bluuebunny/crossref_metadata_2025 huggingface dataset:
Velocity-shear instability of relativistic electron beams: a systematic exploration of the entire(ω, k) space
Authors: Heristchi D. , Cuperman S.
Published: March 2009

Using the exact solution methods developed and illustrated previously, we systematically explore (ω, k) space in order to determine all the unstable modes of velocity-shear-produced electromagnetic waves in magnetized, relativistic electron beams (here ω and k are the complex frequency and real wavenumber). This is done for various physical parameters of the problem, namely shear factor and ratio of pipe to beam radii. The solution of the dispersion relation is supplemented by a marginal stability analysis based on the behaviour of the turning points of the purely real-frequency solution ωr, (k): these are bifurcation points where complex solutions originate or terminate.
Parametric instabilities of finite-amplitude, circularly polarized Alfvén waves in an anisotropic plasma
Authors: Hamabata Hiromitsu
Published: March 2009

A class of parametric instabilities of finite-amplitude, circularly polarized Alfvén waves in a plasma with pressure anisotropy is studied by application of the CGL equations. A linear perturbation analysis is used to find the dispersion relation governing the instabilities, which is a fifth-order polynomial and is solved numerically. A large-amplitude, circularly polarized wave is unstable with respect to decay into three waves: one sound-like wave and two side-band Alfvén-like waves. It is found that, in addition to the decay instability, two new instabilities that are absent in the framework of the MHD equations can occur, depending on the plasma parameters.
Generation of quasi-stationary magnetic fields by external electromagnetic radiation in a cold magneto-active plasma
Authors: Aliev Yu. M. , Jovanović B. M. , Frolov A. A.
Published: March 2009

Starting from the hydrodynamic equations for a cold electron fluid, we develop a theory of the ponderomotive effect of the interaction of high-frequency electromagnetic radiation with a magneto-active plasma. Using the method of averaging on high-frequency oscillations, we obtain expressions for the low- frequency nonlinear currents. We also obtain an equation describing the space–time evolution of a quasi-stationary magnetic field in the plasma.
On radio-frequency wave-induced radial transport and wave helicity
Authors: Petržílka V. A.
Published: March 2009

Expressions for wave-induced radial transport are derived, allowing simple estimates to be obtained. The transport is enhanced owing to the presence of a poloidal magnetostatic field and in the vicinity of ion-cyclotron resonance. The direction of the wave-induced transport also depends on the wave polarization.
Spontaneous free-electron two-quantum Stark emission in an arbitrary direction from a zero-temperature electron beam
Authors: Kim S. H.
Published: March 2009

Emissions from relativistic electrons travelling in periodic electrostatic fields were observed by Smith and Purcell (1953) and Doucas et at. (1992) as extraordinarily (e.g. 1018 times) stronger than any emission that can be conceived with classical electrodynamics under any equivalent condition. The mechanism is identified as the free-electron two-quantum Stark (FETQS) emission generated by the axial uniform motion, which cannot be radiated in the axial direction. From the excellent agreement between the theoretical result for FETQS emission driven by the axial uniform motion, and the experimental observations and many emission phenomena in plasmas, it is concluded that a high-energy electron follows a quantum-mechanically derived formula (without taking the classical limit ←0), although this diverges in the classical limit. From the extraordinarily large FETQS emission due to macroscopic motion, we speculate that even the FETQS emission generated by electron spin can be macroscopically observable. The spin-generated FETQS emission in an arbitrary direction is calculated using relativistic quantum mechanics. It is found that the total power of this emission scales as γ2 times the emission power in the equivalent magnetic wiggler, where γ is the Lorentz factor of the electron, and the emission is practically confined in a cone of angle 1/γ about the axial direction.
Theory of modulational interaction of two coupled waves. Part 2. Instability of pump Langmuir waves
Authors: Vladimirov Sergey V. , Tsytovich Vadim N.
Published: March 2009

The modulational instability of two pump Langmuir waves is investigated for the case of comparatively small instability rates, when ‘renormalized’ expressions for linear and nonlinear plasma polarization responses should be used. The rates found are compared with the modulational instability rates for a single monochromatic Langinuir mode.
Rayleigh surface waves along the boundary between a plasma and a metallic screen
Authors: Ivanov S. T. , Ivanova K. M. , Alexov E. G.
Published: March 2009

Electromagnetic wave propagation along the interface between a magnetoactive plasma and a metallic screen is investigated analytically and numerically. It is shown that the waves have a Rayleigh character: they are superpositions of two partial waves. It is concluded that electromagnetic waves propagate only at frequencies lower than min (ωp, ωc), where ωpis the plasma frequency and ωcis the cyclotron frequency. The field topology is found, and the physical character of the waves is discussed.
On the existence of weak stationary electron-acoustic double layers
Authors: Mace R. L. , Hellberg M. A.
Published: March 2009

