bluuebunny/crossref_metadata_2025 huggingface dataset:
Forced reconnexion by fast magnetosonic waves in a current sheet with stagnation-point flows
Authors: Sakai Jun-Ichi
Published: March 2009

Forced reconnexion due to tearing modes driven by fast magnetosonic waves in a current sheet with stagnation-point flows is discussed. The current sheet with stagnation-point flows which is weakly unstable against tearing modes can be strongly destabilized by vortex motions due to the ponderomotive force of the fast magnetosonic wave. This forced tearing instability can be driven when the incident fast magnetosonic wave intensity, I, exceeds a critical value given by where is the Alfvén velocity, vg the group velocity of the fast wave, vo the background inflow velocity, l the thickness of the current sheet and k the wavenumber of the forced tearing mode. The growth rate is estimated. Applications to solar flares and magnetopause reconnexion processes are briefly discussed.
Transport by weak electrostatic density drift fluctuations
Authors: Sanderson J. J. , Gary S. Peter
Published: March 2009

Expressions are derived for transport by weak electrostatic microfluctuations driven unstable by a density gradient perpendicular to a uniform magnetic field in a Vlasov plasma. These expressions, which are evaluated for the universal and lower-hybrid density drift instabilities, provide improved physical interpretations for various wave-particle exchange frequencies previously reported. It is demonstrated that wave-particle effects generally reduce the free energy driving these instabilities, and that some forms of steady-state distribution functions are inappropriate for nonlinear theory.
Linear conversion in a magnetized plasma with density gradient parallel to the magnetic field
Authors: Mjølhus E.
Published: March 2009

The problem of linear conversion of an ordinary polarized electromagnetic wave in a magnetized plasma with density gradient parallel to the magnetic field is considered. An expression for the conversion coefficient as a function of angle of incidence, WKB parameter and magnetic field is obtained. The magnetic field leads to a narrowing of the range of angles of incidence leading to linear conversion, compared with the unmagnetized case.
Nonlinear variations of the reflexion coefficient for lower-hybrid heating
Authors: Petržílka V. A. , Klíma R. , Pavlo P.
Published: March 2009

The effect of the ponderomotive depression of the plasma density on the power reflexion coefficient is calculated in the first space harmonic approximation. Numerical results demonstrate a considerable growth of the reflexion coefficient when increasing the RF power. Exceptionally, the reflexion coefficient can be almost invariable up to high RF power densities (≃ 10kW cm-2), if the unperturbed plasma density gradient is high (≳ 5 × 1012 cm-4). The relation of the present theory to experiments is discussed.
Excitation of upper-hybrid waves by a thermal parametric instability
Authors: Lee M. C. , Kuo S. P.
Published: March 2009

A purely growing instability characterized by a four-wave interaction has been analysed in a uniform, magnetized plasma. Up-shifted and down-shifted upper-hybrid waves and a non-oscillatory mode can be excited by a pump wave of ordinary rather than extraordinary polarization in the case of ionospheric heating. The differential Ohmic heating force dominates over the ponderomotive force as the wave–wave coupling mechanism. The beating current at zero frequency produces a significant stabilizing effect on the excitation of short-scale modes by counterbalancing the destabilizing effect of the differential Ohmic heating. The effect of ionospheric inhomogeneity is estimated, showing a tendency to raise the thresholds of the instability. When applied to ionospheric heating experiments, the present theory can explain the excitation of field-aligned plasma lines and ionospheric irregularities with a continuous spectrum ranging from metre-scale to hundreds of metre-scale. Further, the proposed mechanism may become a competitive process to the parametric decay instability and be responsible for the overshoot phenomena of the plasma line enhancement at Arecibo.
Theory of weakly nonlinear VLF interactions
Authors: Murdoch N.
Published: March 2009

A mathematical description of the weakly nonlinear regime of the coherent cyclotron resonance instability in the magnetosphere which has hitherto been neglected in the literature has been developed. Using the concept that the fundamental role played by the inhomogeneity of the Earth's magnetic field is to limit the interaction time between the resonance electrons and whistler mode wave, perturbation expansions of the current equations to third order have been obtained. It is then found that the real component of resonance current can qualitatively account for excess growth found in some magnetospheric transmissions while the imaginary component gives a good quantitative estimate of frequency shifts observed in transmissions of the mains harmonics.
Coupling to Z mode near critical angle
Authors: Mjølhus E.
Published: March 2009

The transmission and reflexion coefficients are calculated for the coupling of an ordinary polarized wave to the Z mode when the angle of incidence is near the critical angle.
Ion temperatures inferred from neutron yield measurements in the TRISOPS IIX plasma vortex structure generator
Authors: Wells D. R. , Ziajka P. , Tunstall J.
Published: March 2009

