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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
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NASA NTRS1980

On self-consistent waves and their stability in warm plasmas. II - Instability of circularly polarized waves both in the presence and the absence of an ambient magnetic field

The stability of a self-consistent, large-amplitude, circularly polarized wave in a warm plasma is investigated. For perturbations to the system propagating normal to the plane of circular polarization, a dispersion relation is derived employing an expansion in the nonlinear wave amplitude and the momentum of the plasma particles in the plane of polarization. Instability results both in the absence and presence of a large-scale magnetic field with a growth rate of the order of the nonlinear wave amplitude.

Lee, M. A.↗
NASA NTRS1981

The lower hybrid density drift instability with cold plasma

The linear Vlasov dispersion relation for the lower hybrid density drift instability is studied in a four component (hot electrons and protons, cold electrons and protons) plasma. The introduction of a cold ion population monotonically reduces the maximum growth rate of the instability. Reduction of the ratio of temperatures of the cold and hot plasmas reduces both the real frequency and the growth rate of the instability. Near a ratio of the cold and hot plasma temperatures of 0.01 a higher frequency branch of this instability emerges and for a fixed ratio of cold and hot electron density exhibits an increasing maximum growth rate as the ratio of the cold and hot plasma temperatures decreases further. The ratio of the cold and hot plasma temperatures for the ions is the crucial parameter and deserves detailed magnetospheric studies.

Gary, S. P.↗
NASA NTRS1981

The effect of microscopic turbulence on magnetosphere-ionosphere coupling

The effect of turbulence on the coupling of the magnetosphere and ionosphere has been investigated by including an effective collision frequency in the electron equation of motion. When this term is combined with the continuity equation, the ion equation of motion and Maxwell's equations, a dispersion relation for the kinetic Alfven wave including effective collisions is found. The wave-particle interaction leads to a strong damping of the wave. Inclusion of the effects of plasma sheet kinetics yields a scale size transverse to the magnetic field which corresponds to the size of visual auroral arcs.

Lysak, R. L.↗
NASA NTRS1981

Linear theory of tearing in a high-beta plasma

The linear dispersion relation for a collisionless plasma in a sheared one-dimensional current sheet is calculated with reference to conditions in the daytime magnetopause. Calculations are extended to include plasmas with beta approximately equal to 1. It is found that the tearing mode eigenstructure and temporal growth rate are a sensitive function of the ratios l sub s/l sub G, l sub s/l sub T, and l sub s/l sub n, where l sub s is the shearing length of the magnetic field, l sub G is the gradient scale length, and l sub T is the temperature gradient scale length. In particular, if beta is approximately equal to 1, and l sub s is less than l sub G, l sub n, and l sub T, then the thickness of the layer over which particles are resonantly accelerated by the induction magnetic field is approximately rho, a thermal gyroradius. If the above conditions are not satisfied, plasma gradients may electrostatically stabilize the mode.

Quest, K. B.↗
NASA NTRS1981

Instabilities of low frequency, parallel propagating electromagnetic waves in the earth's foreshock region

An instability analysis is presented for parallel and antiparallel propagating electromagnetic waves generated by reflected and diffuse suprathermal ions upstream of the earth's bow shock. Calculations are performed on the basis of upstream particle observations made by the ISEE 1 Quadrispheric Lepedea instrument and low-energy electron measurements made by the ISEE 1 electron spectrometer for a single period. The electromagnetic dispersion relation is computed and the unstable modes and growth times of the fastest growing waves are determined. It is found that the reflected ions destabilize the plasma most strongly at a wave frequency 0.1 that of the ion gyrofrequency by a resonant ion beam instability for waves propagating upstream and by a nonresonant firehose-like instability for waves propagating downstream. The diffuse ions also destabilize the plasma most strongly at the same frequency by means of resonant instabilities of both right- and left-hand polarized waves propagating away from the bow shock.

Sentman, D. D.↗
NASA NTRS1981

The source mechanism of auroral kilometric radiation

A cyclotron maser mechanism for the auroral kilometric radiation (AKR) proposed by Wu and Lee (1979) and Lee et al. (1980) is reviewed with a discussion of recent developments concerning the quasilinear saturation of AKR and its energy conversion. Emphasis is on the case in which the population of the secondary electrons prevails over that of the energetic electrons. The limit of zero secondary electrons is examined, and it is found that the dispersion relation of the radiation is significantly modified.

