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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

MMS Observations of Electron-Scale Filamentary Currents in the Reconnection Exhaust and Near the X Line

We report Magnetospheric Multiscale observations of macroscopic and electron-scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was approximately 10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron-scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super-Alfvenic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion-scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line.

Phan, T. D.↗
NASA NTRS1992

Numerical simulation of conservation laws

A new numerical framework for solving conservation laws is being developed. This new approach differs substantially from the well established methods, i.e., finite difference, finite volume, finite element and spectral methods, in both concept and methodology. The key features of the current scheme include: (1) direct discretization of the integral forms of conservation laws, (2) treating space and time on the same footing, (3) flux conservation in space and time, and (4) unified treatment of the convection and diffusion fluxes. The model equation considered in the initial study is the standard one dimensional unsteady constant-coefficient convection-diffusion equation. In a stability study, it is shown that the principal and spurious amplification factors of the current scheme, respectively, are structurally similar to those of the leapfrog/DuFort-Frankel scheme. As a result, the current scheme has no numerical diffusion in the special case of pure convection and is unconditionally stable in the special case of pure diffusion. Assuming smooth initial data, it will be shown theoretically and numerically that, by using an easily determined optimal time step, the accuracy of the current scheme may reach a level which is several orders of magnitude higher than that of the MacCormack scheme, with virtually identical operation count.

Chang, Sin-Chung↗
NASA NTRS1987

Shallow n(+) diffusion into InP by an open-tube diffusion technique

Very shallow n(+) layers have been obtained in InP by using gallium sulfide as a source for sulfur diffusion, and chemically vapor-deposited SiO2 as a cap. Diffusions were carried out from 585 to 725 C in an open-tube system with a nitrogen ambient. The doping profile of sulfur in InP is estimated to be of the complementary error function type with a surface concentration of 5.6 x 10 to the 18th/cc and a diffusion constant of 1.1 x 10 to the -14th sq cm/s at 670 C. Diodes made on n(+)-p junctions obtained by this diffusion technique show ideality factors close to unity and saturation current densities as low as 3.4 x 10 to the -15th A/sq cm, signifying the presence of a defect-free junction. These diffusions, with junction depths in the 400-700 A range, are ideal for solar cell applications.

Ghandhi, Sorab K.↗
NASA NTRS2002

Effects of Lateral Plasma Density and Temperature Diffusion on VCSEL Performance

The hydrodynamic model is further verified by applying to a gain-guided single mode VCSEL. DC effects of D(sub NN): (1) increase threshold current J(sub th) and decrease slope efficiency; (2) within the studied range (50% pumping within threshold and realistic diffusion coefficient for a single mode), the L-I relation scales with the relative Injection current (J/J(sub th) - 1). AC effects of D(sub NN): (1) decrease spectral bandwidth and responsivity of direct-current modulation; (2) within the studied range, the frequency response follows the same formal dependence as predicted without diffusion and under a linear gain model, while the resonant frequency position similarly scales with the relative injection current; (3) therefore, it is concluded that the AC effects of D(sub NN) is purely of static nature and reflected via its influence on ot and J(sub th). Within this study, the nonlinear effects of D(sub NN) are mostly reproducible with an equivalent constant diffusion coefficient.

Li, Jian-Zhong↗
NASA NTRS1974

Methods of improving the efficiency of photovoltaic cells

Work on aluminum-alloyed silicon grating cells is continued. Optimization of the geometry (grating line width and spacing) confirms the analysis of such cells. A 1 sq cm grating cell was fabricated and its i-V characteristic was measured under an AMO solar simulator. It is found that the efficiency of this cell would be about 7.9%, if it were covered by the usual antireflection coating. The surface of the cell is not covered by a diffused junction. The response is blue shifted; the current is somewhat higher than that produced by a commercial Si cell. However, the open circuit voltage is low, and attempts to optimize the open circuit voltage of the aluminum-alloy junctions are described. A preliminary X-ray topographic examination of GaAs specimens of the type commonly used to make solar cells is studied. The X-ray study shows that the wafers are filled with regions having strain gradients, possibly caused by precipitates. It is possible that a correlation exists between the presence of low mechanical perfection and minority carrier diffusion lengths of GaAs crystals.

