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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Raman scattering measurements in flames using a tunable KrF excimer laser

A narrow-band tunable KrF excimer laser is used as a spontaneous vibrational Raman scattering source to demonstrate that single-pulse concentration and temperature measurements, with only minimal fluorescence interference, are possible for all major species (O2, N2, H2O, and H2) at all stoichiometries (fuel-lean to fuel rich) of H2-air flames. Photon-statistics-limited precisions in these instantaneous and spatially resolved single-pulse measurements are typically 5 percent, which are based on the relative standard deviations of single-pulse probability distributions. In addition to the single-pulse N2 Stokes/anti-Stokes ratio temperature measurement technique, a time-averaged temperature measurement technique is presented that matches the N2 Stokes Raman spectrum to theoretical spectra by using a single intermediate state frequency to account for near-resonance enhancement. Raman flame spectra in CH4-air flames are presented that have good signal-to-noise characteristics and show promise for single-pulse UV Raman measurements in hydrocarbon flames.

Wehrmeyer, Joseph A.↗
NASA NTRS1990

The effect of molecular relaxation processes in air on the rise time of sonic booms

A theory is developed to explain the effect of molecular relaxation processes on the rise time of sonic booms. To determine the rise time of sonic booms, both O2 and N2 relaxation processes must be included. The N2 relaxation process delays the shock pressure reaching the maximum pressure, and the O2 relaxation process causes a shock profile to have a gentle slope. The N2 relaxation controls the lower part of overpressure; the O2 relaxation controls the higher part. The constant rise time curves show that the rise times increase as the overpressures and humidity decrease. The present approach gives longer rise times than those acquired by Bass et al. for given shock overpressures.

Kang, Jongmin↗
NASA NTRS1992

Measured rotational and vibrational temperature differences in arc jet shock layers

Shock-layer-radiation emission spectra from the first negative system of the nitrogen molecular ion N2(+) are analyzed over the wavelength region between 370 and 430 nm. The shock layer is produced by a blunt body placed in the expanded flow from an arc heater employing either pure nitrogen or a nitrogen-oxygen air mixture. A rotational temperature is obtained from analysis of (0,1) band emission from N2(+). Vibrational temperatures are extracted from population distributions obtained through spectral fitting of the N2(+) (B-X) band sequences. Rotational temperatures up to 9500 K near the front of the shocks are determined for different energy and flow conditions. The rotational temperatures are found to be lower than those obtained for vibration confirming previous measurements and contrary to expectation based on theory.

Blackwell, Harvel E.↗
NASA NTRS1991

Theoretical studies of important processes in planetary and comet atmospheres

This is the fifth semi-annual progress report describing research on dissociative recombination reactions in planetary and comet atmospheres. The Appendix has two papers that describe NASA supported research. Both papers have been recently accepted for publication. The first paper, 'The Generation of O(S-1) from the Dissociative Recombination of O2(+)', describes in detail the Multichannel Quantum Defect (MQDT) theory used for the calculation of dissociative recombination (DR) cross sections and rates. The application to the generation of the upper state of the atomic oxygen green line emission is of great importance to the modelling of planetary atmospheres. The second paper in the Appendix, 'Dissociative Recombination of the Ground State of N2(+)', applies the methods described in the first paper to N2(+). We find remarkable agreement with the prior microwave afterglow experiments for both the total recombination rate and for its electron temperature dependence. However, the results disagree with recent merged beams results which gave cross sections that are a factor of five below the microwave afterglow experiments and the current results. DR of N2(+) is an important mechanism for generating energetic N atoms which can escape the atmosphere of Mars. Currently we are also continuing additional work on the DR of O2(+) which is aimed at calculating both the total DR rate as a function of ion vibrational level and the rate for production of O(D-1).

Guberman, Steven L.↗
NASA NTRS1992

Imaging of four planetary nebulae in the Magellanic Clouds using the Hubble Space Telescope Faint Object Camera

Using the Faint Object Camera on-board the Hubble Space Telescope, we have obtained images of four planetary nebulae (PNe) in the Magellanic Clouds, namely N2 and N5 in the SMC and N66 and N201 in the LMC. Each nebula was imaged through two narrow-band filters isolating forbidden O III 5007 and H-beta, for a nominal exposure time of 1000 s in each filter. In forbidden O III, SMC N5 shows a circular ring structure, with a peak-to-peak diameter of 0.26 arcsec and a FWHM of 0.35 arcsec while SMC N2 shows an elliptical ring structure with a peak-to-peak diameter of 0.26 x 0.21. The expansion ages corresponding to the observed structures in SMC N2 and N5 are of the order of 3000 yr. LMC N201 is very compact, with a FWHM of 0.2 arcsec in H-beta. The Type I PN LMC N66 is a multipolar nebula, with the brightest part having an extent of about 2 arcsec and with fainter structures extending over 4 arcsec.

