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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

A K-band GaAs FET amplifier with 8.2-W output power

An 8.2-W GaAs FET amplifier with 38.6 + or - 0.5-dB gain over a 17.7-19.1-GHz frequency band has been developed. This amplifier combines the outputs of eight multistage amplifier modules utilizing a radial combiner. This state-of-the art power level has been achieved with AM/PM of less than 2 deg/dB. The third-order intermodulation products at 1-dB gain compression were 20 dBc, and variation in group delay over the frequency band was less than + or - 0.25 ns. Tests show that the amplifier is unconditionally stable and follows the graceful-degradation principle.

Goel, J.↗
NASA NTRS1986

Mantle viscosity beneath the Galapagos 95.5 deg W propagating rift

Detailed geophysical surveys in the vicinity of the Galapagos 95.5 deg W propagating rift tip establish the opening history of the rift and its velocity of propagation. These data together with a theory for mantle upwelling into slowly widening lithospheric cracks constrain the viscosity of the asthenosphere beneath the propagating rift to be less than about 10 to the 17th to 10 to the 18th Pa s.

Schubert, G.↗
NASA NTRS1986

Influence of process parameter variation on the reflectivity of sputter-deposited W-C multilayer diffraction gratings

Multilayer W-C diffraction gratings with nominal d spacings of 35 A have been fabricated by magnetron sputter deposition. The peak and integrated reflectivities of these films have been measured with Al K-alpha X-rays and compared to theoretical values. The rms surface roughness has been evaluated. The influence of several sputtering-system process parameters on the reflectivities has been investigated.

Sager, B.↗
NASA NTRS1986

The luminosities of the binary cepheids SU Cyg, SU Cas and W Sgr

Absolute magnitudes for binary classical Cepheids were derived by ratioing IUE low dispersion spectra in regions where the blue companion dominates with spectra of main sequence spectral type standards. The IUE spectra are also used to determine the magnitude difference between the Cepheid and the blue companion. Absolute magnitudes determined for SU Cyg and W Sgr agree with the absolute magnitudes of Sandage and Tammann and Caldwell within the + or - 0m2 estimated uncertainty, but not with the fainter absolute magnitudes of Schmidt. The absolute magnitude of SU Cas is in better agreement with the PLC relations if it is pulsating in the first overtone mode.

Evans, Nancy Remage↗
NASA NTRS1987

Design, fabrication and performance of small, graphite electrode, multistage depressed collectors with 200-W, CW, 8- to 18-GHz traveling-wave tubes

Small multistage depressed collectors (MDC's) which used pyrolytic graphite, ion-beam-textured pyrolytic graphite, and isotropic graphite electrodes were designed, fabricated, and evaluated in conjuntion with 200-W, continuous wave (CW), 8- to 18-GHz traveling-wave tubes (TWT's). The design, construction, and performance of the MDC's are described. The bakeout performance of the collectors, in terms of gas evolution, was indistinguishable from that of typical production tubes with copper collectors. However, preliminary results indicate that some additional radiofrequency (RF) and dc beam processing time (and/or longer or higher temperature bakeouts) may be needed beyond that of typical copper electrode collectors. This is particularly true for pyrolytic graphite electrodes and for TWT's without appendage ion pumps. Extended testing indicated good long-term stability of the textured pyrolytic graphite and isotropic graphite electrode surfaces. The isotropic graphite in particular showed considerable promise as an MDC electrode material because of its high purity, low cost, simple construction, potential for very compact overall size, and relatively low secondary electron emission yield characteristics in the as-machined state. However, considerably more testing experience is required before definitive conclusions on its suitability for electronic countermeasure systems and space TWT's can be made.

