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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Stability and Characteristics of Lower-hybrid Drift Waves: Dependence on Electron Beta and Cross-field Relative Drift

Lower-hybrid drift waves (LHDWs) are frequently observed microinstabilities in both space and laboratory plasmas. Despite decades of study, the relationship between electrostatic (ES-LHDW) and electromagnetic (EM-LHDW) variants and the plasma parameters controlling their stability remains unclear. Here, we systematically examine LHDW behavior by solving the local linear dispersion relation over a wide range of plasma and field conditions. Our results demonstrate that ES-LHDWs and EM-LHDWs are not distinct modes but rather two different regimes of the same drift wave whose character evolves smoothly with electron beta (β e ) and the cross-field electron drift velocity relative to ions, normalized to the ion sound speed (u 0x /C s ). The nature of the waves changes from electrostatic to electromagnetic when β e increases. Growth rates increase with u 0x /C s but decrease with β e , while the most unstable wavelength remains nearly universal, with kρ e ∼ 0.8 (k is the magnitude of the wave vector and ρ e is the electron gyroradius). We further present quasi-linear estimates of nonlinear saturation properties, including energy partition among electric fields, magnetic fields, and particle kinetic responses. We show that ES-LHDWs reach higher electric-field saturation amplitudes, whereas EM-LHDWs generate strong magnetic perturbations and parallel electric fields that may enable efficient particle heating. Comparisons with the classical model reveal that retaining electromagnetic effects is essential for accurate predictions of frequency, growth rate, and the propagation angle. These findings provide a unified framework for understanding LHDWs across diverse collisionless plasma environments, including current sheets of magnetic reconnection, shear layers, collisionless shocks, and boundary regions.

Solar coronal waves↗
DOE OSTI2023

The Kelvin–Helmholtz Instability at the Boundary of Relativistic Magnetized Jets

We study the linear stability of a planar interface separating two fluids in relative motion, focusing on conditions appropriate for the boundaries of relativistic jets. The jet is magnetically dominated, whereas the ambient wind is gas-pressure-dominated. We derive the most general form of the dispersion relation and provide an analytical approximation of its solution for an ambient sound speed much smaller than the jet Alfvén speed v A , as appropriate for realistic systems. The stability properties are chiefly determined by the angle ψ between the wavevector and the jet magnetic field. For ψ = π/2, magnetic tension plays no role, and our solution resembles the one of a gas-pressure-dominated jet. Here, only sub-Alfvénic jets are unstable ($0\lt {M}_{e}\equiv (v/{v}_{{\rm{A}}})\cos \theta \lt 1$ , where v is the shear velocity and θ the angle between the velocity and the wavevector). For ψ = 0, the free energy in the velocity shear needs to overcome the magnetic tension, and only super-Alfvénic jets are unstable ($1\lt {M}_{e}\lt \sqrt{{(1+{{\rm{\Gamma }}}_{w}^{2})/[1+({v}_{{\rm{A}}}/c)}^{2}{{\rm{\Gamma }}}_{w}^{2}]}$, with Γ w the wind adiabatic index). Our results have important implications for the propagation and emission of relativistic magnetized jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind

We use in situ measurements from the first 19 encounters of Parker Solar Probe and the most recent five encounters of Solar Orbiter to study the evolution of the turbulent sonic Mach number M t (the ratio of the amplitude of velocity fluctuations to the sound speed) with radial distance and its relationship to density fluctuations. We focus on the near-Sun region with radial distances ranging from about 11 to 80 R ⊙ . Our results show that (1) the turbulent sonic Mach number M t gradually moves toward larger values as it approaches the Sun, until at least 11 R ⊙ , where M t is much larger than the previously observed value of 0.1 at and above 0.3 au; (2) transonic turbulence with M t ∼ 1 is observed in situ for the first time and is found mostly near the Alfvén critical surface; (3) Alfvén Mach number of the bulk flow M A shows a strong correlation with the plasma beta, indicating that most of the observed sub-Alfvénic intervals correspond to a low-beta plasma; (4) the scaling relation between density fluctuations and M t gradually changes from a linear scaling at larger radial distances to a quadratic scaling at smaller radial distances; and (5) transonic turbulence is more compressible than subsonic turbulence, with enhanced density fluctuations and slightly flatter spectra than subsonic turbulence. A systematic understanding of compressible turbulence near the Sun is necessary for future solar wind modeling efforts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Plasma electron hole kinematics. Hole tracking Particle-In-Cell simulation

