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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Instabilities in uranium plasma and the gas-core nuclear rocket engine

The nonlinear evolution of unstable sound waves in a uranium plasma has been calculated using a multiple time-scale asymptotic expansion scheme. The fluid equations used include the fission power density, radiation diffusion, and the effects of the changing degree of ionization of the uranium atoms. The nonlinear growth of unstable waves is shown to be limited by mode coupling to shorter wavelength waves which are damped by radiation diffusion. This mechanism limits the wave pressure fluctuations to values of order delta P/P approximates 0.00001 in the plasma of a typical gas-core nuclear rocket engine. The instability is thus not expected to present a control problem for this engine.

Tidman, D. A.↗
NASA NTRS1971

Instabilities in uranium plasma.

The nonlinear evolution of unstable sound waves in a uranium plasma has been calculated using a multiple time-scale asymptotic expansion scheme. The fluid equations used include the fission power density, radiation diffusion, and the effects of the changing degree of ionization of the uranium atoms. The nonlinear growth of unstable waves is shown to be limited by mode coupling to shorter wavelength waves which are damped by radiation diffusion. This mechanism limits the wave pressure fluctuations to values of order delta P/P equal to about .00001 in the plasma of a typical gas-core nuclear rocket engine. The instability is thus not expected to present a control problem for this engine.

Tidman, D. A.↗
NASA NTRS1974

Ground-state energies of simple metals

A structural expansion for the static ground-state energy of a simple metal is derived. Two methods are presented, one an approach based on single-particle band structure which treats the electron gas as a nonlinear dielectric, the other a more general many-particle analysis using finite-temperature perturbation theory. The two methods are compared, and it is shown in detail how band-structure effects, Fermi-surface distortions, and chemical-potential shifts affect the total energy. These are of special interest in corrections to the total energy beyond third order in the electron-ion interaction and hence to systems where differences in energies for various crystal structures are exceptionally small. Preliminary calculations using these methods for the zero-temperature thermodynamic functions of atomic hydrogen are reported.

Hammerberg, J.↗
NASA NTRS1974

Study of ceramic products and processing techniques in space

An analysis of the solidification kinetics of beta alumina in a zero-gravity environment was carried out, using computer-simulation techniques, in order to assess the feasibility of producing high-quality single crystals of this material in space. The two coupled transport processes included were movement of the solid-liquid interface and diffusion of sodium atoms in the melt. Results of the simulation indicate that appreciable crystal-growth rates can be attained in space. Considerations were also made of the advantages offered by high-quality single crystals of beta alumina for use as a solid electrolyte; these clearly indicate that space-grown materials are superior in many respects to analogous terrestrially-grown crystals. Likewise, economic considerations, based on the rapidly expanding technological applications for beta alumina and related fast ionic conductors, reveal that the many superior qualities of space-grown material justify the added expense and experimental detail associated with space processing.

Markworth, A. J.↗
NASA NTRS1984

Core saturation in a moving medium

A numerical technique for solving the line transfer equation of a two-level atom in static equilibrium is presented. Complete redistribution of emitted photons is assumed, as is saturation at the line core. Emission intensity is calculated either by a generalized Eddington-Barber relation, a first-order differential equation for the specific intensity, or by a formal transfer integral. Sample calculations are performed of the line transfer equation in a semi-infinite atmosphere with a constant Planck function of the collision parameter and for the Mg II resonance line in a model solar atmosphere experiencing shocks. Attention is focused on the line wings in the latter problem. The first order differential equation approach yields the best intensity values and temperature structure.

Kalkofen, W.↗
NASA NTRS1987

Ginzburg-Landau theory for the solid-liquid interface of bcc elements

Consideration is given to a simple order-parameter theory for the interfacial tension of body-centered-cubic solids in which the principal order parameter is the amplitude of the density wave at the smallest nonzero reciprocal-lattice vector of the solid. The parameters included in the theory are fitted to the measured heat of fusion, melting temperature, and solid-liquid density difference, and to the liquid structure factor and its temperature derivative at freezing. Good agreement is found with experiment for Na and Fe and the calculated anisotropy of the surface tension among different crystal faces is of the order of 2 percent. On the basis of various assumptions about the universal behavior of bcc crystals at melting, the formalism predicts that the surface tension is proportional to the heat of fusion per surface atom.

Shih, W. H.↗
NASA NTRS1990

Positronium formation in Ss state in e+-Li scattering

There are ample theoretical reasons to investigate positron-alkali atom scattering. Moreover, recent measurement on positron-alkali atom system by a Detroit group has renewed much interest in investigating these processes. Positronium (Ps) formation in excited 2s state in positron-Li scattering at intermediate and high energies were studied including second order effects following Basu and Ghosh.