Ebihara, Ben T.↗
NASA NTRS1987

Hemispherical emissivity of V, Nb, Ta, Mo, and W from 300 to 1000 K

The hemispherical emissivities of five transition elements, V, Nb, Ta, Mo, and W, have been measured from 300 to 1000 K, complementing earlier higher-temperature results. These low-temperature data, which are similar, are fitted to a Drude model in which the room-temperature parameters have been obtained from optical measurements and the temperature dependence of the dc resistivity is used as input to calculate the temperature dependence of the emissivity. A frequency-dependent free-carrier relaxation rate is found to have a similar magnitude for all these elements. For temperatures larger than 1200 K the calculated emissivity is always greater than the measured value, indicating that the high-temperature interband features of transition elements are much weaker than those determined from room-temperature measurements.

Cheng, S. X.↗
NASA NTRS1988

Performance of a small, graphite electrode, multistage depressed collector with a 500-W, continuous wave, 4.8- to 9.6-GHz traveling wave tube

A small, isotropic graphite multistage depressed collector (MDC) and a short permanent magnet refocuser were designed, fabricated, and evaluated in conjunction with a 500-W, continuous-wave (CW), 4.8 to 9.6 GHz traveling wave tube (TWT). A novel performance optimization system and technique were used to optimize the TWT-MDC performance for saturated broad-band operation. The MDC performance was evaluated in both four- and three-stage configurations. Average TWT overall and four-stage collector efficiencies of 43.8 and 82.6 percent, respectively, were obtained for saturated octave-bandwidth operation. The isotropic graphite electrode material performed well, and shows considerable promise. However, considerably more test experience is required before definitive conclusions on its suitability for space and airborne TWT's can be made.

Ramins, Peter↗
NASA NTRS1987

Isotopic studies of Mg, Fe, Mo, Ru and W in Fremdlinge from Allende refractory inclusions

This paper presents the first isotopic measurements, made with an ion microprobe, of Mg, Fe, Mo, Ru, and W in Fremdlinge and refractory metal nuggets from Allende meteorite. No non-mass-dependent deviations exceeding 1 percent from normal solar isotopic abundances were found for any of the elements analyzed, despite the unusual mineralogy of Fremdlinge and the extreme enrichment in refractory siderophile elements. It is concluded that the chemical, petrologic, and isotopic data on Fremdlinge are most plausibly interpreted in terms of the origin of Fremdlinge within the solar nebula.

Hutcheon, Ian D.↗
NASA NTRS1987

The luminosities of the binary Cepheids SU Cyg, SU Cas, and W Sgr

Absolute magnitudes for binary classical Cepheids were derived by ratioing IUE low dispersion spectra in regions where the blue companion dominates with spectra of main sequence spectral type standards. The IUE spectra are also used to determine the magnitude difference between the Cepheid and the blue companion. Absolute magnitudes determined for SU Cyg and W Sgr agree with the absolute magnitudes of Sandage and Tammann and Caldwell within the + or - Om2 estimated uncertainty, but not with the fainter absolute magnitudes of Schmidt. The absolute magnitude of SU Cas is in better agreement with the PLC relations if it is pulsating in the first overtone mode.

Evans, Nancy Remage↗
NASA NTRS1988

Mapping of surface activity on the W UMa-type system VW Cephei

After multifilter photometry of the W UMa-type contact binary VW Cep (P = 6.67 hr; G5V + K0V) in 1986/87 revealed large asymmetries in the light curves believed to be caused by large, cool starspot regions on the surface of the larger stars, in Apr. 1987 it was observed with IUE to study the chromospheres and transition regions of the components. During one complete orbital cycle, three SWP and four LWP low dispersion spectra were obtained, including and then excluding the suspected active region. Phase dependent TR line emission strengths were found, most notably C IV, which is 50 percent stronger when the spot region is most visible. The results could be important because VW Cep represents an extreme case for studying stellar dynamo theory and observations can play a crucial role in the unterstanding of magnetic fields and activity cycles in rapidly rotating solar-like stars.

