355 research outputs found
Completion of the Long Duration Wear Test of the NASA HERMeS Hall Thruster
The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5-kW Hall thruster has been the subject of extensive technology maturation by NASA GRC and JPL in preparation for development into a flight propulsion system. As part of this effort, a series of wear tests have been conducted to identify erosion phenomena and the accompanying failure modes as well as to validate service-life models for magnetically-shielded thrusters. This work presents a summary of the results obtained during the Long Duration Wear Test (LDWT), which was the third in this wear test series. The LDWT accumulated approximately 3,570 hours of operation and had the overall goal to identify and correct design or facility issues prior to the flight qualification campaign. Thruster performance, stability, and plume properties were invariant throughout the duration of the LDWT and consistent with measurements acquired during previous HERMeS performance and wear characterizations. Average erosion rates of a carbon-carbon composite pole cover were found to match those measured with graphite to within the empirical uncertainty while the previously observed time-dependence of pole cover erosion rates was linked to changes in pole cover roughness. Azimuthal variations in keeper wear rate were observed including deposition on one of the azimuthal-facing sides of the keeper mask. This strongly suggests the presence of an azimuthal component in the process driving keeper erosion
Matrix concentration inequalities via the method of exchangeable pairs
This paper derives exponential concentration inequalities and polynomial
moment inequalities for the spectral norm of a random matrix. The analysis
requires a matrix extension of the scalar concentration theory developed by
Sourav Chatterjee using Stein's method of exchangeable pairs. When applied to a
sum of independent random matrices, this approach yields matrix generalizations
of the classical inequalities due to Hoeffding, Bernstein, Khintchine and
Rosenthal. The same technique delivers bounds for sums of dependent random
matrices and more general matrix-valued functions of dependent random
variables.Comment: Published in at http://dx.doi.org/10.1214/13-AOP892 the Annals of
Probability (http://www.imstat.org/aop/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Lot-to-lot Variability of BN Grades for Space Electric Propulsion Applications
Historically, several grades of hot pressed hexagonal boron nitride have been used for space electric propulsion applications. This study investigates the material properties of a selection of commercially available boron nitride grades including HP, M26, M, BNXX, and Shapal Hi-M. This work complements data which was presented at JANNAF 2018. The grades selected for this study are of interest because their available billet size is sufficient for space electric propulsion. This research investigates a range of material properties, tailored and focused on enhancing performance, reliability, and economics of electric propulsion thrusters. This work builds upon previous efforts by studying lot-to-lot variability of some of the properties of interest. Coefficient of friction, moisture absorption, moisture sensitivity, hot press directionality, flexural strength, compression strength, elastic modulus, thermal conductivity, thermal emissivity, thermal expansion, density, X-ray diffraction phase, microstructure, and chemical composition were all investigated across two lots of materials to help guide the selection of advanced ceramics. Variability between lots is the critical concern of the study
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Investigation of eight candidate genes on chromosome 1p36 for autosomal dominant total congenital cataract
Purpose: To identify the causative gene for autosomal dominant total congenital cataract in a six-generation Australian family displaying linkage to chromosome 1p36. Methods: Eight candidate genes (HSPB7, FBXO42, EFHD2, ZBTB17, CAPZB, FBLIM1, ALDH4A1, and MFAP2) from within the previously defined linkage interval were selected based on expression in lens and their known or putative function. The coding exons were sequenced in multiple affected family members and compared to the reference sequence. Results: No segregating mutations were identified in any of the eight genes. Thirty-one polymorphisms were detected, 20 of which were in the exons and 11 in the flanking introns. Conclusions: Coding mutations in HSPB7, FBXO42, EFHD2, ZBTB17, CAPZB, FBLIM1, ALDH4A1, and MFAP2 do not account for congenital cataract in this family
Altered resting state neuromotor connectivity in men with chronic prostatitis/chronic pelvic pain syndrome: A MAPP: Research Network Neuroimaging Study.
