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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Electron energization in the geomagnetic tail current sheet

Electron motion in the geomagnetic tail current sheet is examined analytically. An estimation of the energization of electrons trapped by the convective field is made independent of a guiding center approximation. The electrons in the energy range 100 eV - 100 keV have their pitch angle scattered and trapped in the current sheet until accelerated by the convective field. Predictions are made for the electrons that escape the current sheet, electron precipitation in diffuse auroral phenomena and current sheet interactions as a source for plasma sheet electrons and ions.

Lyons, L. R.↗
NASA NTRS1991

High-gain AlGaAs/GaAs double heterojunction Darlington phototransistors for optical neural networks

High-gain MOCVD-grown (metal-organic chemical vapor deposition) AlGaAs/GaAs/AlGaAs n-p-n double heterojunction bipolar transistors (DHBTs) and Darlington phototransistor pairs are provided for use in optical neural networks and other optoelectronic integrated circuit applications. The reduced base doping level used results in effective blockage of Zn out-diffusion, enabling a current gain of 500, higher than most previously reported values for Zn-diffused-base DHBTs. Darlington phototransitor pairs of this material can achieve a current gain of over 6000, which satisfies the gain requirement for optical neural network designs, which advantageously may employ neurons comprising the Darlington phototransistor pairs in series with a light source.

Kim, Jae H.↗
NASA NTRS1984

Ambipolar diffusion in the middle atmosphere

In the middle atmosphere above 60 km, the electron concentration increases with altitude, reaching values of 10 to the 10th per cu m in the daytime ionospheric E region near 100 km. The electrons are more mobile than the ions and diffuse more rapidly through the neutral atmosphere. The electron diffusion polarizes the medium, causing an electric field to develop that acts to retard the electron diffusion and enhance the conduction current of ions. A global zonally averaged numerical model of atmosheric electricity from the ground to 100 km is used to examine the effect of ambipolar diffusion and the earth's geomagnetic field on the currents and fields in the middle atmosphere. The results show that above about 65 km, ambipolar diffusion generates local electric fields and conduction currents that balance electron diffusion currents. The electric fields and conduction currents are a few orders of magnitude larger than the vertical fields and currents calculated from the downward mapping of the ionospheric potential without taking electron diffusion into account. Ambipolar diffusion does not alter the total current flowing in the global circuit. It is a local effect where enhanced conduction currens flow to balance the electron diffusion current.

Tzur, I.↗
NASA NTRS1992

Ion tearing in a magnetotail configuration with an embedded thin current sheet

The ion tearing instability is investigated in a magnetotail configuration that consists of a diffuse plasma sheet current and an embedded, thin current sheet with a strong current. For historical reasons, the thin embedded current sheet will be called a 'neutral sheet', even though the normal component of the magnetic field, Bn, is nonzero. In particular, we assume that the current within the thin current sheet is due to the acceleration of 'Speiserlike' ion trajectories by a cross-tail electric field Ey. It is found that the strong current within the neutral sheet is essentially unimportant to the growth rate of the tearing instability, and that the growth rate scales as (lambda(0)/Lz) squared, where Lz is the overall half thickness of the plasma sheet and lambda (0) is the ion inertial length. In the absence of the current outside the neutral sheet, current filamentation is stable.

Burkhart, G. R.↗
NASA NTRS1985

Current understanding of point defects and diffusion processes in silicon

The effects of oxidation of Si which established that vacancies (V) and Si self interstitials (I) coexist in Si at high temperatures under thermal equilibrium and oxidizing conditions are discussed. Some essential points associated with Au diffusion in Si are then discussed. Analysis of Au diffusion results allowed a determination of the I component and an estimate of the V component of the Si self diffusion coefficient. A discussion of theories on high concentration P diffusion into Si is then presented. Although presently there still is no theory that is completely satisfactory, significant progresses are recently made in treating some essential aspects of this subject.

Tan, T. Y.↗
NASA NTRS2016

Currents and Associated Electron Scattering and Bouncing Near the Diffusion Region at Earth's Magnetopause

Based on high-resolution measurements from NASA's Magnetospheric Multlscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of Inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90 degrees away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).

Lavraud, B.↗
NASA NTRS1992

Anomalous plasma diffusion and the magnetopause boundary layer

An overview of the current state of anomalous diffusion research at the magnetopause and its role in the formation of the magnetopause boundary layer is presented. Plasma wave measurements in the boundary layer indicate that most of the relevant unstable wave modes contribute negligibly to the diffusion process at the magnetopause under magnetically undisturbed northward IMF conditions. The most promising instability is the lower hybrid drift instability, which may yield diffusion coefficients of the right order if the highest measured wave intensities are assumed. It is concluded that global stationary diffusion due to wave-particle interactions does not take place at the magnetopause. Microscopic wave-particle interaction and anomalous diffusion may contribute to locally break the MD frozen-in conditions and help in transporting large amounts of magnetosheath plasma across the magnetospheric boundary.

