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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

A far-infrared study of the N/O abundance ratio in galactic H II regions

New measurements are presented for W43 (G30.8-0.0), Orion A, and G75.84+0.4, which are located at widely varying distances from the galactic center. The combination of the forbidden N III 57.3 microns and forbidden O III 88.4 and 51.8 microns yields measurements of N(++)/O(++) that are for the most part insensitive to electron temperature and density uncertainties and to clumping of the ionized gas. This is due to the similarity of the critical densities for these transitions. It is contended that for the observed nebulae, N(++)/O(++) should be indicative of N/O, a ratio that is of signal importance in nucleosynthesis theory. The measurements are compared with previous measurements of M17 and W51, which lie at intermediate galactocentric distances. For nebulae in the solar circle, it is found that N(++)/O(++) is greater than the N/O values derived from optical studies of N(+)/O(+) in low-ionization zones of the same nebulae. Possible sources of this discrepancy are considered. It is found that N(++)/O(++) in W43 is significantly higher than for the other H II regions in the sample. Since W43 is located at R = 5 kpc, which is the smallest galactocentric distance in the sample, the data are seen as consistent with the presence of a negative abundance gradient d(N/O)/dR.

Lester, D. F.↗
NASA NTRS1987

On the 3d6 4s2 (5D) - 3d7 4s1 (5F) separation in Fe

Full CI calculations are used to calibrate SDSI treatment of the 5D-5F separation in the Fe atom. The (5s4p2d), (5s4p3d), and (5s4p2d1f) basis sets were used for the FCI calculations. The importance of the 3s and 3p correlation and the relativistic effects are studied at SDCI level. The role of polarization functions in computing atomic separations in transition metal atoms is discussed. It is observed that the FCI separation is slightly smaller than the SDCI results, and the SDCI data yield an adequate description of the 5D-5F separation with only 8 electrons correlated. It is concluded that the differential effect of higher excitations becomes more important when the 3s and 3p electrons are correlated.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Crab pulsar giant pulses: Simultaneous radio and GRO observations

Observations are reported of the Crab pulsar made at radio frequencies concurrent with Oriented Scintillation Spectrometer Experiment (OSSE) observations from 15 to 27 May 1991. Using the 43 m telescope at Green Bank at 0.8 and 1.4 GHz, samples were made continuously for 10 hrs/day at intervals of 100 to 300 microsecs. The analysis of the radio data includes calculation of histograms of pulse intensities, absolute timing to about 20 microsec precision, and characterization of intensity variations on time scales from the 33 ms spin period to days. The most detailed analysis is presented made of giant pulses. The ultimate goal is to bin the radio data into giant and nongiant pulses and to form average waveforms of OSSE data for the corresponding pulse periods. A test is done to see whether the violet radio fluctuations (which are not seen in other radio pulsars to the same degree) are correlated with low energy gamma rays, yielding constraints on the radio coherence mechanism and the steadiness of the electron-positron outflow in the magnetosphere. Timing analysis of the radio data provides a well defined ephemeris over the specified range of epochs. The gamma ray pulse phase was predicted with an error of less than 70 microsecs.

Lundgren, Scott C.↗
NASA NTRS1992

Radio synthesis imaging during the GRO solar campaign

The Owens Valley (OVRO) Solar Array was recently expanded to 5 antennas. Using frequency synthesis, the 5-element OVRO Solar Array has up to 450 effective baselines, which can be employed as necessary to make maps at frequencies in the range 1 to 18 GHz. Fortuitously, the last of the 5 antennas was completed and brought into operation on 7 Jun., just in time for the Gamma Ray Observatory (GRO)/Max 1991 observing campaign. Many events were observed jointly with OVRO and the BATSE experiment on GRO, including the six larger events that are presented in tabular form. Unfortunately, the X flares that occurred during the campaign all occurred outside the OVRO time range. The UV coverage of the newly expanded solar array, combined with frequency synthesis, should give a more complete view of solar flares in the microwave range by providing simultaneous spatial and spectral resolution. A promising application of MEM (maximum entropy) is also being pursued that will use smoothness criteria in both the spatial and spectral domains to give brightness temperature maps at each observed frequency (up to 45 frequencies every 10 s). Such maps can be compared directly with the theory of microwave emission to yield plasma parameters in the source - notably the number and energy distribution of electrons, for comparison with the x ray and gamma ray results from GRO.

