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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

The Investigation of the Effects of Gravity on Single Bubble Sonoluminescence

In single bubble following it's rapid collapse each cycle of oscillation of an ultrasonic field. Since widely varying length and time scales affect the bubble dynamics and optical emission processes, it is difficult to anticipate the importance of the effects of gravity present for observations on earth. Our bubble is driven in an acoustically resonating cavity at it's first harmonic mode. The acoustical radiation pressure (Bjerknes force) will then keep it suspended in the center near the pressure antinode. When driven in a region where the diffusive processes balance the bubble it acts in a nonlinear but regular way, emitting a short (approx. 200ps) burst of light each acoustic cycle. Balancing the Bjerknes force with buoyancy, as in, we can see that the bubble should be displaced from the velocity node approximately 20m at normal gravity. Therefore, water flows past the bubble at the time of collapse. Gravitation also changes the ambient pressure at the bubble's location, as Delta.P = rho.g.h this gives a change of approximately -0.5% in our experiment when going from 1.8g to 0g. Studies of ambient pressure changes were also done in order to assess these effects. Inside a pressure sealed chamber a spherical glass cell is filled with distilled water which has been degassed to 120mmHg. A bubble is then trapped in the center and driven by a piezoelectric transducer at 32.2kHz attached to the side of the cell. An optical system is then set up to take strobbed video images along and light emission data simultaneously. Temperature, pressure, drive voltage, and listener voltage are also monitored. PMT output in Volts The radii of the bubbles for both experiment s are fit using the Rayleigh-Plesset equation and the acoustic drive amplitude and the ambient bubble radius are found. There is little change in the acoustic drive amplitude as we expect, since we are not varying the drive voltage. However. the ambient bubble radius goes up considerably. These changes (increased light output, increased maximum bubble radius, and increased ambient bubble radius) are also observed when the ambient pressure is varied in the laboratory by an amount similar to that due to gravitation. The changes in the ambient bubble radius and light output with a change in ambient pressure are predicted by the "dissociation hypothesis" and have been observed by other groups in the laboratory. It seems clear that buoyancy's effect on light output and bubble radius, are at best on the same order as the effects of ambient pressure.

Dzikowicz, Ben↗
NASA NTRS2004

Magnectic Probing of Core Geodynamics

To better understand geomagnetic theory and observation, we can use spatial magnetic spectra for the main field and secular variation to test core dynamical hypotheses against seismology. The hypotheses lead to theoretical spectra which are fitted to observational spectra. Each fit yields an estimate of the radius of Earth s core and uncertainty. If this agrees with the seismologic value, then the hypotheses pass the test. A new way to obtain theoretical spectra extends the hydromagnetic scale analysis of Benton to scale-variant field and flow. For narrow scale flow and a dynamically weak field by the top of Earth s core, this yields a JGR-PI, and a secular variation spectrum modulated by a cubic polynomial in spherical harmonic degree n. The former passes the tests. The latter passes many tests, but does not describe rapid dipole decline and quadrupole rebound; some tests suggest it is a bit hard, or rich in narrow scale change.In a core geodynamo, motion of the fluid conductor does work against the Lorentz force. This converts kinetic into magnetic energy which, in turn, is lost to heat via Ohmic dissipation. In the analysis at length- scale l/k, if one presumes kinetic energy is converted in either eddy- overturning or magnetic free-decay time-scales, then Kolmogorov or other spectra in conflict with observational spectra can result. Instead, the rate work is done roughly balances the dissipation rate, which is consistent with small scale flow. The conversion time-scale depends on dynamical constraints. These are summarized by the magneto- geostrophic vertical vorticity balance by the top of the core, which includes anisotropic effects of rotation, the magnetic field, and the core-mantle boundary. The resulting theoretical spectra for the core- source field and its SV are far more compatible with observation. The conversion time-scale of order 120 years is pseudo-scale-invariant. Magnetic spectra of other planets may differ; however, if a transition to non-conducting fluid hydrogen in Jupiter acts as barrier to vertical flow, as well as current, then the shape of the jovi-magnetic spectrum could be remarkably Earth-like.