Brain network activity associated with altered motor control in individuals with chronic pain is not well understood. Chronic Prostatitis/Chronic Pelvic Pain Syndrome (CP/CPPS) is a debilitating condition in which previous studies have revealed altered resting pelvic floor muscle activity in men with CP/CPPS compared to healthy controls. We hypothesized that the brain networks controlling pelvic floor muscles would also show altered resting state function in men with CP/CPPS. Here we describe the results of the first test of this hypothesis focusing on the motor cortical regions, termed pelvic-motor, that can directly activate pelvic floor muscles. A group of men with CP/CPPS (NÂ =Â 28), as well as group of age-matched healthy male controls (NÂ =Â 27), had resting state functional magnetic resonance imaging scans as part of the Multidisciplinary Approach to the Study of Chronic Pelvic Pain (MAPP) Research Network study. Brain maps of the functional connectivity of pelvic-motor were compared between groups. A significant group difference was observed in the functional connectivity between pelvic-motor and the right posterior insula. The effect size of this group difference was among the largest effect sizes in functional connectivity between all pairs of 165 anatomically-defined subregions of the brain. Interestingly, many of the atlas region pairs with large effect sizes also involved other subregions of the insular cortices. We conclude that functional connectivity between motor cortex and the posterior insula may be among the most important markers of altered brain function in men with CP/CPPS, and may represent changes in the integration of viscerosensory and motor processing
SkyMapper Southern Survey: First Data Release (DR1)
We present the first data release (DR1) of the SkyMapper Southern Survey, a
hemispheric survey carried out with the SkyMapper Telescope at Siding Spring
Observatory in Australia. Here, we present the survey strategy, data
processing, catalogue construction and database schema. The DR1 dataset
includes over 66,000 images from the Shallow Survey component, covering an area
of 17,200 deg in all six SkyMapper passbands , while the full area
covered by any passband exceeds 20,000 deg. The catalogues contain over 285
million unique astrophysical objects, complete to roughly 18 mag in all bands.
We compare our point-source photometry with PanSTARRS1 DR1 and note an
RMS scatter of 2%. The internal reproducibility of SkyMapper photometry is on
the order of 1%. Astrometric precision is better than 0.2 arcsec based on
comparison with Gaia DR1. We describe the end-user database, through which data
are presented to the world community, and provide some illustrative science
queries.Comment: 31 pages, 19 figures, 10 tables, PASA, accepte
The VMC survey - XI : Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae
Copyright American Astronomical SocietyWe present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, Ks survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.Peer reviewe
Polarization as an indicator of intrinsic alignment in radio weak lensing
We propose a new technique for weak gravitational lensing in the radio band
making use of polarization information. Since the orientation of a galaxy's
polarized emission is both unaffected by lensing and is related to the galaxy's
intrinsic orientation, it effectively provides information on the unlensed
galaxy position angle. We derive a new weak lensing estimator which exploits
this effect and makes full use of both the observed galaxy shapes and the
estimates of the intrinsic position angles as provided by polarization. Our
method has the potential to both reduce the effects of shot noise, and to
reduce to negligible levels, in a model-independent way, all effects of
intrinsic galaxy alignments. We test our technique on simulated weak lensing
skies, including an intrinsic alignment contaminant consistent with recent
observations, in three overlapping redshift bins. Adopting a standard weak
lensing analysis and ignoring intrinsic alignments results in biases of 5-10%
in the recovered power spectra and cosmological parameters. Applying our new
estimator to one tenth the number of galaxies used for the standard case, we
recover both power spectra and the input cosmology with similar precision as
compared to the standard case and with negligible residual bias, even in the
presence of a substantial (astrophysical) scatter in the relationship between
the observed orientation of the polarized emission and the intrinsic
orientation. Assuming a reasonable polarization fraction for star-forming
galaxies, and no cosmological conspiracy in the relationship between
polarization direction and intrinsic morphology, our estimator should prove a
valuable tool for weak lensing analyses of forthcoming radio surveys, in
particular, deep wide field surveys with e-MERLIN, MeerKAT and ASKAP and
ultimately, definitive radio lensing surveys with the SKA.Comment: 18 pages, 10 figures, submitted to MNRA
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