Blades, J. C.↗
NASA NTRS1992

The influence of thermal inertia on temperatures and frost stability on Triton

It is presently argued, in view of (1) a thermal inertia model for the surface of Triton which (like previous ones) predicts a monotonic recession of permanent N2 deposits toward the poles and very little seasonal N2 frost in the southern hemisphere, and (2) new spectroscopic evidence for nonvolatile CO2 on Triton's bright southern hemisphere, that much of that bright southern material is not N2. Such bright southern hemisphere volatiles may allow the formation of seasonal frosts, thereby helping to explain the observed spectroscopic changes of Triton during the last decade.

Spencer, John R.↗
NASA NTRS1993

Plasma-puff initiation of high Coulomb transfer switches

The plasma-puff triggering mechanism based on a hypocycloidal pinch geometry was investigated to determine the optimal operating conditions for an azimuthally uniform surface flashover which initiates plasma-puff under wide ranges of fill gas pressures of Ar, He and N2. The optimal fill gas pressures for the azimuthally uniform plasma-puff were about 120 mTorr less than P(opt) less than 450 Torr for He and N2. For Argon 120 mTorr less than P(opt) less than 5 Torr for argon. The inverse pinch switch was triggered with the plasma-puff and the switching capability under various electrical parameters and working gas pressures of Ar, He and N2 was determined. It was also shown that the azimuthally uniform switching discharges were dependent on the type of fill gas and its fill pressure. A new concept of plasma-focus driven plasma-puff was also discussed in comparison with hypocycloidal pinch plasma-puff triggering. The main discharge of the inverse pinch switch with the plasma-focus driven plasma-puff trigger is found to be more azimuthally uniform than that with the hypocycloidal pinch plasma-puff trigger in a gas pressure region between 80 mTorr and 1 Torr. In order to assess the effects of plasma current density on material erosion of electrodes, emissions from both an inverse-pinch plasma switch (INPIStron) and from a spark gap switch under test were studied with an optical multichannel analyzer (OMA). The color temperature of the argon plasma was approximately 4,000 K which corresponded with the peak continuum emission near 750 nm. There are the strong line emissions of argon in the 650 - 800 nm range and a lack of line emissions of copper and other solid material used in the switch. This indicates that the plasma current density during closing is low and the hot spot or hot filament in the switch is negligible. This result also indicates considerable reduction of line emission with the INPIStron switch over that of a spark-gap switch. However, a strong carbon line emission exists due to vaporization of the plastic insulator used. In order to reduce the vaporization of the insulator, the plexiglass insulating material of INPIStron was replaced with Z-9 material. A comparative study of the INPIStron and a spark gap also reveals that the INPIStron, with a low impedance of Z equals 9 ohms, can transfer a high voltage pulse with a superior pulse-shape fidelity over that of a spark gap with Z equals 100 ohms.

Venable, D. D.↗
NASA NTRS1993

Plasma-puff initiation of high Coulomb transfer switches

The plasma-puff triggering mechanism based on a hypocycloidal pinch geometry was investigated to determine the optimal operating conditions for an azimuthally uniform surface flashover which initiates plasma-puff under wide ranges of fill gas pressures of Ar, He and N2. The optimal fill gas pressures for the azimuthally uniform plasma-puff were about 120 mTorr less than P(sub opt) less than 450 Torr for He and N2. For Argon 20 mTorr is less than P(sub opt) is less than 5 Torr. The inverse pinch switch was triggered with the plasma-puff and the switching capability under various electrical parameters and working gas pressures of Ar, He and N2 was determined. It was also shown that the azimuthally uniform switching discharges were dependent on the type of fill gas and its fill pressure. A new concept of plasma-focus driven plasma-puff was also discussed in comparison with hypocycloidal pinch plasma-puff triggering. The main discharge of the inverse pinch switch with the plasma-focus driven plasma-puff trigger is found to be more azimuthally uniform than that with the hypocycloidal pinch plasma-puff trigger in a gas pressure region between 80 mTorr and 1 Torr. In order to assess the effects of plasma current density on material erosion of electrodes, emissions from both an inverse-pinch plasma switch (INPIStron) and from a spark gap switch under test were studied with an optical multichannel analyzer (OMA). The color temperature of the argon plasma was approximately 4,000 K which corresponded with the peak continuum emission near 750 nm. There are the strong line emissions of argon in the 650 - 800 nm range and a lack of line emissions of copper and other solid material used in the switch. This indicates that the plasma current density during closing is low and the hot spot or hot filament in the switch is negligible. This result also indicates considerable reduction of line emission with the INPIStron switch over that of a spark-gap switch. However, a strong carbon line emission exists due to vaporization of the plastic insulator used. In order to reduce the vaporization of the insulator, the plexiglass insulating material of INPIStron was replaced with Z-9 material. A comparative study of the INPIStron and a spark gap also reveals that the INPIStron, with a low impedance of Z = 9 ohms, can transfer a high voltage pulse with a superior pulse-shape fidelity over that of a spark gap with Z = 100 ohms.

