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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

PoET: Polarimeters for Energetic Transients

This presentation focuses on PoET (Polarimeters for Energetic Transients): a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment - GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The PoET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. PoET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.

McConnell, Mark↗
NASA NTRS1972

Some Concepts in Reaction Dynamics

In 1929 London 1 published a very approximate solution of the Schroedinger equation for a system of chemical interest: H3. To the extent that chemistry can be regarded as existing separately from physics, this was a landmark in the history of chemistry, comparable in importance to the landmark in the history of physics marked by the appearance of the Heitler-London equation for H2. The expression for H3, was, of necessity, even less accurate than that for H2, but chemists, like the habitual poor, were accustomed to this sort of misfortune. Together with the physicists they enjoyed the sensation of living in a renaissance. The physicists still could not calculate a great deal that was of interest to them, and the chemists could calculate less, but both could now dream. It would be too easy to say that their dreams were dreams of unlimited computer time. Their dreams were a lot more productive than that. Two years after London published his equation, H. Eyring and M. Polanyi obtained the first numerical energy surface for H3. They infused the London equation with a measure of empiricism to produce an energy surface which, whether or not it was correct in its details, provided a basis for further speculations of an important sort. The existence of a tangible energy surface in 1931 stimulated speculation along two different lines. The following year Pelzer and Wigner used this London-Eyring-Polanyi (LEP) energy surface for a thermodynamic treatment of the reaction rate in H + H2. This important development reached its full flowering a few years later. In these remarks I shall be concerned with another line of development. A second more-or-less distinct category of speculation that began with (and, indeed, in) the 1931 paper has to do with the dynamics of individual reactive encounters under the influence of specified interaction potentials.

Polannyi, John C.↗
NASA NTRS2016

Comparison of Quantum Mechanical and Empirical Potential Energy Surfaces and Computed Rate Coefficients for N2 Dissociation

Comparisons are made between potential energy surfaces (PES) for N2 + N and N2 + N2 collisions and between rate coefficients for N2 dissociation that were computed using the quasiclassical trajectory method (QCT) on these PESs. For N2 + N we compare the Laganas empirical LEPS surface with one from NASA Ames Research Center based on ab initio quantum chemistry calculations. For N2 + N2 we compare two ab initio PESs (from NASA Ames and from the University of Minnesota). These use different methods for computing the ground state electronic energy for N4, but give similar results. Thermal N2 dissociation rate coefficients, for the 10,000K-30,000K temperature range, have been computed using each PES and the results are in excellent agreement. Quasi-stationary state (QSS) rate coefficients using both PESs have been computed at these temperatures using the Direct Molecular Simulation of Schwartentruber and coworkers. The QSS rate coefficients are up to a factor of 5 lower than the thermal ones and the thermal and QSS values bracket the results of shock-tube experiments. We conclude that the combination of ab initio quantum chemistry PESs and QCT calculations provides an attractive approach for the determination of accurate high-temperature rate coefficients for use in aerothermodynamics modeling.

quasiclassical trajectory method↗
DOE OSTI2025

Bayesian optimization of PYTHIA 8 tunes

A new tune (set of model parameters) is found for the six most important parameters of the PYTHIA 8 final state parton shower and hadronization model using Bayesian optimization. The tune fits the Large Electron-Positron collider (LEPI) data from ALEPH better than the default tune in PYTHIA 8. To the best of our knowledge, we present the most comprehensive application of Bayesian optimization to the tuning of a parton shower and hadronization model using the LEPI data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1971

Influence of thermal anisotropy on best-fit estimates of shock normals

The influence of thermal anisotropy on the estimates of interplanetary shock parameters and the associated normals is discussed. A practical theorem is presented for quantitatively correcting for anisotropic effects by weighting the before and after magnetic fields by the same anisotropy parameter h. The quantity h depends only on the thermal anisotropies before and after the shock and on the angles between the magnetic fields and the shock normal. The theorem can be applied to most slow shocks, but in those cases h usually should be lower, and sometimes markedly lower, than unity. For the extreme values of h, little change results in the shock parameters or in the shock normal.

