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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
DOE OSTI2014

Solidification and fcc to metastable hcp phase transition in krypton under variable compression rates

In this work, we present time-resolved synchrotron x-ray diffraction measurements to study kinetics associated with the liquid-solid and solid-solid high-pressure phase transitions in Kr under dynamic loading in a dynamic-diamond anvil cell. The results show a strong compression-rate dependence of the solidification/melting process in liquid Kr. The analysis of the compression-rate dependent melting/solidification, using an Avrami equation with the parameter n=1, indicates a spontaneous nucleation and one-dimensional growth mechanism. In contrast, the face-centered-cubic to metastable hexagonal close-packed transition in solid Kr occurs rapidly at ~0.8 GPa near the melting line, which has negligible compression-rate dependence within the range of compression rates studied (0.004–13 GPa/s).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

SAGAbg. I. A Near-unity Mass-loading Factor in Low-mass Galaxies via Their Low-redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate

Measuring the relation between star formation and galactic winds is observationally difficult. In this work we make an indirect measurement of the mass-loading factor (the ratio between the mass outflow rate and star formation rate) in low-mass galaxies using a differential approach to modeling the low-redshift evolution of the star-forming main sequence and mass–metallicity relation. We use Satellites Around Galactic Analogs (SAGA) background galaxies, i.e., spectra observed by the SAGA Survey that are not associated with the main SAGA host galaxies, to construct a sample of 11,925 spectroscopically confirmed low-mass galaxies from 0.01 ≲ z ≤ 0.21 and measure auroral line metallicities for 120 galaxies. The crux of the method is to use the lowest-redshift galaxies as the boundary condition of our model, and to infer a mass-loading factor for the sample by comparing the expected evolution of the low-redshift reference sample in stellar mass, gas-phase metallicity, and star formation rate against the observed properties of the sample at higher redshift. We infer a mass-loading factor of η m = $0.92^{+1.76}_{-0.74}$, which is in line with direct measurements of the mass-loading factor from the literature despite the drastically different sets of assumptions needed for each approach. While our estimate of the mass-loading factor is in good agreement with recent galaxy simulations that focus on resolving the dynamics of the interstellar medium, it is smaller by over an order of magnitude than the mass-loading factor produced by many contemporary cosmological simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PHIBSS: MOLECULAR GAS, EXTINCTION, STAR FORMATION, AND KINEMATICS IN THE z = 1.5 STAR-FORMING GALAXY EGS13011166

We report matched resolution imaging spectroscopy of the CO 3-2 line (with the IRAM Plateau de Bure millimeter interferometer) and of the H{alpha} line (with LUCI at the Large Binocular Telescope) in the massive z = 1.53 main-sequence galaxy EGS 13011166, as part of the ''Plateau de Bure high-z, blue-sequence survey'' (PHIBSS: Tacconi et al.). We combine these data with Hubble Space Telescope V-I-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction, and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similar and are well modeled by a turbulent, globally Toomre unstable, rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/optical light distribution and peaks in an obscured, star-forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a ''mixed'' extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation, N = dlog{Sigma}{sub starform}/dlog{Sigma}{sub molgas}, depends strongly on the adopted extinction model, and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model, we find N = 1.14 {+-} 0.1.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Compact, ∼1 kpc Host Galaxy of a Quasar at a Redshift of 7.1

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C ii] fine-structure line and the underlying far-infrared (FIR) dust continuum emission in J1120+0641, the most distant quasar currently known (z=7.1). We also present observations targeting the CO(2–1), CO(7–6), and [C i] 369 μm lines in the same source obtained at the Very Large Array and Plateau de Bure Interferometer. We find a [C ii] line flux of F{sub [CII]}=1.11±0.10 Jy km s{sup −1} and a continuum flux density of S{sub 227GHz}=0.53±0.04 mJy beam{sup −1}, consistent with previous unresolved measurements. No other source is detected in continuum or [C ii] emission in the field covered by ALMA (∼ 25″). At the resolution of our ALMA observations (0.″23, or 1.2 kpc, a factor of ∼70 smaller beam area compared to previous measurements), we find that the majority of the emission is very compact: a high fraction (∼80%) of the total line and continuum flux is associated with a region 1–1.5 kpc in diameter. The remaining ∼20% of the emission is distributed over a larger area with radius ≲4 kpc. The [C ii] emission does not exhibit ordered motion on kiloparsec scales: applying the virial theorem yields an upper limit on the dynamical mass of the host galaxy of (4.3±0.9)×10{sup 10} M{sub ⊙}, only ∼20 × higher than the central black hole (BH). The other targeted lines (CO(2–1), CO(7–6), and [C i]) are not detected, but the limits of the line ratios with respect to the [C ii] emission imply that the heating in the quasar host is dominated by star formation, and not by the accreting BH. The star formation rate (SFR) implied by the FIR continuum is 105–340 M{sub ⊙} yr{sup −1}, with a resulting SFR surface density of ∼100–350 M{sub ⊙} yr{sup −1} kpc{sup −2}, well below the value for Eddington-accretion-limited star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2013

