bluuebunny/crossref_metadata_2025 huggingface dataset:
Rayleigh Taylor instability of a viscous Hall plasma with magnetic field
Authors: Bhowmik G.
Published: March 2009

The influence of finite Larmor frequency on the stability of a viscous, finitely conducting liquid in a downward gravitational field under the influence of a uniform magnetic field directed along or normal to gravity, is investigated. The solution in each case is shown to be characterized by a variational principle Based on the variational principle, an approximate solution is obtained for the stability of a layer of fluid of constant kinematic viscosity and an exponentia density distribution. It has been found that finite resistivity and finite Larmor frequency do not introduce any instabifity in a potentially stable configuration. However, for a potentially unstable configuration we find that, for an ideal Hal plasma, the results depend on the orientation of the magnetic field, though the instability persists for all wave-numbers in the presence of non-ideal (finite resistivity and viscosity) effects. For the field aligned with gravity, it is found that a potentially unstable field-free configuration is stabilized if the buoyancy number B ( = gβ/12 V2) is less than unity. For B > 1, the instability arises for wave-numbers exceeding a critical value, which decreases on allowing for Hall terms in the generalized Ohm's law, suggesting a destabilizing influence of finite Larmor frequency. For an ambient horizontal magnetic field, it is found that an ideal plasma is stable, even for B > 0, for perturbations confined to a cone about the magnetic field vector. The angle of the cone of stable propagation, however, decreases on account of finite Larmor frequency.
Magnetic structure of ionizing shock waves Part 2. Oblique shocks
Authors: Leonard B. P.
Published: March 2009

Oblique ionizing shock waves are considered as a subclass of the skew shocks discussed in part 1. It is shown explicitly that, for these cases, when the electrical conductivity is a scalar, the magnetic field and velocity vectors lie in a single plane throughout the shock structure. Details of the shock structure are given in the zero-magnetic-Prandtl-number limit for a range of values of the upstream magnetic pressure ratio as the transverse electric field is varied parametrically. Shocks possessing a magnetic structure are necessarily sub-Alfv énic downstream. Thus some structures are trans-Alfv énic (i.e. super-Alfv énic upstream), and others are completely sub-Alfv énic. Unlike the pre-ionized case, the former are stable to rotational Alfv én disturbances because of their ability to form skew shocks. Many features are qualitatively similar to those of skew shocks. For example, very high downstream temperatures may be obtained by Joule heating, and, in some cases, overall expansion shocks may occur. Although magnetic structures can always be found for a range of shock Alfvén numbers from zero up to a value greater than unity, there is an upper limit, above which only the gas shock exists, corresponding to a specific value of the electric field. There is no upper limit on the gas shook speeds. This contrasts with the skew shock case in which the corresponding (saddle-point) solution has an upper Alfvén number limit (slightly above two for the monatomic, infinite-Mach-number case), above which no skew shock solutions exist at all. Finally, the results of previous studies are reviewed in light of the structural requirements of ionizing shock waves.
Effects of parallel waves in homogeneous plasmas in uniform magnetic fields
Authors: Salu Y.
Published: March 2009

We study the effects of circularly polarized plasma waves in anisotropic homogeneous plasmas. The waves propagate in the direction of an external uniform magnetic field. It is shown that in many cases only one mode, with wavenumber k, is present in the plasma. We present a computer model suitable for the study of such cases. The model is used to simulate linearly stable and unstable phenomena. A good agreement with previous predictions of linear theory is obtained. Nonlinear effects are studied and mechanisms that stop instabilities are described.
Numerical studies of magnetic field annihilation
Authors: Stevenson J. C.
Published: March 2009

The behaviour of a plasma permeated by a magnetic field, where the field possessess a hyperbolic neutural point, is considered. Results from numerical solutions of the magnetohydrodynamic formulation for such flows are reported. Problems are posed with the solar flare models of Dungey, Sweet & Petschek in mind. No evidence is found to support the idea that compression of the field lines near a hyperbolic null, in the presence of electrical resistance, can radically alter the geometry of those field lines (e.g. the formation of switch-off shocks). These computations do show that, for large values of the magnetic Reynolds number, a rate of annihilation, more rapid than that derived from order-of-magnitude estimates, is possible.
On some solutions representing a dipole field immersed in an infinite plasma
Authors: Sozou C.
Published: March 2009

It is shown that some recent solutions representing a dipole field immersed in an infinite plasma, which is perfectly conducting, belong to the vorticity equation in steady inviscid hydrodynamics. Further solutions of this equation, referring to the same problem, are constructed and discussed.
Nonlinear motions and trapping of particles in a quasi-monochromatic wave
Authors: Klíma R.
Published: March 2009

An asymptotic solution is given of hte particle motion in an arbitrary quasimonochromatic wave. Zero stationary magnetic field and untrapped particles are considered. The number of adiabatically trapped particles, the total momentum conservation and the space-averaged distribution function of non-resonant particles are derived for the case of an electrostatic wave with time-dependent amplitude.
An improved hierarchy for turbulent magnetized plasma Part 1. Theory
Authors: Linnebur Eldon J. , Kammash Terry
Published: March 2009

