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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Influence of electrical field on the susceptibility of gallium nitride transistors to proton irradiation

Radiation susceptibility of electronic devices is commonly studied as a function of radiation energetics and device physics. Often overlooked is the presence or magnitude of the electrical field, which we hypothesize to play an influential role in low energy radiation. Accordingly, we present a comprehensive study of low-energy proton irradiation on gallium nitride high electron mobility transistors (HEMTs), turning the transistor ON or OFF during irradiation. Commercially available GaN HEMTs were exposed to 300 keV proton irradiation at fluences varying from 3.76 × 10 12 to 3.76 × 10 14 cm 2 , and the electrical performance was evaluated in terms of forward saturation current, transconductance, and threshold voltage. The results demonstrate that the presence of an electrical field makes it more susceptible to proton irradiation. The decrease of 12.4% in forward saturation and 19% in transconductance at the lowest fluence in ON mode suggests that both carrier density and mobility are reduced after irradiation. Additionally, a positive shift in threshold voltage (0.32 V and 0.09 V in ON and OFF mode, respectively) indicates the generation of acceptor-like traps due to proton bombardment. high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy analysis reveal significant defects introduction and atom intermixing near AlGaN/GaN interfaces and within the GaN layer after the highest irradiation dose employed in this study. According to in-situ Raman spectroscopy, defects caused by irradiation can lead to a rise in self-heating and a considerable increase in (∼750 times) thermoelastic stress in the GaN layer during device operation. The findings indicate device engineering or electrical biasing protocol must be employed to compensate for radiation-induced defects formed during proton irradiation to improve device durability and reliability.

gallium nitride↗
DOE OSTI2025

Tuning the low-energy band structure in twisted bilayer WSe2

Tuning the electronic structures of two-dimensional (2D) material-based heterostructures is of crucial importance for their use in functional next-generation electronics. Here, through angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nano-ARPES), we systematically track the evolution of the near-Fermi-level electronic structure of bilayer WSe2 over a large range of twist angle. While the momentum positioning of the valence-band maxima (VBM) is independent of twist angle, we find that the energetic separation between the hole bands at the K point of the Brillouin zone and the higher binding-energy hole band at Γ can be varied in excess of 100 meV. We explore the mechanisms underpinning this evolution and discuss the implications for tuning both the size of the band gaps, and the efficiency of the spin-dependent electron-phonon coupling channels in homobilayer transition-metal dichalcogenide devices.

Vu, T-H-Y↗
NASA NTRS1974

Errors induced in the measurement and azimuth directions of morphological features imaged on oblique Lunar Orbiter photographs

Many quantitative lunar studies, e.g., the morphology and dimensions of craters, crater density and distribution, have been performed using oblique Lunar Orbiter photographs. If the inherent change in scale and azimuth direction of features imaged on these photographs are not corrected, the measurement can be in considerable error and the resulting statistical inferences may be invalid. The magnitude of this error is dependent upon: the depression angle of the camera, the flight height of the spacecraft, the focal length of the camera, and the position and orientation of the object on the ground. The errors introduced by using unrectified oblique photographs as though they were vertical photographs are examined for several Lunar Orbiter high resolution NASA LRC Enhancement photographic prints taken at various depression angles.

Siegal, B. S.↗
NASA NTRS1977

Extreme ultraviolet spectroheliograph ATM experiment S082A

The XUV spectroheliograph, Apollo Telescope Mount experiment S082A, is described. The instrument was a slitless Wadsworth grating spectrograph that employed photographic recording. The grating was of 4-m radius, ruled with 3600 grooves/mm. By rotating the grating to positions where the normal was at 255 A or 400 A, the spectral ranges 175-335 A and 320-480 A, respectively, were covered with 2-sec of arc spatial resolution. Close to the normal the resolution reached 2 sec of arc, but at the extreme limit, 630 A, it was 10 sec of arc or worse. The aberrations of the instrument are discussed in detail as are the provisions necessary to maintain optimum imagery and reliability in a space environment. During the mission about 1020 exposures were made covering 171-335 A or 320-630 A.

Tousey, R.↗
NASA NTRS1976

The Seasat-A satellite scatterometer

This report describes the methods used to develop performance requirements and design characteristics of a microwave scatterometer wind sensor planned for Seasat-A. User requirements such as wind speed accuracy, resolution cell size, grid spacing, and swath width of the measurements formed the basis for defining instrument characteristics. Results are presented that show scatterometer accuracy as a function of orbit position satisfies User requirements for nominal orbit conditions.

