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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

The P/Halley: Spatial distribution and scale lengths for C2, CN, NH2, and H2O

From P/Halley long slit spectroscopic exposures on 12 dates, extending from Oct. 1985 to May 1986, spatial profiles were obtained for emissions by C2, CN, NH2, and OI(1D). Haser model scale lengths were fitted to these data. The extended time coverage allowed the checking for consistency between the various dates. The time varying production rate of P/Halley severely affected the profiles after perihelion, which is shown in two profile sequences on adjacent dates. Because of the time varying production rate, it was not possible to obtain reliable Haser model scale lengths after perihelion. The pre-perihelion analysis yielded Haser model scale lengths of sufficient consistency that they can be used for production rate determinations, whenever it is necessary to extrapolate from observed column densities within finite observing apertures. Results of scale lengths reduced to 1 AU are given and discussed.

Fink, Uwe↗
NASA NTRS1991

GaAs solar cells for laser power beaming

Efforts to develop GaAs solar cells for coupling to laser beams in the wavelength range of 800 to 840 nm are described. This work was motivated primarily by interests in space-tp-space power beaming applications. In particular, the Battelle Pacific Northwest Laboratories is conducting studies of the utilization of power beaming for several future space missions. Modeling calculations of GaAs cell performance were carried out using PC-1D to determine an appropriate design for a p/n cell structure. Epitaxial wafers were grown by MOCVD and cells fabricated at WSU Tri-Cities. Under simulated conditions, an efficiency of 53 percent was achieved for a cell coupled to 806 nm light at 400 mW/sq cm.

Olsen, Larry C.↗
NASA NTRS1991

Comparative modeling of InP solar cell structures

The comparative modeling of p(+)n and n(+)p indium phosphide solar cell structures is studied using a numerical program PC-1D. The optimal design study has predicted that the p(+)n structure offers improved cell efficiencies as compared to n(+)p structure, due to higher open-circuit voltage. The various cell material and process parameters to achieve the maximum cell efficiencies are reported. The effect of some of the cell parameters on InP cell I-V characteristics was studied. The available radiation resistance data on n(+)p and p(+)p InP solar cells are also critically discussed.

Jain, R. K.↗
NASA NTRS1990

Simultaneous, in situ measurements of OH, HO2, O3, and H2O - A test of modeled stratospheric HO(x) chemistry

Simultaneous, in situ measurements of OH, HO2, H2O, and O3 from 37-23 km are reported. The partitioning between OH and HO2 and the total HO(x) concentration are compared with expected steady-state values. The ratio of HO2 to OH varies from less than 2 at 36 km to more than 3 at 25 km; in the lower stratosphere this ratio is nearly a factor of two less than predicted. The data are used to calculate HO(x) production and loss rates. The measured HO(x) mixing ratio is consistent with production dominated by the reaction of O(1D) with H2O, and loss controlled by NOy below 28 km and HO(x) above 30 km. The steady-state concentration of H2O2 is inferred from the measured HO2 concentration and calculated photolysis rate. The maximum H2O2 mixing ratio (at 33 km) is predicted to be less than 0.2 ppb.

Wennberg, P. O.↗
NASA NTRS1990

The production rate and spatial distribution of H2O for Comet P/Halley

Spectroscopic observations of the O I 1D lines for Comet P/Halley are used to derive water-production rates; 43 exposures during the period August 1985 to June 1986 are reduced. The reduction procedure involves sky subtraction, ratios with solar analog comparison stars, subtraction of contaminating NH2 emissions, and absolute flux calibration. The flux collected within the 2.5-arcsec-wide x 151-arcsec-long integration aperture is converted to a total comet O I 6300-A luminosity using a cometary gas outflow model. Water-production rates for 10 preperihelion dates and seven postperihelion dates are derived. An error budget yields an estimated uncertainty of + 30 percent in H2O production rates with a maximum error of + or - 50 percent. For the preperihelion period, a good fit of the water-production rate as a function of heliocentric distance can be obtained as Q(H2O) = 4.36 x 10 to the 29th x r exp -2.54. A postperihelion enhancement by about a factor of 1.8, which appears to dissipate by June 1986, is observed. Several values of enhanced water production agree well with the observed light curve of P/Halley. Comparison of H2O production rates with those deduced by other methods shows quite reasonable agreement.

Fink, Uwe↗
NASA NTRS1991

Oxygen isotope fractionation in stratospheric CO2

A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO2 samples collected between 26 and 35.5 km show a mass-independent enrichment in both O-17 and O-18 of about 11 per mil above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16 percent in O3-50 and 8 to 11 percent in O3-49, respectively (Schueler et al., 1990). A mechanism to explain the isotope enrichment in CO2 has been recently proposed by Yung et al. (1991). The model is based on the isotope exchange between CO2 and O3 via O(1D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichment and measured values agree well.

