bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal effects on parallel-propagating electron cyclotron waves
Authors: Robinson P. A.
Published: March 2009

Thermal effects on the dispersion of right-handed (RH) electron cyclotron waves propagating parallel to a uniform, ambient magnetic field are investigated in the strictly non-relativistic (‘classical’) and weakly relativistic approximations for real frequency and complex wave vector. In each approximation, the two branches of the RH mode reconnect near the cyclotron frequency as the plasma temperature is increased or the density is lowered. This reconnection occurs in a manner different from that previously assumed at parallel propagation and from that at perpendicular propagation, giving rise to a new mode near the cold plasma cut-off frequency ωxC. For both parallel and perpendicular propagation, it is noted that reconnection occurs approximately when the cyclotron line-width equals the width of the stop-band in the cold plasma dispersion relation. Inclusion of weakly relativistic effects is found to be necessary for quantitative calculations and for an accurate treatment of the new mode near ωxC. Weakly relativistic effects also modify the analytic properties of the dispersion relation so as to introduce a new family of weakly damped and undamped solutions.
Convective cell and Alfvén vortices in an inhomogeneous rotating cold magnetoplasma
Authors: Shukla P. K. , Bharuthram R.
Published: March 2009

It is shown that double vortices are a special class of stationary solutions of the set of nonlinear equations that governs the dynamics of modified convective cells and shear Alfvén waves in a cold rotating magnetized plasma. Criteria for the existence of dipole vortices as well as several analytical expressions for the vortex profiles are presented. It is suggested that modified convective cell and Alfvén dipole vortices may cause anomalous cross-field particle transport in a low-β plasma, such as the ionosphere.
Almost perpendicular electromagnetic wave transformation in a weakly relativistic inhomogeneous plasma
Authors: Sazhin S. S.
Published: March 2009

It is shown that for certain values of the parameters of weakly relativistic plasma, the perpendicular ordinary and extraordinary refractive indices are equal. This permits a transformation between these waves when the wave normal angle θ is close but not equal to ½π Explicit formulae for the corresponding transformation coefficients are derived.
Dispersion of electron Bernstein waves including weakly relativistic and electromagnetic effects. Part 1. Ordinary modes
Authors: Robinson P. A.
Published: March 2009

Ordinary solutions of the weakly relativistic, electromagnetic dispersion relation are investigated for waves propagating perpendicular to a uniform magnetic field in a Maxwellian plasma. Weakly relativistic resonance broadening, frequency downshift and damping are found to alter dramatically the dispersion predicted by the corresponding strictly non-relativistic (‘classical’) theory in the neighbourhood of harmonics of the cyclotron frequency Ωe. All classical resonances and cut-offs are removed except the cut-off at the plasma frequency ωp. At frequencies above ωp the infinite family of classically predicted modes is replaced by a single weakly damped mode whose dispersion differs only slightly from that predicted by cold plasma theory. No weakly damped modes exist in the range of harmonics s satisfying (ωp/Ωe)⅔/8 ≲ S < Ωp/Ωe, however, one such mode is located immediately below each harmonic for s ≲ (ωp/Ωe)⅔/8. A companion paper investigates extraordinary solutions of the dispersion relation.
Ponderomotive force near cyclotron resonance
Authors: Kono M. , Sanuki H.
Published: March 2009

The ponderomotive force in a magnetized plasma is derived by carrying out the renormalization of wave–particle interactions based on the Vlasov equation. A significant feature of the resuit is non-singular behaviour at resonance even in the case of perpendicular propagation. This is shown to be related to the onset of the diffusive motion of particles due to the orbit instability near resonance, where the ponderomotive force is eventually small.
On the theory of electromagnetic waves and instabilities in multi-species weakly ionized plasmas in external d.c. electric field
Authors: Milić B. S. , Krstić S. R.
Published: March 2009

Electromagnetic modes with the electric vector of the wave lying in the plane defined by the wave vector and the external d.c. electric field are studied theoretically in weakly ionized plasmas with several sorts of ions. The corresponding instability criteria are also given. The analysis is based on kinetic equations with BGK model collision integrais for the one-particle distribution functions of the charged constituents (electrons and all sorts of ions). The linear theory of perturbation is applied. Special attention is given to the long-wave domain (modal wavelengths much larger than the electron mean free path): it is in this domain that the instabilities are found to set in first as the electron drift is gradually increased. The increments of these instabilities are, however, smaller than in the short-wave domain which was mainly studied before. Apart from the modes which exist in weakly ionized plasmas with only one ion species, and which re-appear in the case studied here with somewhat altered characteristics, a new slow mode, specifie for multi-species weakly ionized plasmas, is found. Its phase velocity is below the electron thermal velocity, and its existence, as well as the conditions of excitation of the corresponding instability, depend on the plasma composition and non-isothermality (viz. the ratio Te/Ti). In some situations, two such modes are possible. The analysis of the instabilities is completed by a brief description of two (sometimes three) branches of ion acoustic instability, specific for the multi-species plasmas and propagating strictly along the direction of the external electric field.
Non-relativistic thermal effects on parallel-propagating ion cyclotron waves
Authors: Ball L. T.
Published: March 2009