The kinetic equations for infinite homogeneous turbulent plasma in a magnetic field are analyzed using a projection operator which allows the time dependence to be maintained in a more exact and consistent manner than has been possible heretofore. By introducing approximations on the multi-time correlation function rather than the fluctuations, as is conventionally done, a hierarchy of equations is obtained which predicts different behaviour for the system especially in connexion with wave-wave interations. These effects are further highlighted by showing how the present results reduce to those obtained by various authors.
Effects of individual linear damping on nonlinear instability
Authors: Weiland Jan
Published: March 2009

A computer investigation has been made for the problem of three coherent waves in a plasma system assuming a nonlinearly unstable situation and mutually different linear damping of the three interacting waves. The results agree satisfactorily with those of an analytic approach to the problem (Wilhelmsson 1970). In the present paper the solution near the explosion time has been further examined.
Excitation of nanosecond waves on positive columns
Authors: Aničin B. A. , Babovič V. M. , Lonngren K. E.
Published: March 2009

Nanosecond pulses are launched, and propagate on a positive column in the form of electron waves. The dispersion of the pulses is studied experimentally, and is found to be in agreement with monochromatic experiments and theory. Apart from wave attenuation, which is observed to depress signals of higher frequencies, Fourier analysis of the received pulses has revealed that the launching mechanism favours frequencies in the vicinity of 1/6ωp A full-wave theory, based on an extension of Goubau's excitation theorem, is derived, and successfully contrasted with experiment.
Neoclassical diffusion of a finite-β, tokamak plasma
Authors: Nocentini Aldo
Published: March 2009

The neoclassical diffusion in a tokamak is calculated for a finite-β plasma. The flux of the particles is shown to be substantially unchanged, with respect to the low-β case. The ‘equilibrium’ equation is derived. From it the magnetic surfaces can be obtained, once the external electromagnetic field and the density and temperature profiles are given. The case of magnetic surfaces with circular cross-section, in the small aspect ratio limit, is considered in detail.
Temporal electromagnetic echoes oblique to an external magnetic field
Authors: Furutani Y.
Published: March 2009

An exact Green'ls function of the Lenard-Bernstein equation for a magnetoplasma subjected to an external d.c. electric field is obtained. The result is applied to the problem of temporal electromagnetic echoes which can be detected at any angle of observation with respect to the external magnetic field B0.
Nonlinear evolution of firehose-unstable Afvén waves
Authors: Elsässer K. , Schamel H.
Published: March 2009

The time evolution of firehose-unstable Alfvén waves is calculated within the usual weak-turbulence scheme. In § 2 the amplitude equations are established using a third-order solution of Vlasov's equation. From these a spectrum equation is obtained by discarding four-wave correlations; in addition, we derive an equation which characterizes the truncation error ( §3). Both these equations are integrated numerically, together with the velocity-moment equations (§ 4), forward in time. The results as presented in § 5 correspond to a rapid relaxation of the plasma to equilibrium. For large wave-vectors the relaxation time, as well as the equilibrium wave energy, are in good agreement with the quasilinear treatment, and the truncation error is small. But for low wave-numbers the relaxation is much faster, and the wave energy grows higher as predicted by quasilinear theory. This is because the nonlinear particle current leads to a high effective growth rate, especially at small wave-numbers. In this region the truncation error grows appreciably, and may sometimes reach the order of magnitude of the spectrum. But the overall picture as given by quasilinear theory has been confirmed. In § 6 comparison is made with a macroscopic model of Berezin & Sagdeev (1969), where the plasma noise was simulated by ‘computer noise’. For low amplitudes both methods agree qualitatively.
Ergodic behaviour of nonlinear hydromagnetic waves in a cold collisionless plasma
Authors: Inoue Youshinori , Kimura Noriaki
Published: March 2009

Nonlinear hydromagnetic waves propagating along a magnetic field in a cold collisionless plasma are investigated. A criterion for ergodicity of the waves is obtained using the usual method of analytical dynamics. According to this criterion, it is found that, if a parameterb00is a rational number, the wave is periodic, and that, ifb00is an irrational number, the wave is ergodic. Therefore, the waves are almost always ergodic, i.e. the trajectory of the wave fills up a region in the phase plane of the magnetic field. On the other hand, periodic solutions can exist only with measure zero.
Nonlinear radio-frequency response of a non-uniform plasma slab–condenser system with realistic density and velocity profiles
Authors: Ballieu R. , Messiaen A. M. , Vandenplas P. E.
Published: March 2009

The nonlinear behaviour of a realistic one-dimensional bounded plasma (specifically, the classical plasma slab–condenser system) is computed by an iterative perturbation method. The results indicate, somewhat unexpectedly, that the influence of the r.f. field on the static density profile and on the resonance spectrum is much smaller than would have been inferred from a previous analysis of an unbounded plasma. However, this approach is inherently limited by the fact that, even for not too high r.f. fields, the electron density perturbations can become of the same order of magnitude as the static density in the tenuous plasma sheath near the wall. The resonance curves obtained with this sophisticated model show quite remarkable agreement with existing experimental data.
Turbulence in weakly ionized plasma
Authors: Bugnolo D. S.
Published: March 2009

