117 research outputs found
Two-fluid magnetic island dynamics in slab geometry: I - Isolated islands
A set of reduced, 2-D, two-fluid, drift-MHD equations is derived. Using these
equations, a complete and fully self-consistent solution is obtained for an
isolated magnetic island propagating through a slab plasma with uniform but
different ion and electron fluid velocities. The ion and electron fluid flow
profiles around the island are uniquely determined, and are everywhere
continuous. Moreover, the island phase-velocity is uniquely specified by the
condition that there be zero net electromagnetic force acting on the island.
Finally, the ion polarization current correction to the Rutherford island width
evolution equation is evaluated, and found to be stabilizing provided that the
anomalous perpendicular ion viscosity significantly exceeds the anomalous
perpendicular electron viscosity
A non-standard matter distribution in the RS1 model and the coupling constant of the radion
In the zero mode approximation we solve exactly the equations of motion for
linearized gravity in the Randall-Sundrum model with a non-standard
distribution of matter in the neighbourhood of the negative tension brane. It
is shown that the form of this distribution can strongly affect the coupling of
the radion to matter. We believe that such a situation can arise in models with
a realistic mechanisms of matter localization.Comment: 12 pages, LaTe
Physical degrees of freedom in stabilized brane world models
We consider brane world models with interbrane separation stabilized by the
Goldberger-Wise scalar field. For arbitrary background, or vacuum
configurations of the gravitational and scalar fields in such models, we
construct the second variation Lagrangian, study its gauge invariance, find the
corresponding equations of motion and decouple them in a suitable gauge. We
also derive an effective four-dimensional Lagrangian for such models, which
describes the massless graviton, a tower of massive gravitons and a tower of
massive scalars. It is shown that for a special choice of the background
solution the masses of the graviton excitations may be of the order of a few
TeV, the radion mass of the order of 100 GeV, the inverse size of the extra
dimension being tens of GeV. In this case the coupling of the radion to matter
on the negative tension brane is approximately the same as in the unstabilized
model with the same values of the fundamental five-dimensional energy scale and
the interbrane distance.Comment: 17 pages, LaTeX, corrected typos, amended the normalization constants
of the scalar modes and their coupling constants to matte
Effective Lagrangians for physical degrees of freedom in the Randall-Sundrum model
We derive the second variation Lagrangian of the Randall-Sundrum model with
two branes, study its gauge invariance and diagonalize it in the unitary gauge.
We also show that the effective four-dimensional theory looks different on
different branes and calculate the observable mass spectra and the couplings of
the physical degrees of freedom of 5-dimensional gravity to matter.Comment: 22 pages, LaTeX, typos correcte
Low-energy gluon contributions to the vacuum polarization of heavy quarks
We calculate a correction to the electromagnetic current induced by a heavy
quark loop. The contribution of this correction to the vacuum polarization
function appears at the O(alpha_s^3) order of perturbation theory and has a
qualitatively new feature -- its absorptive part starts at zero energy in
contrast to other contributions where the absorptive parts start at the
two-particle threshold. Our result imposes a constraint on the order n of the
moments used in the heavy-quark sum rules, n<4.Comment: 8 pages in LaTeX, 1 PostScript figure included in the tex
Hierarchy problem and the cosmological constant in a five-dimensional Brans-Dicke brane world model
We discuss a new solution, admitting the existence of dS_{4} branes, in
five-dimensional Brans-Dicke theory. It is shown that, due to a special form of
a bulk scalar field potential, for certain values of the model parameters the
effective cosmological constant can be made small on the brane, where the
hierarchy problem of gravitational interaction is solved. We also discuss new
stabilization mechanism which is based on the use of auxiliary fields.Comment: 11 page
Understanding the effect of sheared flow on microinstabilities
The competition between the drive and stabilization of plasma
microinstabilities by sheared flow is investigated, focusing on the ion
temperature gradient mode. Using a twisting mode representation in sheared slab
geometry, the characteristic equations have been formulated for a dissipative
fluid model, developed rigorously from the gyrokinetic equation. They clearly
show that perpendicular flow shear convects perturbations along the field at a
speed we denote by (where is the sound speed), whilst parallel
flow shear enters as an instability driving term analogous to the usual
temperature and density gradient effects. For sufficiently strong perpendicular
flow shear, , the propagation of the system characteristics is
unidirectional and no unstable eigenmodes may form. Perturbations are swept
along the field, to be ultimately dissipated as they are sheared ever more
strongly. Numerical studies of the equations also reveal the existence of
stable regions when , where the driving terms conflict. However, in both
cases transitory perturbations exist, which could attain substantial amplitudes
before decaying. Indeed, for , they are shown to exponentiate
times. This may provide a subcritical route to turbulence in
tokamaks.Comment: minor revisions; accepted to PPC
Linearized gravity on the Randall-Sundrum two-brane background with curvature terms in the action for the branes
We study gravitational perturbations in the Randall-Sundrum two-brane
background with scalar-curvature terms in the action for the branes, allowing
for positive as well as negative bulk gravitational constant. In the zero-mode
approximation, we derive the linearized gravitational equations, which have the
same form as in the original Randall-Sundrum model but with different
expressions for the effective physical constants. We develop a generic method
for finding tachyonic modes in the theory, which, in the model under
consideration, may exist only if the bulk gravitational constant is negative.
In this case, if both brane gravitational constants are nonzero, the theory
contains one or two tachyonic mass eigenvalues in the gravitational sector. If
one of the brane gravitational constants is set to zero, then either a single
tachyonic mass eigenvalue is present or tachyonic modes are totally absent
depending on the relation between the nonzero brane gravitational constant and
brane separation. In the case of negative bulk gravitational constant, the
massive gravitational modes have ghost-like character, while the massless
gravitational mode is not a ghost in the case where tachyons are absent.Comment: 23 pages, revtex, published versio
Brane world corrections to Newton's law
We discuss possible variations of the effective gravitational constant with
length scale, predicted by most of alternative theories of gravity and unified
models of physical interactions. After a brief general exposition, we review in
more detail the predicted corrections to Newton's law of gravity in diverse
brane world models. We consider various configurations in 5 dimensions (flat,
de Sitter and AdS branes in Einstein and Einstein-Gauss-Bonnet theories, with
and without induced gravity and possible incomplete graviton localization), 5D
multi-brane systems and some models in higher dimensions. A common feature of
all models considered is the existence of corrections to Newton's law at small
radii comparable with the bulk characteristic length: at such radii, gravity on
the brane becomes effectively multidimensional. Many models contain superlight
perturbation modes, which modify gravity at large scale and may be important
for astrophysics and cosmology.Comment: Brief review, 16 pages, 92 references. Some description and
references adde
Secondary instability in drift wave turbulence as a mechanism for zonal flow and avalanche formation
The article reports on recent developments in the theory of secondary instability in drift-ion temperature gradient turbulence. Specifically, the article explores secondary instability as a mechanism for zonal flow generation, transport barrier dynamics and avalanche formation. These in turn are related to the space-time statistics of the drift wave induced flux, the scaling of transport with collisionality and β, and the spatio-temporal evolution of transport barriers
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