Bradstreet, David H.↗
NASA NTRS1989

Design, fabrication, and performance of brazed, graphite electrode, multistage depressed collectors with 500-W, continuous wave, 4.8- to 9.6-GHz traveling-wave tubes

A small, isotropic graphite electrode, multistage depressed collector (MDC) was designed, fabricated, and evaluated in conjunction with a 500-W, continuous wave (CW), 4.8- to 9.6-GHz traveling-wave tube (TWT). The carbon electrode surfaces were used to improve the TWT overall efficiency by minimizing the secondary electron emission losses in the MDC. The design and fabrication of the brazed graphite MDC assembly are described. The brazing technique, which used copper braze filler metal, is compatible with both vacuum and the more commonly available hydrogen atmosphere brazing furnaces. The TWT and graphite electrode MCC bakeout, processing, and outgassing characteristics were evaluated and found to be comparable to TWT's equipped with copper electrode MDC's. The TWT and MDC performance was optimized for broadband CW operation at saturation. The average radiofrequency (RF), overall, and MDC efficiencies were 14.9, 46.4, and 83.6 percent, respectively, across the octave operating band. A 1500-hr CW test, conducted without the use of an appendage ion pump, showed no gas buildup and excellent stability of the electrode surfaces.

Ramins, Peter↗
NASA NTRS1988

Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate - Application to the 95.5 deg W Galapagos propagator

Possible rigid plate models and shear models for the formation of curved seafloor lineaments by rift propagation are investigated and are applied to the Galapagos propagation rift system at 95 deg W. It is shown that the geometry noted at the Galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years. It is postulated that the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate. The data interpreted as requiring a shear zone are found to be equally consistent with two distinct models based on rigid plate tectonics.

Acton, Gary↗
NASA NTRS1989

Supersymmetric dark matter above the W mass

The cosmological consequences are studied for the minimal supersymmetric extension of the standard model in the case that the neutralino is heavier than W. The cross section was calculated for annihilation of heavy neutralinos into final states containing gauge and Higgs bosons (XX yields WW, ZZ, HH, HW, HZ), where X is the lightest, nth neutralino and the results are compared with the results with those previously obtained for annihilation into fermions to find the relic cosmological abundance for the most general neutralino. The new channels are particularly important for the Higgsino-like and mixed-state neutralinos, but are sub-dominant (to the fermion-antifermion annihilation channels) in the case that the neutralino is mostly a gaugino. The effect of the top quark mass is also considered. Using these cross sections and the cosmological constraint omega(sub X)h squared is less than or approximately 1, the entire range of cosmologically acceptable supersymmetric parameter space is mapped and a very general bound on the neutralino mass is discovered. For a top quark mass of less than 180 GeV, neutralinos heavier than 3200 GeV are cosmologically inconsistent, and if the top quark mass is less than 120 GeV, the bound is lowered to 2600 GeV. Neutralino states that are mostly gaugino are constrained to be lighter than 550 GeV. It is found that a heavy neutralino that contributes omega(sub X) is approximately 1 arises for a very wide range of model parameters and makes, therefore, a very natural and attractive dark matter candidate.

Griest, Kim↗
NASA NTRS1990

Spent-beam refocusing analysis and multistage depressed collector design for a 75-W, 59- to 64-GHz coupled-cavity traveling-wave tube

A computational design technique for coupled-cavity tubes (TWTs) equipped with spent-beam refocusers (SBRs) and multistage depressed collectors (MDCs) is described. A large-signal multidimensional computer program was used to analyze the TWT-SBR performance and to generate the spent-beam models used for MDC design. The results of a design involving a 75-W, 59 to 64 GHz TWT are presented. The SBR and MDC designs are shown, and the computed TWT, SBR, and MDC performances are described. Collector efficiencies in excess of 94 percent led to projected overall TWT efficiencies in the 40-percent range.

Wilson, Jeffrey D.↗
NASA NTRS1989

C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95 deg W - A laser decrepitation mass spectrometry study

Ratios of C, O, and H dissolved in silicate glass inclusions and pillow rind glasses in samples from the Galapagos Spreading Center near 95 deg W were analyzed (using laser volatilization and mass spectrometry), and the data were assessed in terms of mantle source compositions, oxygen fugacity, kinetic fractionation, and magmatic degassing. It was found that glass inclusions in the Galapagos failing rift lavas are higher and more variable in CO2/H2O (about 0.641) relative to inclusions in propagating rift lavas (about 0.245). This difference is considered to reflect different degrees of degassing during contrasting magmatic histories of the two regions.