The results of two experiments in a weakly ionized turbulent argon plasma are compared with a theory for the spacewisespectrum of the electron – ion density fluctuations of the plasma. It would appear that the theoretical predictions are in reasonable agreement with the results of these experiments. The theory, based on a quasi-static formulation of the Boltzmann equations, is a direct consequence of the Boltzman equations for the electrons, ions and neutrals in theweakly ionized plasma. A Heisenberg model is used for the turbulent diffusion coefficient of the plasma.
Nonlinear stabilization of cold beam-plasma instability
Authors: Gupta M. R.
Published: March 2009

Nonlinear stabilization of cold beam plasma instability are studied on the basis of Dupree's theory of strong plasma turbulence. Calculations are carried out to second order in the perturbed electric field, and it is found that, for a narrow spectrum satisfying δκ/κ≪1, stabilization occurs for the trapping frequency ωT ť ∣qeκΕk/me∣½ ť γL the linear growth rate. Nonlinear frequency correction, and also the changes in the kinetic and thermal energy densities of the beam, are calculated as the waves grow. Some points of difference between the resonant and non-resonant instabilities are discussed.
Cyclotron resonance in an inhomogeneous plasma
Authors: Laird M. J.
Published: March 2009

The motion of a charged particle in a transverse wave of varying amplitude, wavelength and phase speed βp, propagating along a uniform magnetic field, together with a longitudinal electric field, is investigated. The equations of motion, in Hamiltonian form, are reduced to a system with two degrees of freedom in which integrable cases appear naturally. It is shown that particles may be locked in resonance with the wave, and expressions are found for the energy and momentum of such particles in terms of βp.
Characteristics of the electric field far from and close to a radiating antenna around the lower hybrid resonance in the ionospheric plasma
Authors: Beghin C. , Debrie R.
Published: March 2009

From a basic discussion of the behaviour of VLF waves propagated almost perpendicularly to the earth 's magnetic field, the lower hybrid resonance (LHR) frequency is seen as a boundary between compressional Alfv én waves and electrostatic sound waves. The disparity of the wavelengths involved leads to a fundamental difference between the characteristics of the AC electric field far from and close to the source.

Some detailed observations of this field are presented, from an experiment that used both a mother-daughter technique for large distance measurements and a quadrupole probe technique for the local field, and that was performed in the equatorial ionosphere. The frequency dispersion of quasi-transverse (QT) waves received at a large distance and the amplitude-frequency response of the VLF quadrupole probe are compared with a theoretical analysis for the warm ionospheric plasma. From this analysis, the possibility is established of using the properties of fast and slow waves in active experiments, for measuring simultaneously the LHR, the electron density, a hybrid ion-electron thermal velocity and possibly the plasma drift velocity.
Cylindrically symmetric waves in the magnetohydrodynamic approximation
Authors: Woolley M. L.
Published: March 2009

The equations of motion of an ideally conducting medium, in the magnetohydrodynamic approximation, are solved exactly under the hypotheses that (i) one component of the magnetic field is constant everywhere, (ii) the magnetic and hydrodynamic pressures are in equilibrium, and (iii) the solution is invariant under a continuous group of transformations which preserves the symmetry of the uniform magnetic field. It is seen that the invariants of the group of transformations form the basis for a parametric description of the full solution that describes the propagation of cylindrically symmetric magneto-sonic waves in the direction of the uniform field. A number of general features of the motion are deduced, and an integral expression is given for the amplitude of the component of velocity, perpendicular to the uniform magnetic field, which undergoes potential well oscillations.
Kinetic theory of a two-dimensional magnetized plasma. Part 2. Balescu-Lenard limit
Authors: Vahala George
Published: March 2009

The kinetic theory of a two-dimensional one-species plasma in a uniform d.c. magnetic field is investigated in the small plasma parameter limit. The plasma consists of charged rods interacting through the logarithmic Coulomb potential. Vahala & Montgomery earlier derived a Fokker –;Planck equation for this system, but it contained a divergent integral, which had to be cut-off on physical grounds. This cut-off is compared to the standard cut-off introduced in the two-dimensional unmagnetized Fokker –;Planck equation. In the small plasma parameter limit, it is shown (under the assumption that for large integer n, γn/γn+1 = O(np), with p < 2, where γn = ωn −nΩ. with ωn the nth. Bernstein mode and Q the electron gyro frequency) that the Balescu-Lenard collision term is zero in the long time average limit if one considers only two-body interactions. The energy transfer from a test particle to an equilibrium plasma is discussed and also shown to be zero in the long time average limit. This supports the unexpected result of zero Balescu-Lenard collision term.
Ion cyclotron instability in current-carrying plasmas with anisotropic temperatures
Authors: Lee Kai Fong
Published: March 2009

