1,391 research outputs found
A summary of the ECAS performance and cost results for MHD systems
The potential is examined of various advanced power plant concepts using coal and coal-derived fuel. The results indicate that open cycle coal fired direct preheat MHD systems have potentially one of the highest coal-pile-to-bus-bar efficiencies and also one of the lowest costs of electricity (COE) of the systems studied. Closed cycle MHD systems may have the potential to approach the efficiency and COE of open cycle MHD. The 1200-1500 F liquid metal MHD systems studied do not appear to have the potential of exceeding the efficiency or competing with the COE of advanced steam plants
Normal-metal quasiparticle traps for superconducting qubits
The presence of quasiparticles in superconducting qubits emerges as an
intrinsic constraint on their coherence. While it is difficult to prevent the
generation of quasiparticles, keeping them away from active elements of the
qubit provides a viable way of improving the device performance. Here we
develop theoretically and validate experimentally a model for the effect of a
single small trap on the dynamics of the excess quasiparticles injected in a
transmon-type qubit. The model allows one to evaluate the time it takes to
evacuate the injected quasiparticles from the transmon as a function of trap
parameters. With the increase of the trap size, this time decreases
monotonically, saturating at the level determined by the quasiparticles
diffusion constant and the qubit geometry. We determine the characteristic trap
size needed for the relaxation time to approach that saturation value.Comment: 11 pages, 5 figure
Electron self-energy in A3C60 (A=K, Rb): Effects of t1u plasmon in GW approximation
The electron self-energy of the t1u states in A3C60 (A=K, Rb) is calculated
using the so-called GW approximation. The calculation is performed within a
model which considers the t1u charge carrier plasmon at 0.5 eV and takes into
account scattering of the electrons within the t1u band. A moderate reduction
(35 %) of the t1u band width is obtained.Comment: 4 pages, revtex, 1 figure more information at
http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
Aims. We aim to provide observational constraints on diffusion models that
predict peculiar chemical abundances in the atmospheres of Am stars. We also
intend to check if chemical peculiarities and slow rotation can be explained by
the presence of a weak magnetic field.
Methods. We have obtained high resolution, high signal-to-noise ratio spectra
of eight previously-classified Am stars, two normal A-type stars and one Blue
Straggler, considered to be members of the Praesepe cluster. For all of these
stars we have determined fundamental parameters and photospheric abundances for
a large number of chemical elements, with a higher precision than was ever
obtained before for this cluster. For seven of these stars we also obtained
spectra in circular polarization and applied the LSD technique to constrain the
longitudinal magnetic field.
Results. No magnetic field was detected in any of the analysed stars. HD
73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to
have the abundances of a normal A-type star. Am classification is not confirmed
for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry
that is in good agreement with the observations. There is a generally good
agreement between abundance predictions of diffusion models and values that we
have obtained for the remaining Am stars. However, the observed Na and S
abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.
Li appears to be overabundant in three stars of our sample.Comment: Accepted for publication on A&
Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition
On 11 March 2011, an earthquake occurred about 130 km off the Pacific coast of Japan's main island Honshu, followed by a large tsunami. The resulting loss of electric power at the Fukushima Dai-ichi nuclear power plant developed into a disaster causing massive release of radioactivity into the atmosphere. In this study, we determine the emissions into the atmosphere of two isotopes, the noble gas xenon-133 (<sup>133</sup>Xe) and the aerosol-bound caesium-137 (<sup>137</sup>Cs), which have very different release characteristics as well as behavior in the atmosphere. To determine radionuclide emissions as a function of height and time until 20 April, we made a first guess of release rates based on fuel inventories and documented accident events at the site. This first guess was subsequently improved by inverse modeling, which combined it with the results of an atmospheric transport model, FLEXPART, and measurement data from several dozen stations in Japan, North America and other regions. We used both atmospheric activity concentration measurements as well as, for <sup>137</sup>Cs, measurements of bulk deposition. Regarding <sup>133</sup>Xe, we find a total release of 15.3 (uncertainty range 12.2–18.3) EBq, which is more than twice as high as the total release from Chernobyl and likely the largest radioactive noble gas release in history. The entire noble gas inventory of reactor units 1–3 was set free into the atmosphere between 11 and 15 March 2011. In fact, our release estimate is higher than the entire estimated <sup>133</sup>Xe inventory of the Fukushima Dai-ichi nuclear power plant, which we