane and axisymmetric jets with a rarefied background gas
Authors: Bergers D.
Published: July 2002

The interaction of a supersonic plane or axially symmetric jet with a background gas of equal or different species is investigated in the transition regime between continuum and free-molecule flow by solving a set of Boltzmann equations with simplified collision terms. In the solution, the discrete ordinate method is applied and extended to axially symmetric flow by introducing an approximation to the curvature terms. The rate of convergence of the solution is increased by using the deviations of the distribution functions from the equilibrium solutions as dependent variables. The results show that three flow regions characterized by the ratio of cross to self-collisions can be identified in the jet structure. The penetration of the jet by background molecules is studied with regard to isotope separation. The influence of various assumptions for the boundary conditions on the solution is discussed. Density profiles computed for axially symmetric flow are compared with recent experimental data.
Wave propagation in a bubbly liquid with finite-amplitude asymmetric bubble oscillations
Authors: Miksis Michael J. , Ting Lu
Published: July 2002

Effective equations for nonlinear wave propagation in a bubbly liquid are derived by the method of homogenization for the case where the volume fraction of the gas is finite. The analysis is valid when the ratio of the mean interbubble distance to wavelength is small. The effective equations are coupled with a canonical (cell) problem on the microscopic scale. The dominant mode of oscillation of a bubble is volume preserving and asymmetric with finite amplitude. The cell problem and the averaging in the effective equations can be simplified by the method of matched asymptotics for the case of small volume fraction. The cell problem can be further simplified by using the small gas-to-liquid density ratio. Approximate solutions are then constructed to the cell problem. These are coupled to the effective equations on the macroscopic scale to form a closed system. The solutions of this system are analyzed in the linear and weakly nonlinear regime.
g modes in the presence of magnetic shear
Authors: Bhattacharyya B.
Published: July 2002

In a closed toroidal system, the occurrence of the g modes in an inhomogeneous plasma immersed in gravitational and sheared magnetic fields is studied. Both linear and nonlinear solutions for these modes have been evaluated. Also, the stable and unstable criteria of these modes have been set up analytically.
A kinetic equation for spin-polarized plasmas
Authors: Cowley S. C. , Kulsrud R. M. , Valeo E.
Published: July 2002

The usual kinetic description of a plasma is extended to include variables to describe the nuclear spin. The distribution function, over phase space and the new spin variables, provides a sufficient description of a spin-polarized plasma. The evolution for the distribution function is given. The equations derived are used to calculate depolarization caused by four processes: inhomogeneous fields, collisions, collisions in inhomogeneous fields, and waves. It is found that depolarization by field inhomogeneity on scales large compared with the gyroradius is totally negligible. The same is true for collisional depolarization. Collisions in inhomogeneous fields yield a depolarization rate of order 10−3 sec−1 for deuterons and a negligible rate for tritons in a typical fusion reactor design. This is still sufficiently small on reactor time scales. However, small-amplitude magnetic fluctuations (of order 1 G) resonant with the spin precession frequency can lead to significant depolarization (depolarizes triton in 10 sec and deuteron in 100 sec).
Thermocapillary convection in a rectangular cavity with a deformable interface
Authors: Sen Asok K.
Published: July 2002

Steady thermocapillary convection is examined in a differentially heated rectangular cavity of small aspect ratio A. It is shown that when the capillary number is C=O(A3), the interface undergoes an O(1) deformation from its flat position and the flow inside the cavity becomes nonparallel everywhere. The velocity and temperature profiles and the shape of the deformed interface are derived using the method of matched asymptotic expansions.
Magnetic curvature drift instability
Authors: Finn John M. , Drake J. F.
Published: July 2002

The semicollisional tearing mode is found to be strongly destabilized by electron temperature gradient in the presence of curvature or cross-field transport of particles or heat. The mode is unstable over a very wide range of parameters for reversed field pinches or spheromaks and appears to be more likely to explain anomalous transport than resistive interchanges. The mode is not so strongly unstable in tokamaks, because of their long shear length, but can be destabilized in a limited range by toroidal effects, principally Pfirsch–Schlüter transport.
Non-neutral whistler envelope solitons
Authors: Shukla P. K.
Published: July 2002

