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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 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Magnetic fields interacting with nonlinear compressible convection

Two-dimensional numerical simulations are used to study fully compressible convection in the presence of an imposed magnetic field. Highly nonlinear flows are considered that span multiple density scale heights. The convection tends to sweep the initially uniform vertical magnetic field into concentrated flux sheets with significant magnetic pressures. These flux sheets are partially evacuated, and effects of buoyancy and Lorentz forces there can serve to suppress motions. The flux sheets can be surrounded by a sheath of descending flow. If the imposed magnetic field is sufficiently strong, the convection can become oscillatory. The unstably stratified fluid layer has an initial density ratio (bottom to top of layer) of 11. Surveys of solutions at fixed Rayleigh number sample Chandrasekhar numbers from 1 to 1000 and magnetic Prandtl numbers from 1/16 to 1. These nonlinear simulations utilize a two-dimensional numerical scheme based on a modified two-step Lax-Wendroff method.

Hulburt, Neal E.↗
NASA NTRS1988

Magnetic clouds, geomagnetic disturbances, and cosmic ray decreases

Nineteen magnetic clouds are identified in the years from 1978 through 1982 and studied using superposed epoch analysis. The magnetic-field intensity, proton density and proton temperature are enhanced ahead of magnetic clouds that are preceded by shock, while strong magnetic-field intensity and low proton temperature are observed within the clouds. A relatively large (about 2.5 percent) decrease in cosmic-ray intensity is associated with magnetic clouds that are preceded by a shock, perhaps caused by the turbulent sheath behind an interplanetary shock ahead of the magnetic cloud, whereas only a small (0.5 percent) decrease in intensity is associated with the magnetic cloud itself. Magnetic clouds can produce geomagnetic activity with a decrease in Dst index of the order 100 gamma. The magnitude of the change in the Dst index for the case when southward fields arrive first is comparable to that for the case when northward fields arrive first, and the phase is such that geomagnetic activity is associated with southward fields.

Zhang, G.↗
NASA NTRS1987

Bidirectional electron anisotropies in the distant tail - ISEE 3 observations of polar rain

A detailed observational treatment of bidirectional electrons (50 approx. 500 eV) in the distant magnetotail (or greater than or equal to 100 R sub E) is presented. It is found that electrons in this energy range commonly exhibit strong, field aligned anisotropies in the tail lobes. Because of large tail motions, the ISEE-3 data provide extensive sampling of both the north and south lobes in rapid succession, demonstrating directly the strong asymetries that exist between the north and south lobes at any one time. The bidirectional fluxes are found to occur predominantly in the lobe directy connected to the sunward IMF in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Electron anisotropy and magnetic field data are presented which show the transition from unidirectional (sheath) electron populations to bidirectional (lobe) populations. Taken together, the present evidence suggests that the bidirectional electrons that we observe in the distant tail are closely related to the Polar rain electrons observed previously at lower altitudes. Furthermore, these data provide strong evidence that the distant tail is comprised largely of open magnetic field lines in contra distinction to some recently advanced models.

Baker, D. N.↗
NASA NTRS1988

Electron beam experiments at high altitudes

Experiments with the electron gun on the SCATHA satellite produced evidence of beam-plasma interactions, and heating of the low energy electrons around the satellite. These experiments were conducted near geosynchronous orbit, in the dusk bulge, and plasma sheet, with one short operation in the lobe regions, providing a range of ambient plasma densities. The electron gun was operated at 50 eV, with beam currents of 1, 10, and 100 micro-A. Data from electrostatic analyzers and the DC electric field experiment show that the satellite charged to near the beam energy in sunlight, if the beam current was sufficient. Higher ambient densities required higher beam currents. The electrostatic analyzers showed distribution functions which had peaks, or plateaus, at energies greater than the satellite potential. These measurements indicate heating of the ambient plasma at several Debye lengths from the satellite, with the heated plasma then accelerated into the satellite. It is likely that the ambient plasma is in fact the photoelectron sheath generated by the satellite.

Olsen, R. C.↗
NASA NTRS1988

Exceptional preservation of fossils in an Upper Proterozoic shale

An exceptionally well-preserved and distinctive assemblage of Late Proterozoic fossils from subtidal marine shales is reported. In addition to the spheromorphic acritarchs and cyanobacteria sheaths routinely preserved in Proterozoic rocks, this assemblage includes multicellular algae, a diverse assortment of morphologically complex protistan vesicles, and probable heterotrophic bacteria. Thus, it provides one of the clearest and most taxonomically varied views of Proterozoic life yet reported.

Butterfield, N. J.↗
NASA NTRS1989

Making Multicore, Multishell Microspheres

Multiple streams of fluids combined to produce various structures. Spherical or nearly spherical micro-shells containing two or more discrete inclusions produced by proposed technique. Pairs of coaxial nozzles combined in arrays of two or more. Each pair of nozzles generate inner jet surrounded by sheath of fluid. Multiple jets combined to form droplets composed of shells and cores. Spheres designed so cores combine to mix or react only after leaving shell generator. Similarly, two or more shell-wall materials mix or react after leaving generator.

