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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
NASA NTRS1986

The wave magnetic field amplitude threshold for nonlinear trapping of energetic gyroresonant and Landau resonant electrons by nonducted VLF waves in the magnetosphere

A recently developed theory is used to calculate the wave magnetic field amplitude threshold, B-tau, necessary to allow the nonlinear trapping of energetic gyroresonant and Landau resonant electrons by VLF whistler mode waves in the magnetosphere propagating at an arbitrary angle, psi, with respect to the earth's magnetic field. A detailed raytracing study is carried out to establish the variation of psi with position along each magnetic shell in the range L = 2-5 and for frequencies 5-17.8 kHz. It is found that the minimum values of B-tau along each L shell generally occur at points of second-order resonance. In general, for nonducted fixed frequency signals there is only one point of second-order resonance on each L shell, and this is located within a few degrees of the magnetic equator. However, over a narrow range of L, there are as many as three points of second-order resonance and as many as three associated minima in B-tau. At least one of these points is located more than 10 deg from the magnetic equator.

Bell, T. F.↗
NASA NTRS1998

Technologies for Non-Destructive Evaluation of Surfaces and Thin Coating

A study was made on two commercially available terbium-doped silicate glasses. There is an increased interest in silicate glasses doped with rare-earth ions for use in high-energy particle detection and radiographic applications. These glasses are of interest due to the fact that they can be formed into small fiber sensors; a property that can be used to increase the spatial resolution of a detection system. Following absorption of radiation, the terbium ions become excited and then emit photons via 4f-4f electronic transitions as they relax back to the ground state. The lifetime of these transitions is on the order of milliseconds. A longer decay component lasting on the order of minutes has also been observed. While radiative transitions in the 4f shell of rare-earth ions are generally well understood by the Judd-Olfelt theory, the presence of a longer luminescence decay component is not. Experimental evidence that the long decay component is due, in part, to the thermal release of trapped charge carriers will be presented. In addition, a theoretical model describing the time evolution of the radiation-induced luminescence will be presented.

Manos, Dennis M.↗
NASA NTRS1996

Time Evolution of Radiation-Induced Luminescence in Terbium-Doped Silicate Glass

A study was made on two commercially available terbium-doped silicate glasses. There is an increased interest in silicate glasses doped with rare-earth ions for use in high-energy particle detection and radiographic applications. These glasses are of interest due to the fact that they can be formed into small fiber sensors; a property that can be used to increase the spatial resolution of a detection system. Following absorption of radiation, the terbium ions become excited and then emit photons via 4f-4f electronic transitions as they relax back to the ground state. The lifetime of these transitions is on the order of milliseconds. A longer decay component lasting on the order of minutes has also been observed. While radiative transitions in the 4f shell of rare-earth ions are generally well understood by the Judd-Olfelt theory, the pr'esence of a longer luminescence decay component is not. Experimental evidence that the long decay component is due, in part, to the thermal release of trapped charge carriers will be presented. In addition, a theoretical model describing the time evolution of the radiation-induced luminescence will be presented.

West, Michael S.↗
NASA NTRS1993

Infrared coronal emission lines and the possibility of their maser emission in Seyfert nuclei