Grantham, W. L.↗
NASA NTRS1979

Comparison of atmospheric density data from mass spectrometers and atmospheric drag on the Aeros satellites

The comparison of perigee density data near 230 km for satellite Aeros-A has been extended to the complete mission time. The average ratio between orbital drag derived density and mass spectrometer measurements is very near to 1 with a large scatter reflected by a standard deviation of the order of 20%. A method of comparison was developed and tested which uses the observed rate of change of orbital period in comparison with the orbital decay computed from the actual mass spectrometer data measured at discrete positions along the orbit. This method proves that deviations from the average ratio of 1 between perigee densities from drag and mass spectrometers are due to the smoothing and poor resolution of the orbital drag technique.

Roemer, M.↗
NASA NTRS1981

Numerical prediction of the monsoon depression of 5-7 July 1979

A well defined monsoon depression was used for two assimilation and forecast experiments: (1) using conventional surface and upper air data, (2) using these data plus Monex data. The data sets were assimilated and used with a general circulation model to make numerical predictions. The model, the analysis and assimilation procedure, the differences in the analyses due to different data inputs, and the differences in the numerical predictions are described. The MONEX data have a positive impact, although the differences after 24 hr are not significant. The MONEX assimilation does not agree with manual analysis location of depression center. The 2.5 x 3 deg horizontal resolution of the prediction model is too coarse. The assimilation of geopotential height data derived from satellite soundings generated gravity waves with amplitudes similar to the meteorologically significant features investigated.

Shukla, J.↗
NASA NTRS1991

Advanced infrared astronomy

This task supports the application of infrared heterodyne spectroscopy and other high resolution techniques, as well as infrared arrays to ultra-high resolution studies of molecular constituents of planetary atmospheres. High spectral and spatial resolution measurement and analysis of individual spectral lines permits the retrieval of distributions of atmospheric molecular abundances and temperatures and thus, information on local photochemical processes. Determination of absolute line positions to better than 10(exp -8) permits direct measurement of gas velocities to a few m/sec and thus, the study of dynamics. Observations are made from ground based observatories.

Kostiuk, Theodor↗
NASA NTRS1992

A survey of the milliarcsecond polarization properties of BL Lacertae objects at 5 GHz

Consideration is given to millarcsecond-resolution total-intensity and linear polarization maps at 5 GHz presented for 11 BL Lacertae objects. In every BL Lacertae object in which polarization structure was detected, the polarization position angle of the knots is nearly parallel to the VLBI structural axis. The direction of the polarization in the cores of these sources appears to be random. The preferred polarization direction in the jets is explained by the fact that plane perpendicular shock waves are common in these sources; the origin of the absence of the preferred polarization direction in the core components is unclear. These results support an early conclusion that it cannot be the case that a significant number of BL Lacertae objects are gravitationally microlensed images of more distant quasars, since the characteristic VLBI polarization structures observed in these two types of objects are very different.

Gabuzda, D. C.↗
NASA NTRS1993

Magnetic fields in massive cloud cores - Comparison of MILLIPOL and IRAS results

The MILLIPOL polarimeter has been used to obtain 30-arcsec resolution 1300-micron data toward 10 cloud cores; the seven that were detected are associated with compact H II regions in massive molecular clouds. Cloud-core axis ratios and position angles were derived by examining the morphologies of IRAS-traced dust opacity structures. The MILLIPOL-detected cloud cores exhibit 1.5-3.2 core axis ratio values. The magnetic field is found to be generally perpendicular to the core dust distributions.

Kane, Brian D.↗
NASA NTRS1994

Gravity over coronae and chasmata on Venus

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m/km. The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction of underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fauta Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Wester Parga Chasmata are about 75 km. The GTRs at Fauta Corona, the four coronae in eastern Eistla Regio, and the accurate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m/km. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Coronam, and elsewhere along Parga and Hecate Chasmata.