The recent interest in the electron-acoustic wave as a source of broad-band electrostatic noise in the terrestrial magnetosphere makes it interesting to ask whether it can support stationary electrostatic double layers. We investigate this problem in a fluid plasma composed of cool ions, cool electrons and a hot Boltzmann electron component – which is known to support electron-acoustic waves. Although a formal application of the reductive perturbation technique to our dynamical equations leads to an mKdV equation for electron-acoustic waves, it is found that within the present physical model the consistency conditions and required ordering of the coefficients cannot be satisfied simultaneously for reasonable parameter values. As a consequence, it is shown that the neglect of the φ(2)term in deriving the mKdV equation is unjustified under general circumstances, and furthermore that the cubic nonlinearity introduced by the mKdV equation is negligible when compared with this term. Finally, we are led to conclude that stationary, weak electron-acoustic double layers cannot exist in such a plasma.
Ion modes in strongly coupled two-component plasmas
Authors: Berkovsky M. A.
Published: March 2009

A generalized hydrodynamic model is used to evaluate the frequency spectra of longitudinal and transverse modes in dense strongly coupled two-component plasmas. The results are compared with available computer simulation data.
Wave properties of a cylindrical antenna immersed in a magneto-active plasma
Authors: Azarenkov N. A. , Denisenko I. B. , Ostrikov K. N.
Published: March 2009

The dispersion properties and electromagnetic field topography of surface waves propagating along and in the azimuthal direction with respect to a cylindrical metal antenna immersed in a magneto-active plasma are investigated. The external magnetic field is directed along the antenna axis. The presence of a vacuum-gap sheath region separating the antenna from the plasma is assumed. The sheathless case is also considered. The dependence of the surface-wave dispersion properties on magnetic field intensity, plasma density, antenna radius and sheath thickness is presented for structure parameters close to experimental data. The results show qualitative agreement between our theory and experiment. The antenna surface impedance is calculated as well.
Relaxed state of a toroidal fusion plasma with stationary flows
Authors: Żelazny R. , Gałkowski A.
Published: March 2009

Relaxation of a plasma in the presence of zeroth-order flows is studied in a realistic toroidal geometry. The field-aligned flow allows equilibria with finite pressure gradient but homogeneous temperature distribution. Numerical calculations have been performed for the first and second ellipticity regimes of the extended Grad–Shafranov–Schlüter equation. Two distinct classes of solutions exist, depending on the value of the safety factor: one with horizontally stratified density and pressure and one with concentric surfaces of constant density. A comparison of these results is made with those of Finn and Antonsen.
Nonlinear hydrodynamic Rayleigh—Taylor instability of viscous magnetic fluids: effect of a tangential magnetic field
Authors: El-Dib Yusry O.
Published: March 2009

The nonlinear Rayleigh—Taylor instability of viscous magnetic fluids is considered under the influence of gravity and surface tension in the presence of a constant tangential magnetic field. The method of multiple-scales expansion is employed. A nonlinear Schrödinger equation with complex coefficients is imposed from the solvability conditions and used to analyse the stability of the system. A quadratic dispersion relation with complex coefficients is obtained. The Hurwitz criterion for a quadratic polynomial with complex coefficients is used to control the stability of the system. It is found that an increase in the viscosity increases the extent of the stable region in the presence of a magnetic field. Finally it is shown that the magnetic permeability of the fluid affects the stability conditions.
Induced photon decay and photon-beam-induced Langmuir turbulence
Authors: Melrose D. B.
Published: March 2009

A pair of quasi-linear-like equations is derived to describe the effect of three-wave interactions between high-frequency photons and Langmuir waves in a differential approximation. Induced photon decay leads to terms analogous to those for spontaneous emission, but involving the square of the photon occupation number. The effect on the Langmuir waves is evaluated for axisymmetric photons. The effect on the photons is shown to be similar to but weaker than induced Compton scattering by thermal electrons. The absorption coefficient for the Langmuir waves is evaluated for an axisymmetric distribution of photons, and used to discuss a photon-beam-induced instability. Possible astrophysical applications to solar ‘spike’ bursts, the eclipse of a radio pulsar and the variable low-frequency emission from some active galactic nuclei are discussed briefly, and it is concluded that the process can account for the observed properties of the eclipse of PSR 1957 + 20.
The Kelvin–Helmholtz instability in a compressible plasma: the role of the orientation of the magnetic field with respect to the flow
Authors: Gozález A. G. , Gratton J.
Published: March 2009

The hydromagnetic Kelvin–Helmholtz instability is relevant in many complex situations in astrophysical and laboratory plasmas. Many cases of interest are very complicated, since they involve the combined roles of velocity shear, density and magnetic field stratification, and various geometries in compressible plasmas. The present work is part of a systematic investigation of the influence of the various physical and geometrical parameters characterizing the plasmas on the Kelvin–Helmholtz modes. The general dispersion relation for ideal compressible MHD modes localized near a velocity discontinuity between two uniform plasmas is derived. The existence and characteristics of the modes and their stability are studied analytically for any relative orientation of B, u and k, for continuous B and ρ. It is shown that the stability of a given configuration cannot be determined by considering only special orientations of k (say flute or parallel modes). The results obtained here may serve as a guide to interpret results in more complicated and realistic situations, such as those occurring in experiments and natural plasmas.
Electron-Cyclotron maser emission from streaming distributions
Authors: Willes A. J. , Robinson P. A.
Published: March 2009