Energetic emitting regions have traditionally been studied via x-ray, UV and optical emission lines of highly ionized intermediate mass elements. Such lines are often referred to as 'coronal lines' since the ions, when produced by collisional ionization, reach maximum abundance at electron temperatures of approx. 10(exp 5) - 10(exp 6) K typical of the sun's upper atmosphere. However, optical and UV coronal lines are also observed in a wide variety of Galactic and extragalactic sources including the Galactic interstellar medium, nova shells, supernova remnants, galaxies and QSOs. Infrared coronal lines are providing a new window for observation of energetic emitting regions in heavily dust obscured sources such as infrared bright merging galaxies and Seyfert nuclei and new opportunities for model constraints on physical conditions in these sources. Unlike their UV and optical counterparts, infrared coronal lines can be primary coolants of collisionally ionized plasmas with 10(exp 4) less than T(sub e)(K) less than 10(exp 6) which produce little or no optical or shorter wavelength coronal line emission. In addition, they provide a means to probe heavily dust obscured emitting regions which are often inaccessible to optical or UV line studies. In this poster, we provide results from new model calculations to support upcoming Infrared Space Observatory (ISO) and current ground-based observing programs involving infrared coronal emission lines in AGN. We present a complete list of infrared (lambda greater than 1 micron) lines due to transitions within the ground configurations 2s(2)2p(k) and 3s(2)3p(k) (k = 1 to 5) or the first excited configurations 2s2p and 3s3p of highly ionized (x greater than or equal to 100 eV) astrophysically abundant (n(X)/n(H) greater than or equal to 10(exp -6)) elements. Included are approximately 74 lines in ions of O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni spanning a wavelength range of approximately 1 - 280 microns. We present new results from detailed balance calculations, new critical densities for collisional de-excitation, intrinsic photon rates, branching ratios, and excitation temperatures for the majority of the compiled transitions. The temperature and density parameter space for dominant cooling via infrared coronal lines is presented, and the relationship of infrared to optical coronal lines is discussed.

Greenhouse, Matthew A.↗
NASA NTRS1986

The clumpy circumstellar medium around young supernova remnants

Each of the youngest supernova remnants known in the Milky Way, Cas A, Kepler's SNR, and Tycho's SNR, shows a different morphological structure caused by different conditions in the progenitor stars and their surroundings. In all three cases, however, the observed shells have a thickness of about 1/4 the radius, a sharp outer edge, and significant brightness irregularities. These features require that the circumstellar medium be highly clumped. To investigate the phenomenon, models of the expansion have been constructed using a one-dimensional spherical hydrodynamic code. As a supernova shock moves down the external density gradient of the star, material behind the shock begins to go into free expansion. Then as surrounding material is encountered a reverse shock moving back into the ejectum will be formed. Until the expansion has swept up about eight times the ejected mass when the situation can be considered as a point explosion in its surroundings, the dynamics are controlled by conditions between the shocks. The region is also where the synchrotron radio emission from relativistic electrons trapped in magnetic fields arises. Initial particles and fields are accelerated and amplified by eddy motion at the interface between the ejected and swept-up material and at the boundaries of clumps. Polarimetry shows that these SNR have a net radial orientation of their magnetic fields apparently from stretching by Rayleigh-Taylor instabilities at the contact surfaces. Without clumps the observed shell is much too narrow and steep on the inside.

Dickel, J. R.↗
NASA NTRS1977

On determining magnetospheric diffusion coefficients from the observed effects of Jupiter's satellite Io

A method is derived for determining the radial diffusion coefficient from observed satellite effects of the inner Jovian satellites on the energetic particle fluxes. The method is based on data from L values which are significantly removed from the actual sweeping region. With regard to the large losses to the protons at Io's L shell, it is suggested that in addition to satellite sweepup, the losses may be associated with an enhanced precipitation due to resonant interaction with ion cyclotron waves near Io's orbit. It is noted that such additional loss mechanisms may also apply to electrons, and that such losses may significantly affect the estimated diffusion coefficient.

Thomsen, M. F.↗
NASA NTRS1978

Consequences of narrow cyclotron emission from Hercules X-1

Explanations of the line feature in the hard X-ray (58 keV) spectrum of the compact X-ray source Hercules X-1 are presented. The most important characteristic of the emission (interpreted as a cyclotron emission from a strong magnetic field, i.e., B greater than one trillion Gauss) is that the FWHM is less than 12 keV, with a total observed luminosity in the line of approximately 1% of the total X-ray luminosity. If the source region is only at the magnetic poles of a neutron star, the electrons are confined to a path nearly parallel to the B-field lines, and the angle of emission will be narrow. Photon emission under such circumstances is also discussed; and it is suggested that photons can leave either through holes in the shell formed by the hard X-rays, or can escape through the shell itself. Attention is given to the accretion flow structure near the magnetopause, as a critical parameter of narrow line emission.

