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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

On the optical properties of carbon nanotubes. Part I. A general formula for the dynamical optical conductivity

This paper is the first one in a series of two articles in which we revisit the optical properties of single-walled carbon nanotubes (SWNTs). Produced by rolling up a graphene sheet, SWNTs owe their intriguing properties to their cylindrical quasi-one-dimensional (quasi-1D) structure (the ratio length/radius is experimentally of order of 10{sup 3}). We model SWNT by circular cylinders of small diameters on the surface of which the conduction electron gas is confined by the electric field generated by the fixed carbon ions. The pair-interaction potential considered is the 3D Coulomb potential restricted to the cylinder. To reflect the quasi-1D structure, we introduce a 1D effective many-body Hamiltonian which is the starting-point of our analysis. To investigate the optical properties, we consider a perturbation by a uniform time-dependent electric field modeling an incident light beam along the longitudinal direction. By using Kubo’s method, we derive within the linear response theory an asymptotic expansion in the low-temperature regime for the dynamical optical conductivity at fixed density of particles. The leading term only involves the eigenvalues and associated eigenfunctions of the (unperturbed) 1D effective many-body Hamiltonian and allows us to account for the sharp peaks observed in the optical absorption spectrum of SWNT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Atmosphere expansion and mass loss of close-orbit giant exoplanets heated by stellar XUV. II. Effects of planetary magnetic field; structuring of inner magnetosphere

This is the second paper in a series where we build a self-consistent model to simulate the mass-loss process of a close-orbit magnetized giant exoplanet, so-called hot Jupiter (HJ). In this paper we generalize the hydrodynamic (HD) model of an HJ's expanding hydrogen atmosphere, proposed in the first paper, to include the effects of intrinsic planetary magnetic field. The proposed self-consistent axisymmetric 2D magnetohydrodynamics model incorporates radiative heating and ionization of the atmospheric gas, basic hydrogen chemistry for the appropriate account of major species composing HJ's upper atmosphere and related radiative energy deposition, and H{sub 3}{sup +} and Lyα cooling processes. The model also takes into account a realistic solar-type X-ray/EUV spectrum for calculation of intensity and column density distribution of the radiative energy input, as well as gravitational and rotational forces acting in a tidally locked planet–star system. An interaction between the expanding atmospheric plasma and an intrinsic planetary magnetic dipole field leads to the formation of a current-carrying magnetodisk that plays an important role for topology and scaling of the planetary magnetosphere. A cyclic character of the magnetodisk behavior, composed of consequent phases of the disk formation followed by the magnetic reconnection with the ejection of a ring-type plasmoid, has been discovered and investigated. We found that the mass-loss rate of an HD 209458b analog planet is weakly affected by the equatorial surface field <0.3 G, but is suppressed by an order of magnitude at the field of 1 G.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Probing the inert doublet dark matter model with Cherenkov telescopes

We present a detailed study of the annihilation signals of the inert dark matter doublet model in its high mass regime. Concretely, we study the prospects to observe gamma-ray signals of the model in current and projected Cherenkov telescopes taking into account the Sommerfeld effect and including the contribution to the spectrum from gamma-ray lines as well as from internal bremsstrahlung. We show that present observations of the galactic center by the H.E.S.S. instrument are able to exclude regions of the parameter space that give the correct dark matter relic abundance. In particular, models with the charged and the neutral components of the inert doublet nearly degenerate in mass have strong gamma-ray signals. Furthermore, for dark matter particle masses above 1 TeV, we find that the non-observation of the continuum of photons generated by the hadronization of the annihilation products typically give stronger constraints on the model parameters than the sharp spectral features associated to annihilation into monochromatic photons and the internal bremsstrahlung process. Lastly, we also analyze the interplay between indirect and direct detection searches for this model, concluding that the prospects for the former are more promising. In particular, we find that the upcoming Cherenkov Telescope Array will be able to probe a significant part of the high mass regime of the model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Viscous Properties of a Degenerate One-Dimensional Fermi Gas

In this paper, we study the viscous properties of a system of weakly interacting spin-1/2 fermions in one dimension. Accounting for the effect of interactions on the quasiparticle energy spectrum, we obtain the bulk viscosity of this system at low temperatures. Our result is valid for frequencies that are small compared with the rate of fermion backscattering. For frequencies larger than this exponentially small rate, the excitations of the system become decoupled from the center of mass motion, and the fluid is described by two-fluid hydrodynamics. We calculate the three transport coefficients required to describe viscous dissipation in this regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Dark-matter-enhanced probe of relic neutrino clustering

We propose heavy decaying dark matter (DM) as a new probe of the cosmic neutrino background (C⁢𝜈⁢B). Heavy DM, with mass ≳ 10 9 GeV, decaying into neutrinos can be a new source of ultrahigh-energy (UHE) neutrinos. Including this contribution along with the measured astrophysical and predicted cosmogenic neutrino fluxes, we study the scattering of UHE neutrinos with the C⁢𝜈⁢B via standard weak interactions mediated by the 𝑍 boson. We solve the complete neutrino transport equation, taking into account both absorption and reinjection effects, to calculate the expected spectrum of UHE neutrino flux at future neutrino telescopes, such as the IceCube-Gen2 radio. We argue that such observations can be used to probe the C⁢𝜈⁢B properties and, in particular, local C⁢𝜈⁢B clustering. We find that, depending on the absolute neutrino mass and the DM mass and lifetime, a local C⁢𝜈⁢B overdensity ≳ 10 6 can be probed at the IceCube-Gen2 radio within ten years of data taking.

