Index of content:
Volume 3, Issue 12, December 1996

Tearing mode with strong flow shear in the viscosity‐dominated limit
View Description Hide DescriptionUsing a standard boundary layer approach, the tearing mode with shear flow comparable with shear magnetic field and fluidviscosity much larger than resistivity has been studied analytically. The results show that, the growth rate in this case scales as ν^{2/3} _{ν}, where ν_{ν} is the normalized viscosity, in agreement with the numerical results of Einaudi and Rubini [Phys. Fluids B 1, 2224 (1989)]. It is found analytically that large viscosity may have a destabilizing effect on the instability.

Solitary kinetic Alfvén waves on the ion‐acoustic velocity branch in a low‐β plasma
View Description Hide DescriptionStarting with an exact nonlinear equation governing solitary kinetic Alfvén waves (SKAWs), a new type of SKAW on the ion‐acoustic velocity branch is obtained in a low‐β plasma. It propagates in a direction oblique to the ambient magnetic field, has a dispersion originating from the finite perpendicular wavelength, and is accompanied by a density hump soliton similar to that of the SKAWs on the Alfvén velocity branch in the case of 1≫β≫m _{ e }/m _{ i }. It has a perturbed electric field with the same order as that of the latter SKAWs, but its perturbed magnetic field is weaker by a factor of (β/2)^{1/2}, and in the small amplitude limit, the analytical result also leads to the Korteweg–de Vries soliton.

Evidence of low‐frequency oscillations in heavy ion plasmas heated by electron cyclotron resonance
View Description Hide DescriptionTime‐resolved experiments (bremsstrahlung, diamagnetism, electron, and ion end‐loss currents) have been carried out at the Electron Cyclotron ResonanceIon Source Quadrumafios [A. Girard, et al., Rev. Sci. Instrum. 65, 1714 (1994)] continuously operated with heavy neutral gases. All the diagnostics reveal oscillations (on the time scale of about 1 s) which could be characterized by variations of the hot electron perpendicular temperature. The onset of this low‐frequency periodic regime with increasing heating powers and/or decreasing gas injection pressures is shown to limit the highly charged ion currents available. The addition of a lighter gas, e.g., He into Kr, delays the appearance of the periodic regime and enables one to extract higher currents.

Suppression of secondary electron emission from the material surfaces with grazing incident magnetic field in the plasma
View Description Hide DescriptionSuppression of secondary electron emission from the material surfaces with an obliquely incident magnetic field is demonstrated experimentally in a plasma containing hot electrons.

Shock waves in plasmas containing variable‐charge impurities
View Description Hide DescriptionShock structures in plasmas containing variable‐charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter.

Direct determination of ion wave fields in a hot magnetized and weakly collisional plasma
View Description Hide DescriptionElectrostatic ion cyclotron wave fields are determined in a magnetized and weakly collisional plasma. A phased‐locked Laser Induced Fluorescence(LIF)diagnostic is used to directly measure the wave perturbed ion velocity distribution. Comparing these local LIFmeasurements with a theoretical model uniquely determines the wave parameters, such as the wave potential, the three‐dimensional wave vector, and the effective wave damping. The self‐consistent wave–particle interaction is modeled by Boltzmann–Poisson equations in the limit of weak collisions. The wave parameters determined from local measurements agree with those determined from spatial scans.

Onsager symmetry for inhomogeneous magnetized plasmas
View Description Hide DescriptionThe symmetry properties of the linear dielectric function for an inhomogeneous magnetized plasma are studied based on the local mode approximation as well as on the modified local mode approximation. In the local mode approximation applied to the electrostatic or electromagnetic waves, the dielectrictensor lacks both the Hermitian symmetry and the symmetry of the Onsager relations. The lack of the symmetry properties arises because Fourier decomposition into plane waves is not a correct method for an inhomogeneous medium. The modified local mode approximation completely recovers the symmetry properties for the electromagnetic waves. However, it still lacks the symmetry properties for the electrostatic waves, due to the diamagnetic current. Some of the controversies previously given for an inhomogeneous plasma are also discussed.

Rayleigh–Taylor instability in a finely structured medium
View Description Hide DescriptionThe Rayleigh–Taylor instability in a finely layered material, with the layers oriented normally to the direction of the gravity force, is considered. It turns out that, in such a system, velocity perturbation in the most dangerous modes contains a significant and strongly nonuniform shear component that causes an increase of the viscous dissipation. Growth rates of the Rayleigh–Taylor instability for some specific examples of these fine structures are found. The conclusion is drawn that, although the viscous dissipation indeed increases, it remains insufficient to strongly reduce the growth rate of the large‐scale perturbations. Possible situations where this conclusion would become invalid (and where the fine structure would produce a stronger stabilization of the global mode) are discussed.