The ion temperature history of the plasma fireball in TRISOPS IIX has been measured by comparing the time-resolved neutron yield from a D–D reaction and a D–T reaction. This measurement results in a reading of ion temperature that is independent of any assumptions of plasma density or fireball volume. The results verify and clarify previously reported TRISOPS parameters. In a separate set of measurements using a D–D reaction, the ion temperature was determined by comparisons of Doppler broadening and neutron yield. It was found that the ion temperature rises in 5 μs to 6 keV with a corresponding density of 1015 ions cm3. After 8 μs it falls rapidly to 1 keV as the density rises to 1017 ions cm–3. The 1 keV temperature is held for about 20 μs. These temperatures and densities yield an equilibration time for ions and electrons that is long compared with the duration of the experiment.
The effect of fast-time-scale turbulence on magnetohydrodynamical behaviour
Authors: Dendy R. O. , Haar D. Ter
Published: March 2009

We show what corrections have to be made to the equations of ideal magneto-hydrodynamics when there is fast-time-scale turbulence present in a magnetized plasma. We show how the dispersion relations for the ideal Alfvén and magnetoacoustic MHD normal modes are modified when such turbulence is present. Finally, we discuss the relation of our work to that of other authors.
Nonlinear dispersion of surface waves in a plasma column
Authors: Grozev D. , Shivarova A.
Published: March 2009

The effect of the nonlinear changes of the dispersion characteristics of highfrequency azimuthally symmetric surface waves in a plasma column is investigated theoretically. Both (ω + ω) – ω and (ω – ω) + ω nonlinear interactions of the third order (in relation to the fundamental wave amplitude) are considered here. These two types of interactions influence the wave phase velocity and the collisional damping rate in opposite ways. When ω2Pi/v2 < 1, the contribution of the (ω – ω) – ω interaction is negligible and the (ω + ω) – ω interaction results in an increase of the phase velocity and a decrease of the time damping rate. When ω2pi/v2 > 1, both interactions are involved; the (ω – ω) + ω interaction associated with the ponderomotive force becomes more important, decreasing the phase velocity and increasing the time damping rate (ωpi and v are ion plasma and electron-neutral collision frequencies, respectively).
Pseudo-three-dimensional convective cell motion in a magnetized plasma
Authors: Shukla P. K. , Yu M. Y.
Published: March 2009

Linear and nonlinear mechanisms for generating convective cells with finite but small parallel (to the external magnetic field B0) wavelength are presented. The problems of mode-coupling as well as quasi-steady nonlinear mode structures are analytically studied. Possible applications in space plasmas are discussed.
Distribution function and diffusion of α-particles in DT fusion plasma
Authors: Liberman M. A. , Velikovich A. L.
Published: March 2009

In the Fokker-Planck approximation, a general solution of the kinetic equation for the distribution function of non-thermal α-particles in spatially uniform DT plasma is obtained. The previously published solutions are then derived as special cases. Transport equations describing diffusion of non-thermal α-particles are obtained and diffusion coefficients are calculated. To compare the hydro-dynamic and kinetic descriptions of non-thermal α-particle energy transport, the critical ignition length of the overheating instability in DT fusion plasma, determined by non-local plasma heating due to α-particle diffusion, is calculated using both approaches. The two results are in good agreement.
Temporal Alfvén wave echoes in an inhomogeneous plasma
Authors: Rae I. C.
Published: March 2009

If an external current pulse is applied to a diffuse plasma sheet pinch, surface wave modes are generated, which decay by collisionless damping, leaving only oscillations of the Alfvén continuum along the Alfvén resonance surface. The transverse perturbations within this surface phase-mix to zero. It is shown that perturbations induced by an initial pulse are modulated by a (later applied) second pulse of different wavelength, to yield non-vanishing second-order transverse perturbations, even though the first-order transverse perturbations have phase-mixed to zero. This analysis shows the importance of nonlinear effects in the evolution of inhomogeneous magnetohydrodynamic motions.
A general method for calculating the self-induced magnetic force of an axisymmetric toroidal current
Authors: Bobbio S. , Rubinacci G.
Published: March 2009

A method is presented for computing the class of axisymmetric current distributions flowing in a torus whose peripheral surface is a flux surface for the magnetic field produced by the current itself. The method allows the correct calculation of the ‘self-induced’ magnetic forces arising from the interaction between these currents and their own field. The general expression for the self-induced force is given and an approximate formula is presented in the large aspect-ratio limit.
Nonlinear generation of electromagnetic waves in a magnetoplasma
Authors: Murtaza G. , Shukla P. K.
Published: March 2009

It is shown that a finite-amplitude upper-hybrid wave can decay into a perpendicularly (to an external magnetic field B0zˇ) propagating electrostatic lower-hybrid/ion-cyclotron wave and a left-handed circularly polarized (LHCP) electromagnetic wave (along B0) or an ordinary mode radiation (across B0). Nonlinear dispersion relations and the growth rates are obtained. It is suggested that the present nonlinear instability can be responsible for the enhanced narrow band radiation which is frequently observed in the Earth's magnetosphere.
Some properties of solitary BGK waves in a collisionless plasma
Authors: Shah H. A. , Turikov V. A.
Published: March 2009

