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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

The formation of arcs in the dynamic spectra of Jovian decameter bursts

A model is presented that accounts for several features of the dynamic spectral arcs observed at decameter wavelengths by the planetary radio astronomy experiment on Voyagers 1 and 2. It is shown that refraction of an extraordinary mode wave initially excited and nearly orthogonal to the local magnetic field is significantly influenced by the local plasma density. The source of the radiation is on the L = 6 flux, and the emission cone angle of the sheet is chosen to vary with frequency so that it is relatively small at both high and low frequencies while about 80 deg at intermediate frequencies. This functional dependence of cone angle on frequency appears to be consistent with estimates of refraction of the ordinary mode in a source region where the electron density is greater than a few hundred per cu cm.

Goldstein, M. L.↗
NASA NTRS1982

Electromagnetic cyclotron-loss-cone instability associated with weakly relativistic electrons

The amplification of fast extraordinary mode waves at frequencies very close to the electron cyclotron frequency, due to the presence of a population of energetic electrons with a loss-cone type distribution, is studied. Low-energy background electrons are included in the analysis. Two types of loss-cone distribution functions are considered, and it is found that the maximum growth rates for both distribution functions are of the same order of magnitude. When the thermal effects of the energetic electrons are included in the dispersion equation, the real frequencies of the waves are lower than those obtained by using the cold plasma approximation. This effect tends to enhance the growth rate. An idealized case including a parallel electric field such that the distribution function of the trapped energetic electrons is modified is also considered. It is assumed that the parallel electric field can remove the low-energy background electrons away from the source region of radiation. Both these effects increase the growth rate.

Wong, H. K.↗
NASA NTRS1983

Influence of radiative recombination on the minority-carrier transport in direct band-gap semiconductors

When a semiconductor sample is irradiated by means of an external source, emitting photons or electrons, excess carriers are produced which distribute themselves throughout the sample. One of the parameters which determine the distribution of the carriers is the surface recombination velocity. The present investigation is concerned with the recombination lifetime tau. The predominant mechanism for recombination in wide band-gap semiconductors is described by the Shockley-Read-Hall (SRH) theory. The transport equations are derived for free carriers and the radiation field. The considered theory is applied to a semiinfinite, one-dimensional semiconductor slab irradiated by light of a given frequency. Some numerical considerations based on n-type GaAs are presented. Attention is given to a determination of the radiation transmitted through the surface of the sample.

Von Roos, O.↗
NASA NTRS1985

The quasars 1038 + 528 A and B

The results of VLBI observations of the quasars 1038 + 528 A and B at 2.8, 3.6, 13, and 18 cm at various times between November 1979 and March 1981 are reported. The observations and data calibration are described, as are the mapping and astrometric techniques applied in the study. Both quasars are found to have 'core-jet' morphologies. The core of the A quasar dominates its morphology at centrimetric wavelengths with the brightness temperature of its 400 pc long jet being about 1/100 that of the core. By contrast, the 'jet' in the B quasar is very short (about 70 pc); the tail of this jet has the steepest spectral index found to date in extragalactic compact sources, indicating that high electron losses are responsible for the shortness of the jet. No evidence for appreciable morphological change in the B quasar was found over the time span of the study, whereas a new feature may be emerging from the A quasar core at superluminal speed.

Marcaide, J. M.↗
NASA NTRS1986

Experimental basis for a Titan probe organic analysis

The recent Voyager flyby of Titan produced evidence for at least nine organic compounds in that atmosphere that are heavier than methane. Several models of Titan's atmosphere, as well as laboratory simulations, suggest the presence of organics considerably more complex that those observed. To ensure that the in situ measurements are definitive with respect to Titan's atmosphere, experiment concepts, and the related instrumentation, must be carefully developed specifically for such a mission. To this end, the possible composition of the environment to be analyzed must be bracketed and model samples must be provided for instrumentation development studies. Laboratory studies to define the optimum flight experiment and sampling strategy for a Titan entry probe are currently being conducted. Titan mixtures are being subjected to a variety of energy sources including high voltage electron from a DC discharge, high current electric shock, and laser detonation. Gaseous and solid products are produced which are then analyzed. Samples from these experiements are also provided to candidate flight experiments as models for instrument development studies. Preliminary results show that existing theoretical models for chemistry in Titan's atmosphere cannot adequetely explain the presence and abundance of all trace gases observed in these experiments.