Motivated by the need to explain observed elliptically polarized emission from Jupiter, the mechanism of electron-cyclotron maser emission is considered for drifting electron distributions, where the electrons stream with a non-zero mean velocity parallel to the magnetic field lines. An analytical expression for the semirelativistic growth rate is derived and its properties analysed in detail for waves generated in the magneto-ionic modes. The main features of the growth rate are discussed, on the basis of a geometric analysis using resonant ellipses.
Surface currents on models of force-free solar magnetic flux tubes
Authors: Melrose D. B. , Nicholls Jennifer , Broderick N. G.
Published: March 2009

A model of a cylindrically symmetric, force-free magnetic field consisting of a sequence of concentric layers with piecewise-constant α is used to construct models of the surface currents on isolated, force-free magnetic flux tubes. Two boundary conditions are considered: a current-neutralized flux tube (Bφ = 0, Bz φ 0, Bz ≠ O at r > r0), and an isolated current-carrying flux tube (Bφ ≠ 0, Bz = 0 at r > r0). A single-a model that is current-neutralized is a reverse-field pinch, and is unacceptable as a model for a solar flux tube. Examples of two-α models for a current-neutralized flux tube are presented. The models of the surface currents satisfying either boundary condition are shown to simplify considerably when the surface layer is thin. A model consisting of several layers, with piecewise-constant α, may be used to find an approximate solution for a force-free flux tube with an arbitrarily specified current profile.
Strongly nonlinear stationary Langmuir waves (T ≪ mc2)
Authors: Aleshin I. M. , Drofa M. A. , Kuzmenkov L. S.
Published: March 2009

Extremely large-amplitude Langmuir waves are investigated on the basis of relativistic kinetic theory. Analytical formulae for the electron density and the electric field of the wave differ essentially from earlier results obtained using the cold-hydrodynamical equations.
Sensitivity of global energy confinement to the boundary condition due to coupling of MHD and transport processes
Authors: Deng X. H. , Zhang C. , Huo Y. P. , Wang J. F. , Wang Shui
Published: March 2009

MHD instabilities are governed by the transport-determined plasma profiles, and the transport process is affected in turn by the instabilities. Using a one-dimensional code, we have investigated the inter-relationship between instabilities, transport and plasma profile in a tokamak discharge. The results show that the global energy confinement becomes strongly dependent on the boundary transport condition owing to strong coupling between them, and a higher edge temperature would ensure a higher core temperature and hence greater global energy confinement.
Specular reflection as a probe for diagnostics of laser-produced plasmas
Authors: Desai T. , Pant H. C. , Khan M. , Sarkar S. , Chakraborty B.
Published: March 2009

The characteristics of the specularly reflected laser light (1.06 μm) from a copper plasma have been studied experimentally. The diffusivity and depolarization of specularly reflected radiation can be explained by the density contours and self-generated magnetic field at the radiation turning point in the plasma layer.
Ion-cyclotron modes in weakly relativistic plasmas
Authors: Venugopal Chandu , Kurian P. J. , Renuka G.
Published: March 2009

We derive a dispersion relation for the perpendicular propagation of ioncyclotron waves around the ion gyrofrequency ω+ in a weaklu relaticistic anisotropic Maxwellian plasma. These waves, with wavelength greater than the ion Larmor radius rL+ (k⊥ rL+ < 1), propagate in a plasma characterized by large ion plasma frequencies (). Using an ordering parameter ε, we separated out two dispersion relations, one of which is independent of the relativistic terms, while the other depends sensitively on them. The solutions of the former dispersion relation yield two modes: a low-frequency (LF) mode with a frequency ω < ω+ and a high-frequency (HF) mode with ω > ω+. The plasma is stable to the propagation of these modes. The latter dispersion relation yields a new LF mode in addition to the modes supported by the non-relativistic dispersion relation. The two LF modes can coalesce to make the plasma unstable. These results are also verified numerically using a standard root solver.
Effect of high-frequency magnetic fluctuations on MHD waves
Authors: Nakamura Tadas K. , Kennel C. F.
Published: March 2009

We have shown earlier that compressional magnetic fluctuations modify the properties of MHD waves. The modifications can be large even when the fluctuation level is much smaller than the ambient magnetic field. There are two kinds of effects: resonant and non-resonant. The former is a result of the so- called parametric cyclotron resonance, and enables energy transfer from the wave to particle Larmor motion. This transfer causes the damping of waves even when their frequency is so small that ordinary cyclotron damping is negligible. The non-resonant effect does not cause energy exchange, but it does modify wave dispersion and causes mode coupling between the shear Alfvén mode and magnetosonic modes.
Nonlinear Landau damping in a relativistic plasma
Authors: Mukherjee Joydeep , Chowdhury A. Roy
Published: March 2009

