741 research outputs found
Stochastic electron heating in the laser and quasi-static electric and magnetic fields
The dynamics of relativistic electrons in the intense laser radiation and
quasi-static electromagnetic fields both along and across to the laser
propagating direction are studied in the 3/2 dimensional Hamiltonian framework.
It is shown that the unperturbed oscillations of the relativistic electron in
these electric fields could exhibit a long tail of harmonics which makes an
onset of stochastic electron motion be a primary candidate for electron
heating. The Poincar\'e mappings describing the electron motions in the laser
and electric fields only are derived from which the criterions for instability
are obtained. It follows that for both transverse and longitudinal electric
fields, there exist upper limits of the stochastic electron energy depending on
the laser intensity and electric field strength. Specifically, these maximum
stochastic energies are enhanced by a strong laser intensity but weak electric
field. Such stochastic heating would be reduced by the superluminal phase
velocity in both cases. The impacts of the magnetic fields on the electron
dynamics are different for these two cases and discussed qualitatively. These
analytic results are confirmed by the numerical simulations of solving the 3/2D
Hamiltonian equations directly
Axisymmetric equilibria of a gravitating plasma with incompressible flows
It is found that the ideal magnetohydrodynamic equilibrium of an axisymmetric
gravitating magnetically confined plasma with incompressible flows is governed
by a second-order elliptic differential equation for the poloidal magnetic flux
function containing five flux functions coupled with a Poisson equation for the
gravitation potential, and an algebraic relation for the pressure. This set of
equations is amenable to analytic solutions. As an application, the
magnetic-dipole static axisymmetric equilibria with vanishing poloidal plasma
currents derived recently by Krasheninnikov, Catto, and Hazeltine [Phys. Rev.
Lett. {\bf 82}, 2689 (1999)] are extended to plasmas with finite poloidal
currents, subject to gravitating forces from a massive body (a star or black
hole) and inertial forces due to incompressible sheared flows. Explicit
solutions are obtained in two regimes: (a) in the low-energy regime
, where
, , , and are related to the thermal,
poloidal-current, flow and gravitating energies normalized to the
poloidal-magnetic-field energy, respectively, and (b) in the high-energy regime
. It turns out
that in the high-energy regime all four forces, pressure-gradient,
toroidal-magnetic-field, inertial, and gravitating contribute equally to the
formation of magnetic surfaces very extended and localized about the symmetry
plane such that the resulting equilibria resemble the accretion disks in
astrophysics.Comment: 12 pages, latex, to be published in Geophys. Astrophys. Fluid
Dynamic
Computational study of boron nitride nanotube synthesis: how catalyst morphology stabilizes the boron nitride bond
In an attempt to understand why catalytic methods for the growth of boron
nitride nanotubes work much worse than for their carbon counterparts, we use
first-principles calculations to study the energetics of elemental reactions
forming N2, B2 and BN molecules on an iron catalyst. We observe that in the
case of these small molecules, the catalytic activity is hindered by the
formation of B2 on the iron surface. We also observe that the local morphology
of a step edge present in our nanoparticle model stabilizes the boron nitride
molecule with respect to B2 due to the ability of the step edge to offer sites
with different coordination simultaneously for nitrogen and boron. Our results
emphasize the importance of atomic steps for a high yield chemical vapor
deposition growth of BN nanotubes and may outline new directions for improving
the efficiency of the method.Comment: submitted to physical review
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