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At least 1,081 records · Page 60

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NASA NTRS1976

Investigation of the double exponential in the current-voltage characteristics of silicon solar cells

A theoretical analysis is presented of certain peculiarities of the current-voltage characteristics of silicon solar cells, involving high values of the empirical constant A in the diode equation for a p-n junction. An attempt was made in a lab experiment to demonstrate that the saturation current which is associated with the exponential term qV/A2kT of the I-V characteristic, with A2 roughly equal to 2, originates in the space charge region and that it can be increased, as observed on ATS-1 cells, by the introduction of additional defects through low energy proton irradiation. It was shown that the proton irradiation introduces defects into the space charge region which give rise to a recombination current from this region, although the I-V characteristic is, in this case, dominated by an exponential term which has A = 1.

Wolf, M.↗

NASA NTRS1977

Observation of preplanetary disks around MWC 349 and LKH-alpha 101

Infrared spectra of three highly reddened emission-line objects (MWC 349, MWC 297, and LKH-alpha 101) have been obtained from both aircraft and ground-based observatories. These spectra show bright emission lines which are consistent with recombination in an H II region. Additional optical data for MWC 349 yield a model which consists of a central O6.5 star with circumstellar disk seen face-on, a surrounding H II region, and circumstellar dust. The parameters of the disk are discussed in terms of the viscous dissipation model of Lynden-Bell and Pringle (1974). The observations are in excellent agreement with the predictions of the model and represent the first substantial case for a newly formed star with a preplanetary disk.

Thompson, R. I.↗

NASA NTRS1977

Molecular clouds

An attempt is made to understand star formation in the context of the dense interstellar molecular gas from which stars are made. Attention is given to how molecular observations (e.g., UV spectroscopy and radio 21-cm and recombination line observations) provide data on the physical state of the dense interstellar gas; observations of H II regions, stellar associations, and dark nebulae are discussed. CO clouds are studied with reference to radial velocity, temperature, density, ionization, magnetic field.

Thaddeus, P.↗

NASA NTRS1978

Direct nuclear pumping by a volume source of fission fragments

A detailed kinetic model is presented for the analysis of nuclear pumped lasers when the pumping is a result of a volume source of fission fragments. The results of the model are employed to study a He-3 - Xe laser. For the range of pressures, neutron fluxes and mixtures considered, the gain and power calculations are in good agreement with experiment. Moreover, based on these calculations, it appears that the collisional recombination is the dominant pumping mechanism for 7p-7s transitions while direct excitation is the dominant pumping mechanism for the 5d-6p transitions.

Deese, J. E.↗

NASA NTRS1978

Artificially created holes in the ionosphere

The artificial creation of ionospheric holes by the release of highly reactive molecules into the F region is investigated. Through ion-atom interchange or charge transfer reactions, H2 or H2O reacts with O(+) to form OH(+) or H2O(+), respectively, which subsequently dissociatively recombines with electrons at a very rapid rate. The diffusion of H2 is also modified by chemical loss to the ambient atomic oxygen atmosphere. The limited spatial and temporal extent of the hole-making process allows several approximations to be made which permit three-dimensional analytic solutions of the continuity equations for the released particles, the O(+) and e(-) densities, and the intermediary molecular ions. A versatile formalism is developed whereby the hole-making capability of virtually any spatial-temporal configuration of released particles can be determined by convolving a set of destruction operators which can be viewed as Green's functions for the problem. As a specific application of the techniques developed, the modification of a winter nighttime ionosphere is described by simulating the release of 1000 kg of water vapor near a height of 300 km.

Mendillo, M.↗

NASA NTRS1978

The O I /6300 A/ airglow

The production and destruction of O(D-1) atoms in the thermosphere are studied on the basis of observations conducted by the Atmosphere Explorer-C spacecraft. In particular, measurements of the O I transition at 6300 A provide a means of calculating the strength of the O2(+) dissociative recombination source of O(D-1), which amounts to 1.33. A value of 3 times 10 to the minus 11th power cu cm/sec is established for the quenching rate of O(D-1) by molecular nitrogen, while the photodissociation rate for O2 is set at three millionths per sec. These determinations are in agreement with the conventional theory of the day 6300-A airglow.