Morrison, Andrew D.↗
NASA NTRS1988

Townsend coefficients for electron scattering over dielectric surfaces

A method for describing the probability of initiating flashover discharges across dielectric surfaces is presented in which a transport coefficient for electron multiplication similar to the Townsend coefficient used for gas discharges is defined. The coefficient is a function of the scaling parameter (charge released from the cathode)/(cathode-anode separation) and is also a measure of the growth of the sheath on the dielectric surface resulting from electron scattering. Results are discussed as to when the source of seed electrons does not necessarily depend upon field emission at the cathode-vacuum-dielectric triple point. For these conditions, there is a different functional dependence of flashover probability on voltage and geometry than when field emission provides the seed electrons. As a result, criteria previously used to predict flashover discharges may not apply.

Peck, Timothy L.↗
NASA NTRS1989

Electrostatic charging of spacecraft in response to electron beam injection

Electron beam injections from spacecraft now constitute a major activity in space research. Here, the charging level of a conducting surface when an electron beam is injected from it is investigated. Injections into both vacuum and an ambient plasma are considered. When a Maxwellian beam is injected into vacuum, the surface changes to a potential much greater than the average beam energy. The dependence of this excess is examined by considering beams with water-bag types of velocity distribution functions in which no electron has a velocity V(max) above a certain value. The electric field distribution in the electron sheath near the surface is determined by the pressure distribution. Thus, the surface potential is determined not only by V(max) but by all the beam parameters. The ambient plasma reduces the charging level and causes an oscillation in the surface potential. The oscillation frequency is the electron-plasma frequency associated with the ambient plasma.

Singh, Nagendra↗
NASA NTRS1990

Automatic Tension Adjuster For Flexible-Shaft Grinder

Flexible shaft of grinding tool automatically maintained in tension by air pressure. Probelike tool bent to reach hard-to-reach areas for grinding and polishing. Unless shaft held in tension, however, it rubs against its sheath, overheating and wearing out quickly. By taking up slack in flexible cable, tension adjuster reduces friction and enables tool to operate more efficiently, in addition to lengthening operating life.

Burley, Richard K.↗
NASA NTRS1990

Implantable Electrode For Critical Locations

Implantable electrode holds itself in place until body tissue grows around it and retains it. Sheath covers electrode during implantation. Partially retracted after implantation and completely retracted after growth of tissue onto electrode. Intended to provide electrical stimuli to muscles having become inactive through accident or disease. Helps paraplegics and others to exercise voluntary control over arms and legs.

Collins, Earl R., Jr.↗
NASA NTRS1989

VLA observations of the coronal plasma

VLA observations at 20 cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft x ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio or x ray wavelengths while remaining invisible in the other spectral domain, and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90 cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying H alpha filament seen in absorption. The 20 cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources. The 90 cm VLA data indicate that time-correlated radio bursts can occur in active regions on opposite sides of the solar equator. These regions are apparently linked by large scale, trans-equatorial magnetic loops at least 2.6 x 10(exp 5) km (or 6 feet) long; these loops act as magnetic conduits for relativistic electrons moving at one-third the velocity of light.

Lang, Kenneth R.↗
NASA NTRS1989

Space Station Freedom solar array panels plasma interaction test facility

The Space Station Freedom Power System will make extensive use of photovoltaic (PV) power generation. The phase 1 power system consists of two PV power modules each capable of delivering 37.5 KW of conditioned power to the user. Each PV module consists of two solar arrays. Each solar array is made up of two solar blankets. Each solar blanket contains 82 PV panels. The PV power modules provide a 160 V nominal operating voltage. Previous research has shown that there are electrical interactions between a plasma environment and a photovoltaic power source. The interactions take two forms: parasitic current loss (occurs when the currect produced by the PV panel leaves at a high potential point and travels through the plasma to a lower potential point, effectively shorting that portion of the PV panel); and arcing (occurs when the PV panel electrically discharges into the plasma). The PV solar array panel plasma interaction test was conceived to evaluate the effects of these interactions on the Space Station Freedom type PV panels as well as to conduct further research. The test article consists of two active solar array panels in series. Each panel consists of two hundred 8 cm x 8 cm silicon solar cells. The test requirements dictated specifications in the following areas: plasma environment/plasma sheath; outgassing; thermal requirements; solar simulation; and data collection requirements.

