5,742 research outputs found
High-temperature electric stator
Thermal vacuum tests are conducted on high-temperature stator without a rotor. Electrical characteristics remained satisfactory during tests, insulation improved slightly, and conductor resistance remained constant. The assembly included a bore seal for hermetically sealing the stator from the rotor cavity
Braze alloys for high temperature service
Two groups of refractory metal compositions have been developed that are very useful as high temperature brazing alloys for sealing between ceramic and metal parts. Each group consists of various compositions of three selected refractory metals which, when combined, have characteristics required of good braze alloys
Bimetallic junctions
The formation of voids through interdiffusion in bimetallic welded structures exposed to high operating temperatures is inhibited by utilizing an alloy of the parent materials in the junction of the parent materials or by preannealing the junction at an ultrahigh temperature. These methods are also used to reduce the concentration gradient of a hardening agent
Robust autoresonant excitation in the plasma beat-wave accelerator: a theoretical study
A modified version of the Plasma Beat-Wave Accelerator scheme is introduced
and analyzed, which is based on autoresonant phase-locking of the nonlinear
Langmuir wave to the slowly chirped beat frequency of the driving lasers via
adiabatic passage through resonance. This new scheme is designed to overcome
some of the well-known limitations of previous approaches, namely relativistic
detuning and nonlinear modulation or other non-uniformity or non-stationarity
in the driven Langmuir wave amplitude, and sensitivity to frequency mismatch
due to measurement uncertainties and density fluctuations and inhomogeneities
Self-consistent Langmuir waves in resonantly driven thermal plasmas
The longitudinal dynamics of a resonantly driven Langmuir wave are analyzed
in the limit that the growth of the electrostatic wave is slow compared to the
bounce frequency. Using simple physical arguments, the nonlinear distribution
function is shown to be nearly gaussian in the canonical particle action, with
a slowly evolving mean and fixed variance. Self-consistency with the
electrostatic potential provide the basic properties of the nonlinear
distribution function including a frequency shift that agrees well with driven,
electrostatic particle simulations. This extends earlier work on nonlinear
Langmuir waves by Morales and O'Neil [G. J. Morales and T. M. O'Neil, Phys.
Rev. Lett. 28, 417 (1972)], and could form the basis of a reduced kinetic
treatment of Raman backscatter in a plasma.Comment: 11 pages, 4 figures, submitted to Physics of Plasma
Protein folding rates correlate with heterogeneity of folding mechanism
By observing trends in the folding kinetics of experimental 2-state proteins
at their transition midpoints, and by observing trends in the barrier heights
of numerous simulations of coarse grained, C-alpha model, Go proteins, we show
that folding rates correlate with the degree of heterogeneity in the formation
of native contacts. Statistically significant correlations are observed between
folding rates and measures of heterogeneity inherent in the native topology, as
well as between rates and the variance in the distribution of either
experimentally measured or simulated phi-values.Comment: 11 pages, 3 figures, 1 tabl
Time dependence of Bragg forward scattering and self-seeding of hard x-ray free-electron lasers
Free-electron lasers (FELs) can now generate temporally short, high power
x-ray pulses of unprecedented brightness, even though their longitudinal
coherence is relatively poor. The longitudinal coherence can be potentially
improved by employing narrow bandwidth x-ray crystal optics, in which case one
must also understand how the crystal affects the field profile in time and
space. We frame the dynamical theory of x-ray diffraction as a set of coupled
waves in order to derive analytic expressions for the spatiotemporal response
of Bragg scattering from temporally short incident pulses. We compute the
profiles of both the reflected and forward scattered x-ray pulses, showing that
the time delay of the wave is linked to its transverse spatial shift
through the simple relationship , where
is the grazing angle of incidence to the diffracting planes. Finally,
we apply our findings to obtain an analytic description of Bragg forward
scattering relevant to monochromatically seed hard x-ray FELs.Comment: 11 pages, 6 figure
Establishment of norms in specific areas of the visual field for critical fusion frequency as determined by a modified stroboscope
Establishment of norms in specific areas of the visual field for critical fusion frequency as determined by a modified stroboscop
Marine extinction risk shaped by trait-environment interactions over 500 million years
Perhaps the most pressing issue in predicting biotic responses to present and future global change is understanding how environmental factors shape the relationship between ecological traits and extinction risk. The fossil record provides millions of years of insight into how extinction selectivity (i.e., differential extinction risk) is shaped by interactions between ecological traits and environmental conditions. Numerous paleontological studies have examined trait-based extinction selectivity; however, the extent to which these patterns are shaped by environmental conditions is poorly understood due to a lack of quantitative synthesis across studies. We conducted a meta-analysis of published studies on fossil marine bivalves and gastropods that span 458 million years to uncover how global environmental and geochemical changes covary with trait-based extinction selectivity. We focused on geographic range size and life habit (i.e., infaunal vs. epifaunal), two of the most important and commonly examined predictors of extinction selectivity. We used geochemical proxies related to global climate, as well as indicators of ocean acidification, to infer average global environmental conditions. Life-habit selectivity is weakly dependent on environmental conditions, with infaunal species relatively buffered from extinction during warmer climate states. In contrast, the odds of taxa with broad geographic ranges surviving an extinction ( \u3e 2500km for genera, \u3e 500km for species) are on average three times greater than narrow-ranging taxa (estimate of odds ratio: 2.8, 95% confidence interval=2.3-3.5), regardless of the prevailing global environmental conditions. The environmental independence of geographic range size extinction selectivity emphasizes the critical role of geographic range size in setting conservation priorities
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