Gary, Dale E.↗
NASA NTRS1991

Cloud screening Coastal Zone Color Scanner images using channel 5

Clouds are removed from Coastal Zone Color Scanner (CZCS) data using channel 5. Instrumentation problems require pre-processing of channel 5 before an intelligent cloud-screening algorithm can be used. For example, at intervals of about 16 lines, the sensor records anomalously low radiances. Moreover, the calibration equation yields negative radiances when the sensor records zero counts, and pixels corrupted by electronic overshoot must also be excluded. The remaining pixels may then be used in conjunction with the procedure of Simpson and Humphrey to determine the CZCS cloud mask. These results plus in situ observations of phytoplankton pigment concentration show that pre-processing and proper cloud-screening of CZCS data are necessary for accurate satellite-derived pigment concentrations. This is especially true in the coastal margins, where pigment content is high and image distortion associated with electronic overshoot is also present. The pre-processing algorithm is critical to obtaining accurate global estimates of pigment from spacecraft data.

Eckstein, B. A.↗
NASA NTRS1992

Diode laser stabilization

Center-frequency drift in diode lasers can be reduced by stabilizing the laser to an optical cavity with a low bandwidth feedback loop that acts on the injection current. Resonant optical feedback from a confocal or ring cavity can reduce high frequency noise, yielding linewidths in the 10 kHz range. With either an optical or an electronic lock to a cavity, the laser's wavelength is restricted to those regions that can be reached by temperature and current tuning of the solitary diode; this can be extended to full coverage by means of an extended cavity with a frequency-selective component, such as a diffraction grating.

Fox, R. W.↗
NASA NTRS1993

First in-situ observations of neutral and plasma density fluctuations within a PMSE layer

The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, Cornell University Portable Radar Interferometer (CUPRI) and European Incoherent Scattter facility (EISCAT) observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.

Lubken, Franz-Josef↗
NASA NTRS1998

Electroformed Nickel Mirrors for the Next Generation Space Telescope

This paper summarizes the work to date on a novel mirror fabrication technique being developed at the Marshall Space Flight Center for potential use on the Next Generation Space Telescope (NGST). This technique involves forming an extremely lightweight mirror by electroplating nickel and nickel based alloys onto a highly polished precision mandrel. The resulting mirror shell can then be backed up with or attached to a lightweight structure to produce a mirror element that is on the order of 15 kg/sq m areal density. Since the mirrors are fabricated from a mandrel (or master), subsequent mirrors can be made with very high economy; this technique is particularly suited to segmented mirrors schemes whereby large apertures are achieved through the deployment of smaller segments. Control of the electroplating process is the key element for producing high quality optics; bath chemistry and real time control of the plating current density yields uniform grained electroforms with zero residual stress. To accomplish this, a special electronic sensor was developed whereby the residual stress can be monitored as the nickel is electrolytically deposited. This information is used in a feedback loop to modulate current density which, in turn, directly governs the residual stress. Details pertaining to this and other aspects of the fabrication of a half meter mirror will be published along with test results and metrology data.