Thiemens, M. H.↗
NASA NTRS1990

Effect of emitter parameter variation on the performance of heteroepitaxial indium phosphide solar cells

Metalorganic chemical-vapor-deposited heteroepitaxial indium phosphide (InP) solar cell experimental results were simulated by using a PC-1D computer model. The effect of emitter parameter variation on the performance of n(+)/p/p(+) heteroepitaxial InP/GaAs solar cell was presented. The thinner and lighter doped emitters were observed to offer higher cell efficiencies. The influence of emitter thickness and minority carrier diffusion length on the cell efficiency with respect to dislocation density was studied. Heteroepitaxial cells with efficiencies similar to present day homojunction InP efficiencies (greaater than 16 percent AM0) were shown to be attainable if a dislocation density lower than 10(exp 6)/sq cm could be achieved. A realistic optimized design study yielded InP solar cells of over 22 percent AM0 efficiency at 25 C.

Jain, R. K.↗
NASA NTRS1990

Electromagnetic axisymmetric finite elements based on a gauged four-potential variational principle

Electromagnetic finite elements are derived based on a variational principle that uses the electromagnetic four-potential as a primary variable. The Lorentz gage normalization is incorporated as a constraint condition through a Lagrange multiplier field to construct elements suitable for downstream coupling with mechanical and thermal finite elements for the analysis of high-temperature superconductor devices with aerospace applications. The main advantages are: jump discontinuities on interfaces are naturally handled; no a priori approximations are invoked; and the number of degrees of freedom per node remains modest as the problem dimensionality increases. The new elements are tested on two magnetostatic axisymmetric problems. The results are in excellent agreement with analytical solutions and previous solutions for the 1D problem of a conducting infinite wire, in which case the multiplier field has no effect. For materials of widely different permeability, jump conditions are naturally accommodated by the present formulation.

Schuler, J.↗
NASA NTRS1991

The ionopause current layer at Venus

The results of recent theoretical models for the magnetic field in the ionosphere of Venus are reviewed, with special attention given to the 1D MHD model of Shinagawa and Cravens (1989) and the 2D kinematic dynamo model of Cravens et al. (1990). The results are used to examine the behavior of the ionopause, and it is suggested that the sensitivity of horizontal plasma transport to the presence of a magnetic field and the convergence of the vertical flow explain the narrowness of the Venus ionopause. During high solar wind dynamic pressure conditions, ion-neutral friction also plays an important role in determining the thickness of the ionopause.

Cravens, T. E.↗
NASA NTRS1991

The evolution of void-filled cosmological structures

1D, 2D, and 3D simulations are used here to investigate the salient features in the evolution of void-filled cosmological structures in universes with arbitrary values of Omega. It is found that the growth of a void as a function of time decreases significantly at the time corresponding to Omega = 0.5. In models constructed in 2D and 3D, suitable initial conditions lead to cellular structure with faceted voids similar to those observed in redshift surveys. Matter compressed to planes flows more rapidly toward condensations at the intersections than would be expected for spherical infall. The peculiar streaming velocities for void diameters of 5000 km/s should be observable. The simulations provide a more physical basis and dynamics for the bubbly and Voronois tesselation models used to derive statistical properties of cellular large-scale structure.

Regos, Eniko↗
NASA NTRS1991

A test of odd-oxygen photochemistry using Spacelab 3 atmospheric trace molecule spectroscopy observations

The ozone profile calculated using a 1D time-dependent photochemical model, with recommended absorption cross sections and reaction rate constants, is systematically less than Spacelab 3 (May 1985) atmospheric trace molecule spectroscopy (ATMOS) observations at sunset, 30 deg N latitude, throughout the upper stratosphere and mesosphere. Model results are about 9 percent smaller than the nominal ATMOS O3 value at 40 km and 50 percent less at 76 km. Given the uncertainties associated with the ATMOS profile, the model 'deficit' may be as large as 15 percent at 36 km, increasing to about 70 percent at 76 km. A similar model O3 deficit exists at sunrise, 47 deg S latitude, in the stratosphere: nominal differences of not more than 25 percent, increasing to not more than 45 percent when the ATMOS uncertainties are considered. Model results closely approximate ATMOS observations in both the upper stratosphere and mesosphere upon enhancing odd-oxygen production by increasing the O2 Schumann-Runge band and Herzberg-continuum cross section values used in the calculations.

Allen, Mark↗
NASA NTRS1991

Ozone budget over the Amazon - Regional effects from biomass-burning emissions

Data from the NASA dry-season Amazon boundary layer experiment (ABLE2A) is used with a 1D tropospheric photochemical model to analyze the atmospheric chemistry in the region and determine the impact of the long-range transport of biomass-burning emissions. Inputs of surface sources and the deposition of various species measured during ABLE2A are employed to simulate the background atmosphere, and haze characteristics are introduced for a 12-hr simulation. The in situ ozone production rate doubles during the period of haze when hydrocarbons are present. The model predicts that the production of ozone is enhanced during the dry season, and that increased ozone during the southern tropical burning season is related to the regional transport of haze.