Han, Kwang S.↗
NASA NTRS1993

Characteristics of plasma-puff trigger for a inverse-pinch plasma switch

The plasma-puff triggering mechanism based on a hypocycloidal pinch geometry was investigated to determine the optimal operating conditions for the azimuthally uniform surface flashover which initiates plasma-puff under wide ranges of fill gas pressure of Ar, He and N2. The optimal fill-gas pressure range for the azimuthally uniform plasma-puff was about 120 mTorr less than or equal to P(sub op) less than or equal to 450 Torr for He and N2. For Argon 120 mTorr is less than or equal to P(sub op) is less than or equal to 5 Torr. The inverse-pinch switch was triggered with the plasma-puff and the switching capability under various electrical parameters and working gas pressures of Ar, He and N2 was determined. The azimuthally uniform switching discharges were dependent on the type of fill gas and its fill pressure. A new concept of plasma-focus driven plasma-puff will be discussed in comparison with the current hypocycloidal-pinch plasma-puff triggering.

Choi, Eun H.↗
NASA NTRS1994

Formation and chemical reactivity of carbon fibers prepared by defluorination of graphite fluoride

Defluorination of graphite fluoride (CFX) by heating to temperatures of 250 to 450 C in chemically reactive environments was studied. This is a new and possibly inexpensive process to produce new carbon-based materials. For example, CF 0.68 fibers, made from P-100 carbon fibers, can be defluorinated in BrH2C-CH = CH-CH2Br (1,4-dibromo-2butene) heated to 370 C, and graphitized to produce fibers with an unusually high modulus and a graphite layer structure that is healed and cross-linked. Conversely, a sulfur-doped, visibly soft carbon fiber was produced by defluorinating CF 0.9 fibers, made from P-25, in sulfur (S) vapor at 370 C and then heating to 660 C in nitrogen (N2). Furthermore, defluorination of the CF 0.68 fibers in bromine (Br2) produced fragile, structurally damaged carbon fibers. Heating these fragile fibers to 1100 C in N2 caused further structural damage, whereas heating to 150 C in bromoform (CHBr3) and then to 1100 C in N2 healed the structural defects. The defluorination product of CFX, tentatively called activated graphite, has the composition and molecular structure of graphite, but is chemically more reactive. Activated graphite is a scavenger of manganese (Mn), and can be intercalated with magnesium (Mg). Also, it can easily collect large amounts of an alloy made from copper (Cu) and type 304 stainless steel to form a composite. Finally, there are indications that activated graphite can wet metals or ceramics, thereby forming stronger composites with them than the pristine carbon fibers can form.

Hung, Ching-Cheh↗
NASA NTRS1994

The temperature of nitrogen on Pluto

With Hapke scattering theory and absorption coefficients derived from our laboratory measurements of solid N2 we have modeled the spectrum of Triton. By comparing a Hapke scattering model to the measured spectrum from Triton, we determined the temperature of the N2 on the satellite's surface to be 38 (+2, -1) K which is in accord with the measurements of Voyager 2. Applying this technique to Pluto we find that the temperature of N2 on that body is 40 +/- 2 K. Other aspects of this investigation are discussed.

Tryka, K. A.↗
NASA NTRS1993

Reevaluation of the O(+)(2P) reaction rate coefficients derived from Atmosphere Explorer C observations

O(+)(2P) is an important species for studies of the ionosphere and thermosphere: its emission at 7320 A can be used as a diagnostic of the thermospheric atomic oxygen density. Unfortunately, there are no laboratory measurements of the O and N2 reaction rates which are needed to determine the major sinks of (O+)(2p). We have recalculated the O and N2 reaction rates for O(+) (2P) using recent improvements in the solar EUV flux, cross sections, and photoelectron fluxes. For the standard solar EUV flux, the new N2 reaction rate of 3.4 +/- 1.5 x 10 exp -10 cu cm/s is close to the value obtained by Rusch et al. (1977), but the new O reaction rate of 4.0 +/- 1.9 x 10 exp -10 cu cm/sec is about 8 times larger. These new reaction rates are derived using neutral densities, electron density, and solar EUV fluxes measured by Atmosphere Explorer C in 1974 during solar minimum. The new theoretical emission rates are in good agreement with the data for the two orbits studied by Rusch et al.

