3,527 research outputs found
Retention and application of Skylab experiment experiences to future programs
Problems encountered on Skylab Experiments are listed in order that these experiences and associated recommendations might help to prevent similar problems on future programs. The criteria for selection of the data to be utilized was to identify the problem areas within the Skylab Program which would be of major significance with respect to future programs. Also, the problem had to be unique in that it would help identify to a designer/manufacturer an unforeseen or unanticipated occurrence which could cause failures, delays, or additional cost. Only those unexpected problems that may occur due to the nature of aerospace experiment environmental and operational requirements are included
Retention and application of Skylab experiences to future programs
The problems encountered and special techniques and procedures developed on the Skylab program are described along with the experiences and practical benefits obtained for dissemination and use on future programs. Three major topics are discussed: electrical problems, mechanical problems, and special techniques. Special techniques and procedures are identified that were either developed or refined during the Skylab program. These techniques and procedures came from all manufacturing and test phases of the Skylab program and include both flight and GSE items from component level to sophisticated spaceflight systems
Symmetry Relations for Trajectories of a Brownian Motor
A Brownian Motor is a nanoscale or molecular device that combines the effects
of thermal noise, spatial or temporal asymmetry, and directionless input energy
to drive directed motion. Because of the input energy, Brownian motors function
away from thermodynamic equilibrium and concepts such as linear response
theory, fluctuation dissipation relations, and detailed balance do not apply.
The {\em generalized} fluctuation-dissipation relation, however, states that
even under strongly thermodynamically non-equilibrium conditions the ratio of
the probability of a transition to the probability of the time-reverse of that
transition is the exponent of the change in the internal energy of the system
due to the transition. Here, we derive an extension of the generalized
fluctuation dissipation theorem for a Brownian motor for the ratio between the
probability for the motor to take a forward step and the probability to take a
backward step
Internal thermal noise in the LIGO test masses : a direct approach
The internal thermal noise in LIGO's test masses is analyzed by a new
technique, a direct application of the Fluctuation-Dissipation Theorem to
LIGO's readout observable, (longitudinal position of test-mass face,
weighted by laser beam's Gaussian profile). Previous analyses, which relied on
a normal-mode decomposition of the test-mass motion, were valid only if the
dissipation is uniformally distributed over the test-mass interior, and they
converged reliably to a final answer only when the beam size was a
non-negligible fraction of the test-mass cross section. This paper's direct
analysis, by contrast, can handle inhomogeneous dissipation and arbitrary beam
sizes. In the domain of validity of the previous analysis, the two methods give
the same answer for , the spectral density of thermal noise, to within
expected accuracy. The new analysis predicts that thermal noise due to
dissipation concentrated in the test mass's front face (e.g. due to mirror
coating) scales as , by contrast with homogeneous dissipation, which
scales as ( is the beam radius); so surface dissipation could
become significant for small beam sizes.Comment: 6 pages, RevTex, 1 figur
Quantum Stress Focusing in Descriptive Chemistry
We show that several important concepts of descriptive chemistry, such as
atomic shells, bonding electron pairs and lone electron pairs, may be described
in terms of {\it quantum stress focusing}, i.e. the spontaneous formation of
high-pressure regions in an electron gas. This description subsumes previous
mathematical constructions, such as the Laplacian of the density and the
electron localization function, and provides a new tool for visualizing
chemical structure. We also show that the full stress tensor, defined as the
derivative of the energy with respect to a local deformation, can be easily
calculated from density functional theory.Comment: 5 pages, 2 figure
Electro-optic time profile monitors for femtosecond electron bunches at the soft x-ray free-electron laser FLASH