Weaver, R. P.↗
NASA NTRS1995

Nature and location of the source of plasma sheet boundary layer ion beams

Onsager et al. (1991) have put forward a model of the formation of the plasma sheet boundary layer (PSBL) which relies on a steady source of plasma from a spatially extended plasma sheet, together with steady equatorward and earthward ExB convection of field lines due to reconnection at a downtail neutral line. This model is a synthesis of earlier proposals and it explains such features as an electron layer exterior to the ion boundary layer, ion velocity dispersion, counter streaming beams, low-speed cutoffs in the beams. It also explains the apparent evolution of the ion beams through 'kidney bean' shaped velocity-space distributions toward quasi-isotropic shells without invoking pitch angle scattering or energy diffusion. In this paper we explore two ramifications of the model. In principle we can map, as a function of time, the downtail neutral line distance and establish whether or not it is retreating during substorm recovery. We can also reconstruct the plasma distribution function near the neutral line to see if it is most consistent with mantle or plasma sheet plasma. We perform this analysis using International Sun Earth Explorer (ISEE) Fast Plasma Experiment (FPE) data for two plasma sheet recovery events, one on March 1, 1978, and the other on April 18, 1978. On March 1, 1978, we find evidence for an initial retreat from around 110 to 160 R(sub E) in the first 15 min; little further retreat occurs thereafter. On April 18, 1978, the neutral line location ranges from as little as 40 R(sub E) tailward of the satellite to as much as 200 R(sub E), but there is no evidence for a systematic retreat. The reconstructed ion distributions for these events are most consistent with a plasma sheet origin for the March 1 case and possibly plasma mantle or low-latitude boundary layer for the April 18 case.

Elphic, R. C.↗
NASA NTRS2009

Dynamic Mechanical Properties and Fracture Surface Morphologies of Core-Shell Rubber (CSR) Toughened Epoxy at Liquid Nitrogen (Ln2) Temperatures

The dynamic mechanical properties and fracture surface morphologies were evaluated for a commercial epoxy resin toughened with two types of core-shell rubber (CSR) toughening agents (Kane Ace(Registered TradeMark) MX130 and MX960). The impact resistance (R) was evaluated by the resulting breaking energy measured in Charpy impact tests conducted on an instrumented drop tower. The resulting fracture surface morphologies were examined using Scanning Electron Microscopy (SEM). Fractographic observations of the CSR toughened epoxy tested at ambient temperature, showed a fracture as characterized by slender dendrite textures with large voids. The increasing number of dendrites and decreasing size of scale-like texture with more CSR particles corresponded with increased R. As the temperature decreased to Liquid Nitrogen (LN 2), the fracture surfaces showed a fracture characterized by a rough, torn texture containing many river markings and deep furrows.

Wang, J.↗
NASA NTRS1986

On the applicability of relativistic dispersion to auroral zone electron distributions

An examination is made to identify appropriate times to include weakly relativistic wave dispersion in calculations of auroral zone electron distributions. Two dispersion relations are examined. First, solutions are found for a ring distribution of particles around a cold ambient plasma. The solutions are found only for modes near the electron gyrofrequency. A trapped mode which is decoupled from the freely propagating R-X mode of cold plasmas is found to be the most unstable mode. Wave dispersion is examined with a spherically symmetric Dory-Guest-Harris (DGH) distribution, which describes a shell distribution that has a peak momentum larger than the thermal spread. The trapped mode is again found to be the most unstable. The analysis indicates it is beneficial to include relativistic effects whenever calculating auroral plasma distributions, and that a simple model will have sufficient accuracy.