Voorhies, Coerte↗
DOE OSTI2023

Propellant Mass Gauging in a Spherical Tank under Micro-Gravity Conditions Using Capacitance Plate Arrays and Machine Learning

Propellant mass gauging under micro-gravity conditions is a challenging task due to the unpredictable position and shape of the fuel body inside the tank. Micro-gravity conditions are common for orbiting satellites and rockets that operate on limited fuel supplies. Capacitance sensors have been investigated for this task in recent years; however, the effect of various positions and shapes of the fuel body is not analyzed in detail. In this paper, we investigate this with various fill types, such as annular, core-annular, and stratified fills at different positions. We compare the performance among several curve-fitting-based approaches and a machine-learning-based approach, the latter of which offers superior performance in estimating the fuel content.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Overview of the Langmuir probe system on the Mega Ampere Spherical Tokamak (MAST) Upgrade

A detailed description of the Langmuir probe system on Mega Ampere Spherical Tokamak Upgrade is presented. The system features 850 tile-embedded probes and 40 bespoke electronic modules that each have the capability to drive and acquire data from up to 16 probes in a time-multiplexed manner. The system provides spatiotemporal-resolved measurements (1 cm and ~1 ms, respectively) in the divertor region of ion saturation current, electron temperature, and floating potential. The standard interpretation of current-voltage (IV) characteristics is to apply a four-parameter fit, based on unmagnetized probe theory, which includes a linear model for the ion saturation region. In conclusion, to mitigate the effect of the magnetic field, analysis is restricted to the region of the IV characteristic, which is sensitive to only the tail of the electron energy distribution function.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

No One Size Fits All: Semiconductor Nanocrystal Sizing Curves

Although colloidal quantum dots or nanocrystals (NCs) are now long past their infancy, there remain basic issues that must be addressed to fully establish their fundamental, size-dependent properties. This is especially true for model systems such as CdSe where confinement effects are responsible for changes in their optical response and where significant effort has been invested in developing theories of their underlying electronic structure. Surprisingly, there still appears to be a lack of consensus on one of the most fundamental metrics for NCs, their sizing curves. As shown here, these are empirical functions that link the size of isotropically shaped semiconductor NCs (such as the diameter, d, for spherical NCs and edge length, l, for cube-shaped ones) to corresponding changes in optical response.

36 MATERIALS SCIENCE↗
NASA NTRS1997

Multiwavelength Modeling of Nova Atmospheres

We have analyzed the early optically thick ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: 5,000 less than or equal to T(sub model) less than or equal to 60,000K, solar abundances, (rho)(alpha) r(sup -3), v(sub max) = 2000 km/s, L = 6 x 10(exp 4) solar luminosity, and a statistical or microturbulent velocity of 50 km/s. We used the synthetic spectra to estimate the model parameters corresponding to the observed IUE spectra. The fits to the observations were then iteratively improved by changing the parameters of the model atmospheres, in particular T(sub model) and the abundances, to arrive at the best fits to the optically thick pseudo-continuum and the features found in the IUE spectra. The IUE spectra show two different optically thick subphases. The earliest spectra, taken a few days after maximum optical light, show a pseudo-continuum created by overlapping absorption lines. The later observations, taken approximately 3 weeks after maximum light, show the simultaneous presence of allowed, semi-forbidden, and forbidden lines in the observed spectra. Analysis of these phases indicate that OS And 86 had solar metallicities except for Mg which showed evidence of being underabundant by as much as a factor of 10. We determine a distance of 5.1 kpc to OS And 86 and derive a peak bolometric luminosity of approximately 5 x 10(exp 4) solar luminosity. The computed nova parameters provide insights into the physics of the early outburst and explain the spectra seen by IUE. Lastly, we find evidence in the later observations for large non-LTE effects of Fe II which, when included, lead to much better agreement with the observations.