Sarkar, K. P.↗
NASA NTRS1990

Development of mid-infrared solid state lasers for spaceborne lidar

Researchers investigated laser performance of Ho(3+):Tm(3+):Cr(3+):YAG crystals under both Cr:GSAG laser and flashlamp pumping. A flashlamp pumped Cr:GSAG laser was built to simulate high power quasi-CW laser diode pumping of a 2.1 micron holmium laser. The 2.1 micron output laser energy exceeded more than 14 mJ, the highest value reported to date under laser pumping near 785 nm. This was obtained in a pulse length of nearly 650 microns from a 3 x 3 mm Ho:Tm:Cr:YAG rod by using the flashlamp-pumped Cr:GSAG laser as a pumping source at the diode laser wavelength, 785 microns. In addition, Ho:Tm:Cr:YAG crystals with various Tm(3+) concentrations have been evaluated for flashlamp-pumped normal mode and Q-switched 2.1 micron laser operations under a wide variety of experimental conditions in order to understand internal dynamic processes among the ions and to determine an optimum lasing condition. An increase of the laser slope efficiency was observed with the increase of the Tm(3+) concentration from 2.5 atomic percent to 4.5 atomic percent. The thermal dependence of the laser performance was also investigated. Q-switched laser output energies corresponding to nearly 100 percent of the normal-mode laser energies were obtained in a strong single spike of 200 ns pulse length by optimizing the opening time of a lithium niobate Q-switch.

Whitney, Donald A.↗
NASA NTRS1990

The kinetics of the Au-InP interaction

An analysis of the reaction of Au and Au-In alloys with InP has permitted the identification of the mechanisms occurring during the first two stages of the Au-InP interaction. The first stage of the interaction, during which the Au is converted to a saturated Au (In) solution, is controlled by the vacancy-generation rate at the free surface of the metallization. The activation energy for this process is the activation energy for Au self-diffusion. Evidence is presented for the existence of large localized variations in this value due to surface related effects. At the completion of stage I stage II becomes active and continues until the metallization is converted to Au3In. This process, proceeding via an interstitial interchange mechanism, is many orders of magnitude slower than stage I. The rate-limiting step, with an activation energy of 2.8 eV, is shown to be the diffusion of In from the InP-metal interface. The P atoms that are released when In enters the metallization during stage I leave the system without reacting, whereas in stage II they form a compound (Au2P3) at the InP-metal interface. The presence of the Au2P3 severely retards the stage II interaction rate.

Fatemi, Navid S.↗
NASA NTRS1991

Intrusive and nonintrusive measurements of flow properties in arc jets

While numerous diagnostic techniques are available for determining the flow properties of arc jets, these have to be used complementarily in order to cover all the requisite information. Although intrusive techniques disturb the flow, they yield much information. The determination of total enthalpy remains a major challenge, and accurate heat-flux measurements entail knowledge of atom recombination and chemical energy recombination coefficients. Such state-specific methods as the spectroscopic and laser techniques are useful in understanding the chemistry and nonequilibrium reaction and excitation rates of the flow.

Scott, Carl D.↗
NASA NTRS2011

Effects of Atomic Oxygen and Grease on Outgassing and Adhesion of Silicone Elastomers for Space Applications

An investigation of silicone elastomers for seals used in docking and habitat systems for future space exploration vehicles is being conducted at NASA. For certain missions, NASA is considering androgynous docking systems where two vehicles each having a seal would be required to: dock for a period of time, seal effectively, and then separate with minimum push-off forces for undocking. Sili-cone materials are generally chosen for their wide operating temperatures and low leakage rates. However silicone materials are often sticky and usually exhibit considerable adhesion when mated against metals and silicone surfaces. This paper investigates the adhesion unit pressure for a space rated silicone material (S0383-70) for either seal-on-seal (SoS) or seal-on-aluminum (SoAl) operation modes in the following conditions: as-received, after ground-based atomic-oxygen (AO) pre-treatment, after application of a thin coating of a space-qualified grease (Bray-cote 601EF), and after a combination of AO pre-treatment and grease coating. In order of descending adhesion reduction, the AO treatment reduced seal adhesion the most, followed by the AO plus grease pre-treatment, followed by the grease treatment. The effects of various treatments on silicone (S0383-70 and ELA-SA-401) outgassing properties were also investigated. The leading adhesion AO pre-treatment reduction led to a slight decrease in outgassing for the S0383-70 material and virtually no change in ELA-SA-401 outgassing.

