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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Resistive decay of Alfven waves in a non-uniform plasma

The effect of resistive dissipation on the propagation of an MHD disturbance in a nonuniform plasma is examined. The present analysis, based on a boundary-layer technique, shows the existence of resistive normal modes with complex eigenfrequencies. The real part of the eigenfrequency is associated with an oscillatory behavior and defines the location in space of the layer where resistivity is important. The dissipation mechanism is responsible for the damping of the wave, in contrast with previous works in which the ideal MHD theory was used.

Mok, Y.↗
NASA NTRS1995

The inner magnetosheath of Venus: An analogue for Earth?

The unmagnetized planets provide examples of solar wind interactions that are free from the complications associated with magnetopause reconnection and with sensitive obstacle response to incident solar wind pressure changes. Using the Venus magnetosheath as a testbed, we search for evidence of standing slow mode 'transitions' in the inner subsolar region as reported for Earth by Song et al. Although the system at Venus is much smaller in scale, the Pioneer Venus Orbiter magnetometer data indicate that for perpendicular interplanetary magnetic field conditions the general behavior of the plasma in the magnetosheath is as expected from the simple depletion layer model. In examples of magnetic field measurements chosen for the apparently steady interplanetary conditions during the spacecraft pass, there is no clear evidence for a slow mode structure near the ionopause as might be expected on the basis of the Song et al. study. The implication is that some aspect of the Venus magnetosheath (such as its small size or the presence of local planetary ion production) makes it physically different from Earth's, that the conditions of the magnetosheath during Song's study differed significantly from those in the Venus study, or that the observations of Song et al. do not represent a steady state.

Luhmann, J. G.↗
NASA NTRS1999

Key Durability Issues with Mullite-Based Environmental Barrier Coatings for Si-Based Ceramics

Plasma-sprayed mullite (3Al2O3 central dot 2SiO2) and mullite/yttria-stabilized-zirconia (YSZ) dual layer coatings have been developed to protect silicon-based ceramics from environmental attack. Mullite-based coating systems show excellent durability in air. However, in combustion environments, corrosive species such as molten salt or water vapor penetrate through cracks in the coating and attack the Si-based ceramics along the interface, Thus modification of the coating system for enhanced crack-resistance is necessary for long-term durability in combustion environments. Other key durability issues include interfacial contamination and coating/substrate bonding. Interfacial contamination leads to enhanced oxidation and interfacial pore formation, while weak coating/substrate bonding leads to rapid attack of the interface by corrosive species, both of which can cause premature failure of the coating. Interfacial contamination can be minimized by limiting impurities in coating and substrate materials. The interface may be modified to improve the coating/substrate bond.

Lee, Kang N.↗
NASA NTRS1995

Kinetic structure of intermediate shocks: Implications for the magnetopause

A general study of the structure and stability of intermediate shocks (IS) in an isotropic plasma is presented using a hybrid as well as a resistive Hall MHD code. Special emphasis is put on the question of whether the rotational layers observed at the magnetopause can be intermediate shocks. The shocks are formed dynamically by the interaction between a flowing plasma and a stationary piston. Coplanar ISs (both strong and weak) are found to be stable in a collisionless plasma. The existence of slow shocks in a high beta plasma is also established for the first time. Noncoplanar ISs are found to be time-dependent, evolving toward a rotational discontinuity (RD) after some characteristic time tau which can be quite long (1000 Omega(exp -1), where Omega is the ion gyrofrequency). The value tau is larger the closer the rotation angle is to 180 deg. Rotations larger than 180 deg are found to be unstable, decaying into a state of minimum shear (i.e., rotation angle less than 180 deg). There are various length scales associated with an IS in the kinetic regime. The shortest scale is found to be the length scale over which rotation of the transverse component of the magnetic field takes place. This scale can have a half width as small as one ion inertial length (c/omega(sub p)) for electron sense rotations and 3c/omega(sub p) for ion sense rotations, for an upstream ion beta of unity. Both of these scales are consistent with the observed thickness at the magnetopause and identical to the corresponding RD scales. A detailed study of the mode conversion of the Alfven ion cyclotron waves (A/IC) waves across both slow and intermediate shocks and the resulting downstream wave spectrum are presented. The possibility that the large number of relfected ions observed at the magnetopause may be due to the presence of strong ISs is considered. The identification of strong ISs and their distinction from RDs should be possible in observations due to significant differences that exist between jump conditions and overall structure of the two discontinuities. The jumps in the plasma parameters across a weak IS are typically small. This together with the fact that the weak ISs and RDs have very similar thickness and other overall properties makes the distinction between weak ISs and RDs in the observations largely inconsequential. However, at large noncoplanarity angles the weak IS approaches the RD limit in a relatively short time (approximately less than 100 Omega (exp -1)). Thus, magnetopause rotations with large noncoplanarity angles are most likely either RDs or strong ISs. Finally, direct comparisons between fluid (resistive Hall MHD) and kinetic simulations show that fluid theory is not applicable to study of ISs in a collisionless plasma.