The effect of thermal anisotropies on the Drummond-Rosenbiuth two-stream ion cyclotron instability is studied. The properties of the instability are found to be dependent on the ratio of parallel electron temperature to perpendicular ion temperature, T∥e /T⊥ ι which can alternatively be viewed as the product ofT∥e /T⊥ ι and the ion anisotropyT∥e /T⊥ ιThe critical drift velocity decreases and the growth rate increases with T∥e /T⊥ ι. However, the dependence on T∥e /T⊥ ι becomes less sensitive as this temperature ratio increases, and, in the limit of T∥e /T⊥ ι>1, the instability becomes independent of the perpendicular temperatures.
The instability of hydrodynamic longitudinal oscillations in a non-uniform magnetoactive plasma
Authors: Demchenko V. V. , El-Naggar I. A.
Published: March 2009

The stability of hydrodynamic longitudinal oscillations of an inhomogeneous magnetoactive plasma is analysed. If the non-uniformity of density is taken into account, it is shown to result in parametric-like instability at sub- and ultra- harmonics of natural plasma oscillation frequencies. The conditions for the development of unstable oscillations and their growth rates are determined.
Absorption and transformation of electrostatic surface waves in the transition layer of a magneto-active plasma
Authors: Demchenko† V. V. , Zayed K. E.
Published: March 2009

We investigate electrostatic surface-wave damping in strongly inhomogeneous magneto-active plasma with collisions. We demonstrate that such damping occurs at wave frequencies close to the natural (hybrid) frequencies of the plasma oscillations. We obtain dispersion equations and expressions for the damping coefficients of surface waves propagating along (i) the interface of semi-infinite plasmas, (ii) a plane plasma layer, and (iii) a vacuum layer in the plasma. We also study the linear transformation of surface waves into longitudinal oscillations (concentrated in the transition layer and directed along the gradient of plasma density) caused by the initial perturbation.
Electric field fluctuations in turbulent plasmas
Authors: Cook I. , Taylor J. B.
Published: March 2009

It is well known that the properties of stable quiescent plasma may be calculated by a method known as the ‘superposition of dressed particles’. In this paper an analogous superposition principle is constructed for turbulent plasmas. This shows that the turbulent fluctuations may suppress the growth of the instabilities that give rise to them. The fluctuation spectrum, diffusion coefficient and dielectric constant of turbulent plasma are calculated for the guiding centre model. The relationship of this theory to that of Dupree and Weinstock is discussed.
Observation of ‘switch-on’ shocks in a magnetized plasma
Authors: Craig A. D. , Paul J. W. M.
Published: March 2009

The Charybdis experiment is designed to produce shock waves propagating into a magnetized hydrogen plasma in directions parallel and almost parallel to the initial field and, in particular, the ‘switch-on’ shock. An initial plasma

is produced in a pyrex chamber (0.46 m dia., 125 m long) containing an axial magnetic field (Bz up to 0.28 T): a magnetic piston is produced by a fast-rising radial discharge between a short central and an annular outer electrode at one end of the chamber; the Lorentz force on the current sheet causes axial propagation, and under certain conditions a curved shock front propagates ahead of the piston. The Alfvén Mach number of the flow is varied by changing the axial field strength: at high Mach number, where a parallel shock should be gas dynamic, no separated shock is found; at low Mach number, where theory predicts‘switchon’ shock behaviour, a clearly separated curved shock front is found which passes through a point of parallel propagation. Magnetic and electric probe measurements at the point of parallel propagation show the presence of a ‘switchon’ shock. It is suggested that the ‘switch-on’ and adjacent oblique shocks are matched together by an intermediate wave propagating behind the latter and merging with the former.
Comparison of electron and electronic temperatures in recombining nozzle flow of ionized nitrogen—hydrogen mixture. Part 1. Theory
Authors: Park Chul
Published: March 2009

This is part 1 of a two-part paper. In this part, relaxation of the population distribution of electronic states is studied theoretically for a highly-ionized nitrogen- hydrogen mixture expanding through a nozzle, wherein the hydrogen content is less than 0.1 %. The analysis incorporates quantum-mechanical excitation rate coefficients, and considers the effects of wall cooling and absorption of radiation. Calculations are carried out for a condition produced experimentally, the experiment being the subject of part 2. The electronic excitation temperatures are deduced from the computed population distributions along the nozzle, and are compared with the calculated electron temperatures; this shows a large discrepancy between the two.
Stability of a vortex sheet between collisionless plasmas
Authors: Rajaram Ramaswamy , Kalra G. L. , Tandon J. N.
Published: March 2009

The problem of stability of a vortex sheet between two collisionless plasma media described by Chew et al. (1956) equations is examined. The relevance of these investigations to the internal structure of the solar wind and to the stability of the magnetosphere–solar wind boundary is discussed.
Wavenumber space analysis of oscillations in weakly non-uniform magnetoplasmas
Authors: Brownell J. H.
Published: March 2009

A differential equation in wavenumber space is used to find the electric field excited by an external source in a weakly non-uniform, low-β, neutral magneto- plasma slab (with B perpendicular to ∇n). The analysis is general enough to allow one to judge the accuracy of previously used long-wavelength methods. Electrostatic waves which propagate across the magnetic field parallel to the density gradient are considered, and the impedance of a grid-plasma system is determined. The calculation is specialized to the case of a low density plasma with applied frequencies near the upper hybrid.
Transverse ionizing MHD detonation waves. Part 1. Structure, Hugoniot curves and Chapman–Jouguet limit
Authors: Frankenthal Shimshon , Friedland Allan B. , Abarbanel Saul
Published: March 2009

