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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

All-Solid-State 2.45-to-2.78-THz Source

Sources in the THz range are required in order for NASA to implement heterodyne instruments in this frequency range. The source that has been demonstrated here will be used for an instrument on the SOFIA platform as well as for upcoming astrophysics missions. There are currently no electronic sources in the 2 3- THz frequency range. An electronically tunable compact source in this frequency range is needed for lab spectroscopy as well as for compact space-deployable heterodyne receivers. This solution for obtaining useful power levels in the 2 3- THz range is based on utilizing power-combined multiplier stages. Utilizing power combining, the input power can be distributed between different multiplier chips and then recombined after the frequency multiplication. A continuous wave (CW) coherent source covering 2.48 2.75 THz, with greater than 10 percent instantaneous and tuning bandwidth, and having l 14 W of output power at room temperature, has been demonstrated. This source is based on a 91.8 101.8-GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid-state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source has enabled wideband, high-resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by other existing systems, providing new insight in the physics of these molecules. Further - more, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 1012, and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 108. The harmonic purity is better than 25 dB. This source can serve as a local oscillator for a variety of heterodyne systems, and can be used as a method for precision control of more powerful but much less frequency-agile quantum mechanical terahertz sources.

Mehdi, Imran↗
NASA NTRS1979

A secondary tracer approach to the derivation of galactic cosmic-ray source isotopic abundances

A formalism has been developed for deriving cosmic-ray source isotopic abundances from observed local abundances using a purely secondary nuclide as a tracer of spallation production during propagation. Although the formalism is based on the leaky-box model of cosmic-ray propagation, it is shown that source abundances derived by the tracer technique are reasonably independent of detailed propagation models. The tracer formalism also permits a quantitative evaluation of the effects of observational uncertainties on deduced source abundances. It is shown that statistical errors in the observed abundances and uncertainties in the spallation cross sections are at present the dominant sources of uncertainty. The latter error can be reduced with increased detector size or exposure time, while the former can be minimized by measurements of the relative production cross sections. As a specific example, the tracer technique is applied to the isotopes of sulfur and calcium, and the level of uncertainties which must be achieved to distinguish evolutionary differences between solar-system material and cosmic ray-source material are established.

Stone, E. C.↗
NASA NTRS1988

Free-flying experiment to measure the Schawlow-Townes linewidth limit of a 300 THz laser oscillator

Recent advances in laser diode-pumped solid state laser sources permit the design and testing of laser sources with linewidths that approach the Schawlow-Townes limit of 1 Hz/mW of output power. Laser diode pumped solid state ring oscillators have been operated with CW output power levels of 25 mW at electrical efficiencies that exceed 6 percent. These oscillators are expected to operate for lifetimes that approach those of the laser diode sources which is now approaching 20,000 hours. The efficiency and lifetime of these narrow linewidth laser sources will enable space measurements of gravity waves, remote sensing applications (including local range rate and measurements), and laser sources for frequency and time standards. A free-flight experiment, 'SUNLITE', is being designed to measure the linewidth of this all-solid-state laser system.

Byer, R. L.↗
NASA NTRS1982

A feedback model for the source of auroral kilometric radiation

It is noted that in order to compensate for the wave refraction inside the source, the boundary reflection surfaces must converge with altitude, and this implies that the most likely auroral kilometric radiation source would be a thin, local density enhancement, since the refractive index contours at its boundaries would be expected to slope inward. The ISEE observations of multiple spectral components, which are attributed to separate oscillations at different altitudes in the same enhancement, indicate a source thickness as small as 25 km and an internal wave growth threshold of roughly 40 dB, rather than the 70-120 dB previously believed necessary to account for auroral kilometric radiation without feedback. What is considered more significant is that the feedback model accounts for numerous aspects of the auroral kilometric radiation behavior, predicts emission at the wave growth saturation level, and leads to the conclusion that auroral kilometric radiation originates at many compact sites, each emitting a nearly monochromatic wave.