Mckay, C. P.↗
NASA NTRS1986

Wide field and diffraction limited array camera for SIRTF

The Infrared Array Camera for the space Infrared Telescope Facility (SIRTF/IRAC) is capable of two-dimensional photometry in either a wide field or diffraction-limited mode over the wavelength interval from 2 to 30 microns. Three different two-dimensional direct readout (DRO) array detectors will be used: Band 1-InSb or Si:In (2-5 microns) 128 x 128 pixels, Band 2-Si:Ga (5-18 microns) 64 x 64 pixels, and Band 3-Si:Sb (18-30 microns) 64 x 64 pixels. The hybrid DRO readout architecture has the advantages of low read noise, random pixel access with individual readout rates, and nondestructive readout. The scientific goals of IRAC are discussed, which are the basis for several important requirements and capabilities of the array camera: (1) diffraction-limited resolution from 2-30 microns, (2) use of the maximum unvignetted field of view of SIRTF, (3) simultaneous observations within the three infrared spectral bands, and (4) the capability for broad and narrow bandwidth spectral resolution. A strategy has been developed to minimize the total electronic and environmental noise sources to satisfy the scientific requirements.

Fazio, G. G.↗
NASA NTRS1986

Wide field and diffraction limited array camera for SIRTF

The Infrared Array Camera for the Space Infrared Telescope Facility (SIRTF/IRAC) is capable of two-dimensional photometry in either a wide field or diffraction-limited mode over the wavelength interval from 2 to 30 microns. Three different two-dimensional direct readout (DRO) array detectors are being considered: Band 1-InSb or Si:In (2-5 microns) 128 x 128 pixels, Band 2-Si:Ga (5-18 microns) 64 x 64 pixels, and Band 3-Si:Sb (18-30 microns) 64 x 64 pixels. The hybrid DRO readout architecture has the advantages of low read noise, random pixel access with individual readout rates, and nondestructive readout. The scientific goals of IRAC are discussed, which are the basis for several important requirements and capabilities of the array camera: (1) diffraction-limited resolution from 2-30 microns, (2) use of the maximum unvignetted field of view of SIRTF, (3) simultaneous observations within the three infrared spectral bands, and (4) the capability for broad and narrow bandwidth spectral resolution. A strategy has been developed to minimize the total electronic and environmental noise sources to satisfy the scientific requirements.

Fazio, G. G.↗
NASA NTRS1987

High spectral resolution measurements of a solar flare hard X-ray burst

Observations are reported of an intense solar flare hard X-ray burst on June 27, 1980, made with a balloon-borne array of liquid nitrogen-cooled Ge detector which provided unprecedented spectral resolution (no more than 1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 0.1-1 billion K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting about 3-15 sec, which have a hard spectrum and a break energy of 30-65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 kev to at least 100 keV through the event. The double power-law shape indicates that DC electric field acceleration, similar to that occurring in the earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission.

Lin, R. P.↗
NASA NTRS1987

Constraints on possible precursor AGN sources of the cosmic X-ray background

New limits on small-scale surface brightness fluctuations of the residual cosmic X-ray background are used to reevaluate the conjecture that unresolved precursor AGN sources could be the origin of the flux. The large number of these predominantly X-ray-emitting objects thereby required imposes that their lifetime be longer than previously considered. Attention is drawn to the fact that the reduction in the effective Eddington luminosity limit for such sources due to electron-positron pair opacity could have the effect of increasing their lifetime to an appropriate level.

Boldt, Elihu↗
NASA NTRS1989

Vacuum microelectronics for beam power and rectennas

Vacuum Microelectronic devices can be described as vacuum transistors or micro-miniature vacuum tubes, as one chooses. The fundamental reason behind this new technology is the very large current densities available from field emitters, namely as high as 10(8) A/sq cm. Array current densities as high as 1000 A/sq cm have been measured. Total electron transit times from source to drain for 1 micron feature size devices have been predicted to be about 150fs. This very short transit time implies the possibility of submillimeter wave transmitters and rectennas in devices which can operate with reasonably high voltages and which are small in size and are lightweight. In addition, they are expected to be extremely radiation hard and very temperature insensitive. That is, they are expected to have radiation hardness characteristics similar to vacuum tubes, and both the high temperature and low temperature limits should be determined by the package. That is, there should be no practical intrinsic temperature or carrier freezeout problems for devices based on metals or composites. But the technology is difficult to implement at the present time because it is based on 300 to 500 angstrom radius field emitters which must be relatively uniform. There is also the need to understand the non-equilibrium transport physics in the near-surface regions of the field emitters.

Gray, Henry F.↗
NASA NTRS1989

Use of a krypton isotope for rapid ion changeover at the Lawrence Berkeley Laboratory 88-inch cyclotron

An isotope of krypton, Kr86, has been combined with a mix of Ar, Ne, and N ions at the electron cyclotron resonance (ECR) source, at the Lawrence Berkeley Laboratory cyclotron, to provide rapid ion changeover in Single Event Phenomena (SEP) testing. The new technique has been proved out successfully by a recent Jet Propulsion Laboratory (JPL) test in which it was found that there was no measurable contamination from other isotopes.