Schubert, Gerald↗
NASA NTRS2002

Numerical Simulations of Vortex Generator Vanes and Jets on a Flat Plate

Numerical simulations of a single low-profile vortex generator vane, which is only a small fraction of the boundary-layer thickness, and a vortex generating jet have been performed for flows over a flat plate. The numerical simulations were computed by solving the steady-state solution to the Reynolds-averaged Navier-Stokes equations. The vortex generating vane results were evaluated by comparing the strength and trajectory of the streamwise vortex to experimental particle image velocimetry measurements. From the numerical simulations of the vane case, it was observed that the Shear-Stress Transport (SST) turbulence model resulted in a better prediction of the streamwise peak vorticity and trajectory when compared to the Spalart-Allmaras (SA) turbulence model. It is shown in this investigation that the estimation of the turbulent eddy viscosity near the vortex core, for both the vane and jet simulations, was higher for the SA model when compared to the SST model. Even though the numerical simulations of the vortex generating vane were able to predict the trajectory of the stream-wise vortex, the initial magnitude and decay of the peak streamwise vorticity were significantly under predicted. A comparison of the positive circulation associated with the streamwise vortex showed that while the numerical simulations produced a more diffused vortex, the vortex strength compared very well to the experimental observations. A grid resolution study for the vortex generating vane was also performed showing that the diffusion of the vortex was not a result of insufficient grid resolution. Comparisons were also made between a fully modeled trapezoidal vane with finite thickness to a simply modeled rectangular thin vane. The comparisons showed that the simply modeled rectangular vane produced a streamwise vortex which had a strength and trajectory very similar to the fully modeled trapezoidal vane.

Allan, Brian G.↗
NASA NTRS2005

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

The center of our Galaxy is a known strong source of electron-positron 511- keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| greater than 40 degrees) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.

Teegarden, B. J.↗
NASA NTRS2006

Lunar Geodetic Opportunities with the Laser Altimeter on LRO

The Lunar Reconnaissance Orbiter (LRO) is to be launched at the end of 2008 and will carry 7 instruments, one of which is a laser altimeter (LOLA), and obtain observations of the Moon for a period of 1 year. The orbit will be near polar and approximately circular at 50 km altitude with monthly orbital adjustments to maintain the mean altitude. The LOLA instrument has a -10 cm single-shot accuracy, with 5 beams, and operates at 28 Hz. It provides 5 adjacent profiles, each approximately 12 to 15 meters apart with a swath of approximately 65 meters. The 5 beams are arranged in a cross-shaped pattern that provides simultaneous along and cross track altimetry and providing slopes in 2 orthogonal directions every 50 meters along track. In combination with the LRO tracking data LOLA will be used to improve the model of the lunar gravity by using the altimeter on both the lunar near-side and far-side as an additional tracking system to enable precise positioning of the LRO spacecraft at about the 50 meter level rms. The instrument is expected to provide full polar coverage at very high northern and southern latitudes with spatial resolutions of 25 meters or better. In addition to the range to the surface LOLA measures the surface roughness from the spreading of the laser pulse and also the reflectance of the surface at 1064 nm. These measurements in conjunction with the altimetry will assist in the selection of future landing sites for future robotic and human missions to the Moon.

Zuber, Maria T.↗
DOE OSTI2022

From ground to excited electronic state dynamics of electron and ion irradiated graphene nanomaterials

Microscopes utilizing convergent electron and ion beams are emerging as powerful tools for both imaging and manipulating two-dimensional materials with atomic resolution, allowing the ultimate limits of nanofabrication to be realized. In this chapter, we detail the use of time-dependent electronic structure theory to determine the excited state properties and reactivity of functionalized graphene nanostructures. A time-dependent density functional theory treatment of electronic excitations of materials is presented, with specific emphasis on predicting the position-dependent electronic response of two-dimensional nanomaterials to electron beams. The method is demonstrated in a study highlighting the important role that electronic excitation can play in opening reaction pathways relevant to atomically precise defect manipulation in graphene. Finally, we provide some perspective on future development directions for methods of simulating nonequilibrium electronic and vibrational dynamics induced by electron/ion beams.