Following the method developed by Bernstein, Greene & Kruskal we obtain expressions for the distribution function of the trapped particles for a collisionless plasma, for cases when the plasma is unbounded and bounded (in a cylindrical waveguide). Figures are drawn showing the relationship between the width and amplitude of the solitary BGK wave for various cases. Analytic expressions depicting this relationship are also derived.
On whistler-mode trapping in the magnetospheric ducts
Authors: Sazhin S. S.
Published: March 2009

Conditions for whistler-mode trapping in the magnetospheric ducts are considered. The plasma model includes electron temperature and anisotropy, and there are no restrictions on the value of the wave normal angle. It is pointed out that the range of the wave normal angles for which whistler-mode waves can be trapped in the ducts with enhanced electron density increases when the electron temperature and (or) anisotropy increases; the corresponding increase also takes place for the average wave normal angle when whistler-mode waves are trapped in the ducts that have a deficiency in electron density. The limits of applicability of a simplified formula derived by Sazhin & Sazhina, for whistler-mode propagation in a hot anisotropic plasma, are clarified.
Particle losses due to RF-enhanced pitch-angle scattering into velocity space loss regions
Authors: Anderson D. , Lisak M.
Published: March 2009

The application of RF heating (ICRH or LHH) to a fusion plasma creates high-energy tails on the distribution function of the heated particle species. In the presence of loss regions in velocity space, e. g. due to particle drifts in the toroidal magnetic field ripple of a tokamak, the collisional pitch-angle scattering losses may be significantly enhanced. The present work assesses the importance of such RF-enhanced losses and explicit expressions are derived for the evolution of the loss fraction. In particular, for parameters typical of PLT we find that the loss fraction could rapidly reach high values.
Properties of Alfvén Solitons in a Finite-Beta Plasma J. Plasma Phys. vol. 27, 1982, p. 193
Authors: Spangler Steven R. , Sheerin James P.
Published: March 2009

In the aforementioned paper we obtained an equation for non-linear Alfvén waves in a finite-β plasma, and investigated envelope soliton solutions thereof. The purpose of this note is to point out an error in the derivation of the soliton envelopes, and present corrected expressions for these solitons.

The error arises from our assumption of translational invariance of both the envelope and phase of an envelope soliton expressed in equations (16) and (17). Rather, the phase is related to the amplitude by

where y ≡ x — VEt is a comoving co-ordinate, and all other quantities are defined in the above paper.
Nonlinear evolution of double layers and electron vortices in an unstable plasma diode
Authors: Iizuka Satoru , Tanaca Hiroshi
Published: March 2009

Nonlinear evolution of a large-amplitude relaxation oscillation in an unstable plasma diode is investigated by a computer simulation using a particle model. The oscillation is closely related to double-layer dynamics accompanied by a negative potential dip on the low-potential tail. A vortex is formed in the electron velocity phase space during the potential evolution, which causes a topological rearrangement of the phase space in the high-potential plasma.
Oscillation-centre fluid theory of the ponderomotive force
Authors: Kentwell G. W. , Jones D. A.
Published: March 2009

A new fluid-theory derivation of the time-dependent ponderomotive force is presented in the oscillation-centre representation. We show that the velocity renormalization adopted in this paper is equivalent to the oscillation-centre representation and is the velocity required so that the mean density obeys the continuity equation exactly. We also show that previous fluid theory results may be recovered by adopting a different velocity renormalization and by redefining the ponderomotive force.
A study of plasma expansion phenomena in laboratory generated plasma wakes: preliminary results
Authors: Wright K. H. , Stone N. H. , Samir U.
Published: March 2009

The plasma expansion into the wake of a large rectangular plate immersed in a single-ion, collisionless, streaming plasma has been investigated in the laboratory. Several characteristics of the process involved in ‘plasma expansion into vacuum’ that have been predicted theoretically were observed, including the creation and motion of a rarefaction wave disturbance; the creation and motion of an expansion front; and the acceleration of ions into the wake at speeds above the ion-acoustic speed. The expansion was limited to early times; i.e. a few ion plasma periods, by the combination of plasma drift speed and vacuum chamber size. This prevented detailed comparison with self-similar theory, but results are in good agreement with numerical simulations and other laboratory experiments for the early time expansion. The conclusion is that the plasma expansion process is the dominant wake filling mechanism in the near wake of a body, whose potential is approximately the plasma space potential.
Modulational instability of obliquely modulated ionacoustic waves in a two-electron-temperature plasma
Authors: Yashvir  , Bhatnagar T. N. , Sharma S. R.
Published: March 2009

The unstable domain in the (k, Ø) plane for oblique modulation of ion-acoustic waves, in a two-electron-temperature plasma, is investigated using the KBM perturbation technique. It is shown that, in a collisionless plasma, the maximum growth rate for the modulational instability, for large carrier-wave amplitudes (a0 ≳ 0·1), exceeds the electron Landau damping rate for sufficiently oblique modulation.
Stability of strong electromagnetic waves in overdense plasmas against long-wavelength perturbations
Authors: Romeiras F. J. , Rowlands G.
Published: March 2009