Hauschildt, P. H.↗
NASA NTRS2004

A Geomagnetic Estimate of Mean Paleointensity

To test a statistical hypothesis about Earth's magnetic field against paleomagnetism, the present field is used to estimate time averaged paleointensity. The estimate uses the modem magnetic multipole spectrum R(n), which gives the mean square induction represented by spherical harmonics of degree n averaged over the sphere of radius a = 6371.2 km. The hypothesis asserts that the low degree multipole powers of the core-source field are distributed as chi-squared with 2n+l degrees of freedom and expectation values {R(n)} = K[(n+l/2)/n(n+l](c/a)(sup 2n+4), where c is the 3480 km radius of Earth's core. (This is compatible with a usually mainly geocentric axial dipolar field). Amplitude K is estimated by fitting theoretical to observational spectra through degree 12. The resulting calibrated expectation spectrum is summed through degree 12 to estimate expected square intensity {F(sup 2)}. The sum also estimates {F(sup 2)} averaged over geologic time, in so far as the present magnetic spectrum is a fair sample of that generated in the past by core geodynamic processes.

Voorhies, Coerte↗
NASA NTRS1978

Spectral line shapes in spherically symmetric radially moving clouds

We present a method for the analysis of spectral line shapes arising in homogeneous, moving gas clouds in which velocity and molecular line excitation have power-law dependences on radial distance from the center. Analytical expressions are obtained for radial flows, for both optically thick and optically thin lines. The additional case of an optically thick line from a differentially rotating cloud is considered qualitatively. The method is applied to the interpretation of the C(12)O line observed in the direction of the Kleinmann-Low infrared nebula in Orion. While gravitational collapse and accelerated outflows would produce lines qualitatively similar to the observed profile, it does not appear to be possible to fit either model to the observations in detail.

Kuiper, T. B. H.↗
NASA NTRS1987

Substorm-associated lower hybrid waves in the plasma sheet observed by ISEE 1

Observations of the electric field at frequencies from 2-128 Hz, using the burst mode of the spherical double probe on ISEE 1, have been examined for a time period previously identified as containing the traversal of a near-earth neutral line past the satellite. Intense waves (3 to over 30 mV/m) at approximately half the lower hybrid frequency were observed throughout the plasma sheet from the neutral sheet to the boundary, but only during the period of the large dc electric field and E x B velocity associated with the substorm neutral line. The wave number deduced from linear fits of the data was comparable to the inverse electron gyroradius. These results are consistent with the lower-hybrid drift instability. Although peaks between the ion and electron plasma frequency were sometimes observed simultaneously, the integrated power below 100 Hz was usually at least one to two orders of magnitude greater than that above 100 Hz. Although theoretical work has suggested that the instability would be suppressed at the neutral sheet, the largest waves observed occurred right at the neutral sheet when the southward component of the magnetic field was 6 gamma. The observed waves could provide an anomalous resistivity of about (3-1000) x 10 to the -7th S (compared to the classical value of 1 x 10 to the -18th S).

Cattell, C. A.↗
NASA NTRS1995

GHRS observations of cool, low-gravity star. 2: Flow and turbulent velocities in the outer atmosphere of gamma CRU CIS (M3.4 III)

