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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

The effect of shear flow on the resistive tearing instability

We develop a new scaling theory for the resistive tearing mode instability of a current sheet with a strong shear flow across the layer. The growth rate decreases with increasing flow shear and is completely stabilized as the shear flow becomes Alfvénic: both in the constant-Ψ regime, as in previous results, but we also show that the growth rate is in fact suppressed more strongly in the nonconstant-Ψ regime. As a consequence, for sufficiently large flow shear, the maximum of the growth rate is always affected by the shear suppression, and the wavenumber at which this maximum growth rate is attained is an increasing function of the strength of the flow shear. These results may be important for the onset of reconnection in imbalanced MHD turbulence.

astrophysical plasmas↗
NASA NTRS1979

Generalized lower-hybrid-drift instability

The theory of lower-hybrid-drift instability is extended to include a finite value of the component of wave vector parallel to the ambient magnetic field so that the analysis bridges the usual lower-hybrid-drift instability of flute modes and the modified-two-stream instability. The present theory also includes electromagnetic and ambient magnetic field-gradient effects. It is found that in the cold-electron limit the density and magnetic gradients can qualitatively modify the conclusion obtained in the early theory of the modified-two-stream instability. For example, even if the relative drift far exceeds the Alfven speed of the plasma, the instability may still persist. This result is in contrast to that established in the literature. When the electron temperature is finite, the problem is complicated. Numerical solutions are obtained for a number of cases.

Hsia, J. B.↗
DOE OSTI2025

Suppression of the collisionless tearing mode by flow shear: implications for reconnection onset in the Alfvénic solar wind

We analyse the collisionless tearing mode instability of a current sheet with a strong shear flow across the layer. The growth rate decreases with increasing shear flow, and is completely stabilised as the shear flow becomes Alfvénic. We also show that, in the presence of strong flow shear, the tearing mode growth rate decreases with increasing background ion-to-electron temperature ratio, the opposite behaviour to the tearing mode without flow shear. We find that even a relatively small flow shear is enough to dramatically alter the scaling behaviour of the mode, because the growth rate is small compared with the shear flow across the ion scales (but large compared with shear flow across the electron scales). Our results may explain the relative absence of reconnection events in the near-Sun Alfvénic solar wind observed recently by NASA’s Parker Solar Probe.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1976

Thermal instability in post-flare plasmas

The cooling of post-flare plasmas is discussed and the formation of loop prominences is explained as due to a thermal instability. A one-dimensional model was developed for active loop prominences. Only the motion and heat fluxes parallel to the existing magnetic fields are considered. The relevant size scales and time scales are such that single-fluid MHD equations are valid. The effects of gravity, the geometry of the field and conduction losses to the chromosphere are included. A computer code was constructed to solve the model equations. Basically, the system is treated as an initial value problem (with certain boundary conditions at the chromosphere-corona transition region), and a two-step time differencing scheme is used.

Antiochos, S. K.↗
NASA NTRS2017

Saturn's Rings and Associated Ring Plasma Cavity: Evidence for Slow Ring Erosion

We re-examine the radio and plasma wave observations obtained during the Cassini Saturn orbit insertion period, as the spacecraft flew over the northern ring surface into a radial distance of 1.3 Rs (over the C-ring). Voyager era studies suggest the rings are a source of micro-meteoroid generated plasma and dust, with theorized peak impact-created plasma outflows over the densest portion of the rings (central B-ring). In sharp contrast, the Cassini Radio and Plasma Wave System (RPWS) observations identify the presence of a ring-plasma cavity located in the central portion of the B-ring, with little evidence of impact-related plasma. While previous Voyager era studies have predicted unstable ion orbits over the C- ring, leading to field-aligned plasma transport to Saturns ionosphere, the Cassini RPWS observations do not reveal evidence for such instability-created plasma fountains. Given the passive ring loss processes observed by Cassini, we find that the ring lifetimes should extend >10(exp 9) years, and that there is limited evidence for prompt destruction (loss in <100 Myrs).

