2,191 research outputs found
Almost reducibility for finitely differentiable SL(2,R)-valued quasi-periodic cocycles
Quasi-periodic cocycles with a diophantine frequency and with values in
SL(2,R) are shown to be almost reducible as long as they are close enough to a
constant, in the topology of k times differentiable functions, with k great
enough. Almost reducibility is obtained by analytic approximation after a loss
of differentiability which only depends on the frequency and on the constant
part. As in the analytic case, if their fibered rotation number is diophantine
or rational with respect to the frequency, such cocycles are in fact reducible.
This extends Eliasson's theorem on Schr\"odinger cocycles to the differentiable
case
Instability and dynamics of two nonlinearly coupled laser beams in a plasma
We investigate the nonlinear interaction between two laser beams in a plasma
in the weakly nonlinear and relativistic regime. The evolution of the laser
beams is governed by two nonlinear Schroedinger equations that are coupled with
the slow plasma density response. We study the growth rates of the Raman
forward and backward scattering instabilities as well of the Brillouin and
self-focusing/modulational instabilities. The nonlinear evolution of the
instabilities is investigated by means of direct simulations of the
time-dependent system of nonlinear equations.Comment: 18 pages, 8 figure
High-quality ion beams by irradiating a nano-structured target with a petawatt laser pulse
We present a novel laser based ion acceleration scheme, where a petawatt
circularly polarized laser pulse is shot on an ultra-thin (nano-scale)
double-layer target. Our scheme allows the production of high-quality light ion
beams with both energy and angular dispersion controllable by the target
properties. We show that extraction of all electrons from the target by
radiation pressure can lead to a very effective two step acceleration process
for light ions if the target is designed correctly. Relativistic protons should
be obtainable with pulse powers of a few petawatt. Careful analytical modeling
yields estimates for characteristic beam parameters and requirements on the
laser pulse quality, in excellent agreement with one and two-dimensional
Particle-in Cell simulations.Comment: 18 pages, 7 figures, accepted in New. J. Phy
Electrostatic pair creation and recombination in quantum plasmas
The collective production of electron-positron pairs by electrostatic waves
in quantum plasmas is investigated. In particular, a semi-classical governing
set of equation for a self-consistent treatment of pair creation by the
Schwinger mechanism in a quantum plasma is derived.Comment: 4 pages, 3 figures, to appear in JETP Letter
A possibility to measure elastic photon--photon scattering in vacuum
Photon--photon scattering in vacuum due to the interaction with virtual
electron-positron pairs is a consequence of quantum electrodynamics. A way for
detecting this phenomenon has been devised based on interacting modes generated
in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo,
Phys. Rev. Lett. \textbf{87} 171801 (2001)]. Here we materialize these ideas,
suggest a concrete cavity geometry, make quantitative estimates and propose
experimental details. It is found that detection of photon-photon scattering
can be within the reach of present day technology.Comment: 7 pages, 3 figure
Kolmogorov condition near hyperbolic singularities of integrable Hamiltonian systems
In this paper we show that, if an integrable Hamiltonian system admits a
nondegenerate hyperbolic singularity then it will satisfy the Kolmogorov
condegeneracy condition near that singularity (under a mild additional
condition, which is trivial if the singularity contains a fixed point)Comment: revised version, 11p, accepted for publication in a sepecial volume
in Regular and Chaotic Dynamics in honor of Richard Cushma
Instability and Evolution of Nonlinearly Interacting Water Waves
We consider the modulational instability of nonlinearly interacting
two-dimensional waves in deep water, which are described by a pair of
two-dimensional coupled nonlinear Schroedinger equations. We derive a nonlinear
dispersion relation. The latter is numerically analyzed to obtain the regions
and the associated growth rates of the modulational instability. Furthermore,
we follow the long term evolution of the latter by means of computer
simulations of the governing nonlinear equations and demonstrate the formation
of localized coherent wave envelopes. Our results should be useful for
understanding the formation and nonlinear propagation characteristics of large
amplitude freak waves in deep water.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
New Precision Electroweak Tests in Supergravity Models
We update the analysis of the precision electroweak tests in terms of 4
epsilon parameters, , to obtain more accurate experimental
values of them by taking into account the new LEP data released at the 28th
ICHEP (1996, Poland). We also compute and in the
context of the no-scale supergravity model to obtain the
updated constraints by imposing the correlated constraints in terms of the
experimental ellipses in the plane and also by imposing
the new bound on the lightest chargino mass, .
Upon imposing these new experimental results, we find that the situations in
the no-scale model are much more favorable than those in the standard model,
and if , then the allowed regions at the 95% C.~L. in
the no-scale model are and for , which are in fact much more stringent than in
our previous analysis. Therefore, assuming that , if the
lightest chargino mass bound were to be pushed up only by a few GeV, the sign
on the Higgs mixing term in the no-scale model could well be determined
from the constraint to be positive at the 95% C.~L. At
any rate, better accuracy in the measured from the Tevatron in the near
future combined with the LEP data is most likely to provide a decisive test of
the no-scale supergravity model.Comment: 15 pages, REVTEX, 1 figure (not included but available as a ps file
from [email protected]
Electromagnetic waves and bursty electron acceleration: implications from Freja
International audienceDispersive Alfvén wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extremely low frequency (ELF) emissions below the lower hybrid cutoff frequency (a few kHz). Large-scale density depletions/cavities, as determined by the Langmuir probe measurements, and strong electrostatic emissions are often observed simultaneously. A correlation study has been carried out between the E and B field fluctuations below 64 Hz and 10 Hz, respectively, (the DC instruments upper threshold) and the characteristics of the precipitating electrons. This study revealed that the energisation of electrons is indeed related to the broadband ELF emissions and that the electrostatic component plays a predominant role during very active magnetospheric conditions. Furthermore, the effect of the ELF electromagnetic emissions on the larger scale field-aligned current systems has been investigated, and it is found that such an effect cannot be detected. Instead, the Alfvénic activity creates a local region of field-aligned currents. It is suggested that dispersive Alfvén waves set up these local field-aligned current regions and, in turn, trigger more electrostatic emissions during certain conditions. In these regions, ions are transversely heated, and large-scale density depletions/cavities may be created during especially active periods
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