Whistler envelope solitons are reconsidered including the charge separation effects in the dynamics of quasistatic plasma slow response. The coupled nonlinear ordinary differential equations are solved with the help of a perturbation scheme in which the slow ambipolar potential is expanded in terms of the whistler wave intensity. The soliton profiles are obtained analytically.
The structure of the turbulent boundary layer on a cylinder in axial flow
Authors: Lueptow Richard M. , Haritonidis Joseph H.
Published: July 2002

The thick, turbulent boundary layer, which develops as a fluid flows parallel to a cylinder, has been experimentally characterized for the case where the boundary layer is thick compared to the radius of transverse curvature. Measurements of the turbulence intensity, velocity spectra, and intermittency are qualitatively similar to those for the planar boundary layer. Although measurements of wall shear stress using several different techniques have substantial scatter, the wall shear stress appears to be larger than that for the turbulent boundary layer on a flat plate at the same Reynolds number based on streamwise distance. The variable interval time averaging (VITA) and uv-quadrant techniques were used to detect the burst cycle near the wall. Conditionally averaged velocities were similar to those for a flat plate boundary layer indicating a similar burst cycle near the wall. However, the VITA frequency scaling indicates an interaction between the flow in the wall region and the outer flow. Flow visualization was used to observe the crossflow of structures in the boundary layer of a cylinder moving through a tank of quiescent water. Large-scale structures were observed moving from the outer region on one side of the cylinder to the outer region on the opposite side of the cylinder suggesting that the wall may be less important in controlling the size and motion of coherent structures in the cylindrical boundary layer than in the planar boundary layer.
Nonlinearly driven tearing instabilities by kinetic Alfvén waves
Authors: Kar Chitra , Sundaram A. K. , Sen A.
Published: July 2002

In this work, a nonlinear coupling mechanism, in which the mode-converted kinetic Alfvén waves beat with each other to resonantly modify the resistive tearing mode characteristics, has been investigated. Using the variational as well as the asymptotic methods, it is shown that new m=1 and m=2 tearing instabilities are excited with significantly enhanced growth rates varying as fractional powers of the pump amplitude. For rather moderate Alfvén wave intensity and a typical choice of tokamak parameters, the calculated growth rates turn out to be large and fall in the range 104–106 sec−1.
Development of pseudo-two-dimensional turbulent wakes
Authors: Sharma S. D.
Published: July 2002

Pseudo-two-dimensional wakes are generated by introducing spanwise cellular structures in the otherwise plane turbulent wake by means of the castellated blunt trailing edges of different configurations. The transverse growths of these coflowing cellular wakes are found to be independent of each other without any noticeable spanwise interaction. This wake growth is examined in the light of the plane equilibrium wake analysis. Though these wakes are not found to be exactly self-similar, their growth shows a nonmonotonous approach toward the asymptotic state appropriate to that of a plane wake. The dye emission in the wakes illustrated a coherent vortical structure in the transverse plane, similar to that of the usual two-dimensional wake, in spite of the initial spanwise irregularities.
Stimulated Raman scattering in cannonball targets
Authors: Sakawa Y. , Tanaka K. A. , Nishimura H. , Nakai M. , Yabe T. , Sakurai H. , Izawa Y. , Kato Y. , Mochizuki T. , Nakatsuka M. , Yamanaka C.
Published: July 2002

Stimulated Raman scattering (SRS) in cannonball targets was experimentally investigated. Above the observed threshold laser intensity, the energy conversion to superhot electrons (E=100–800 keV) shows an increase of five orders of magnitude without saturation. Major differences in observing SRS from cannonball targets and directly driven targets lie in the background electron temperature and the electron plasma density where SRS occurs. A simple model calculation is presented and is compared with the data to infer coronal temperatures. The electron density ranges from 5×1019 to 3×1020 cm−3 and the coronal temperature is 0.2 keV for cannonball targets including cavity targets, while the density ranges from 3×1020 to 6×1020 cm−3 and the temperature is 1 keV for the directly driven target. The generation efficiency of SRS light is strongly correlated with superhot electron generation.
Averaged cold plasma equations for ion cyclotron waves in stellarators
Authors: Riedel K. S.
Published: July 2002