Dark matter↗
DOE OSTI2013

Optical nutation in the exciton range of spectrum

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

High-frequency solitons in media with induced scattering from damped low-frequency waves with nonuniform dispersion and nonlinearity

The dynamics of high-frequency field solitons is considered using the extended nonhomogeneous nonlinear Schrödinger equation with induced scattering from damped low-frequency waves (pseudoinduced scattering). This scattering is a 3D analog of the stimulated Raman scattering from temporal spatially homogeneous damped low-frequency modes, which is well known in optics. Spatial inhomogeneities of secondorder linear dispersion and cubic nonlinearity are also taken into account. It is shown that the shift in the 3D spectrum of soliton wavenumbers toward the short-wavelength region is due to nonlinearity increasing in coordinate and to decreasing dispersion. Analytic results are confirmed by numerical calculations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Validation of KENO Delayed Neutron Fraction Capabilities

The delayed neutron fraction, which is necessary for the safe operation of reactors, varies by fissionable material and neutron spectrum. A prompt fission neutron spectrum has been included in the SCALE 6.2 release to account for the removal of delayed neutrons from precursor materials from the neutron balance. This paper describes how experimental measurements from criticality safety benchmarks were used to validate the SCALE calculational procedure for the delayed neutron fraction. Additionally, the effect of nuclear data uncertainty on the KENO calculation of β eff was examined with the SAMPLER sequence.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1984

The case for antiparticles in the extragalactic cosmic radiation

The presence of an excess of low-energy antiprotons in the primary cosmic radiation has given rise to several possible explanations, some of which involve exotic processes such as mini-black holes and extragalactic antiparticles. The latter possibility is considered, and it is shown that there are interesting implications for the cosmic radiation at higher energies. Indeed, it may be possible to account for a previously puzzling feature of the cosmic ray spectrum (a 'bump' in the range between 10 to the 14th and 10 to the 15th eV) by hypothesizing a primary extragalactic origin for the bulk of the observed cosmic ray antiprotons, although such an explanation is not unique. In this model, most of the cosmic rays above 10 to the 15th eV are extragalactic. A method of testing this hypothesis experimentally is described.

Stecker, F. W.↗
NASA NTRS1984

Magnetohydrodynamic model of Crab nebula radiation

The properties of the synchrotron continuum are calculated on the basis of a steady spherically symmetric MHD model of the flow in the Crab nebula. The model accounts for the spectral distribution of the continuum from the optical to gamma-rays, and semiquantitatively for its observed spatial distribution. The model does not account for the radio portion of the Crab nebula's spectrum.

Kennel, C. F.↗
NASA NTRS1983

Origin of diffuse gamma-ray background

It is shown that the observed spectrum of the diffuse gamma-ray background could be accounted for by unresolved active galaxies even with no evolution, if their spectra are described by a spherical accretion/shock acceleration model (1983) and are steepened by photon-photon pair production interactions at high energies. The model is based on HEAO-1 and SAS-2 observations in the region of 2 to 165 keV and above 30 MeV. The model predicts a higher flux of high-energy neutrinos from active galaxies than models in which electrons are accelerated directly. It is noted that several active galaxies should be detectable by the Gamma-Ray Observatory, making it possible to determine the actual contribution of active galaxies to the diffuse gamma-ray background.

Protheroe, R. J.↗
NASA NTRS1989

The composition of heavy molecular ions inside the ionopause of Comet Halley

The RPA2-PICCA instrument aboard the Giotto spacecraft obtained 10-210 amu mass spectral of cold thermal molecular ions in the coma of Comet Halley. The dissociation products of the long chain formaldehyde polymer polyoxymethylene (POM) have recently been proposed as the dominant complex molecules in the coma of Comet Halley; however, POM alone cannot account for all of the features of the high resolution spectrum. An important component of the dust at Comet Halley is particles highly enriched in carbon, hydrogen, oxygen, and nitrogen relative to the composition of carbonaceous chondrites. Since this dust could be a source for the heavy molecules observed by PICCA, a search was conducted for other chemical species by determining all the molecules with mass between 20 and 120 amu which can be made from the relatively abundant C, H, O, and N, without regard to chemical structure.