Calvert, W.↗
NASA NTRS2000

A Unified Wall Boundary Treatment for Viscous and Inviscid Flows in the CE/SE Method

In the setting of the conservation element-solution element (CE/SE) method, a new and unified wall boundary treatment for the Navier-Stokes and Euler Equations is proposed. In essence, the shear stress exerted on the fluid by a wall is modeled as a source term as a part of local spacetime flux conservation in the vicinity of a wall boundary. When the fluid is inviscid, the source term vanishes and the boundary condition reduces to the usual 'slip' condition. On the other hand, when the fluid is viscous, the source-term effect is consistent with the traditional no-slip condition. Numerical results show that the new treatment is robust, efficient, and accurate for viscous and inviscid flows.

Chang, Sin-Chung↗
NASA NTRS2001

The Effect of Doppler Frequency Shift, Frequency Offset of the Local Oscillators, and Phase Noise on the Performance of Coherent OFDM Receivers

This paper first shows that the Doppler frequency shift affects the frequencies of the RF carrier, subcarriers, envelope, and symbol timing by the same percentage in an Orthogonal Frequency Division Multiplexing (OFDM) signal or any other modulated signals. Then the SNR degradation of an OFDM system due to Doppler frequency shift, frequency offset of the local oscillators and phase noise is analyzed. Expressions are given and values for 4-, 16-, 64-, and 256-QAM OFDM systems are calculated and plotted. The calculations show that the Doppler shift of the D3 project is about 305 kHz, and the degradation due to it is about 0.01 to 0.04 dB, which is negligible. The degradation due to frequency offset and phase noise of local oscillators will be the main source of degradation. To keep the SNR degradation under 0.1 dB, the relative frequency offset due to local oscillators must be below 0.01 for the 16 QAM-OFDM. This translates to an offset of 1.55 MHz (0.01 x 155 MHz) or a stability of 77.5 ppm (0.01 x 155 MHz/20 GHz) for the DI project. To keep the SNR degradation under 0.1 dB, the relative linewidth (0) due to phase noise of the local oscillators must be below 0.0004 for the 16 QAM-OFDM. This translates to a linewidth of 0.062 MHz (0.0004 x 155 MHz) of the 20 GHz RIF carrier. For a degradation of 1 dB, beta = 0.04, and the linewidth can be relaxed to 6.2 MHz.

Xiong, Fuqin↗
NASA NTRS1994

The Arctic Boundary Layer Expedition (ABLE-3B): July - August 1990

The Arctic Boundary Layer Expedition (ABLE) 3B used data from ground-based, aircraft, and satellite platforms to characterize the chemistry and dynamics of the troposphere in subarctic and Arctic regions of midcontinent and eastern Canada during July - August 1990. This paper reports the experimental design for ABLE 3B and a brief overview of results. The detailed results are presented in a series of papers in this issue. The chemical composition of the atmospheric mixed layer over remote tundra, boreal wetland, and forested environments was influenced by emissions of CH4 and nonmethane hydrocarbons from biogenic sources, emissions of gases and aerosols from local biomass burning, and transport of pollutants into the study areas from urban/industrial sources. Minimum concentrations of both trace gas and aerosol species in boundary layer air were associated with Arctic source areas. In the free troposphere the biospheric influence was undetectable, and major sources of chemical variability were related to long-range transport of pollutants into the study areas from biomass burning and industrial sources in Alaska and the Great Lakes regions, respectively. Minimum concentrations of both trace gas and aerosol species in the free troposphere were associated with a persistent, widespread air mass which both chemistry and air mass trajectory analyses suggested had originated in the tropical Pacific. Subsidence of air from the upper troposphere and lower stratosphere frequently enhanced ozone and influenced other trace gas and aerosol species at midtropospheric altitudes. The North American Arctic is a complex dynamical and chemical environment with considerable spatial and temporal variability in aerosol and trace gas concentrations. The use of atmospheric chemical indicators for climate change detection will require a much more comprehensive Arctic monitoring program than currently exists.

Harriss, R. C.↗
NASA NTRS1993

Microjansky source counts and spectral indices at 8.44 GHz

Deep images were made with 10 arcsec resolution at 8.44 GHz with microjansky sensitivity of two regions previously imaged at several lower frequencies. The differential microjansky source count fits a power-law distribution with slope 2.3 +/- 0.2. This slope is roughly constant between 1.41 and 8.44 GHz and between S(v) = about 4 and X(v) = 1000 microJy. Unless the majority of radio sources at microJy levels are local, their average luminosity and/or space density probably has evolved with cosmic epoch in order to sustain the steep and nearly Euclidean value of the slope over such a large range in flux density. The 8.44 GHz counts probably converge at less than the 1 microjansky level if the evolutionary properties of these weak sources are similar to those of luminous gE radio galaxies and quasars. The median angular size of sources detected at 8.44 GHz at microjansky levels is 2.6 +/- 1.4 arcsec, and 40 percent of the source are larger than 5 arcsec.

