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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Mechanism for the Green Glow of the Upper Ionosphere

The generation of the green line of atomic oxygen by dissociative recombination of 02 plus occurs by the capture of an electron into a repulsive state of 02 followed by dissociation along another state of a different electronic symmetry. The two states are coupled together by mixed symmetry Rydberg states. Quantum chemical calculations give a rate coefficient at room temperature of (0.39 (+ 0.31 or -0.19)) x 10 exp -8 cubic centimeters per second. The quantum yield of excited oxygen is within the range deduced from ground, rocket, and satellite observations. The rate coefficients and yields are needed in models of the optical emission, chemistry, and energy balance of planetary ionospheres.

Guberman, Steven L.↗
NASA NTRS2000

Error Estimates for Emission Lines in the Hydrogen and Helium Isosequences

Emission lines from hydrogen and helium isosequence are among tile strongest in X-ray spectra; they will soon be used to measure the temperature, density, and equilibrium state of collisionally excited, astrophysical plasmas. We have created a new plasma code, APEC, which calculates the emission from such a plasma. APEC calculates the line emission from the direct electron and proton excitation rate and the radiative and dielectronic recombination rate. We show how different collisional plasma codes give varying emissivities for some strong lines of O VII and Fe XXVI, where direct excitation is the. primary effect. This variation is partly due to simple differences in the plasma code. However, the primary reason is that much work remains to be done on experimental and theoretical calculations of the, atomic rates. Large (approximately 50%) differences exist even for excitation rates for hydrogenic ions.

Smith, R. K.↗
NASA NTRS2013

Charge Yield at Low Electric Fields: Considerations for Bipolar Integrated Circuits

A significant reduction in total dose damage is observed when bipolar integrated circuits are irradiated at low temperature. This can be partially explained by the Onsager theory of recombination, which predicts a strong temperature dependence for charge yield under low-field conditions. Reduced damage occurs for biased as well as unbiased devices because the weak fringing field in thick bipolar oxides only affects charge yield near the Si/SiO2 interface, a relatively small fraction of the total oxide thickness. Lowering the temperature of bipolar ICs - either continuously, or for time periods when they are exposed to high radiation levels - provides an additional degree of freedom to improve total dose performance of bipolar circuits, particularly in space applications.

ionizing radiation↗
NASA NTRS1985

DLTS analysis of radiation-induced defects in one-MeV electron irradiated germanium and Alsub0.17Gasub0.83As solar cells

The radiation-induced deep-level defects in one-MeV electron-irradiated germanium and AlxGal-xAs solar cell materials using the deep-level transient spectroscopy (DLTS) and C-V techniques were investigated. Defect and recombination parameters such as defect density and energy levels, capture cross sections and lifetimes for both electron and hole traps were determined. The germanium and AlGaAs p/n junction cells were irradiated by one-MeV electrons. The DLTS, I-V, and C-V measurements were performed on these cells. The results are summarized as follows: (1) for the irradiated germanium samples, the dominant electron trap was due to the E sub - 0.24 eV level with density around 4x10 to the 14th power 1/cu cm, independent of electron fluence, its origin is attributed to the vacancy-donor complex defect formed during the electron irradiation; (2) in the one-MeV electron irradiated Al0.17Ga0.83 as sample, two dominant electron traps with energies of Ec-0.19 and -0.29 eV were observed, the density for both electron traps remained nearly constant, independent of electron fluence. It is shown that one-MeV electron irradiation creates very few or no new deep-level traps in both the germanium and AlxGa1-xAs cells, and are suitable for fabricating the radiation-hard high efficiency multijunction solar cells for space applications.

Li, S. B.↗
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.↗
NASA NTRS2004

Charge Storage, Conductivity and Charge Profiles of Insulators as Related to Spacecraft Charging