Wu, C. S.↗
NASA NTRS1982

Modulation of energetic particle fluxes by a mixed mode of transverse and compressional waves

Modulation characteristics of particle fluxes in the presence of a mixed mode of compressional and transverse magnetic waves at hydromagnetic frequencies are investigated through kinetic perturbation of the distribution function. The magnetospheric medium where the particles are modulated contains both the magnetic and pressure gradients. The modulation features are found to be strongly dependent on the energy and pitch angle of the particles. Drifting particles can resonate with waves whose phase velocities are close to their drift velocities. When this occurs, the modulation amplitudes become significantly large and large phase shifts will occur. It is pointed out that resonance is important for particles with mid pitch angles (40-70 deg). The phase shift between the particle modulations and the magnetic field oscillations are strongly controlled by the combined effects of transverse and compressional wave components and/or the occurrence of drift resonance. Numerical calculations are performed using the dispersion relation of drift mirror Alfven waves as an example of waves with both compressional and transverse components.

Lin, C. S.↗
NASA NTRS1982

Flute mode waves near the lower hybrid frequency excited by ion rings in velocity space

Discrete emissions at the lower hybrid frequency are often seen on the S3-3 satellite. Simultaneous observation of perpendicularly heated ions suggests that these ions may provide the free energy necessary to drive the instability. Studies of the dispersion relation for flute modes excited by warm ion rings in velocity space show that waves are excited with real frequencies near the lower hybrid frequency and with growth rates ranging from about 0.01 to 1 times the ion cyclotron frequency. Numerical results are therefore consistent with the possibility that the observed ions are the free energy source for the observed waves.

Cattell, C.↗
NASA NTRS1982

A dispersion law for solar oscillations

The simple model of p-mode solar oscillations of Liebacher and Stein (1981), in which the acoustic vibrations are trapped in a resonant cavity taking the form of a spherical shell below the solar surface, is compared with Doppler shift observations of vertical velocities. The model is shown to predict a modified dispersion law in which the sound travel time across the cavity is a function of the ratio of the temporal frequency to the horizontal wavenumber, resulting in a single curve when the temporal frequency is plotted against the wavenumber. Frequencies derived from a two-dimensional power spectrum of velocity observations are found to conform to a modified version of the dispersion relation, and that only when the fundamental mode is excluded. Results thus suggest that all modes with the same frequency/wavenumber ratio are trapped in an identical cavity, or, more plausibly, that the difference in upper boundary conditions for different modes has minimal effect on the resulting frequencies.

Duvall, T. L., Jr.↗
NASA NTRS1983

Magneto-atmospheric waves

A theoretical treatment of magneto-atmospheric waves is presented and applied to the modelling of waves in the solar atmosphere. The waves arise in compressible, stratified, electrically conductive atmospheres within gravitational fields when permeated by a magnetic field. Compression, buoyancy, and distortion of the magnetic field all contribute to the existence of the waves. Basic linearized equations are introduced to describe the waves and attention is given to plane-stratified atmospheres and their stability. A dispersion relation is defined for wave propagation in a plane-stratified atmosphere when there are no plane-wave solutions. Solutions are found for the full wave equation in the presence of either a vertical or a horizontal magnetic field. The theory is applied to describing waves in sunspots, in penumbrae, and flare-induced coronal disturbances.

Thomas, J. H.↗
NASA NTRS1983

Excitation of an electrostatic wave by a cold electron current sheet of finite thickness

Calculations for the threshold of current-driven instabilities and the growth rates of ion acoustic and electrostatic ion cyclotron instabilities in a magnetized plasma driven a current sheet with a finite width are presented. Maxwellian equations are employed to model the velocity distributions of electrons and ions in a direction perpendicular to the sheet. A dispersion relation is defined for the regions of instability, and boundary conditions are characterized in order to obtain a set of eigenvalue equations. Thresholds are delineated for various regions, including ducted mode solutions where only ion-acoustic waves are excited in areas where the frequency range significantly exceeds the ion cyclotron frequency. When a constant electron drift velocity is present, a thick current sheet is more unstable than a thin one. Fewer modes become unstable with a thinner sheet.

Hwang, K. S.↗
NASA NTRS1983

Surface-polaritonlike waves guided by thin, lossy metal films

Surface-plasmon polaritons guided by thin, lossy metallic films bounded by dissimilar dielectric media are investigated. New solutions to the dispersion relation are found, representing waves that are leaky (radiative) in one of the dielectrics. The new waves are interpreted in terms of the coupling of a damped surface plasmon at one interface with continuum modes at the other. Their excitation by end-fire coupling techniques is suggested.