Redmon, John W.↗
NASA NTRS2003

Plasma Interactions with High Voltage Solar Arrays for a Direct Drive Hall Effect Thruster System

The Environmental Effects Group of NASA s Marshall Space Flight Center (MSFC) is conducting research into the effects of plasma interaction with high voltage solar arrays. These high voltage solar arrays are being developed for a direct drive Hall Effect Thruster propulsion system. A direct drive system configuration will reduce power system mass by eliminating a conventional power-processing unit. The Environmental Effects Group has configured two large vacuum chambers to test different high-voltage array concepts in a plasma environment. Three types of solar arrays have so far been tested, an International Space Station (ISS) planar array, a Tecstar planar array, and a Tecstar solar concentrator array. The plasma environment was generated using a hollow cathode plasma source, which yielded densities between 10(exp 6) - 10(exp 7) per cubic centimeter and electron temperatures of 0.5-1 eV. Each array was positioned in this plasma and biased in the -500 to + 500 volt range. The current collection was monitored continuously. In addition, the characteristics of arcing, snap over, and other features, were recorded. Analysis of the array performance indicates a time dependence associated with the current collection as well as a tendency for "conditioning" over a large number of runs. Mitigation strategies, to reduce parasitic current collection, as well as arcing, include changing cover-glass geometry and layout as well as shielding the solar cell edges. High voltage performance data for each of the solar array types tested will be presented. In addition, data will be provided to indicate the effectiveness of the mitigation techniques.

Schneider, T.↗
NASA NTRS2003

The Athena Microscopic Imager Investigation

The Athena science payload on the Mars Exploration Rovers (MER) includes the Microscopic Imager (MI) [1]. The MI is a fixed-focus camera mounted on the end of an extendable instrument arm, the Instrument Deployment Device (IDD; see Figure 1).The MI was designed to acquire images at a spatial resolution of 30 microns/pixel over a broad spectral range (400 - 700 nm; see Table 1). Technically, the microscopic imager is not a microscope: it has a fixed magnification of 0.4 and is intended to produce images that simulate a geologist s view through a common hand lens. In photographers parlance, the system makes use of a macro lens. The MI uses the same electronics design as the other MER cameras [2, 3] but has optics that yield a field of view of 31 31 mm across a 1024 1024 pixel CCD image (Figure 2). The MI acquires images using only solar or skylightillumination of the target surface. A contact sensor is used to place the MI slightly closer to the target surface than its best focus distance (about 66 mm), allowing concave surfaces to be imaged in good focus. Because the MI has a relatively small depth of field (3 mm), a single MI image of a rough surface will contain both focused and unfocused areas. Coarse focusing will be achieved by moving the IDD away from a rock target after the contact sensor is activated. Multiple images taken at various distances will be acquired to ensure good focus on all parts of rough surfaces. By combining a set of images acquired in this way, a completely focused image can be assembled. Stereoscopic observations can be obtained by moving the MI laterally relative to its boresight. Estimates of the position and orientation of the MI for each acquired image will be stored in the rover computer and returned to Earth with the image data. The MI optics will be protected from the Martian environment by a retractable dust cover. The dust cover includes a Kapton window that is tinted orange to restrict the spectral bandpass to 500-700 nm, allowing color information to be obtained by taking images with the dust cover open and closed. The MI will image the same materials measured by other Athena instruments (including surfaces prepared by the Rock Abrasion Tool), as well as rock and soil targets of opportunity. Subsets of the full image array can be selected and/or pixels can be binned to reduce data volume. Image compression will be used to maximize the information contained in the data returned to Earth. The resulting MI data will place other MER instrument data in context and aid in petrologic and geologic interpretations of rocks and soils on Mars.

K E Herkenhoff↗
NASA NTRS2005

Solid State Digital Propulsion "Cluster Thrusters" For Small Satellites Using High Performance Electrically Controlled Extinguishable Solid Propellants (ECESP)

