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Surface mineral crusts: A priority target in search for life on Mars
Mineral crusts are strong candidates in the search for evidence of life during planetary exploration, and should be an important target for examination in impact craters. Crusts in the Haughton crater readily yield a biological signature
Two-dimensional photonic crystal polarizer
A novel polarizer made from two-dimensional photonic bandgap materials was
demonstrated theoretically. This polarizer is fundamentally different from the
conventinal ones. It can function in a wide frequency range with high
performance and the size can be made very compact, which renders it useful as a
micropolarizer in microoptics.Comment: 8 pages, RevTex, 4 figure
Stochastic theory of large-scale enzyme-reaction networks: Finite copy number corrections to rate equation models
Chemical reactions inside cells occur in compartment volumes in the range of
atto- to femtolitres. Physiological concentrations realized in such small
volumes imply low copy numbers of interacting molecules with the consequence of
considerable fluctuations in the concentrations. In contrast, rate equation
models are based on the implicit assumption of infinitely large numbers of
interacting molecules, or equivalently, that reactions occur in infinite
volumes at constant macroscopic concentrations. In this article we compute the
finite-volume corrections (or equivalently the finite copy number corrections)
to the solutions of the rate equations for chemical reaction networks composed
of arbitrarily large numbers of enzyme-catalyzed reactions which are confined
inside a small sub-cellular compartment. This is achieved by applying a
mesoscopic version of the quasi-steady state assumption to the exact
Fokker-Planck equation associated with the Poisson Representation of the
chemical master equation. The procedure yields impressively simple and compact
expressions for the finite-volume corrections. We prove that the predictions of
the rate equations will always underestimate the actual steady-state substrate
concentrations for an enzyme-reaction network confined in a small volume. In
particular we show that the finite-volume corrections increase with decreasing
sub-cellular volume, decreasing Michaelis-Menten constants and increasing
enzyme saturation. The magnitude of the corrections depends sensitively on the
topology of the network. The predictions of the theory are shown to be in
excellent agreement with stochastic simulations for two types of networks
typically associated with protein methylation and metabolism.Comment: 13 pages, 4 figures; published in The Journal of Chemical Physic
Global Alfven Eigenmodes in the H-1 heliac
Recent upgrades in H-1 power supplies have enabled the operation of the H-1
experiment at higher heating powers than previously attainable. A heating power
scan in mixed hydrogen/helium plasmas reveals a change in mode activity with
increasing heating power. At low power (<50 kW) modes with beta-induced Alfven
eigenmode (BAE) frequency scaling are observed. At higher power modes
consistent with an analysis of nonconventional Global Alfven Eigenmodes (GAEs)
are observed, the subject of this work. We have computed the mode continuum,
and identified GAE structures using the ideal MHD solver CKA and the
gyrokinetic code EUTERPE. An analytic model for ICRH-heated minority ions is
used to estimate the fast ion temperature from the hydrogen species. Linear
growth rate scans using a local flux surface stability calculation, LGRO, are
performed. These studies demonstrate growth from circulating particles whose
speed is significantly less than the Alfven speed, and are resonant with the
mode through harmonics of the Fourier decomposition of the strongly-shaped
heliac magnetic field. They reveal drive is possible with a small, hot
energetic tail of the hydrogen species. Local linear growth rate scans are also
complemented with global calculations from CKA and EUTERPE. These qualitatively
confirm the findings from the LGRO study, and show that the inclusion of finite
Larmor radius effects can reduce the growth rate by a factor of three, but do
not affect marginal stability. Finally, a study of damping of the global mode
with the thermal plasma is conducted, computing continuum, and the damping
arising from parallel electric fields. We find that continuum damping is of
order 0.1% for the configuration studied. The inclusion of resistivity lifts
the damping to 19%. Such large damping is consistent with experimental
observations that in absence of drive the mode decays rapidly (~0.1 ms).Comment: 18 pages, 15 figures, submitted 07/04/2017 to Plasma Physics and
Controlled Fusio
Survival of Organic Materials in Hypervelocity Impacts of Ice on Sand, Ice, and Water in the Laboratory
The survival of organic molecules in shock impact events has been investigated in the laboratory. A frozen mixture of anthracene and stearic acid, solvated in dimethylsulfoxide (DMSO), was fired in a two-stage light gas gun at speeds of ?2 and ?4?km s?1 at targets that included water ice, water, and sand. This involved shock pressures in the range of 2–12 GPa. It was found that the projectile materials were present in elevated quantities in the targets after impact and in some cases in the crater ejecta as well. For DMSO impacting water at 1.9?km s?1 and 45° incidence, we quantify the surviving fraction after impact as 0.44±0.05. This demonstrates successful transfer of organic compounds from projectile to target in high-speed impacts. The range of impact speeds used covers that involved in impacts of terrestrial meteorites on the Moon, as well as impacts in the outer Solar System on icy bodies such as Pluto. The results provide laboratory evidence that suggests that exogenous delivery of complex organic molecules from icy impactors is a viable source of such material on target bodies
How accurate are the non-linear chemical Fokker-Planck and chemical Langevin equations?
The chemical Fokker-Planck equation and the corresponding chemical Langevin
equation are commonly used approximations of the chemical master equation.
These equations are derived from an uncontrolled, second-order truncation of
the Kramers-Moyal expansion of the chemical master equation and hence their
accuracy remains to be clarified. We use the system-size expansion to show that
chemical Fokker-Planck estimates of the mean concentrations and of the variance
of the concentration fluctuations about the mean are accurate to order
for reaction systems which do not obey detailed balance and at
least accurate to order for systems obeying detailed balance,
where is the characteristic size of the system. Hence the chemical
Fokker-Planck equation turns out to be more accurate than the linear-noise
approximation of the chemical master equation (the linear Fokker-Planck
equation) which leads to mean concentration estimates accurate to order
and variance estimates accurate to order . This
higher accuracy is particularly conspicuous for chemical systems realized in
small volumes such as biochemical reactions inside cells. A formula is also
obtained for the approximate size of the relative errors in the concentration
and variance predictions of the chemical Fokker-Planck equation, where the
relative error is defined as the difference between the predictions of the
chemical Fokker-Planck equation and the master equation divided by the
prediction of the master equation. For dimerization and enzyme-catalyzed
reactions, the errors are typically less than few percent even when the
steady-state is characterized by merely few tens of molecules.Comment: 39 pages, 3 figures, accepted for publication in J. Chem. Phy
Vibrational Stability of NLC Linac and Final Focus Components
Vertical vibration of linac components (accelerating structures, girders and
quadrupoles) in the NLC has been studied experimentally and analytically.
Effects such as structural resonances and vibration caused by cooling water
both in accelerating structures and quadrupoles have been considered.
Experimental data has been compared with analytical predictions and simulations
using ANSYS. A design, incorporating the proper decoupling of structure
vibrations from the linac quadrupoles, is being pursued.Comment: 3 pages, 8 figures presented at the LINAC 2002 conference, Gyeongju
Kore
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