Mitchell, David L.↗
NASA NTRS1991

UGC galaxies stronger than 25 mJy at 4.85 GHz

UGC galaxies in the declination band +5 to +75 deg were identified by position coincidence with radio sources stronger than 25 mJy on the Green Bank 4.85 GHz sky maps. Candidate identifications were confirmed or rejected with the aid of published aperture-synthesis maps and new 4.86 GHz VLA maps having 15 or 18 arcsec resolution, resulting in a sample of 347 nearby radio galaxies plus five new quasar-galaxy pairs. The radio energy sources in UGC galaxies were classified as 'starbursts' or 'monsters' on the basis of their infrared-radio flux ratios, infrared spectral indices, and radio morphologies. The rms scatter in the logarithmic infrared-radio ratio q is not more than 0.16 for starburst galaxies selected at 4.85 GHz. Radio spectral indices were obtained for nearly all of the UGC galaxies, and S0 galaxies account for a disproportionate share of the compact flat-spectrum (alpha less than 0.5) radio sources. The extended radio jets and lobes produced by monsters are preferentially, but not exclusively, aligned within about 30 deg of the optical minor axes of their host galaxies. The tendency toward minor-axis ejection appears to be independent of radio-source size and is strongest for elliptical galaxies.

Condon, J. J.↗
NASA NTRS1991

Relativistic dynamics of expanding sources

Given a relativistically expanding source, the transformation of the emission spectrum from the comoving frame of the source to the observer's frame is calculated. In particular, two special cases are dealt with, that of a uniformly expanding sphere and that of a cylinder. In each case a power-law spectrum with two distinct regions - the optically thick and thin parts - is considered, and those regions are transformed separately. The results are discussed, and a generalized procedure for transforming a spectrum of unspecified form is outlined. This procedure takes into account any bulk motion by the source as well as expansion. Finally, applications of this procedure to astrophysical sources are briefly discussed.

Sadun, A. C.↗
NASA NTRS1992

Stratification of chemical elements in the atmosphere of the Ap star 53 Camelopardalis

A detailed abundance analysis of Cr, Fe, Mn, Ca, and Ti in the Ap star 53 Cam from IUE high-resolution spectra has been performed. It is found that UV lines give significantly lower abundance than visible lines for Cr and Fe. Theoretical spectra of Cr II line calculated with the stratified chromium abundance distributions predicted a priori by the diffusion-mass loss model match well both visible and UV lines. The spectral shape of the Ca II K line and its variation are well accounted for by the diffusion model with mass loss. A spectrum synthesis with a stratified abundance well reproduces the observations. These findings provide convincing evidence for stratification in the photosphere of 53 Cam.

Babel, J.↗
NASA NTRS1994

Analysis of small-scale microwave background radiation anisotropy in the presence of foreground contamination

Many of the current round of experiments searching for anisotropies in the microwave background radiation (MBR) are confronting the problem of how to disentangle the cosmic signal from contamination due to Galactic and intergalactic foreground sources. Here we show how commonly used likelihood function techniques can be generalized to account for foreground. Specifically we set some restrictions on the spectrum of foreground contamination but allow the amplitude to vary arbitrarily. The likelihood function thus generalized gives reasonable limits on the MBR anisotropy which, in some cases, are not much less restrictive than what one would get from more detailed modeling of the foreground. Furthermore, the likelihood function is exactly the same as one would obtain by simply projecting out foreground contamination and looking at the reduced data set. We apply this generalized analysis to the recent medium-angle data sets of ACME-HEMT (Gaier et al. 1992; Schuster et al. 1993) and MAX (Meinhold et al. 1993; Gunderson et al. 1993). The resulting analysis constrains the one free parameter in the standard cold dark matter theory to be Q(sub rms-ps) = 18(sub -5 sup +8) microKelvin. This best fit value, although in striking agreement with the normalization from Cosmic Background Explorer (COBE), is not a very good fit, with an overall chi-squared/degrees of freedom = 208/168. We also argue against three commonly used methods of dealing with foreground: (1) ignoring it completely; (2) subtracting off a best-fit foreground and treating the residuals as if uncontaminated; and (3) culling data which appears to be contaminated by foreground.

Dodelson, Scott↗
NASA NTRS2010

Relation Between Type II Bursts and CMEs Inferred from STEREO Observations

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe coronal mass ejections (CMEs) a in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be approximately 1.5Rs (solar radii), which coincides with the distance at which the Alfv?n speed profile has a minimum value. We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfv?n speed peaks (?3Rs ? 4Rs). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5Rs to 4Rs. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (approximately 1.5 Rs) of STEREO CMEs at the time of type II bursts is smaller than that (2.2 Rs) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was a diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1Rs - 2Rs by the time the CME left the LASCO field of view.

Gopalswamy, N.↗
NASA NTRS2009

Relation Between Type II Bursts and CMEs Inferred from STEREO Observations

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe CMEs in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be approx. 1.5Rs (solar radii), which coincides with the distance at which the Alfven speed profile has a minimum value.We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfven speed peaks (approx. 3Rs - 4Rs). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5Rs to 4Rs. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (approx 1.5Rs) of STEREO CMEs at the time of type II bursts is smaller than that (2.2Rs) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was a diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1Rs - 2Rs by the time the CME left the LASCO field of view.

Solar Energetic Particles↗