Yonover, Robert N.↗
NASA NTRS1990

Conceptual design of a 0.1 W magnetic refrigerator for operation between 10 K and 2 K

The design of a magnetic refrigerator for space applications is discussed. The refrigerator is to operate in the temperature range of 10 K-2 K, at a 2 K cooling power of 0.10 W. As in other magnetic refrigerators operating in this temperature range GGG has been selected as the refrigerant. Crucial to the design of the magnetic refrigerator are the heat switches at both the hot and cold ends of the GGG pill. The 2 K heat switch utilizes a narrow He II filled gap. The 10 K heat switch is based on a narrow helium gas gap. For each switch, the helium in the gap is cycled by means of activated carbon pumps. The design concentrates on reducing the switching times of the pumps and the switches as a whole. A single stage system (one magnet; one refrigerant pill) is being developed. Continuous cooling requires the fully stationary system to have at least two stages running parallel/out of phase with each other. In order to conserve energy, it is intended to recycle the magnetic energy between the magnets. To this purpose, converter networks designed for superconducting magnetic energy storage are being studied.

Helvensteijn, Ben P. M.↗
NASA NTRS1992

Primary production estimates from recordings of solar-stimulated fluorescence in the equatorial Pacific at 150 deg W

Biological, optical, and hydrographical data were collected on the WEC88 cruise along 150 deg W and during a 6-day time-series station on the equator during February/March 1988. This area was characterized by a subsurface chlorophyll maximum (SCM), located at 50-70 m depth at the equator and descending down to 120-125 m at the north and south end of the transect. Highest primary production rates were near-surface and confined to the equatorial region and stations between 7 deg and 11 deg N. To determine the relationship between solar-stimulated fluorescence (centered at 683 nm wavelength) and primary production, a production-fluorescence model based on phytoplankton physiology and marine optics is described. Results of model calculations predict that there is a linear relation between production and fluorescence. A comparison between morning and midday measurements of the production-fluorescence relation showed that there was some difference between the two, whereas evening measurements, on the other hand, were distinctly different from the morning/midday ones. This seems to suggest that diurnal variations contribute significantly to variability in the quantum yield of photochemical processes. The ratio of the quantum yield of photosynthesis to the quantum yield of fluorescence ranged between 0.24 and 0.44 molC/Ein for all stations. The highest value for this ratio occurred at the equatorial stations, indicating that latitudinal variability could have an effect on the production-fluorescence relation.

Stegmann, P. M.↗
NASA NTRS1992

Estimating new production in the equatorial Pacific Ocean at 150 deg W

A major goal of the WEC88 cruise of the R/V Wecoma to the equatorial Pacific (made in February-March 1988) was to establish rates of new production along a meridional section at 150 deg W and to compare these measured rates with the relatively high values for the equatorial Pacific that had been reported previously using indirect methods and models. Production values were obtained from the traditional approach using N-15 labeled nitrate uptake, and by using C-14 fixation values multiplied by f (proportion of new production) from various sources: from N-15 data, from a C-14 fixation-versus-f relationship, or from a nitrate-versus-f relationship. The ratios of directly measured nitrate and carbon uptake and the ratios of nitrate to nitrate plus ammonium uptake, i.e., values of f, agree well; values of f calculated from carbon uptake or from nitrate concentration are overestimates for the equatorial upwelling region. Carbon-to-nitrogen uptake ratios measured with C-14 and N-15, respectively, approximate the Redfield molar ratio, 6.6 C:N. The overall mean value of f (0.17) helps confirm the view that the low primary production in the enriched eastern equatorial Pacific is due to failure of the nitrate-uptake system.

Dugdale, Richard C.↗