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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Microanalysis of extended-test xenon hollow cathodes

Four hollow cathode electron sources were analyzed via boroscopy, scanning electron microscopy, energy dispersive x ray analysis, and x ray diffraction analysis. These techniques were used to develop a preliminary understanding of the chemistry of the devices that arise from contamination due to inadequate feed-system integrity and improper insert activation. Two hollow cathodes were operated in an ion thruster simulator at an emission current of 23.0 A for approximately 500 hrs. The two tests differed in propellant-feed systems, discharge power supplies, and activation procedures. Tungsten deposition and barium tungstate formation on the internal cathode surfaces occurred during the first test, which were believed to result from oxygen contamination of the propellant feed-system. Consequently, the test facility was upgraded to reduce contamination, and the test was repeated. The second hollow cathode was found to have experienced significantly less tungsten deposition. A second pair of cathodes examined were the discharge and the neutralizer hollow cathodes used in a life-test of a 30-cm ring-cusp ion thruster at a 5.5 kW power level. The cathodes' test history was documented and the post-test microanalyses are described. The most significant change resulting from the life-test was substantial tungsten deposition on the internal cathode surfaces, as well as removal of material from the insert surface. In addition, barium tungstate and molybdate were found on insert surfaces. As a result of the cathode examinations, procedures and approaches were proposed for improved discharge ignition and cathode longevity.

Verhey, Timothy R.↗
NASA NTRS1990

Xenon fractionation in porous planetesimals

The distinctively fractionated Xe on Mars and earth may have its root in a common source from which both planets accreted. Beginning with Ozima and Nakazawa's (1980) hypothesis that terrestrial Xe fractionation was caused by gravitational separation of adsorbed solar nebular gases inside large porous planetesimals, it is pointed out that Xe would have been trapped as the planetesimal grew and pores were squeezed shut by lithostatic pressure. It is shown that enough fractionated Xe to supply the earth could have been trapped this way. The degree of fractionation is controlled by the lithostatic pressure at the pore-closing front and so would have been roughly the same for all large planetesimals. The predicted degree of fractionation agrees well with that preserved in terrestrial and Martian Xe. Relative to Xe, this source is strongly depleted in other noble gases. In contrast to the original Ozima and Nakazawa hypothesis, the present hypothesis predicts the observed fractionation, and it allows planetary accretion to occur after the dissipation of the solar nebula.

Zahnle, Kevin↗
NASA NTRS1991

Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

INAA, noble gas, and petrographic studies conducted on samples of 18 chondrules and matric material from the Chainpur (LL3) indicate that the I-129/I-127 ratio, R(0), varies by a factor of more than 10 among the chondrules. This corresponds to a greater-than-50 Ma span in apparent I-Xe ages. Models which invoke either gas-dust mixing or nebular heterogeneity cannot satisfactorily explain these data, any more than can hypotheses which attribute the variations to differences in formation age, metamorphic rate, or time of aqueous alteration. It is alternatively suggested that the variations represent periods of low-grade shock events.

Swindle, T. D.↗
NASA NTRS1991

A 5,000 hour xenon hollow cathode life test

A cathode life test voluntary terminated after 5024 hours of operation at 25 A is reported. The cathode including a 6.35-mm diameter by 57.12-mm long molybdenum tube with a nominal wall thickness of 0.38 mm is described along with a test facility and start-up procedure. It is shown that over the time of the experiment, the cathode-orifice diameter eroded from 1.80 mm to 2.08 mm, which corresponds to a ratio of the emission current to the orifice diameter at the end of the test of 12.0 A/mm. Tungsten deposits on the interior surface of the insert are detected in post-test analyses, and complete depletion of the original impregnate is suggested by X-ray diffraction analyses. A cathode-jet phenomenon in which energetic ions are produced during hollow-cathode operation at emission currents above 20 A is confirmed.

