2,151 research outputs found
Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster
NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation
The band structure of BeTe - a combined experimental and theoretical study
Using angle-resolved synchrotron-radiation photoemission spectroscopy we have
determined the dispersion of the valence bands of BeTe(100) along ,
i.e. the [100] direction. The measurements are analyzed with the aid of a
first-principles calculation of the BeTe bulk band structure as well as of the
photoemission peaks as given by the momentum conserving bulk transitions.
Taking the calculated unoccupied bands as final states of the photoemission
process, we obtain an excellent agreement between experimental and calculated
spectra and a clear interpretation of almost all measured bands. In contrast,
the free electron approximation for the final states fails to describe the BeTe
bulk band structure along properly.Comment: 21 pages plus 4 figure
Sporobolus stapfianus: Insights into desiccation tolerance in the resurrection grasses from linking transcriptomics to metabolomics
Predominant clusters of SDATs that share distinct patterns of abundance during dehydration: A. Predominant patterns of abundance for transcripts in clusters that exhibited increased abundance during dehydration. B. Predominant patterns of abundance for transcripts in clusters that exhibited a decreased abundance during dehydration. (PDF 226 kb
H I content in Coma cluster substructure
Galaxy clusters are some of largest structures in the universe. These very
dense environments tend to be home to higher numbers of evolved galaxies that
what is found in lower density environments. It is well known that dense
environments can influence the evolution of galaxies through the removal of the
neutral gas (HI) reservoirs which fuel star formation. It is unclear which
environment has a stronger effect: the local environment (i.e. the substructure
within the cluster), or the cluster itself. Using the new HI data from the
Westerbork Coma Survey, we explore the average HI content of galaxies across
the cluster comparing galaxies that reside in substructure to those that do
not. We apply to the Dressler-Shectman test to our newly compiled redshift
catalogue of the Coma cluster to search for substructure. With so few of the
Coma galaxies directly detected in HI, we use the HI stacking technique to
probe average HI content below what can be directly detected. Using the
Dressler-Shectman test, we find 15 substructures within the footprint of the
Westerbork Coma Survey. We compare the average HI content for galaxies within
substructure to those not in substructure. Using the HI stacking technique, we
find that the Coma galaxies (for which are not detected in HI) are more than
10--50 times more HI deficient than expected which supports the scenario of an
extremely efficient and rapid quenching mechanism. By studying the galaxies
that are not directly detected in HI, we also find Coma to be more HI deficient
than previously thought.Comment: 18 pages (+ 21 page appendix), 23 figures, accepted for publication
in A&
Large enhancement of the thermopower in NaCoO at high Na doping
Research on the oxide perovskites has uncovered electronic properties that
are strikingly enhanced compared with those in conventional metals. Examples
are the high critical temperatures of the cuprate superconductors and the
colossal magnetoresistance in the manganites. The conducting layered cobaltate
displays several interesting electronic phases as is varied
including water-induced superconductivity and an insulating state that is
destroyed by field. Initial measurements showed that, in the as-grown
composition, displays moderately large thermopower and
conductivity . However, the prospects for thermoelectric cooling
applications faded when the figure of merit was found to be small at this
composition (0.60.7). Here we report that, in the poorly-explored
high-doping region 0.75, undergoes an even steeper enhancement. At the
critical doping 0.85, (at 80 K) reaches values 40 times
larger than in the as-grown crystals. We discuss prospects for low-temperature
thermoelectric applications.Comment: 6 pages, 7 figure
PatientĂą reported outcomes in a large North American cohort living with chronic hepatitis B virus: a crossĂą sectional analysis
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153747/1/apt15618_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153747/2/apt15618.pd
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Microstructure and texture analysis of 304 austenitic stainless steel using Bragg edge transmission imaging
Bragg edge imaging using pulsed neutrons is a non-destructive technique for studying microstructure and texture of materials. It provides two-dimensional visualization of crystallographic information using a pixelated gas electron multiplier detector and a time-of-flight method. In this work, the properties of type 304 austenitic stainless steel samples were studied via Bragg edge imaging. The samples included hot-rolled, cold-rolled and heat-treated specimens, which were characterized to investigate texture, phase fraction and grain growth. The results showed that the crystallite size increased with increasing annealing temperature. The cold-rolled and annealed samples exhibited strong textures, while the hot-rolled sample showed no preferred orientation. The phase volume fraction of induced martensite in the cold-rolled sample was also obtained. Two-dimensional maps of microstructures and textures were obtained without destructive processes. The results were validated by electron backscatter diffraction and found to be consistent. This work provides valuable information for non-destructive characterization of bulk materials by performing Bragg edge imaging using the Hokkaido University compact accelerator neutron source
The WEBT Campaign on the Blazar 3C279 in 2006
The quasar 3C279 was the target of an extensive multiwavelength monitoring
campaign from January through April 2006, including an optical-IR-radio
monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration.
In this paper we focus on the results of the WEBT campaign. The source
exhibited substantial variability of optical flux and spectral shape, with a
characteristic time scale of a few days. The variability patterns throughout
the optical BVRI bands were very closely correlated with each other. In
intriguing contrast to other (in particular, BL Lac type) blazars, we find a
lag of shorter- behind longer-wavelength variability throughout the RVB ranges,
with a time delay increasing with increasing frequency. Spectral hardening
during flares appears delayed with respect to a rising optical flux. This, in
combination with the very steep IR-optical continuum spectral index of ~ 1.5 -
2.0, may indicate a highly oblique magnetic field configuration near the base
of the jet. An alternative explanation through a slow (time scale of several
days) acceleration mechanism would require an unusually low magnetic field of <
0.2 G, about an order of magnitude lower than inferred from previous analyses
of simultaneous SEDs of 3C279 and other FSRQs with similar properties.Comment: Accepted for publication in Ap
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