We consider the stability against long-wavelength small parallel perturbations of a class of exact standing wave solutions of the equations that describe an unmagnetized relativistic overdense cold electron plasma. The main feature of these nonlinear waves is a circularly polarized transverse component of the electric field periodically modulated in the longitudinal direction. Using an analytical method developed by Rowlands we obtain a dispersion relation valid for long-wavelength perturbations. This dispersion relation is a biquadratic equation in the phase velocity of the perturbations whose coefficients are very complicated functions of the two parameters used to define the nonlinear waves: the normalized ion density and a quantity related to the modulation depth. This dispersion relation is discussed for the whole range of the two parameters revealing, in particular, the existence of a region in parameter space where the nonlinear waves are stable.
Kinetic theory of stimulated Raman scattering from a magnetized plasma
Authors: Boyd T. J. M. , Rankin R.
Published: March 2009

Finite Larmor radius corrections have been considered in the effects of strong magnetic fields on stimulated Raman scattering. A nonlinear dispersion relation describing the various channels of decay has been derived from the Vlasov-Maxwell equations and frequencies and growth rates determined for the decay of incident laser light in the extraordinary mode into scattered extraordinary mode radiation and electron Bernstein waves. A relativistic one-and-a-half dimensional particle code has been used to simulate the scattering process and the results from the numerical experiments have been compared with those obtained analytically, the agreement being generally good. Growth rates of the Bernstein waves are substantial when sufficiently strong magnetic fields are present in hot plasmas. Under these conditions the kinetic analysis shows that, in contrast to the predictions of fluid theory, the scattered light emitted from densities well below the quarter-critical layer can have a frequency less than ½ω0 where ω0 is the laser frequency. In an unmagnetized plasma this occurs only when the plasma has a finite temperature.
Resonance cone structure in a warm inhomogeneous bounded plasma with lower-hybrid resonance layers
Authors: Grabbe Crockett L.
Published: March 2009

The problem of the wave fields excited by a gap source at the edge of an inhomogeneous, magnetized plasma with a pair of lower-hybrid resonance layers present and bounded by conducting walls is solved. The approach used is that of a solution as a sum of multiply-reflected extraordinary mode and ion-thermal resonance cones as an alternative to the guided-wave mode approach. This is achieved by dividing up the plasma into cross-sections where WKB solutions are valid, and into lower-hybrid resonance layers where asymptotic methods are employed and connexion coefficients between each region are obtained. A diagrammatic scheme for writing the solution is introduced which can in principle be used to write down the solution for any problem of this general type once the connexion coefficients across a resonance layer have been determined via asymptotic analysis. This allows a determination in great detail of the structure and properties of the resonance cones in our model and the way they transform across the back-to-back hybrid layers. Evanescent resonance cones are found to exist in the high-density region between the hybrid resonance layers and tunnel through to the other side, maintaining their general structure if the layer is relatively thin.
Plasmoid motion across a tangential discontinuity (with application to the magnetopause)
Authors: Lemaire J.
Published: March 2009

The motion of a plasmoid (plasma-field entity) across an inhomogeneous magnetic field distribution of which the direction and strength change along the penetration trajectory has been studied. The bulk velocity decreases when the plasma element penetrates into a region of increasing magnetic field. The critical magnetic field intensity where a plasmoid is stopped or deflected is found to be the same critical field as that which has been observed in laboratory experiments for a non-rotating B-field distribution. The polarization electric field induced inside a moving plasma element has been determined for both low-β and high-β plasmoids. The momentum density vector of a plasmoid is deflected in the – B × ∇B and – B × (B. ∇)B directions as it penetrates into an inhomogeneous B-field distribution. This kinetic model has been applied to the impulsive penetration of solar wind plasma irregularities impinging on the earth's geomagnetic field with an excess momentum density. As a consequence of impulsive penetration, a plasma boundary layer is formed where the intruding plasmoids are deflected eastward. Magnetospheric plasma is dragged in the direction parallel to the flanks of the average magnetopause surface. Diamagnetic effects of these impulsively penetrating plasmoids into the magnetosphere are also briefly discussed.
Turbulent diffusion in two-dimensional, strongly magnetized plasmas
Authors: Pécseli H. L. , Mikkelsen T.
Published: March 2009

Particle diffusion is investigated in a strictly two-dimensional collisionless guiding-centre model for a strongly magnetized plasma. An analytical expression is presented for the entire time variation of the mean square test-particle displacement in the limit of low-frequency, strongly turbulent, electric field fluctuations. The analysis relies on an explicit integral expression for the Lagrangian autocorrelation function in terms of the Eulerian wavenumber spectrum and a time-varying weight function. Bohm diffusion is discussed by means of a simple model spectrum. The analysis applies for turbulent transport associated with electrostatic convective cells, magnetostatic cells and drift wave turbulence with the assumption of local homogeneity and isotropy in two dimensions.
Electromagnetic solitary waves in magnetized plasmas
Authors: Hazeltine R. D. , Holm D. D. , Morrison P. J.
Published: March 2009