deGroh, Henry C., III↗
NASA NTRS2004

Progress in Fabrication of Rocket Combustion Chambers by VPS

Several documents in a collection describe aspects of the development of advanced materials and fabrication processes intended to enable the manufacture of advanced rocket combustion chambers and nozzles at relatively low cost. One concept discussed in most of the documents is the fabrication of combustion-chamber liners by vacuum plasma spraying (VPS) of an alloy of 88Cu/8Cr/4Nb (numbers indicate atomic percentages) -- a concept that was reported in "Improved Alloy for Fabrication of Combustion Chambers by VPS" (MFS-26546). Another concept is the deposition of graded-composition wall and liner structures by VPS in order to make liners integral parts of wall structures and to make oxidation- and thermal-protection layers integral parts of liners: The VPS process is started at 100 percent of a first alloy, then the proportion of a second alloy is increased gradually from zero as deposition continues, ending at 100 percent of the second alloy. Yet another concept discussed in one of the documents is the VPS of oxidation-protection coats in the forms of nickel-and-chromium-containing refractory alloys on VPS-deposited 88Cu/8Cr/4Nb liners.

Holmes, Richard R.↗
NASA NTRS2018

Molecular Dynamics Simulations of Liquid and Polymer Electrolytes for Energy Storage Devices

Advancing beyond current lithium-ion technology is necessary in order to enable energy storage devices for electric airplanes. Electrolyte stability is a key limiting factor, yet the design of improved electrolytes remains a formidable challenge. Molecular dynamics (MD) simulations are a powerful tool for studying electrolytes, since they can be used to evaluate structural, thermodynamic, and transport properties, and can provide molecular-level detail often inaccessible to experimental techniques. Our computational materials groups at the NASA Ames Research Center has developed models and methods to accurately simulate both liquid and polymer electrolytes.We report the results from atomistic MD simulations of several electrolyte materials, with lithium salts dissolved in ionic liquids, dimethoxyethane (DME), and polyethylene oxide (PEO). For improved accuracy, we employ polarizable models, where each atom is given an environment-dependent atomic dipole. The simulations accurately predict bulk transport properties, including viscosity, diffusion, and ionic conductivity, in quantitative agreement with available experimental data. Moreover, the simulations provide important insights into the solvation structure of the lithium ions.We also report the results from coarse-grained MD simulations of polyanion electrolytes. In order to more efficiently capture the longer length- and time-scales of these systems, we employ a generic bead-spring model. These simulations provide important insight into how the polymer chain architecture and ionic interaction strengths affect the ionic aggregation behavior and cation dynamics. Despite the simplicity of the model, the simulations yield qualitative agreement with experimental data for similar systems.

Abbott, Lauren J.↗
NASA NTRS2009

Ikhana: Unmanned Aircraft System Western States Fire Missions. Monographs in Aerospace History, Number 44

In 2006, NASA Dryden Flight Research Center, Edwards, Calif., obtained a civil version of the General Atomics MQ-9 unmanned aircraft system and modified it for research purposes. Proposed missions included support of Earth science research, development of advanced aeronautical technology, and improving the utility of unmanned aerial systems in general. The project team named the aircraft Ikhana a Native American Choctaw word meaning intelligent, conscious, or aware in order to best represent NASA research goals. Building on experience with these and other unmanned aircraft, NASA scientists developed plans to use the Ikhana for a series of missions to map wildfires in the western United States and supply the resulting data to firefighters in near-real time. A team at NASA Ames Research Center, Mountain View, Calif., developed a multispectral scanner that was key to the success of what became known as the Western States Fire Missions. Carried out by team members from NASA, the U.S. Department of Agriculture Forest Service, National Interagency Fire Center, National Oceanic and Atmospheric Administration, Federal Aviation Administration, and General Atomics Aeronautical Systems Inc., these flights represented an historic achievement in the field of unmanned aircraft technology.

Western States fire missions↗
NASA NTRS1995

Composition of interstellar clouds in the disk and halo. 3: HD 149881

The spectrum of HD 149881, a halo star 1300 pc above the Galactic plane, has been analyzed, based on high-resolution GHRS data from Hubble Space Telescope (HST) together with 21 cm emission observations and profiles of Na I and Ca II features. The results reveal 11 absorption components, of which nine are produced in H I regions, with log N(H(sup 0) in each region mostly approximately 19.5; one component clearly originates in an H II region. Detailed column densities show that (D(Zn)), the absolute value of the depletion D, for Zn, like that for S, does not much exceed 0.1 dex. With Zn used as a standard of reference, D(Fe) varies from -1.1 to -0.6 dex; (D(Cr)) and (D(Mn)) average less than (D(Fe)) by about 0.1 and 0.2 dex, respectively, with dispersions of about 0.1 dex in this difference. As in Papers I and II, the measured D(Si) is consistent with a 2 to 1 ratio of Fe to Si atoms in grains. The kinetic temperature is these H I regions varies widely among components. Maximum temperatures, found from the widths of 21 cm emission components, range from about 100 to 10,000 K, while actual temperatures, found by comparing the H I widths with the b-values for the ultraviolet features, are about equal or less than 1000 K for six of the H I components. In contrast, toward the halo star HD 93521, temperatures of about 6000 K were found for most of the components. With use of these HD 149881 temperatures, values of n(sub e) were obtained from the ionization equilibrium of Ca(+) and also of Na(0). These values range over an order of magnitude, from about 4.9 x 10(exp -2) to 1.3 x 10(exp -3)/cu cm, systematically smaller (but also less certain) than in HD 93521. For the low values of n(sub e) found in the coldest components, the electrons can be accounted for by ionization of C atoms, if n(sub H) approximately equals 10/cu cm. For the warmest component, an ionization probability of beta(H(sup 0)) approximately equals 3 x 10(exp -15)/s is required, with lower values for other H I components.