Reconstruction of the Mars Science Laboratory Parachute Performance and Comparison to the Descent Simulation

The Mars Science Laboratory used a single mortar-deployed disk-gap-band parachute of 21.35 m nominal diameter to assist in the landing of the Curiosity rover on the surface of Mars. The parachute system s performance on Mars has been reconstructed using data from the on-board inertial measurement unit, atmospheric models, and terrestrial measurements of the parachute system. In addition, the parachute performance results were compared against the end-to-end entry, descent, and landing (EDL) simulation created to design, develop, and operate the EDL system. Mortar performance was nominal. The time from mortar fire to suspension lines stretch (deployment) was 1.135 s, and the time from suspension lines stretch to first peak force (inflation) was 0.635 s. These times were slightly shorter than those used in the simulation. The reconstructed aerodynamic portion of the first peak force was 153.8 kN; the median value for this parameter from an 8,000-trial Monte Carlo simulation yielded a value of 175.4 kN - 14% higher than the reconstructed value. Aeroshell dynamics during the parachute phase of EDL were evaluated by examining the aeroshell rotation rate and rotational acceleration. The peak values of these parameters were 69.4 deg/s and 625 deg/sq s, respectively, which were well within the acceptable range. The EDL simulation was successful in predicting the aeroshell dynamics within reasonable bounds. The average total parachute force coefficient for Mach numbers below 0.6 was 0.624, which is close to the pre-flight model nominal drag coefficient of 0.615.

Cruz, Juan R.↗
NASA NTRS1999

Heart rate dynamics before spontaneous onset of ventricular fibrillation in patients with healed myocardial infarcts

The traditional methods of analyzing heart rate (HR) variability have failed to predict imminent ventricular fibrillation (VF). We sought to determine whether new methods of analyzing RR interval variability based on nonlinear dynamics and fractal analysis may help to detect subtle abnormalities in RR interval behavior before the onset of life-threatening arrhythmias. RR interval dynamics were analyzed from 24-hour Holter recordings of 15 patients who experienced VF during electrocardiographic recording. Thirty patients without spontaneous or inducible arrhythmia events served as a control group in this retrospective case control study. Conventional time- and frequency-domain measurements, the short-term fractal scaling exponent (alpha) obtained by detrended fluctuation analysis, and the slope (beta) of the power-law regression line (log power - log frequency, 10(-4)-10(-2) Hz) of RR interval dynamics were determined. The short-term correlation exponent alpha of RR intervals (0.64 +/- 0.19 vs 1.05 +/- 0.12; p <0.001) and the power-law slope beta (-1.63 +/- 0.28 vs -1.31 +/- 0.20, p <0.001) were lower in the patients before the onset of VF than in the control patients, but the SD and the low-frequency spectral components of RR intervals did not differ between the groups. The short-term scaling exponent performed better than any other measurement of HR variability in differentiating between the patients with VF and controls. Altered fractal correlation properties of HR behavior precede the spontaneous onset of VF. Dynamic analysis methods of analyzing RR intervals may help to identify abnormalities in HR behavior before VF.

Non-NASA Center↗
DOE OSTI2016

Indirect excitation of ultrafast demagnetization

Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. As a result, our data thus confirm recent theoretical predictions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Energy relaxation dynamics in a nodal-line semimetal

Here we study the temperature relaxation dynamics of nodal-line semimetals after a sudden excitation in the presence of acoustic and optical phonon modes. We find that the nodal line constrains the electron momenta in scattering processes, and as a result, the temperature relaxation due to acoustic phonons is exponential as a function of time. However, depending on initial conditions, other functional forms are possible. In typical pump-probe experiments, the temperature relaxation is linear due to acoustic phonons with rates that vary as ~n 1/2 with density. The temperature relaxation due to optical phonons is also linear with rates ~n -1/2 or ~n .