Hays, P. B.↗

NASA NTRS1978

The far-ultraviolet flux from Lambda Orionis A

Digicon observations of emission-line intensities in the H II region around Lambda Ori are used to determine the effective UV ionizing flux from Lambda Ori A. It appears that the flux shortward of 353 A (and perhaps shortward of 504 A) is significantly less than predicted from calculations of static stellar atmospheres. The discrepancy between theory and observation may be a consequence of the existence of an extended atmosphere due to radiation-pressure-driven mass loss. The results are consistent with those obtained from UV absorption-line studies made with the Copernicus satellite and with radio recombination-line data for compact H II regions.

Harms, R. J.↗

NASA NTRS1978

Interpretation of scanning electron microscope measurements of minority carrier diffusion lengths in semiconductors

In scanning electron microscope (SEM) injection measurements of minority carrier diffusion lengths some uncertainties of interpretation exist when the response current is nonlinear with distance. This is significant in epitaxial layers where the layer thickness is not large in relation to the diffusion length, and where there are large surface recombination velocities on the incident and contact surfaces. An image method of analysis is presented for such specimens. A method of using the results to correct the observed response in a simple convenient way is presented. The technique is illustrated with reference to measurements in epitaxial layers of GaAs. Average beam penetration depth may also be estimated from the curve shape.

Flat, A.↗

NASA NTRS1978

Spread F bubbles - Nonlinear Rayleigh-Taylor mode in two dimensions

The paper discusses long-wavelength developed bottomside spread F which has been attributed to the Rayleigh-Taylor instability. The nonlinear saturation amplitude and the k spectrum of the inertia-dominated Rayleigh-Taylor instability is found in two directions: east-west and vertical. As in the collisional case (Chaturvedi and Ossakow, 1977), the dominant nonlinearity is found to be two-dimensional. It is found that the linearly most unstable modes, which are primarily horizontal, saturate by the nonlinear generation of vertical spatial harmonics. The harmonics are damped by diffusion or recombination. The resulting amplitude spectrum indicates that bubbles are vertically elongated in both inertial and collisional regimes.

Hudson, M. K.↗

NASA NTRS1978

Global circulation and distribution of hydrogen in thermosphere of Venus

The global density distribution of atomic H in the upper atmosphere of Venus is determined in terms of a global circulation model. It is shown that H produced on the dayside is efficiently advected to the nightside by the major gas CO2 where it is then convected to lower altitudes and recombines. A maximum night/day H density ratio of about 5 is derived, in which case exospheric return flow from night to day strongly limits the nightside density enhancement. A previously postulated constraint connecting the eddy diffusion coefficient with the escape flux of H in one dimensional models is no longer required when global circulation of H is considered.

Hartle, R. E.↗

NASA NTRS1978

Review of rate coefficients of ionic reactions determined from measurements made by the atmosphere explorer satellites

The large data base of aeronomic parameters measured by the Atmosphere Explorer C, D, and E satellites since December 1973 has been used to determine a number of reaction rate coefficients highly relevant to our understanding of thermospheric chemistry. In this paper the results are reviewed for ionic rate coefficients for recombination of NO(+), O2(+), for reactions of O(+) + N2, N2(+) + O, and O(++) + O, and for various reactions involving O(+)(2D) and O(+)(2P) ions with O and N2.

Torr, D. G.↗

NASA NTRS1978

Hydrogen emission-line spectra in quasars and active galactic nuclei

An attempt is made to account for the observational fact that the intensity ratios of the lower Balmer lines seen in emission from QSOs, type 1 Seyfert galaxies, and other related active galaxies are not what would be expected from the conventional picture of lines generated in a recombining hydrogen plasma. Previous observations of emission-line intensities in QSOs and Seyfert galaxies are reviewed. The various processes included in the rate coefficients of the calculations are then described, viz., free-free, bound-free, and bound-bound transitions, radiative transfer, and angular-momentum mixing. The strengths of the various emission lines are calculated by using more accurate values for the transition rates, including excitation and deexcitation simultaneously, and providing for finite rates in optically forbidden transitions. Numerical results are presented for the cases of ground-state collisional excitation, reabsorption of the upper Balmer lines, and collisional deexcitation. The results are applied to construct a model for the emission-line gas in a QSO and to discuss limits on models of Seyfert galaxies.

