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At least 847 records · Page 47

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NASA NTRS1995

Some wave-particle effects on large-scale Alfven wave propagation and damping

Phase mixing can reduce greatly the torsional Alfven wave's dissipation length for propagation in complex magnetic field-line geometries. This phase mixing causes significant energy transfers from large to small scales where a conversion from ordered wave energy into a particle kinetic form occurs. This conversion during its initial stages is an entropy conserving process well described by Vlasov theory, Nonlinear stages of wave-particle resonance, particle trapping, and collisional resistivity are often invoked as processes eventually responsible for converting ordered wave motions into random thermal motion. Strictly speaking, this entropy producing phase cannot be described within Vlasov theory, and the large-scale effects of these microscopic events resides at the difficult frontier between generalized fluid and kinetic theories. We attempt to describe certain aspects of such resonances within the framework of fluid theory focusing on torsional Alfven wave energy transport and deposition within flux tubes.

Siregar, E.↗

NASA NTRS1992

Nonlinear vibration and radiation from a panel with transition to chaos induced by acoustic waves

The dynamic response of an aircraft panel forced at resonance and off-resonance by plane acoustic waves at normal incidence is investigated experimentally and numerically. Linear, nonlinear (period doubling) and chaotic responses are obtained by increasing the sound pressure level of the excitation. The response time history is sensitive to the input level and to the frequency of excitation. The change in response behavior is due to a change in input conditions, triggered either naturally or by modulation of the bandwidth of the incident waves. Off-resonance, bifurcation is diffused and difficult to maintain, thus the panel response drifts into a linear behavior. The acoustic pressure emanated by the panel is either linear or nonlinear as is the vibration response. The nonlinear effects accumulate during the propagation with distance. Results are also obtained on the control of the panel response using damping tape on aluminum panel and using a graphite epoxy panel having the same size and weight. Good agreement is obtained between the experimental and numerical results.

Maestrello, Lucio↗

NASA NTRS1974

Observations of enhanced ion line frequency spectrum during Arecibo ionospheric modification experiment

The Arecibo 430 MHz incoherent scatter radar (ISR) was used to monitor the effects of modifying the ionosphere by a high power HF transmitter feeding the 305 m reflector antenna. When in the ordinary magnetoionic mode parametric instabilities develop in the ionosphere near the reflection level. Manifestations of these instabilities are the strong enhancement of Langmuir oscillations in the direction of the ISR beam at a wavelength of 35 cm and the simultaneous much weaker enhancement of ion oscillations in that direction. The spectral analysis of the enhanced peak with a height resolution of 2.4 km shows that the ionic mode enhancement most often has a double humped frequency spectrum corresponding to up- and down-going ion acoustic waves. The shape of the frequency spectrum is interpreted in terms of a stable oscillation which is driven by a secondary electrostatic field caused by nonlinear interaction of Langmuir waves within a cone centered on the magnetic field and by the scattering of the pump field on stable Langmuir waves travelling along the direction of the ISR.

Hagfors, T.↗

DOE OSTI2020

Self-Guiding of Long-Wave Infrared Laser Pulses Mediated by Avalanche Ionization

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest, as it promises high power transmission without beam breakup and multifilamentation. Central to self-guiding is the mechanism for the arrest of self-focusing collapse. Here, we show that discrete avalanche sites centered on submicron aerosols can arrest self-focusing, providing a new mechanism for self-guided propagation of moderate intensity LWIR pulses in outdoor environments. In this work, our conclusions are supported by simulations of LWIR pulse propagation using an effective index approach that incorporates the time-resolved plasma dynamics of discrete avalanche breakdown sites.

47 OTHER INSTRUMENTATION↗

NASA NTRS1966

Waves in materials with memory. V.

One-dimensional acceleration waves and higher order waves propagating in general nonlinear Maxwellian materials with fading memory

VISCOELASTICITY↗

NASA NTRS1984

Surface solitary waves and solitons

The solar atmosphere and solar wind are magnetically structured. The structuring can include tangential discontinuities, which can support surface waves. Such waves can be dispersive. This means that dispersion and nonlinearity can balance in such a way that solitary waves (or solitons) can result. This general point is illustrated by a two-dimensional nonlinear analysis which explicitly demonstrates the presence of long-wavelength solitary waves propagating on tangential discontinuities. If the waves are only weakly nonlinear, then they obey the Korteweg-de Vries equation and are true solitons.

Hollweg, J. V.↗

NASA NTRS1986

Wave-interactions in three-dimensional boundary layers

A Fourier-Chebyshev spectral method is used to study the nonlinear stability and disturbance wave-interactions in a two-dimensional boundary layer on a curved wall and a three-dimensional boundary layer on a rotating disk. Goertler and crossflow instability, respectively, are the dominant mechanisms in these boundary layers. For the range of parameters considered, the present computational model does not support the Goertler/TS interaction of the type proposed by Nayfeh (1981). According to his model, oblique TS waves with spanwise wavelengths twice that of the finite-amplitude Goertler vortex are excited in the boundary layer. The present nonlinear calculations show that both the Goertler and crosssflow vortices develop strong instabilities in their harmonics with half the wavelength of the primary vortices.

Malik, M. R.↗

NASA NTRS1993

Wave generation by contaminant ions

Investigations dealing with the generation of waves by contaminant ions are reported. The studies included the properties of the velocity distribution function of such ions. It was found that it takes about one ion-cyclotron period for the distribution function to transform from a beam to a ring distribution. A linear instability analysis was performed to examine the possibility of wave excitation by an ion beam streaming perpendicular to the ambient magnetic field in an ionospheric type of plasma. A simulation code was developed to study the nonlinear behavior of the waves excited by beam and ring types of ion distributions. The code treats both electrons and ions as particles kinetically to also factor in the possible involvement of lower-hybrid waves.

Singh, N.↗

NASA NTRS1972

Instabilities in uranium plasma and the gas-core nuclear rocket engine

The nonlinear evolution of unstable sound waves in a uranium plasma has been calculated using a multiple time-scale asymptotic expansion scheme. The fluid equations used include the fission power density, radiation diffusion, and the effects of the changing degree of ionization of the uranium atoms. The nonlinear growth of unstable waves is shown to be limited by mode coupling to shorter wavelength waves which are damped by radiation diffusion. This mechanism limits the wave pressure fluctuations to values of order delta P/P approximates 0.00001 in the plasma of a typical gas-core nuclear rocket engine. The instability is thus not expected to present a control problem for this engine.

Tidman, D. A.↗

NASA NTRS1971

Instabilities in uranium plasma.

The nonlinear evolution of unstable sound waves in a uranium plasma has been calculated using a multiple time-scale asymptotic expansion scheme. The fluid equations used include the fission power density, radiation diffusion, and the effects of the changing degree of ionization of the uranium atoms. The nonlinear growth of unstable waves is shown to be limited by mode coupling to shorter wavelength waves which are damped by radiation diffusion. This mechanism limits the wave pressure fluctuations to values of order delta P/P equal to about .00001 in the plasma of a typical gas-core nuclear rocket engine. The instability is thus not expected to present a control problem for this engine.

Tidman, D. A.↗

NASA NTRS1981

Auroral kilometric radiation - A theoretical review

Auroral kilometric radiation (AKR) is a high-density radio wave radiation in the frequency band from 50 to 750 kHz, with a peak around 250 kHz, that has been observed emanating from the auroral zone. In connection with its low frequency, the radiation can not penetrate through the ionosphere to earth, so all observations have been made by satellite. The AKR is closely correlated with the occurrence of discrete auroral arcs, which are believed to be generated by intense inverted V electron precipitation bands. A review is presented of several theories which have been proposed to explain the observed AKR. Attention is given to the conversion of electron cyclotron wave to O mode, the coherent amplification of gyroemission by velocity space instabilities, beam-driven electromagnetic instability via low-frequency turbulence, soliton radiation, loss cone instability, nonlinear beating of electrostatic waves, and the beam amplification of electromagnetic wave via coherent EIC density fluctuations.

Grabbe, C. L.↗

NASA NTRS1991

Subharmonic Route to Boundary-Layer Transition - Critical Layer Nonlinearity

The linear and nonlinear dynamics of a triad of initially linear stability waves comprising a single plane wave at fundamental frequency and two symmetric oblique waves with half the frequency and streamwise wave number of the plane wave are presented. Analysis is performed for the initial nonlinear development of the waves where the order of the oblique waves' amplitude is equal to or less than that of the plane wave. Results show that the fundamental basically follows the linear theory, while the subharmonic follows an exponential-of-an-exponential growth.

Mankbadi, Reda R.↗

DOE OSTI2025

Nonlocal Coherent Optical Nonlinearities of a Macroscopic Quantum System

The optical responses of solids are typically understood to be local in space. Whether locality holds for the optical response of a macroscopic quantum system has, however, been largely unexplored. Here, we use multidimensional coherent spectroscopy at the optical diffraction limit to demonstrate nonlocal optical nonlinearities in a semiconductor microcavity. In conclusion, these nonlocal optical responses are both coherent and quantum in nature, deriving from the macroscopic length scale of confined exciton-polariton wave functions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1978

Conversion loss and noise of microwave and millimeter-wave mixers. I - Theory. II - Experiment

The conversion loss and noise of microwave and millimeter-wave mixers are analyzed. Nonlinear capacitance, arbitrary embedding impedances, as well as shot, thermal and scattering noise arising in the diode, figure in the analysis. The anomalous mixer noise noted in millimeter-wave mixers by Kerr (1975) is shown to be explainable in terms of the correlation of down-converted components of the time-varying shot noise. A digital computer analysis of the conversion loss, noise, and output impedance of an 80-120-GHz mixer is also conducted.

Held, D. N.↗

NASA NTRS1992

Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.

Vishniac, Ethan T.↗

NASA NTRS1982

Dissipation of ionospheric irregularities by wave-particle and collisional interactions

The nonlinear dissipation of plasma irregularities aligned parallel to an ambient magnetic field is studied numerically using a model which employs both wave-particle and collisional diffusion. A wave-particle diffusion coefficient derived from a local theory of the universal drift instability is used. This coefficient is effective in regions of nonzero plasma gradients and produces triangular-shaped irregularities with spectra which vary as f to the -4th, where f is the spatial frequency. Collisional diffusion acts rapidly on the vertices of the irregularities to reduce their amplitude. The simultaneous action of the two dissipative processes is more efficient than collisions acting alone. In this model, wave-particle diffusion mimics the forward cascade process of wave-wave coupling.

Bernhardt, P. A.↗

DOE OSTI2020

Lightwave terahertz quantum manipulation of nonequilibrium superconductor phases and their collective modes

In this work, we present a gauge-invariant density matrix description of nonequilibrium superconductor (SC) states with spatial and temporal correlations driven by intense terahertz (THz) lightwaves. We derive superconductor Bloch-Maxwell equations of motion that extend Anderson pseudospin models to include the Cooper pair center-of-mass motion and electromagnetic propagation effects. We thus describe quantum control of dynamical phases, collective modes, quasiparticle coherence, and high nonlinearities during cycles of carrier wave oscillations, which relates to our recent experiments. Coherent photogeneration of a nonlinear supercurrent with a dc component, achieved via condensate acceleration by an effective lightwave field, dynamically breaks the equilibrium inversion symmetry. Experimental signatures include high harmonic light emission at equilibrium-symmetry-forbidden frequencies, Rabi-Higgs collective modes and quasiparticle coherence, and nonequilibrium moving condensate states tuned by few-cycle THz fields. We use such lightwaves as an oscillating accelerating force that drives strong nonlinearities and anisotropic quasiparticle populations to control and amplify different classes of collective modes, e.g., damped oscillations, persistent oscillations, and overdamped dynamics via Rabi flopping. Recent phase-coherent nonlinear spectroscopy experiments can be modeled by solving the full nonlinear quantum dynamics including self-consistent light-matter coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1975

Stability of localized plasma model in two and three dimensions

Stability against catastrophic radial collapse is demonstrated for initially well-behaved, localized solutions in a class of two- and three-dimensional wave equations characterized by saturating nonlinearities, which model various electrostatic and electromagnetic 'caviton' structures in an asymptotically uniform plasma. This result contrasts sharply with mathematical predictions of equations with low-order nonlinearities. Discrete classes of possible quasistationary modes exhibit features comparable to atomic wave functions and differ qualitatively from one-dimensional structures.

Wilcox, J. Z.↗