Spitzer, Lyman, Jr.↗
NASA NTRS1978

The abundance of boron in B- and A-type stars

Results are reported for Copernicus observations of the 1362.46-A resonance line of B II in 16 normal B and A stars. An average for the stars in which a boron abundance has been determined yields a B/H abundance ratio of 2 x 10 to the -10th; this value is taken to be the cosmic abundance of boron. The observed abundances are compared with those predicted by the galactic-cosmic-ray (GCR) spallation theory for the 'best fit' GCR flux, and remarkably good agreement is obtained. The value derived for the cosmic boron abundance is shown to support the theory that spallation reactions by high-energy GCRs on atoms of C, N, and O in the interstellar gas are responsible for the origin of B and Be as well as at least some Li. It is noted that the meteorite boron abundance is an order of magnitude greater than the cosmic abundance derived from stars, which implies an anomalous local enrichment in boron and possibly in the B-11/B-10 ratio during the formation of carbonaceous chondrites.

Boesgaard, A. M.↗
NASA NTRS2001

Long-Term Reliability of SiGe/Si HBTs From Accelerated Lifetime Testing

Accelerated lifetime tests were performed on double-mesa structure Si(0.7)Ge(0.3)/Si npn heterojunction bipolar transistors, grown by molecular beam epitaxy, in the temperature range of 175 C-275 C. The transistors (with 5x20 sq micron emitter area) have DC current gains approx. 40-50 and f(sub T) and f(sub max) of up to 22 GHz and 25 GHz, respectively. It is found that a gradual degradation in these devices is caused by the recombination enhanced impurity diffusion (REID) of boron atoms from the p-type base region and the associated formation of parasitic energy barriers to electron transport from the emitter to collector layers. This REED has been quantitatively modeled and explained, to the first order of approximation, and the agreement with the measured data is good. The mean time to failure (MTTF) of these devices at room temperature under 1.35 x 10(exp 4) A/sq cm current density operation is estimated from the extrapolation of the Arrhenius plots of device lifetime versus reciprocal temperature. The results of the reliability tests offer valuable feedback for SiGe heterostructure design in order to improve the long-term reliability of the devices and circuits made with them. Hot electron induced degradation of the base-emitter junction was also observed during the accelerated lifetime testing. In order to improve the HBT reliability endangered by the hot electrons, deuterium sintered techniques have been proposed. The preliminary results from this study show that a deuterium-sintered HBT is, indeed, more resistant to hot-electron induced base-emitter junction degradation.

Bhattacharya, Pallab↗
NASA NTRS1998

Flow Characteristics of UHV Nitrogen Arcjet

Flow characteristics in the nozzle of a nitrogen arcjet have been simulated by a model. Electrons, ions, and neutral atoms and molecules are represented as separate species. Dissociation, ionization, and recombination are treated as separate reactions. Thermal non-equilibrium is represented by a two-temperature model. Energy input to plasma from the arc is modeled as a source determined by local current and electrical conductivity. Momentum and energy losses and recombinations at the nozzle wall are included as source/sink determined by fluxes to the wall. The results show essentially frozen gas-phase reactions and thermal nonequilibrium due to Joule heating of electrons and in the expansion part of the nozzle. Wall interactions have strong effects on the results, indicating that they play important roles in the flow due to very high surface area relative to the volume of the plasma. For example, plasma velocity decreases from 6000 m per s without wall interactions to 3000 m per s with modest amount of wall interactions. These results also show reasonable agreement with optical emission measurements, which confirms that the arcjet plasma is far from LTE. The spectra suggest nitrogen dissociation levels of 0.3% - 9%, depending on nitrogen flow rate and arc plasma current, which also determine the relative amounts of excited atomic and molecular nitrogen. Langmuir probe studies of the source show that electron and ion fluxes increase with increasing power, and that the ion energy distribution shifts to lower energies. Typical ion fluxes were on the order of 4E-9 A/cm**2 with a maximum ion kinetic energy of 3.5eV. The median electron energy was 1eV, with a maximum of less than 4eV.

Chang, C. H.↗