A Hamiltonian formulation, in terms of a non-canonical Poisson bracket, is presented for a nonlinear fluid system that includes reduced magnetohydro-dynamics and the Hasegawa–Mima equation as limiting cases. The single-helicity and axisymmetric versions possess three nonlinear Casimir invariants, from which a generalized potential can be constructed. Variation of the generalized potential yields a description of exact nonlinear stationary states. The new equilibria, allowing for plasma flow as well as partial electron adiabaticity, are distinct from those found in conventional magnetohydrodynamic theory. They differ from electrostatic stationary states in containing plasma current and magnetic field excitation. One class of steady-state solutions is shown to provide a simple electromagnetic generalization of drift-solitary waves.
Low- and medium-energy kinetic theory of a particle-beam generated plasma
Authors: Peyraud N.
Published: March 2009

The electron distribution fonction in a particle-beam generated plasma is derived in its low- and medium-energy portion (under the first excitation threshold) from a Boltzmann equation, which contains elastic and inelastic collisional processes and a continuous source term. This equation is analytically solved and we discuss the importance of the non-Maxwellian deviations and their consequences for the determination of macroscopic values such as electron density or ‘temperature’. A numerical discussion is included for the case of an argon plasma created by a high-energy electron beam between 103and 106eV.
Three-wave interaction in a cold plasma column
Authors: Lindgren T. , Stenflo L. , Kostov N. , Zhelyazkov I.
Published: March 2009

We derive the coupled-mode equations for the non-resonant interaction of three high-frequency surface waves propagating along a cold plasma column with sharp movable boundary. The plasma is then supposed to be surrounded by vacuum. Our boundary conditions take into account the effects of the presence of surface charges, as well as the movements of the boundary. The coupling coefficients are expressed in explicit forms and compared with previous results.
On beam-generated electrostatic electron waves in magnetized warm plasma
Authors: Hultqvist Bengt
Published: March 2009

The linear dispersion relation for electrostatic electron waves is analysed in some detail. A method is described which allows the dispersion equation to be solved and the attenuation rate/growth rate to be derived for waves generated by electrons flowing through a stationary plasma along the field lines of a static magnetic field. The solution is derived from three simple curves (one for each of the real and imaginary parts of the dispersion equation and one for the growth rate) and is applicable for any propagation angle relative to the magnetic field lines (below a limiting value), for any wavelength, any magnetic field intensity, any electron density and temperature and any flow speed of a beam plasma component. The effects of cyclotron resonance start to be significant at an angle from the direction of the magnetostatic field lines that depends on most of the variables mentioned, but only in one specific combination, which is the measure of the ratio between thermal velocity of the electrons and the phase velocity of the wave at the cyclotron frequency. In contrast to numerical methods, the method described here provides a parameterization of the solutions to the dispersion equation and of the growth rate. It gives an overview of the dependencies on the variables in the equation over a large part of parameter space without any other computations than those involved in adjusting scale factors.
Quasi-linear theory of the loss-cone instability
Authors: Galeev A. A.
Published: March 2009

The loss-cone instability of plasma confined in a mirror-type trap is considered. Relaxation of the particle distribution in the trap with a length larger than the mean free path between ‘turbulent’ collisions is described by conventional quasilinear theory. A quasi-linear equation for the ion distribution is solved analytically for the case of a small loss-cone volume of particles in velocity space (it takes place, for instance, in the case of a trap with a larger mirror ratio).
Combined Taylor and Helmholtz instability in hydromagnetics including Hall effect
Authors: Talwar S. P. , Kalra G. L.
Published: March 2009

The problem of combined Rayleigh—Taylor and Kelvin—Helmholtz instability for incompressible plasmas carrying a uniform magnetic field is investigated taking account of the Hall current. The resistivity and the finite ion Larmor radius effects are left out. It is found that the finite Larmor frequency is destabilizing in nature. The growth rate of an otherwise (without the Hall current) unstable configuration is increased, and unstable modes may be produced in otherwise stable situations for reasonably large values of the Hall current. The Hall effect also results in overstable modes.
Radiative behaviour of a shock-heated argon plasma flow
Authors: Horn K. P. , Wong H. , Bershader D.
Published: March 2009

Experimental and theoretical studies were made of the decrease in the degree of ionization of shock-heated agron resulting from radiative emission. the studies were based on measurment of the time history of the electron density by means of optical interferometry used in conjunction with a rotating mirror camera. These measurments apply to the quasi-euilibrium region which follows the short electronic relaxation zone behind strong shocks. Loss of energy by rediation led to cooling of the gas as evidenced by the decreasing electron density. The analytical studies were based on the assumption that the dense plasma readiated throught both continuu, emission and emission from spectral lines. Comparison was made with experimental results, and theory and experiment were in good agreement, for a realistic choice of the parameters contained in the continuum theory. The analysis indicated that line radiation losses could not be disregarded. Sizeable loss resulted from lower level bound-bound transitions even though these lines experienced absorption.
A single-fluid model for shock formation in MHD shock tubes
Authors: Hoffman Alan L.
Published: March 2009