Strangeway, R. J.↗
NASA NTRS1995

A microwave plasma cleaning apparatus

In a microwave electron cyclotron resonance plasma source, reactive plasmas of oxygen and its mixtures of argon have been used for evaluating plasma cleaning technologies. Small aluminum samples (0.95 x 1.9 cm) were coated with thin films (less than or equal to 20 micrometers in thickness) of Shell Vitrea oil and cleaned with reactive plasmas. The discharge parameters, such as gas pressure, magnetic field, substrate biasing, and microwave power, were varied to change cleaning conditions. A mass spectroscopy (or residual gas analyzer) was used to monitor the status of plasma cleaning. Mass loss of the samples after plasma cleaning was measured to estimate cleaning rates. Measured cleaning rates of low-pressure (0.5-m torr) argon/oxygen plasmas were as high as 2.7 micrometers/min. X-ray photoelectron spectroscopy was used to determine cleanliness of the sample surfaces. In this paper, significant results of the plasma cleaning are reported and discussed.

Tsai, C. C.↗
NASA NTRS2001

Physical and Electronic Isolation of Carbon Nanotube Conductors

Multi-walled nanotubes are proposed as a method to electrically and physically isolate nanoscale conductors from their surroundings. We use tight binding (TB) and density functional theory (DFT) to simulate the effects of an external electric field on multi-wall nanotubes. Two categories of multi-wall nanotube are investigated, those with metallic and semiconducting outer shells. In the metallic case, simulations show that the outer wall effectively screens the inner core from an applied electric field. This offers the ability to reduce crosstalk between nanotube conductors. A semiconducting outer shell is found not to perturb an electric field incident on the inner core, thereby providing physical isolation while allowing the tube to remain electrically coupled to its surroundings.

OKeeffe, James↗
NASA NTRS1976

AE 6: A model environment of trapped electrons for solar maximum

A projected inner zone electron model environment, AE 6, for the epoch 1980 is presented. It is intended to provide estimates of mission fluxes that spacecraft will encounter in the coming solar maximum years. AE 6 is presented by graphs of omnidirectional integral flux as a function of L shell, the ambient magnetic field B, and the energy E. Results of orbital integrations for altitudes from 150 n.m. to 18,000 n.m. are given for circular orbits with four different inclinations, using the AE 6 and the AE 4 solar maximum models for the inner and outer zones, respectively. The derivation of AE 6 is described, and a brief comparison is given of the radial profiles of equatorial fluxes from several related models. A short summary of the associated computer programs is included.

Teague, M. J.↗
NASA NTRS2020

Polyvinyl Alcohol and Medium Density Polyethylene Composite Materials for Neutron Radiation Shielding

Manned spacecraft venturing beyond the Earth’s Van Allen Belts will be subjected to high levels of radiation from both energetic particles emanating from the sun and from galactic atomic events, such as supernovae. The metal shell of a spacecraft offers some protection from gamma radiation and high energy particles originating from both the sun and cosmic sources. The interaction of these high energy particles and radiation with the spacecraft hull create secondary radiation and neutrons, additional hazards for astronauts and electronic systems. Shielding of neutrons is the most challenging of the high energy radiation, since neutrons have zero charge and are least affected by electrical fields, magnetic fields, and atomic forces. Previous studies showed that polyethylene is an effective neutron shielding material, due to its high hydrogen content. High energy neutrons are slowed to thermal neutrons through multiple collisions with the hydrogen atoms, increasing the effectiveness of neutron capture by boron-10 atoms. This study discusses the fabrication and testing of polyvinyl alcohol (PVOH) samples as a neutron shielding material, with comparison to medium density polyethylene (MDPE) composite samples to be used for neutron shielding for spacecraft. PVOH, an aqueous polymer, was chosen for hydrogen content and for enabling the use of organic reinforcement such as cellulose for secondary structural composites. OLTARIS (On-Line Tool for the Assessment of Radiation in Space) was used for radiation shielding simulation and compared with experimental data. Modeling indicates that the most effective shielding from radiation was in an aluminum layer backed by a polymer layer, either MDPE or PVOH. Cross-linked PVOH was also included in the OLTARIS modeling. All 3 polymer cases showed shielding improvement over aluminum alone, with similar dose equivalent reduction for galactic cosmic radiation(GCR)and for radiation from a solar particle event (SPE). Fourier Transform Infrared (FTIR) Spectroscopy was used to confirm the cross-linking in the PVOH sample and to determine the uniformity through the thickness. PVOH, cross-linked PVOH, and MDPE samples were tested for neutron shielding effectiveness using a 1 curie Americium-Beryllium neutron source. Test results indicated the most shielding in the cross-linked PVOH, followed by the pristine PVOH, and MDPE. Neutron testing suggests that polyvinyl alcohol is a good alternative to polyethylene for neutron shielding. The neutron shielding materials, PVOH and cross-linked PVOH developed under this research will enable the safe operation of spacecraft beyond the Earth’s protective Van Allen Radiation Belts. It will be crucial for NASA’s manned missions to the Moon or Mars to protect the health of astronauts and equipment from the harmful effects of excessive radiation.

