Engineering topics

Summers, A.

Publications and source records attributed to Summers, A..

11 records

DOE OSTI2022

Few-femtosecond resolved imaging of laser-driven nanoplasma expansion

Abstract The free expansion of a planar plasma surface is a fundamental non-equilibrium process relevant for various fields but as-yet experimentally still difficult to capture. The significance of the associated spatiotemporal plasma motion ranges from astrophysics and controlled fusion to laser machining, surface high-harmonic generation, plasma mirrors, and laser-driven particle acceleration. Here, we show that x-ray coherent diffractive imaging can surpass existing approaches and enables the quantitative real-time analysis of the sudden free expansion of laser-heated nanoplasmas. For laser-ionized SiO 2 nanospheres, we resolve the formation of the emerging nearly self-similar plasma profile evolution and expose the so far inaccessible shell-wise expansion dynamics including the associated startup delay and rarefaction front velocity. Our results establish time-resolved diffractive imaging as an accurate quantitative diagnostic platform for tracing and characterizing plasma expansion and indicate the possibility to resolve various laser-driven processes including shock formation and wave-breaking phenomena with unprecedented resolution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1983

Models of the formation of the solar nebula

Protostellar cloud collapse and solar nebula formation models indicate that the size of the nebula produced will be larger in terms of both gas centrifugal balance R(CF) and collapse time diffusion length R(V). From this, it can be deduced that low mass nebulas are produced if (R(V)/R(CF))-squared is much greater than unity, while nebulas result for values lower than approximately unity. The total angular momentum value distinguishes most current models of the solar nebula. Analytic expressions for the surface density, nebular mass flux and photospheric temperature distributions during the formation stage are presented for simple modes illustrating and general properties of growing protostellar disks.

Cassen, P.↗

NASA NTRS1979

Convection and lunar thermal history

The effects of solid interior convection on the thermal history of the moon are examined. Convective models of lunar evolution are calculated to demonstrate the influence of various viscosities, radioactive heat source distributions and initial temperature profiles and tested by means of a thermal history simulation code. Results indicate that solid convection does not necessarily produce a quasi-steady thermal balance between heat sources and surface losses. The state of the lithosphere is found to be sensitive to the efficiency of heat source redistribution, while that of the convecting interior depends primarily on rheology. Interior viscosities of 10 to the 21st to 10 to the 22nd cm/sec are obtained, along with a central temperature above 1100 C. It is suggested that mare flooding could have been the result of magma production by pressure release melting in the upwelling region of convection cells.

Cassen, P.↗

NASA NTRS1976

Stratospheric aerosols and climatic change

Generated primarily by volcanic explosions, a layer of submicron silicate particles and particles made of concentrated sulfuric acids solution is present in the stratosphere. Flights through the stratosphere may be a future source of stratospheric aerosols, since the effluent from supersonic transports contains sulfurous gases (which will be converted to H2SO4) while the exhaust from Space Shuttles contains tiny aluminum oxide particles. Global heat balance calculations have shown that the stratospheric aerosols have made important contributions to some climatic changes. In the present paper, accurate radiative transfer calculations of the globally-averaged surface temperature (T) are carried out to estimate the sensitivity of the climate to changes in the number of stratospheric aerosols. The results obtained for a specified model atmosphere, including a vertical profile of the aerosols, indicate that the climate is unlikely to be affected by supersonic transports and Space Shuttles, during the next decades.

Baldwin, B.↗

NASA NTRS1976

Estimates of the climatic impact of aerosols produced by Space Shuttles, SST's, and other high flying aircraft

Solar and terrestrial radiative transfer calculations are performed to evaluate the effect of additional aerosols (sulfuric acid, aluminum oxide) produced by aircraft and Space Shuttles flying through the stratosphere on the global heat balance. The results are presented by plotting the dependence of various quantities of interest as a function of the change in the optical depth of the stratosphere at a reference wavelength of 0.55 micron. Perturbation optical depths that will result from the amount of emission expected from supersonic transports (SSTs) and Space Shuttles over the next several decades are determined. The magnitude and importance of the surface temperature change resulting from the added aerosols are assessed. The effect of added aerosols on ozone destruction is evaluated. It is shown that the aerosols produced by SSTs, other high flying aircraft, and Space Shuttles over the next several decades would not seriously alter the climate. However, the effect of SSTs is sufficiently close to the threshold limit, which requires reevaluation as new data are available.