A simple single-fluid model is presented which accounts for the behaviour of MHD shock producing devices in which the shock and the driving current sheet are unseparated. By means of both characteristic and similarity solutions, it is shown that unusual plasma behaviour is a simple consequence of the fluid equations of motion. The important parameters in determining the performance of MilD shock tubes are the width of the current sheet at breakdown (related to the magnetic Reynolds number) and the current sheet Mach number with respect to the gas at rest. It is shown that, under many conditions, separation is more likely to be achieved with a decrease, rather than an increase, in current sheet speed. Lowering the Mach number is more influential in causing the shock to form at the front of the current sheet than raising the magnetic Reynolds number.
On ion—electron beams emitted by a plane
Authors: Winograd Yeshaiahu Y.
Published: March 2009

The non-linear one-dimensional steady-state equations which govern the flow of an ion—electron beam emitted from a plane are solved in the phase plane, and it is shown that a perfectly neutralized beam follows for a large range of injection velocities of the electrons. When the velocity of the ions is less than the electron sound speed the transition region for the neutralization has a length of the order of a Debye length λD = (kT)½ (4πNe2)–½, which is a typical plasma sheath. The maximum velocity of injection of the electrons for which neutralization is predicted is, in this case, the sound speed of the electrons. If the electrons are injected with a supersonic speed, they cannot be decelerated continuously to the subsonic speed corresponding to the velocity of the ions. No bound is set on the electron injection velocity from below. When the velocity of the ion beam is greater than the electron sound speed, oscillations with an amplitude which depends on the velocity of injection of the electrons, and a wavelength which depends on the ratio of the ion velocity to the electron speed of sound, are found. In this case the injection speed of the electrons needed to obtain the steady-state oscillatory solution is bounded both from above and from below. Subsonic electrons cannot be accelerated continuously to the supersonic velocity required to match the velocity of the ions, and within the supersonic range there is shown to be a limit (depending on the ratio of the ion velocity to the electron speed of sound, so that faster electrons cannot be decelerated continuously to match the (supersonic) ion velocity.
Model Fokker—Planck Equations: Part 3. Application to transport phenomena
Authors: Dougherty J. P. , Watson S. R. , Hellberg M. A.
Published: March 2009

The Chapman—Enskog expansion is applied to the model Fokker—Planck equation for a plasma, derived in part 2. It is shown that the complete set of transport coefficients can be calculated without further approximations. Results are derived first in the absence of any external magnetic field. The transport coefficients are also derived when there is a strong magnetic field, in which case they become anisotropic.
Propagation of hydromagnetic waves through an anisotropic plasma
Authors: Abraham-Shrauner Barbara
Published: March 2009

The propagation of small amplitude hydromagnetic waves of a collisionless, anisotropic plasma in a strong magnetic field is treated using the Chew—Goldberger—Low (CGL) fluid equations. The Alfvén, fast and slow magnetoacoustic, and entropy modes are analysed by phase speed and wave front diagrams and compared to their counterparts in magnetohydrodynamics (MHD) where collisions maintain an isotropic pressure. Two main classes of CGL waves are identified, pseudo-MHD and reverse-MHD where the latter has the slow wave speed greater than the Alfvén wave speed. Significant differences of the CGL hydromagnetic waves as compared to MHD waves are found which have special relevance for a study of infinitesimal and finite discontinuities in space plasmas.
Shock jump conditions for an anisotropic plasma
Authors: Abraham-Shrauner Barbara
Published: March 2009

The jump conditions for finite discontinuities in a collisionless plasma are derived in the Chew—Goldberger—Low approximation.
A continuum theory of electrostatic probes in a slightly ionized gas
Authors: Toba K. , Sayano S.
Published: March 2009

A systematic continuum theory based on the method of matched asymptotic expansions is developed to deal with electrostatic probes in a slightly ionized gas. To the first approximation of thin space charge sheath, the sheath solution is shown to become common to the geometries considered, i.e. plane, cylindrically and spherically symmetric probe surfaces: the main interest lies in the last geometry. The second-order approximation strongly affects the last two geometries. Detailed numerical calculations are presented to illustrate the effects of a finite ratio of the Debye length to the characteristic body dimension; including a contraction of the sheath region and enhancement of the electric field from the asymptotic (first-order approximation) values.
Stability of magnetohydrodynamic shock waves
Authors: Lessen Martin , Deshpande Narayan V.
Published: March 2009

The stability of oblique magnetohycirodynamic shock waves is studied with respect to a disturbance that excites magneto-acoustic waves. The problem is solved numerically by the normal mode analysis and it is shown that slow shocks are unstable in the sense that the disturbance grows exponentially with time. Growth rates are calculated for a particular Mach number and for different values of the magnetic field and obliqueness. The fast shock appears to be stable.
Time-dependent motions of the cavity formed when a uniform corpuscular flux is incident on the magnetic field of a line current
Authors: Hewson-Browne R. C. , Burghes D. N.
Published: March 2009

This paper concerns the time-dependent motions of the cavity formed when a uniform corpuscular flux is incident on the magnetic field of a line current. The two-dimensional problem is formulated and solved with two particular classes of solutions being given; namely, the steady-state solutions and the time-history of the interaction with a cloud of flux.
A note on the two-stream instability
Authors: Clemmow P. C.
Published: March 2009