The structure and jump relations across a one-dimensional ionizing magnetohydrodynamic detonation wave with transverse electromagnetic fields are studied. It is shown that the downstream point in a three-dimensional phase space, constructed from the gas velocity, magnetic field and burned fraction of gas, is of a singular nature and leads to a unique solution. In the zero viscosity, infinite transitional electrical conductivity limit, the ‘fast wave’ degenerates into a pre-compression ionizing shock followed by a perfectly-conducting deflagration zone, and the ‘slow wave’ degenerates into an ordinary gas dynamic pre-compression shock followed by a deflagration zone in which the transition to perfect conductivity occurs. In both cases, a relationship is derived connecting the electric field (which cannot be arbitrarily imposed) and both the upstream variables and parameters describing the ionization transition point. This relationship must be appended to the Rankine–Hugoniot jump relations and a Chapman-Jouguet condition (or its equivalent) to provide a unique relationship between the assumed unknown shock-speed and the downstream state as a function of the assumed known upstream state.

The ionization temperature is found to affect the jump and determines the electric field developed upstream. It is found that the Hugoniot consists of two branches: a ‘normal’ one which retains the character of an ordinary detonation Hugoniot, and an ‘anomalous’ one which does not. Although both branches represent detonations endowed with a transition structure, it is suggested that ‘anomalous’ detonations are either unstable or non-evolutionary. The evolution of the topology of the Hugoniot, as a function of the upstream magnetic field and ionizing temperature, is studied and correlated with Chapman-Jouguet propagations. Results are presented for the Chapman-Jouguet jump values and detonation speeds, as functions of dimensionless parameters that represent the upstream magnetic and electric field. Relationships between this upstream electric field and ionization temperature is derived and found to differ according as ionization precedes or follows ignition.
Anisotropy of turbulence in a collisionless shock
Authors: Muraoka K. , Murray E. L. , Paul J. W. M. , Summers D. D. R.
Published: March 2009

We have measured the level of density fluctuations 〈δne2(K)〉 for ‖K‖ ∼ 1/λD as a function of the angle between k and the current within a collisionless shock. The cut-off angle is in good agreement with that predicted for current-driven ion wave turbulence and one-dimensional heating of the ions. This agreement provides further evidence that ion-wave instability is responsible for the observed turbulence within the shock.
Potential created by a test particle in one-, two- and three-dimensions in a flowing ion-electron plasma
Authors: Chenevier P. , Dolique J. M. , Perès H.
Published: March 2009

In a plasma at rest, the electrostatic potential around a point charge obeys the well-known Debye shielding law. Modifications, for the ease of a flowing plasma, have been studied in the limiting cases of either long or short distances for subsonic and hypersonic velocities. Only recently this was extended to include all distances, restricted to a two-dimensional case. However, neither of these modifications includes the ionic motions. In this paper, we derive general expressions for the electrostatic potential which are valid for all distances and flow velocities in one-, two- and three-dimensional cases. This is done first with unperturbed ions, and then for the case where ionic motions, which are seen to be important for mesothermic flow velocities, are included. In both cases, the results are discussed and illustrated.
Mutual influence between two spherical conducting grids in a warm isotropic plasma
Authors: Chasseriaux J. M. , Odero D.
Published: March 2009

The multiple water-bag model can be used to describe a uniform warm plasma as accurately as one wishes. Moreover, it makes boundary-value problems in such a plasma tractable. As a special case, the problems of the influence of a point charge on a spherical grid and of the mutual influence between two spherical grids are solved. From these results, the self-impedance of a double-sphere dipole antenna is deduced. If the spheres are widely separated it exhibits a resonance at the plasma frequency, whereas if the spheres are close there is an anti-resonance at this frequency. The mutual impedance of a quadrupole probe comprising two double-sphere antennae is also studied, and is found, usually, to be not very sensitive to the finite radius of the spheres. For determining the electron temperature of the plasma, this last device seems better than a double-sphere antenna alone.
Anomalous skin effect for a plasma column in a magnetic field
Authors: Storer R. G. , Meaney C.
Published: March 2009

The influence of a steady axial magnetic field on the anomalous penetration of low frequency electromagnetic fields into a cylindrical plasma column is investigated by considering a plasma with a Gaussian electron density distribution. For this model, a complete solution is obtained for Boltzmann's equation coupled to Maxwell's equations, and the fields calculated exactly. The results show dramatic changes of the internal fields for small changes of the applied magnetic field when the average Lamour radius of the electrons is of the order of the plasma radius.
Mutual impedance of two small dipoles in warm streaming plasma to measure a Cherenkov cone of plasma waves
Authors: Fiala Vladimir
Published: March 2009

