663 research outputs found
Electron bunch injection at an angle into a laser wakefield
External injection of electron bunches longer than the plasma wavelength in a
laser wakefield accelerator can lead to the generation of femtosecond
ultrarelativistic bunches with a couple of percent energy spread. Extensive
study has been done on external electron bunch (e.g. one generated by a
photo-cathode rf linac) injection in a laser wakefield for different
configurations. In this paper we investigate a new way of external injection
where the electron bunch is injected at a small angle into the wakefield. This
way one can avoid the ponderomotive scattering as well as the vacuum-plasma
transition region, which tend to destroy the injected bunch. In our
simulations, the effect of the laser pulse dynamics is also taken into account.
It is shown that injection at an angle can provide compressed and accelerated
electron bunches with less than 2% energy spread. Another advantage of this
scheme is that it has less stringent requirements in terms of the size of the
injected bunch and there is the potential to trap more charge
A Mixed Hybrid Algorithm for Integral Wind Farm Optimum Design
. Nowadays, the weight of the generation of
renewable energies has grown spectacularly with regard to
other conventional energies. This is due to diverse factors: a
bigger environmental concern (Kyoto protocol, white-book in
EU, etc.), the raising prices of the traditional fuels, etc. On the
other hand, wind power has experienced a bigger grown, among
the renewable energies. That is why the development of an
effective tool for the design and lay-out of wind farms has a
special relevance.
This paper present a mix of evolutionary algorithms to look for
the optimum integral design of the wind park taking into
account all the part involved: number of wind generators, the
type and the height of the wind generators, its location, the
number and location of substations, the best layout of lowvoltage and high-voltage lines among wind generators,
substations and the existing transmission lines, etc.
Due to problem complexity, and as a first approach, the global
optimization problem has been splitted in two main (uncoupled)
parts, first the wind generators lay out and then the network
configurationMinisterio de Ciencia y Tecnología ENE2004-03342/CONMinisterio de Ciencia y Tecnología DPI2002-04416-C04-02Junta de Andalucía ACC-1021-TIC-200
Density Waves in Layered Systems with Fermionic Polar Molecules
A layered system of two-dimensional planes containing fermionic polar
molecules can potentially realize a number of exotic quantum many-body states.
Among the predictions, are density-wave instabilities driven by the anisotropic
part of the dipole-dipole interaction in a single layer. However, in typical
multilayer setups it is reasonable to expect that the onset and properties of a
density-wave are modified by adjacent layers. Here we show that this is indeed
the case. For multiple layers the critical strength for the density-wave
instability decreases with the number of layers. The effect depends on density
and is more pronounced in the low density regime. The lowest solution of the
instability corresponds to the density waves in the different layers being
in-phase, whereas higher solutions have one or several adjancet layers that are
out of phase. The parameter regime needed to explore this instability is within
reach of current experiments.Comment: 7 pages, 4 figures. Final version in EPJD, EuroQUAM special issue
"Cold Quantum Matter - Achievements and Prospects
Thermodynamics of Dipolar Chain Systems
The thermodynamics of a quantum system of layers containing perpendicularly
oriented dipolar molecules is studied within an oscillator approximation for
both bosonic and fermionic species. The system is assumed to be built from
chains with one molecule in each layer. We consider the effects of the
intralayer repulsion and quantum statistical requirements in systems with more
than one chain. Specifically, we consider the case of two chains and solve the
problem analytically within the harmonic Hamiltonian approach which is accurate
for large dipole moments. The case of three chains is calculated numerically.
Our findings indicate that thermodynamic observables, such as the heat
capacity, can be used to probe the signatures of the intralayer interaction
between chains. This should be relevant for near future experiments on polar
molecules with strong dipole moments.Comment: 15 pages, 5 figures, final versio
Microstructural analysis of Ta-containing NiCoCrAlY bond coats deposited by HVOF on different Ni-based superalloys
Towards a Realistic Neutron Star Binary Inspiral: Initial Data and Multiple Orbit Evolution in Full General Relativity
This paper reports on our effort in modeling realistic astrophysical neutron
star binaries in general relativity. We analyze under what conditions the
conformally flat quasiequilibrium (CFQE) approach can generate
``astrophysically relevant'' initial data, by developing an analysis that
determines the violation of the CFQE approximation in the evolution of the
binary described by the full Einstein theory. We show that the CFQE assumptions
significantly violate the Einstein field equations for corotating neutron stars
at orbital separations nearly double that of the innermost stable circular
orbit (ISCO) separation, thus calling into question the astrophysical relevance
of the ISCO determined in the CFQE approach. With the need to start numerical
simulations at large orbital separation in mind, we push for stable and long
term integrations of the full Einstein equations for the binary neutron star
system. We demonstrate the stability of our numerical treatment and analyze the
stringent requirements on resolution and size of the computational domain for
an accurate simulation of the system.Comment: 22 pages, 18 figures, accepted to Phys. Rev.
