7,732 research outputs found
Spin contribution to the ponderomotive force in a plasma
The concept of a ponderomotive force due to the intrinsic spin of electrons
is developed. An expression containing both the classical as well as the
spin-induced ponderomotive force is derived. The results are used to
demonstrate that an electromagnetic pulse can induce a spin-polarized plasma.
Furthermore, it is shown that for certain parameters, the nonlinear
back-reaction on the electromagnetic pulse from the spin magnetization current
can be larger than that from the classical free current. Suitable parameter
values for a direct test of this effect are presented.Comment: 4 pages, 2 figures, version accepted for publication in Physical
Review Letter
Evidence for a connection between the gamma-ray and the highest energy cosmic-ray emissions by BL Lacertae objects
A set of potentially gamma-ray--loud BL Lac objects is selected by
intersecting the EGRET and BL Lac catalogs. Of the resulting 14 objects, eight
are found to correlate with arrival directions of ultra--high-energy cosmic
rays (UHECRs), with significance of the order of 5 sigma. This suggests that
gamma-ray emission can be used as a distinctive feature of those BL Lac objects
that are capable of producing UHECR.Comment: 11 pages, 1 figure, version published in APJ Letter
Two-body Photodisintegration of He with Full Final State Interaction
The cross sections of the processes He()H and
He()He are calculated taking into account the full final
state interaction via the Lorentz integral transform (LIT) method. This is the
first consistent microscopic calculation beyond the three--body breakup
threshold. The results are obtained with a semirealistic central NN potential
including also the Coulomb force. The cross sections show a pronounced dipole
peak at 27 MeV which lies within the rather broad experimental band. At higher
energies, where experimental uncertainties are considerably smaller, one finds
a good agreement between theory and experiment. The calculated sum of three--
and four--body photodisintegration cross sections is also listed and is in fair
agreement with the data.Comment: 18 pages, 6 figure
Laser-plasma interactions with a Fourier-Bessel Particle-in-Cell method
A new spectral particle-in-cell (PIC) method for plasma modeling is presented
and discussed. In the proposed scheme, the Fourier-Bessel transform is used to
translate the Maxwell equations to the quasi-cylindrical spectral domain. In
this domain, the equations are solved analytically in time, and the spatial
derivatives are approximated with high accuracy. In contrast to the
finite-difference time domain (FDTD) methods that are commonly used in PIC, the
developed method does not produce numerical dispersion, and does not involve
grid staggering for the electric and magnetic fields. These features are
especially valuable in modeling the wakefield acceleration of particles in
plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the
test simulations of laser plasma interactions are compared to the ones done
with the quasi-cylindrical FDTD PIC code CALDER-CIRC.Comment: submitted to Phys. Plasma
Ultra cold neutron trap as a tool to search for dark matter with long-range radius of forces
The problem of possible application of an ultracold neutron (UCN) trap as a
detector of dark matter particles with long-range radius of forces has been
considered. Transmission of small recoil energy in scattering is a
characteristic of long-range forces. The main advantage of the ultracold
neutron technique lies in possibility of detecting recoil energy as small as
eV. Here are presented constraints on the interaction potential
parameters: for dark matter particles and
neutrons, as well as on the density value of long-range dark matter on the
Earth. The possible mechanism of accumulation of long-range dark matter on the
Earth surface is considered and the factor of density increase on the Earth
surface is evaluated. The results of the first experiment on search of
astronomical day variation of ultracold neutron storage time are under
discussion.Comment: 17 pages, 19 figures. arXiv admin note: substantial text overlap with
arXiv:1109.339
Laser beam coupling with capillary discharge plasma for laser wakefield acceleration applications
One of the most robust methods, demonstrated up to date, of accelerating
electron beams by laser-plasma sources is the utilization of plasma channels
generated by the capillary discharges. These channels, i.e., plasma columns
with a minimum density along the laser pulse propagation axis, may optically
guide short laser pulses, thereby increasing the acceleration length, leading
to a more efficient electron acceleration. Although the spatial structure of
the installation is simple in principle, there may be some important effects
caused by the open ends of the capillary, by the supplying channels etc., which
require a detailed 3D modeling of the processes taking place in order to get a
detailed understanding and improve the operation. However, the discharge
plasma, being one of the most crucial components of the laser-plasma
accelerator, is not simulated with the accuracy and resolution required to
advance this promising technology. In the present work, such simulations are
performed using the code MARPLE. First, the process of the capillary filling
with a cold hydrogen before the discharge is fired, through the side supply
channels is simulated. The main goal of this simulation is to get a spatial
distribution of the filling gas in the region near the open ends of the
capillary. A realistic geometry is used for this and the next stage
simulations, including the insulators, the supplying channels as well as the
electrodes. Second, the simulation of the capillary discharge is performed with
the goal to obtain a time-dependent spatial distribution of the electron
density near the open ends of the capillary as well as inside the capillary.
