966 research outputs found
The VISTA spacecraft: Advantages of ICF (Inertial Confinement Fusion) for interplanetary fusions propulsion applications
Inertial Confinement Fusion (ICF) is an attractive engine power source for interplanetary manned spacecraft, especially for near-term missions requiring minimum flight duration, because ICF has inherent high power-to-mass ratios and high specific impulses. We have developed a new vehicle concept called VISTA that uses ICF and is capable of round-trip manned missions to Mars in 100 days using A.D. 2020 technology. We describe VISTA's engine operation, discuss associated plasma issues, and describe the advantages of DT fuel for near-term applications. Although ICF is potentially superior to non-fusion technologies for near-term interplanetary transport, the performance capabilities of VISTA cannot be meaningfully compared with those of magnetic-fusion systems because of the lack of a comparable study of the magnetic-fusion systems. We urge that such a study be conducted
Nested : Assessing the convergence of Markov chain Monte Carlo when running many short chains
The growing availability of hardware accelerators such as GPUs has generated
interest in Markov chains Monte Carlo (MCMC) workflows which run a large number
of chains in parallel. Each chain still needs to forget its initial state but
the subsequent sampling phase can be almost arbitrarily short. To determine if
the resulting short chains are reliable, we need to assess how close the Markov
chains are to convergence to their stationary distribution. The
statistic is a battle-tested convergence diagnostic but unfortunately can
require long chains to work well. We present a nested design to overcome this
challenge, and introduce tuning parameters to control the reliability, bias,
and variance of convergence diagnostics
The economic burden of stroke care in England, Wales and Northern Ireland: using a national stroke register to estimate and report patient level health economic outcomes in stroke
Synthesis of the elements in stars: forty years of progress
Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B2FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments
Periodic Variability During the X-ray Decline of 4U 1636-53
We report the onset of a large amplitude, statistically significant
periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U
1636-53, the X-ray flux of which has been gradually declining over the last
four years. This behaviour is remarkably similar to that observed in the
neutron star LMXB KS 1731-260, which is a long-term transient. We also report
on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and
find that the hard X-ray flux (20-100 keV) indicates an apparent
anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is
transiting from activity to quiescence, as occurred in KS 1731-260. We also
suggest that the variability during the X-ray decline is the result of an
accretion rate variability related to the X-ray irradiation of the disc.Comment: 6 pages. Accepted by MNRA
The International Land Model Benchmarking (ILAMB) System: Design, Theory, and Implementation
The increasing complexity of Earth system models has inspired efforts to quantitatively assess model fidelity through rigorous comparison with best available measurements and observational data products. Earth system models exhibit a high degree of spread in predictions of land biogeochemistry, biogeophysics, and hydrology, which are sensitive to forcing from other model components. Based on insights from prior land model evaluation studies and community workshops, the authors developed an open source model benchmarking software package that generates graphical diagnostics and scores model performance in support of the International Land Model Benchmarking (ILAMB) project. Employing a suite of in situ, remote sensing, and reanalysis data sets, the ILAMB package performs comprehensive model assessment across a wide range of land variables and generates a hierarchical set of web pages containing statistical analyses and figures designed to provide the user insights into strengths and weaknesses of multiple models or model versions. Described here is the benchmarking philosophy and mathematical methodology embodied in the most recent implementation of the ILAMB package. Comparison methods unique to a few specific data sets are presented, and guidelines for configuring an ILAMB analysis and interpreting resulting model performance scores are discussed. ILAMB is being adopted by modeling teams and centers during model development and for model intercomparison projects, and community engagement is sought for extending evaluation metrics and adding new observational data sets to the benchmarking framework.Key PointThe ILAMB benchmarking system broadly compares models to observational data sets and provides a synthesis of overall performancePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146994/1/jame20779_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146994/2/jame20779.pd
The Optical/Infrared Astronomical Quality of High Atacama Sites. II. Infrared Characteristics
We discuss properties of the atmospheric water vapor above the high Andean
plateau region known as the Llano de Chajnantor, in the Atacama Desert of
northern Chile. A combination of radiometric and radiosonde measurements
indicate that the median column of precipitable water vapor (PWV) above the
plateau at an elevation of 5000 m is approximately 1.2 mm. The exponential
scaleheight of the water vapor density in the median Chajnantor atmosphere is
1.13 km; the median PWV is 0.5 mm above an elevation of 5750 m. Both of these
numbers appear to be lower at night. Annual, diurnal and other dependences of
PWV and its scaleheight are discussed, as well as the occurrence of temperature
inversion layers below the elevation of peaks surrounding the plateau. We
estimate the background for infrared observations and sensitivities for broad
band and high resolution spectroscopy. The results suggest that exceptional
atmospheric conditions are present in the region, yielding high infrared
transparency and high sensitivity for future ground-based infrared telescopes.Comment: 25 pages, 11 figures, to appear in the PASP (July 2001
Superior Uptake and Outcomes of Early Infant Diagnosis of HIV Services at an Immunization Clinic Versus an âUnder-Fiveâ General Pediatric Clinic in Malawi
Although the Malawian government recommends HIV-exposed infants receive early infant diagnosis of HIV (EID) at âunder-fiveâ pediatric clinics (U5Cs), most never enroll. Therefore, we evaluated the integration of EID testing into an immunization clinic (IC) compared to the current standard of EID testing at an U5C
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