Karimabadi, H.↗
NASA NTRS1999

Probing Ganymede's Magnetosphere with Field Line Resonances

We report on the spectrum of field line resonances identified in data acquired by the Galileo spacecraft within Ganymede's magnetosphere on a relatively low latitude pass. We infer properties of the plasma distribution and its transport from the observed spectrum. The harmonic structure in the spectrum of the magnetometer data agrees very well with the frequencies predicted for resonances of a dipole field. The spectrum implies a density of 2 amu per cubic centimeter near the equator on closed field lines of Ganymede's magnetic field inside of 2 RG (Ganymede radii). This density is significantly reduced relative to the local density of the Jovian plasma sheet near Ganymede (less than or equal to 8 electrons per cubic centimeter, or approximately equal to 100 amu per cubic centimeter for an average ion mass per charge of 20 amu and average charge of 1.5 electron charges). A shadowing effect of Ganymede for the flow of particles injected at a reconnection layer on the side of the moon downstream relative to the direction of torus plasma flow accounts for the marked density depletion. Implications for conducting paths near Ganymede's surface are considered.

Volwerk, M.↗
NASA NTRS1997

Gravity Wave Seeding of Equatorial Plasma Bubbles

Some examples from the Atmosphere Explorer E data showing plasma bubble development from wavy ion density structures in the bottomside F layer are described. The wavy structures mostly had east-west wavelengths of 150-800 km, in one example it was about 3000 km. The ionization troughs in the wavy structures later broke up into either a multiple-bubble patch or a single bubble, depending upon whether, in the precursor wavy structure, shorter wavelengths were superimposed on the larger scale wavelengths. In the multiple bubble patches, intrabubble spacings vaned from 55 km to 140 km. In a fully developed equatorial spread F case, east-west wavelengths from 690 km down to about 0.5 km were present simultaneously. The spacings between bubble patches or between bubbles in a patch appear to be determined by the wavelengths present in the precursor wave structure. In some cases, deeper bubbles developed on the western edge of a bubble patch, suggesting an east-west asymmetry. Simultaneous horizontal neutral wind measurements showed wavelike perturbations that were closely associated with perturbations in the plasma horizontal drift velocity. We argue that the wave structures observed here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity waves seed equatorial spread F irregularities.

Singh, Sardul↗
NASA NTRS1991

Current collection by conducting bodies in the low earth orbit

Collection of electrons by a conducting cylinder in a flowing magnetized plasma is studied by means of numerical simulations. Transport of electrons across magnetic field lines in regions of large electric fields in the sheath structure of the cylinder is demonstrated. The potential structures, current flow in the plasma and plasma distributions around the cylinder are presented with and without a release of a neutral gas plume near the cylinder. The simulation with the plume shows current enhancement and formation of a double layer, which is found to be highly dynamic. The simulation with the neutral gas plume simulates the plasma processes occurring in a plasma contactor.

Singh, Nagendra↗
NASA NTRS1981

A magnetospheric signature of some F layer positive storms

Calculations of electron density distributions in the global thermosphere-ionosphere system perturbed by high-latitude thermospheric heating are presented which indicate a link between the heating and magnetospheric plasma disturbances near the equator. The calculations were made using a self-consistent model of the global sunlit thermosphere-ionosphere system describing the evolution of equatorial plasma disturbances. The heat input is found to cause electron density enhancements that propagate along magnetic field lines from the F2 maximum over mid-latitudes to the equator in the magnetosphere and which correspond to the positive phase of an F layer storm. The positive phase is shown to be generated by the induction of equatorward winds that raise the mid-latitude F layer through momentum transfer from neutral atoms to ionospheric ions, which ions pull electrons with them. Model results are used to identify plasma signatures of equatorward winds and an intensified magnetospheric electric field in Explorer 45 and Arial 4 measurements taken during the positive phase of an F layer storm.

