650 research outputs found
Perturbative approach to the nonlinear saturation of the tearing mode for any current gradient
Within the traditional frame of reduced MHD, a new rigorous perturbation
expansion provides the equation ruling the nonlinear growth and saturation of
the tearing mode for any current gradient. The small parameter is the magnetic
island width w. For the first time, the final equation displays at once terms
of order w ln(1/w) and w which have the same magnitude for practical purposes;
two new O(w) terms involve the current gradient. The technique is applicable to
the case of an external forcing. The solution for a static forcing is computed
explicitly and it exhibits three physical regimes.Comment: 4 pages, submitted to Physical Review Letter
Direct path from microscopic mechanics to Debye shielding, Landau damping, and wave-particle interaction
The derivation of Debye shielding and Landau damping from the -body
description of plasmas is performed directly by using Newton's second law for
the -body system. This is done in a few steps with elementary calculations
using standard tools of calculus, and no probabilistic setting. Unexpectedly,
Debye shielding is encountered together with Landau damping. This approach is
shown to be justified in the one-dimensional case when the number of particles
in a Debye sphere becomes large. The theory is extended to accommodate a
correct description of trapping and chaos due to Langmuir waves. Shielding and
collisional transport are found to be two related aspects of the repulsive
deflections of electrons, in such a way that each particle is shielded by all
other ones while keeping in uninterrupted motion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.3096,
arXiv:1210.154
THE PHASE DIAGRAM FOR THE SINE-GORDON MODEL WITH TWO UMKLAPP TERMS
We study the Landau free energy for a uniaxial ordering, taking into account
two Umklapp terms of comparable strengths (those of the third and fourth
order). Exploring the analogy with the well-known nonintegrable classical
mechanical problem of two mixed nonlinear resonances, we complete the previous
studies of the corresponding phase portrait by calculating numerically periodic
solutions, including those far from the separatrices. It is shown that in the
physical range of parameters only periodic configurations are absolutely
stable. We determine for the first time the complete thermodynamic phase
diagram and show that, in contrast to some earlier claims, the wave number of
the ordering does not pass through the devil's staircase, but through a finite
number of steps which decreases as the amplitudes of the Umklapp terms
increase.Comment: 10 pages, RevTeX 3.0, 5 PostScript figures uuencoded and compressed,
to be published in Phys. Letters
Microturbulence studies in RFX-mod
Present-days Reversed Field Pinches (RFPs) are characterized by quasi-laminar
magnetic configurations in their core, whose boundaries feature sharp internal
transport barriers, in analogy with tokamaks and stellarators. The abatement of
magnetic chaos leads to the reduction of associated particle and heat transport
along wandering field lines. At the same time, the growth of steep temperature
gradients may trigger drift microinstabilities. In this work we summarize the
work recently done in the RFP RFX-mod in order to assess the existence and the
impact upon transport of such electrostatic and electromagnetic
microinstabilities as Ion Temperature Gradient (ITG), Trapped Electron Modes
(TEM) and microtearing modes.Comment: Work presented at the 2010 Varenna workshop "Theory of Fusion
Plasmas". To appear in Journal of Physics Conference Serie
Optimization-Based Control for Dynamic Legged Robots
In a world designed for legs, quadrupeds, bipeds, and humanoids have the
opportunity to impact emerging robotics applications from logistics, to
agriculture, to home assistance. The goal of this survey is to cover the recent
progress toward these applications that has been driven by model-based
optimization for the real-time generation and control of movement. The majority
of the research community has converged on the idea of generating locomotion
control laws by solving an optimal control problem (OCP) in either a
model-based or data-driven manner. However, solving the most general of these
problems online remains intractable due to complexities from intermittent
unidirectional contacts with the environment, and from the many degrees of
freedom of legged robots. This survey covers methods that have been pursued to
make these OCPs computationally tractable, with specific focus on how
environmental contacts are treated, how the model can be simplified, and how
these choices affect the numerical solution methods employed. The survey
focuses on model-based optimization, covering its recent use in a stand alone
fashion, and suggesting avenues for combination with learning-based
formulations to further accelerate progress in this growing field.Comment: submitted for initial review; comments welcom
Controlling chaos in area-preserving maps
We describe a method of control of chaos that occurs in area-preserving maps.