Loferski, J. J.↗
NASA NTRS1986

Gravity waves in the middle atmosphere: Recent progress and needed studies

The recent recognition of the important role played by gravity waves in the large-scale circulation and thermal structure of the mesosphere and lower thermosphere has stimulated considerable research on their properties and their middle atmosphere effects. For example, these studies have begun to provide important information on gravity wave scales, propagation, filtering, and the processes responsible for saturation and turbulent diffusion. There remain, however, many areas in which our current understanding of middle atmosphere gravity waves is deficient. The purpose here is to review the progress that has been made to date and to suggest areas in which additional studies are most needed. Gravity wave scales, gravity wave saturation mechanisms, turbulence production and turbulent diffusion, and distribution of gravity wave energies and momentum fluxes with height and time are discussed.

Fritts, D. C.↗
NASA NTRS2004

Refining the W1 and SE1 Facilities

The Engine Research Building (ERB) houses more than 60 test rigs that study all aspects of engine development. By working with Mary Gibson in the SE1 and W1A Turbine Facilities, I became aware of her responsibilities and better acquainted with the inner workings of the ERB. The SE1 Supersonic/Subsonic Wind Tunnel Facility contains 2 small wind tunnels. The first tunnel uses an atmospheric inlet, while the second uses treated 40-psig air. Both of the tunnels are capable of subsonic and supersonic operation. An auxiliary air supply and exhaust piping providing both test sections with suction, blowing, and crossfire capabilities. The current configuration of SE1 consists of a curved diffuser that studies the blockage along the endwalls. The W1A Low Speed Compressor Facility provides insight for the complex flow phenomena within its 4-stage axial compressor, sand the data obtained from W 1A is used to develop advanced models for fluid dynamic assessment. W1A is based off of a low speed research compressor developed by GE in the 1950's. This compressor has a removable casing treatment under rotor 1, which allows for various tip treatment studies. The increased size and low speed allows instrumentation to be located in the compressor s complex flow paths. Air enters the facility through a filtered roof vent, conditioned for temperature and turbulence, and then passed through the compressor W1A is described as a dynamic facility with many projects taking place simultaneously. This current environment makes it challenging to follow the various affairs that are taking place within the area. During my first 4 weeks at the NASA Glenn Research Center, I have assisted Mary Gibson in multiple tasks such as facility documents, record keeping, maintenance and upgrades. The facility has lube systems for its gearbox and compressor. These systems are critical in the successful operation of the facility. I was assigned the task of creating a facility estimate list, which included the filters and strainers required for the compressor. For my remaining time spent here, we expect to complete a facility parts listing and a virtual project summary so that W1A and SE1 will become ergonomic facilities that will make it easier for people to observe the capabilities and history of the area and the employees that operate. Bolstering our efforts in achieving these goal are the online technical tutorials, software such as Microsoft Excel. Macromedia Flash MX Macromedia Dreamweaver MX, Photoshop 6.0 and the assistance of several NASA employees.

Chambers, Rodney D.↗
NASA NTRS1990

A technique to measure the thermal diffusivity of high T(sub c) superconductors

High T(sub c) superconducting electrical current leads and ground straps will be used in cryogenic coolers in future NASA Goddard Space Flight Center missions. These superconducting samples will be long, thin leads with a typical diameter of two millimeters. A longitudinal method is being developed to measure the thermal diffusivity of candidate materials for this application. This technique will use a peltier junction to supply an oscillatory heat wave into one end of a sample and will use low mass thermocouples to follow the heat wave along the sample. The thermal diffusivity will be calculated using both the exponential decay of the heat wave and the phase shift of the wave. Measurements will be done in a cryostat between 10 K and room temperature.

