8,722 research outputs found
Plasma wake inhibition at the collision of two laser pulses in an underdense plasma
An electron injector concept for laser-plasma accelerator was developed in
ref [1] and [2] ; it relies on the use of counter-propagating ultrashort laser
pulses. In [2], the scheme is as follows: the pump laser pulse generates a
large amplitude laser wakefield (plasma wave). The counter-propagating
injection pulse interferes with the pump laser pulse to generate a beatwave
pattern. The ponderomotive force of the beatwave is able to inject plasma
electrons into the wakefield. We have studied this injection scheme using 1D
Particle in Cell (PIC) simulations. The simulations reveal phenomena and
important physical processes that were not taken into account in previous
models. In particular, at the collision of the laser pulses, most plasma
electrons are trapped in the beatwave pattern and cannot contribute to the
collective oscillation supporting the plasma wave. At this point, the fluid
approximation fails and the plasma wake is strongly inhibited. Consequently,
the injected charge is reduced by one order of magnitude compared to the
predictions from previous models.Comment: 4 pages, 4 figure
Quasimonoenergetic electron beams produced by colliding cross-polarized laser pulses in underdense plasmas
The interaction of two laser pulses in an underdense plasma has proven to be
able to inject electrons in plasma waves, thus providing a stable and tunable
source of electrons. Whereas previous works focused on the "beatwave" injection
scheme in which two lasers with the same polarization collide in a plasma, this
present letter studies the effect of polarization and more specifically the
interaction of two colliding cross-polarized laser pulses. It is shown both
theoretically and experimentally that electrons can also be pre-accelerated and
injected by the stochastic heating occurring at the collision of two
cross-polarized lasers and thus, a new regime of optical injection is
demonstrated. It is found that injection with cross-polarized lasers occurs at
higher laser intensities.Comment: 4 pages, 4 figure
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
Upper bound on the density of Ruelle resonances for Anosov flows
Using a semiclassical approach we show that the spectrum of a smooth Anosov
vector field V on a compact manifold is discrete (in suitable anisotropic
Sobolev spaces) and then we provide an upper bound for the density of
eigenvalues of the operator (-i)V, called Ruelle resonances, close to the real
axis and for large real parts.Comment: 57 page
Collisional excitation of doubly and triply deuterated ammonia NDH and ND by H
The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the NDH and ND isotopologues. These calculations are based on
the most accurate NH--H potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below 100 cm. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NHD, NDH and ND
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NHD and NHD are found to be consistent with
the statistical ratios
Mechanical Properties and Oxidation Behaviour of Electroconductive Ceramic Composites
International audienceDense electroconductive ceramic-ceramic composites silicon carbide-hafnium diboride (SiC-HfB2) and silicon carbide-hafnium carbide (SiC-HfC) were obtained by Hot Pressing (HP). In view of the results, the high performance composite grade SiC-HfB2 has also been elaborated by Hot Isostatic Pressing (HIP). For 25 mol % HfC or HfB2 content, the resistivity was low enough to allow electrodischarged machining (EDM). The mechanical and thermal properties as well as the wear and oxidation behaviours were evaluated and compared. The electroconductive boride composite (75-25 mol% SiC-HfB2) exhibits high mechanical properties. The benefit of the diboride phase's presence is also noticed in fluent oxygen, up to 1450°C. The SiC-HfB2 composite is as resistant as silicon carbide. This behaviour may be related to the formation of a borosilicate based oxide layer containing hafnium phases, which plays the role of a coating and which limits the B2O3 evaporation
Probing the Slope of Cluster Mass Profile with Gravitational Einstein Rings: Application to Abell 1689
The strong lensing modelling of gravitational ``rings'' formed around massive
galaxies is sensitive to the amplitude of the external shear and convergence
produced by nearby mass condensations. In current wide field surveys, it is now
possible to find out a large number of rings, typically 10 gravitational rings
per square degree. We propose here, to systematically study gravitational rings
around galaxy clusters to probe the cluster mass profile beyond the cluster
strong lensing regions. For cluster of galaxies with multiple arc systems, we
show that rings found at various distances from the cluster centre can improve
the modelling by constraining the slope of the cluster mass profile. We outline
the principle of the method with simple numerical simulations and we apply it
to 3 rings discovered recently in Abell~1689. In particular, the lens modelling
of the 3 rings confirms that the cluster is bimodal, and favours a slope of the
mass profile steeper than isothermal at a cluster radius \sim 300 \kpc. These
results are compared with previous lens modelling of Abell~1689 including weak
lensing analysis. Because of the difficulty arising from the complex mass
distribution in Abell~1689, we argue that the ring method will be better
implemented on simpler and relaxed clusters.Comment: Accepted for publication in MNRAS. Substantial modification after
referee's repor
NH_3(1_0-0_0) in the pre-stellar core L1544
Pre-stellar cores represent the initial conditions in the process of star and
planet formation, therefore it is important to study their physical and
chemical structure. Because of their volatility, nitrogen-bearing molecules are
key to study the dense and cold gas present in pre-stellar cores. The NH_3
rotational transition detected with Herschel-HIFI provides a unique combination
of sensitivity and spectral resolution to further investigate physical and
chemical processes in pre-stellar cores. Here we present the velocity-resolved
Herschel-HIFI observations of the ortho-NH_3(1_0-0_0) line at 572 GHz and study
the abundance profile of ammonia across the pre-stellar core L1544 to test
current theories of its physical and chemical structure.
Recently calculated collisional coefficients have been included in our
non-LTE radiative transfer code to reproduce Herschel observations. A gas-grain
chemical model, including spin-state chemistry and applied to the (static)
physical structure of L1544 is also used to infer the abundance profile of
ortho-NH_3 . The hyperfine structure of ortho-NH_3(1_0-0_0) is resolved for the
first time in space. All the hyperfine components are strongly self-absorbed.
The profile can be reproduced if the core is contracting in quasi-equilibrium,
consistent with previous work, and if the NH_3 abundance is slightly rising
toward the core centre, as deduced from previous interferometric observations
of para-NH_3(1,1). The chemical model overestimates the NH_3 abundance at radii
between ~ 4000 and 15000 AU by about two orders of magnitude and underestimates
the abundance toward the core centre by more than one order of magnitude. Our
observations show that chemical models applied to static clouds have problems
in reproducing NH_3 observations.Comment: accepted for publication in A&A Letter
- âŠ