Conditions for the instability of longitudinal waves in a single-component two-stream plasma, in which the streams have the same velocity spread, are given in the form of simple relations between the density ratio and the mean velocity difference of the streams.
Radio frequency properties of a plane grid capacitor immersed in a hot collision-free plasma
Authors: Buckley R.
Published: March 2009

A pair of plane parallel grids is inserted in a hot plasma, and an oscillatory voltage is applied across them. The electric field excited in the plasma, and the complex admittance of the grid/plasma system, are computed for applied frequencies high enough to justify neglect of ion response. The grids are electrically, but not mechanically, coupled to the plasma, which is assumed to be in a spatially uniform collision free Maxwellian equilibrium state. The field is computed as a function of distance from the grid plates over a range of frequencies covering the plasma frequency, and the complex admittance of the system is computed as a function of frequency at various grid separation distances. The real component of the field and the capacitive component of the admittance are subject to three major effects: cold plasma dielectric behaviour, oscillatory Debye sheaths on the grids, and (above the plasma frequency), longitudinal plasma waves. The imaginary field component and the conductive admittance component are produced by spatial Landau damping. In an accompanying paper (Freeston 1968), the computed admittance is compared with laboratory measurements made in a situation approximating well to the idealized problem considered here.
Reflection of waves at an interface in a streaming plasma
Authors: Clemmow P. C. , Ott E.
Published: March 2009

A uniform, unbounded, cold plasma stream, with relativistic velocity (0,U, 0), traverses a vacuum half-spacey> 0 and a dielectric half-spacey< 0. A plane electromagnetic wave in the plasma stream iny> 0 is incident obliquely on the face of the dielectric. The criterion for determining which characteristic waves are present in each half space is discussed, and it is shown that, for the polarization in which B is parallel toy= 0, there is one reflected wave and three transmitted waves. One of the latter can be an amplifying wave. A first approximation to the transmission and reflexion coefficients is found in the case when the density of the stream is low.
Effects of inelastic collisions in low-temperature, diffusion-dominated discharges
Authors: Crescentini A. Airoldi , Maroli C.
Published: March 2009

A numerical iteration procedure has been used to obtain the solution of an integrodifferential (linear) equation which gives the energy distribution of the electrons of a low-temperature, diffusion-dominated discharge. The equation takes explicitly into account the effects of inelastic electron-neutral collisions. The results presented here have been obtained, as an instance, for helium and neon discharges, in presence of a static and uniform magnetic field.
Propagation of electromagnetic waves normal to a magnetic field in a uniform anisotropic Maxwellian plasma
Authors: Stewart P.
Published: March 2009

Closed forms are found for the dispersion relations describing the propagation through a uniform anisotropic plasma of the three modes of electromagnetic waves whose wave vectors are perpendicular to a steady and uniform magnetic field. Such relations are found to be conveniently expressed in terms of hypergeometric functions of the second order.
A converging, cylindrical, ionizing shock
Authors: Sloan D. M.
Published: March 2009

The investigation deals with a collapsing, cylindrical, ionizing shock in an applied axial magnetic field. The magnetic field in the nonconducting gas ahead of the shock increases as the shock converges and the consequent increase in the Lorentz force behind the shock produces a retardation. It is shown that no accelerating self-similar solution exists and the governing equations are then solved numerically.
A modified truncation procedure for the BBGKY hierarchy
Authors: Myerscough C. J.
Published: March 2009

The approximations usually made to truncate the BBGKY hierarchy for a plasma are discussed; their failure at small inter-particle separations leads to divergence of the Balescu—Lenard collision integral. A number of authors have obtained convergent kinetic equations, often by rather complicated methods.

It is shown here that, if the standard truncation procedure is modified in a way which makes it less obviously inconsistent for close approaches, the standard methods maybe closely followed in deriving a convergent collision integral which agrees to dominant order with the ‘cutoff’ Balescu—Lenard integral and with the other work on the problem. In fact, the kinetic equation obtained is identical with the Balescu—Lenard equation except that the Coulomb potential is replaced by another that is non-singular at the origin. A physical interpretation of this result is suggested.
Kinetic theory of the stability of collisional plasma in curved magnetic field
Authors: Jungwirth K.
Published: March 2009

The effect of curvature of magnetic field lines on plasma instabilities due to a large gradient of the ion temperature has been studied. It is shown that none of the discussed effects can be obtained so long as the curvature of magnetic field lines is simulated by a fictional gravitation field. In configurations with large enough minimum-B, the usual temperature-gradient instability is stabilized in collisionless plasma. However, a new dissipative instability arises due to ion–ion collisions with the maximum growth rate γmax ˜ Vii/40.
Ion density and current distributions in a propagating current sheet, determined by microwave reflection technique
Authors: Ellis W. R. , Jahn R. G.
Published: March 2009

Electron and ion density distributions and the problem of electron/ion current partitioning are studied in a large radius Z-pinch in argon. Radial scans of the electron density and temperature proffles in the propagating sheet are obtained with a multi-purpose microwave interferometer which measures the complex refiexion coefficient from the plasma. At 70 GHz the probe resolves densities up to 1017 cm−3 over dimensions of ˜ 4 mm. Proffles of the electric and magnetic field components are obtained using standard probe techniques, and these data are used to calculate ion and electron current components in the sheet.

