Engineering topics

Rose, M. Franklin

Publications and source records attributed to Rose, M. Franklin.

194 records · Page 11

NASA NTRS2000

Scattering/Transmission of Energetic H(sup +), He(sup +), and O(sup +) Through a Thin Composite Si/Lexan/C Foil

The need for a thin foil to serve both as an extreme ultraviolet (EUV) radiation attenuator and as a secondary electron generator for time-of-flight (TOF) instruments and energetic neutral atom (ENA) imagers has led us to test the scattering and transmission characteristics of a layered foil of nominal aerial thicknesses of 5 microns/sq cm Si, 4 microns/sq.cm Lexan ((C16)H(14)O3), and 1 micron/sq.cm C. We report the scattering and transmission results of subjecting this foil to H(sup +), He(sup +), and O(sup +) beams of energy less than 120 keV. The scattering of these particles, defined by the half-angular-width at half-maximum (HWHM), decreases with increasing energy and is found to be 10 deg for 10 keV H(sup +), 22 keV for He(sup +), and 100 keV for O(sup +). The 50% transmission threshold for these particles was found to be 3.9 keV for H(sup +), 8.6 keV for He(sup +), and 49.5 keV for O(sup +). Additionally, ion transmission shows evidence of resonance scattering from a rectangular potential independent of incident ion mass. Particularly evident is a local transmission minimum at 70 keV possessing a Gaussian-width of 20 keV. For incident energies below 50 keV, computer simulations of H+-foil interactions using SRIM96 agree well with the H(sup +) data, while yielding less scattering and higher transmission than indicated by the O(sup +) data.

Adrian, M. L.↗

NASA NTRS2000

Sunyaev - Zel'dovich Effect Imaging of Massive Clusters of Galaxies at Redshift > 0.8

We present Sunyaev-Zel'dovich Effect (SZE) imaging observations of three distant (z greater than 0.8) and highly X-ray luminous clusters of galaxies, C1J1226+33, C1J0152-13 and MS1054-03. Two of the clusters, C1J1226+33 and C1J0152-13, were recently discovered in the WARPS X-ray survey. Their high X-ray luminosity suggests they are massive systems and, if confirmed, would provide strong constraints on the cosmological parameters of structure formation models. Our Sunyaev-Zel'dovich Effect data provide confirmation that they are massive clusters similar to the well studied cluster MS1054-03. Assuming the clusters have the same gas mass fraction derived from SZE measurements of eighteen known massive clusters, we are able to infer their mass and electron temperatures from the SZE data. The derived electron temperatures are 10.0, 8.5, and 10.3 KeV, respectively, and we infer total masses of approximately 2 x 10(circumflex) 14 h(circumflex) -l M - sun within a radius of 65" for all three clusters. For C1J0152-13 and MS1054-03 we find good agreement between our SZE derived temperatures and those derived from X-ray spectroscopic measurements. No X-ray derived temperatures are available for C1J1226+33, and thus the SZE data provide the first confirmation that it is indeed a massive system. The demonstrated ability to determine cluster temperatures and masses from SZE observations without access to X-ray data illustrates the power of using deep SZE surveys to probe the distant universe.

Joy, Marshall↗

NASA NTRS2000

The Immediate Auroral Response to Interplanetary Shock Event

The auroral response to the dynamic solar wind is generally thought of as having a time lag of several minutes to hours. However, with the simultaneous and continuous availability of global images and solar wind data for several hours has enabled the direct and immediate response of the aurora to be observed. Localized as well as large scale phenomena have been observed nearly simultaneously with sudden variances in the solar wind density and velocity. A review of the historical and recent progress in this area of research will be presented.

Spann, James F.↗

NASA NTRS2000

Behavior of the Aurora During 10-12 May, 1999 When The Solar Wind Nearly Disappeared

The aurora was still active with occasional pseudobreakup events when the solar wind density diminished to unusually small densities (0.2 cc -1) during May 10-12, 1999. The aurora was observed at high magnetic latitudes indicating that the electron precipitation source moved northward as the geomagnetic activity decreased. The events we have studied indicate that the solar wind density alone is not the primary parameter that controls the auroral activity. The weak auroral activity was observed with 150 nT magnetic bays and when the interplanetary magnetic field (IMF) Bz was small and positive resulting in small _ parameter. A new auroral feature was observed on May 11, 1999, between 0900-2000 UT. The electron precipitation was energetic, uniform, and covered the region commonly identified as the polar cap. This precipitation lasted for more than 10 hours and was stable over time scales of tens of minutes. On May 12, as the solar wind began to recover, a prolonged period of dayside activity occurred and was followed by a typical aurora at 0500 UT.