Chang, T.↗
NASA NTRS1993

Wave-like perturbations observed at low altitudes by the Pioneer Venus Orbiter Neutral Mass Spectrometer during orbiter entry

Wave-like perturbations have been observed in the nightside neutral density data acquired for He, N, O, N2 and CO2 by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) during entry in late 1992. The data cover an altitude range of 133-200 km from 0.5-4.5 hours local solar time and occur at medium solar activity (F(sub 10.7) = 120). The perturbations, with an effective wavelength along the orbit of about 100 to 600 km, have similar amplitudes for the various species and helium is out of phase with respect to the heavier mass species. The measurements are comparable to those observed in 1978-80 at solar maximum activity (F(sub 10.7) = 200) above 145 km. Between 133 and 160 km the rms amplitudes grow with altitude at a rate of about (1.6, 2.1, 2.7, 4.4) x 10(exp 3)/km for N, O, N2, and CO2, respectively. The average rms amplitudes above 145 km of 0.08 for N and O and 0.1 for N2 are comparable in magnitude to those observed in the earlier 1978-80 data of 0.06, 0.08, and 0.095 respectively. CO2 is an exception for which the entry value is 0.17 compared to 0.09 earlier. By combining the two overlapping data sets there is a suggestion that the CO2 amplitudes grow in value with altitude up to about 140-170 km and then decrease in amplitude. Like the earlier data, the entry data are consistent with the interpretation that the neutral density perturbations are due to gravity waves propagating upward from the lower thermosphere.

Kasprzak, W. T.↗
NASA NTRS1994

Middle- and low-latitude emissions from energetic neutral atom precipitation seen from ATLAS 1 under quiet magnetic conditions

During the ATLAS 1 mission spectral observations were made at middle and low latitudes of features expected from the precipitation of energetic neutral atoms. The Imaging Spectrometric Observatory was used at night in the UV and visible with maximum gain. The tangent ray heights of the look directions ranged from near 100 km to near 200 km, and the geomagnetic conditions were quiet during the observations, which were made March 28 to April 3, 1992. The N2(+) 1N 391.4-nm and O I 130.4 and 135.6-nm emissions were observed at all latitudes, with lower emission rates at lower magnetic dip latitudes, except that enhancements in the O I lines were seen within 30 deg of the dip equator to radiative recombination of ionospheric plasma. The latitude profile observed for the N2(+) 1N emission did not show an equatorial or midlatitude peak. This implies that the source of energetic neutrals is more consistent with prompt charge exchange loss of freshly injected trapped ions with relatively low mirror heights (i.e., ions on higher L shells with equatorial pitch angle distributions nearly isotropic to the loss cone) than loss of highly eroded populations of particles with high mirror heights (i.e., ions on lower L shells with pancake equatorial pitch angle distributions). The N2(+) 1N emission rates have been compared with models of atmospheric emission due to fluxes of O/O(+) and H/H(+) in the thermosphere, as produced by energetic neutral oxygen or hydrogen atom precipitation. Energy deposition rates are inferred.

Tinsley, B. A.↗
NASA NTRS1994

Electron transport and energy degradation in the ionosphere: Evaluation of the numerical solution, comparison with laboratory experiments and auroral observations

Auroral electron transport calculations are a critical part of auroral models. We evaluate a numerical solution to the transport and energy degradation problem. The numerical solution is verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N2(+) first negative emissions. Our numerical solutions agree with range measurements in collision chambers. The calculated N(2)2P to N2(+)1N emission ratio is independent of the spectral characteristics of the incident electrons, and agrees with the value observed in aurora. Using different sets of energy loss cross sections and different functions to describe the energy distribution of secondary electrons that emerge from ionization collisions, we discuss the uncertainties of the solutions to the electron transport equation resulting from the uncertainties of these input parameters.