36 MATERIALS SCIENCE↗
NASA NTRS1994

Evolution of solitary density waves in stellar winds of early-type stars: A simple explanation of discrete absorption component behavior

We model the evolution of a density shell propagating through the stellar wind of an early-type star, in order to investigate the effects of such shells on UV P Cygni line profiles. Unlike previous treatments, we solve the mass, momentum, and energy conservation equations, using an explicit time-differencing scheme, and present a parametric study of the density, velocity, and temperature response. Under the assumed conditions, relatively large spatial scale, large-amplitude density shells propagate as stable waves through the supersonic portion of the wind. Their dynamical behavior appears to mimic propagating 'solitary waves,' and they are found to accelerate at the same rate as the underlying steady state stellar wind (i.e., the shell rides the wind). These hydrodynamically stable structures quantitatively reproduce the anomalous 'discrete absorption component' (DAC) behavior observed in the winds of luminous early-type stars, as illustrated by comparisons of model predictions to an extensive International Ultraviolet Explorer (IUE) time series of spectra of zeta Puppis (O4f). From these comparisons, we find no conclusive evidence indicative of DACs accelerating at a significantly slower rate than the underlying stellar wind, contrary to earlier reports. In addition, these density shells are found to be consistent within the constraints set by the IR observations. We conclude that the concept of propagating density shells should be seriously reconsidered as a possible explanation of the DAC phenomenon in early-type stars.

Waldron, Wayne L.↗
NASA NTRS1978

Gamma ray lines from novae

Activities identify the mechanisms responsible for the eruptions characterizing cataclysmic variables are concerned with close binary systems, the properties of which are dominated by the presence of mass transfer from a late-type star onto a white dwarf via an accretion disk. Theories for the outbursts vary in their identification of the eruption mechanism with either gravitational or nuclear energy sources. Existing models for the dwarf novae invoke a dynamical instability in the envelope of the red star which triggers episodic Roche lobe overflow and a concomitant increase in the rate of mass transfer and the outbursts are associated with an increase in the accretion luminosity liberated in the disk. The most successful theoretical models for common nova events involve thermonuclear runaways proceeding in accreted hydrogen envelopes on the white dwarf components of these systems. The characteristics of such runaway events, which can give rise to detectable gamma ray fluxes are discussed. Salient features of existing runaway models are reviewed. Gamma fluxes from Na-22 decay, positron annihilation, and shock induction are predicted.

Truran, J. W.↗
DOE OSTI2024

Upgrade of the Lyman-alpha diagnostic system on DIII-D for main chamber edge neutral studies

The LLAMA (Lyman Alpha Measurement Apparatus) pinhole camera diagnostic had previously been deployed on DIII-D to measure radial profiles of the Lyman-α (Ly-α) deuterium neutral line brightness across the plasma boundary in the lower chamber to infer neutral deuterium density and ionization rate profiles. This system has recently been upgraded with a new diagnostic head, named ALPACA, that also encloses two pinhole cameras and duplicates the LLAMA views in the upper chamber. Similar to LLAMA, ALPACA provides two times 20 lines of sight, viewing the plasma edge on the inboard and outboard sides with a radial resolution of ~2.5 cm (FWHM) and an effective time resolution of ~1 ms that allows for the investigation of inter-ELM dynamics. The extended Ly-α system provides better coverage to study neutrals in experiments with various plasma shapes utilizing both the upper and lower divertors. Furthermore, post-campaign calibration of the LLAMA diagnostic has successfully been demonstrated for the first time. This was facilitated by various upgrades to the calibration set-up and detailed measurements of the emissivity distribution of the Ly-α calibration source using a pinhole collimator. It was found that the sensitivity of the inboard LLAMA pinhole camera was reduced by a factor of 2.0 ± 0.2 over the course of six months of plasma operation in 2021. In conclusion, the upgraded Ly-α system, equipped with improved absolute calibration, will provide key input for neutral fueling and pedestal particle transport studies and for 2D edge transport code validation on the DIII-D tokamak.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