A generalized non-local nonlinear Schrödinger equation describing the phenomenon of nonlinear Landau damping in a relativistic two-component plasma is deduced using the kinetic-theory approach of Vlasov. Parameters appearing in the equation are evaluated explicitly for the case of a Maxwellian distribution function.
Nonlinear evolution of Langmuir and electromagnetic pulses in a warm, unmagnetized plasma: modulational instability, integrability and self-focusing in 2 + 1 dimensions
Authors: Kates Ronald E. , Kaup D. J.
Published: March 2009

The nonlinear dynamics of wave envelopes modulated in 2 + 1 dimensions is considered for two systems in plasma physics: (i) Langmuir pulses and (ii) intense (but weakly relativistic) electromagnetic (EM) pulses. Using singular perturbation techniques applied to an envelope approximation, both problems are reduced to the two-dimensional nonlinear Schrödinger (2DNLS) system, which describes the dynamics of two coupled slowly varying potentials. The general 2DNLS system exhibits a rich variety of phenomena, including enhanced (compared with ‘longitudinal’ propagation) modulational stability and (1D) soliton formation; decay of 1D solitons over long time scales; self- focusing regimes (determined by a virial-type condition); as well as integrability and 2D solitons. Applying our recent results on the 2DNLS system, we determine which of these phenomena can actually occur here and compute the parameter regimes. (i) The 2DNLS system for the Zakharov equations is modulationally unstable for all parameter values. It also has an integrable sector and a self-focusing regime. (ii) The 2DNLS system describes coupled ‘longitudinal’ and ‘transverse’ modulations of linearly or circularly polarized EM pulses propagating through a warm unmagnetized two-component neutral plasma with arbitrary masses (i.e. electron—positron or electron—ion). The pulse can accelerate particles to weakly (but not fully) relativistic velocities; relativistic, ponderomotive and harmonic effects all contribute to the nonlinear terms. The resulting 2DNLS system does not admit a self-focusing regime. Parameter values leading to an integrable case (the so-called ‘Davey—Stewartson I’ equations, which admit 2D soliton solutions) are computed; however, the required values would not be attainable in a laboratory or astrophysical setting. None the less, the existence of new nonlinear modulational instabilities associated with the second spatial degree of freedom already represents an important potential limitation on any (1 + 1)-dimensional approach to nonlinear evolution and modulational instability of plasma EM waves.
Rotating magnetohydrodynamics
Authors: Shan X. , Montgomery D.
Published: March 2009

It is shown that rotation of a periodic column of magnetohydrodynamic fluid can be stabilized against current-driven instabilities by rotation. The rotation is considered as resulting from a slight departure from overall charge neutrality, and is braked by wall friction (loss of angular momentum to the cylinder wall). Both effects are modelled phenomenologically in the equation of motion, and the emphasis is on their bulk dynamic effects rather their microscopic kinetic theory origins. Much as rotation is known to stabilize a Navier—Stokes fluid against transverse displacements, it is shown that sufficient rotation will suppress the helical vortices and helical deformations of the current channel that are known to result when the axial current in the column exceeds its stability threshold.
Anisotropic self-defocusing of a Trivelpiece-Gould mode in a plasma
Authors: Sharma A. K. , Konar S.
Published: March 2009

The paper presents a paraxial-ray theory of self-defocusing of obliquely propagating Trivelpiece-Gould (TG) modes in a homogeneous plasma. The TG mode with Gaussian distribution of intensity, in a direction perpendicular to its phase velocity, creates a ponderomotive force on electrons. The electrons are redistributed through ambipolar diffusion. The redistributed electrons are responsible for defocusing of the TG mode. The importance of our study is pointed out.
Electron-cyclotron absorption by inhomogeneous current-carrying plasmas
Authors: Cavalcanti C. J. H. , Schneider R. S. , Ziebell L. F.
Published: March 2009

We consider effects of inhomogeneity on the absorption of high-frequency electromagnetic waves, propagating at arbitrary angles relative to the magnetic field, by current-carrying plasmas. An inhomogeneous current is assumed to be immersed in an otherwise homogeneous background, and the absorption of fundamental electron-cyclotron waves is discussed, with emphasis on the dependence of the inhomogeneity effect on wave frequency and angle of propagation.
Resonant absorption of Alfvén waves near a neutral point
Authors: Uberoi C.
Published: March 2009

It is shown that, although the mathematical analysis of the Alfvén-wave equation does not show any variation at non-zero or zero singular points, the role of surface waves in the physical mechanism of resonant absorption of Alfvén waves is very different at these points. This difference becomes even greater when resistivity is taken into account. At the neutral point the zero-frequency surface waves that are symmetric surface modes of the structured neutral layer couple to the tearing mode instability of the layer. The importance of this study for the energy balance in tearing modes and the association of surface waves with driven magnetic reconnection is also pointed out.
The role of a density jump in the Kelvin—Helmholtz instability of compressible plasma
Authors: González A. G. , Gratton J.
Published: March 2009