Pollack, J. B.↗

NASA NTRS1976

Volcanic explosions and climatic change - A theoretical assessment

A theoretical assessment is presented of the influence of volcanic activity on the climate. The methodology of radiative transfer calculations is described and the sources of the adopted parameters are considered. The dependence of various quantities of interest is plotted as a function of the change in the optical depth of the stratosphere at a reference wavelength. An investigation is conducted concerning the increase in optical depth produced by volcanic explosions. An estimate is obtained regarding the magnitude of the mean surface temperature change resulting from the added aerosols.

Pollack, J. B.↗

NASA NTRS1975

Jupiter - Its infrared spectrum from 16 to 40 micrometers

Spectral measurements of the thermal radiation from Jupiter in the band from 16 to 40 micrometers are analyzed under the assumption that pressure-broadened molecular hydrogen transitions are responsible for the bulk of the infrared opacity over most of this spectral interval. Both the vertical pressure-temperature profile and the molecular hydrogen mixing ratio are determined. The derived value of the molecular hydrogen mixing ratio, 0.89 plus or minus 0.11, is consistent with the solar value of 0.86.

Houck, J. R.↗

NASA NTRS1975

Jupiter: Its infrared spectrum from 10 to 40 microns

Spectral measurements of the thermal radiation from Jupiter in the 16-40 micrometer band were analyzed under the assumption that pressure broadened H2 transitions are responsible for the bulk of the infrared opacity over most of this spectral interval. Both the vertical pressure-temperature profile and the hydrogen mixing ratio were determined. The derived value of the molecular hydrogen mixing ratio, 0.89 + or - 0.11, is consistent with the solar value, 0.86.

Houck, J. R.↗

NASA NTRS1973

Estimates of the size of the particles in the rings of Saturn and their cosmogonic implications

Observations are discussed which are consistent with the ring particles having a very high single scattering albedo at radio wavelengths, with multiple scattering effects being important. Comparison of scattering calculations for ice and silicate particles with the radio and radar observations imply a mean particle radius of about 1 cm. The ice bands observed in the near-infrared reflectivity spectra of the rings are formed by scattering within a microstructure on the surface of the ring particles, with the scattering centers being 25 to 125 micrometers in size. The Poynting-Robertson effect has caused a significant spiraling-in of the ring particles, probably resulting in a broadening of the rings. The inferred mean size is consistent with a model in which meteoroid impacts have caused a substantial reduction in the mean particle size from its initial value.

Pollack, J. B.↗

NASA NTRS1972

An evolutionary calculation of Jupiter.

A preliminary evolutionary calculation has been made for a stellar object of 0.001 solar mass composed of pure hydrogen. The star undergoes the gravitational contraction from an initial radius of 35 times the present radius of Jupiter (Rj). We assume the interior to be in convective equilibrium throughout the evolution. The evolution has been followed for 1 b.y. at which time R = 2.8 Rj and the central temperature is 18,000 K.

Grossman, A. S.↗

NASA NTRS1972

An evolutionary calculation of Jupiter

A preliminary evolutionary calculation was made for a stellar object of 0.001 solar mass composed of pure hydrogen. The star undergoes the gravitational contraction from an initial radius of 35 times the present radius of Jupiter (R sub Jup). The interior is assumed to be in convective equilibrium throughout the evolution. The evolution has been followed for 10 to the ninth power y at which time R = 2.8 R sub Jup and the central temperature is 18000 K. The log of the luminosity (in units of solar luminosity) and effective temperature due to the internal energy sources are log L/solar luminosity = -9.0 and log T sub e = 1.77.

Grossman, A. S.↗