Dissipation of charges built up near the surface of insulators due to space environment interaction is central to understanding spacecraft charging. Conductivity of insulating materials is key to determine how accumulated charge will distribute across the spacecraft and how rapidly charge imbalance will dissipate. To understand these processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and charge transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators. One must consider generation of mobile electrons and holes, their trapping, thermal de-trapping, mobility and recombination. Conductivity is more appropriately measured for spacecraft charging applications as the "decay" of charge deposited on the surface of an insulator, rather than by flow of current across two electrodes around the sample. We have found that conductivity determined from charge storage decay methods is 102 to 104 smaller than values obtained from classical ASTM and IEC methods for a variety of thin film insulating samples. For typical spacecraft charging conditions, classical conductivity predicts decay times on the order of minutes to hours (less than typical orbit periods); however, the higher charge storage conductivities predict decay times on the order of weeks to months leading to accumulation of charge with subsequent orbits. We found experimental evidence that penetration profiles of radiation and light are exceedingly important, and that internal electric fields due to charge profiles and high-field conduction by trapped electrons must be considered for space applications. We have also studied whether the decay constants depend on incident voltage and flux or on internal charge distributions and electric fields; light-activated discharge of surface charge to distinguish among differing charge trapping centers; and radiation-induced conductivity. Our experiments also show that "Malter" electron emission occurs for hours after turning off the electron beam. This Malter emission similar to emission due to negative electron affinity in semiconductors is a result of the prior radiation or optical excitations of valence electrons and their slow drift among traps towards the surface where they are subsequently emitted. This work is supported through funding from the NASA Space Environments and Effects Program.

Dennison, J. R.↗
NASA NTRS1977

The influence of autoionization accompanied by excitation on dielectronic recombination and ionization equilibrium

In the process of dielectronic recombination, the doubly excited state formed by radiationless capture may autoionize preferentially into an excited state of the recombining ion. This additional autoionization process has not been discussed in previous treatments of dielectronic recombination. The dielectronic recombination rates for certain nonhydrogenic Fe ions, although still larger than the direct radiative recombination rates, are found to be substantially reduced by the inclusion of the additional autoionization rate in the branching ratio for the stabilizing radiative transition. Consequently, the temperatures of maximum equilibrium abundance are significantly lower than those predicted by recent calculations. Finally, the radiative energy loss rate coefficients are calculated for radiation processes involving electron Fe-ion collisions in high-temperature plasmas. Electron impact excitation of resonance line radiation is the dominant radiative cooling mechanism in steady-state plasmas at temperatures where ions with bound electrons are abundant. However, it is found that the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.

Jacobs, V. L.↗
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 NTRS1980

Efficient ionisation of calcium, strontium and barium by resonant laser pumping

Efficient ionization has been observed when an atomic vapor of strontium, barium or calcium was illuminated with a long pulse tunable laser at the frequency of the atomic resonance line. The variation in the degree of ionization with neutral density and laser intensity has been measured using the 'hook' method. The maximum ionization observed was 94%. Excited state populations were measured yielding an excitation temperature (depending on exact experimental conditions) in the region of 0.4 eV. The decay of ion density after the laser pulse was monitored and the recombination coefficients determined. The results are interpreted in terms of an electron heating model.

Skinner, C. H.↗
NASA NTRS1982

Thickness dependences of solar cell performance

The significance of including factors such as the base resistivity loss for solar cells thicker than 100 microns and emitter and BSF layer recombination for thin cells in predicting the fill factor and efficiency of solar cells is demonstrated analytically. A model for a solar cell is devised with the inclusion of the dopant impurity concentration profile, variation of the electron and hole mobility with dopant concentration, the concentration and thermal capture and emission rates of the recombination center, device temperature, the AM1 spectra and the Si absorption coefficient. Device equations were solved by means of the transmission line technique. The analytical results were compared with those of low-level theory for cell performance. Significant differences in predictions of the fill factor resulted, and inaccuracies in the low-level approximations are discussed.

Sah, C. T.↗
NASA NTRS1990

The red and green lines of atomic oxygen in the nightglow of Venus

O(1D) and O(1S), the excited states that give rise to the atomic oxygen red and green lines, are produced in the Venus nightglow in dissociative recombination of O2(+). The emissions should also be excited by precipitation of soft electrons, the suggested source of the 'auroral' emission features of atomic oxygen at 1304 and 1356 A, which have been reported from observations of the Pioneer Venus Orbiter Ultraviolet Spectrometer. No emisison at 6300 or 5577 A was detected, however, by the visible spectrophotometers on the Soviet spacecraft Veneras 9 and 10; upper limits have been placed on the intensities of these features. The constraints placed on models for the auroral production mechanism by the Venera upper limits by modeling the intensities of the red and green lines in the nightglow are evaluated, combining a model for the vibrational distribution of O2(+) on the nightside of Venus with rate coefficients recently computed by Guberman for production of O(1S) and O(1D) in dissociative recombination of O2(+) from different vibrational levels. The integrated overhead intensities are 1 - 2 R for the green line and about 46 R for the red line.