The field of a small (Hertzian) dipole excited on a fixed frequency in warm electron plasma streaming atsuperthermal velocity is approximated electrostatically. The plasma is described by a hydrodynamic model, without collisions or magnetic field. The mutual impedance for two such dipoles is studied, to determine the geometrical structure of the field, and in particular to locate the Cherenkov cone of slow plasma waves excited by the source.
Stimulated backscattering of electromagnetic waves from ion–ion hybrid waves in a magnetized plasma
Authors: Lee Kai Fong
Published: March 2009

In a high-density magnetized plasma composed of two ion species of different charge-to-mass ratios, electrostatic waves propagating across the magnetic field exhibit a resonance at the Buchsbaum or ion-ion hybrid frequency, in addition to the resonances at the upper and lower hybrid frequencies. In this paper, the possibility of stimulated scattering of electromagnetic waves incident normal to the magnetic field from electrostatic waves at the ion-ion hybrid frequency is investigated. Based on the cold-plasma equations, it is found that such a process is theoretically possible. Formulas for the threshold power and growth rate are obtained, which show that the threshold power is much greater, and the growth rate much less, than those of stimulated scattering from upper and lower hybrid waves.
Plasma heating with multiple ion-cyclotron resonance
Authors: Dolgopolov V. V. , Hussein A. M. , Khalil SH. M.
Published: March 2009

We consider the electromagnetic field produced by an azimuthal current of arbitrary shape, external to a plasma. In particular, we study absorption of the field energy by a radially inhomogeneous plasma cylinder with double ioncyclotron resonance. We calculate the power absorbed by plasma ions for an arbitrary ion distribution function. We show that, in some regions of the plasma it is possible for the work done by the field on the currents to be negative. We also show that, if the plasma has regions with a large density gradient (e.g. a narrow transitional plasma-vacuum layer), power absorption increases sharply. We consider, too, the exciting of the natural oscillations of the cylinder by external currents. In this case, power absorption is independant of the dissipative properties of the plasma.
An absolute Raman instability in an inhomogeneous plasma
Authors: Cairns R. A.
Published: March 2009

We examine stimulated Raman scattering in an inhomogeneous plasma, and demonstrate that there may be an exponentially growing decay, which owes its existence to reflexion of the electromagnetic decay wave. Unlike absolute instabilities, which depend on simultaneous vanishing of the wavenumber mismatch and its derivative, it can occur over a wide spectrum of decay wave frequencies, and may, therefore, be of greater practical importance than those instabilities.
Ion-acoustic solitons excited by a single grip
Authors: Watanabe S.
Published: March 2009

Ion-acoustic solitons are observed to form when a large-amplitude positive step voltage is applied to a single mesh grid. For the formation of the soliton, the sheath about the grid plays an important rôle. The number and the amplitude of the solitons are examined experimentally as a function of the nonlinear index, and are compared with theory. The observed width and the velocity of the soliton are well explained by the theory of a hot-ion fluid and Boltzmann electrons. The phase delay of two solitons by a head-on collision is also observed.
Generation of monochromatic electrostatic waves of large amplitude in a bounded beam–plasma system
Authors: Bitter M. , Paris P. J.
Published: March 2009

Monochromatic electrostatic waves of large amplitude were excited by the interaction of an electron beam with a bounded plasma. These waves were identified as resonant beam modes, which are amplified by multiple reflexion in a cavity. Nonlinear effects, such as the generation of harmonies and sidebands, were observed.
Hydromagnetic stability of a compressible jet
Authors: Chakraborty B. B. , Iyengar H. K. S.
Published: March 2009

This paper studies the hydromagnetic stability of a cylindrical jet of a perfectly-conducting, inviscid and compressible fluid. The fluid velocities and magnetic fields, inside and outside the jet, are uniform and in the axial direction, with possible discontinuities in their values across the jet surface. For large wavelength disturbances, the jet behaves as though it were incompressible. Numerical evaluation of the roots of the dispersion relation for a number of different magnetic-field strengths and jet velocities, but for disturbances of finite ranges of wavenumbers, indicates that the jet is stable against axisymmetric disturbances, but instability is present for asymmetric disturbances when the magnetic fields are sufficiently small. The magnetic field is found to have a stabilizinginfluence when compressibility is not very large; for high compressibility, it may have even a destabilizing effect. The paper explains physically the roles of compressibility and the magnetic field in bringing about the stability of the jet. When the wavelengths of disturbances are small, the dispersion relation reduces to that for a two-dimensional jet and a vortex sheet; and the results for these cases are known from earlier studies.
Electromagnetic wave backscattering across a magnetic field in a bounded plasma
Authors: Willett Joseph E. , Bilikmen Sinan , Maraghechi Behrooz
Published: March 2009

The stimulated backscattering of electromagnetic ordinary waves from extraordinary waves propagating normal to a magnetic field in a plasma of finite length is studied. A pair of coupled differential equations for the amplitudes of the backscattered and scatterer waves is derived from Maxwell's equations and the moment equations for an inhomogeneous magnetized plasma. Solution of the coupled equations for a homogeneous plasma yields an expression for the growth rate of the absolute instability as a function of plasma length and damping rates of the product waves. The convective regime in which only spatial amplification occurs is discussed. A numerical study of the effects of the magnetic field on Raman and Brillouin backscattering is presented.
Oscillation centres and mode coupling in non-uniform Vlasov plasma
Authors: Johnston Shayne , Kaufman Allan N.
Published: March 2009

