651 research outputs found
Estimate of convection-diffusion coefficients from modulated perturbative experiments as an inverse problem
The estimate of coefficients of the Convection-Diffusion Equation (CDE) from
experimental measurements belongs in the category of inverse problems, which
are known to come with issues of ill-conditioning or singularity. Here we
concentrate on a particular class that can be reduced to a linear algebraic
problem, with explicit solution. Ill-conditioning of the problem corresponds to
the vanishing of one eigenvalue of the matrix to be inverted. The comparison
with algorithms based upon matching experimental data against numerical
integration of the CDE sheds light on the accuracy of the parameter estimation
procedures, and suggests a path for a more precise assessment of the profiles
and of the related uncertainty. Several instances of the implementation of the
algorithm to real data are presented.Comment: Extended version of an invited talk presented at the 2012 EPS
Conference. To appear in Plasma Physics and Controlled Fusio
A symplectic, symmetric algorithm for spatial evolution of particles in a time-dependent field
A symplectic, symmetric, second-order scheme is constructed for particle
evolution in a time-dependent field with a fixed spatial step. The scheme is
implemented in one space dimension and tested, showing excellent adequacy to
experiment analysis.Comment: version 2; 16 p
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
When can Fokker-Planck Equation describe anomalous or chaotic transport?
The Fokker-Planck Equation, applied to transport processes in fusion plasmas,
can model several anomalous features, including uphill transport, scaling of
confinement time with system size, and convective propagation of externally
induced perturbations. It can be justified for generic particle transport
provided that there is enough randomness in the Hamiltonian describing the
dynamics. Then, except for 1 degree-of-freedom, the two transport coefficients
are largely independent. Depending on the statistics of interest, the same
dynamical system may be found diffusive or dominated by its L\'{e}vy flights.Comment: 4 pages. Accepted in Physical Review Letters. V2: only some minor
change
High-Resolution Chandra Spectroscopy Of Tau Scorpii: A Narrow-Line X-Ray Spectrum From A Hot Star
Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, tau Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of tau Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the hot plasma on this star being within a few stellar radii of the photosphere, from at least one He-like complex there is evidence that the X-ray emitting plasma is located more than a stellar radius above the photosphere. The Chandra spectrum of Sco is harder and more variable than those of other hot stars, with the exception of the young magnetized O star theta(1) Ori C. We discuss these new results in the context of wind, coronal, and hybrid wind-magnetic models of hot-star X-ray emission
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
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
Foliar lead uptake by lettuce exposed to atmospheric fallouts
Metal uptake by plants occurs by soil−root transfer but also by direct transfer of contaminants from the atmosphere to the shoots. This second pathway may be particularly important in kitchen gardens near industrial plants. The mechanisms of foliar uptake of lead by lettuce (Lactuca sativa) exposed to the atmospheric fallouts of a lead-recycling plant were studied. After 43 days of exposure, the thoroughly washed leaves contained 335 ± 50 mg Pb kg−1 (dry weight). Micro-X-ray fluorescence mappings evidenced Pb-rich spots of a few hundreds of micrometers in diameter located in necrotic zones. These spots were more abundant at the base of the central nervure. Environmental scanning electron microscopy coupled with energy dispersive X-ray microanalysis showed that smaller particles (a few micrometers in diameter) were also present in other regions of the leaves, often located beneath the leaf surface. In addition, submicrometric particles were observed inside stomatal openings. Raman microspectrometry analyses of the leaves identified smelter-originated Pb minerals but also secondary phases likely resulting from the weathering of original particles. On the basis of these observations, several pathways for foliar lead uptake are discussed. A better understanding of these mechanisms may be of interest for risk assessment of population exposure to atmospheric metal contamination
Landau model for uniaxial systems with complex order parameter
We study the Landau model for uniaxial incommensurate-commensurate systems of
the I class by keeping Umklapp terms of third and fourth order in the expansion
of the free energy. It applies to systems in which the soft mode minimum lies
between the corresponding commensurate wave numbers. The minimization of the
Landau functional leads to the sine-Gordon equation with two nonlinear terms,
equivalent to the equation of motion for the well-known classical mechanical
problem of two mixing resonances. We calculate the average free energies for
periodic, quasiperiodic and chaotic solutions of this equation, and show that
in the regime of finite strengths of Umklapp terms only periodic solutions are
absolute minima of the free energy, so that the phase diagram contains only
commensurate configurations. The phase transitions between neighboring
configurations are of the first order, and the wave number of ordering goes
through harmless staircase with a finite number of steps. These results are the
basis for the interpretation of phase diagrams for some materials from the I
class of incommensurate-commensurate systems, in particular of those for
ABX and BCCD compounds. Also, we argue that chaotic barriers which
separate metastable periodic solutions represent an intrinsic mechanism for
observed memory effects and thermal hystereses.Comment: 12 pages, 14 figures, LaTeX, to be published in Phys. Rev.
Scaling laws for the largest Lyapunov exponent in long-range systems: A random matrix approach
We investigate the laws that rule the behavior of the largest Lyapunov
exponent (LLE) in many particle systems with long range interactions. We
consider as a representative system the so-called Hamiltonian alpha-XY model
where the adjustable parameter alpha controls the range of the interactions of
N ferromagnetic spins in a lattice of dimension d. In previous work the
dependence of the LLE with the system size N, for sufficiently high energies,
was established through numerical simulations. In the thermodynamic limit, the
LLE becomes constant for alpha greater than d whereas it decays as an inverse
power law of N for alpha smaller than d. A recent theoretical calculation based
on Pettini's geometrization of the dynamics is consistent with these numerical
results (M.-C. Firpo and S. Ruffo, cond-mat/0108158). Here we show that the
scaling behavior can also be explained by a random matrix approach, in which
the tangent mappings that define the Lyapunov exponents are modeled by random
simplectic matrices drawn from a suitable ensemble.Comment: 5 pages, no figure
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