Treumann, Rudolf A.↗
NASA NTRS1992

Ion radial diffusion in an electrostatic impulse model for stormtime ring current formation

Two refinements to the quasi-linear theory of ion radial diffusion are proposed and examined analytically with simulations of particle trajectories. The resonance-broadening correction by Dungey (1965) is applied to the quasi-linear diffusion theory by Faelthammar (1965) for an individual model storm. Quasi-linear theory is then applied to the mean diffusion coefficients resulting from simulations of particle trajectories in 20 model storms. The correction for drift-resonance broadening results in quasi-linear diffusion coefficients with discrepancies from the corresponding simulated values that are reduced by a factor of about 3. Further reductions in the discrepancies are noted following the averaging of the quasi-linear diffusion coefficients, the simulated coefficients, and the resonance-broadened coefficients for the 20 storms. Quasi-linear theory provides good descriptions of particle transport for a single storm but performs even better in conjunction with the present ensemble-averaging.

Chen, Margaret W.↗
NASA NTRS1987

Determination of diffusion coefficients in polypyrrole thin films using a current pulse relaxation method

The current pulse E sub oc relaxation method and its application to the determination of diffusion coefficients in electrochemically synthesized polypyrrole thin films is described. Diffusion coefficients for such films in Et4NBF4 and MeCN are determined for a series of submicron film thicknesses. Measurement of the double-layer capacitance, C sub dl, and the resistance, R sub u, of polypyrrole thin films as a function of potential obtained with the galvanostatic pulse method is reported. Measurements of the electrolyte concentration in reduced polypyrrole films are also presented to aid in the interpretation of the data.

Penner, Reginald M.↗
NASA NTRS1993

Calculation methods for rotor wake formation in forward flight

The primary focus of this research centered on improving the vorticity-capturing properties of Eulerian flow solvers in convection-dominated flows. The work was motivated by the fact that excessive numerical diffusion observed in current Euler solvers on grids of feasible size is a major obstacle to accurately computing the flow about helicopter rotors, especially in the forward flight case. The work was originally supported by NASA Grant NAG2-421 for two years. The research then continued for four more years under an ONR contract. The research goal of reduction of numerical diffusion in Euler solvers remained the same, although the target application was now the prediction of internal vortical secondary flows. This was part of a larger ONR-sponsored research program at MIT aimed at reducing the noise of turbopumps. The research work culminated in the PhD thesis titled 'A Coupled Eulerian/Lagrangian Method for the Solution of Three-Dimensional Vortical Flows'.

Drela, Mark↗
NASA NTRS1984

Analysis of the electron-beam-induced current of a polycrystalline p-n junction when the diffusion lengths of the material on either side of a grain boundary differ

The short circuit current generated by the electron beam of a scanning electron microscope in p-n junctions is reduced by enhanced recombination at grain boundaries in polycrystalline material. Frequently, grain boundaries separate the semiconductor into regions possessing different minority carrier life times. This markedly affects the short circuit current I(sc) as a function of scanning distance from the grain boundary. It will be shown theoretically that (1) the minimum of the I(sc) in crossing the grain boundary with the scanning electron beam is shifted away from the grain boundary toward the region with smaller life time (shorter diffusion length), (2) the magnitude of the minimum differs markedly from those calculated under the assumption of equal diffusion lengths on either side of the grain boundary, and (3) the minimum disappears altogether for small surface recombination velocities (s less than 10,000 cm/s). These effects become negligible, however, for large recombination velocities s at grain boundaries. For p-type silicon this happens for s not less than 100,000 cm/s.

Von Roos, O.↗
NASA NTRS2002

Static and Dynamic Effects of Lateral Carrier Diffusion in Semiconductor Lasers

Electron and hole diffusions in the plane of semiconductor quantum wells play an important part in the static and dynamic operations of semiconductor lasers. It is well known that the value of diffusion coefficients affects the threshold pumping current of a semiconductor laser. At the same time, the strength of carrier diffusion process is expected to affect the modulation bandwidth of an AC-modulated laser. It is important not only to investigate the combined DC and AC effects due to carrier diffusion, but also to separate the AC effects from that of the combined effects in order to provide design insights for high speed modulation. In this presentation, we apply a hydrodynamic model developed by the present authors recently from the semiconductor Bloch equations. The model allows microscopic calculation of the lateral carrier diffusion coefficient, which is a nonlinear function of the carrier density and plasma temperature. We first studied combined AC and DC effects of lateral carrier diffusion by studying the bandwidth dependence on diffusion coefficient at a given DC current under small signal modulation. The results show an increase of modulation bandwidth with decrease in the diffusion coefficient. We simultaneously studied the effects of nonlinearity in the diffusion coefficient. To clearly identify how much of the bandwidth increase is a result of decrease in the threshold pumping current for smaller diffusion coefficient, thus an effective increase of DC pumping, we study the bandwidth dependence on diffusion coefficient at a given relative pumping. A detailed comparison of the two cases will be presented.