The Goddard High Resoulution Spectrograph (GHRS) on the Hubble Space Telescope (HST) has been used to obtain medium (R = 20,000) and high (R = 85,000) resoultion UV spectra of chromosphere emission features for the M3.4 III star gamma Cru. Small Science Aperture (SSA) G270M and Echelle-B spectra of selected regions in the 2300-2850 A range were obtained to determine the kinematics of the chromosphere using lines of C2), Fe2, Co2, Si1/2), Ni2, Mn2, and Mg2. Profiles of C2) (UV 0.01) lines and fluorescently excited lines of low optical depth indicate average turbulent velocities (Doppler FWHM) of 30.2 +/- 1.3 and 28.8 +/- 1.3 km/s, respectively. The fluorescent emission lines (mean RV = 21.3 +/- 0.9 km/s) and the wings of the emission components of Fe2 lines (mean RV = 22.8 +/- 0.4 km/s) are approximately at rest relative to the radial velocity of the star (21 km/s), while the C2) lines show a modest inflow (mean RV = 23.1 +/- 0.9 km/s). The more opaque lines of Fe2 and Mg2 exhibit complex profiles resulting from line formation in an optically thick, extended expanding atmosphere. The emission wings of these lines are broadened by multiple scattering, and they are centered near the photospheric radial velocity. Closer to line center, these strong lines show a strong blueshifted self-absorption feature (already seen in IUE data), indicative of formation in an expanding chromosphere, and a previously unseen dip in the profiles on the red side of line center. The absorption components, when extracted using simple Gaussian fits, show strong correlations with the relative optical depths of the lines. The derived absorption flow velocities converge to the photospheric velocity as one examines spectra features formed deeper in the atmosphere. The blueward abosrption velocity increases in magnitude from about 7 to 14 km/s with increasing line optical depth - the strong absorptions directly map the acceleration of the outflowing stellar wind, while the interpretation of the weaker redshifted abosrptions is more ambiguous, indicating either an inflow of material or formation in an extended, spherically expanding outflow. The Mg2 and Fe2 profiles, taken together, imply that the wind speed decreases between the atmospheric layers where the Mg2 and Fe2 self-absorption components are formed. Interstellar absorptions are seen in the resonance lines of Mg2 (UV 1) and Fe2 (UV1) with zero-volt lower levels, at about -3 km/s, consistent with models of the interstellar medium in the direction of gamma Cru. Finally, we have detected the Mg2 'satellite lines' seen in solar spectra obtained above the limb. In gamma Cru these lines are probably fluorescently excited by H Ly beta.

Carpenter, Kenneth G.↗
NASA NTRS2008

Photo-Double Ionization: Threshold Law and Low-Energy Behavior

The threshold law for photoejection of two electrons from atoms (PDI) is derived from a modification of the Coulomb-dipole (C-D) theory. The C-D theory applies to two-electron ejection from negative ions (photo-double detachment:PDD). The modification consists of correctly accounting for the fact that in PDI that the two escaping electrons see a Coulomb field, asymptotically no matter what their relative distances from the residual ion are. We find in the contralinear spherically symmetric model that the analytic threshold law Q(E),i. e. the yield of residual ions, to be Qf(E)approaches E + CwE(sup gamma(w)) + CE(sup 5/4)sin[1/2 ln(E + theta)]/ln(E). The first and third terms are beyond the Wannier law. Our threshold law can only be rigorously justified for residual energies less than or equal to 10(exp -3) eV. Nevertheless in the present experimental range (0.1 - 4 eV), the form, even without the second term, can be fitted to experimental results of PDI for He, Li, and Be, in contrast to the Wannier law which has a larger deviation from the data for Li and Be, for both of which the data show signs of modulation.

Bhatia, Anand↗
NASA NTRS2007

Photo-Double Ionization: Threshold Law and Low-Energy Behavior

The threshold law for photoejection of two electrons from atoms (PDI) is derived from a modification of the Coulomb-dipole (C-D) theory. The C-D theory applies to two-electron ejection from negative ions (photo-double detachment:PDD). The modification consists of correctly accounting for the fact that in PDI that the two escaping electrons see a Coulomb field, asymptotically no matter what their relative distances from the residual ion are. We find in the contralinear spherically symmetric model that the analytic threshold law Q(E), i.e. the yield of residual ions, to be Q Integral of (E) varies as E + (C(sub w) E(sup gamma W)) +CE(sup 5/4) sin [1/2 ln E + phi]/ln(E). The first and third terms are beyond the Wannier law. Our threshold law can only be rigorously justified for residual energies <= 10(exp -3) eV. Nevertheless in the present experimental range (0.1 - 4 eV), the form, even without the second term, can be fitted to experimental results of PDI for He, Li, and Be, in contrast to the Wannier law which has a larger deviation from the data for Li and Be.