Steppes, savannahs, forests and phytodiversity reservoirs during the Pleistocene in the Iberian Peninsula
A palaeobotanical analysis of the Pleistocene floras and vegetation in the Iberian Peninsula shows the existence of patched landscapes with Pinus woodlands, deciduous and mixed forests, parklands (savannah-like), shrublands, steppes and grasslands. Extinctions of Arctotertiary woody taxa are recorded during the Early and Middle Pleistocene, but glacial refugia facilitated the survival of a number of temperate, Mediterranean and Ibero-North African woody angiosperms. The responses of Iberian vegetation to climatic changes during the Pleistocene have been spatially and temporarily complex, including rapid changes of vegetation in parallel to orbital and suborbital variability, and situations of multi-centennial resilience or accommodation to climatic changes. Regional characteristics emerged as soon as for the Middle Pleistocene, if not earlier: Ericaceae in the Atlantic coast indicating wetter climate, thermo-mediterranean elements in the south as currently, and broad-leaf trees in the northeastern. Overall, steppe landscapes and open Pinus woodlands prevailed over many continental regions during the cold spells of the Late Pleistocene. The maintenance of a high phytodiversity during the glacials was linked to several refuge zones in the coastal shelves of the Mediterranean and intramountainous valleys. Northern Iberia, especially on coastal areas, was also patched with populations of tree species, and this is not only documented by palaeobotanical data (pollen, charcoal) but also postulated by phylogeographical models
Layers of Cold Dipolar Molecules in the Harmonic Approximation
We consider the N-body problem in a layered geometry containing cold polar
molecules with dipole moments that are polarized perpendicular to the layers. A
harmonic approximation is used to simplify the hamiltonian and bound state
properties of the two-body inter-layer dipolar potential are used to adjust
this effective interaction. To model the intra-layer repulsion of the polar
molecules, we introduce a repulsive inter-molecule potential that can be
parametrically varied. Single chains containing one molecule in each layer, as
well as multi-chain structures in many layers are discussed and their energies
and radii determined. We extract the normal modes of the various systems as
measures of their volatility and eventually of instability, and compare our
findings to the excitations in crystals. We find modes that can be classified
as either chains vibrating in phase or as layers vibrating against each other.
The former correspond to acoustic and the latter to optical phonons.
Instabilities can occur for large intra-layer repulsion and produce diverging
amplitudes of molecules in the outer layers. Lastly, we consider experimentally
relevant regimes to observe the structures.Comment: 17 pages, 20 figures, accepted versio
Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum
We report a first measurement for ultra-high energy cosmic rays of the
correlation between the depth of shower maximum and the signal in the water
Cherenkov stations of air-showers registered simultaneously by the fluorescence
and the surface detectors of the Pierre Auger Observatory. Such a correlation
measurement is a unique feature of a hybrid air-shower observatory with
sensitivity to both the electromagnetic and muonic components. It allows an
accurate determination of the spread of primary masses in the cosmic-ray flux.
Up till now, constraints on the spread of primary masses have been dominated by
systematic uncertainties. The present correlation measurement is not affected
by systematics in the measurement of the depth of shower maximum or the signal
in the water Cherenkov stations. The analysis relies on general characteristics
of air showers and is thus robust also with respect to uncertainties in
hadronic event generators. The observed correlation in the energy range around
the `ankle' at differs significantly from
expectations for pure primary cosmic-ray compositions. A light composition made
up of proton and helium only is equally inconsistent with observations. The
data are explained well by a mixed composition including nuclei with mass . Scenarios such as the proton dip model, with almost pure compositions, are
thus disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray
flux at Earth.Comment: Published version. Added journal reference and DOI. Added Report
Numbe
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