Brophy, John R.↗
NASA NTRS1991

Microanalyses of extended-test xenon hollow cathodes

Four hollow cathodes have been examined using boroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction analysis to characterize the life-limiting phenomena that resulted from their extended operation. The first pair of cathodes was operated for approximately 500 h in an ion thruster simulator at a 5.0 kW-equivalent power level using different cathode activation and propellant feed-system procedures. The second pair of cathodes was the main discharge cathode and neutralizer cathode from an ion thruster which have completed an 89-hour life-test at a 5.5 kW power level. The greatest change to the cathode condition was found to be removal of refractory metals, primarily tungsten, and their subsequent deposition on other cathode surfaces. Barium tungstate and barium molybdate were also found in all four cathodes. Criteria for improved cathode longevity and discharge operation are proposed for use in hollow cathode and full thruster testing facilities.

Verhey, Timothy R.↗
NASA NTRS1991

Iodine-xenon, chemical, and petrographic studies of Semarkona chondrules - Evidence for the timing of aqueous alteration

The relationship of the I-Xe system of the Semarkona meteorite to other measured properties is investigated via INAA, petrographic, and noble-gas analyses on 17 chondrules from the meteorite. A range of not less than 10 Ma in apparent I-Xe ages is observed. The three latest apparent ages fall in a cluster, suggesting the possibility of a common event. It is argued that the initial I-129/I-127 ratio (R0) is related to chondrule type and/or mineralogy, with nonporphyritic and pyroxene-rich chondrules showing evidence for lower R0s than porphyritic and olivine-rich chondrules. Chondrules with sulfides on or near the surface have lower R0s than other chondrules. The He-129/Xe-132 ratio in the trapped Xe component anticorrelates with R0, consistent with the evolution of a chronometer in a closed system or in multiple systems. It is concluded that the variations in R0 represent variations in ages, and that later events, possibly aqueous alteration, preferentially affected chondrules with nonporphyritic textures and/or sulfide-rich exteriors about 10 Ma after the formation of the chondrules.

Swindle, T. D.↗
NASA NTRS1992

Solar-type xenon - Isotopic abundances in Pesyanoe

Elemental and isotopic abundances of Ar and Xe were measured in three grain-size separates in the dark phase of the enstatite achondrite Pesyanoe by stepwise heating, using a combination of pyrolysis and combustion steps. The data reveal a low-temperature gas fraction with Ar/Xe ratios close to the solar ratio and isotopically similar to solar-type Ar and Xe observed in lunar samples. The gas released at intermediate temperature steps shows that Ar and Xe are isotopically fractionated, compared to the low-temperature components. Pesyanoe does not have excess Ar-40 and fission Xe, which means its parent body was too small for ion reimplantation. It is concluded that solar-type Xe isotopic abundances can be inferred from the low-temperature component. A comparison of Pesyanoe and lunar data shows that isotopic signatures of solar-wind Xe, as sampled at two different points in solar system space and time, are identical within experimental error.

Kim, J. S.↗
NASA NTRS1992

A comparison of solar wind and estimated solar system xenon abundances - A test for solid/gas fractionation in the solar nebula

The solar Xe elemental abundance is determined here using solar wind measurements from lunar ilmenites which are normalized to Si by spacecraft data. The results are compared with estimated abundances assuming no fractionation. When corrected for solar wind/photospheric fractionation, the Xe-130 abundance given by surface layer oxidation of ilmenite from solid 71501 exposed within the last 200 m.y. is 0.24 +/- 0.09 normalized to Si = 10 exp 6. This is indistinguishable from estimates made assuming no solid/gas fractionation. Results from breccia 79035 ilmenite exposed at least 1 Gyr ago indicate that the solar wind Xe flux may have been significantly higher relative to other noble gases, perhaps due to more efficient Xe ionization. If this is true, fluxes of C and S, which have first ionization potentials similar to Xe, should also be higher in the ancient solar wind from the same time period.