Finally, to evaluate effectiveness of the beam coupling with the channeling
plasma wave guide and electron acceleration, modeling of laser-plasma
interaction was performed with the code INF&RNOComment: 11 pages, 9 figure
Velocity of Light in Dark Matter with Charge
We propose an interesting mechanism to reconcile the recent experiments of
the Michelson-Morley type and slowdown of the velocity of light in dark matter
with a fractional electric charge when the index of refraction of dark matter
depends on the frequency of a photon. After deriving the formula for the
velocity of light in a medium with the index of refraction in a
relativistic regime, it is shown that the local anisotropy of the light speed
is proportional to the second order in . This result implies
that the experiments of the Michelson-Morley type do not give rise to a
stringent constraint on the slowdown of the velocity of light in dark matter
with electric charge.Comment: 10 page
Instabilities for a relativistic electron beam interacting with a laser irradiated plasma
The effects of a radiation field (RF) on the unstable modes developed in
relativistic electron beam--plasma interaction are investigated assuming that
, where is the frequency of the RF and
is the plasma frequency. These unstable modes are parametrically
coupled to each other due to the RF and are a mix between two--stream and
parametric instabilities. The dispersion equations are derived by the
linearization of the kinetic equations for a beam--plasma system as well as the
Maxwell equations. In order to highlight the effect of the radiation field we
present a comparison of our analytical and numerical results obtained for
nonzero RF with those for vanishing RF. Assuming that the drift velocity
of the beam is parallel to the wave vector of the
excitations two particular transversal and parallel configurations of the
polarization vector of the RF with respect to are
considered in detail. It is shown that in both geometries resonant and
nonresonant couplings between different modes are possible. The largest growth
rates are expected at the transversal configuration when is
perpendicular to . In this case it is demonstrated that in general
the spectrum of the unstable modes in -- plane is split into two
distinct domains with long and short wavelengths, where the unstable modes are
mainly sensitive to the beam or the RF parameters, respectively. In parallel
configuration, , and at short wavelengths
the growth rates of the unstable modes are sensitive to both beam and RF
parameters remaining insensitive to the RF at long wavelengths.Comment: 23 pages, 5 figure
Lorentz-violation and cosmological perturbations: a toy brane-world model
We study possible effects of Lorentz-violation on the generation of
cosmological perturbations at inflation by introducing a simple inflating
five-dimensional brane-world setup with violation of four-dimensional
Lorentz-invariance at an energy scale . We consider massless scalar field,
meant to mimic perturbations of inflaton and/or gravitational field, in this
background. At three-momenta below , there exists a zero mode localized on
the brane, whose behaviour coincides with that in four-dimensional theory. On
the contrary, at three-momenta above , the localized mode is absent and
physics is entirely five-dimensional. As three-momenta get redshifted, more
modes get localized on the brane, the phenomenon analogous to ``mode
generation''. We find that for , where is the inflationary Hubble
scale, the spectrum of perturbations coincides with that in four-dimensional
theory. For and time-dependent bulk parameters, the spectrum deviates,
possibly strongly, from the flat spectrum even for pure de Sitter inflation.Comment: 5 figures, iopart, minor changes, appendix adde
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