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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

A numerical study of scalar gradients in Kelvin-Helmholtz billows

A high resolution numerical technique is used to model the development of a periodically perturbed shear layer imbedded in an initially vertical gradient of a passive scalar. The technique follows the development of the vorticity through an initial linear growth state and well into the nonlinear development of Kelvin-Helmholtz billows, in the zero-viscosity, zero-diffusion limit. The resulting scalar distribution rapidly develops regions of extremely sharp scalar gradients, which wind around the periodically spaced vortical low gradient cores. Vertical cross sections through different parts of the billow structure are presented and compared with rocket measurements of electron density fine structure in the mesosphere. Gradient limits imposed by finite diffusion are calculated, and implications for atmospheric radar observations are discussed.

Parker, J. W.↗
NASA NTRS1988

Oscillator strengths for Si II

New accurate results are presented for absorption oscillator strengths of Si II resonance lines. In these ab initio calculations, an extensive basis set of 67 electronic configuration are employed, including fine-structure states and relativistic effects in the Breit-Pauli approximation. For the strong Si II lines, good agreement is found with previous LS-coupling f-values of Dufton et al. (1983). Oscillator strengths of selected weaker lines disagree in some cases with empirically determined f-values by Shull, Snow, and York (1981) and by Van Buren (1986). The theoretical results are combined with curve-of-growth analyses of Copernicus and IUE data to yield a list of recommended Si II oscillator strengths consistent with the errors in both theoretical and observational analyses. These f-values have been used (Van Steenberg and Shull) for a Galactic survey of interstellar silicon abundances with the International Ultraviolet Explorer satellite.

Luo, Ding↗
NASA NTRS1992

Interstellar absorption along the line of sight to Theta Carinae using Copernicus observations

A profile fitting technique is employed to identify the velocities and Doppler b values for H I and H II clouds along the line of sight to Theta Car. Total abundances and depletions for 12 elements, plus column densities for the J = 0 to J = 5 rotational levels of H2 are obtained. Electron densities for both clouds are calculated from the ratios of the fine-structure levels of C II and N II, obtaining 0.08/cu cm and 1.2/cu cm. The fine-structure levels of C I, which led to 120/cu cm, are used to calculate the neutral hydrogen density for the H I region. D I is also present in the data from the Theta Car line of sight, yielding a D/H ratio of 5 x 10 exp -6. Elemental depletions are calculated for the H I region as well. Comparison of the results for Theta Car and those for Zeta Oph and Alpha Vir shows that the absolute depletions are different; however, the relative depletions are remarkably stable for different physical conditions.

Allen, M. M.↗
NASA NTRS1998

Electro-mechanical Properties of Carbon Nanotubes

We present a simple picture to understand the bandgap variation of carbon nanotubes with small tensile and torsional strains, independent of chirality. Using this picture, we are able to predict a simple dependence of d(Bandoap)$/$d(strain) on the value of $(N_x-N_y)*mod 3$, for semiconducting tubes. We also predict a novel change in sign of d(Bandgap)$/$d(strain) as a function of tensile strain arising from a change in the value of $q$ corresponding to the minimum bandgap. These calculations are complemented by calculations of the change in bandgap using energy minimized structures, and some important differences are discussed. The calculations are based on the $i$ electron approximation.

Anantram, M. P.↗
NASA NTRS2009

Radiation from Relativistic Shocks with Turbulent Magnetic Fields

Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked region. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants. New recent calculation of spectra with various different Lorentz factors of jets (two electrons) and initial magnetic fields. New spectra based on small simulations will be presented.

Nishkawa, K.↗
NASA NTRS1989

LEO high voltage solar array arcing response model, continuation 5

The modeling of the Debye Approximation electron sheaths in the edge and strip geometries was completed. Electrostatic potentials in these sheaths were compared to NASCAP/LEO solutions for similar geometries. Velocity fields, charge densities and particle fluxes to the biased surfaces were calculated for all cases. The major conclusion to be drawn from the comparisons of our Debye Approximation calculations with NASCAP-LEO output is that, where comparable biased structures can be defined and sufficient resolution obtained, these results are in general agreement. Numerical models for the Child-Langmuir, high-voltage electron sheaths in the edge and strip geometries were constructed. Electrostatic potentials were calculated for several cases in each of both geometries. Velocity fields and particle fluxes were calculated. The self-consistent solution process was carried through one cycle and output electrostatic potentials compared to NASCAP-type input potentials.