Lummerzheim, D.↗
NASA NTRS1995

Sensitivity of the 6300 A twilight airglow to neutral composition

The field line interhemispheric plasma (FLIP) model is used to study the 6300 A line intensity measured during three morning twilights from the McDonald Observatory in Texas. The Imaging Spectrometric Observatory (ISO) measured the 6300 A intensity during the winter of 1987 and the spring and summer of 1988. The FLIP model reproduces the measured intensity and its variation through the twilight well on each day using neutral densities from the MSIS-86 empirical model. This is in spite of the fact that different component sources dominate the integrated volume emission rate on each of the days analyzed. The sensitivity of the intensity to neutral composition is computed by varying the N2, O2, and O densities in the FLIP model and comparing to the intensity computed with the unmodified MSIS-86 densities. The ion densities change self-consistently. Thus the change in neutral composition also changes the electron density. The F2 peak height is unchanged in the model runs for a given day. The intensity changes near 100 deg SZA are comparable to within 10% when either (O2), (N2), or (O) is changed, regardless of which component source is dominant. There is strong sensitivity to changes in (N2) when dissociative recombination is dominant, virtually no change in the nighttime (SZA greater than or equal to 108 deg) intensity with (O2) doubled, and sensitivity of over 50% to doubling or halving (O) at night. When excitation by conjugate photoelectrons is the dominant nighttime component source, the relative intensity change with (O) doubled or halved is very small. This study shows the strong need for simultaneous measurements of electron density and of emissions proportional to photoelectron fluxes if the 6300 A twilight airglow is to be used to retrieve neutral densities.

Melendez-Alvira, D. J.↗
NASA NTRS1995

Thermal Stability of Hi-Nicalon SiC Fiber in Nitrogen and Silicon Environments

The room temperature tensile strength of uncoated and two types of pyrolytic boron nitride coated (PBN and Si-rich PBN) Hi-Nicalon SiC fibers was determined after 1 to 400 hr heat treatments to 1800 C under N2 pressures of 0.1, 2, and 4 MPa, and under 0.1 Mpa argon and vacuum environments. In addition, strength stability of both uncoated and coated fibers embedded in silicon powder and exposed to 0.1 MPa N2 for 24 hrs at temperatures to 1400 C was investigated. The uncoated and both types of BN coated fibers exposed to N2 for 1 hr showed noticeable strength degradation above 1400 C and 1600 C, respectively. The strength degradation appeared independent of nitrogen pressure, time of heat treatment, and surface coatings. TEM microstructural analysis suggests that flaws created due to SiC grain growth are responsible for the strength degradation. In contact with silicon powder, the uncoated and both types of PBN coated fibers degrade rapidly above 1350 C.

Bhatt, R. T.↗
NASA NTRS1995

High Modulus and High T(sub g) Thermally Stable Polymers from p-Ethynyl-Terminated Rigid-Rod Monomers

Novel p-ethynyl-substituted rigid rod monomers were studied by pressure Differential Scanning Calorimetry (DSC), ThermoGravimetric Analysis (TGA), IsothermoGravimetric Analysis (IGA), and TGA-IR. These monomers, 4,4'-bis(((4-ethynylphenyl)carbonyl)oxy)biphenyl, designated 1, 1,5-bis-(((4-ethynylphenyl)carbonyl)oxy)naphthalene, designated 2, and bis(4-ethynylphenyl)pyromellitimide, designated 3, were polymerized in the solid state. Thermal polymerization in N2 or air produced highly cross-linked resins with polymerization exotherms centered between 212 and 276 C. The (delta)H's of polymerization of these resins in air were found to be double those in N2. When monomers 1 and 2 were heated in air from 23 to 750 C at 10 C/min, the main decomposition product was carbon dioxide, evolving at a maximum rate between 500 and 600 C; water was also detected as a decomposition product. Void-free neat resin moldings, designated lp to 3p, were made by compression molding the monomers and then heating them. The resulting polymers were highly cross-linked, and their glass transition temperatures (T(sub g)) were much higher than their polymerization temperatures. Using ThermoMechanical Analysis (TMA), we found that polymers lp to 3p had T(sub g)'s of 422, 329, and 380 C, respectively. The thermal and thermooxidative stabilities improved when lp to 3p were postcured in N2 (the postcured polymers were designated 1pp to 3pp). The Linear Thermal Expansions (LTE) for lp and 3p were 1% between 23 and 420 C. Using rheological analysis, we could not clearly detect the T(sub g)'s of lp to 3p because their moduli dropped only slightly between 23 and 490 C and the changes in tan beta were very low. Because of high cross-link density, their moduli changed little as the resins went from a glassy to a rubbery state. Their shear storage moduli in air ranged from 0.82 (3p, 3pp) to 1.6 GPa (lpp) at 23 C, from 0.16 (2p) to 0.7GPa(lpp) at 380 C, and from 0.18(lpp)to 0.6GPa(2p) at 490 C. Finally, these novel-p-ethynyl polymers exhibited an excellent combination of high T(sub g), low LTE, and high thermooxidative stability. Most notably, 3pp lost only 3% of its initial weight when it was aged for 500 h at 288 C in air.

Melissaris, Anastasios P.↗