Fox, J. L.↗
NASA NTRS1986

Solar flare ionization in the mesosphere observed by coherent-scatter radar

The coherent-scatter technique, as used with the Urbana radar, is able to measure relative changes in electron density at one altitude during the progress of a solar flare when that altitude contains a statistically steady turbulent layer. This work describes the analysis of Urbana coherent-scatter data from the times of 13 solar flares in the period from 1978 to 1983. Previous methods of measuring electron density changes in the D-region are summarized. Models of X-ray spectra, photoionization rates, and ion-recombination reaction schemes are reviewed. The coherent-scatter technique is briefly described, and a model is developed which relates changes in scattered power to changes in electron density. An analysis technique is developed using X-ray flux data from geostationary satellites and coherent scatter data from the Urbana radar which empirically distinguishes between proposed D-region ion-chemical schemes, and estimates the nonflare ion-pair production rate.

Parker, J. W.↗
NASA NTRS1981

Coplanar back contacts for thin silicon solar cells

The type of coplanar back contact solar cell described was constructed with interdigitated n(+) and p(+) type regions on the back of the cell, such that both contacts are made on the back with no metallization grid on the front. This cell construction has several potential advantages over conventional cells for space use namely, convenience of interconnects, lower operating temperatures and higher efficiency due to the elimination of grid shadowing. However, the processing is more complex, and the cell is inherently more radiation sensitive. The latter problem can be reduced substantially by making the cells very thin (approximately 50 micrometers). Two types of interdigitated back contact cells are possible, the types being dependent on the character of the front surface. The front surface field cell has a front surface region that is of the same conductivity type as the bulk but is more heavily doped. This creates an electric field at the surface which repels the minority carriers. The tandem junction cell has a front surface region of a conductivity type that is opposite to that of the bulk. The junction thus created floats to open circuit voltage on illumination and injects carriers into the bulk which then can be collected at the rear junction. For space use, the front surface field cell is potentially more radiation resistant than the tandem junction cell because the flow of minority carriers (electrons) into the bulk will be less sensitive to the production of recombination centers, particularly in the space charge region at the front surface.

Storti, G.↗
NASA NTRS1982

Grown-in defects and defects produced by 1-Me electron irradiated in Al0.3Ga0.7As P-N junction solar cells

Studies of grown-in defects and defects produced by the one-MeV electron irradiation in Al sub 0.3 Ga sub 0.7As p-n junction solar cells fabricated by liquid phase epitaxial (LPE) technique were made for the unirradiated and one-MeV electron irradiated samples, using DLTS and C-V methods. Defect and recombination parameters such as energy level, defect density, carrier capture cross sections and lifetimes were determined for various growth, annealing, and irradiation conditions.

Li, S. S.↗
NASA NTRS1984

Explanation of the Normal Winter Anomaly from the Seasonal Variation of Short Wave Absorption

The frequency dependence of the winter anomaly (WA) of radio wave absorption indicates the altitude range where the considered seasonal variation of absorption, L, takes place: 75-95 km. In this height region considerable seasonal variations of ionic composition and effective recombination coefficient, alpha sub e, exist, which can cause seasonal variations of electron concentration, N, and absorption, L. An attempt to render a qualitative estimation of the normal WA, i.e., the increased ratio of winter over summer absorption, L sub w/L sub s, at medium latitudes 40 deg and 50 deg, for solar zenith angles CHi = 60 deg and 75 deg is made. This is compared with existing experimental data.

Velinov, P. J.↗
NASA NTRS1985

EBIC Characterization and Hydrogen Passivation in Silicon Sheet

As a general qualitative tool, the electron beam induced current (EBIC) method can be very useful in imaging recombination in silicon sheet used for solar cells. Work using EBIC on EFG silicon ribbon is described. In particular, some efforts at making the technique more quantitative and hence more useful, some limitations of the method, and finally specific application to hydrogen passivation is treated. Some brief remarks are made regarding the technique itself.

Hanoka, J. I.↗
NASA NTRS1986

Improved High/Low Junction Silicon Solar Cell

Air-mass-zero open-circuit voltage increased to 650 mV. Oxide-chargeinduced high/low emitter solar cell, electron accumulation layer induced by positive charge. High/low emitter suppresses dark emitter recombination current, resulting in power-conversion efficiencies significantly higher than previously achieved.

Neugroschel, A.↗