Metz, Roger N.↗
NASA NTRS1975

Widths of atomic 4s and 4p vacancy states, 46 less than or equal to Z less than or equal to 50

Auger and X-ray photoelectron spectra involving N1, N2, and N3 vacancy states of Pd, Ag, Cd, In, and Sn were measured and compared with results of free atom calculations. As previously observed in Cu and Zn Auger spectra that involve 3d-band electrons, free-atom characteristics with regard to widths and structure were found in the Ag and Cd M4-N4,5N4,5 and M5-N4,5N4,5 Auger spectra that arise from transitions of 4d-band electrons. Theoretical N1 widths computed with calculated free-atom Auger energies agree well with measurements. Theory however predicts wider N2 than N3 vacancy states (as observed for Xe), while the measured N2 and N3 widths are nearly equal to each other and to the average of the calculated N2 and N3 widths. The calculations are made difficult by the exceedingly short lifetime of some 4p vacancies and by the extreme sensitivity of super-Coster-Kronig rates, which dominate the deexcitation, to the transition energy and to the fine details of the atomic potential.

Hsiungchen, M.↗
NASA NTRS1992

Resistivity due to weak double layers - A model for auroral arc thickness

We have calculated the resistivity due to a sequence of fluctuating weak double layers aligned parallel to the ambient magnetic field line. The average response of an electron drifting through a 1D randomly oriented array of WDLs is studied using a test particle approach. The average is taken over the randomly fluctuating values of the electric field associated with the double layers. Based on our calculations, we estimate that a 350 eV electron energy the thickness of the visual auroral arc is about 2.5 km and that of the auroral fine structure as about 250 m when mapped down to the ionosphere. The significance of our calculations is discussed in the context of magnetosphere-ionosphere coupling.

Prakash, Manju↗
NASA NTRS2001

Ab Initio and Improved Empirical Potentials for the Calculation of the Anharmonic Vibrational States and Intramolecular Mode Coupling of N-Methylacetamide

The second-order Moller-Plesset ab initio electronic structure method is used to compute points for the anharmonic mode-coupled potential energy surface of N-methylacetamide (NMA) in the trans(sub ct) configuration, including all degrees of freedom. The vibrational states and the spectroscopy are directly computed from this potential surface using the Correlation Corrected Vibrational Self-Consistent Field (CC-VSCF) method. The results are compared with CC-VSCF calculations using both the standard and improved empirical Amber-like force fields and available low temperature experimental matrix data. Analysis of our calculated spectroscopic results show that: (1) The excellent agreement between the ab initio CC-VSCF calculated frequencies and the experimental data suggest that the computed anharmonic potentials for N-methylacetamide are of a very high quality; (2) For most transitions, the vibrational frequencies obtained from the ab initio CC-VSCF method are superior to those obtained using the empirical CC-VSCF methods, when compared with experimental data. However, the improved empirical force field yields better agreement with the experimental frequencies as compared with a standard AMBER-type force field; (3) The empirical force field in particular overestimates anharmonic couplings for the amide-2 mode, the methyl asymmetric bending modes, the out-of-plane methyl bending modes, and the methyl distortions; (4) Disagreement between the ab initio and empirical anharmonic couplings is greater than the disagreement between the frequencies, and thus the anharmonic part of the empirical potential seems to be less accurate than the harmonic contribution;and (5) Both the empirical and ab initio CC-VSCF calculations predict a negligible anharmonic coupling between the amide-1 and other internal modes. The implication of this is that the intramolecular energy flow between the amide-1 and the other internal modes may be smaller than anticipated. These results may have important implications for the anharmonic force fields of peptides, for which N-methylacetamide is a model.

Gregurick, Susan K.↗
NASA NTRS2006

New Accurate Oscillator Strengths and Electron Excitation Collision Strengths for N1