We investigate strictly non-relativistic thermal effects on the dispersion of lefthanded (LH) ion cyclotron waves (ICW's) with real frequency and complex wave vector, propagating parallel to a uniform ambient magnetic field. Changes to the topology of the cold-plasma dispersion relations in the vicinity of the ion gyrofrequencies are studied in plasmas consisting predominantly of protons with a small admixture of a heavy ion. The two branches of the LH mode reconnect near the heavy-ion gyrofrequency as the heavy-ion temperature is increased or its relative density is reduced. The reconnection results in one mode in which waves can propagate at all frequencies below the proton gyrofrequency and another which allows propagation only in a narrow frequency range extending upwards from the cut-off frequency to a regime where strong damping occurs. The topology of the reconnected dispersion curves is quite different from that seen in the real wave vector – complex frequency case. This work is relevant to theories of ion heating and acceleration in multi-ion plasmas as are found in the solar wind, in solar and stellar flares, and in the Earth's magnetosphere. In particular, strongly species-dependent heating and acceleration can arise from wave–particle interactions between the various ionic species, and ICW's at frequencies near the respective ion gyrofrequencies. These interactions depend critically on the wave dispersion.
Some remarks on spontaneous magnetic field generation and the nonlinear dynamics of a Langmuir plasma
Authors: Berezhiani V. I. , Tskhakaya D. D. , Auer G.
Published: March 2009

It is shown that, in the hydrodynamic approximation, the spontaneous generated magnetic field does not act directly on the development of Langmuir plasma turbulence. It is shown that besides the magneto-modulational effects, it is necessary to take into account the purely relativistic effect, i.e. the dependence of the electron mass on velocity. In most cases the contribution of this relativistic effect prevails over the spontaneous magnetic field contribution to the development of the modulational instability of HF waves.
Kinetic and transport theory for a non-neutral plasma taking account of strong gyration and non-uniformities on the collisional scale
Authors: Øien Alf H.
Published: March 2009

From the BBGKY equations for a pure electron plasma a derivation is made of a collision integral that includes the combined effects of particle gyration in a strong magnetic field and non-uniformities of both the distribution function and the self-consistent electric field on the collisional scale. A series expansion of the collision integral through the distribution function and the electric field on the collisional scale is carried out to third order in derivatives of the distribution function and to second order in derivatives of the electric field. For the strong-magnetic-field case when collision-term contributions to only first order in 1/B are included, a particle flux transverse to the magnetic field proportional to l/B2 is derived. The importance of long-range collective collisions in this process is shown. The result is in contrast with the classical l/B4 proportionality, and is in accordance with earlier studies.
A two-dimensional ion acoustic solitary wave in a weakly relativistic plasma
Authors: Nejoh Yasunori
Published: March 2009

The two-dimensional Korteweg-de Vries equation is first derived for a weakly relativistic ion acoustic wave propagating in a collisionless plasma. We show that the relativistic effect greatly influences the phase velocity, the amplitude and the width of a solitary wave solution, and that the presence of streaming ions gives rise to the formation of a precursor. We also discuss three limiting cases of the present results.
Effects of parallel ion dynamics on drift-Alfvén vortices in plasmas
Authors: Shukla P. K.
Published: March 2009

Drift-Alfvén vortices are investigated, taking into account the nonlinear ion dynamics parallel to the external magnetic field. It is found that the parallel ion motion restricts the vortex speed.
Resistively induced plasma diffusion due to the trivial marginal modes of ideal MHD
Authors: Liebert W. , Rebhan E.
Published: March 2009

The influence of nonlinearities and plasma resistivity on the so-called trivial marginal modes of ideal MHD is investigated. It turns out that to lowest significant order of a reductive perturbation analysis nonlinearities have no influence, while in toroidal confinement configurations resistivity induces a local plasma diffusion across the magnetic surfaces. This is demonstrated for tokamaks with zero poloidal current density. In addition, the appearance of singularities in the plasma motion parallel to the magnetic field lines suggests a profile condition at the edge of the plasma. Heat conduction would directly tend to keep these effects at lower level, while indirectly it creates a reinforcement mechanism through the coupling to usual cross diffusion.
Models of laser-plasma ablation. Part 3. Steady-state theory: deflagration flow
Authors: Pert G. J.
Published: March 2009

The theory of plasma ablation by laser irradiation from cylindrical and spherical solid targets is considered when thermal conduction is dominant and absorption is local at the critical density. Analytic solutions for both inhibited and uninhibited heat fluxes are developed, but only investigated in detail when flux limiting does not introduce a step discontinuity. In most cases it is found that only a restricted region of flow is steady, and must be terminated by a rarefaction wave. The transition from quasi-planar to strongly divergent flow is shown to depend on a characteristic parameter, which represents the ratio of the thermal conduction length to the target radius.
Time evolution of the velocity distribution of neutral-beam-injected ions heated by ICRH in a two-component plasma
Authors: Anderson D. , Eriksson L.-G. , Lisak M.
Published: March 2009