Parks, G.↗

NASA NTRS2000

Electron and Proton Auroral Dynamics

Data from the Wide-band Imaging Camera (WIC) sensitive to far ultraviolet auroras and from the Spectrographic Imager (SI) channel SI12, sensitive to proton precipitation induced Lyman alpha were analyzed during a high altitude orbit segment of the IMAGE spacecraft. This segment began during the expansive phase of a substorm. The aurora changed into a double oval configuration, consisting of a set of discrete pole-ward forms and a separate diffuse auroral oval equatorwards, Although IMF Bz was strongly southward considerable activity could be seen poleward of the discrete auroras in the region that was considered to be the polar cap. The SI12 Doppler shifted Lyman alpha signature of precipitating protons show that the proton aurora is on the equatorward side of the diffuse aurora. In the following several hours the IMF Bz field changed signed. Although the general character of the proton and electron aurora did not change, the dayside aurora moved equatorward when the Bz was negative and more bright aurora was seen in the central polar cap during periods of positive Bz.

Mende, S. B.↗

NASA NTRS2000

Estimating Cosmic Ray Spectral Parameters From Simulated Detector Responses With Detector Design Implications

A simple power law model consisting of a single spectral index alpha (sub 1), is believed to be an adequate description of the galactic cosmic ray (GCR) proton flux at energies below 10(exp 13) eV, with a transition at knee energy E(sub k) to a steeper spectral index alpha(sub 2) greater than alpha(sub 1) above E(sub k). The maximum likelihood procedure is developed for estimating these three spectral parameters of the broken power law energy spectrum from simulated detector responses. These estimates and their surrounding statistical uncertainty are being used to derive the requirements in energy resolution, calorimeter size, and energy response of a proposed sampling calorimeter for the Advanced Cosmic ray Composition Experiment for the Space Station (ACCESS). This study thereby permits instrument developers to make important trade studies in design parameters as a function of the science objectives, which is particularly important for space-based detectors where physical parameters, such as dimension and weight, impose rigorous practical limits to the design envelope.

Howell, L. W.↗

NASA NTRS2000

Correlations and Gravitational Temperature in Settling Suspensions

Particle Image Velocimetry (PIV) is used to study the slow settling motions of spheres in suspensions ranging from dilute to highly concentrated, 0.05 less than phi less than 0.50. During sedimentation, velocity fluctuations are found to be organized into regions of characteristic size L approximately equal to 11 a phi(exp -1/3) and buoyant mass DELTA m given by the rms density fluctuations in a region of size L(exp 3) assuming random statistics. A simple model incorporating the suspension viscosity and excluded volume effects accurately predicts the magnitudes of the observed velocity fluctuations DELTA V. These lead to a universal relation for particle diffusion that can be written in a Stokes-Einstein type form as D approximately equal to (DELTA m gL)/(6 pi (eta)L).

Segre, Philip N.↗

NASA NTRS2000

Glass/Jamming Transition in Colloidal Aggregation

We have studied colloidal aggregation in a model colloid plus polymer system with short-range attractive interactions. By varying the colloid concentration and the strength of the attraction, we explored regions where the equilibrium phase is expected to consist of colloidal crystallites in coexistance with colloidal gas (i.e. monomers). This occurs for moderate values of the potential depth, U approximately equal to 2-5 kT. Crystallization was not always observed. Rather, over an extended sub-region two new metastable phases appear, one fluid-like and one solid-like. These were examined in detail with light scattering and microscopy techniques. Both phases consist of a near uniform distribution of small irregular shaped clusters of colloidal particles. The dynamical and structural characteristics of the ergodic-nonergodic transition between the two phases share much in common with the colloidal hard sphere glass transition.

Segre, Philip N.↗

NASA NTRS2000

High Undercooling of Ni59Nb41 Alloy in a Containerless Electrostatic Levitation Facility

Utilizing the containerless electrostatic levitation facility at NASA/MSFC, we were able to undercool the Ni59Nb41 (atomic) alloy by 210 K which was 160 degrees farther than the results of previous flight experiments. Undercoolings were clustered around 200 K during the repeated melting-freezing cycles on a single sample. Prior to this work, a metastable liquid separation had been presumed to limit the undercooling of this alloy. However, microstructural observations have revealed that undercooling was limited by crystal nucleation.

Robinson, M. B.↗

NASA NTRS2000

Detached and Floating-Zone Growth of Semiconductor Crystals on the ISS

Understanding the mechanism of detached Bridgman growth and establishing the growth of large scale germanium-silicon crystals by the float-zone technique are the key points of the project "RDGS - Reduction of Defects in Germanium-Silicon". The contact angle of the melt and the growth angle of the crystal are essential parameters which allow a controlled use of detached growth. The contact angle was determined for a variety of different substrates and melt compositions; pBN showed the highest value for pure germanium as well as for germanium-rich GeSi melts. The growth angle of Ge(sub 0.95) Si(sub 0.05) was measured to be 8.5-10.5 degrees which concurs with the values of pure germanium and silicon, respectively. The temperature dependence and the concentration dependence of the surface tension were determined for concentrations up to 10at% silicon (partial derivative (gamma)/partial derivative T=-0.08 (raised dot) 10(exp -3)N/m (raised dot) K, partial derivative (gamma)/partial derivative (C)=2.2 (raised dot) 10(exp -3)N/m (raised dot) at%). Using these values, the critical Marangoni number indicating the transition to time-dependent thermocapillary flow will be exceeded for the growth of large scale float-zone crystals onboard the ISS. Therefore, suitable tools for flow control are required.