Richardson, Jennifer L.↗
NASA NTRS1991

Effect of chemical kinetics uncertainties on calculated constituents in a tropospheric photochemical model

Random photochemical reaction rates are employed in a 1D photochemical model to examine uncertainties in tropospheric concentrations and thereby determine critical kinetic processes and significant correlations. Monte Carlo computations are used to simulate different chemical environments and their related imprecisions. The most critical processes are the primary photodissociation of O3 (which initiates ozone destruction) and NO2 (which initiates ozone formation), and the OH/methane reaction is significant. Several correlations and anticorrelations between species are discussed, and the ozone/transient OH correlation is examined in detail. One important result of the modeling is that estimates of global OH are generally about 25 percent uncertain, limiting the precision of photochemical models. Techniques for reducing the imprecision are discussed which emphasize the use of species and radical species measurements.

Thompson, Anne M.↗
NASA NTRS1990

The Nuclear Astrophysics Explorer

The Nuclear Astrophysics Explorer (NAE) is a concept for a possible future NASA Explorer mission which would obtain high resolution, E/Delta E about 500, observations of gamma-ray lines in order to study many fundamental problems in astrophysics. It operates from 15 keV to 10 MeV with a 3-sigma sensitivity of about 3 x 10 to the -6th ph/sq cm-s in a 10 to the 6th s observation. This is 100 times below the presently known gamma-ray line fluxes. The NAE uses a heavily shielded array of nine cooled Ge detectors in a very low background configuration. Its 10-deg field of view contains a versatile coded mask system which provides 2D imaging with 4-deg resolution, 1D imaging with 2-deg resolution and efficient measurements of emission from diffuse and point sources. The late 1990s is the earliest the NAE mission could begin. The scientific motivation, instrument concept, mission concept and expected results, and status and plans for the NAE are presented.

Matteson, James L.↗
NASA NTRS1991

Chaotic jumps in the generalized first adiabatic invariant in current sheets

The present study examines how the changes in the generalized first adiabatic invariant J derived from the separatrix crossing theory can be incorporated into the drift variable approach to generating distribution functions. A method is proposed for determining distribution functions for an ensemble of particles following interaction with the tail current sheet by treating the interaction as a scattering problem characterized by changes in the invariant. Generalized drift velocities are obtained for a 1D tail configuration by using the generalized first invariant. The invariant remained constant except for the discrete changes caused by chaotic scattering as the particles cross the separatrix.

Brittnacher, M. J.↗
NASA NTRS1991

Numerical models of giant planet formation with rotation

A 1D quasi-spherical approximation is presently used under the assumption of hydrostatic equilibrium to model giant-planet formation processes that encompass the accretion and transport of angular momentum for both radiative and convective zones. These calculations have been conducted up to masses comparable to those of Saturn. Angular momentum transport is strong, leading to strong concentration of angular momentum in the outer convective zones which develop envelope masses greater than 30 earth masses. When the models are allowed to contract near the calculation's end, the ratio of centrifugal force to gravity at the outer radius rises sufficiently to validate the quasi-spherical approximation.

Korycansky, D. G.↗
NASA NTRS1991

New results concerning the Galactic fountain

The time-dependent nonequilibrium thermal and ionization history is traced of hot optically-thin radiatively-cooling gas in a 1D planar steady-state flow model of the Galactic fountain. The calculation of such a flow which neglects photoionization is found to be unable to simultaneously reproduce the observed C IV, Si IV, and N V column densities through Galactic halo gas. It is shown that, when photoionization is taken into account, a fountain flow can match the observations of both UV absorption and emission lines for a range of ionizing fluxes which depends on the characteristic size of the cooling regions within the flow. What levels of the external photoionizing flux, such as that contributed by Galactic starlight, supernova remnants, and the metagalactic radiation background, can produce agreement with observational results are determined. The ionizing radiation emitted by the cooling gas itself is found to be sufficient to cause the flow to match the observed column densities and line emission.

Shapiro, Paul R.↗
NASA NTRS1991

Monte Carlo analysis of dissociation and recombination behind strong shock waves in nitrogen

Computations are presented for the relaxation zone behind strong, 1D shock waves in nitrogen. The analysis is performed with the direct simulation Monte Carlo method (DSMC). The DSMC code is vectorized for efficient use on a supercomputer. The code simulates translational, rotational and vibrational energy exchange and dissociative and recombinative chemical reactions. A model is proposed for the treatment of three body-recombination collisions in the DSMC technique which usually simulates binary collision events. The model improves previous models because it can be employed with a large range of chemical-rate data, does not introduce into the flow field troublesome pairs of atoms which may recombine upon further collision (pseudoparticles) and is compatible with the vectorized code. The computational results are compared with existing experimental data. It is shown that the derivation of chemical-rate coefficients must account for the degree of vibrational nonequilibrium in the flow. A nonequilibrium-chemistry model is employed together with equilibrium-rate data to compute the flow in several different nitrogen shock waves.

Boyd, I. D.↗