When the toroidally varying component of a stellarator equilibrium is much smaller (larger) than the helically varying component, the cold plasma equations can be averaged over the helical (toroidal) angle to yield a two-dimensional description of ion cyclotron wave propagation. Higher order corrections may be computed iteratively; at each step only a two-dimensional operator need be inverted. Since the cold plasma equations have a singularity at the cyclotron resonance, a special coordinate system is introduced that exactly represents the resonance surface.
Droplet size control in liquid jet breakup
Authors: Hilbing James H. , Heister Stephen D.
Published: July 2002

A Boundary Element Method (BEM) has been developed to investigate the nonlinear evolution of the surface of liquid jets injected from circular orifices under unsteady inflow conditions. For fixed wavelength perturbations, the model predicts the formation of main and satellite droplets. The size of the droplets is affected by changes in the perturbation wavelength, perturbation magnitude and Weber number. Satellite droplet velocities are less than main droplet velocities due to the sequential shedding of droplets from the orifice. Using this information, one can determine the likelihood of droplet recombinations downstream of the initial pinching event.
Conjugate flows and flat solitary waves for a continuously stratified fluid
Authors: Lamb Kevin G. , Wan Bangjun
Published: July 2002

The concept of conjugate flows is used to determine the vertical structure of solitary internal waves which are horizontally uniform in their center. Continuously stratified fluids are considered and solutions obtained with and without the Boussinesq approximation are compared. Only mode-1 waves are considered. For stratifications with a single pycnocline, conjugate flow solutions are obtained provided the pycnocline is not too close to the upper or lower boundaries. The parallel shear flow in the center of a flat solitary wave is potentially unstable (minimum Richardson number less than 1/4) if the upstream pycnocline is sufficiently narrow. For stratifications with two pycnoclines, cases with three mode-1 conjugate flow solutions have been found. Some conjugate flow solutions for the two-pycnocline case do not seem to correspond to a flat solitary wave. Non-Boussinesq effects were found to be small if the surface to bottom density difference is about 4% for stratifications with one or two pycnoclines.
Cylindrical Couette flows of a rarefied gas with evaporation and condensation: Reversal and bifurcation of flows
Authors: Sone Yoshio , Sugimoto Hiroshi , Aoki Kazuo
Published: July 2002

A rarefied gas between two coaxial circular cylinders made of the condensed phase of the gas is considered, where each cylinder is kept at a uniform temperature and is rotating at a constant angular velocity around its axis (cylindrical Couette flows of a rarefied gas with evaporation or condensation on the cylinders). The steady behavior of the gas, with special interest in bifurcation of a flow, is studied on the basis of kinetic theory from the continuum to the Knudsen limit. The solution shows profound variety: reversal of direction of evaporation-condensation with variation of the speed of rotation of the cylinders; contrary to the conventional cylindrical Couette flow without evaporation and condensation, bifurcation of a flow in a simple case where the state of the gas is circumferentially and axially uniform.
Fluid dynamics of a double emulsion droplet in an electric field
Authors: Ha Jong-Wook , Yang Seung-Man
Published: July 2002

One of general free boundary problems concerning the electrohydrodynamic effects on a concentric double emulsion drop is studied theoretically for the three constituent phases of leaky dielectric fluids. In order to proceed the problem analytically, the domain perturbation procedure is utilized in the small deformation limit. The patterns of electric-field-driven flow are successfully characterized by examining the distribution of induced surface charges at the inner and outer drop interfaces. The second recirculating flow is generated in the annular phase when the inner and outer interfaces are charged with the same sense. The deformation type of inner and outer interfaces can be roughly interpreted by the flow patterns, although the exact description on the deformation requires consideration of the combined contributions from both electric and flow fields. In addition, the presence of double emulsion droplets alters the stress field of the continuous phase. The electric-field-induced “particle stress” not only changes the effective viscosity of dispersion of the double emulsion droplets but yields the normal stress difference, which is typical of a viscoelastic fluid. Finally, the heat transfer rate enhanced by the electric-field-driven flow is also considered.
Space–time characteristics of the wall shear-stress fluctuations in a low-Reynolds-number channel flow
Authors: Jeon Sejeong , Choi Haecheon , Yoo Jung Yul , Moin Parviz
Published: July 2002

