27,039 research outputs found
An overview of the development of remote sensing techniques for the screwworm eradication program
The current status of remote sensing techniques developed for the screwworm eradication program of the Mexican-American Screwworm Eradication Commission was reported. A review of the type of data and equipment used in the program is presented. Future applications of remote sensing techniques are considered
Vortex density fluctuations in quantum turbulence
We compute the frequency spectrum of turbulent superfluid vortex density
fluctuations and obtain the same Kolmogorov scaling which has been observed in
a recent experiment in Helium-4. We show that the scaling can be interpreted in
terms of the spectrum of reconnecting material lines. The calculation is
performed using a vortex tree algorithm which considerably speeds up the
evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure
Turbulent transport in tokamak plasmas with rotational shear
Nonlinear gyrokinetic simulations have been conducted to investigate
turbulent transport in tokamak plasmas with rotational shear. At sufficiently
large flow shears, linear instabilities are suppressed, but transiently growing
modes drive subcritical turbulence whose amplitude increases with flow shear.
This leads to a local minimum in the heat flux, indicating an optimal E x B
shear value for plasma confinement. Local maxima in the momentum fluxes are
also observed, allowing for the possibility of bifurcations in the E x B shear.
The sensitive dependence of heat flux on temperature gradient is relaxed for
large flow shear values, with the critical temperature gradient increasing at
lower flow shear values. The turbulent Prandtl number is found to be largely
independent of temperature and flow gradients, with a value close to unity.Comment: 4 pages, 5 figures, submitted to PR
High Rate Discharge Studies of LI/SO2 Batteries
A battery composed of twelve lithium/sulfur dioxide D size cells in series is forced discharged at 21 amperes. This current is established by the proposed use of the battery and represented a discharge condition which might produce venting. Discharge of the battery into voltage reversal results not only in cells venting but also in the violent rupture of at least one cell
PEPSI deep spectra. III. A chemical analysis of the ancient planet-host star Kepler-444
We obtained an LBT/PEPSI spectrum with very high resolution and high
signal-to-noise ratio (S/N) of the K0V host Kepler-444, which is known to host
5 sub-Earth size rocky planets. The spectrum has a resolution of R=250,000, a
continuous wavelength coverage from 4230 to 9120A, and S/N between 150 and
550:1 (blue to red). We performed a detailed chemical analysis to determine the
photospheric abundances of 18 chemical elements, in order to use the abundances
to place constraints on the bulk composition of the five rocky planets. Our
spectral analysis employs the equivalent width method for most of our spectral
lines, but we used spectral synthesis to fit a small number of lines that
require special care. In both cases, we derived our abundances using the MOOG
spectral analysis package and Kurucz model atmospheres. We find no correlation
between elemental abundance and condensation temperature among the refractory
elements. In addition, using our spectroscopic stellar parameters and isochrone
fitting, we find an age of 10+/-1.5 Gyr, which is consistent with the
asteroseismic age of 11+/-1 Gyr. Finally, from the photospheric abundances of
Mg, Si, and Fe, we estimate that the typical Fe-core mass fraction for the
rocky planets in the Kepler-444 system is approximately 24 per cent. If our
estimate of the Fe-core mass fraction is confirmed by more detailed modeling of
the disk chemistry and simulations of planet formation and evolution in the
Kepler-444 system, then this would suggest that rocky planets in more
metal-poor and alpha-enhanced systems may tend to be less dense than their
counterparts of comparable size in more metal-rich systems.Comment: in press, 11 pages, 3 figures, data available from pepsi.aip.d
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Zero-Turbulence Manifold in a Toroidal Plasma
Sheared toroidal flows can cause bifurcations to zero-turbulent-transport
states in tokamak plasmas. The maximum temperature gradients that can be
reached are limited by subcritical turbulence driven by the parallel velocity
gradient. Here it is shown that q/\epsilon (magnetic field pitch/inverse aspect
ratio) is a critical control parameter for sheared tokamak turbulence. By
reducing q/\epsilon, far higher temperature gradients can be achieved without
triggering turbulence, in some instances comparable to those found
experimentally in transport barriers. The zero-turbulence manifold is mapped
out, in the zero-magnetic-shear limit, over the parameter space (\gamma_E,
q/\epsilon, R/L_T), where \gamma_E is the perpendicular flow shear and R/L_T is
the normalised inverse temperature gradient scale. The extent to which it can
be constructed from linear theory is discussed.Comment: 5 Pages, 4 Figures, Submitted to PR
Transport Bifurcation in a Rotating Tokamak Plasma
The effect of flow shear on turbulent transport in tokamaks is studied
numerically in the experimentally relevant limit of zero magnetic shear. It is
found that the plasma is linearly stable for all non-zero flow shear values,
but that subcritical turbulence can be sustained nonlinearly at a wide range of
temperature gradients. Flow shear increases the nonlinear temperature gradient
threshold for turbulence but also increases the sensitivity of the heat flux to
changes in the temperature gradient, except over a small range near the
threshold where the sensitivity is decreased. A bifurcation in the equilibrium
gradients is found: for a given input of heat, it is possible, by varying the
applied torque, to trigger a transition to significantly higher temperature and
flow gradients.Comment: 4 pages, 4 figures, submitted to PR
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