Bhatia, A. K.↗
NASA NTRS2011

Improved Systematic Pointing Error Model for the DSN Antennas

New pointing models have been developed for large reflector antennas whose construction is founded on elevation over azimuth mount. At JPL, the new models were applied to the Deep Space Network (DSN) 34-meter antenna s subnet for corrections of their systematic pointing errors; it achieved significant improvement in performance at Ka-band (32-GHz) and X-band (8.4-GHz). The new models provide pointing improvements relative to the traditional models by a factor of two to three, which translate to approximately 3-dB performance improvement at Ka-band. For radio science experiments where blind pointing performance is critical, the new innovation provides a new enabling technology. The model extends the traditional physical models with higher-order mathematical terms, thereby increasing the resolution of the model for a better fit to the underlying systematic imperfections that are the cause of antenna pointing errors. The philosophy of the traditional model was that all mathematical terms in the model must be traced to a physical phenomenon causing antenna pointing errors. The traditional physical terms are: antenna axis tilts, gravitational flexure, azimuth collimation, azimuth encoder fixed offset, azimuth and elevation skew, elevation encoder fixed offset, residual refraction, azimuth encoder scale error, and antenna pointing de-rotation terms for beam waveguide (BWG) antennas. Besides the addition of spherical harmonics terms, the new models differ from the traditional ones in that the coefficients for the cross-elevation and elevation corrections are completely independent and may be different, while in the traditional model, some of the terms are identical. In addition, the new software allows for all-sky or mission-specific model development, and can utilize the previously used model as an a priori estimate for the development of the updated models.

Rochblatt, David J.↗
DOE OSTI2015

Pushing core-collapse supernovae to explosions in spherical symmetry. I. The model and the case of SN 1987A

We report on a method, PUSH, for artificially triggering core-collapse supernova explosions of massive stars in spherical symmetry. We explore basic explosion properties and calibrate PUSH to reproduce SN 1987A observables. Our simulations are based on the GR hydrodynamics code AGILE combined with the neutrino transport scheme isotropic diffusion source approximation for electron neutrinos and advanced spectral leakage for the heavy flavor neutrinos. To trigger explosions in the otherwise non-exploding simulations, the PUSH method increases the energy deposition in the gain region proportionally to the heavy flavor neutrino fluxes. We explore the progenitor range 18–21 M{sub ⊙}. Our studies reveal a distinction between high compactness (HC; compactness parameter ξ{sub 1.75}>0.45) and low compactness (LC; ξ{sub 1.75}<0.45) progenitor models, where LC models tend to explode earlier, with a lower explosion energy, and with a lower remnant mass. HC models are needed to obtain explosion energies around 1 Bethe, as observed for SN 1987A. However, all the models with sufficiently high explosion energy overproduce {sup 56}Ni and fallback is needed to reproduce the observed nucleosynthesis yields. {sup 57–58}Ni yields depend sensitively on the electron fraction and on the location of the mass cut with respect to the shell structure of the progenitor. We identify a progenitor and a suitable set of parameters that fit the explosion properties of SN 1987A assuming 0.1 M{sub ⊙} of fallback. We predict a neutron star with a gravitational mass of 1.50 M{sub ⊙}. We find correlations between explosion properties and the compactness of the progenitor model in the explored mass range. However, a more complete analysis will require exploring of a larger set of progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Seismic Moment Tensor Screening on the Hypersphere: Application to an Earthquake Population

A population of earthquakes is considered in the moment tensor screening frame- work. First, the members of the population are tested using the spherical distance statistic discussed in Ford and Kraft. All members are screened out, which is to say the explosion hypothesis is rejected at high confidence.Then, to gain insight to non-explosion populations, we estimate the distribution parameters for the earthquake population. Descriptive parameters of the population suggest it is ill-suited for description by a unimodal distribution on the hypersphere such as the von Mises Fisher distribution that described the explosion population. Nevertheless, we define an earthquake screening statistic based on the best-fit distribution parameters. No explosions are screened out, which is to say the earthquake hypothesis is not rejected for these events resulting in a poor screening performance.