Lingerfelt, David↗
DOE OSTI2021

Molecular imaging of plant-microbe interactions on the Brachypodium seed surface

Plant growth-promoting rhizobacteria (PGPR) play a crucial role in biological control and pathogenic defense on and with-in plant tissues, however the mechanism(s) by which plants associate with PGPR to elicit such beneficial effects needs further study. Here, we present high mass resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging of Brachypodium distachyon (Brachypodium) seeds with and without exposure to two model PGPR, i.e., Gram-negative Pseudomonas fluorescens SBW25::GFP (P.) and Gram-positive Arthrobacter chlorophenolicus A6::GFP (A.). Delayed image extraction was used to image PGPB-treated seed sections to reveal morphological changes. ToF-SIMS spectral compari-son, principal component analysis (PCA), and two-dimensional (2D) imaging show that the selected PGPR have different effects on the host seed surface, resulting in changes in chemical composition and morphology. Metabolite prod-ucts and biomarkers, such as flavonoids, phenolic com-pounds, fatty acids, and indole-3-acetic acid (IAA), were ob-served on the PGPR-treated seed surfaces. These compounds had different distributions on the Brachypodium seed surface for the two PGPR, indicating that the different bacteria elicited distinct responses from the host. Our results illustrate that ToF-SIMS is an effective tool to study plant-microbe interac-tions and to provide insightful information with submicrome-ter lateral resolution of the chemical distributions associated with morphological features, potentially offering a new way to study the mechanisms underlying beneficial roles of PGPR.

PGPR, Brachypodium distachyon, ToF-SIMS, metabolit↗
NASA NTRS1990

Planetary circulations in the presence of transient and self-induced heating

The linearized global primitive equation model of Salby and Garcia (1987) was modified to study tropospheric circulations and their interaction with tropical convection. The vertical resolution was increased to approximately 1.5 km, with the effective vertical domain extending the approximately 40 km (higher altitudes in the calculation are used to ensure the radiation upper boundary condition is satisfied). A formal wave conditional instability of the second kind (CISK) parameterization was introduced. That parameterization allows low-frequency disturbances to interact with convection by organizing surface moisture. Boundary layer friction has two important effects in these calculations: it exaggerates surface convergence near the equator by driving the flow out of geostrophic balance; and it introduces a phase shift between the convergence pattern and the circulation, which drives the heating out of quadrature with the temperature field and allows a positive feedback to take place. The observational complement of this theoretical work involves Global Cloud Imagery (GCI) composited from six satellites in the International Satellite Cloud Climatology Project (ISCCP) which were simultaneously observing the Earth's cloud field. Having global coverage, horizontal resolution of half a degree, and produced synoptically every 3 hours, the GCI affords an unprecendented view of global convection. A full year (1983 to 1984) of GCI has now been successfully created. Among other phenomena, that imagery reveals a regular progression of easterly waves in the convective pattern of the Western Pacific. Those disturbances have odd symmetry about the equator and organize convection into a pattern with similar symmetry. In time-mean maps of cloud cover, these propagating disturbances produce a split Inter-Tropical Convergence Zone (ITCZ) in the western Pacific.

Salby, Murry L.↗
NASA NTRS1990

The molecular spiral arms of NGC 6946

High resolution observations of molecules in external galaxies are essential to understanding physical processes leading to the formation of stars. One question is whether there is a spiral structure in the molecular gas, but it was not possible to resolve the spiral arms of external galaxies until the advent of large millimeter-wave telescopes. With the Institute for Radio Astronomy in the Millimeter Range (IRAM) 30 m telescope, researchers are carrying through the mapping of NGC 6946 in the CO-12(1-0) and (2-1) lines. This galaxy is a large, gas-rich Scd spiral with a strong star formation activity. NGC 6946 is well studied at radio and optical wavelengths, so that it is possible to compare the location of the spiral arms tracers: HI ridge, HII regions and molecular clouds. The disk CO emission is very contrasted (no lines for some positions, 1 K in CO(1-0) for some others) and correlated with the optical spiral arms: this clearly shows up in a figure which presents superimposed contours of CO(2-1) integrated emissivity and of H alpha line emission. The agreement is very good, and there is no displacement across the arm between the CO, HI and H alpha ridges of emission. The arms are barely resolved by the 23 inch beam and the molecular contrast averaged over the map is about 4. The CO(2-1) maxima are closer to the position of the HII regions than those of CO(1-0), which could be due to variations of excitation conditions. The CO excitation in the disk of NGC 6946 is low: when all data are convolved to the same resolution of 23 inches the CO(2-1) lines are about 0.45 times fainter than the CO(1-0) ones, while in the nucleus they have roughly the same intensity. This suggests that in the disk of NGC 6946 most of the CO emission comes from cold optically thick gas located in cloud envelopes rather than from cloud cores. The molecular and atomic component in the observed regions of NGC 6946 seems to be organized in large gaseous complexes.

Casoli, F.↗