Krolik, J. H.↗

NASA NTRS1978

Atmospheric chemistry - Response to human influence

Global atmospheric chemistry is surveyed, and the agreement of models with observed distribution of gases is considered. The influence of human perturbations due to combustion, agriculture, and chloro-carbon releases is examined with emphasis on ozone. Effects of combustion-related releases of CO on the abundances of other gases as well as possible effects of CO on tropospheric ozone are discussed. Other topics include the contribution of the chlorocarbon industry to stratospheric chloride and the recombination of nitrogen fixed by agriculture and combustion.

Logan, J. A.↗

NASA NTRS1978

Ion convection and the formation of the mid-latitude F region ionization trough

Total ion concentration and ion drift velocity data from Atmospheric Explorer C are used to examine the convective flow of plasma in the vicinity of the midlatitude F region ionization trough. It is found that the low-latitude portion of the premidnight trough is a region of slow eastward plasma drift while the poleward portion is characterized by generally westward flow. The reversal from eastward to westward flow is described. It is proposed that the plasma flux tubes within the premidnight trough have corotated eastward into the evening local time sector from dusk before stagnating and flowing westward at higher latitudes in the nighttime sector. The role of F region recombination in trough formation is considered.

Spiro, R. W.↗

NASA NTRS1977

Studies of silicon pn junction solar cells

Modifications of the basic Shockley equations that result from the random and nonrandom spatial variations of the chemical composition of a semiconductor were developed. These modifications underlie the existence of the extensive emitter recombination current that limits the voltage over the open circuit of solar cells. The measurement of parameters, series resistance and the base diffusion length is discussed. Two methods are presented for establishing the energy bandgap narrowing in the heavily-doped emitter region. Corrections that can be important in the application of one of these methods to small test cells are examined. Oxide-charge-induced high-low-junction emitter (OCI-HLE) test cells which exhibit considerably higher voltage over the open circuit than was previously seen in n-on-p solar cells are described.

Lindholm, F. A.↗

NASA NTRS1979

The prediction of two-dimensional airfoil stall progression

A generalized boundary condition potential flow calculation method was combined with a momentum integral boundary layer method and a base flow theory of separation to predict airfoil viscous-inviscid interference up to and beyond stall. The resultant program considers laminar and turbulent separation and is, therefore, applicable to thin or thick airfoil stall. The calculated flow field includes the airfoil and the separation bubble recombination region behind the airfoil. Calculated pressure distributions and equivalent airfoil shapes, including the displacement thickness of the viscous regions, are compared with flow field measurements for several airfoils. The measured displacement thicknesses and wake centerlines corroborate the calculated shape. The comparison also suggests the use of the analytical solution to evaluate the measurements.

Gross, L. W.↗

NASA NTRS1979

Low energy proton radiation damage to (AlGa)As-GaAs solar cells

Twenty-seven 2 times 2 sq cm (AlGa)As-GaAs solar cells were fabricated and subjected to 50 keV, 100 keV, and 290 keV of proton irradiation along with eighteen high efficiency silicon solar cells. The results of the study further corroborate the advantages for space missions offered by GaAs cells over state of the art silicon cells. Thus, even though the GaAs cells showed greater degradation when irradiated by protons with energy less than 5 MeV, the solar cells were normally protected from these protons by the glass covers used in space arrays. The GaAs cells also offered superior end of life power capability compared with silicon. The change in the open circuit voltage, short circuit current, spectral response, and dark 1-5 characteristics after irradiation at each proton energy and fluence were found to be consistent with the explanation of the effect of the protons. Also dark 1-5 characteristics showed that a new recombination center dominates the current transport mechanism after irradiation.

Loo, R.↗

NASA NTRS1979

Silicon solar cell fabrication technology

The laser cell scanner was used to characterize a number of solar cells made in various materials. An electron beam-induced current (EBIC) study was performed using a stereoscan scanning electron microscope. Planar p-n junctions were analyzed. A theory for the EBIC based on the analytical solution of the ambipolar diffusion equation under the influence of electron beam excitation parameter z (which is related to beam penetration), the junction depth Z sub j, the beam current and the surface recombination, was formulated and tested. The effect of a grain boundary was studied.

Stafsudd, O. M.↗