A database obtained by direct numerical simulation of turbulent channel flow is used to compute the frequency/wave number spectra of the wall shear-stress fluctuations. Auto- and cross-correlations of the wall shear-stress and pressure fluctuations are obtained in space and time domains. The convection velocities as a function of frequency, wave number, and spatial and temporal separations are calculated. The overall convection velocities of wall shear-stress fluctuations for the most energetic structures are obtained and the validity of Taylor’s hypothesis is confirmed.
Symmetry of periodic structures in a 3D mixing cavity flow
Authors: Galaktionov O. S. , Anderson P. D. , Peters G. W. M.
Published: July 2002

Symmetry concepts are applied to analyze three-dimensional time-periodic mixing Stokes flows in a cubic domain. Usage of simple map algebra, developed by Franjione et al. [Phys. Fluids A 11, 1772–1783 (1989); Philos. Trans. R. Soc. London, Ser. A 338, 301–323 (1992)], allows us to reveal the symmetry in the arrangement of periodic points in such a flow. Symmetries reveal essential features of the underlying physics, and with manipulation of symmetries mixing protocols can be made efficient over the entire flow domain.
Kinetic investigation of the effect of lower hybrid current drive on tearing mode instability
Authors: Sheng Z. M. , Yu G. Y.
Published: July 2002

Effect of lower-hybrid current drive on tearing mode instability is investigated in a drift kinetic formalism. In collisional regime the co-driven current plays a stabilizing role to tearing mode but a destabilizing role to drift tearing mode. A competitive effect among the gradients of density, temperature and driven current at the rational surface may exist. In semicollisional regime, lower hybrid current drive increases the growth rate of tearing mode and reduces the growth rate of drift tearing mode.
Effect of a plasma sheath and ion injection on axial particle and energy confinement in a collisional mirror plasma
Authors: Ordonez C. A.
Published: July 2002

Continual scattering of particles into the loss cone of a simple magnetic mirror used to confine a collisional, fully Maxwellian plasma results in exceedingly large particle and energy loss rates. However, a considerable increase in confinement is found when a material wall is placed at the high field side of the mirror. A plasma sheath forms which reflects a large fraction of the electrons which pass through the mirror. Further, the sheath potential can be increased by axial ion injection. With optimized axial ion injection, the energy loss rate for a deuterium–tritium plasma decreases to 1.5% of its value without sheath reflection and near perfect electron confinement is obtained.
Response to ‘‘Comment on ‘Theory and three-dimensional simulation of light filamentation in laser-produced plasmas’ ’’ [Phys. Fluids B 5, 2243 (1993)]
Authors: Berger R. L. , Lasinski B. F. , Kaiser T. B. , Williams E. A. , Langdon A. B. , Cohen B. I.
Published: July 2002

In response to the comment on theory and simulation of light filamentation in laser-produced plasma the authors believe that the criticism overstates the outcome of choosing one form of thermal conduction over another. (AIP)
Dynamo power balance in a reversed-field pinch
Authors: Nordlund P. , Mazur S.
Published: July 2002

A study of the steady-state magnetohydrodynamic dynamo power balance in the Extrap T1 reversed-field pinch [Nucl. Fusion 34, 427 (1994)] is reported. The power transfer between the poloidal and toroidal magnetic fields is discussed in terms of a dynamo efficiency which is degraded with increasing pinch parameter Θ=Bθ(a)/〈Bφ〉 as a consequence of a larger fraction of the input power (up to 50% at high Θ) being dissipated by the fluctuations. It is shown that this anomalous (non-Spitzer) input power scales specifically with the power in nonlinearly driven dissipative m∈[0,2] modes. This is consistent with numerical results indicating the dynamo arises primarily from the m=1 modes.
Numerical simulation of Čerenkov whistler emission by a modulated beam
Authors: Krafft C. , Matthieussent G. , Lembège B.
Published: July 2002

