718 research outputs found
On the master equation approach to kinetic theory: linear and nonlinear Fokker--Planck equations
We discuss the relationship between kinetic equations of the Fokker-Planck
type (two linear and one non-linear) and the Kolmogorov (a.k.a. master)
equations of certain N-body diffusion processes, in the context of Kac's
"propagation of chaos" limit. The linear Fokker-Planck equations are
well-known, but here they are derived as a limit N->infty of a simple linear
diffusion equation on (3N-C)-dimensional N-velocity spheres of radius sqrt(N)
(with C=1 or 4 depending on whether the system conserves energy only or energy
and momentum). In this case, a spectral gap separating the zero eigenvalue from
the positive spectrum of the Laplacian remains as N->infty,so that the
exponential approach to equilibrium of the master evolution is passed on to the
limiting Fokker-Planck evolution in R^3. The non-linear Fokker-Planck equation
is known as Landau's equation in the plasma physics literature. Its N-particle
master equation, originally introduced (in the 1950s) by Balescu and Prigogine
(BP), is studied here on the (3N-4)-dimensional N-velocity sphere. It is shown
that the BP master equation represents a superposition of diffusion processes
on certain two-dimensional sub-manifolds of R^{3N} determined by the
conservation laws for two-particle collisions. The initial value problem for
the BP master equation is proved to be well-posed and its solutions are shown
to decay exponentially fast to equilibrium. However, the first non-zero
eigenvalue of the BP operator is shown to vanish in the limit N->infty. This
indicates that the exponentially fast approach to equilibrium may not be passed
from the finite-N master equation on to Landau's nonlinear kinetic equation.Comment: 20 pages; based on talk at the 18th ICTT Conference. Some typos and a
few minor technical fixes. Modified title slightl
TOPICA/TORIC integration for self-consistent antenna and plasma analysis
Sheets presentatio
TOPLHA: an accurate and efficient numerical tool for analysis and design of LH antennas
This paper presents a self-consistent, integral-equation approach for the analysis of plasma-facing lower hybrid (LH) launchers; the geometry of the waveguide grill structure can be completely arbitrary, including the non-planar mouth of the grill. This work is based on the theoretical approach and code implementation of the TOPICA code, of which it shares the modular structure and constitutes the extension into the LH range. Code results are validated against the literature results and simulations from similar code
Phase transition in the collisionless regime for wave-particle interaction
Gibbs statistical mechanics is derived for the Hamiltonian system coupling
self-consistently a wave to N particles. This identifies Landau damping with a
regime where a second order phase transition occurs. For nonequilibrium initial
data with warm particles, a critical initial wave intensity is found: above it,
thermodynamics predicts a finite wave amplitude in the limit of infinite N;
below it, the equilibrium amplitude vanishes. Simulations support these
predictions providing new insight on the long-time nonlinear fate of the wave
due to Landau damping in plasmas.Comment: 12 pages (RevTeX), 2 figures (PostScript
Post-collapse dynamics of self-gravitating Brownian particles in D dimensions
We address the post-collapse dynamics of a self-gravitating gas of Brownian
particles in D dimensions, in both canonical and microcanonical ensembles. In
the canonical ensemble, the post-collapse evolution is marked by the formation
of a Dirac peak with increasing mass. The density profile outside the peak
evolves self-similarly with decreasing central density and increasing core
radius. In the microcanonical ensemble, the post-collapse regime is marked by
the formation of a ``binary''-like structure surrounded by an almost uniform
halo with high temperature. These results are consistent with thermodynamical
predictions
Impact of contractile reserve on acute response to cardiac resynchronization therapy
Background: Cardiac resynchronization therapy (CRT) provides benefit for congestive heart failure, but still 30% of patients failed to respond to such therapy. This lack of response may be due to the presence of significant amount of scar or fibrotic tissue at myocardial level. This study sought to investigate the potential impact of myocardial contractile reserve as assessed during exercise echocardiography on acute response following CRT implantation.
Methods: Fifty-one consecutive patients with heart failure (LV ejection fraction 27% ± 5%, 67% ischemic cardiomyopathy) underwent exercise Doppler echocardiography before CRT implantation to assess global contractile reserve (improvement in LV ejection fraction) and local contractile reserve in the region of the LV pacing lead (assessed by radial strain using speckle tracking analysis). Responders were defined by an increase in stroke volume ≥15% after CRT.
Results: Compared with nonresponders, responders (25 patients) showed a greater exercise-induced increase in LV ejection fraction, a higher degree of mitral regurgitation and a significant extent of LV dyssynchrony. The presence of contractile reserve was directly related to the acute increase in stroke volume (r = 0.48, p<0.001). Baseline myocardial deformation as well as contractile reserve in the LV pacing lead region was greater in responders during exercise than in nonresponders (p<0.0001).
Conclusions: Heart failure patients referred to CRT have less chance of improving under therapy if they have no significant mitral regurgitation, no LV dyssynchrony and no contractile myocardial recruitment at exercise
Long-time discrete particle effects versus kinetic theory in the self-consistent single-wave model
The influence of the finite number N of particles coupled to a monochromatic
wave in a collisionless plasma is investigated. For growth as well as damping
of the wave, discrete particle numerical simulations show an N-dependent long
time behavior resulting from the dynamics of individual particles. This
behavior differs from the one due to the numerical errors incurred by Vlasov
approaches. Trapping oscillations are crucial to long time dynamics, as the
wave oscillations are controlled by the particle distribution inhomogeneities
and the pulsating separatrix crossings drive the relaxation towards thermal
equilibrium.Comment: 11 pages incl. 13 figs. Phys. Rev. E, in pres
Phosphorylated cofilin-2 is more prone to oxidative modifications on Cys39 and favors amyloid fibril formation
Cofilins are small protein of the actin depolymerizing family. Actin polymerization/depolymerization is central to a number of critical cellular physiological tasks making cofilin a key protein for several physiological functions of the cell. Cofilin activity is mainly regulated by phosphorylation on serine residue 3 making this post-translational modification key to the regulation of myofilament integrity. In fact, in this form, the protein segregates in myocardial aggregates in human idiopathic dilated cardiomyopathy. