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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Applications of the concept of generalized vorticity to space plasmas

A reformulation of the momentum equation for electrons or ions in a collisionless plasma leads to an equation which describes the behavior of the plasma in terms of a generalized vorticity. This vorticity is both divergence-free and conserved along plasma flow streamlines. When the plasma has zero vorticity, a special relation is established which appears to have application to small scale magnetic features within both conventional space plasmas and superconductors.

Banks, P. M.↗
NASA NTRS1986

Plasma wave signatures of collisionless shocks and the role of plasma wave turbulence in shock formation

Observations of the plasma waves associated with collisionless shocks are reviewed, and the understanding of their generation mechanisms and their importance to shock physics are summarized. The emphasis is on waves generated directly at the shock, especially ion acoustic and lower-hybrid-like modes. The observations are discussed in the context of shock structure, with attention given to the distinctions between waves generated in the shock foot and ramp. The behavior of resistive, dispersive, and supercritical quasi-perpendicular shocks is contrasted. Evidence for the operation of various generation mechanisms, including interactions with cross-field currents, gyrating reflected ions, and field-aligned electron beams, are summarized. The various forms of plasma heating which are actually observed are outlined, and the role of the various wave modes in this heating is discussed. Conclusion, it is argued that, while plasma wave turbulence may play a vital role in plasma heating for some special shocks, it is of second-order importance in most cases.

Mellott, M. M.↗
NASA NTRS1973

Application of cylindrical Langmuir probes to streaming plasma diagnostics.

The current-voltage characteristics of cylindrical probes in a high velocity collisionless plasma flow have been investigated experimentally and theoretically. The plasma was generated by a focused laser pulse incident on a metallic target in vacuum. An analysis, developed from a stationary plasma analog to the flowing case, demonstrated a failure of plasma shielding of probe potential in the electron attracting region. Modifications of relatively simple previous treatments were found to be valid for computing electron current to a probe. The electron characteristics derived from the present analysis agree well with experimental results. The ion and electron portions of the characteristics are consistent with each other and with independent diagnostic measurements.

Segall, S. B.↗
NASA NTRS1976

Nonlinear interaction between two electrostatic harmonics in a plasma

The problem of harmonic and subharmonic generation of electrostatic waves in a general collisionless plasma is treated using coupled-mode theory based on two time scales. A novel feature is that one of the two interacting waves may be a negative-energy wave. Since the model describing the medium need not be specified, only a general linear and nonlinear conductivity or an equivalent description is required. Just by invoking wave-energy conservation, the coupled-mode equations are obtained in such a way that unequivocal conclusions can be drawn. When both waves have positive energy, they exchange part of it in a periodic fashion, provided that both have some energy initially. If initially all the energy is in the fundamental, all of it will eventually end up (irreversibly) in the second harmonic. If all the energy is in the harmonic initially, no generation of the fundamental (or subharmonic) will take place. If one of the two waves is a negative-energy wave, an explosive instability develops, regardless of initial values. For comparable conditions, the instability time depends on whether the negative-energy wave is in the fundamental or in the upper harmonic.

Verheest, F.↗
NASA NTRS1989

Structure of slow magnetosonic shocks in low beta plasmas

Slow magnetosonic shocks are an efficient way in which magnetic energy in a collisionless plasma is converted into particle flow and thermal energy. Previous analytic and simulation studies of slow shocks have suggested that their structure consists of a damped wavetrain beginning at the shock transition and extending into the downstream region. Spacecraft observations in the solar wind and the earth's magnetotail have found structures that resemble slow shocks except that most of them do not possess a trailing wavetrain. To resolve the conflict between theory and observations of slow shocks, new simulations have been performed which correct some of the previous results and show that depending on the sonic Mach number and the ratio of electron to ion temperature, slow shocks may or may not possess a wavetrain.

Omidi, N.↗
NASA NTRS1972

RF sheath and admittance characteristics of a spherical plasma probe.

Development of a radio-frequency sheath model for a spherical probe in a collisionless plasma. The method of solution is based on the quasi-static approximation and the electrostatic probe theory of Bernstein and Rabinowitz (1959). The resistive part of the admittance is ascribed to the sheath transit-time collisionless dissipation mechanism suggested by Mayer (1963) and developed by Gould (1964). Expressions are obtained for the effective sheath thickness and the equivalent resistance of the transit-time dissipation. The sheath model and, hence, the admittance are completely determined in terms of the bias potential, the probe radius, the plasma frequency, and the Debye length - i.e., there are no adjustable parameters in the proposed theory which are to be determined by experiment. The results obtained agree favorably with Cohen and Bekefi's (1971) experimental data on the conductance resonant frequency and the width of the conductance peak.

Kan, J. R.↗
NASA NTRS1985

Explosive tail reconnection - The growth and expansion phases of magnetospheric substorms

In the current 'conceptual' model of magnetospheric substorms, the growth phase terminates and the expansion phase commences with the onset of rapid reconnection at a new, near-earth X-type neutral line. Physical concepts developed in the analysis of the collisionless tearing mode and the flow of collisionless plasma in weakly magnetized, thin current sheets are combined to construct a model of purely collisionless, time-dependent, ion-dominated reconnection. Formulated in the context of time-dependent magnetospheric convection, the model describes the reconnection collapse of the initially thick plasma sheet. In the nonlinear phase the reconnection rate grows explosively in time and saturates into a steady collisionless reconnection flow when the initial magnetic flux in the current sheet has reconnected; at saturation the reconnection rate is comparable to the maximum Petschek rate. The time scale and dynamics of the explosive reconnection model are broadly consistent with observations of substorm growth phase and expansion phase onset. For typical plasma sheet parameters the explosive reconnection electromotive force across the tail approaches 1 MV at saturation.

Coroniti, F. V.↗
DOE OSTI2023

Spatial Intermittency of Particle Distribution in Relativistic Plasma Turbulence

Abstract Relativistic magnetically dominated turbulence is an efficient engine for particle acceleration in a collisionless plasma. Ultrarelativistic particles accelerated by interactions with turbulent fluctuations form nonthermal power-law distribution functions in the momentum (or energy) space, f ( γ ) d γ ∝ γ − α d γ , where γ is the Lorenz factor. We argue that in addition to exhibiting non-Gaussian distributions over energies, particles energized by relativistic turbulence also become highly intermittent in space. Based on particle-in-cell numerical simulations and phenomenological modeling, we propose that the bulk plasma density has lognormal statistics, while the density of the accelerated particles, n , has a power-law distribution function, P ( n ) dn ∝ n − β dn . We argue that the scaling exponents are related as β ≈ α + 1, which is broadly consistent with numerical simulations. Non-space-filling, intermittent distributions of plasma density and energy fluctuations may have implications for plasma heating and for radiation produced by relativistic turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1968

Cross-stream instability in a plasma.

Mixed hydromagnetic-electrostatic instability propagating perpendicular to aligned magnetic field and electron streaming in collisionless plasma

Sen, A. K.↗
DOE OSTI2021

Plasma sheath and presheath development near a partially reflective surface

This work addresses one-dimensional evolution of a collisionless plasma next to a solid surface that is immersed into the plasma instantaneously. In particular, we consider how the self-similar rarefaction wave establishes dynamically and how the electron reflection from the surface modifies the structure of the rarefaction wave and the Debye sheath. Here, we demonstrate that a sufficiently strong reflection eliminates the Debye sheath and changes the wall potential and the plasma flow parameters significantly. The paper presents numerical results that illustrate the developed analytical theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