58 GEOSCIENCES↗
DOE OSTI2018

Analysis of textural properties of CaO-based CO 2 sorbents by ex-situ USAXS

Here in this work, the micro-textural properties of CaO-based CO 2 sorbents prepared in a thermo-gravimetric reactor were investigated by ex situ synchrotron-based Small Angle X-ray Scattering (SAXS), Ultra Small Angle X-ray Scattering (USAXS) and Wide Angle X-ray Scattering (WAXS). Six sorbent samples were produced by calcination in vacuum at different temperatures and durations of the heating step; additionally, four partially carbonated samples were produced, after calcination in vacuum, at different CaO conversions. The scattering intensity profiles were analyzed by means of the Unified Fit model. Bimodal pore size distributions (micro-pores and meso-pores) were identified and the micro-textural properties (porosity, specific surface and pore radius of gyration) of each of the two pore populations were quantified both for the calcined and for the carbonated samples. The micro-textural properties of a few calcined samples were compared with N 2 -adsorption measurements. For each pore population, the trends of porosity, specific surface and pore radius of gyration, in addition to the pore number density and sphericity factor, were obtained versus time during the calcination and versus the CaO conversion during the carbonation. The relevance of micro-pores with respect to macro-pores was assessed both as far as the porosity impact on the CO 2 capture capacity and as far as the specific surface impact on the carbonation kinetics. For the calcined samples, the micro-textural properties of a partially calcined sample were compared with those of the completely calcined samples, and pore correlation distances were determined. Finally, for the carbonated samples, the specific surface per unit of particle volume results linearly dependent on the CaO conversion and, if expressed per unit of mass of unreacted CaO, does not change significantly for most of the CaO carbonation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Multi-frequency observations of a superbubble in the LMC: The case of LHA 120-N 70

We present a detailed study of new Australia Telescope Compact Array and XMM-Newton observations of LHA 120-N 70 (hereafter N 70), a spherically shaped object in the Large Magellanic Cloud, classified as a superbubble. Both archival and new observations were used to produce high quality radio continuum, X-ray, and optical images. The radio spectral index of N 70 is estimated to be α = –0.12 ± 0.06, indicating that while a supernova (SN) or supernovae have occurred in the region at some time in the distant past, N 70 is not the remnant of a single specific SN. N 70 exhibits limited polarization with a maximum fractional polarization of 9% in a small area of the northwest limb. We estimate the size of N 70 to have a diameter of 104 pc (±1 pc). The morphology of N 70 in X-rays closely follows that in radio and optical, with most X-ray emission confined within the bright shell seen at longer wavelengths. Purely thermal models adequately fit the soft X-ray spectrum which lacks harder emission (above 1 keV). We also examine the pressure output of N 70 where the values for the hot (P {sub X}) and warm (P{sub H} {sub II}) phases are consistent with other studied H II regions. However, the dust-processed radiation pressure (P {sub IR}) is significantly smaller than in any other object studied in Lopez et al. N 70 is a very complex region that is likely to have had multiple factors contributing to both the origin and evolution of the entire region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Robust inference of the Galactic Centre gamma-ray excess spatial properties

ABSTRACT The gamma-ray Fermi-LAT Galactic Centre excess (GCE) has puzzled scientists for over 15 yr. Despite ongoing debates about its properties, and especially its spatial distribution, its nature remains elusive. We scrutinize how the estimated spatial morphology of this excess depends on models for the Galactic diffuse emission, focusing particularly on the extent to which the Galactic plane and point sources are masked. Our main aim is to compare a spherically symmetric morphology – potentially arising from the annihilation of dark matter (DM) particles – with a boxy morphology – expected if faint unresolved sources in the Galactic bulge dominate the excess emission. Recent claims favouring a DM-motivated template for the GCE are shown to rely on a specific Galactic bulge template, which performs worse than other templates for the Galactic bulge. We find that a non-parametric model of the Galactic bulge derived from the VISTA Variables in the Via Lactea survey results in a significantly better fit for the GCE than DM-motivated templates. This result is independent of whether a galprop-based model or a more non-parametric ring-based model is used to describe the diffuse Galactic emission. This conclusion remains true even when additional freedom is added in the background models, allowing for non-parametric modulation of the model components and substantially improving the fit quality. When adopted, optimized background models provide robust results in terms of preference for a boxy bulge morphology for the GCE, regardless of the mask applied to the Galactic plane.

Astronomy & Astrophysics↗