12,914 research outputs found
Relative periodic orbits in point vortex systems
We give a method to determine relative periodic orbits in point vortex
systems: it consists mainly into perform a symplectic reduction on a fixed
point submanifold in order to obtain a two-dimensional reduced phase space. The
method is applied to point vortices systems on a sphere and on the plane, but
works for other surfaces with isotropy (cylinder, ellipsoid, ...). The method
permits also to determine some relative equilibria and heteroclinic cycles
connecting these relative equilibria.Comment: 27 pages, 17 figure
U(N) tools for Loop Quantum Gravity: The Return of the Spinor
We explore the classical setting for the U(N) framework for SU(2)
intertwiners for loop quantum gravity (LQG) and describe the corresponding
phase space in terms of spinors with appropriate constraints. We show how its
quantization leads back to the standard Hilbert space of intertwiner states
defined as holomorphic functionals. We then explain how to glue these
intertwiners states in order to construct spin network states as wave-functions
on the spinor phase space. In particular, we translate the usual loop gravity
holonomy observables to our classical framework. Finally, we propose how to
derive our phase space structure from an action principle which induces
non-trivial dynamics for the spin network states. We conclude by applying
explicitly our framework to states living on the simple 2-vertex graph and
discuss the properties of the resulting Hamiltonian.Comment: 23 page
New tools for Loop Quantum Gravity with applications to a simple model
Loop Quantum Gravity is now a well established approach to quantum gravity.
One of the main challenges still faced by the theory is constructing a
consistent dynamics which would lead back to the standard dynamics of the
gravitational field at large scales. Here we will review the recent U(N)
framework for Loop Quantum Gravity and the new spinor representation (that
provides a classical setting for the U(N) framework). Then, we will apply these
techniques to a simple model in order to propose a dynamics for a symmetry
reduced sector of the theory. Furthermore, we will explore certain analogies of
this model with Loop Quantum Cosmology.Comment: 4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011
(ERE 2011) held in Madrid, Spai
Kinetic modelling of runaway electron avalanches in tokamak plasmas
Runaway electrons (REs) can be generated in tokamak plasmas if the
accelerating force from the toroidal electric field exceeds the collisional
drag force due to Coulomb collisions with the background plasma. In ITER,
disruptions are expected to generate REs mainly through knock-on collisions,
where enough momentum can be transferred from existing runaways to slow
electrons to transport the latter beyond a critical momentum, setting off an
avalanche of REs. Since knock-on runaways are usually scattered off with a
significant perpendicular component of the momentum with respect to the local
magnetic field direction, these particles are highly magnetized. Consequently,
the momentum dynamics require a full 3-D kinetic description, since these
electrons are highly sensitive to the magnetic non-uniformity of a toroidal
configuration. A bounce-averaged knock-on source term is derived. The
generation of REs from the combined effect of Dreicer mechanism and knock-on
collision process is studied with the code LUKE, a solver of the 3-D linearized
bounce-averaged relativistic electron Fokker-Planck equation, through the
calculation of the response of the electron distribution function to a constant
parallel electric field. This work shows that the avalanche effect can be
important even in non-disruptive scenarios. RE formation through knock-on
collisions is found to be strongly reduced when taking place off the magnetic
axis, since trapped electrons cannot contribute to the RE population. The
relative importance of the avalanche mechanism is investigated as a function of
the key parameters for RE formation; the plasma temperature and the electric
field strength. In agreement with theoretical predictions, the simulations show
that in low temperature and E-field knock-on collisions are the dominant source
of REs and can play a significant role for RE generation, including in
non-disruptive scenarios.Comment: 23 pages, 12 figure
Compton telescope with coded aperture mask: Imaging with the INTEGRAL/IBIS Compton mode
Compton telescopes provide a good sensitivity over a wide field of view in
the difficult energy range running from a few hundred keV to several MeV. Their
angular resolution is, however, poor and strongly energy dependent. We present
a novel experimental design associating a coded mask and a Compton detection
unit to overcome these pitfalls. It maintains the Compton performance while
improving the angular resolution by at least an order of magnitude in the field
of view subtended by the mask. This improvement is obtained only at the expense
of the efficiency that is reduced by a factor of two. In addition, the
background corrections benefit from the coded mask technique, i.e. a
simultaneous measurement of the source and background. This design is
implemented and tested using the IBIS telescope on board the INTEGRAL satellite
to construct images with a 12' resolution over a 29 degrees x 29 degrees field
of view in the energy range from 200 keV to a few MeV. The details of the
analysis method and the resulting telescope performance, particularly in terms
of sensitivity, are presented
A Study of the Formation of Single- and Double-Walled Carbon Nanotubes by a CVD Method
The reduction in H2/CH4 atmosphere of aluminum-iron oxides produces metal particles small enough to catalyze the formation of single-walled carbon nanotubes. Several experiments have been made using the same temperature profile and changing only the maximum temperature (800-1070 °C). Characterizations of the catalyst materials are performed using notably 57Fe Mo¨ssbauer spectroscopy. Electron microscopy and a macroscopical method are used to characterize the nanotubes. The nature of the iron species (Fe3+, R-Fe, ç-Fe-C, Fe3C) is correlated to their location in the material. The nature of the particles responsible for the high-temperature formation of the nanotubes is probably an Fe-C alloy which is, however, found as Fe3C by postreaction analysis. Increasing the reduction temperature increases the reduction yield and thus favors the formation of surface-metal particles, thus producing more nanotubes. The obtained carbon nanotubes are mostly single-walled and double-walled with an average diameter close to 2.5 nm. Several formation mechanisms are thought to be active. In particular, it is shown that the second wall can grow inside the first one but that subsequent ones are formed outside. It is also possible that under given experimental conditions, the smallest (<2 nm) catalyst particles preferentially produce double-walled rather than single-walled carbon nanotubes
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
Revealing the active galactic nucleus in the superantennae through L-band spectroscopy
We present an L-band spectrum of the Ultraluminous Infrared Galaxy IRAS
19254-7245 (the Superantennae), obtained with VLT-ISAAC. The high signal to
noise ratio allows a study of the main spectral features with unprecedented
detail for an extragalactic source. We argue that the main energy source in the
IR is an obscured AGN. This is indicated by the low equivalent width of the 3.3
micron PAH feature, the broad absorption feature at 3.4 um, and the steep
continuum at lambda>3.7 um (f_lambda ~lambda^(2.7)). The substructure of the
3.4 um absorption feature indicates that the absorption is due to hydrocarbon
chains of 6-7 carbon atoms.Comment: 12 pages, 3 figures. ApJ Letters, in pres
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