The time evolution of the distribution function of the beam-injected particles in the presence of ICRH in a two-component plasma is determined. Consideration is restricted to the time development during two complementary time periods: (i) the early time period, i.e. 0 ≤t ≪ TS, and (ii) the quasi-steady state, i.e. t > Ts, where Ts is the slowing-down time for beam-ion-electron collisions. Explicit analytical solutions are obtained for anisotropic as well as isotropie beam injection.
Nonlinear interaction of obliquely propagating Bernstein waves with electrons in a plasma
Authors: Sugawa Masao
Published: March 2009

We obtain analytical expression for the interaction of obliquely propagating Bernstein waves with electrons by using the monochromatic wave approximation for quasi-linear theory in a weakly turbulent plasma. A numerical analysis is also carried out. The waves show initially strong damping and irregular amplitude oscillation, and the electron velocity distribution shows a variation corresponding to one of the waves. These are results of the energy exchange between waves and electrons. Despite the use of the monochromatic wave approximation, strongly scattering electrons with a broad velocity spread about the resonant velocity by the wave is seen.
Analytical solutions for the current driven by a rotating magnetic field in a cylindrical plasma with azimuthal field
Authors: Watterson Peter A.
Published: March 2009

The generation of steady currents by a rotating magnetic field (RMF) in a cylindrical plasma permeated by a steady azimuthal (or toroidal) magnetic field is studied analytically. Solutions are presented for the following limiting cases:

(1) high resistivity, when the penetration of the RMF and current drive are confined to a skin depth layer;

(2) low resistivity and weak toroidal field (small compared with the RMF), when the RMF fully penetrates the plasma and the toroidal current is that due to nearly synchronous rotation of the electron fluid with the RMF;

(3) low resistivity and intermediate toroidal field (comparable to the axial field associated with synchronous current), when the toroidal current is a significant fraction of its synchronous value, but large oscillating fields are generated; and

(4) strong toroidal field, when the RMF fully penetrates the plasma but current is only driven in a boundary layer at the plasma edge.

The applicability of these solutions is governed by the relative sizes of three dimensionless parameters.
The structure of reconnection layers
Authors: Heyn Martin F. , Biernat Helfried K. , Rijnbeek Richard P. , Semenov Vladimir S.
Published: March 2009

Using nonlinear ideal MHD equations, we analyse the structure of the convection region within a Petschek-type model for reconnection. We show how, assuming that the normal field and flow components remain small and using simple wave analysis, the structure of the reconnection layer as well as the behaviour of the tangential field and plasma parameters can be specified in terms of the external parameters in the inflow regions. Equations for the normal field and flow components and the angular width of the reconnection layer (assuming planar symmetry and a steady state) are also given in terms of the value of the electric field along the reconnection line. The model is suited to application at the earth's magnetopause and in the distant magnetotail. In particular, it lends itself to an investigation of the requirements for reconnection to occur and the general validity of MHD reconnection models in practical applications.
Helicity injection and relaxation in a solar-coronal magnetic loop with a free surface
Authors: Browning P. K.
Published: March 2009

A solar-coronal magnetic loop is rooted in the photosphere, where motions shuffle the footpoints of the field, generating currents in the corona. The dissipation of these currents provides a possible mechanism for heating the solar corona. A theory is described based on a generalization of Taylor's hypothesis, predicting that as the loop is twisted up, it relaxes towards a minimum-energy state V × B = μB. The footpoint motions inject helicity as well as energy, and the evolution is determined through a helicity-injection equation. The loop is modelled as a straight magnetic-flux tube, with twisting motions at the ends, confined by a constant external pressure at the curved surface, which is a free boundary. The problem of the loop evolution in response to given footpoint motions is solved, and an interesting example of multiple equilibria arises. The heating rate is calculated for an almost-potential loop. The model may also be regarded as representing a laboratory experiment: in particular, a simple idealization of a spheromak, with the footpoint motions replaced by an applied voltage.
Weak nonlinear electromagnetic waves and low-frequency magnetic-field generation in electron-positron-ion plasmas
Authors: Rizzato F. B.
Published: March 2009

The weakly nonlinear localization of obliquely modulated high-frequency electromagnetic waves in an electron-positron-ion plasma is considered. It is shown that the amplitude of the wave turns out to be a strongly dependent function of the angle between the slow modulations and the fast spatial variations and that the possibility appears of spontaneous generation of low-frequency magnetic fields. These magnetic fields are also functions of this angle and of the high-frequency wave polarization. The analysis of colinear modulation in electron-positron plasmas shows that some restriction must be made regarding the validity of previous calculations.
The effect of lower-hybrid waves on the propagation of hydromagnetic waves
Authors: Hamabata Hiromitsu , Namikawa Tomikazu , Mori Kazuhiro
Published: March 2009