The nonorthogonal orbitals technique in a multiconfiguration Hartree-Fock approach is used to calculate oscillator strengths and transition probabilities of N(I) lines. The relativistic effects are allowed by means of Breit-Pauli operators. The length and velocity forms of oscillator strengths show good agreement for most transitions. The B-spline R-matrix with pseudostates approach has been used to calculate electron excitation collision strengths and rates. The nonorthogonal orbitals are used for an accurate description of both target wave functions and the R-matrix basis functions. The 24 spectroscopic bound and autoionizing states together with 15 pseudostates are included in the close-coupling expansion. The collision strengths for transitions between fine-structure levels are calculated by transforming the LS-coupled K-matrices to K-matrices in an intermediate coupling scheme. Thermally averaged collision strengths have been determined by integrating collision strength over a Maxwellian distribution of electron energies over a temperature range suitable for the modeling of astrophysical plasmas. The oscillator strengths and thermally averaged collision strengths are presented for transitions between the fine-structure levels of the 2s(sup 2)p(sup 3) (sup 4)S(sup 0), (sup 2)D(sup 0), (sup 2)P(sup 0), 2s2p(sup 4) (sup 4)P, 2s(sup 2)2p(sup 2)3s (sup 4)P, and (sup 2)P terms and from these levels to the levels of the 2s(sup 2)2p(sup 2)3p (sup 2)S(sup 0), (sup 4)D(sup 0), (sup 4)P(sup 0), (sup 4)S(sup 0), (sup 2)D(sup 0), (sup 2)P(sup 0),2s(sup 2)2p(sup 2)3s(sup 2)D, 2s(sup 2)2p(sup 2)4s(sup 4)P, (sup 2)P, 2s(sup 2)2p(sup 2)3d(sup 2)P, (sup 4)F,(sup 2)F,(sup 4)P, (sup 4)D, and (sup 2)D terms. Thermally averaged collision strengths are tabulated over a temperature range from 500 to 50,000 K.

Tayal, S. S.↗
NASA NTRS1981

Extended adiabatic blast waves and a model of the soft X-ray background

An analytical approximation is generated which follows the development of an adiabatic spherical blast wave in a homogeneous ambient medium of finite pressure. An analytical approximation is also presented for the electron temperature distribution resulting from coulomb collisional heating. The dynamical, thermal, ionization, and spectral structures are calculated for blast waves of energy E sub 0 = 5 x 10 to the 50th power ergs in a hot low-density interstellar environment. A formula is presented for estimating the luminosity evolution of such explosions. The B and C bands of the soft X-ray background, it is shown, are reproduced by such a model explosion if the ambient density is about .000004 cm, the blast radius is roughly 100 pc, and the solar system is located inside the shocked region. Evolution in a pre-existing cavity with a strong density gradient may, it is suggested, remove both the M band and OVI discrepancies.

Cox, D. P.↗
NASA NTRS1981

The Voyager 1 EUV spectrum of the Io plasma torus

The Voyager 1 EUV spectrum of the hot Io plasma torus obtained before encounter in 1979 at a range of 4,800,000 km is analyzed in terms of a collisional model with improved collision strengths. Model calculations are based entirely on steady-state equilibrium between electron excitation-deactivation and radiative decay in the subspecies structure, and plasma temperature and species number densities are derived by fitting model spectra to the observations. Lines of S II, S III, S IV, O II, and O III are found to be dominant in the plasma torus, at torus half width densities of 44, 160, 220, 50, and 340/cu cm, respectively, and lines of S V and K III may also be present. An effective electron temperature of 80,000 K is estimated for the central dense region near 6 Jupiter radii. Model calculations are in substantial agreement with other spectroscopic observations of the hot torus; however, the observations exhibit strong morphological differences with recent ground-based observations of S II emission, and calculated ion densities differ from in situ Voyager 1 plasma science measurements.

Shemansky, D. E.↗
NASA NTRS1977

Pioneer Venus spacecraft charging model

Five environmental models were constructed to represent the solar wind and the upper, middle, and lower ionosphere of Venus. The spacecraft structure was modeled with over 140 passive electrical elements representing structural elements of the spacecraft. Electron, ion, secondary electron, and photocurrents to the spacecraft from the plasma were calculated, ignoring sheath effects. In all but one case, potentials of interest were less than 1 volt. Potential differences between widely separated points on the equipment shelf were less than 1 mV. The one area of concern is the solar panel potential when the orbiter is passing through the bowshock region.

Robinson, P. A., Jr.↗
NASA NTRS1990

Spectrophotometry of the shell around AG Carinae

Spatially-resolved long-slit spectrophotometry are presented for two regions of the shell nebula around the P-Cygni variable star AG Carinae. The spectra cover the 3700-6800 A wavelength range. Emission-line diagnostics are used to derive extinction, electron temperatures, and densities for various positions in the nebula. The chemical abundances and ionization structure are calculated and compared with other types of planetary nebulae and shells around other luminous stars. It is found that the N/O and N/S ratios of Ag Car are high compared to solar neighborhood ISM values. The O/H depletion found for the AG Car shell approaches that found in the condensations of the Eta Car system.