Within the approximations of MHD theory, calculations show that the overall axial current pattern is built up of four interior zones alternately dominated by electron or ion current. This structure is observed to be closely correlated with variations of the Hall parameter, Wb/vc, about the value of unity through the sheet, which suggests that this parameter might be used as a guide to the relative importance of ion conduction in pinch discharge current sheets.
The growth and decay of resonant plasma oscillations excited by a small pulsed dipole
Authors: Fejer J. A. , Yu Wai-Mao
Published: March 2009

The application of integration by the method of stationary phase to resonant oscillations excited by a small pulsed dipole is outlined. Both the growth and the decay of the oscillations near the plasma frequency are determined by this method at a fixed distance from the dipole, first in the absence and then in the presence of an external magnetic field. It is shown that Landau damping must be taken into account in the calculation of the growth but not of the decay. The oscillations are shown to spread out with a speed that is about half the mean thermal speed of electrons.

Only the decay, not the growth, of the oscifiations near harmonics of the cyclotron frequency can be calculated by the same method. It is shown, moreover, that the amplitude, calculated for an observation point that moves away with sateffite velocity in an ionospheric environment, is only valid for time delays longer than about a minute. Such a result is therefore of no practical interest because the resonances observed from sateffites only last a few milliseconds. The erroneous nature of using such a result and the need for a different approach, such as that used in earlier work by the first author, are thus demonstrated.
Possibility of three waves coupling
Authors: Pointu A. M. , Pomeau Y. , Constantin A.
Published: March 2009

In the course of an experimental investigation of the Manley Rowe oscillations, we have been led to define some of the conditions required for an ideal coupling between three travelling waves. It is shown that, because of homogeneity conditions, the only possible pumping wave is the electron cyclotron wave. Experimental values are given to within an order of magnitude for finite geometry.
Higher order resonances in a plasma
Authors: Lewak G. J. , Chen C. S.
Published: March 2009

Resonances of a plasma driven into a weakly non-linear régime are investigated by two different models.

(1) A cold plasma driven by an extended wave source is considered. It is found that resonances can occur at fractions as well as multiples of the normal mode resonances. For the fractional resonances the response amplitude grows linearly with time, whereas for the harmonic resonances it grows exponentially with time.

(2) An unmagnetized Vlasov plasma is excited by a two-grid source below the plasma frequency. A resonance is shown to occur when the driving frequency approaches one half the plasma frequency. It is argued that such non-linear resonances may be responsible for the Alouette II observations of resonances at fractions of the plasma frequency.
A collisionless shock in a plasma which is almost stable
Authors: Smith A.
Published: March 2009

The possibility of a collisionless shock in a plasma not subject to electrostatic instabilities nor immersed in a permanent magnetic field is investigated. The equations governing a steady shock in an electron-ion plasma are deduced and their limitations are discussed. Attention is given to the fact that the velocity distribution in a collisionless plasma may not be Maxwellian. This yields a parameter which is a measure of high-energy particle density. It is found that almost always no steady shock exists. The inverse of the electron plasma wave-number k0=ωp/c provides a scale length for the shock, though the dampening effect of the ions indicates a shock thickness much in excess of . It is concluded that the mechanism for such shocks may be important in a gas such as the solar wind.
The interface of a uniform plasma enveloped by the magnetic field of a system of line currents
Authors: Sozou C.
Published: March 2009

It is shown that complex variable transformations, suitable for obtaining the solution for the field boundary of a system of line currents confined in one cavity by a perfectly conducting uniform plasma, can be used for obtaining the solution to the inverse problem where a perfectly conducting uniform plasma is confined in one cavity by a system of line currents. It is deduced that the minimum number of line currents for confining (not stably) a plasma is two. The equilibrium configurations for several special but simple cases are investigated and discussed.
Plasma containment in a spherical multipole magnetic trap
Authors: Sadowski M.
Published: March 2009

A method of plasma containment by means of spherical multipole (SM) magnetic fields has been developed. The analysis of the confining field geometry has proved that the high-order SM-configurations constitute minimum-B traps with a given number of point cusps instead of linear loss gaps which occur in a standard minimum-B geometry. Injection and trapping of a gun-produced plasma in the SM magnetic trap have been investigated and compared with those in a spindle-cusp trap.
Reflexion of a plane wave at a plasma half-space
Authors: Clemmow P. C. , Karunarathne V. B.
Published: March 2009

The problem of a monochromatic plane electromagnetic wave incident from vacuum on the plane boundary of a plasma half-space is considered. The method of solution is based on the representation of the disturbance in the plasma half- space as that generated by some current source in the complementary half-space into which, for the purpose of the representation, the plasma is conceived to extend. The specular reflexion boundary conditions and the diffuse scattering boundary conditions are treated in turn, first for normal incidence and then for oblique incidence.