Electrically controlled extinguishable solid propellants (ESCSP) are capable of multiple ignitions, extinguishments and throttle control by the application of electrical power. Both core and end burning no moving parts ECESP grains/motors to three inches in diameter have now been tested. Ongoing research has led to a newer family of even higher performance ECESP providing up to 10% higher performance, manufacturing ease, and significantly higher electrical conduction. The high conductivity was not found to be desirable for larger motors; however it is ideal for downward scaling to micro and pico- propulsion applications with a web thickness of less than 0.125 inch/ diameter. As a solid solution propellant, this ECESP is molecularly uniform, having no granular structure. Because of this homogeneity and workable viscosity it can be directly cast into thin layers or vacuum cast into complex geometries. Both coaxial and grain stacks have been demonstrated. Combining individual propellant coaxial grains and/or grain stacks together form three-dimensional arrays yield modular cluster thrusters. Adoption of fabless manufacturing methods and standards from the electronics industry will provide custom, highly reproducible micro-propulsion arrays and clusters at low costs. These stack and cluster thruster designs provide a small footprint saving spacecraft surface area for solar panels and/or experiments. The simplicity of these thrusters will enable their broad use on micro-pico satellites for primary propulsion, ACS and formation flying applications. Larger spacecraft may find uses for ECESP thrusters on extended booms, on-orbit refueling, pneumatic actuators, and gas generators.

Sawka, Wayne N.↗
NASA NTRS2012

High Operating Temperature Midwave Quantum Dot Barrier Infrared Detector (QD-BIRD)

The nBn or XBn barrier infrared detector has the advantage of reduced dark current resulting from suppressed Shockley-Read-Hall (SRH) recombination and surface leakage. High performance detectors and focal plane arrays (FPAs) based on InAsSb absorber lattice matched to GaSb substrate, with a matching AlAsSb unipolar electron barrier, have been demonstrated. The band gap of lattice-matched InAsSb yields a detector cutoff wavelength of approximately 4.2 ??m when operating at ~150K. We report results on extending the cutoff wavelength of midwave barrier infrared detectors by incorporating self-assembled InSb quantum dots into the active area of the detector. Using this approach, we were able to extend the detector cutoff wavelength to ~6 ?m, allowing the coverage of the full midwave infrared (MWIR) transmission window. The quantum dot barrier infrared detector (QD-BIRD) shows infrared response at temperatures up to 225 K.

quantum dot↗
NASA NTRS2013

Azimuthal Spoke Propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E x B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E x B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Cylindrical Hall Thruster (CHT)↗
NASA NTRS2013

Azimuthal spoke propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E × B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E × B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Polk, James E.↗
NASA NTRS2003

A Study of the Non-Thermal X-Ray Emission of Shell-Type Supernova Remnants

We present an analysis of the X-ray spectrum of the Galactic shell-type SNR G347.3-0.5 (RX 51713.7-3946). This SNR is a member of a growing class of SNRs which are dynamically young, shell-type sources that emit non-thermal X-rays from specific regions on their outer shells. By performing a joint spectral analysis of data from observations made of G347.3-0.5 using the ROSAT PSPC, the ASCA GIS and the RXTE PCA, we have fit the spectra of particular regions of this SNR (including the bright northwestern and southwestern rims, the northeast rim and the interior diffuse emission) over the approximate energy range of 0.5 through 30 keV. We find that fits to the spectra of this SNR over this energy range using the SRCUT model were superior to a simple power law model or the SRESC model. We find that the inclusion of a thermal model with the SRCUT model helps to improve the fit to the observed X-ray spectrum: this represents the first detection of thermal X-ray emission from G347.3-0.5. Thermal emission appears to be more clearly associated with the diffuse emission in the interior of the SNR than with the bright X-ray emitting rims. A weak emission feature seen near 6.4 keV in the RXTE PCA spectrum most likely originates from diffuse X-ray emission from the surrounding Galactic Ridge rather than from G347.3-0.5 itself. We have analyzed our RXTE PCA data to search for pulsations from a recently discovered radio pulsar (PSR 51713-3949) which may be associated with G347.3-0.5, and we do not detect any X-ray pulsations at the measured radio period of 392 ms. Using the best-fit parameters obtained from the SRCUT model, we estimate the maximum energy of cosmic-ray electrons accelerated by the rims of G347.3-0.5 to be 19-25 TeV (assuming a magnetic field strength of B = 10muG), consistent with the results of Ellison et al. We present a broadband (radio to gamma ray) photon energy-flux spectrum for the northwestern rim of G347.3-0.5, where we have fit the spectrum using a more sophisticated synchrotron-inverse Compton model with a variable magnetic field strength. Our fit derived from this model yields a maximum energy of only 8.8 (+4.1) (-3.4) TeV for the accelerated cosmic-ray electrons and a much greater magnetic field strength of 150 (+250) (-80). Finally, we compare the gross properties of G347.3-0.5 with other SNRs known to possess X-ray spectra dominated by non-thermal emission.