Wiens, Roger C.↗
NASA NTRS1993

Mass Spectrometric Measurement of Martian Krypton and Xenon Isotopic Abundance

The Viking gas chromatograph mass spectrometer experiment provided significant data on the atmospheric composition at the surface of Mars, including measurements of several isotope ratios. However, the limited dynamic range of this mass spectrometer resulted in marginal measurements for the important Kr and Xe isotopic abundance. The Xe-129 to Xe-132 ratio was measured with an uncertainty of 70%, but none of the other isotope ratios for these species were obtained. Accurate measurement of the Xe and Kr isotopic abundance in this atmosphere provides an important data point in testing theories of planetary formation and atmospheric evolution. The measurement is also essential for a stringent test for the Martian origin of the SNC meteorites, since the Kr and Xe fractionation pattern seen in gas trapped in glassy nodules of an SNC (EETA 79001) is unlike any other known solar system resevoir. Current flight mass spectrometer designs combined with the new technology of a high-performance vacuum pumping system show promise for a substantial increase in gas throughput and the dynamic range required to accurately measure these trace species. Various aspects of this new technology are discussed.

Mahaffy, P.↗
NASA NTRS1992

Xenon critical velocity releases from the ACTIVNY satellite - Discussion of attempted optical observations

The ACTIVNY satellite launched on Sept. 30, 1989 was a platform for a suite of active plasma experiments, including releases of Xe gas to test Alfven's critical velocity effect. Three releases were done over Alaska and three over Utah, all fully illuminated by sunlight. We attempted optical observations from ground stations in the dark, using various low light level imagers and spectrographs, with narrow bandpass filters on XeI and XeII lines and also in white light. Viewing conditions were very good on several releases, the satellite could tracked easily, but we saw no optical evidence of the releases from any wavelength. We discuss the reasons why optical diagnostics in the visible and near infrared failed to detect emissions from the releases.

Wescott, E. M.↗
NASA NTRS1992

Trapped xenon in ordinary chondrites

A trapped component of heavy noble gates representing a distinct solar system (presumably asteroid belt) reservoir is known to be concentrated in carbonaceous carriers of chondritic meteorites. The isotopic signature of trapped Xe in separates of the H4 chondrite Forest Vale (FV) was determined by combusting its carrier phases at 600 C in oxygen, discriminating against in situ produced nucleogenic components which are released above 600 C. The isotopic abundances of FV combustion-Xe (FVC-Xe) are compared to signatures of bulk trapped Xe in chondritic meteorites. We conclude that FVC-Xe represents the predominant trapped component in ordinary chondrites (OC) for which we adopt the term OC-Xe. Its isotopic signature differs from Xe in ureilites, in 'average carbonaceous chondrites', in earth's atmosphere, and in the solar wind. Additional minor Xe components were identified in type 3 chondrites and in the metal phase of chondrites. We discuss relationships among solar system Xe reservoirs and show that OC-Xe signature is consistent with a mixture of HL-Xe with slightly mass fractionated solar-type Xe.

Lavielle, Bernard↗
NASA NTRS1993

Isotopic signatures and distribution of nitrogen and trapped and radiogenic xenon in the Acapulco and FRO90011 meteorites

Acapulco metal and silicate show distinct N isotopic signatures. Trapped heavy noble gases are carried by 'magnetic' opx and radiogenic Xe-129 excesses are observed in phosphate and in minor surficial phases on metal grains. N and Xe isotopic signatures in FRO90011 do not agree with those observed in Acapulco. The Acapulco meteorite is unique in having achondritic texture and chondritic composition. Its mineralogical study shows the record of high temperature (1100 C) recrystallization. However, this meteorite shows abundances of volatile elements close to the levels observed in carbonaceous chondrites and concentrations of heavy noble gases comparable to those observed in type 4 ordinary chondrites, not expected for a presumed highly equilibrated object. Nitrogen measurements in bulk Acapulco revealed two different isotopic signatures, in apparent conflict with evidence for a high degree of recrystallization. N and Xe were studied in separated mineral phases to search for the carriers in order to better understand the formation and thermal history of the Acapulco parent body.

Kim, Y.↗
NASA NTRS1994

Recycle Requirements for NASA's 30 cm Xenon Ion Thruster

Electrical breakdowns have been observed during ion thruster operation. These breakdowns, or arcs, can be caused by several conditions. In flight systems, the power processing unit must be designed to handle these faults autonomously. This has a strong impact on power processor requirements and must be understood fully for the power processing unit being designed for the NASA Solar Electric Propulsion Technology Application Readiness program. In this study, fault conditions were investigated using a NASA 30 cm ion thruster and a power console. Power processing unit output specifications were defined based on the breakdown phenomena identified and characterized.