Through Čerenkov emission, the interaction of an electron beam with a plasma leads to the excitation of whistler waves. Particle-in-cell simulations show that, when the beam is injected at an angle with respect to the magnetic field, the whistler wave emission of a modulated beam is increased compared to the same beam without modulation. This increased emission is related to the transfer of energy which occurs between the electrostatic mode directly excited by the density modulation and the electromagnetic whistler. This transfer of energy requires that the electrostatic field has a component perpendicular to the magnetic field.
Space-charge plasma-circuit instability in cw CO2 laser axial discharge plasmas
Authors: Tsui K. H. , Massone C. A.
Published: July 2002

The intense voltage spikes of about 10 μs width in high voltage direct current (dc) axial discharges, particularly, in continuous wave (cw) CO2 laser systems, are analyzed by a bound plasma model subject to external circuit constraint. The spikes, whose amplitude grows from cathode to anode, are believed to be a manifestation of the convective space-charge instability of the ion acoustic plasma-circuit mode.
Discrete spectra of compressional Alfvén eigenmodes in inhomogeneous magnetohydrodynamic plasma
Authors: Rutkevich I. M. , Mond M.
Published: July 2002

The spectrum of the compressional Alfvén eigenmodes in a magnetohydrodynamic (MHD) current-carrying inhomogeneous plasma column is obtained analytically. A MHD model that is adequate for calculating the compressional Alfvén spectra in large tokamaks like the Joint European Torus (JET) [Plasma Physics and Controlled Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 27] and Tokamak Fusion Test Reactor (TFTR) [Plasma Physics and Controlled Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] is employed. It is shown that the radial nonuniformities of the plasma density give rise to various types of localization of the eigenmodes. Thus, the global, near-axis, near-wall, and annulus spectra are identified and studied. The eigenfrequencies and domains of localization of the various eigenmodes, as well as their range of existence in parameter and wavelength space, are determined and investigated. The effects of finite ion gyrofrequency on the MHD eigenmodes are examined, and a perturbation scheme for calculating the modified eigenfrequencies and localization domains is presented. Numerical calculations of a set of near-axis eigenmodes for the JET parameters is given as an example for the applicability of the MHD model. Finally, the relevancy of the eigenmodes to plasma heating in large tokamaks is discussed.
Forming and maintaining a potential well in a quasispherical magnetic trap
Authors: Krall Nicholas A. , Coleman M. , Maffei K. , Lovberg J. , Jacobsen R. , Bussard R. W.
Published: July 2002

The formation and maintenance of an electrostatic potential well by injecting electrons into a quasispherical cusped magnetic confinement geometry is studied experimentally, as a function of plasma fill density and of the energy and current of the injected electrons. A model is developed to analyze the experiment. It is found that the potential is comparable to the energy of the injected electrons at low density, and decreases as an increasing density of cold plasma fills the device because of ionization or wall bombardment. Implications for fusion based on electrostatic/magnetic confinement are discussed.
Particle-in-cell simulation of spherical plasma focus diode
Authors: Jiang W. , Sakagami T. , Masugata K. , Yatsui K.
Published: July 2002

A self-magnetically insulated, three-dimensionally focused ion-beam diode, spherical plasma focus diode (SPFD), is studied by numerical simulation using a two-dimensional, electromagnetic, relativistic particle-in-cell computer code. The calculated results of the diode impedance, the ion-current efficiency, and the focusing characteristics of the ion beam are presented. These results, except the data of the ion-beam current, are in good agreement with the experimental results. Since the major ion deflection direction observed experimentally is also obtained in the simulation, it is attempted to optimize the SPFD by correcting the ion deflection with the shape of the diode gap by means of simulations.
A study for the interaction between solar wind irregularities and the magnetosphere through three-dimensional simulations
Authors: Dai Wenlong , Woodward Paul R.
Published: July 2002