Powers, Charles E.↗
NASA NTRS1978

Lifetime and diffusion length measurements on silicon material and solar cells

Experimental methods were evaluated for the determination of lifetime and diffusion length in silicon intentionally doped with potentially lifetime-degrading impurities found in metallurgical grade silicon, impurities which may be residual in low-cost silicon intended for use in terrestrial flat-plate arrays. Lifetime measurements were made using a steady-state photoconductivity method. Diffusion length determinations were made using short-circuit current measurements under penetrating illumination. Mutual consistency among all experimental methods was verified, but steady-state photoconductivity was found preferable to photoconductivity decay at short lifetimes and in the presence of traps. The effects of a number of impurities on lifetime in bulk material, and on diffusion length in cells fabricated from this material, were determined. Results are compared with those obtained using different techniques. General agreement was found in terms of the hierarchy of impurities which degrade the lifetime.

Othmer, S.↗
NASA NTRS1977

Photochemistry of the Martian atmosphere

A critical analysis is carried out for models which may be fine tuned to give agreement with observational constraints for O2, CO, and O3 in the Martian lower atmosphere and which are in accord also with upper atmospheric data for O and CO. The models have a number of features in common: dynamic mixing must be exceedingly rapid at heights above 90 km; if heterogeneous processes are not assumed to play a major role in the removal of atmospheric CO and O2, mixing must be rapid at heights between 30 and 40 km. The value implied for the diffusion coefficient in this region depends on the assumptions made regarding the rates for reaction of OH with HO2 to form H2O and the rate of reaction of HO2 with itself to form H2O2. If these reaction rates are taken to have values similar to those used in current models of the earth's stratosphere, the eddy diffusion coefficient at 40 km above the Martian surface should be about 5 by 10 to the seventh power sq cm/sec.

Kong, T. Y.↗
NASA NTRS1979

Open-circuit voltage improvements in low-resistivity solar cells

Mechanisms limiting the open-circuit voltage in 0.1 ohm-cm solar cells were investigated. It was found that a rather complicated multistep diffusion process could produce cells with significantly improved voltages. The voltage capabilities of various laboratory cells were compared independent of their absorption and collection efficiencies. This was accomplished by comparing the cells on the basis of their saturation currents or, equivalently, comparing their voltage outputs at a constant current-density level. The results show that for both the Lewis diffused emitter cell and the Spire ion-implanted emitter cell the base component of the saturation current is voltage controlling. The evidence for the University of Florida cells, although not very conclusive, suggests emitter control of the voltage in this device. The data suggest further that the critical voltage-limiting parameter for the Lewis cell is the electron mobility in the cell base.

Godlewski, M. P.↗
NASA NTRS2004

Non-Additive Voltametric Currents From a Mixture of Two, Three and Four Redox-Active Compounds and Electroanalytical Implications

We have published recently the effect of dissimilar diffusion coefficients on the size of the voltammetric waves from a mixture of two redox-active compounds. Similarly, at the potential range where three redox-active species, decamethylferrocene (dMeFc), ferrocene (Fc) and N-methylphenothiazine (MePTZ), are oxidized simultaneously with rates controlled by linear diffusion, electrogenerated radicals diffusing outwards from the electrode react with the original species diffusing towards the electrode from the bulk; thus, Fc(+) reacts with dMeFc producing Fc and dMeFc(+), while MePTZ(+) reacts both with dMeFc producing MePTZ and dMeFc(+), and with Fc producing MePTZ and Fc(+). These reactions replace dMeFc with Fc at the second plateau, and both dMeFc and Fc with MePTZ at the third plateau. Since the diffusion coefficients of the three species are not equal, the mass-transfer limited currents of the second and the third oxidation wave plateaus change by approx. 10%. Numerical simulations of the experimental voltamograms support this mechanism. Similar results were also obtained for a mixture of four redoxactive compounds. The implications of this non-additive nature of currents on: (a) the use of internal voltammetric standards for quantitative analysis of a mixture of redox-active compounds; and, (b) the half wave potentials (E1/2) of the 2nd, 3rd and 4th waves for qualitative analysis, will be discussed.