Soli, George A.↗
NASA NTRS1989

Laser light scattering review

Since the development of laser light sources and fast digital electronics for signal processing, the classical discipline of light scattering on liquid systems experienced a strong revival plus an enormous expansion, mainly due to new dynamic light scattering techniques. While a large number of liquid systems can be investigated, ranging from pure liquids to multicomponent microemulsions, this review is largely restricted to applications on Brownian particles, typically in the submicron range. Static light scattering, the careful recording of the angular dependence of scattered light, is a valuable tool for the analysis of particle size and shape, or of their spatial ordering due to mutual interactions. Dynamic techniques, most notably photon correlation spectroscopy, give direct access to particle motion. This may be Brownian motion, which allows the determination of particle size, or some collective motion, e.g., electrophoresis, which yields particle mobility data. Suitable optical systems as well as the necessary data processing schemes are presented in some detail. Special attention is devoted to topics of current interest, like correlation over very large lag time ranges or multiple scattering.

Schaetzel, Klaus↗
NASA NTRS1991

Variable soft X-ray excesses in active galactic nuclei from nonthermal electron-positron pair cascades

In the present study of the formation of steep soft X-ray excesses that are superposed on flatter, hard X-ray power-law spectra in nonthermal electron-positron pair cascade sources, the soft excess in pair-cascade AGN models appears as a steep power law superposed on the tail of the UV bump and the flat nonthermal (hard X-ray) power law. The model-parameter space in which an excess in soft X-rays is visible is ascertained, and the time-variability of soft excesses in pair cascade models is examined. It is established that the parameter space in which soft excesses appear encompasses the range of preferred input parameters for a recently development Compton reflection model of UV and X-ray emission from the central engine of an AGN.

Zdziarski, Andrzej A.↗
NASA NTRS1992

Heavy Ions In Space (HIIS) experiment

The Heavy Ions In Space (HIIS) experiment has two primary objectives: (1) to measure the elemental composition of ultraheavy Galactic cosmic rays, beginning in the tin-barium region of the periodic table; and (2) to study heavy ions which arrive at LDEF below the geomagnetic cutoff, either because they are not fully stripped of electrons or because their source is within the magnetosphere. Both have practical as well as astrophysical consequences. The HIIS experiment used eight thick stacks of plastic track detectors mounted in two trays on the space facing end of LDEF. Since the last LDEF symposium, the statistics were increased of the observations and have extended the analysis to a second stack and to detector sheets near the top of a stack. New results are reported on the detector resolution and on the observations of both stopping and relativistic particles.

Adams, James H., Jr.↗
NASA NTRS1992

Atomic oxygen effects on thin film space coatings studied by spectroscopic ellipsometry, atomic force microscopy, and laser light scattering

The University of Nebraska is currently evaluating Low Earth Orbit (LEO) simulation techniques as well as a variety of thin film protective coatings to withstand atomic oxygen (AO) degradation. Both oxygen plasma ashers and an electron cyclotron resonance (ECR) source are being used for LEO simulation. Thin film coatings are characterized by optical techniques including Variable Angle Spectroscopic Ellipsometry, Optical spectrophotometry, and laser light scatterometry. Atomic Force Microscopy (AFM) is also used to characterize surface morphology. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated AO exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.

Synowicki, R. A.↗
NASA NTRS1993

Low earth simulation and materials characterization

Oxygen plasma ashers and an electron cyclotron resonance (ECR) sources are currently being used for low Earth orbit (LEO) simulation. The suitability of each of these simulation techniques is considered. Thin film coatings are characterized by optical techniques, including variable-angle spectroscopic ellipsometry, optical spectrophotometry, and laser light scatterometry. Atomic force microscopy (AFM) has been used to characterize the surface morphology of thin aluminum films as a function of substrate temperature during deposition. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated atomic oxygen (AO) exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible as Kapton to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.

Synowicki, R. A.↗
NASA NTRS1995

Atomic Oxygen Durability Evaluation of Protected Polymers Using Thermal Energy Plasma Systems

The durability evaluation of protected polymers intended for use in low Earth orbit (LEO) has necessitated the use of large-area, high-fluence, atomic oxygen exposure systems. Two thermal energy atomic oxygen exposure systems which are frequently used for such evaluations are radio frequency (RF) plasma ashers and electron cyclotron resonance plasma sources. Plasma source testing practices such as ample preparation, effective fluence prediction, atomic oxygen flux determination, erosion measurement, operational considerations, and erosion yield measurements are presented. Issues which influence the prediction of in-space durability based on ground laboratory thermal energy plasma system testing are also addressed.

Banks, Bruce A.↗