Plasma↗
NASA NTRS1989

Nonlinear Parker instability of isolated magnetic flux in a plasma

The nonlinear evolution of the Parker instability in an isolated horizontal magnetic-flux sheet embedded in a two-temperature layer atmosphere is studied by using a two-dimensional MHD code. In the solar case, this two-layer model is regarded as a simplified abstraction of the sun's photosphere/chromosphere and its overlying much hotter (coronal) envelope. The horizontal flux sheet is initially located in the lower temperature atmosphere so as to satisfy magnetostatic equilibrium under a constant gravitational acceleration. Ideal MHD is assumed, and only perturbations with k parallel to the magnetic-field lines are investigated. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. In the nonlinear regime of the instability, both the rise velocity of a magnetic loop and the local Alfven velocity at the top of the loop increase linearly with height and show self-similar behavior with height as long as the wavelength of the initial perturbation is much smaller than the horizontal size of the computing domain.

Shibata, K.↗
NASA NTRS1986

Stimulated scattering instability of lower hybrid waves

The excitation of a low-frequency quasi-mode together with two lower hybrid sidebands by a lower hybrid pump is analyzed for laboratory plasmas. This instability is excited in a narrow spectral range around the scale length of the quasi-mode which is about half the pump wavelength. The growth rates are of order of the ion cyclotron frequency. This instability can modify the intensity distribution of the pump wave and as a consequence affect the energy deposition of the pump wave in plasmas. The smaller parallel phase velocities of the forwardscattered lower hybrid sidebands are conducive to the acceleration of bulk electrons along the magnetic field.

Kuo, S. P.↗
NASA NTRS1986

The electron-cyclotron maser instability as a source of plasma radiation

The generation of continuum bursts from the sun at dm and m wavelengths (in particular, type IV bursts) via the electron-cyclotron-maser instability is examined. The maser instability can be driven by an electron distribution with either a loss-cone anisotropy or a peak at large pitch angles. For omega(p)/Omega(e) much greater than 1, the maser emission is produced by electrons interacting through a harmonic (cyclotron) resonance and is electrostatic, being in the upper hybrid mode at frequencies approximately equal to omega(p). Coalescence processes are required to convert the electrostatic waves into transverse radiation which can escape from the source region. Whether the resultant spectrum is nearly a smooth continuum or has a zebra-stripe pattern (both of which occur in type IV bursts) depends on the form of the electron distribution, inhomogeneities in the density and magnetic field, and whether the maser reaches saturation. For at least the case of some type IV dm bursts with fine structure, comparison with observations seems to indicate that the electrons producing the emission are more likely to have a loss-cone distribution, and that the maser instability is not at saturation.

Winglee, R. M.↗
DOE OSTI2023

Perspectives on relativistic electron–positron pair plasma experiments of astrophysical relevance using high-power lasers

The study of relativistic electron–positron pair plasmas is both of fundamental physics interest and important to understand the processes that shape the magnetic field dynamics, particle acceleration, and radiation emission in high-energy astrophysical environments. Although it is highly desirable to study relativistic pair plasmas in the laboratory, their generation and control constitutes a critical challenge. Significant experimental and theoretical progress has been made over recent years to explore the use of intense lasers to produce dense relativistic pair plasma in the laboratory and study the basic collective plasma processes associated with these systems. Important challenges remain in terms of improving the number of pairs, system size, and control over the charge neutrality required to establish laboratory platforms that can expand our understanding of relativistic pair plasma and help validate underlying models in conditions relevant to high-energy astrophysical phenomena. Furthermore, we highlight recent progress in this field, discuss the main challenges, and the exciting prospects for studying relativistic pair plasmas and astrophysics relevant instabilities in the laboratory in the near future.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Wave-kinetic approach to zonal-flow dynamics: Recent advances