This method is based on small modifications of the original map by addition of
a small control term. We apply this control technique to the standard map and
to the tokamap
Relevance of a photo-Fenton like technology based on peroxymonosulphate for 17b-estradiol removal from wastewater
The objective of this work was to evaluate the effectiveness of sulphate radical based homogeneous
advanced oxidation technologies (SR-AOTs) involving peroxymonosulphate (PMS) as an oxidant and ferrous
iron (Fe(II)) as a catalyst, for the removal of 17b-estradiol (E2) from wastewater effluents collected
downstream of a biological WWTP in Perpignan (France). This molecule is used as a surrogate for endocrine
disrupting compounds (EDCs) due to its high biological activity at very low concentration levels (ng/
L). For this purpose, two different laboratory-scale devices have been employed, one for indoor experiments
working with controlled and artificial UV light centered on k = 365 nm emission, and the other
at a larger scale for outdoor experiments using direct solar irradiation. Comparison of kinetic studies with
those obtained with commonly used hydroxyl radical based advanced oxidation technologies (HR-AOTs),
i.e., UV–Vis/H2O2/Fe(II) and UV/TiO2 revealed the higher efficiency of the former over the latter ones. Estrogenicity
measurement through bioassays confirmed the complete removal of 17b-estradiol after only a
few minutes treatment. Determination of E2 transformation pathways upon sulphate radical reactivity
through intermediates identification by mass spectrometry revealed that the oxidation of phenol moiety
into quinone might be the main step responsible for the decrease in estrogenicity. UV–Vis/PMS/Fe(II) system
appears to be the most suitable method for the treatment of aqueous solutions containing E2
Timing Signatures of the Internal-Shock Model for Blazars
We investigate the spectral and timing signatures of the internal-shock model
for blazars. For this purpose, we develop a semi-analytical model for the
time-dependent radiative output from internal shocks arising from colliding
relativistic shells in a blazar jet. The emission through synchrotron and
synchrotron-self Compton (SSC) radiation as well as Comptonization of an
isotropic external radiation field are taken into account. We evaluate the
discrete correlation function (DCF) of the model light curves in order to
evaluate features of photon-energy dependent time lags and the quality of the
correlation, represented by the peak value of the DCF. The almost completely
analytic nature of our approach allows us to study in detail the influence of
various model parameters on the resulting spectral and timing features. This
paper focuses on a range of parameters in which the gamma-ray production is
dominated by Comptonization of external radiation, most likely appropriate for
gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL
Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability
of the optical emission is highly correlated with the X-ray and high-energy
(HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the
optical variability with respect to the higher-energy bands by 1 - 2 hours and
of the HE gamma-rays before the X-rays by about 1 hour. We show that variations
of certain parameters may lead to changing signs of inter-band time lags,
potentially explaining the lack of persistent trends of time lags in most
blazars.Comment: Accepted for publication in Ap
A Renormalization Group for Hamiltonians: Numerical Results
We describe a renormalization group transformation that is related to the
breakup of golden invariant tori in Hamiltonian systems with two degrees of
freedom. This transformation applies to a large class of Hamiltonians, is
conceptually simple, and allows for accurate numerical computations. In a
numerical implementation, we find a nontrivial fixed point and determine the
corresponding critical index and scaling. Our computed values for various
universal constants are in good agreement with existing data for
area-preserving maps. We also discuss the flow associated with the nontrivial
fixed point.Comment: 11 Pages, 2 Figures. For future updates, check
ftp://ftp.ma.utexas.edu/pub/papers/koch
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