766 research outputs found
Modelling the electric field applied to a tokamak
The vector potential for the Ohmic heating coil system of a tokamak is
obtained in semi-analytical form. Comparison is made to the potential of a
simple, finite solenoid. In the quasi-static limit, the time rate of change of
the potential determines the induced electromotive force through the
Maxwell-Lodge effect. Discussion of the gauge constraint is included.Comment: 13 pages, 7 figures, final versio
Classical Lattice Gauge Fields with Hard Thermal Loops
We propose a formulation of the long-distance dynamics of gauge theories at
finite temperature on a lattice in Minkowski space, including the effects of
hard thermal loops on the dynamics of the long wavelength modes. Our approach
is based on the dual classical limits of quantum fields as waves and particles
in the infrared and ultraviolet limits, respectively. It exhibits manifest
invariance under space-dependent lattice gauge transformations and conserves
Gauss' law.Comment: 11 pages, plain latex file, with major revisions to include details
on the algorith
Spatial distribution of photoelectrons participating in formation of x-ray absorption spectra
Interpretation of x-ray absorption near-edge structure (XANES) experiments is
often done via analyzing the role of particular atoms in the formation of
specific peaks in the calculated spectrum. Typically, this is achieved by
calculating the spectrum for a series of trial structures where various atoms
are moved and/or removed. A more quantitative approach is presented here, based
on comparing the probabilities that a XANES photoelectron of a given energy can
be found near particular atoms. Such a photoelectron probability density can be
consistently defined as a sum over squares of wave functions which describe
participating photoelectron diffraction processes, weighted by their normalized
cross sections. A fine structure in the energy dependence of these
probabilities can be extracted and compared to XANES spectrum. As an
illustration of this novel technique, we analyze the photoelectron probability
density at the Ti K pre-edge of TiS2 and at the Ti K-edge of rutile TiO2.Comment: Journal abstract available on-line at
http://link.aps.org/abstract/PRB/v65/e20511
The lifecycle of powerful AGN outflows
During the course of this conference, much evidence was presented that points
to an intimate connection between the energetic outflows driven by AGN and the
energy budget and quite possibly also the evolution of their gaseous
environments. However, it is still not clear if and how the AGN activity is
triggered by the cooling gas, how long the activity lasts for and how these
effects give rise to the observed distribution of morphologies of the outflows.
In this contribution we concentrate on the high radio luminosity end of the AGN
population. While most of the heating of the environmental gas may be due to
less luminous and energetic outflows, these more powerful objects have a very
profound influence on their surroundings. We will describe a simple model for
powerful radio galaxies and radio-loud quasars that explains the dichotomy of
their large-scale radio morphologies as well as their radio luminosity
function.Comment: 6 pages, contribution to 'Heating vs. coooling in galaxies and galaxy
clusters', Garching 2006, proceedings to be published by Springer (ESO
Astrophysics Symposia), eds. H. Boehringer, P. Schuecker, G.W. Pratt & A.
Finogueno
Nonlinear Realization of Chiral Symmetry on the Lattice
We formulate lattice theories in which chiral symmetry is realized
nonlinearly on the fermion fields. In this framework the fermion mass term does
not break chiral symmetry. This property allows us to use the Wilson term to
remove the doubler fermions while maintaining exact chiral symmetry on the
lattice. Our lattice formulation enables us to address non-perturbative
questions in effective field theories of baryons interacting with pions and in
models involving constituent quarks interacting with pions and gluons. We show
that a system containing a non-zero density of static baryons interacting with
pions can be studied on the lattice without encountering complex action
problems. In our formulation one can also decide non-perturbatively if the
chiral quark model of Georgi and Manohar provides an appropriate low-energy
description of QCD. If so, one could understand why the non-relativistic quark
model works.Comment: 34 pages, 2 figures, revised version to be published in J. High
Energy Phys. (changes in the 1st paragraph, additional descriptions on the
nature of the coordinate singularities in Sec.2, references added
Recent results on multiplicative noise
Recent developments in the analysis of Langevin equations with multiplicative
noise (MN) are reported. In particular, we:
(i) present numerical simulations in three dimensions showing that the MN
equation exhibits, like the Kardar-Parisi-Zhang (KPZ) equation both a weak
coupling fixed point and a strong coupling phase, supporting the proposed
relation between MN and KPZ;
(ii) present dimensional, and mean field analysis of the MN equation to
compute critical exponents;
(iii) show that the phenomenon of the noise induced ordering transition
associated with the MN equation appears only in the Stratonovich representation
and not in the Ito one, and
(iv) report the presence of a new first-order like phase transition at zero
spatial coupling, supporting the fact that this is the minimum model for noise
induced ordering transitions.Comment: Some improvements respect to the first versio
Nonequilibrium phase transitions induced by multiplicative noise: effects of self-correlation
A recently introduced lattice model, describing an extended system which
exhibits a reentrant (symmetry-breaking, second-order) noise-induced
nonequilibrium phase transition, is studied under the assumption that the
multiplicative noise leading to the transition is colored. Within an effective
Markovian approximation and a mean-field scheme it is found that when the
self-correlation time of the noise is different from zero, the transition is
also reentrant with respect to the spatial coupling D. In other words, at
variance with what one expects for equilibrium phase transitions, a large
enough value of D favors disorder. Moreover, except for a small region in the
parameter subspace determined by the noise intensity and D, an increase in the
self-correlation time usually preventsthe formation of an ordered state. These
effects are supported by numerical simulations.Comment: 15 pages. 9 figures. To appear in Phys.Rev.
A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots
A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
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