The basic physics of drift-wave turbulence and zonal flows has long been studied within the framework of the wave-kinetic theory. Recently, this framework has been reexamined from first principles, which has led to more accurate yet still tractable “improved” wave-kinetic equations. In particular, these equations reveal an important effect of the zonal-flow “curvature” (the second radial derivative of the flow velocity) on the dynamics and stability of drift waves and zonal flows. Here, we overview these recent findings and present a consolidated high-level picture of (mostly quasilinear) zonal-flow physics within reduced models of drift-wave turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Upper and lower bounds on phase-space rearrangements

Broad classes of plasma phenomena can be understood in terms of phase-space rearrangements. For example, the net effect of a wave–particle interaction may consist of moving populations of particles from one region of phase space to another. Different phenomena drive rearrangements that obey different rules. When those rules can be specified, it is possible to calculate bounds that limit the possible effects the rearrangement could have (such as limits on how much energy can be extracted from the particles). This leads to two problems. The first is to understand the mapping between the allowed class of rearrangements and the possible outcomes that these rearrangements can have on the overall distribution. The second is to understand which rules are appropriate for which physical systems. There has been recent progress on both fronts, but a variety of interesting questions remain unanswered.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1965

Hydromagnetic stability

Stability of magnetized plasmas - gravitational instability, effects of finite Larmor radius, frequency, and resistivity

MAGNETOHYDRODYNAMIC STABILITY↗
NASA NTRS1977

Three-dimensional ion sound turbulence

A fast cold large diameter electron beam is injected into a uniform background plasma. The injected beam current is balanced by a field-aligned return current which gives rise to an ion acoustic instability. The plasma is essentially unbounded and collisionless on the time scale of the pulsed experiment. The fluctuations are analyzed by probe techniques in real time and space as well as in the frequency and wavenumber domains. Strong density fluctuations are observed in the entire ion acoustic spectrum reaching peak amplitudes at the low frequency end. The propagation of phase coherent test waves in the current carrying plasma is investigated. For the highly turbulent regime strong damping is observed while at lower drift velocities direction is observed. Oblique propagation of two strong test waves at different frequencies indicate the role of nonlinear ion Landau damping in absorption of wave energy.

Stenzel, R. L.↗
NASA NTRS2011

AXIOM: Advanced X-Ray Imaging of the Magnetosphere

Planetary plasma and magnetic field environments can be studied in two complementary ways by in situ measurements, or by remote sensing. While the former provide precise information about plasma behaviour, instabilities and dynamics on local scales, the latter offers the global view necessary to understand the overall interaction of the magnetospheric plasma with the solar wind. Some parts of the Earth's magnetosphere have been remotely sensed, but the majority remains unexplored by this type of measurements. Here we propose a novel and more elegant approach employing remote X-ray imaging techniques, which are now possible thanks to the relatively recent discovery of solar wind charge exchange X-ray emissions in the vicinity of the Earth's magnetosphere. In this article we describe how an appropriately designed and located X-ray telescope, supported by simultaneous in situ measurements of the solar wind, can be used to image the dayside magnetosphere, magnetosheath and bow shock, with a temporal and spatial resolution sufficient to address several key outstanding questions concerning how the solar wind interacts with the Earth's magnetosphere on a global level. Global images of the dayside magnetospheric boundaries require vantage points well outside the magnetosphere. Our studies have led us to propose AXIOM: Advanced X-ray Imaging Of the Magnetosphere, a concept mission using a Vega launcher with a LISA Pathfinder-type Propulsion Module to place the spacecraft in a Lissajous orbit around the Earth Moon L1 point. The model payload consists of an X-ray Wide Field Imager, capable of both imaging and spectroscopy, and an in situ plasma and magnetic field measurement package. This package comprises a Proton-Alpha Sensor, designed to measure the bulk properties of the solar wind, an Ion Composition Analyser, to characterize the minor ion populations in the solar wind that cause charge exchange emission, and a Magnetometer, designed to measure the strength and direction of the solar wind magnetic field. We also show simulations that demonstrate how the proposed X-ray telescope design is capable of imaging the predicted emission from the dayside magnetosphere with the sensitivity and cadence required to achieve the science goals of the mission.

Branduardi-Raymont, G.↗