11,857 research outputs found
Self-Diffusion of a Polymer Chain in a Melt
Self-diffusion of a polymer chain in a melt is studied by Monte Carlo
simulations of the bond fluctuation model, where only the excluded volume
interaction is taken into account. Polymer chains, each of which consists of
segments, are located on an simple cubic lattice
under periodic boundary conditions, where each segment occupies unit cells. The results for
and 512 at the volume fraction are reported, where
for and L=192 for . The -dependence of the
self-diffusion constant is examined. Here, is estimated from the mean
square displacements of the center of mass of a single polymer chain at the
times larger than the longest relaxation time. From the data for , 384
and 512, the apparent exponent , which describes the apparent power
law dependence of on as , is estimated as
. The ratio seems to be a
constant for and 512, where and
denote the longest relaxation time and the mean square end-to-end distance,
respectively.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
Using the local gyrokinetic code, GS2, to investigate global ITG modes in tokamaks. (I) s- model with profile and flow shear effects
This paper combines results from a local gyrokinetic code with analytical
theory to reconstruct the global eigenmode structure of the linearly unstable
ion-temperature-gradient (ITG) mode with adiabatic electrons. The simulations
presented here employ the s- tokamak equilibrium model. Local
gyrokinetic calculations, using GS2 have been performed over a range of radial
surfaces, x, and for ballooning phase angle, p, in the range -, to map out the complex local mode frequency, . Assuming a quadratic radial profile for the
drive, namely , (holding constant all other equilibrium
profiles such as safety factor, magnetic shear etc.), has a
stationary point. The reconstructed global mode then sits on the outboard mid
plane of the tokamak plasma, and is known as a conventional or isolated mode,
with global growth rate, ~ Max[], where
is the local growth rate. Taking the radial variation in
other equilibrium profiles (e.g safety factor q(x)) into account, removes the
stationary point in and results in a mode that peaks
slightly away from the outboard mid-plane with a reduced global growth rate.
Finally, the influence of flow shear has also been investigated through a
Doppler shift, , where n
is the toroidal mode number and incorporates the effect of
flow shear. The equilibrium profile variation introduces an asymmetry to the
growth rate spectrum with respect to the sign of ,
consistent with recent global gyrokinetic calculations.Comment: 10 pages, 8 figures and 1 tabl
Kinetic instabilities that limit {\beta} in the edge of a tokamak plasma: a picture of an H-mode pedestal
Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak (MAST)
have sufficient resolution to capture plasma evolution during the short period
between edge-localized modes (ELMs). Immediately after the ELM steep gradients
in pressure, P, and density, ne, form pedestals close to the separatrix, and
they then expand into the core. Local gyrokinetic analysis over the ELM cycle
reveals the dominant microinstabilities at perpendicular wavelengths of the
order of the ion Larmor radius. These are kinetic ballooning modes (KBMs) in
the pedestal and microtearing modes (MTMs) in the core close to the pedestal
top. The evolving growth rate spectra, supported by gyrokinetic analysis using
artificial local equilibrium scans, suggest a new physical picture for the
formation and arrest of this pedestal.Comment: Final version as it appeared in PRL (March 2012). Minor improvements
include: shortened abstract, and better colour table for figures. 4 pages, 6
figure
Intracranial pressure monitoring in normal dogs using subdural and intraparenchymal miniature strain-gauge transducers.
BackgroundMonitoring of intracranial pressure (ICP) is a critical component in the management of intracranial hypertension. Safety, efficacy, and optimal location of microsensor devices have not been defined in dogs.Hypothesis/objectiveAssessment of ICP using a microsensor transducer is feasible in anesthetized and conscious animals and is independent of transducer location. Intraparenchymal transducer placement is associated with more adverse effects.AnimalsSeven adult, bred-for-research dogs.MethodsIn a prospective investigational study, microsensor ICP transducers were inserted into subdural and intraparenchymal locations at defined rostral or caudal locations within the rostrotentorial compartment under general anesthesia. Mean arterial pressure and ICP were measured continuously during physiological maneuvers, and for 20 hours after anesthesia.ResultsBaseline mean ± SD values for ICP and cerebral perfusion pressure were 7.2 ± 2.3 and 78.9 ± 7.6 mm Hg, respectively. Catheter position did not have a significant effect on ICP measurements. There was significant variation from baseline ICP accompanying physiological maneuvers (P < .001) and with normal activities, especially with changes in head position (P < .001). Pathological sequelae were more evident after intraparenchymal versus subdural placement.Conclusions and clinical importanceUse of a microsensor ICP transducer was technically straightforward and provided ICP measurements within previously reported reference ranges. Results support the use of an accessible dorsal location and subdural positioning. Transient fluctuations in ICP are normal events in conscious dogs and large variations associated with head position should be accounted for when evaluating animals with intracranial hypertension
A Radio Determination of the Time of the New Moon
The detection of the New Moon at sunset is of importance to communities based
on the lunar calendar. This is traditionally undertaken with visual
observations. We propose a radio method which allows a higher visibility of the
Moon relative to the Sun and consequently gives us the ability to detect the
Moon much closer to the Sun than is the case of visual observation. We first
compare the relative brightness of the Moon and Sun over a range of possible
frequencies and find the range 5--100\,GHz to be suitable. The next
consideration is the atmospheric absorption/emission due to water vapour and
oxygen as a function of frequency. This is particularly important since the
relevant observations are near the horizon. We show that a frequency of GHz is optimal for this programme. We have designed and constructed a
telescope with a FWHM resolution of 0.6 and low sidelobes to
demonstrate the potential of this approach. At the time of the 21 May 2012 New
Moon the Sun/Moon brightness temperature ratio was in agreement
with predictions from the literature when combined with the observed sunspot
numbers for the day. The Moon would have been readily detectable at from the Sun. Our observations at 16\,hr\,36\,min UT indicated that
the Moon would have been at closest approach to the Sun 16\,hr\,25\,min
earlier; this was the annular solar eclipse of 00\,hr\,00\,min\,UT on 21 May
2012.Comment: 11 pages, 15 figures, accepted for publication in MNRA
X-ray observations of highly obscured τ_(9.7 μm) > 1 sources: an efficient method for selecting Compton-thick AGN?
Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ_(9.7 μm) > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine)
in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si
absorption, with the most notable example being NGC1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey.
All these have been selected to have no PAH features (EW_(6.2 μm) 10^(42) erg s^(−1)) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L_(X)/L_(6 μm) luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN
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