Windhorst, Rogier A.↗
NASA NTRS1978

IUE observations of X-ray sources - HD153919 /4U1700-37/, HDE226868 /Cyg X-1/, HZ Her /Her X-1/

The first UV spectroscopic measurements of binary X-ray sources with IUE show highly variable emission from a photoionized plasma in the object HZ Her, and give evidence for localized circumsystem material in the binary source Cygnus X-1. In addition, a substantial stellar wind is found in one of the brightest identified X-ray sources HD153919. This system may be surrounded by an extensive H II region unusual in its content of high excitation ion species.

Dupree, A. K.↗
NASA NTRS1983

Plasma jet effects on the ionospheric plasma

Heavy ion beams were injected into the ionospheric plasma (experiments ARCS 1 and ARCS 2). In ARCS 1, operation of a 25eV argon ion source, mounted on a plasma diagnostic payload, produced an accelerated electron population; broadband electric field turbulence; large, spin synchronized electric field perturbations; and depletions of thermal ions. In ARCS 2, the ion source was deployed upward along the local magnetic field direction away from the diagnostic payload, and observed effects are contained within several meters of the ion source. However, enhanced wave levels near the LHR frequency are observed at distances up to 1 km, as are the injected ions themselves. A measurement of the dominant wavelength of the enhanced waves is consistent with an inference based upon the accelerated electron population seen in ARCS 1. This electron population is not evident during ARCS 2.

Moore, T. E.↗
NASA NTRS2024

Temporary Atmospheres Produced By Human Activities on the Moon

Outgassing from materials, whether through the ascent/descent stages of lunar vehicles, airlock depressurizing, rover or astronaut suit outgassing, may cause an effect of unwanted accumulation of volatiles at the surface and exosphere. This is especially important at (or proximal) to permanently shadowed regions (PSRs) at the lunar poles. Herein, we provide estimates of expected outgassing from various human-landed objects on the Moon, including backpacks, airlocks, rovers, landers, trash and mining operations. Astronaut suits produce some level of oxygen outgassing (Helou et al., 2022), which may transport and condense in these PSRs, even in micro- cold traps (Glavin et al., 2010).We estimate the outgassing from drill mining and trash-to-gas conversion assuming a specific technology is operating. These outgassing systems can create local, temporary atmospheres in the vicinity (∼100 km radius) of the sources. The atmosphere may be particularly high within meters of the source. To obtain column densities for these temporary atmospheres, we first bracket ranges for the gas number loss as a function of time. We then derive the maximum distance traveled and the time the released molecules remain in the exosphere for a single ballistic hop, assuming the molecules are ejected from the surface of the object at its surface temperature. In some cases, such as the astronaut backpack and the rover, the temperature is that of the source. Given this information, an average and peak local exospheric density and column density can be estimated. We find that backpacks, airlock releases, and the Starship lander can create relatively high-density local atmospheres, with local near-lander outgassing water densities exceeding 107/cm3. This local water exosphere is over 106 times greater than the LADEE-derived lower limit of the natural water exosphere at ∼3/cm3. Thus, the anthropogenic temporary water exosphere will likely dominate the environment near the lander, making an assessment of the natural exospheric water environment difficult.