explain with the decay of iodine-133 (half-life of 20.8 h) into <sup>133</sup>Xe. There is strong evidence that the <sup>133</sup>Xe release started before the first active venting was made, possibly indicating structural damage to reactor components and/or leaks due to overpressure which would have allowed early release of noble gases. For <sup>137</sup>Cs, the inversion results give a total emission of 36.6 (20.1–53.1) PBq, or about 43% of the estimated Chernobyl emission. Our results indicate that <sup>137</sup>Cs emissions peaked on 14–15 March but were generally high from 12 until 19 March, when they suddenly dropped by orders of magnitude at the time when spraying of water on the spent-fuel pool of unit 4 started. This indicates that emissions may not have originated only from the damaged reactor cores, but also from the spent-fuel pool of unit 4. This would also confirm that the spraying was an effective countermeasure. We explore the main dispersion and deposition patterns of the radioactive cloud, both regionally for Japan as well as for the entire Northern Hemisphere. While at first sight it seemed fortunate that westerly winds prevailed most of the time during the accident, a different picture emerges from our detailed analysis. Exactly during and following the period of the strongest <sup>137</sup>Cs emissions on 14 and 15 March as well as after another period with strong emissions on 19 March, the radioactive plume was advected over Eastern Honshu Island, where precipitation deposited a large fraction of <sup>137</sup>Cs on land surfaces. Radioactive clouds reached North America on 15 March and Europe on 22 March. By middle of April, <sup>133</sup>Xe was fairly uniformly distributed in the middle latitudes of the entire Northern Hemisphere and was for the first time also measured in the Southern Hemisphere (Darwin station, Australia). In general, simulated and observed concentrations of <sup>133</sup>Xe and <sup>137</sup>Cs both at Japanese as well as at remote sites were in good quantitative agreement. Altogether, we estimate that 6.4 PBq of <sup>137</sup>Cs, or 18% of the total fallout until 20 April, were deposited over Japanese land areas, while most of the rest fell over the North Pacific Ocean. Only 0.7 PBq, or 1.9% of the total fallout were deposited on land areas other than Japan
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
We study how chemical abundances of late B-, A- and early F-type stars evolve
with time, and we search for correlations between the abundance of chemical
elements and other stellar parameters, such as effective temperature and Vsini.
We have observed a large number of B-, A- and F-type stars belonging to open
clusters of different ages. In this paper we concentrate on the Praesepe
cluster (log t = 8.85), for which we have obtained high resolution, high
signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am
star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For
all the observed stars, we have derived fundamental parameters and chemical
abundances. In addition, we discuss another eight Am stars belonging to the
same cluster, for which the abundance analysis had been presented in a previous
paper. We find a strong correlation between peculiarity of Am stars and Vsini.
The abundance of the elements underabundant in Am stars increases with Vsini,
while it decreases for the overabundant elements. Chemical abundances of
various elements appear correlated with the iron abundance.Comment: Accepted for publication on A&
Advanced Diagnostics for the Study of Linearly Polarized Emission. II: Application to Diffuse Interstellar Radio Synchrotron Emission
Diagnostics of polarized emission provide us with valuable information on the
Galactic magnetic field and the state of turbulence in the interstellar medium,
which cannot be obtained from synchrotron intensity alone. In Paper I (Herron
et al. 2017b), we derived polarization diagnostics that are rotationally and
translationally invariant in the - plane, similar to the polarization
gradient. In this paper, we apply these diagnostics to simulations of ideal
magnetohydrodynamic turbulence that have a range of sonic and Alfv\'enic Mach
numbers. We generate synthetic images of Stokes and for these
simulations, for the cases where the turbulence is illuminated from behind by
uniform polarized emission, and where the polarized emission originates from
within the turbulent volume. From these simulated images we calculate the
polarization diagnostics derived in Paper I, for different lines of sight
relative to the mean magnetic field, and for a range of frequencies. For all of
our simulations, we find that the polarization gradient is very similar to the
generalized polarization gradient, and that both trace spatial variations in
the magnetoionic medium for the case where emission originates within the
turbulent volume, provided that the medium is not supersonic. We propose a
method for distinguishing the cases of emission coming from behind or within a
turbulent, Faraday rotating medium, and a method to partly map the rotation
measure of the observed region. We also speculate on statistics of these
diagnostics that may allow us to constrain the physical properties of an
observed turbulent region.Comment: 34 pages, 25 figures, accepted for publication in Ap
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