Stegeman, G. I.↗
NASA NTRS1983

Beam-plasma interactions in a positive ion-negative ion plasma

An electron-free plasma consisting of negative ions /SF6(-)/ and positive ions /Ar(+)/, and negligible neutral-ion collision frequencies has been created in the laboratory. This plasma has a mass ratio of approximately 3.5-similar to many computer particle-in-cell simulated systems. A fluid description of this positive and negative ion confinement (PANIC) plasma is given and compared to experimental measurements of a beam-plasma instability for both beam species and a wide range of beam energies. The fluid dispersion relation and most growing modes are predicted to be insensitive to many parameters of the PANIC beam-plasma system, and found to the consistent with the data.

Intrator, T.↗
NASA NTRS1983

Further observations on resonance cones in non-Maxwellian plasmas

Results on the angular distribution of the electrostatic potential of a pulsating point charge in a warm magnetized plasma permeated by an electron beam are presented. The theoretical formulation for a finite magnetic field is given, and the solution of the resonance cone dispersion relation is presented. Numerical results on the angular distribution of the potential are shown, and the propagation of waves outside the resonance cones is described. It is demonstrated that with the inclusions of a finite magnetic field, the field patterns of a point charge are qualitatively similar to those obtained for a uniaxial plasma. The Cerenkov radiation occurs at angles much smaller than the cold-cone angle, even with the finite magnetic field. When the beam velocity is well above the thermal velocity of the background electrons, a characteristic wave propagation occurs between the cold-cone angles.

Thiemann, H.↗
NASA NTRS1983

Long range surface plasmons in birefringent media

The propagation properties of surface plasmon polaritons guided by thin metal films bounded by birefringent media are investigated. For the very thin (less than or equal to 150 A) films required to produce long propagation distances, the effects of birefringence on the dispersion relations are found to be minimal. A small effect of the birefringence on the mode attenuation is found.

Stegeman, G. I.↗
NASA NTRS1983

Observations of odd-half cyclotron harmonic emissions in a shell-Maxwellian laboratory plasma

During the last 14 years, the subject of odd-half electron cyclotron harmonic emission from plasma has been given considerable attention. It was apparently first reported to occur in a beam plasma machine. The existence of the emission in space has been well documented through observations made with satellites. Because wavelengths are difficult to observe in space, no wave number spectrum has ever been obtained for the half-odd harmonic emission. Such a spectrum together with a frequency spectrum might provide the basis for a successful modeling of the instability. The present investigation is concerned with the design of a laboratory experiment in which a plasma with an anisotropic velocity distribution is produced and measured. In addition, the dispersion relation is directly measured, and a noise analysis is conducted. The obtained plasma, a mixture of shell and Maxwellian distributions, is found to emit waves within the Bernstein wave branches. By correlation measurements, a mode is found which is essentially an absolute instability in a narrow frequency band.

Urrutia, J. M.↗
NASA NTRS1983

Nonlinear theory of type I irregularities in the equatorial electrojet

By allowing for the effect of wave electric fields on electron orbits, a nonlinear dispersion relation for type I irregularities is obtained. This relation predicts (1) isotropy of the Doppler shift with elevation, (2) a limiting phase velocity equal to the ion acoustic speed, and (3) a saturation amplitude which is maximum for horizontally propagating waves and decreases with elevation. With the theory presented here, the in situ electric field measurements of Pfaff et al. (1980) in the electrojet environment could provide a quantitative check for the theory of orbit diffusion by stochastic electric fields.

Sudan, R. N.↗
NASA NTRS1984

Integrated optical signal processing with magnetostatic waves

Magneto-optical devices based on Bragg diffraction of light by magnetostatic waves (MSW's) offer the potential of large time-bandwidth optical signal processing at microwave frequencies of 1 to 20 GHz and higher. A thin-film integrated-optical configuration, with the interacting MSW and guided-optical wave both propagating in a common ferrite layer, is necessary to avoid shape-factor demagnetization effects. The underlying theory of the MSW-optical interaction is outlined, including the development of expressions for optical diffraction efficiency as a function of MSW power and other relevant parameters. Bradd diffraction of guided-optical waves by transversely-propagating magnetostatic waves and collinear TE/TM mode conversion included by MSW's have been demonstrated in yttrium iron garnet (YIG) thin films. Diffraction levels as large as 4% (7 mm interaction length) and a modulation dynamic range of approx 30 dB have been observed. Advantages of these MSW-based devices over the analogous acousto-optical devices include: much greater operating frequencies, tunability of the MSW dispersion relation by varying either the RF frequency or the applied bias magnetic field, simple broad-band MSW transducer structures (e.g., a single stripline), and the potential for very high diffraction efficiencies.

Fisher, A. D.↗