Allen, Glenn E.↗
NASA NTRS2015

Electron Beam Irradiated Intercalated CNT Yarns For Aerospace Applications

Multi-walled CNT yarns have been experimentally and commercially created to yield lightweight, high conductivity fibers with good tensile properties for application as electrical wiring and multifunctional tendons. Multifunctional tendons are needed as the cable structures in tensegrity robots for use in planetary exploration. These lightweight robust tendons can provide mechanical strength for movement of the robot in addition to power distribution and data transmission. In aerospace vehicles, such as Orion, electrical wiring and harnessing mass can approach half of the avionics mass. Use of CNT yarns as electrical power and data cables could reduce mass of the wiring by thirty to seventy percent. These fibers have been intercalated with mixed halogens to increase their specific electrical conductivity to that near copper. This conductivity, combined with the superior strength and fatigue resistance makes it an attractive alternative to copper for wiring and multifunctional tendon applications. Electron beam irradiation has been shown to increase mechanical strength in pristine CNT fibers through increased cross-linking. Both pristine and intercalated CNT yarns have been irradiated using a 5-megavolt electron beam for various durations and the conductivities and tensile properties will be discussed. Structural information obtained using a field emission scanning electron microscope, energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy will correlate microstructural details with bulk properties.

electrical wiring↗
NASA NTRS2021

Day-to-Day Variability of Ionosphere Electron Density During Solar Minimum Derived From FORMOSAT-7/COSMIC-2 Measurements

This study examines the day-to-day variability of low-latitude ionosphere using global ionospheric specification (GIS) electron density profiles derived from FORMOSAT-7/COSMIC-2 radio occultation measurements during a deep solar minimum period of August 2019 to July 2020. The measurements reveal significant daily variations over dayside low latitudes, yielding about 10-20% standard deviation in equinoxes, 20-30% in solstices, reaching 40-50% in winter. The nighttime deviations could be 30-60%, being largest in solstices. Day-to-day variations are also observed in the longitudinal wave-4 structures. The period mostly remained geomagnetically quiet except for some moderate disturbances on a few days. Tidal decomposition of the GIS electron density shows that in-situ forced migrating diurnal (DW1) terdiurnal (TW3) oscillations and the background zonal mean yield only ~25% of the daily variations despite accounting for almost 75-90% of the observed electron density. Thus, forcing from lower atmosphere dominates the contribution (~75%) to the observed daily variations. Only about one third of this lower atmospheric forcing comes from the migrating semidiurnal SW2 and the usually investigated non-migrating diurnal eastward DE2, DE3, stationary planetary wave SPW3, SPW4, and semidiurnal eastward SE1, and SE2 components. The residual tides other than those mentioned above, including secondary waves through non-linear interactions and other planetary waves, thus significantly influence the day-to-day variations in electron density and modify the longitudinal wave structures.

day-to-day variability↗
NASA NTRS1997

Charge Transfer Between Ground-State Si(3+) and He at Electron-Volt Energies

The charge-transfer rate coefficient for the reaction Si(3+)(3s(sup 2)S) + He yields products is measured by means of a combined technique of laser ablation and ion storage. A cylindrical radio-frequency ion trap was used to store Si(3+) ions produced by laser ablation of solid silicon targets. The rate coefficient of the reaction was derived from the decay rate of the ion signal. The measured rate coefficient is 6.27(exp +0.68)(sub -0.52) x 10(exp -10)cu cm/s at T(sub equiv) = 3.9 x 10(exp 3)K. This value is about 30% higher than the Landau-Zener calculation of Butler and Dalgarno and is larger by about a factor of 3 than the recent full quantal calculation of Honvault et al.

Fang, Z.↗