Rosemary M Killen↗
DOE OSTI2024

Harnessing large language models’ zero-shot and few-shot learning capabilities for regulatory research

Abstract Large language models (LLMs) are sophisticated AI-driven models trained on vast sources of natural language data. They are adept at generating responses that closely mimic human conversational patterns. One of the most notable examples is OpenAI's ChatGPT, which has been extensively used across diverse sectors. Despite their flexibility, a significant challenge arises as most users must transmit their data to the servers of companies operating these models. Utilizing ChatGPT or similar models online may inadvertently expose sensitive information to the risk of data breaches. Therefore, implementing LLMs that are open source and smaller in scale within a secure local network becomes a crucial step for organizations where ensuring data privacy and protection has the highest priority, such as regulatory agencies. As a feasibility evaluation, we implemented a series of open-source LLMs within a regulatory agency’s local network and assessed their performance on specific tasks involving extracting relevant clinical pharmacology information from regulatory drug labels. Our research shows that some models work well in the context of few- or zero-shot learning, achieving performance comparable, or even better than, neural network models that needed thousands of training samples. One of the models was selected to address a real-world issue of finding intrinsic factors that affect drugs' clinical exposure without any training or fine-tuning. In a dataset of over 700 000 sentences, the model showed a 78.5% accuracy rate. Our work pointed to the possibility of implementing open-source LLMs within a secure local network and using these models to perform various natural language processing tasks when large numbers of training examples are unavailable.

Biochemistry & Molecular Biology↗
NASA NTRS1977

The energetics of molecular clouds. I - Methods of analysis and application to the S255 molecular cloud

Observations of the S255 molecular cloud in the radio lines of CO, (C-13)O, and H2O as well as in the IR region from 12 to 20 microns are presented. These results show that an extended molecular cloud is associated with the optically visible H II regions S255 - S257, both the kinetic temperature and molecular density are enhanced in an area centered on a compact near-IR source, this source is also coincident with an OH and an H2O maser (within the errors), and the IR source exhibits the silicon absorption feature. First-order techniques which allow physical properties of molecular clouds to be estimated and the energetics of the gas and dust in the clouds to be analyzed are developed and applied to S255. It is concluded that: (1) the gas cooling rate is much less than the dust cooling rate; (2) the primary energy flow is in the IR through dust emission over a range of temperatures; (3) the near-IR source is apparently responsible for the local peak in gas kinetic temperature; and (4) the exciting stars of nearby H II regions appear to be the primary heat sources for the overall molecular cloud.

Evans, N. J., II↗
NASA NTRS1991

Psychophysical evaluation of three-dimensional auditory displays

Work during this reporting period included the completion of our research on the use of principal components analysis (PCA) to model the acoustical head related transfer functions (HRTFs) that are used to synthesize virtual sources for three dimensional auditory displays. In addition, a series of studies was initiated on the perceptual errors made by listeners when localizing free-field and virtual sources. Previous research has revealed that under certain conditions these perceptual errors, often called 'confusions' or 'reversals', are both large and frequent, thus seriously comprising the utility of a 3-D virtual auditory display. The long-range goal of our work in this area is to elucidate the sources of the confusions and to develop signal-processing strategies to reduce or eliminate them.

Wightman, Frederic L.↗
NASA NTRS1992

Two populations of gamma-ray bursts - A solution to the BATSE conundrum

BATSE measurements suggest that the distribution of gamma-ray burst sources is spatially nonuniform, but as yet shows no discernible anisotropy. This is not consistent with the distribution expected solely from a local galactic population of neutron star sources. We suggest, however, that the inconsistency can be reconciled if there are at least two distinct classes of bursts, both of which originate on galactic neutron stars, but which differ by a factor of 10 exp 4, or more, in mean luminosity. We show, as an example, that the observed V/V(max) distributions and anisotropy constraints of gamma-ray bursts are consistent with those expected from a combination of two such classes, resulting from different burst mechanisms on a single galactic, high-velocity, neutron star population.