The general coupling coefficient for three electromagnetic linear modes of an inhomogeneous and relativistic plasma is derived from the oscillation-centre viewpoint. A concise and manifestly symmetric formula is obtained; it is cast in terms of Poisson brackets of the single-particle perturbation Hamiltonian and its convective time-integral along unperturbed orbits. The simplicity of the compact expression obtained is shown to lead to a new insight into the essence of three-wave coupling and of the Manley–Rowe relations governing such interactions. Thus, the interaction Hamiltonian of the three waves is identified as simply the trilinear contribution to the single-particle (new) Hamiltonian, summed over all non-resonant particles. The relation between this work and the Lie-transform approach to Hamiltonian perturbation theory is discussed.
An energy principle for soliton solutions with higher nonlinearities
Authors: Laedke E. W. , Spatschek K. H.
Published: March 2009

The two-dimensional stability of Langmuir solitons in investigated taking into account the dynamic ion response and electron nonlinearities. A variational principle for the growth rate is derived which allows one to determine growth rates and regions of instability by standard numerical procedures. The validity of previous methods using trial functions and the variation of action method, as well as the limitations of the corresponding results for the growth rates, neglecting the dynamic ion response, are critically examined.
Measurements of the impedance of a magneto-acoustic wave-launching antenna
Authors: Blackwell B. D. , Cross B. C.
Published: March 2009

Magnetoacoustic waves were generated in an argon plasma by passing an RF current through a solenoid antenna surrounding the plasma column. The complex impedance of the antenna was measured as a funcion of wave frequency in order to investigate the column geometric resonances and to obtain estimates of ion density and electron temperature. Although this technique yielded reliable estimates of the ion number density, the wave &litudes near resonance were found to be anomalously low and reliable temperature measurements could not be obtained.
Parametric decay of magneto-acoustic oscillations in a cylindrical plasma
Authors: Cramer N. F. , Sy W. N-C.
Published: March 2009

Parametric decay of magneto-acoustic oscillations in a cylindrical plasma column is considered, including the Hall effect and the effects of finite resistivity. Growth rates for axisymmetric modes in decay instabilities are derived and applied to the problems of parametric excitation of Alfvén waves and supplementary heating of plasmas. It is shown that both the slow (ion cyclotron) and fast (compressional) modes may be parametrically excited. The threshold &litudes for nonlinear dissipation of pump field energy are calculated and briefly discussed.
Wave packets in dispersive media: an approximation encompassing both main signal and precursors
Authors: Buckley R. , Sammut C. A. P.
Published: March 2009

The propagation of a linear wave packet in a generally dispersive, but non- absorbing, medium is considered. By an extension of the principle of stationary phase to higher inverse order in time, an approximation is derived that encompasses both the main signal propagating at the group speed, and the precursors that precede and follow it.
Plasma equilibrium in toroidal l = 3 stellarators
Authors: Fielding P. J. , Hitchon W. N. G.
Published: March 2009

The equations of MHD equilibrium are solved by including plasma pressure and current in a large aspect-ratio ordering scheme for the calculation of toroidal, l = 3 stellarator vacuum fields. The extended ordering unifies the low-beta equilibrium theory for tokamaks and l = 3 stellarators, and allows solutions to be obtained simply for arbitrarily prescribed pressure and current density profiles. Expressions are given for the equilibrium magnetic field and the equation for the flux surfaces is calculated, including the effects of l = 3 shaping and toroidal displacement. These results are used to calculate equilibria for the parameters of CLEO stellarator, and we examine the role of an externally applied vertical field in reducing pressure-induced flux surface distortion and destruction.
Slowly propagating instabilities in plasmas with Margenau-Davydov electron distribution function
Authors: Žigman V. J. , Milić B. S.
Published: March 2009

The properties of certain wave modes excited in a weakly ionized plasma placed in an external d.c. electric field are analyzed from the standpoint of the linearized kinetic equation, the electron steady-state distribution function being taken in the form of the extended Margenau–Davydov and, in particular, Druyvesteinian. The presence of absolute stability cones formed by certain propagation directions is found. The corresponding critical values of the electron drift, destabilizing each of the modes considered, is also evaluated for a plasma with a Druyvesteinian distribution.
Spontaneous excitation of magnetic fields and collapse dynamics in a Langmuir plasma
Authors: Kono M. , Škorić M. M. , Haar D. Ter
Published: March 2009

We discuss various aspects of the spontaneous generation of magnetic fields in a Langmuir plasma. We first of all show that the correct general expression for the ponderomotive force leads to the solenoidal current responsible for the magnetic-field generation. We derive the ponderomotive-force expression and also the magnetic-field generation equations from a two-time-scale two-fluid description. We also use a kinetic approach to derive the magnetic-field generation equations. We discuss the stability of monochromatic Langmuir waves and show that they are subject to both the ordinary modulational instability and to a magneto-modulational instability. We show that the coupled nonlinear equations describing the electric field strength amplitude, the plasma density, and the self-generated magnetic field can, under certain conditions, be reduced to a generalized cubic nonlinear Schrodinger equation. We finally show, by using a virial theorem, that the self-generated magnetic field does not stabilize the wave collapse.
Three-wave coupling coefficients for non-uniform plasmas
Authors: Lindgren T. , Larsson J. , Stenflo L.
Published: March 2009