Fast surface waves in an ideal Hall‐magnetohydrodynamic plasma slab
View Description Hide DescriptionThe propagation of fast sausage and kink magnetohydrodynamic(MHD)surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm’s law. It is found that the Hall effect modifies the dispersion characteristics of MHD surface modes when the Hall term scaling length is not negligible (less than, but comparable to the slab thickness). The dispersion relations for both modes have been derived for parallel propagation (along the ambient equilibrium magnetic field lines).The Hall term imposes some limits on the possible wave number range. It turns out that the space distribution of almost all perturbed quantities in sausage and kink surface waves with Hall effect is rather complicated as compared to that of usual fast MHDsurface waves. The applicability to solar wind aspects of the results obtained, is briefly discussed.

Unstable particle drift across a magnetic field caused by oblique electric field gradients
View Description Hide DescriptionThe presence of a positive electric field gradient e dE/dx>0 oblique to a uniform magnetostatic field is shown to cause unstable particle motion across the magnetic lines. Both the drift velocity along ∇ E and the E×B drift velocity exponentiate in time with growth rate proportional to dE/dx^{1/2} sin θ, where π/2−θ is the angle between E and B. Thus the cross‐Btransport due to ∇ E is more severe than the ∇ B and ∂E/∂t effects, that cause constant drift velocity under uniform field variation. The result has implications to a variety of situations involving oblique electric gradients, such as magnetized plasmasheaths, large amplitude drift waves, and possibly tokamak edge transport.

Stability of vortex flows in magnetized plasmas. II. Boltzmann vortices
View Description Hide DescriptionThe stability of elliptical vortexflow to the excitation of low‐frequency electrostaticdrift waves is investigated. It is demonstrated that finite ellipticity of flow drives the secondary instability of drift/drift‐acoustic waves. When flow ellipticity is small, certain matching conditions between the rotation frequency and secondary wave frequency are needed for instability to occur. The case of large eccentricity is solved by asymptotic methods and gives a growth rate for the instability that depends logarithmically on eccentricity. Such secondary instability mechanisms can act as a sink of vortex energy (limiting the vortex condensation at the long‐wavelength end) and thereby help in our understanding of nonlinear saturation of low‐frequency instabilities.

Ion extraction from plasma by using a radio frequency resonant electric field
View Description Hide DescriptionIn order to raise an ion extraction efficiency from a plasma, a new method using an rf field has been proposed and demonstrated. The resonant frequencies of the rf field were theoretically evaluated to excite the eigenwave of the plasma. The lower frequency of the two plasma‐sheath resonances under the magnetic field was selected because it has hardly any dependence on the plasma density when the density is over a critical value. Verification of this method was carried out using Xe discharge plasma (electron density, 1×10^{16} m^{−3}; electron temperature, 8 eV) between the parallel plate electrodes (length, 0.5 m). The resonance was found at about 10 MHz, which agreed with the theoretical result. The ion current at the resonance was anisotropic and was twice as large as the ion saturated current, which is the limiting value of the conventional electrostatic method.

Eikonal differential scattering cross sections for elastic electron–ion collisions in strongly coupled plasmas
View Description Hide DescriptionThe eikonal approximation is applied to investigate the elastic electron–ion collision processes in strongly coupled plasmas. Plasma‐screening effects are investigated for the eikonal differential elastic scattering cross sections. The electron–ion interaction potential in strongly coupled plasmas has been approximated by the ion‐sphere model potential. The classical straight‐line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the eikonal differential elastic scattering cross section as a function of the impact parameter and ion‐sphere radius. A modified eikonal approximation called the Wallace correction is also considered. The eikonal differential elastic scattering cross sections substantially decrease with an increase of the energy of the projectile electron and increase as the plasma‐screening effect decreases through the ion‐sphere radius.

Dispersion properties of low‐frequency waves in magnetized dusty plasmas with dust size distribution
View Description Hide DescriptionDispersion properties of some low‐frequency modes in magnetized dusty plasmas have been investigated theoretically, taking into account the dust size distribution. In contrast to earlier work on dispersion properties of different modes in magnetized dusty plasmas with a single dust size, it is assumed that the dust grains are distributed in size. Assuming that the dust size is given by a power law distribution, with a nonzero minimum and finite maximum grain size, the effect of dust size distribution on the dispersion properties of some electrostatic and electromagnetic modes in magnetized dusty plasmas is investigated. It is seen that in the very low‐frequency (VLF) regime, taking into account the size distribution leads to a new kind of damping for an electrostatic dust‐cyclotron and right‐handed circularly polarized (RCP) electromagnetic Alfvén mode, whereas the left‐handed circularly polarized (LCP) Alfvén mode remains undamped. In the low‐frequency (LF) regime, size distribution results only in the modification of the dispersion relations for all the modes.