Dass, Amala↗
NASA NTRS1989

The effect of ballooning modes on thermal transport and magnetic field diffusion in the solar corona

Presently favored mechanisms of coronal heating (current sheet dissipation and Alfven wave resonant heating) deposit heat in thin layers. Classical thermal conduction cannot explain how heat is transported across the magnetic field. If heating occurs in thin layers, large pressure gradients can be created which can give rise to ballooning modes. These instabilities are caused by the pressure gradient and the curvature of the magnetic field, and are stabilized by magnetic tension. The modes are broad band in wavelength and should produce turbulence. A mixing length expression for the turbulent heat transport shows that it is more than adequate to rapidly convect heat into much broader layers. Furthermore, the turbulent resistivity implies that heating occurs over most of the width of these broadened layers. The broadening also implies that much shorter time scales are required for heating. The beta values in the corona suggest that 1-10 turbulent layers are formed in typical loop or arch structures.

Strauss, H. R.↗
NASA NTRS1992

Magnetic diffusion and flare energy buildup

Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear 2D force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field and photospheric velocity field are used, it is found that 3-4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates.

Wu, S. T.↗
NASA NTRS1993

Positrons from supernovae

Positrons are produced in the ejecta of supernovae by the decay of nucleosynthetic Co-56, Ti-44, and Al-26. We calculate the probability that these positrons can survive without annihilating in the supernova ejecta, and we show that enough of these positrons should escape into the interstellar medium to account for the observed diffuse Galactic annihilation radiation. The surviving positrons are carried by the expanding ejecta into the interstellar medium where their annihilation lifetime of 10 exp 5 - 10 exp 6 yr is much longer than the average supernovae occurrence time of about 100 yr. Thus, annihilating positrons from thousands of supernovae throughout the Galaxy produce a steady diffuse flux of annihilation radiation. We further show that combining the calculated positron survival fractions and nucleosynthetic yields for current supernova models with the estimated supernova rates and the observed flux of diffuse Galactic annihilation radiation suggests that the present Galactic rate of Fe-56 nucleosynthesis is about 0.8 +/- 0.6 solar mass per 100 yr.

Chan, Kai-Wing↗
NASA NTRS1991

High voltage thermally diffused p(+)n solar cells

The possibility of fabricating thermally diffused p(+)n InP solar-cells with high open-circuit voltage without sacrificing the short circuit current is discussed. The p(+)n InP junctions were formed by Cd and Zn diffusion through a 3-5-nm-thick anodic or chemical phosphorus-rich oxide cap layer grown on n:InP:S (with ND-NA = 3.5 x 10 exp 16 and 4.5 x 10 exp 17/cu cm) Czochralski LEC-grown substrates. After thinning the emitter from its initial thickness of 1 to 2.5 micron down to 0.06-0.15 micron, the maximum efficiency was found when the emitter was 0.2 to 0.3 micron thick. Typical AM0, 25 C values of 854-860 mV were achieved for Voc, Jsc values were from 25.9 to 29.1 mA/sq cm using only the P-rich passivating layer left after the thinning process as an antireflection coating.

Faur, M.↗
NASA NTRS1972

Silicon solar cell efficiency: Practice and promise

The maximum efficiency of silicon solar cells was calculated and yielded a value near 18%. Additionally, the performance of these high efficiency cells in a synchronous orbit radiation field was calculated and it is suggested that these cells would be superior to present silicon cells. The performance of conventional cells was analyzed and several areas in which performance gains may be achieved are discussed. These areas include improvements in diffused region profile, in reduction of excess forward currents in cells made from low resistivity material, and in the theory for describing complex solar cell structures.

Brandhorst, H. W., Jr.↗