Mitra, P. Mila↗
NASA NTRS2004

Ceramic Interconnects with Low Sintering Temperature

Ceramic interconnects for use in solid oxide he1 cells are expected to operate between 900 to approximately 1000 C, sinter below 1400 C to allow co-firing and meet a number thermal mechanical requirements. The pervoskite type (ABO3) lanthanum chromite based materials have emerged as a leading candidate that will meet these criteria by varying the composition on the A and B sites. A need therefore exists to determine this material's temperature dependent electrical and mechanical properties with respect to these site substitutions. In this investigation oxide powders were prepared by the glycine-nitrate process. Ionic substitutions were carried out on A sites with Ca or Sr, and B sites with Co and Al, respectively. Only stoichiometric compositions were considered for the sake of stability. The powders and their ability to sinter were investigated by XRD, SEM, dilatometry and density measurements. The sintered materials were further examined by SEM, thermal expansion and electric conductivity measurements in order to elucidate the resulting microstructure, electrical and mechanical properties. In addition quantum mechanical calculations were performed to obtain insight into the effects of these dopants on the materials electronic band structure and lattice parameter.

Zhong, Zhi-Min↗
NASA NTRS2012

Radiation from Accelerated Particles in Shocks and Reconnections

Plasma instabilities are responsible not only for the onset and mediation of collisionless shocks but also for the associated acceleration of particles. We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection and, more generally, relativistic acceleration behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. Our initial results of a jet-ambient interaction with anti-parallelmagnetic fields show pile-up of magnetic fields at the colliding shock, which may lead to reconnection and associated particle acceleration. We will investigate the radiation in a transient stage as a possible generation mechanism of precursors of prompt emission. In our simulations we calculate the radiation from electrons in the shock region. The detailed properties of this radiation are important for understanding the complex time evolution and spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Nishikawa, K. I.↗
NASA NTRS2013

A Binary-Encounter-Bethe Approach to Simulate DNA Damage by the Direct Effect

The DNA damage is of crucial importance in the understanding of the effects of ionizing radiation. The main mechanisms of DNA damage are by the direct effect of radiation (e.g. direct ionization) and by indirect effect (e.g. damage by.OH radicals created by the radiolysis of water). Despite years of research in this area, many questions on the formation of DNA damage remains. To refine existing DNA damage models, an approach based on the Binary‐Encounter‐Bethe (BEB) model was developed[1]. This model calculates differential cross sections for ionization of the molecular orbitals of the DNA bases, sugars and phosphates using the electron binding energy, the mean kinetic energy and the occupancy number of the orbital. This cross section has an analytic form which is quite convenient to use and allows the sampling of the energy loss occurring during an ionization event. To simulate the radiation track structure, the code RITRACKS developed at the NASA Johnson Space Center is used[2]. This code calculates all the energy deposition events and the formation of the radiolytic species by the ion and the secondary electrons as well. We have also developed a technique to use the integrated BEB cross section for the bases, sugar and phosphates in the radiation transport code RITRACKS. These techniques should allow the simulation of DNA damage by ionizing radiation, and understanding of the formation of double‐strand breaks caused by clustered damage in different conditions.

Plante, Ianik↗
NASA NTRS2002

Effect of Electronic Excitation on Hydrogen Atom Transfer (Tautomerization) Reactions for the DNA Base Adenine

Geometrical structures and energetic properties for four different tautomers of adenine are calculated in this study, using multi-configurational wave functions. Both the ground and the lowest single excited state potential energy surface are studied. The energetic order of the tautomers on the ground state potential surface is 9H less than 7H less than 3H less than 1H, while on the excited state surface this order is found to be different: 3H less than 1H less than 9H less than 7H. Minimum energy reaction paths are obtained for hydrogen atom transfer (9 yields 3 tautomerization) reactions in the ground and the lowest excited electronic state. It is found that the barrier heights and the shapes of the reaction paths are different for the ground and the excited electronic state, suggesting that the probability of such tautomerization reaction is higher on the excited state potential energy surface. The barrier for this reaction in the excited state may become very low in the presence of water or other polar solvent molecules, and therefore such tautomerization reaction may play an important role in the solution phase photochemistry of adenine.

Chaban, Galina M.↗