Extension of the ray equations of geometric optics to include diffraction effects
View Description Hide DescriptionThe paper discusses an extension of the ray equations of geometric optics to include diffraction effects for a wave beam propagating in a dispersive anisotropic medium. The diffraction effects are introduced through a complex eikonal function, where the complex part describes the electric field profile of the beam. The ray equations are derived using a formalism that allows for a complex wave vector but yields trajectories of wave propagation in real space. The wavelength, width of the beam, and length scale over which the plasma parameters change are ordered 1: δ ^{−1}:δ ^{−2}, with δ being a small parameter. A consistent treatment of this ordering yields additional terms in the ray equations when compared with expressions in the literature, that arise from corrections to the dispersion relation. It is discussed to what accuracy the rays represent the flux of wave energy. An approximated set of equations that describe the propagation of a Gaussian beam is derived.

An evaluation of different antenna designs for helicon wave excitation in a cylindrical plasma source
View Description Hide DescriptionA magnetohydrodynamic numerical model, based on the finite element method, is employed to analyze the antenna radiation resistance in a cylindrical helicon wave driven plasma source. The antenna radiation resistances of four commonly used antennas are compared. The effects on antenna radiation resistance of frequency, plasma density, density profile and the system dimensions are investigated. It is confirmed that m=+1 is the most strongly excited mode. It is shown that the plasma density gradient tends to suppress the excitation of negative m‐modes. Some wave field patterns are also presented which demonstrate commonly observed features in the experiments such as the beat patterns of copropagating radial modes. The findings highlight the importance of antenna radiation resistance modelling as a first step to a self consistent model of the discharge physics of cool dense helicon wave driven sources.

Cross field diffusion and transport in a relativistic and beam‐driven magnetoplasma: Collisional kinetics
View Description Hide DescriptionStreaming or beam‐driven kinetic theory is formulated for a relativistic and collisional plasma diffusing across magnetic field lines, including the small density and temperature gradient. Explicit formulas for modified transport coefficients are presented in various thermal regimes of interest. Their dependence on streaming velocity (V _{0}) is discussed qualitatively.

Relativistic theory of nonlinear oscillations in magnetoactive and transient plasmas or jets
View Description Hide DescriptionExplicit results pertaining to nonlinear oscillation and triggering of chaos in a relativistic magnetoactive beam–plasma system, which shows transient or streaming phenomena, are analytically presented. The phase‐space mappings and their striking features, both in strong streaming and nonstreaming situations, are shown. A new feature concerning streaming plasmaanalysis is revealed in the form of frequency modulation, especially at the onset of chaos, including varying k _{ z } pertaining to an ensemble of electrostatic waves. It is further shown that the modulation frequency is governed by marked cutoff values, corresponding to varying magnetic field lines, and that it approaches a steady value with the rise in the streaming parameter. Our findings may have important implications for astrophysical settings where there beam–plasma interactions exist.

Periodic nonlinear wave–wave interaction in a plasma discharge with no external oscillatory driving force
View Description Hide DescriptionExperimental measurements of time series and frequency spectra characterize a periodic nonlinear interaction between pairs of self‐excited, propagating, ionization waves simultaneously present in the positive column of a neon glow discharge. No periodically varying external driving force is applied. The interaction is the spatio‐temporal extension of the previously established temporal periodic pulling process, the incomplete entrainment of a driven nonlinear oscillator. The particular mode playing the role of the driving perturbation in the interaction can be selected and is identified by inspection of the asymmetric, multisideband spectrum of light fluctuations, which reflect the system’s dynamics. A comparison between the spatio‐temporal and temporal periodic pulling shows that the former is associated with a relatively strong driving force.

Parametric instability of a large‐amplitude nonmonochromatic Alfvén wave
View Description Hide DescriptionThe parametric instability of a finite‐amplitude Alfvén wave is studied in a one‐dimensional geometry. The pump wave is an exact solution of the nonlinear magnetohydrodynamic(MHD)equations, i.e., the magnetic fieldperturbation has a uniform intensity and rotates in the plane perpendicular to the propagation direction, but its Fourier spectrum contains several wavelengths. The weakly nonmonochromatic regime is first studied by an analytical approach. It is shown that the growth rate of the instability decreases quadratically with a parameter that measures the departure from the monochromatic case. The fully nonmonochromatic case is studied by numerically solving the instability equations, when the phase function of the pump wave has a power‐law spectrum. Though the growth rate is maximum in the monochromatic case, it remains of the same order of magnitude also for wide spectrum pump waves. For quasimonochromatic waves the correction to the growth rate depends only on the spectral index of the phase function.