The kinematics of a 1-D electron hole is studied using a novel Particle-In-Cell simulation code. A hole tracking technique enables us to follow the trajectory of a fast-moving solitary hole and study quantitatively hole acceleration and coupling to ions. We observe a transient at the initial stage of hole formation when the hole accelerates to several times the cold-ion sound speed. Artificially imposing slow ion speed changes on a fully formed hole causes its velocity to change even when the ion stream speed in the hole frame greatly exceeds the ion thermal speed, so there are no reflected ions. The behavior that we observe in numerical simulations agrees very well with our analytic theory of hole momentum conservation and the effects of “jetting.”

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1970

Ultrasonic propagation in gases at high temperatures

Ultrasonic pulse method /1 to 3 MHz/ measures both sound speed and absorption in monatomic and polyatomic gases in a temperature range of 300 to 20000 degrees K at atmospheric pressure. Helium, nitrogen, oxygen, and argon are investigated.

Carey, C.↗
NASA NTRS1970

Dispersion discontinuities of strong collisionless shocks

Linear fluid equations are used to estimate wave train properties of strong collisionless shocks. Fast shocks exhibit several dispersion changes with increasing Mach number. For perpendicular propagation into a finite-beta plasma, an ion cyclotron radius trailing wave train exists only for (M sub F)2 is smaller than 2. Oblique fast shocks have a leading ion inertia wave train if M sub A is smaller than root of M(+)/M(-) cos theta/2 and a trailing electron inertia train if M sub A is greater than root of M(+)/M(-) cos theta/2. If the downstream sound speed exceeds the flow speed, linear wave theory predicts a trailing ion acoustic structure which probably resides within the magnetic shock. For a turbulent shock model in which an effective electron-ion collision frequency exceeds the lower hybrid frequency, ions decouple from the magnetic field; the shock wave train now trails with electron inertia and electron gyroradius lengths. Comparisons of this turbulent model and observations on the earth's bow shock are made.

Coroniti, F. V.↗
NASA NTRS1973

Pressure waves generated by steady flames.

Analysis of pressure waves that can be generated by clouds of explosive gas mixtures in a free atmosphere which is initially at a uniform state. The treatment is restricted only to the final stage of constant flame velocity when the flowfield is self-similar. By the introduction of reduced blast-wave parameters as phase-plane coordinates, the problem is resolved into the determination of the appropriate integral curves on this plane. Results, including space profiles of gasdynamic parameters, have been computed for a specific case of a hydrocarbon-air mixture characterized by a specific heat ratio of 1.3, sound speed at NTP of 345 m/sec, and volumetric expansion ratio corresponding to constant pressure deflagration of 7. Maximum overpressure ratios that can be generated by such flames in point-and line-symmetrical waves range from .00053, for the lower bound in the burning speed, up to 6 for the deflagration, while, for the average speeds of 5 to 10 m/sec, they are at a level of 0.05 to 0.10.

Kuhl, A. L.↗
NASA NTRS1973

Far field effects of weak spatially periodic inhomogeneities.

Small variations in the properties of an acoustic medium produce cumulative effects that significantly alter wave forms propagating over long distances. To investigate some aspects of these effects, without statistical models, the one-dimensional wave equation with a weak spatially periodic variation in the sound speed is treated. An approximation is developed valid for such weak inhomogeneities over large distances based upon the approximate treatment of a linear differential equation with a periodic coefficient. It is applied to the propagation of an initial step wave and identifies the dispersive nature of the medium which accounts for the cumulative effects.

Friedman, M. B.↗
NASA NTRS1974

Role of physical properties of liquids in cavitation erosion

The dependence of erosion rates on the ambient temperature of water is discussed. The assumption that the gas inside the bubble is compressed adiabatically during collapse gives better agreement with experiments than the assumption that the gas is isothermally compressed. Acoustic impedance is an important liquid parameter that governs the erosion intensity in vibratory devices. The investigation reveals that the major physical properties of liquids governing the intensity of erosion include density, sound speed, surface tension, vapor pressure, gas content, and nuclei distribution.