NON-EQUILIBRIUM IONIZATION EFFECTS ON THE DENSITY LINE RATIO DIAGNOSTICS OF O IV

The dynamic timescales in the solar atmosphere are shorter than the ionization and recombination times of many ions used for line ratio diagnostics of the transition region and corona. The long ionization and recombination times for these ions imply that they can be found far from their equilibrium temperatures, and spectroscopic investigations require more care before being trusted in giving correct information on local quantities, such as density and temperature. By solving the full time-dependent rate equations for an oxygen model atom in the three-dimensional numerical model of the solar atmosphere generated by the Bifrost code, we are able to construct synthetic intensity maps and study the emergent emission. We investigate the method of electron density diagnostics through line ratio analysis of the O IV 140.1 nm to the 140.4 nm ratio, the assumptions made in carrying out the diagnostics, and the different interpretations of the electron density. The results show big discrepancies between emission in statistical equilibrium and emission where non-equilibrium (NEQ) ionization is treated. Deduced electron densities are up to an order of magnitude higher when NEQ effects are accounted for. The inferred electron density is found to be a weighted mean average electron density along the line of sight and has no relation to the temperature of emission. This study shows that numerical modeling is essential for electron density diagnostics and is a valuable tool when the ions used for such studies are expected to be out of ionization equilibrium. Though this study has been performed on the O IV ion, similar results are also expected for other transition region ions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Dislocation Networks and the Microstructural Origin of Strain Hardening

In this paper, when metals plastically deform, the density of line defects called dislocations increases and the microstructure is continuously refined, leading to the strain hardening behavior. Using discrete dislocation dynamics simulations, we demonstrate the fundamental role of junction formation in connecting dislocation microstructure evolution and strain hardening in face-centered cubic (fcc) Cu. The dislocation network formed consists of line segments whose lengths closely follow an exponential distribution. This exponential distribution is a consequence of junction formation, which can be modeled as a one-dimensional Poisson process. According to the exponential distribution, two non-dimensional parameters control microstructure evolution, with the hardening rate dictated by the rate of stable junction formation. Among the types of junctions in fcc crystals, we find that glissile junctions make the dominant contribution to strain hardening.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

The time-resolved atomic, molecular and optical science instrument at the Linac Coherent Light Source

The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1975

What we have learned from the magnetosphere

Three significant discoveries resulting from recent studies of earth's magnetosphere are discussed: (1) the central role of magnetic-field-line reconnection in determining the topology, geometry, and dynamics of the magnetosphere; (2) the realization that auroral arcs result from coupling of solar-wind stresses to the ionosphere and neutral atmosphere; and (3) the role of plasma turbulence in affecting both the microscopic state and macroscopic observables of the magnetospheric plasma. Magnetic-field-line reconnection is described in terms of Dungey's (1961) model of the magnetosphere, a simple picture of magnetic-substorm evolution is outlined, and questions concerning the reconnection rate are considered. The production of aurorae by field-aligned electron beams is examined, and an analogy is made between earth's magnetosphere and that of a pulsar with aligned dipole and spin axes. Plasma-turbulence problems are reviewed which concern anomalously fast electron and proton losses from the Van Allen belts, whistler-electron interactions, and anomalous resistance in auroral arcs. The Pioneer 10 encounter with Jupiter's magnetosphere is briefly noted.

Kennel, C. F.↗
NASA NTRS1987

The role of ring current O(+) in the formation of stable auroral red arcs

Observations of thermal and energetic ion populations by the Dynamics Explorer 1 satellite in the magnetospheric energy source region as well as nearly simultaneous Langmuir probe measurements of enhanced electron temperatures by Dynamics Explorer 2 within the stable auroral red (SAR) arc at F region heights are used to examine the role of heavy ions in the formation of SAR arcs. It is found that sufficient energy is transferred to the electron gas at high altitudes via Coulomb collisions between the observed ring current ions and thermal electrons to support the enhanced F region electron temperatures measured on these field lines. The latitudinal variation in the electron heating rates calculated using observed ion populations is consistent with the observed variation in electron temperature across the SAR arc. In all cases, ring current O(+) is the major source of energy for the SAR arcs.

Kozyra, J. U.↗
NASA NTRS1992

Self-tuning control of attitude and momentum management for the Space Station

This paper presents a hybrid state-space self-tuning design methodology using dual-rate sampling for suboptimal digital adaptive control of attitude and momentum management for the Space Station. This new hybrid adaptive control scheme combines an on-line recursive estimation algorithm for indirectly identifying the parameters of a continuous-time system from the available fast-rate sampled data of the inputs and states and a controller synthesis algorithm for indirectly finding the slow-rate suboptimal digital controller from the designed optimal analog controller. The proposed method enables the development of digitally implementable control algorithms for the robust control of Space Station Freedom with unknown environmental disturbances and slowly time-varying dynamics.

Shieh, L. S.↗