Miller, N. J.↗
NASA NTRS1995

Theoretical Plasma Distribution Consistent With Ulysses Magnetic Field Observations in a High-Speed Solar Wind Tangential Discontinuity

The overall multi-layer structure of the magnetic field observed by Ulysses across a broad solar wind tangential discontinuity can be reproduced fairly well by means of a kinetic model. Such a simulation provides complementary information about the velocity distribution functions, which are not always known due to the low time resolution inherent in plasma measurements. The success of such a simulation proves that our kinetic model can be used as a realistic basis for further studies of the structure and stability of tangential discontinuities.

solar wind Ulysses plasma↗
NASA NTRS2011

Dayside Magnetopause Transients Correlated with Changes of the Magnetosheath Magnetic Field Orientation

The paper analyses one long-term pass (26 August 2007) of the THEMIS spacecraft across the dayside low-latitude magnetopause. THEMIS B, serving partly as a magnetosheath monitor, observed several changes of the magnetic field that were accompanied by dynamic changes of the magnetopause location and/or the structure of magnetopause layers observed by THEMIS C, D, and E, whereas THEMIS A scanned the inner magnetosphere. We discuss the plasma and the magnetic field data with motivation to identify sources of observed quasiperiodic plasma transients. Such events at the magnetopause are usually attributed to pressure pulses coming from the solar wind, foreshock fluctuations, flux transfer events or surface waves. The presented transient events differ in nature (the magnetopause surface deformation, the low-latitude boundary layer thickening, the crossing of the reconnection site), but we found that all of them are associated with changes of the magnetosheath magnetic field orientation and with enhancements or depressions of the plasma density. Since these features are not observed in the data of upstream monitors, the study emphasizes the role of magnetosheath fluctuations in the solar wind-magnetosphere coupling.

Tkachenko, O.↗
NASA NTRS1996

Electrical double layers at the oil/water interface

This review presents the historical development and current status of the theory of the electrical double layer at a liquid/liquid interface. It gives rigorous thermodynamic definitions of all basic concepts related to liquid interfaces and to the electrical double layer. The difference between the surface of a solid electrode and the interface of two immiscible electrolyte solutions (ITIES) is analyzed in connection to their electrical properties. The most important classical relationships for the electrical double layer are presented and critically discussed. The generalized adsorption isotherm is derived. After a short review of the classical Gouy-Chapman and Verwey-Niessen models, more recent developments of the double layer theory are presented. These include effects of variable dielectric permittivity, nonlocal electrostatics, hydration forces, the modified Poisson-Boltzmann equation and the ion-dipole plasma. The relative merits of different theories are estimated by comparing them with computer simulation of the ITIES and electrical double layer. Special attention is given to the structure of ITIES and its variation due to adsorption of ions and amphiphilic molecules.

Non-NASA Center↗
NASA NTRS2005

Low-Thermal-Conductivity Pyrochlore Oxide Materials Developed for Advanced Thermal Barrier Coatings

When turbine engines operate at higher temperatures, they consume less fuel, have higher efficiencies, and have lower emissions. The upper-use temperatures of the base materials (superalloys, silicon-based ceramics, etc.) used for the hot-section components of turbine engines are limited by the physical, mechanical, and corrosion characteristics of these materials. Thermal barrier coatings (TBCs) are applied as thin layers on the surfaces of these materials to further increase the operating temperatures. The current state-of-the-art TBC material in commercial use is partially yttria-stabilized zirconia (YSZ), which is applied on engine components by plasma spraying or by electron-beam physical vapor deposition. At temperatures higher than 1000 C, YSZ layers are prone to sintering, which increases thermal conductivity and makes them less effective. The sintered and densified coatings can also reduce thermal stress and strain tolerance, which can reduce the coating s durability significantly. Alternate TBC materials with lower thermal conductivity and better sintering resistance are needed to further increase the operating temperature of turbine engines.