Lingenfelter, R. E.↗
NASA NTRS1991

Virtual acoustic displays

A 3D auditory display can potentially enhance information transfer by combining directional and iconic information in a quite naturalistic representation of dynamic objects in the interface. Another aspect of auditory spatial clues is that, in conjunction with other modalities, it can act as a potentiator of information in the display. For example, visual and auditory cues together can reinforce the information content of the display and provide a greater sense of presence or realism in a manner not readily achievable by either modality alone. This phenomenon will be particularly useful in telepresence applications, such as advanced teleconferencing environments, shared electronic workspaces, and monitoring telerobotic activities in remote or hazardous situations. Thus, the combination of direct spatial cues with good principles of iconic design could provide an extremely powerful and information-rich display which is also quite easy to use. An alternative approach, recently developed at ARC, generates externalized, 3D sound cues over headphones in realtime using digital signal processing. Here, the synthesis technique involves the digital generation of stimuli using Head-Related Transfer Functions (HRTF's) measured in the two ear-canals of individual subjects. Other similar approaches include an analog system developed by Loomis, et. al., (1990) and digital systems which make use of transforms derived from normative mannikins and simulations of room acoustics. Such an interface also requires the careful psychophysical evaluation of listener's ability to accurately localize the virtual or synthetic sound sources. From an applied standpoint, measurement of each potential listener's HRTF's may not be possible in practice. For experienced listeners, localization performance was only slightly degraded compared to a subject's inherent ability. Alternatively, even inexperienced listeners may be able to adapt to a particular set of HRTF's as long as they provide adequate cues for localization. In general, these data suggest that most listeners can obtain useful directional information from an auditory display without requiring the use of individually-tailored HRTF's.

Wenzel, Elizabeth M.↗
NASA NTRS1995

Theoretical overview and modeling of the sodium and potassium atmospheres of mercury

A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmsophere of Mercury is given. Information for these four factors, which control the spatial distribution of these two alkali-group gases about the planet, is incorporated in numerical models. The spatial nature and relative importance of the initial source atom atmosphere and the ambient (ballistic hopping) atom atmosphere are then examined and are shown to be controlled and coupled to a great extent by the extremely large and variable solar radiation acceleration experienced by sodium and potassium as they resonantly scatter solar photons. The lateral (antisunward) transport rate of thermally accommodated sodium and potassium ambient atoms is shown to be driven by the solar radiation acceleration and, over a significant portion of Mercury's orbit about the Sun, is sufficiently rapid to be competitive with the short photoionization lifetimes for these atoms when they are located on the summit surface near or within about 30 deg of the terminator. The lateral transport rate is characterized by a migration time determined by model calculations for an ensemble of atoms initially starting at a point source on the surface (i.e., a numerical spacetime dependent Green's function). Four animations for the spacetime evolution of the sodium (or potassium) atmosphere produced by a point source on the surface are presented on a videotape format. For extended surface sources for sodium and potassium, the local column density is determined by competition between the photoionization lifetimes and the lateral transport times of atoms originating from different surface source locations. Sodium surface source fluxes (referenced to Mercury at perihelion) that are required on the sunlit hemisphere to reproduce the typically observed several megarayleighs of D2 emission-line brightness and the inferred column densities of 1-2 x 10(exp 11) atoms per sq cm range from approximately 2-5 x 10(exp 7) atoms/sq cm/sec. The sodium model is applied to study observational data that document an anticorrelation in the average sodium column density and solar radiation acceleration. Lateral transport driven by the solar radiation acceleration is shown to produce this behavior for combinations of different sources and surface accomodation coefficients. The best fit model fits to the observational data require a significant degree of thermal accommodation of the ambient sodium atoms to the surface and a source rate that decreases as an inverse power of 1.5 to 2 in heliocentric distance.

Smyth, William H.↗
NASA NTRS2019

The Seismic Signatures of Recently Formed Impact Craters on Mars

We investigated the seismic signatures of recent impact crater clusters on Mars that would be recorded by the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight)seismometers. We used a database of 77 measured and dated impact sites, with craters with diameters between 2.1 and 33.8 m, along with inferred impact angle, bolide trajectory, and varying target material properties to empirically scale for the momentum, expected seismic source function, and radiation pattern of impacts. The impact source is simulated in a local 3‐Dfinite difference wave propagation code and coupled to teleseismic distances by scaling the spectra of 1‐D global synthetic seismograms. We use the InSight seismometer noise floors to estimate detectability of impact(s) across azimuth and distance. Our experiments reveal that impact clusters have a higher peak corner frequency resulting from energy contributed by smaller craters to the power spectrum. We also find that the time separation between individual impacts in a cluster is small (<10–15 ms) and requires a seismometer closely situated to the source (<10 km) and a high sampling rate (>100 Hz) to resolve individual impacts within the cluster. Two of the clusters in our database(>20‐m effective diameter) would have been detectable by InSight, with the assumptions that the Martian background noise and seismic attenuation are both low. Joint detection of surface changes from newly formed crater(s) in images and by InSight will provide precise source locations that are crucial for constraining the internal structure of Mars

N. C. Schmerra↗