The hydromagnetic Kelvin–Helmholtz instability is relevant in many complex situations in astrophysical and laboratory plasmas. Many cases of interest are very complicated, since they involve the combined role of velocity shear, of density and magnetic field stratification, and of various geometries in compressible plasmas. In the present work we continue investigating the influence of various physical and geometrical parameters of the plasma on the Kelvin–Helmholtz modes. We use the general dispersion relation for the ideal compressible MHD modes localized near a velocity discontinuity between two uniform plasmas. We study analytically the existence and properties of the modes and their stability, for a velocity jump combined with a density jump, and for any relative orientation of B, u and k (B is continuous). Stability is analysed by means of a general procedure that allows discussion of any configuration and all kinds of perturbations. The boundaries between modes of different kinds are discussed. In contrast to the case of uniform density, for a density jump there are no monotonically unstable modes, only overstabilities. The unstable modes belong to two types. Those with the largest growth rates tend to monotonically unstable modes in the limit of uniform density, and are related to the torsional Alfvén mode. The other overstable modes have no analogue among the purely incompressible modes, and occur in a range of U that is stable in the incompressible limit. We derive bounds for the growth rate of the instability. The present results may serve as a guide to interpret results in more complicated and realistic situations as those occurring in laboratory and natural plasmas.
A Fourier-space description of oscillations in an inhomogeneous plasma. Part 1. Continuous Fourier transformation
Authors: Sedláček Z. , Cally P. S.
Published: March 2009

Oscillations in inhomogeneous cold plasmas or inhomogeneous magnetofluids are interpreted in terms of the dynamics of their spectra in wavenumber space. By Fourier transforming the basic integro-differential equation of the problem, a generalized wave equation in wavenumber space is derived, thus converting the oscillation and phase-mixing processes in the original χ space into processes of dispersive propagation and scattering of the spectrum in wavenumber space. The Barston singular continuum eigenmodes correspond to stationary scattering states of a monochromatic wave in wavenumber space, whereas the damping phenomena in χ space correspond to transient ‘leaking’ phenomena accompanying scattering and dispersive propagation of a wave packet in wavenumber space.
Characteristics and Riemann invariants for multi-ion plasmas in the presence of Alfvén waves
Authors: Ratkiewicz R. , Innes D. E. , McKenzie J. F.
Published: March 2009

In this paper the characteristics for a single- and a bi-ion plasma in the presence of Alfvén waves are given. In the single-ion case, the analysis is extended to the situation where Alfvén waves saturate and dissipatively heat the plasma. When there is no dissipation, there are three sound waves and one entropy wave in the single-ion plasma. Each sound wave is associated with two Riemann invariants relating the changes in density and wave pressure to changes in the flow. In the case when the Alfvén waves saturate and heat the plasma, there are two sound waves and one modified entropy sound wave. Each wave is associated with two Riemann invariants relating changes in density and entropy to changes in the flow. The analysis for the bi-ion plasma is simplified to very sub-Alfvénic flows. In this case the Alfvén waves behave like another plasma component, and both the electric and Alfvén wave forces have the same structure. The system possesses two entropy waves and four sound waves. Each sound wave is associated with two Riemann invariants relating changes in density and flow velocity along the characteristic curves.
A self-similar solution of dissipative MHD for a jet in the boundary-layer approximation
Authors: Gonález Alejandro G. , Heyn Martin
Published: March 2009

A solution of dissipative nonlinear MHD taking account of the balance between viscous drag, the Lorentz force, resistive diffusion and inertia in a boundary- layer approximation is presented. It is a steady solution corresponding to a jet in a conducting fluid with viscosity. The problem is solved using a self-similar variable. An exact analytical solution is possible. The integrals of motion are obtained and their physical meaning is explained. The behaviour of the solutions is described. The entrainment of the jet is observed in some examples after an initial stage dominated by magnetic fields. These solutions are an extension of Bickley's jet for a case with magnetic field and resistivity.
A possible excitation mechanism for a longitudinal wave instability in a plasma by a quasi-static electric field
Authors: Kryshtal A. N. , Kuchernko V. P.
Published: March 2009

A specific type of longitudinal wave instability in a plasma due entirely to an external quasi-static electric field E0 is considered for the cases of ion-acoustic and Langmuir waves. The quasi-static time variation of the field leads to a relatively simple mechanism for the development of instability. For frequency ranges typical of ion-acoustic and Langmuir waves dispersion relations are obtained, with Landau damping and pair Coulomb collisions taken into account in addition to E0. The model integral of Bhatnagar, Gross & Krook is used to describe collisions. Two methods for taking account of the influence of the electric field on development of the instability are compared: that of Pines & Schrieffer and that of Brinca and Dysthe. Expressions are obtained for the growth rates in the framework of linear theory. Numerical simulations of both types of instabilities are performed for specific values of the plasma parameters.
Theory of stimulated scattering of large-amplitude waves
Authors: Stenflo L.
Published: March 2009