Thiruvengadam, A.↗
NASA NTRS1975

Broadband noise generated by turbulent inflow to rotor or stator blades in an annular duct

The Green's function relating the radiated pressure field to the fluctuating forces on rotor or stator blades is developed in the presence of dissipation due to turbulent velocity fluctuations and sound speed fluctuations. The resonances in the output power spectrum which would occur at the cut-off frequencies in the absence of dissipation should be removed and smeared out by the incorporation of dissipation. Wave number dependence is developed for an effective eddy viscosity due to the aforementioned fluctuations in the background medium. The space-time correlation function for blade-normal velocity fluctuations on a single or on two different blades is developed in terms of the velocity correlation tensor for the inflow under the assumptions of isotropy and (Taylor) frozen behavior. The correlation function is then simplified under certain approximations and the behavior of the blade-force correlation function is inferred.

Lnae, F.↗
NASA NTRS1976

Evaporative cooling of flare plasma

A one-dimensional loop model for the evaporative cooling of the coronal flare plasma was investigated. Conductive losses dominated radiative cooling, and the evaporative velocities were small compared to the sound speed. The profile and evolution of the temperature were calculated. The model was in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation was to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma.

Antiochos, S. K.↗
NASA NTRS1977

High-resolution stellar vidicon spectrophotometry. I - Variable mass loss from Arcturus and the hypothesis of giant convective elements

High-resolution spectrophotometry of the variable Ca II K line in the K2 IIIp star Alpha Boo was performed with the McMath Solar Telescope at Kitt Peak National Observatory and an experimental SEC vidicon camera. The results are compared with Copernicus observations of the Mg II h and k lines and with earlier Ca II data. It is found that either of two states may typically occur in the Arcturus chromosphere. From comparison with the results of model calculations for expanding chromospheres, it is concluded that these correspond respectively to a 'normal' state in which the mass loss is less than one billionth of a solar mass per year and an 'abnormal' state in which the mass loss is about 8 billionths of a solar mass per year. In the latter case, the expansion velocity is around 13 km/s at optical depth unity in the K-line, which exceeds the local sound speed. It is suggested that the abnormal state represents the rise to the photosphere of a very large convective element, as hypothesized for red giants by Schwarzschild (1975).

Chiu, H. Y.↗
NASA NTRS1977

Reflection and radiation due to a quadrupole near a fluid interface

The reflection of sound at an interface between two fluid half-space in relative motion (one in motion and the other at rest), but with different density and different sound speed, is considered. The reflection coefficient is found in an explicitly closed form. A brief discussion of the effect of mean velocity, i.e., corresponding Mach number on different noise generating quadrupole directivities concludes the presentation of the paper. The graphs provided illustrate this aspect in a rather more convincing way.

Dash, R.↗
NASA NTRS1978

Evaporative cooling of flare plasma

We investigate a one-dimensional loop model for the evaporative cooling of the coronal flare plasma. The important assumptions are that conductive losses dominate radiative cooling and that the evaporative velocities are small compared with the sound speed. We calculate the profile and evolution of the temperature and verify the accuracy of our assumptions for plasma parameters typical of flare regions. The model is in agreement with soft X-ray observations on the evolution of flare temperatures and emission measures. The effect of evaporation is to greatly reduce the conductive heat flux into the chromosphere and to enhance the EUV emission from the coronal flare plasma.

Antiochos, S. K.↗
NASA NTRS1979

Radiative dominated cooling of the flare corona and transition region

Models in which radiation dominates cooling flare loops are investigated. The radiative models are found to predict a differential emission measure (Q) proportional to T to the (l+1) power, where l measures the dependence of the radiative loss coefficient on temperature, lamda (T) approximately T to the (-l) power. It is concluded that the radiative models are incapable of explaining the observed temperature dependence of Q for flare coronal and transitional plasma. The models suggest that large mass motions (velocities of the order of the sound speed) may be required.

Antiochos, S. K.↗
NASA NTRS1979

A two phase Mach number description of the equilibrium flow of nitrogen in ducts

Some additional thermodynamic properties of the usual two-phase form which is linear in the moisture fraction are derived which are useful in the analysis of many kinds of duct flow. The method used is based on knowledge of the vapor pressure and Gibbs function as functions of temperature. With these, additional two-phase functions linear in moisture fraction are generated, which ultimately reveal that the squared ratio of mixture specific volume to mixture sound speed depends on liquid mass fraction and temperature in the same manner as do many weighted mean two-phase properties. This leads to a simple method of calculating two-phase Mach numbers for various duct flows. The matching of one- and two-phase flows at a saturated vapor point with discontinuous Mach number is also discussed.

Bursik, J. W.↗