neutron, shielding, radiation, polyvinyl alcohol, ↗
NASA NTRS1972

K-shell photoionization cross-sections.

Approximate values for the threshold energies, threshold cross sections, and energy dependence of the cross sections for K-shell photoionization are tabulated for H, He, C, N, O, Ne, Mg, Si, and S in all stages of ionization. The approximation of these data is based on the assumptions that the threshold energy is a simple function of the nuclear charge and the number of electrons present in the atom, and that the threshold values and energy dependence of the cross sections are determined only by the threshold energy.

Daltabuit, E.↗
NASA NTRS1982

IUE observations of proto-planetary and variable planetary nebulae. I - V1016 Cygni, HM Sagittae, and HBV 475. II - A search for variability in IC 4997 and NGC 6905

The IUE satellite has undertaken UV observations of the proto-planetary nebulae V1016 Cyg, HM Sge, and HBV 475, yielding emission line fluxes, line ratios, line profiles, electron densities, and distances from these objects. While levels of increasing excitation and ionization as a function of time are shown by the data for the first two nebulae, the trend for HBV 475 is found to lead in the opposite direction. The formation of a shell is suggested by dramatic changes in the HM Sge UV line profiles over the last four years, including the disappearance of W-R features and the incipient splitting of the semi-forbidden C III 1909 A line. An additional IUE search for UV variability in the planetary nebulae IC 4997 and NGC 6905 has yielded emission line fluxes, line ratios and profiles, and central star temperatures, as well as stratification effects data for several ions in NGC 6905

Feibelman, W. A.↗
NASA NTRS2016

Multilayer Active Shell Mirrors for Space Telescopes

A novel active mirror technology based on carbon fiber reinforced polymer (CFRP) substrates and replication techniques has been developed. Multiple additional layers are implemented into the design serving various functions. Nanolaminate metal films are used to provide a high quality reflective front surface. A backing layer of thin active material is implemented to provide the surface-parallel actuation scheme. Printed electronics are used to create a custom electrode pattern and flexible routing layer. Mirrors of this design are thin (< 1.0 mm), lightweight (2.7 kg/m2 ), and have large actuation capabilities. These capabilities, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Such mirrors could be used as lightweight primaries for small CubeSat-based telescopes or as meter-class segments for future large aperture observatories. Multiple mirrors can be produced under identical conditions enabling a substantial reduction in manufacturing cost and complexity. An overview of the mirror design and manufacturing processes is presented. Predictions on the actuation performance have been made through finite element simulations demonstrating correct abilities on the order of 250−300× for astigmatic modes with only 41 independent actuators. A description of the custom metrology system used to characterize the active mirrors is also presented. The system is based on a Reverse Hartmann test and can accommodate extremely large deviations in mirror figure (> 100 µm PV) down to sub-micron precision. The system has been validated against several traditional techniques including photogrammetry and interferometry. The mirror performance has been characterized using this system, as well as closed-loop figure correction experiments on 150 mm dia. prototypes. The mirrors have demonstrated post-correction figure accuracies of 200 nm RMS (two dead actuators limiting performance).

Reverse Hartmann↗