Propagation characteristics of hydromagnetic waves in a magnetic plasma are investigated using the two-plasma fluid equations including the effect of lower-hybrid waves propagating perpendicularly to the magnetic field. The effect of lower-hybrid waves on the propagation of hydromagnetic waves is analysed in terms of phase speed, growth rate, refractive index, polarization and the amplitude relation between the density perturbation and the magnetic-field perturbation for the cases when hydromagnetic waves propagate in the plane whose normal is perpendicular to both the magnetic field and the propagation direction of lower-hybrid waves and in the plane perpendicular to the propagation direction of lower-hybrid waves. It is shown that hydromagnetic waves propagating at small angles to the propagation direction of lower-hybrid waves can be excited by the effect of lower-hybrid waves and the energy of excited waves propagates nearly parallel to the propagation direction of lower-hybrid waves.
Nonlinear interaction of electrostatic waves in a magnetized Vlasov plasma
Authors: Larsson Jonas
Published: March 2009

A new expression for the second-order dielectric function є(2) (K1,ω1, K2,ω2) is derived from the Vlasov–Poisson equations. The formula is also suitable for the calculation of this quantity in general situations when it is essential to use kinetic theory and when the wave vectors K1 and K2 have arbitrary directions with respect to each other as well as to the external magnetic field. We also consider two earlier formulae for this response function and discuss advantages and disadvantages of the different expressions.
An extended analytical solution of the Boltzmann equation for non-homogeneous fusion and astrophysical plasmas
Authors: Cuperman S. , Zoler D.
Published: March 2009

The perturbative Chapman-Enskog procedure for solving Boltzmann's equation, holding when f1 ≪ f0 (f = f0 + f1 + …), is replaced by a method that is free of such a limitation. This work represents an extension to the case of strongly anisotropic plasma systems and the spherical geometry of that of Campbell (1984, 1986). The solution obtained here is expressed in terms of prescribed ratios of mean free path for collisions, as well as electric and gravitational fields, to the temperature- and density-gradient lengths. This solution is also used to discuss the limitation of the conduction transport coefficients in electron plasmas.
Resonance cones in a drifting plasma
Authors: Fiala V. , Karpman V. I.
Published: March 2009

Lower oblique resonance cones excited by a harmonic point source in a drifting plasma are studied in different approximations. It is shown that the electromagnetic correction to the dispersion, giving finite wave velocities, is necessary for a correct description of the drift effects.
Spectral analysis of turbulent effects on resistivity and the tearing instability
Authors: Deeds D. , van Hoven G.
Published: March 2009

Biskamp and Welter (1983) have defined an anomalous resistivity due to shortwavelength turbulence. They reported that this resistivity can be of either sign, and that negative anomalous resistivity in particular can affect the growth of the tearing instability. We use a spectral numerical-simulation code and ancillary diagnostics to analyse the behaviour of resistive magnetic tearing in the presence of turbulence of the sort postulated by Biskamp and Welter. We find that, in general, the ‘anomalous resistivity’ tends to return quickly towards zero even when artificially supported away from zero, and that its effect on tearing-mode behaviour is not consistent with its interpretation as a resistivity. We investigate analytically the behaviour reported by Biskamp and Welter, and the behaviour we observe. We also argue that, while not meaningful as a true resistivity, the ‘anomalous-resistivity’ parameter is a useful diagnostic showing the energy balance of the System – a property we refer to as Alfvénicity – illustrating, for example, the onset of nonlinearity in the tearing process.
Approximation of the dielectric properties of Maxwellian plasmas: dispersion functions and physical constraints
Authors: Robinson P. A. , Newman D. L.
Published: March 2009

The dielectric properties of Maxwellian plasmas are approximated using both Padé approximants to the dispersion function and direct approximation of the distribution. Physical constraints on permissible approximations are discussed, and it is found that some previously published results can lead to predictions of qualitatively incorrect wave properties, including unphysical negative damping. Approximate dispersion functions for Maxwellian distributions are given explicitly, and some of the effects of these approximations on the resulting dispersion are discussed. The approximations discussed here are of use both in analytic work and in accelerating large-scale numerical computations.
OTSI of lower-hybrid waves in large- and small-size devices
Authors: Tripathi V. K. , Siva Rama Prasad P. V.
Published: March 2009

Oscillating two-stream instability (OTSI) of lower-hybrid pump waves has been studied in both large- and small-size devices. For large-size devices, the instability has been investigated by considering fast-ion-mode side-bands and employing local theory. The convective threshold due to the finite extent of the pump has been calculated. For small-size devices, the instability has been investigated by ignoring thermal effects on the side-bands but employing nonlocal theory. We have found that there is no important change in growth rates in both cases.
Whistler instability in a magnetospheric duct
Authors: Talukdar I. , Tripathi V. K. , Jain V. K.
Published: March 2009

A whistler wave propagating through a preformed magnetospheric duct is susceptible to growth/amplification by an electron beam. The interaction is non-local and could be of Čerenkov or slow-cyclotron type. First-order perturbation theory is employed to obtain the growth rate for flat and Gaussian beam densities.
Two-dimensional hydrodynamic models of laser-produced plasmas
Authors: Pert G. J.
Published: March 2009

Analytic modelling of laser-produced plasmas has generally been restricted to one-dimensional flow. Multi-dimensional hydrodynamic approximations are available, and are explored in this paper. Two configurations are examined. The explosive mode in which the entire body of material is uniformly heated is treated by the self-similar form, and the aspect ratio of the resulting expansion is determined. Ablative flows can be approximated by the hybrid model, and the self-regulating flow from a solid target can be calculated in this way.
Ion- and electron-acoustic waves in sheet plasmas
Authors: Chaudhry M. B. , Tahir M. D.
Published: March 2009