Lepping, R. P.↗
NASA NTRS1972

Interplanetary shock normals

A scheme has been developed to improve the estimate of an interplanetary shock normal by using both magnetic field and plasma data from a single spacecraft. Calculation of the basic shock model employs a subset of the eight magnetohydrodynamic conservation relations for a shock in an isotropic medium. This subset consists of six equations that are devoid of pressure and temperature terms. A sigma-weighted least squares loss function technique is used to best fit the overdetermination equations with respect to the eleven parameters of the system. This procedure usually yields a normal much more accurate than the one obtained using magnetic field values alone. An example is given of the use of the technique on the data for the Pioneer 7 shock of 29 August 1966.

Lepping, R. P.↗
NASA NTRS1972

Reply to a paper on the use of two magnetometers for magnetic field measurements on a spacecraft

The use of two magnetometers for measuring the ambient magnetic field of spacecraft is compared with the single magnetometer variance method. It is shown that the dual magnetometer concept was developed for missions which are conducted with attitude stabilized spacecraft which have been constructed without an adequate magnetics control program. Specific methods for applying the dual magnetometer technique are described and an error analysis is conducted to determine the accuracy of the results.

Ness, N. F.↗
NASA NTRS1972

Influence of thermal anisotropy on best-fit estimates of shock normals.

This paper deals with the influence of thermal anisotropy on least-squares estimates of interplanetary shock parameters and the associated normals by using the Rankine-Hugoniot equations. A practical theorem is given for quantitatively correcting for anisotropic effects by weighting the before and after magnetic fields by the same 'anisotropy parameter' h. The quantity h depends only on the thermal anisotropies before and after the shock and on the angles between the magnetic fields and the shock normal. It is shown that, for fast shocks and for a liberal range of realistic conditions, the quantity h lies in the range from 0.90 to 1.22. The theorem can also be applied to most slow shocks, but in those instances h usually should be lower and sometimes markedly lower than unity.

Lepping, R. P.↗
NASA NTRS1972

A two-spacecraft test of a single spacecraft method of estimating shock normals

By assuming the validity of a subset of the Rankine-Hugoniot conservation relations for interplanetary (IP) shocks in an isotropic medium it has been demonstrated, in principle, that improved shock normals can be calculated by using a least-squares technique on combined magnetic field and plasma data from a single spacecraft. The scheme uses those six conservation relations not involving pressure and temperature. This paper deals with a test of the scheme by examining in detail a shock across which the magnetic field changed direction by a small amount (approximately 10 deg). On January 26, 1968 at about 1430 UT this shock was observed by the plasma and magnetic field instruments in Explorers 33 and 35. The spacecraft were 76.6 and 56.9 R sub E sunward of the earth, respectively (and 43.5 R sub E from each other), and therefore well outside the earth's bow shock region, a necessary condition for a valid test.

Lepping, R. P.↗
NASA NTRS1972

A shock surface geometry: The 15-16 February 1967 event

The flare-associated interplanetary shock observed by Explorer 33 and Pioneer 7 is analyzed to yield an estimation of the ecliptic plane geometry of the shock surface near 1 AU. These spacecraft were separated by 23 deg in the heliocentric longitude and Pioneer 7 was at a distance of 1.12 AU from the sun. Although a data gap occurred at the apparent time of passage of the disturbance at Pioneer 6, which was 85 deg in heliocentric longitude from Pioneer 7 and at 0.83 AU, the recovered data did suggest such a passage. A consistent picture of the shock propagation is given to explain the difference in arrival times at Pioneers 6, 7, and Explorer 33 and the difference of the shock normals observed by Pioneer 7 and Explorer 33. The average shock speed from the sun to each spacecraft and the local speed at Explorer 33 and their relations to the position of the initiating solar flare are obtained and discussed. In the region of space between the earth and Pioneer 7 the shock surface radius of curvature in the ecliptic plane was 0.4 AU or less.

Lepping, R. P.↗
NASA NTRS1973

A reverse shock associated with a stream-stream interaction: The 29th February 1968 event

A clearly defined perpendicular fast reverse shock was found in the interplanetary plasma and magnetic field data from Explorers 33 and 35. An observation by Pioneer 8, which is not unambiguously identifiable because of insufficient data, indicates the evolution of this event. It is suggested that this reverse shock was formed from a stream-stream interaction and that an associated foward shock was developing and probably formed beyond the earth's orbit.

Chao, J. K.↗