By means of the standard fluid equations, we consider the nonlinear coupling between a large-amplitude pump wave and the low-frequency modes in a collisional plasma. We derive the general dispersion relation in order to discuss the case where the pump-wave frequency is not much larger than the electron gyrofrequency.
Analysis of plasma critical flow in ablative discharge capillaries with a non-constant cross-section
Authors: Ashkenazy J. , Zoler D.
Published: March 2009

A quasi-one-dimensional model for the steady-state flow of a plasma in an ablative discharge capillary is presented for capillaries with non-constant cross- section. It is demonstrated that small modifications of the capillary geometry can lead to significant changes in the plasma exit parameters. In this respect, the possibility of obtaining an extended range of plasma parameters makes this type of ablative capillary a useful source of plasma for a variety of applications. Numerical solution of the equations of the model for the critical-flow case allows evaluation of the main fluid-dynamic and thermodynamic parameters of the plasma inside the capillary and at its exit.
Relaxed states of an ideal MHD plasma with external magnetic field
Authors: Knorr G. , Mond M. , Grabbe C.
Published: March 2009

We study an ideal MHD plasma with the non-vanishing invariants energy, crosshelicity and magnetic helicity, confined in a cylinder with infinitely conducting walls and an externally applied magnetic fieldB0. The magnetic and velocity fields are expanded in base vector fields, satisfying Δ × Bλ = Bλ. Boundary conditions are imposed to make the curl a self-adjoint operator. The three invariants depend on the time-dependent coefficients of the base vector fields, and are used to construct the partition function to gather statistical information about the equlibrium thermodynamic state to which the plasma relaxes after a turbulent transition. For zero external magnetic field but large magnetic helicity, the energy resides preferentially in magnetic field fluctuations. A sizeable fraction of the kinetic energy initially present is transformed into magnetic energy. The energy condenses via an inverse cascade predominantly to the lowest energy eigenstate, in agreement with results obtained by Taylor. However, since we consider the whole spectrum of eigenstates, the energy does not exclusively occupy the lowest eigenstate. If the eigenvalues are densely spaced (as in a thin torus), the higher eigenmodes also contain appreciable amounts of energy, resulting in a finite pressure of the plasma. For constant and finite external magnetic field, the average induced magnetic field exactly cancels the external field. This indicates that, on a statistical average, the plasma is diamagnetic or superconducting. Superimposed on the average statistical state are fluctuations that may become large if the magnetic helicity is large.
Semi-analytical modelling of non-local ion-cyclotron resonance heating in toroidal geometry
Authors: van Eester D.
Published: March 2009

A semi-analytical approach is proposed for computing the non-local response of a toroidal plasma to an electromagnetic perturbation. Although the perturbed distribution function as well as the absorbed power are also computed, the focus is on the associated quasi-linear diffusion operator. Different decorrelation models allow one to recapture the well-known ideal collisionless plasma result where the wave–particle energy exchange is exactly at resonance, as well as the more realistic plasma response where the interaction region is widened. The choice of the independent variables also allows one to carry out all computations except the integration of the guiding-centre motion analytically.
Time and space scales of electrical and gasdynamic non-uniformities in a non-equilibrium MHD plasma
Authors: Sokolov A. Yu. , Kabashima S. , Zubstov V. M.
Published: March 2009

The Saha equilibrium approximation and the temperature relaxation approximation to the MHD equations are investigated with regard to the analysis of the formation of nonlinear ionization non-uniformities and their evolution in a non- equilibrium MHD plasma. An analysis of characteristic relaxation times under typical plasma parameters is carried out to verify these approximations. Characteristic space scales of electron thermal conductivity and radiation are evaluated. These space scales determine the characteristic width of nonlinear ionization wave fronts, and are important for the application of numerical simulations.
Whistler and electron-cyclotron instabilities in a plasma duct
Authors: James Litty , Jassal Lalita , Tripathi V. K.
Published: March 2009

A gyrating electron beam propagating through a planar plasma duct can excite a whistler wave or an electron-cyclotron wave, depending on the plasma- density profile of the duct and the energy of the beam. The gyrational motion of the electron beam supplies energy to the wave. In the case of a whistler wave the interaction occurs via coupling of the fast cyclotron beam mode (ω ≈ k2vb + ωc/γ0) to the electromagnetic whistler mode through the Weibel instability mechanism, whereas for an electron-cyclotron wave the coupling is through the negative-mass instability mechanism. It is seen that there exist two diflerent beam-energy regimes for excitation of these waves. The growth rate and the efficiency of conversion for both waves are calculated for typical parameters of a gyrotron and a large-orbit gyrotron.
The influence of azimuthal motion on ion resonance instability for a non-neutral plasma column
Authors: Chen H. C.
Published: March 2009