Ion-acoustic waves in sheet plasmas, which are of thickness of order of the ion Larmor radius ρi, have been investigated numerically. The frequency range considered is less than the ion cyclotron frequency ωci. An integral equation in wavenumber space is derived from the linearized Vlasov-Poisson equations and analysed numerically. Eigenfrequencies and eigenfunctions of the wave have been studied systematically by varying the plasma thickness, plasma density, electron-to-ion temperature ratio and parallel wavenumber. Electron-acoustic waves are found when the parallel wavenumber is very small (e.g. k∥ρi = 0·005) and the electron and ion temperatures are comparable.
The flow field induced by spherical blast waves propagating into perfect and imperfect dusty gases
Authors: Rakib Z. , Mond M. , Ben-Dor G. , Igra O.
Published: March 2009

The flow fields induced by spherical blast waves propagating into dusty gases are investigated numerically. In the case of a moderate blast wave the gaseous phase is assumed to be perfect, while for strong blast waves real-gas effects are accounted for. The effects of the dust loading ratio and the initial energy used to generate the blast on the suspension properties are investigated and discussed in detail. Owing to the fact that the physics involved inside the exploding charge is very complicated, the present solution is appropriate for simulating the flow induced by the blast wave outside the exploding sphere only.
Finite-Larmor-radius effects on z–pinch stability
Authors: Scheffel Jan , Faghihi Mostafa
Published: March 2009

The effect of finite Larmor radius (FLR) on the stability of m = 1 small-axial-wavelength kinks in a z–pinch with purely poloidal magnetic field is investigated. We use the incompressible FLR MHD model; a collisionless fluid model that consistently includes the relevant FLR terms due to ion gyroviscosity, Hall effect and electron diamagnetism. With FLR terms absent, the Kadomtsev criterion of ideal MHD, 2r dp/dr + m2B2/μ0 ≥ 0 predicts instability for internal modes unless the current density is singular at the centre of the pinch. The same result is obtained in the present model, with FLR terms absent. When the FLR terms are included, a normal-mode analysis of the linearized equations yields the following results. Marginally unstable (ideal) modes are stabilized by gyroviscosity. The Hall term has a damping (but not absolutely stabilizing) effect – in agreement with earlier work. On specifying a constant current and particle density equilibrium, the effect of electron diamagnetism vanishes. For a z–pinch with parameters relevant to the EXTRAP experiment, the m = 1 modes are then fully stabilized over the crosssection for wavelengths λ/a ≤ 1, where a denotes the pinch radius. As a general z–pinch result a critical line-density limit Nmax = 5 × 1018 m–1 is found, above which gyroviscous stabilization near the plasma boundary becomes insufficient. This limit corresponds to about five Larmor radii along the pinch radius. The result holds for wavelengths close to, or smaller than, the pinch radius and for realistic equilibrium profiles. This limit is far below the required limit for a reactor with contained alpha particles, which is in excess of 1020 m–1.
Nonlinear gyrokinetic Maxwell-Vlasov equations using magnetic co-ordinates
Authors: Brizard A.
Published: March 2009

A gyrokinetic formalism using magnetic co-ordinates is used to derive self-consistent, nonlinear Maxwell–Vlasov equations that are suitable for particle simulation studies of finite-β tokamak microturbulence and its associated anomalous transport. The use of magnetic co-ordinates is an important feature of this work since it introduces the toroidal geometry naturally into our gyrokinetic formalism. The gyrokinetic formalism itself is based on the use of the action-variational Lie perturbation method of Cary & Littlejohn, and preserves the Hamiltonian structure of the original Maxwell-Vlasov system. Previous nonlinear gyrokinetic sets of equations suitable for particle simulation analysis have considered either electrostatic and shear-Alfvén perturbations in slab geometry or electrostatic perturbations in toroidal geometry. In this present work fully electromagnetic perturbations in toroidal geometry are considered.
A stable stationary solution of compressible magnetohydrodynamics associated with conservation of modified kinetic helicity
Authors: Norwood Joseph
Published: March 2009

It is shown that, in addition to magnetic helicity and cross-helicity, a modified form of kinetic helicity is also conserved in MHD fluids if the Hall effect is taken into account. The consequence of including this modified kinetic helicity as a conservation integral in a variation of the system energy is the emergence of an unconditionally MHD-stable solution that is realized in migma plasmas and may be expected to emerge as a self-organized configuration from an initially turbulent magnetofluid.
Corrigendum
Authors: Kim S. H.
Published: March 2009

A longitudinal undulating electric field in the lasing electron beam direction (or the longitudinal direction) and an electromagnetic wave propagating in the longitudinal direction (laser wave) drive a current that cannot be in the longitudinal direction. Thus this current can emit an electromagnetic wave having the same wave vector and polarization vector as those of the incident electromagnetic wave. This process is called stimulated bremsstrahlung in the longitudinal electric wiggler. Because of recent strong interest in this free electron lasing mechanism, it is appropriate to correct errors in numerical factors in the gain formula and equations in our paper.
The acceleration of cometary ions by Alfvén waves
Authors: Duffy Peter
Published: March 2009