Martin, Donald F.↗
NASA NTRS1990

Kinetics and thermodynamics of ceramic/metal interface reactions related to high T(sub c) superconducting applications

Superconducting ceramic materials, no matter what their form, size or shape, must eventually make contact with non-superconducting materials in order to accomplish current transfer to other parts of a real operating system, or for testing and measurement of properties. Thus, whether the configuration is a clad wire, a bulk superconducting disc, tape, or a thick or thin superconducting film on a substrate, the physical and mechanical behavior of interface (interconnections, joints, etc.) between superconductors and normal conductor materials of all kinds is of extreme importance to the technological development of these systems. Fabrication heat treatments associated with the particular joining process allow possible reactions between the superconducting ceramic and the contact to occur, and consequently influence properties at the interface region. The nature of these reactions is therefore of great broad interest, as these may be a primary determinant for the real capability of these materials. Research related both to fabrication of composite sheathed wire products, and the joining contacts for physical property measurements, as well as, a review of other related literature in the field are described. Comparison are made between 1-2-3, Bi-, and Tl-based ceramic superconductors joined to a variety of metals including Cu, Ni, Fe, Cr, Ag, Ag-Pd, Au, In, and Ga. The morphology of reaction products and the nature of interface degradation as a function of time will be highlighted.

Notis, Michael R.↗
NASA NTRS1989

Measurement of interfacial shear strength in SiC-fiber/Si3N4 composites

An indentation method for measuring shear strength in brittle matrix composites was applied to SiC-fiber/Si3N4-matrix samples. Three methods were used to manufacture the composites: reaction bonding of a Si/SiC preform, hot-pressing, and nitrogen-overpressure sintering. An indentation technique developed by Marshall for thin specimens was used to measure the shear strength of the interface and the interfacial friction stresses. This was done by inverting the sample after the initial push through and retesting the pushed fibers. SEM observations showed that the shear strength was determined by the degree of reaction between the fiber and the matrix unless the fiber was pushed out of its (well-bonded) sheath.

Laughner, James W.↗
NASA NTRS1990

High-voltage plasma interactions calculations using NASCAP/LEO

This paper reviews four previous simulations (two laboratory and two space-flight) of interactions of a high-voltage spacecraft with a plasma under low-earth orbit conditions, performed using a three-dimensional computer code NASCAP/LEO. Results show that NASCAP/LEO can perform meaningful simulations of high-voltage plasma interactions taking into account three-dimensional effects of geometry, spacecraft motion, and magnetic field. Two new calculations are presented: (1) for current collection by 1-mm pinholes in wires (showing that a pinhole in a wire can collect far more current than a similar pinhole in a flat plate); and (2) current collection by Charge-2 mother vehicle launched in December 1985. It is shown that the Charge-2 calculations predicted successfully ion collection at negative bias, the floating potential of a probe outside or inside the sheath under negative bias conditions, and magnetically limited electron collection under electron beam operation at high altitude.

Mandell, M. J.↗
NASA NTRS1990

Structure and properties of the subsolar magnetopause for northward IMF - ISEE observations

This paper describes the structure and the magnetic-field, electric-field, and plasma properties of the ISEE-1 magnetopause crossing on November 5, 1978, which occurred near the subsolar point when the IMF was strongly northward. It was found that the magnetopause was composed of three layers: (1) a sheath transition layer, in which there is a gradual density decrease without a change in temperature and which occurs totally within the magnetosheath plasma; (2) an outer boundary layer, which is dominated by magnetosheath particles; (3) and an inner boundary layer dominated by magnetospheric particles. No significnt heating or cooling was seen across the magnetopause during this crossing. The plasma within each of the layers was quite uniform, and their boundaries were sharp, suggesting that there was very little diffusion present.

Song, P.↗
NASA NTRS1990

Observation of an antenna-plasma instability

This paper investigates the conditions leading to, and the causes of, the phenomenon observed during a rocket flight (to 585-km altitude) of an occasionally occurring narrow-band signal on an electric antenna whose frequency was found to vary as the rocket turned. The amplitude also varied by a factor larger than can be explained by variable coupling to the plasma; maximum oscillation amplitude occurred when the antenna was aligned with the earth's magnetic field. A tentative explanation of this phenomenon is given, suggesting that the signals on the antenna were caused by an interacton of the flowing plasma with sheath waves around the antenna.

Kellogg, P. J.↗
NASA NTRS1989

The Environment-Power System Analysis Tool development program

The Environment Power System Analysis Tool (EPSAT) is being developed to provide engineers with the ability to assess the effects of a broad range of environmental interactions on space power systems. A unique user-interface-data-dictionary code architecture oversees a collection of existing and future environmental modeling codes (e.g., neutral density) and physical interaction models (e.g., sheath ionization). The user-interface presents the engineer with tables, graphs, and plots which, under supervision of the data dictionary, are automatically updated in response to parameter change. EPSAT thus provides the engineer with a comprehensive and responsive environmental assessment tool and the scientist with a framework into which new environmental or physical models can be easily incorporated.

Jongeward, Gary A.↗