A general treatment of ion resonance instability for a non-neutral plasma column is performed using a macroscopic cold-fluid-Maxwell model. The azimuthal motion of the plasma components has an important influence on the behaviour of the instability. When the electrons are in slow rotational equilibrium, the instability occurs in both slow and fast ion rotational equilibrium. However, there is stability when the electrons are in fast rotational equilibrium except that the l = 1 mode becomes unstable and independent of plasma rotation. The kink (l = 1) mode only occurs when the plasma column boundary exceeds a certain threshold value that depends on the ratio of the plasma frequency to the cyclotron frequency.
Plasma biasing by non-resonant ponderomotive forces of radio-frequency waves
Authors: van Nieuwenhove R. , Petržíka V. , Tataronis J. A.
Published: March 2009

A numerical code that computes stationary radial electric fields E0r induced by ponderomotive forces of radio-frequency waves is developed. The computed value of E0r is typically several hundred volts per centimetre when the wave frequency is 1 % lower or higher than the ion-cyclotron frequency. This value of E0r suggests that the ponderomotively induced electric field would be an effective bias in tokamaks. Numerical data are presented to illustrate the properties of this bias field.
Electromagnetic ion-beam instabilities in a cold plasma
Authors: Gnavi G. , Gomberoff L. , Gratton F. T. , Galvão R. M. O.
Published: March 2009

We study the stability of the cold-plasma dispersion relation for circularly polarized waves in a plasma composed of an ion background and an ion beam. The presence of the beam introduces a resonant branch into the dispersion relation for right-hand-polarized waves propagating in the direction of the external magnetic field, which, for V>Vφ, has negative energy (here V is the beam velocityVφ is the wave phase velocity). Therefore this branch may give rise to explosive instabilities when the waves experience parametric decays. It is shown graphically that resonant right-hand-polarized and non-resonant left-hand-polarized waves, propagating parallel to the external magnetic field, can be unstable. It is also shown that the instability region can extend to large frequencies and wavenumnbers, and that the instability regions have a band structure. The parametric dependence of instability thresholds and marginal modes is also studied.
Expansion of a bounded plasma with dust particles
Authors: Chutov Yu. I. , Kravchenko A. Yu. , Schram P. P. J. M.
Published: March 2009

The expansion of a bounded plasma with dust particles is investigated by means of computer modelling, taking into account the dynamics of the dust particle charge as well as the Coulomb collisions of electrons and ions with dust particles. The PIC method is used for the computer modelling. The collection of electrons and ions by dust particles is described in a way similar to orbit-limited probe theory. Coulomb interactions are described in the framework of stochastic differential equations. It is shown that the mean distribution functions of electrons and ions are influenced by the dust particles during plasma expansion. The evolution of the ion distribution function leads to a strong deviation from equilibrium. Dust particles also influence the temporal behaviour of the plasma parameters.
Diocotron instability of a warm electron beam in crossed fields
Authors: Lee Hee J. , Yang Kwang-Sup
Published: March 2009

The warm-fluid equation derived from the drift kinetic equation is solved numerically to investigate the electrostatic low-frequency stability of an electron ribbon beam drifting in the crossed-fields of a planar magnetron. The temperature effect is manifested only for oblique propagation with respect to the drifting beam direction. The dispersion relation takes the form ω = ω(k⊥/ks∥) = ω(ks∥/k⊥), where k⊥ and k⊥ are respectively the components of the surface wave vector k parallel and perpendicular to the magnetic field. The obliqueness of the propagation direction and the non-zero temperature give rise to a resonant instability, in addition to the diocotron instability, and the wavenumber corresponding to the maximum diocotron growth rate shifts as the temperature changes.
Nonlinear MHD waves on an interface of finite thickness with compressibility and resonant damping
Authors: Khrabrov A. V. , Sonnerup B.U.Ö.
Published: March 2009

Weakly nonlinear surface waves rippling an MHD tangential discontinuity (TD) of finite thickness are studied. The waves are two-dimensional and evanescent in the direction normal to the inhomogeneous layer. A novel approach is utilized to treat weakly nonlinear evanescent wave fields in physical space. The evolution equation obtained, which governs the interface displacement as a function of time and position, extends previous results of Ruderman and Goossens, valid for propagation along a unidirectional magnetic field in a stationary incompressible plasma, to a TD of general field geometry in a compressible plasma in the presence of stable velocity shear. Certain conservation properties of the evolution equation are presented, including the Hamiltonian formulation. Exploratory numerical results are reported. The model is of potential use in the analysis of observed quasi-stationary perturbations at the earth's magnetopause.
Steady waves in a cold plasma
Authors: Il'Ichev Andrej
Published: March 2009

The possible crest forms of magnetoacoustic travelling waves of small amplitude satisfying the full system of equations describe the wave propagation in a cold quasineutral collisionless plasma are determined. For a certain range of the inclination angle of the magnetic induction vector we find solitary waves. We find also families of periodic waves of two different types, as well as quasiperiodic waves aiid generalized solitary waves with non-decreasing oscillations at infinity.
Nonlinear stability of Kelvin–Helmholtz waves in magnetic fluids stressed by a time-dependent acceleration and a tangential magnetic field
Authors: El-Dib Yusry O.
Published: March 2009