In this paper the theoretical aspects of the stochastic acceleration of photoionized pick-up ions upstream of a comet's bow shock are examined. The acceleration is due to the resonant interaction of the ions with Alfvén waves. By making physically reasonable assumptions for the turbulent fields and by including an exponential spatial factor in the injection term, it is found that, with spatial diffusion ignored, exact solutions can be found.
Alfvén-wave heating in resistive MHD
Authors: Poedts Stefaan , Kerner Wolfgang , Goossens Marcel
Published: March 2009

Resonant absorption of Alfvén waves in tokamak plasmas is studied numerically using the linearized equations of resistive magnetohydrodynamics. A numerical code based on a finite-element discretization is used for determining the stationary state of a cylindrical plasma column that is excited by an external periodic driver. The energy dissipation rate in the stationary state is calculated and the dependence of the plasma heating on electrical resistivity, the equilibrium profiles, and the wavenumbers and frequency of the external driver is investigated. Resonant absorption is extremely efficient when the plasma is excited with a frequency near that of a so-called ‘collective mode’. The heating of a plasma by driving it at the frequencies of discrete Alfvén waves is also investigated.
Stimulated scattering of an electromagnetic wave by an upper-hybrid wave in a two-electron-temperature plasma
Authors: Guha S. , Asthana Meenu
Published: March 2009

We present an analytical investigation of the stimulated scattering of an electromagnetic wave by an upper-hybrid wave in a two-electron-temperature plasma using a hydrodynamical model. The nonlinear dispersion equation of the upper-hybrid wave is derived and the growth rate is calculated. It is found that increasing the concentration ratio of hot to cold electrons decreases the growth rate of the upper-hybrid wave.
Free double layers in mercury-arc discharges
Authors: Maciel Homero Santiago , Allen J. E.
Published: March 2009

A study has been carried out of free double layers formed within the plasma volume of a low-pressure mercury-arc discharge at high current densities. The free double layer is observed to form as a visible boundary, which drifts slowly from the central section of the discharge. Current-driven instabilities are observed as the discharge current is gradually increased to a critical value, at which current limitation is observed to occur. This process, which is accompanied by high-current spikes, ceases when the free double layer becomes visible as a sharp boundary dividing the discharge column into two regions of different luminosities. The layer is observed to form in the later stages of current limitation, the onset of which occurs for a ratio of drift to thermal speed of electrons of about unity. Electrical energy is converted by the layer into kinetic energy of the charged particles. Accordingly, high-energy ions were measured by means of an electrostatic energy analyser. The multiple-sheath character of the free ‘double layer’, which is inferred from probe measurements of potential profiles, is discussed and comparisons with other space-charge structures with the same topology are made.
Wave properties of an ion-beam system with a strong magnetic field
Authors: Naidu K. , Zank G. P. , McKenzie J. F.
Published: March 2009

This paper develops a theoretical framework for the description and classification of small-amplitude waves with frequencies much less than the ion gyrofrequency, propagating in an ion-beam plasma system. In this respect, the results extend to the strongly magnetized regime the results obtained previously by Zank and McKenzie and applied by Greaves et al. to study wave propagation in such a system for frequencies in excess of the ion gyrofrequency but less than the electron plasma frequency. For completeness, the full wave equation governing an ion-beam plasma system for any strength of applied magnetic field is derived. In specializing to the strong-magnetic-field limit, we find that the class of refractive-index topologies (which characterize the kinematic properties of wave propagation) is less rich than in the un-magnetized case. After investigating the topology of the refractive-index surface and the phase-, ray- and group-velocity surfaces, we construct a CMA diagram appropriate to the strongly magnetized ion-beam plasma system. The temporal stability and spatial amplification of the slow ion-acoustic mode for frequencies less than the stationary ion plasma frequency is investigated. We show that a strong magnetic field normal to the drift direction of the ion beam stabilizes long-wavelength modes that would be unstable in the unmagnetized case.
An equation for the drift velocity of relativistic electrons scattered isotropically and a negative magneto-resistance effect
Authors: Nagata M.
Published: March 2009

Previous work on non-relativistic electrons isotropically scattered by neutral atoms is extended to the relativistic case, and an equation is derived for the electron drift velocity. In the non-relativistic case the drift-velocity equation contains a corrected momentum-transfer term arising from the fact that the collision phenomenon is characterized by the mean free path rather than by the mean collision time. In the present work it is found that the time variation of the electron mass has a very complicated effect on the relativistic equation for the drift velocity. In addition, an unexpected theoretical result is obtained: a negative magneto-resistance effect in which the magneto-resistance decreases as the magnitude of a magnetic field imposed on the plasma increases. However, this effect is not a relativistic one but is rather due to the correction in the momentum-transfer term.
The effect of a velocity-dependent charge-exchange kernel on neutral-atom transport in a half-space plasma: exact solution
Authors: Prinja A. K. , Williams M. M. R.
Published: March 2009