Precise measurements of the temporal proïŹle of ultrashort electron bunches are of high interest for the optimization and operation of ultraviolet and x-ray free-electron lasers. The electro-optic (EO) technique has been applied for a single-shot direct visualization of the time proïŹle of individual electron bunches at FLASH. This paper presents a thorough description of the experimental setup and the results. An absolute calibration of the EO technique has been performed utilizing simultaneous measurements with a transverse-deïŹecting radio-frequency structure that transforms the longitudinal bunch charge distribution into a transverse streak. EO signals as short as 60 fs (rms) have been observed using a gallium-phosphide (GaP) crystal, which is a new record in the EO detection of single electron bunches and close to the physical limit imposed by the EO material properties. The data are in quantitative agreement with a numerical simulation of the EO detection process
Oscillating epidemics in a dynamic network model: stochastic and mean-field analysis
An adaptive network model using SIS epidemic propagation with link-type-dependent link activation and deletion is considered. Bifurcation analysis of the pairwise ODE approximation and the network-based stochastic simulation is carried out, showing that three typical behaviours may occur; namely, oscillations can be observed besides disease-free or endemic steady states. The oscillatory behaviour in the stochastic simulations is studied using Fourier analysis, as well as through analysing the exact master equations of the stochastic model. By going beyond simply comparing simulation results to mean-field models, our approach yields deeper insights into the observed phenomena and help better understand and map out the limitations of mean-field models
The Nondeterministic Waiting Time Algorithm: A Review
We present briefly the Nondeterministic Waiting Time algorithm. Our technique
for the simulation of biochemical reaction networks has the ability to mimic
the Gillespie Algorithm for some networks and solutions to ordinary
differential equations for other networks, depending on the rules of the
system, the kinetic rates and numbers of molecules. We provide a full
description of the algorithm as well as specifics on its implementation. Some
results for two well-known models are reported. We have used the algorithm to
explore Fas-mediated apoptosis models in cancerous and HIV-1 infected T cells
Syntactic Markovian Bisimulation for Chemical Reaction Networks
In chemical reaction networks (CRNs) with stochastic semantics based on
continuous-time Markov chains (CTMCs), the typically large populations of
species cause combinatorially large state spaces. This makes the analysis very
difficult in practice and represents the major bottleneck for the applicability
of minimization techniques based, for instance, on lumpability. In this paper
we present syntactic Markovian bisimulation (SMB), a notion of bisimulation
developed in the Larsen-Skou style of probabilistic bisimulation, defined over
the structure of a CRN rather than over its underlying CTMC. SMB identifies a
lumpable partition of the CTMC state space a priori, in the sense that it is an
equivalence relation over species implying that two CTMC states are lumpable
when they are invariant with respect to the total population of species within
the same equivalence class. We develop an efficient partition-refinement
algorithm which computes the largest SMB of a CRN in polynomial time in the
number of species and reactions. We also provide an algorithm for obtaining a
quotient network from an SMB that induces the lumped CTMC directly, thus
avoiding the generation of the state space of the original CRN altogether. In
practice, we show that SMB allows significant reductions in a number of models
from the literature. Finally, we study SMB with respect to the deterministic
semantics of CRNs based on ordinary differential equations (ODEs), where each
equation gives the time-course evolution of the concentration of a species. SMB
implies forward CRN bisimulation, a recently developed behavioral notion of
equivalence for the ODE semantics, in an analogous sense: it yields a smaller
ODE system that keeps track of the sums of the solutions for equivalent
species.Comment: Extended version (with proofs), of the corresponding paper published
at KimFest 2017 (http://kimfest.cs.aau.dk/
Transition Phenomena Induced by Internal Noise and Quasi-absorbing State
We study a simple chemical reaction system and effects of the internal noise.
The chemical reaction system causes the same transition phenomenon discussed by
Togashi and Kaneko [Phys. Rev. Lett. 86 (2001) 2459; J. Phys. Soc. Jpn. 72
(2003) 62]. By using the simpler model than Togashi-Kaneko's one, we discuss
the transition phenomenon by means of a random walk model and an effective
model. The discussion makes it clear that quasi-absorbing states, which are
produced by the change of the strength of the internal noise, play an important
role in the transition phenomenon. Stabilizing the quasi-absorbing states
causes bifurcation of the peaks in the stationary probability distribution
discontinuously.Comment: 6 pages, 5 figure
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