The nonlinear stability of surface waves propagating between two superposed streaming magnetic fluids is investigated. The fluids are stressed by a constant tangential magnetic field and a vertical periodic acceleration. The solution employs the method of multiple scales. Owing to the periodicity, resonant cases appear. Two parametrically nonlinear Schrödinger equations are derived for the resonant cases to describe the elevation of weakly nonlinear capillary waves. The standard nonlinear Schrödinger equation is satisfied for the non resonant cases. Necessary and sufficient conditions for stability are obtained. A formula for the surface elevation is obtained in each case. It is found that the magnetic field, the velocities and the frequency of the applied periodic force play dual roles in the resonant region. Investigation of the stability criterion by nonlinear perturbation shows that an increase in the acceleration frequency has a stabilizing effect. The stabilizing role of the frequency is due to the destabilizing effect of the amplitude of the periodic acceleration.
A new Hamiltonian formulation for fluids and plasmas. Part 1. The perfect fluid
Authors: Larsson Jonas
Published: March 2009

A new formulation of the Hamiltonian structure underlying the perfect fluid equations is presented. Besides time, a parameter c is also used. Correspondingly, there are two interdependent systems of equations expressing time evolution and e evolution respectively. The accessibility equations define the e dynamics and give the variation in the usual Eulerian fluid variables as determined by the generating functions. The time evolutions of both the Eulerian fluid variables and the generating functions are obtained from an action principle. The consistency of the e and the time dynamics is crucial for this formulation, i.e. the accessibility equations must be propagated in time by the Euler–Lagrange equations. The reason for introducing this new formulation is its power in certain applications where the existing Hamiltonian alternatives seem less convenient to use. In particular, it is a promising tool for Hamiltonian perturbation theory. We consider the small-amplitude expansion, and find, very simply and naturally, the Hermitian structure of the linearized ideal fluid equations as well as coupling coefficients for resonant three-wave interaction exhibiting the Manley–Rowe relations.
A criterion for splitting of a reconnecting current sheet into MHD discontinuities
Authors: Markovskii S. A. , Somov B. V.
Published: March 2009

We consider the splitting of a reconnecting current sheet into MHD discontinuities, which is observed in many numerical simulations of the magnetic reconnection process. We suppose that the splitting takes place as a consequence of non-evolutionarity of the reconnecting current sheet as a onedimensional discontinuity. This means that the problem of the time evolution of its small perturbations does not have a unique solution. Since a physical problem must always have a unique solution, a non-evolutionary discontinuity cannot exist in a real plasma, and splits into evolutionary discontinuities. Solving the linear MHD equations inside and outside the sheet, we show that for large enough plasma conductivity, certain small perturbations interact with the sheet as with a discontinuity. On the basis of the non-evolutionarity criterion, with respect to these perturbations, we obtain a condition on the flow velocity at the sheet surface, under which the splitting takes place.
Resonant electromagnetic wave mixing in a dusty plasma
Authors: Ma Jin-Xiu , Yu M. Y. , Shukla P. K.
Published: March 2009

Wave mixing resulting from the resonance of electromagnetic waves with ion acoustic waves modified by dust charge fluctuations is investigated. The corresponding nonlinear susceptibilities are derived and applied to the study of phase conjugation by four-wave mixing. It is shown that dust charge fluctuations can lead to a filling-up of the resonance-induced splitting of the phase-conjugate reflectivity, making the latter a useful tool for the diagnostics of dusty plasmas.
Dynamics of a magnetized gravitating dusty plasma
Authors: Mahanta L. , Saikia B. J. , Pandey B. P. , Bujarbarua S.
Published: March 2009

Linear stability of a magnetized dusty plasma is studied for the regime with ≈ 1 (where G is the universal gravitational constant, md is the grain mass and qd is the grain charge). We demonstrate that the magnetic force plays a stabilizing role and, together with electrostatic repulsion, counter gravitational condensation. The implications of this result for astrophysical situations are briefly discussed.
A dielectric tensor for a uniform magnetoplasma with a generalized Lorentzian distribution
Authors: Mace R. L.
Published: March 2009

It is demonstrated that the dielectric tensor for a non-relativistic magnetized plasma whose particle velocity distributions can be modelled by isotropic kappa, or generalized Lorentzian, distributions admits an expression similar to that obtained by Trubnikov for a relativistic plasma. The kappa distribution is a useful distribution for modelling space plasmas containing significant numbers of superthermal particles, i.e. those that have energies in excess of the thermal energy. The dielectric tensor is valid for arbitrary wavevectors, and is shown to reproduce the known limiting case of wave propagation parallel to the magnetic field. Even in this limiting case, the results obtained represent a generalization of previous results to arbitrary real values of the index K, the parameter that shapes the superthermal tail on the distribution. The expression for the dielectric tensor might be useful as a starting point for numerical studies of waves and instabilities in plasmas containing superthermal particles.