A symmetric factorization of the velocity-dependent charge-exchange kernel (the so-called separable-kernel model) is used in the Boltzmann equation for neutral atoms to obtain an exact solution for a half-space plasma by the Wiener-Hopf method. This work generalizes earlier work employing constant, velocity-independent charge-exchange interactions to the case of an arbitrary velocity dependence of the Maxwellian averaged charge-exchange reaction rate. The effects of velocity dependence on the speed-angle distribution of escaping neutrals and the total charge-exchange rate in the half-space are shown to be significant. It is also shown how the Wiener-Hopf method can be applied to such problems with a realistic Maxwellian plasma background, without first approximating the ion distribution.
Weakly relativistic dielectric tensor in the presence of temperature anisotropy
Authors: Bornatici M. , Chiozzi G. , de Chiara P.
Published: March 2009

Analytical expressions for the weakly relativistic dielectric tensor near the electron-cyclotron frequency and harmonies are obtained to any order in finite-Larmor-radius effects for a bi-Maxwellian distribution function. The dielectric tensor is written in ternis of generalized Shkarofsky dispersion functions, whose properties are well known. Relevant limiting cases are considered and, in particular, the anti-Hermitian part of the (fully relativistic) dielectric tensor is evaluated for two cases of strong temperature anisotropy.
Potential bounds and estimation for the multiple water-bag plasma
Authors: Chee-Seng Lim
Published: March 2009

Bounds are established for the permanent-state radiation-conditioned response to a vibrating charge in the MWB (‘multiple water-bag’) model of a warm Maxwellian plasma. Those bounds vary with the observation position beyond the charge and, along any radial direction, serve as boundary curves of potential bands containing the exact potential curves. They can therefore be employed for potential estimation. The bounds are actually Poisson potentials and are independent of (the degree of accuracy in) the MWB modelling when the charge frequency ω exceeds the electron plasma frequency ωp. In this case each potential band narrows to improve potential estimation as ω/ωp increases, and in fact a relative error in estimation can be uniformly predetermined to as small as one desires for all MWB models, charge distributions and observation points by setting ω beyond an appropriate level above ωp. The bounds are, however, model-dependent if ωp exceeds ω in magnitude, in which case, they are partially Poissonian; moreover, potential estimation based on them improves as ω/ωp decreases, and an error analysis is again performed. In either case, a sharper estimation is obtained by averaging the bounds to get an estimate; thus, for instance, the associated relative error cannot exceed 0·01, 0·02 and 0·05 when ω/ωp = (101)½, (51)½ and (21)½ respectively. Applications are described.
Electron-temperature-gradient-driven drift waves and anomalous electron energy transport
Authors: Shukla P. K. , Murtaza G. , Weiland J.
Published: March 2009

By means of a kinetic description for ions and Braginskii's fluid model for electrons, we derive three coupled nonlinear equations governing the dynamics of low-frequency short-wavelength electrostatic waves in the presence of equilibrium density, temperature and magnetic-field gradients in a two-component magnetized plasma. In the linear limit we present a dispersion relation that admits new instabilities of drift waves. An estimate of the anomalous electron energy transport due to non-thermal drift waves is obtained by making use of the saturated wave potential, which is deduced from the mixing-length hypothesis. We also present stationary solutions of the nonlinear equations governing the interaction of linearly unstable drift waves. The relevance of our investigation to wave phenomena in space and laboratory plasmas is pointed out.
Monte Carlo calculations for transport due to MHD modes
Authors: Punjabi Alkesh , Boozer Allen , Lam Maria , Kim Myung-Hee , Burke Kathy
Published: March 2009

The three basic mechanisms that produce either classical or anomalous transport are spatial variation of magnetic field strength, spatial variation of electrostatic potential in magnetic surfaces, and loss of magnetic surfaces. A Monte Carlo code is written to study transport due to these three mechanisms interacting with collisional effects. The equations of motion are obtained from the canonical drift Hamiltonian, but non-canonical co-ordinates are used to simplify the integrations. The code is applied to the reversed-field-pinch ZT-40 and the Tokapole II. For ZT-40 the Bessel-function model is used to represent the magnetic field geometry. The effects of pitch-angle scattering, loop voltage and the break-up of magnetic surfaces resulting from resistive MHD perturbations on the drift particle trajectories are illustrated. The particle diffusion coefficients are obtained for varying amplitudes of resistive MHD perturbations. For Tokapole II the spectrum of both the ideal and resistive MHD perturbations is constructed from the experimental data. The drift trajectories for trapped and passing electrons in the presence of such perturbations are obtained. The particle diffusion coefficients for the neo-classical regime in Tokapole II are obtained for varying collision frequency. By comparing the transport coefficients for various groups of particles with the experimental data, we hope to obtain far more information on the transport mechanisms than can be obtained by the standard confinement time measurements. The various groups of particles that can be studied using the code include runaway electrons, thermal electrons, and both passing and trapped diagnostic beam ions.
Transformation approximation method for an electromagnetic ion-cyclotron instability caused by proton temperature anisotropy
Authors: Higuchi Y.
Published: March 2009