Li, Jian-Zhong↗
NASA NTRS1975

Hot-cold plasma interactions in the earth's magnetosphere

The paper gives a synopsis of observations and results obtained from the Explorer-45 satellite which pertain directly to the interaction of the magnetospheric hot and cold plasma populations. The specific case discussed is the interaction of the hot ring current plasma with the cold plasmaspheric plasma in the evening to early morning local time sector during magnetic storm recovery phase. It was found that above the plasmapause region, the hot ring current plasma is stable with negligible losses due to pitch angle diffusion; the hot ring current plasma enters a moderate pitch angle regime in the plasmapause region, whereby the addition of cold plasma destabilizes the hot plasma. Analysis of the energy, spatial, and temporal dependence of the above destabilization along with the ion-cyclotron resonant energy equation, and comparison of this analysis with an in situ estimate of the plasma density strongly indicate that the mechanism responsible for destabilization of the hot plasma is the amplification of ion-cyclotron waves due to the interaction of the cold plasmaspheric plasma with the hot ring current plasma in a manner similar to that discussed by Cornwall et al. (1970).

Williams, D. J.↗
NASA NTRS1978

The effect of nonlinear diffusive heat transport in a simple climate model

The effects of nonlinear diffusive heat transport in North's (1975) model is reexamined. By inserting the nonlinearity into the original energy balance equation rather than the spectrally transformed linear algebraic equations, and including one more mode in the spectral expansion, it is found that nonlinear diffusion does give a significantly different sensitivity for states near the current climate. For linear diffusion, an increase of 0.1% of the solar constant from the present value is sufficient to melt all the ice on the earth, whereas an increase of about five times as large is required for nonlinear diffusion. This difference in sensitivity shows the importance of dynamical modeling in simple climate models.

Lin, C. A.↗
NASA NTRS1973

An auroral breakup mechanism

A purely growing electrostatic drift instability driven by the electron temperature gradient at the inner edge of the plasma sheet can grow for large enough values of the temperature gradient. The parallel electric field associated with the instability is localized near the magnetic equator. The growth of the drift instability leads to enhanced whistler noise and increased electron pitch angle diffusion. If the current limit is exceeded in the ionosphere while the parallel electric field of the drift instability exists along the field line, rapid electron precipitation (the auroral breakup) can result.

Maggs, J. E.↗
NASA NTRS1975

A feasibility study for measuring stratospheric turbulence using metrac positioning system

The feasibility of obtaining measurements of Lagrangian turbulence at stratospheric altitudes is demonstrated by using the METRAC System to track constant-level balloons. The basis for current estimates of diffusion coefficients are reviewed and it is pointed out that insufficient data is available upon which to base reliable estimates of vertical diffusion coefficients. It is concluded that diffusion coefficients could be directly obtained from Lagrangian turbulence measurements. The METRAC balloon tracking system is shown to possess the necessary precision in order to resolve the response of constant-level balloons to turbulence at stratospheric altitudes. A small sample of data recorded from a tropospheric tetroon flight tracked by the METRAC System is analyzed to obtain estimates of small-scale three-dimensional diffusion coefficients. It is recommended that this technique be employed to establish a climatology of diffusion coefficients and to ascertain the variation of these coefficients with altitude, season, and latitude.

Gage, K. S.↗
NASA NTRS1979

Mathematical model investigation of long-term transport of ocean-dumped sewage sludge related to remote sensing

An existing, three-dimensional, Eulerian-Lagrangian finite-difference model was modified and used to examine the transport processes of dumped sewage sludge in the New York Bight. Both in situ and laboratory data were utilized in an attempt to approximate model inputs such as mean current speed, horizontal diffusion coefficients, particle size distributions, and specific gravities. The results presented are a quantitative description of the fate of a negatively buoyant sewage sludge plume resulting from continuous and instantaneous barge releases. Concentrations of the sludge near the surface were compared qualitatively with those remotely sensed. Laboratory study was performed to investigate the behavior of sewage sludge dumping in various ambient density conditions.

Kuo, C. Y.↗