Dold, P.↗

NASA NTRS2000

Effect of Elevated Pressure on the Heat Transfer and Power Requirements During Bridgman Growth of PMN-PT Crystals

Performance of the furnace during Bridgman growth of the lead magnesium niobate-lead titanate crystal (PMN-PT) is analyzed. PMN-PT is electrostrictive ceramic that has near ideal strain-voltage function. Furthermore piezoelectric (2000 to 2300 pC/N) and coupling (92 to 95%) constants are exceptionally good. Due to these properties PMN-PT has wide range of applications - from sonars to transducers in a high precision optical systems. In this research first attempt to crystallize PMN-PT in a Mellen type vertical Bridgman furnace was not successful, as melting temperature of precursor materials was not achieved. At this point choice was between building a new more powerful facility or finding ways to enhance performance of the existing furnace. Besides adjusting power supply to the individual heating elements, redesigning ampoule holding cartridge and improving furnace insulation one more radical improvement was proposed. The entire furnace was placed into the high pressure chamber. Further experiments confirmed that temperature inside the furnace was increased sufficiently to melt precursor materials to obtain PMN-PT. Numerical modeling is undertaken to find limitations of this technique and to predict temperature distribution inside the ampoule. It is of interest also to account for main factors contributing to a higher temperatures achieved in the furnace under the higher pressure (up to 10 atm.). Numerical model of the furnace is based on general purpose finite - element code FIDAP and on previous efforts to model Bridgman type furnace with multiply heaters. In order to account for all heat transfer mechanism involved - conduction, convection and radiation - different parts of the furnace are modeled in accordance with expected dominant mode of heat transfer - conduction in the solid parts, conduction and radiation in the ampoule, gas convection and conduction in the furnace openings complemented with wall-to-wall radiation. Because of these complicating factors, dimensional rather than non-dimensional modeling is performed using steady-state 2-D and 3-D models. Particular attention is paid to the modeling of radiation in a semitransparent material of ampoule 7 sapphire. The radiation model is validated by solving realistic test problem - conduction and radiation heat transfer in the fused quartz. Results are in agreement with both experimental and analytical data.

Bune, Andris↗

NASA NTRS2000

Microgravity and Macromolecular Crystallography

Macromolecular crystal growth has been seen as an ideal experiment to make use of the reduced acceleration environment provided by an orbiting spacecraft. The experiments are small, simply operated and have a high potential scientific and economic impact. In this review we examine the theoretical reasons why microgravity should be a beneficial environment for crystal growth and survey the history of experiments on the Space Shuttle Orbiter, on unmanned spacecraft, and on the Mir space station. Finally we outline the direction for optimizing the future use of orbiting platforms.

Kundrot, Craig E.↗

NASA NTRS2000

Radial Flows in Supergranules

We determine the radial component of the supergranular flow velocity by examining the center-to-limb variation of the Doppler velocity signal. We acquire individual Doppler images obtained with the MDI instrument on the SOHO spacecraft and process them to remove the p-mode oscillation signal, the axisymmetric flows, the convective blueshift signal, and instrumental artifacts. The remaining Doppler signal contains only cellular flow structures. The Doppler signal from the horizontal flows in these cells varies like sin p, where p is the heliocentric angle from disk center. The Doppler signal from radial flows varies like cos p. We fit the center-to-limb variation of the mean squared velocity signal to a straight line in sin(exp 2) rho over the central portion of the disk. The intercept of this line at disk center gives the amplitude of the radial component of the flow. The slope of the line gives the amplitude of the horizontal component. We find that the radial flows for typical supergranules have speeds about 10% that of their associated horizontal flows or about 30 m/s. The ratio of the radial to horizontal flow speed increases from 9% to about 18% as the size of the cells decreases from > 60 Mm to approximately 5 Mm. We use data simulations to check these results and find a ratio that increases from 5% to only about 12% over the same range of cell sizes. These smaller ratios are attributed to an underestimation of the horizontal flow speeds due to the fact that, the transverse component of the horizontal flow is not detected by the Doppler measurements.

Hathaway, D. H.↗

NASA NTRS1999

Discovery of X-Ray Emission from the Crab Pulsar at Pulse Minimum

The Chandra X-ray Observatory observed the Crab Nebula and Pulsar using the Low-Energy Transmission Grating (LETG) with the High-Resolution Camera (HRC). Time-resolved zeroth-order images reveal that the pulsar emits x rays at all pulse phases. Analysis of the flux at minimum -- most likely nonthermal in origin -- places an upper limit (T(sub infinity) < 2.1 MK) on the surface temperature of the underlying neutron star. In addition, analysis of the pulse profile appears to confirm the absolute timing of the Observatory to within about 0.2 ms.

Tennant, Allyn F.↗