The transformation approximation for the plasma dispersion fonction is applied to an electromagnetic ion-cyclotron instability caused by proton temperature anisotropy. The transformation method gives an improved dispersion relation and instability growth rate compared with the asymptotic expansion for the plasma dispersion fonction. It is found that the maximum growth rate is slightly suppressed when the transformation approximation for the plasma dispersion function is used. However, it is shown that the transformation approximation method yields an unreliable estimate of the growth rate for values greater than a critical thermal anisotropy. Cold-heavy-ion effects on the ion-cyclotron instability are also investigated.
Kinetic effects on the ideal Alfvén continuum
Authors: Åkerstedt Hans O.
Published: March 2009

Kinetic effects such as those of finite Larmor radius (FLR) and. wave–particle resonances on the ideal Alfvén-wave continuum in a z-pinch geometry are investigated. The effect of FLR increases the order of the equations and is therefore treated as a singular perturbation. Similar eigenvalue curves are found as for the case when the non-ideal effect is resistivity, with a bifurcation of a global-mode branch into a boundary-mode branch and an internal-mode branch.
Waves in a paramagnetic plasma
Authors: Bantikassegn W. , Mal'nev V. N.
Published: March 2009

The permittivity tensor Є¸(K, ω) of an electron–ion plasma with paramagnetic atoms as neutral component (a paramagnetic plasma) is obtained. Based on this tensor, new types of slow electromagnetic waves in the paramagnetic plasma are predicted.
Stability analysis of sheared non-neutral relativistic cylindrical electron beams in applied magnetic fields
Authors: Zoler D. , Cuperman S.
Published: March 2009

A self-consistent stability analysis of relativistic non-neutral cylindrical electron flows propagating along applied magnetic fields is considered within the framework of the macroscopic cold-fluid-Maxwell equations. The full influence of the equilibrium self-electric and self-magnetic fields is retained. Then the E x B drift (E being the radial electric field created by the uncompensated charge) generates a radial shear, vz(r) and v0(r). The effect of the shear in the axial velocity component, as reflected in the relative axial motion of adjacent concentric layers of beam particles, is investigated. The self-consistent treatment of the problem thus shows that the equilibrium state considered in this paper is unstable.
Kinetic theory of Alfvén waves in plasmas with force-free currents
Authors: Donnelly I. J. , Clancy B. E.
Published: March 2009

Equations are derived for the kinetic-theory analysis of small-amplitude Alfvén waves in cylindrical plasmas carrying force-free currents. The equations, which include ion Larmor-radius effects to second order, are applicable to reversed-field pinches as well as to tokamaks. Fourier mode amplitudes are derived for model antennas with radial current feeds, and a quantitative analysis is made of the antenna resistance and the wave density fields in a small tokamak during Alfvén-wave heating. The effect of the plasma current on the wave thermal energy flux is discussed.
Experiments on the nonlinear evolution of surface waves in an open plasma waveguide
Authors: Grozev D. , Shivarova A. , Tanev S.
Published: March 2009

The nonlinear behaviour of surface waves propagating along a plasma column is investigated. The experiments include (i) measurements of the nonlinear dispersion of the waves and (ii) observation of the evolution of the shape of the wave envelope and of the wave spectrum when the waves are excited with an amplitude-modulated signal. Comparison with theory shows that (i) the measured nonlinear wavelength shift corresponds to increasing wave phase velocity with increasing wave amplitude, (ii) the observed narrowing of the wave envelope can be associated with the formation of periodic nonlinear waves described by dn Jacobi elliptic function solutions of the nonlinear Schrödinger equation and (iii) the observed strong changes of the wave spectrum are mainly related to a nonlinear effect of phase modulation.
Arbitrary-amplitude electron-acoustic solitons in a two-electron-component plasma
Authors: Mace R. L. , Baboolal S. , Bharuthram R. , Hellberg M. A.
Published: March 2009

Motivated by plasma and wave measurements in the cusp auroral region, we have investigated electron-acoustic solitons in a plasma consisting of fluid ions, a cool fluid electron and a hot Boltzmann electron component. A recently described method of integrating the full nonlinear fluid equations as an initial-value problem is used to construct electron-acoustic solitons of arbitrary amplitude. Using the reductive perturbation technique, a Korteweg-de Vries equation, which includes the effects of finite cool-electron and ion temperatures, is derived, and results are compared with the full theory. Both theories admit rarefactive soliton solutions only. The solitons are found to propagate at speeds greater than the electron sound speed (ε0c/ε0ε)½υε, and their profiles are independent of ion parameters. It is found that the KdV theory is not a good approximation for intermediate-strength solitons. Nor does it exhibit the fact that the cool- to hot-electron temperature ratio restricts the parameter range over which electron-acoustic solitons may exist, as found in the arbitrary-amplitude calculations.
Wave collapse in the lower part of the ionosphere
Authors: Stenflo L.
Published: March 2009

Equations are presented for the coupling between high- and low-frequency waves in a collisional plasma that models the ionospheric E region. New collapsing solutions can be found from these equations, even for frequencies smaller than the electron–neutral and ion–neutral collision frequencies.