Pinero, Luis R.↗
NASA NTRS1994

Nitrogen and xenon isotopic disequilibrium in Bachmut (L6) chondrule J2689 and matrix

Chondrule J2689, a large (8-mm) RP chondrule from Bachmut (L6) was previously found to be in disequilibrium with its host in a variety of features: (1) It has a fine-grained hornfelsic texture; (2) it contains low amounts of metal with a low-Ni taenite composition not found in the matrix; (3) the Ni/Co ratio of that metal is close to the solar ratio, which is equal to the bulk Ni/Co ratio (23) of the chondrule; (4) the bulk alkali content of the chondrule is high and the Na/K ratio is fractionated with respect to the average L chondrite ratio of 8; (5) Cr is depleted in spite of the high pyroxene content; (6) the siderophile elements are strongly depleted but are fractionated with their abundances increasing with volatility; and (7) the O isotopes of the chondrule and the host are out of equilibrium. However, the (Fe,Mg) silicates, feldspar, and chromite have chemical compositions indistinguishable from that of the host chondrite. We have studied a chip of the chondrite and of adjacent matrix by stepwise heating and by combustion in O for N and Xe isotopic abundances. (1) The chondrule preserved distinct bulk, metal, and O isotopic compositional features. It is therefore unlikely that the 'equilibration' of the major silicates Fe/Mg ratios could have taken place after accretion; (2) the chondrule was well equilibrated before break-up and exposure to cosmic rays; (3) two N signatures in the matrix also indicate that the matrix is not equilibrated; and (4) all data collected so far point toward the presence of unequilibrated Bachmut components. Very few reactions took place after accretion.

Kim, Y.↗
NASA NTRS1998

Investigation of laser polarized xenon magnetic resonance

Ground-based investigations of a new biomedical diagnostic technology: nuclear magnetic resonance of laser polarized noble gas are addressed. The specific research tasks discussed are: (1) Development of a large-scale noble gas polarization system; (2) biomedical investigations using laser polarized noble gas in conventional (high magnetic field) NMR systems; and (3) the development and application of a low magnetic field system for laser polarized noble gas NMR.

Walsworth, Ronald L.↗
NASA NTRS1998

Destructive Evaluation of a Xenon Hollow Cathode after a 28,000 Hour Life Test

International Space Station (ISS) plasma contactor system requires a hollow cathode assembly (HCA) with a lifetime of at least 18,000 hours. In order to demonstrate the lifetime capability of the HCA, a series of hollow cathode wear tests was performed which included a life test operated at the maximum current of the HCA. This test sought to verify hollow cathode lifetime capability and contamination control protocols. This hollow cathode accumulated 27,800 hours of operation before it failed during a restart attempt. The cathode was subsequently destructively analyzed in order to determine the failure mechanism. Microscopic examination of the cathode interior determined that relatively small changes in the cathode physical geometry had occurred and barium tungstates, which are known to limit the emission process, had formed over a majority of the electron emitter surface. Because the final state of the insert was consistent with expected impregnate chemistry, the hollow cathode was believed to have reached the end of its usable life under the test conditions.

Sarver-Verhey, Timothy R.↗
NASA NTRS1999

Studies of Light and Charge Produced by Alpha-Particles in High-Pressure Xenon

The results of measurements of scintillation light induced by alpha-particles in high-pressure Xe are presented. The ratio of N(sub ex)/N(sub i) and the intensity and time characteristics of the luminescence signal have been measured for different densities up to 0.74 g/cc. The role and mechanisms of the electron-ion recombination in high-pressure gas have been investigated. The existence of clusters and self-trapped excitons above 0.6 g/cc has been observed and discussed. This work is a part of a development program of high-pressure Xe detectors for low-energy gamma rays.

Bolotnikov, Aleksey↗