Bansal, Narottam P.↗
NASA NTRS2001

An Experimental Investigation of the Effects of Charging on the International Space Station

An experimental investigation has been undertaken to determine the effects associated with operating the International Space Station (ISS) without a Plasma Contactor Unit (PCU). The role of the PCU is to maintain the potential of the ISS to within 40 volts of the ambient plasma potential. In the event of a PCU failure, ISS structure may charge to - 160 volts with respect to the Low Earth Orbit (LEO) plasma, due to the use of high voltage photovoltaic solar arrays. Operation without a PCU will result in the charging of the oxide layers on anodized aluminum components facing into the RAM direction. In this investigation, arcs were observed as a result of anodized materials charging in a plasma environment. Actual ISS materials were used in the investigation. The materials included meteoroid and debris shield samples, as well as components from the Extravehicular Mobility Unit (EMU). Results show that the occurrence of arcs was dependent on several factors including material composition and applied voltage. The breakdown thresholds for the materials tested will be shown, as well as the voltage and current waveforms during an arc event. The damage resulting from an arc will also be described. In addition, a description of associated operational hazards resulting from arcing on the spacecraft will be provided.

Schneider, Todd↗
NASA NTRS1973

On the cause of equatorial spread F.

It is suggested that convective electric fields in the equatorial ionosphere are generated by neutral winds (zonal or meridional) which act on regions having structure in their field aligned Pedersen conductivity integrals. The conductivity structure is presumably caused by long-lived metallic ions concentrated irregularly below the F layer at altitudes where their collision and gyrofrequencies are comparable. The resulting plasma convection leads to large-scale irregularities and sets up gradients in the electric field that drive other instability mechanisms to produce small-scale irregularities. The mechanism proposed requires a different velocity above and below 200 km for winds in the magnetic east-west direction, but for winds in the magnetic meridian direction, an altitude gradient is not required.

Hanson, W. B.↗
NASA NTRS1976

Variations in plasma characteristics near D sheets in the solar wind

A strong interplanetary shock was detected by the Pioneer 8 magnetometer, plasma probe, and wave instrument at 0048 UT, on June 11, 1968. During the rest of this day the interplanetary medium was highly disturbed as seven well-defined current layers or D sheets, as well as a number of other localized interaction regions, swept past the spacecraft. The local plasma characteristics that were best correlated with passage of the D sheets appeared to involve changes in the suprathermal electron population. Changes in the 400-Hz wave levels were also detected near the discontinuities during periods when the local ion plasma frequency was near 400 Hz, but the limited measurement capability of the Pioneer 8 wave instrument does not allow an unambiguous identification of the wave-particle interactions associated with these measurements.

Scarf, F. L.↗
NASA NTRS1976

Resolution of electron emission mechanisms in an argon arc with a hot tungsten cathode

The regenerative nature of the interaction between the electron emission processes at the cathode surface and the ion-, excited-atom-, and photon-production processes in the plasma was examined. Semiquantitative estimates of the thickness of the species production layer, the current partition, and the time constants were made. These results were used to interpret experimental measurements of the current decay of an argon arc at 46.7 kPa, 3.2-7.9 A, and 28-18 V, with a self-sustained hot tungsten cathode. After the discharge is suddenly connected to a voltage clamp, the disparity of decay rates for different emission processes permits the separation of thermionic emission from the faster-decaying components. The observed thermionic current constitutes about 30% of the total emission. The high nonthermionic portions of the current could not be explained in terms of the present understanding of the emission processes.

Chen, M. M.↗
NASA NTRS1986

Ion acceleration in laboratory plasmas

Recent experiments which investigate ion acceleration in 'collisionless' laboratory plasmas are described. The experiments fall into two classes - those involving stationary and those involving time dependent processes.Among the stationary structures which accelerate ions are sheaths, presheaths, double layers, multiple double layers and ambipolar potentials. RF is an example of time dependent processes. Both classes of experiments are considered. Experiments with 'collisionless' plasmas in multidipole triple plasma-type devices and novel phenomena associated with the use of Ion Cyclotron Resonance Heating in the inhomogeneous magnetic field of a tandem mirror 'fusion' plasma are described. New RF phenomenon include RF pitch angle scattering, RF electron pumping, and ICRF ponderomotive force.

Hershkowitz, Noah↗