Resonant three-wave interaction in a spatially non-uniform magnetized fluid plasma is considered. We calculate the coupling coefficientsVjfor wave propagation in arbitrary directions and demonstrate that new terms, which appear inVjowing to the presence of density gradients, can dominate actual wave coupling processes.
Steady-state solutions for the penetration of a rotating magnetic field into a plasma column
Authors: Jones Ieuan R. , Hugrass Waheed N.
Published: March 2009

The penetration of an externally applied rotating magnetic field into a plasma cylinder is examined. Steady-state solutions of an appropriate set of magneto-fluid equations show that, provided the amplitude and rotation frequency of the field are suitably chosen, the penetration is not limited by the usual classical skin effect. The enhanced penetration of the rotating field is accompanied by the generation of a unidirectional azimuthal electron current which is totally absent in a purely resistive plasma cylinder.
Magnetic diffusion and current profiles during current reversal
Authors: Shkarofsky I. P. , Shoucri Magdi
Published: March 2009

Computer studies are performed on the temporal changes of magnetic flux surfaces and current density profiles in a tokamak (of 25 cm minor radius) undergoing current reversal. The flux on the plasma boundary is forced to vary in time so as to model a total current reversal from positive to negative in about 5 ms. A two-dimensional computer code with radial and azimuthal spatial variations has been written, as well as a simpler one-dimensional code with only radial variation. The two-dimensional code shows that the flux variation takes place in way showing the formation of magnetic islands. Both codes show that the current penetration in the plasma is much slower than the reversal time. This slow resistive penetration occurs even with an enhanced resistivity factor which increases towards the boundary and with a temperature profile which decays in time towards zero current. After maintaining a constant flat negative current for certain period, results are also obtained on the profiles during reversing back rom negative to positive current and maintaining the constant positive current.
Parametric instability of hydromagnetic waves with oblique propagation
Authors: Yumoto Kiyohumi
Published: March 2009

The parametric excitation of Alfvén waves by two magnetosonic waves, which propagate obliquely to the static magnetic field, is analysed. The theoretical model used is the uniform, unbounded, ideally conducting and compressible one-fluid plasma, with suitable resonance conditions. Our consideration is restricted to determining the conditions under which some initially small perturbation grows, so that the magnetosonic pump wave is regarded as constant. It is found that, both in the case of the standing and the propagating pump, the growth rates of the excited waves depend not only on the pump power but also on β, and that the threshold pump intensities are proportional to β. In the case of the standing pump, the threshold pump intensity of the oscillating instability is zero at perfect matching. The parametric excitation of Alfvén waves by two magnetosonic waves, which propagate obliquely to the static magnetic field, is analysed. The theoretical model used is the uniform, unbounded, ideally conducting and compressible one-fluid plasma, with suitable resonance conditions. Our consideration is restricted to determining the conditions under which some initially small perturbation grows, so that the magnetosonic pump wave is regarded as constant. It is found that, both in the case of the standing and the propagating pump, the growth rates of the excited waves depend not only on the pump power but also on β, and that the threshold pump intensities are proportional to β. In the case of the standing pump, the threshold pump intensity of the oscillating instability is zero at perfect matching.
Parametric study of the relaxation zone behind strong normal shock waves in a dusty ionized monatomic gas
Authors: Igra O. , Ben-dor G.
Published: March 2009

The conservation equations appropriate to a steady, one-dimensional flow of dusty ionized argon were solved numerically. The specific effect of each of the physical parameters of the dust upon the flow properties in the relaxation zone was studied. It was found that increasing the dust particle mass causes an increase in both the kinematic and thermal relaxation lengths. In addition to these changes, the flow field inside the relaxation zone is also affected. An increase in the dust mass (caused either by an increase in the dust density or its diameter) causes an increase in the plasma velocity, temperature and electron number density and a decrease in its density and pressure. Similar effects are encountered when the specific heat capacity of the dust is changed. An increase in the emissivity of the dust causes an increase in the plasma density and pressure and a decrease in its velocity, temperature and electron number density. Increasing the emissivity of the dust results in a decrease in the relaxation zone length.
Grad–Fokker–Planck plasma equations. Part 1. Coffision moments
Authors: Broughan K. A.
Published: March 2009

Thirteen moments are taken of the collision term in the Boltzmann–Fokker– Planck equation for a multi-species, multi-temperature, hot plasma, following the method first developed by Grad for neutral gases. The collision integrals are evaluated for each colliding species pair. These integrals give, in particular, the rate of exchange of momentum and energy produced by collisions. The set of integrals may be combined with moments of the remaining terms in the Boltzmann equation to give thirteen moment equations for each species of particle. To complete the calculation, extensive use was made of the symbolic computer language REDUCE.