In order to understand the interaction between solar wind irregularities and the closed magnetosphere, three-dimensional (3-D) computer simulations for ideal magnetohydrodynamical (MHD) equations are performed. This paper considers only the situation in which the initial interstellar magnetic field is either parallel or antiparallel to the magnetospheric magnetic field. The irregularity is modeled as a ball, which initially moves toward the magnetosphere. The penetration of the irregularity in the 3-D model is found to be less efficient than in a two-dimensional (2-D) model. Through a comparison of the present results from 3-D simulations, not much difference has been found in the efficiency for the irregularity to penetrate into the magnetosphere between two situations in which the interstellar field is parallel or antiparallel to the magnetospheric field.
Interaction between two electromagnetic solitary waves in a plasma
Authors: Kuehl H. H. , Zhang C. Y.
Published: July 2002

From numerical solutions of two colliding electromagnetic solitary waves, it is shown that their shapes are modified, and a trailing wake of plasma oscillations is produced by the collision. They therefore cannot be classified as solitons. For sufficiently large relative velocities, however, it is shown that the modifications to the vector and scalar potentials, as well as to the wake due to the collision, are small.
Surface-wave–particle interactions in a cylindrical plasma submitted to a static magnetic field
Authors: Dengra A.
Published: July 2002

A new theoretical model for the study of the surface-wave–particle interactions in a plasma column in the presence of a constant external magnetic field has been developed. The model is based on the linear resolution of the Vlasov equation by the method of characteristics, with the specular reflection hypothesis at the wall. The expression obtained for the rate of increase of kinetic energy per electron permits the analysis of the influence of the critical parameters in this transference process.
Modified magnetohydrodynamic waves in a current sheet in space
Authors: Yamauchi M. , Lui A. T. Y.
Published: July 2002

Large-scale coherent motions of a current sheet such as flapping or tearing of the entire current sheet are studied. The basic magnetohydrodynamic (MHD) equations are integrated over the thickness of the current sheet, and linear analysis is applied to obtain the modified dispersion relations for the MHD fast, Alfvén, and the slow waves under non-zero background cross-sheet current. The dispersion relation for the fast and slow modes contains an imaginary part, because energy is exchanged between the wave and the background sheet current. A short-wavelength MHD slow wave propagating against/along the magnetic tension force is unstable/stable, whereas the situation is reversed for the MHD fast waves. For a thin current sheet (long-wavelength limit), the MHD slow wave becomes stagnant and very unstable, whereas the MHD fast wave propagates slowly and its stability depends on the strength of the background current.
Radiation transport coefficients for degenerate plasma
Authors: Khalfaoui A. H. , Bennaceur D.
Published: July 2002

A comprehensive analysis of radiation heat conduction is presented for dense plasma at very high temperature. The main objective is to correctly evaluate radiation transport coefficients, and thereby to determine the relative importance of conductive opacity and radiative opacity. Here, the latter is related to the radiative mean free path and therefore to the Rosseland mean opacity. The electron conduction opacity, however, is calculated through a quantum collective approach, taking into account several plasma nonideality effects such as electron–electron (e-e) collisions, dynamic shielding, electron degeneracy, and ion coupling. Opacity ratios are determined as functions of the degree of the plasma degeneracy and coupling strength, hence identifying plasma conditions for which radiation heat conduction should be evaluated with care. A comparison with existing models is also presented. It is shown that, over a wide range of density and temperature, the conductive opacity can be very important and cannot be neglected in radiation physics.
Geometry of singular structures in magnetohydrodynamic flows
Authors: Grauer Rainer , Marliani Christiane
Published: July 2002

The flattening of current sheets is investigated by means of numerical simulations of the ideal incompressible magnetohydrodynamic equations in two dimensions. The use of adaptive mesh refinement techniques allows one to resolve the more and more singular structures and to follow the exponential growth of current density. The numerical results are in good agreement with a scaling ansatz proposed by Sulem et al. [J. Plasma Phys. 33, 191 (1985)]. The geometry of the current sheets is characterized by the alignment properties of the deformation matrices.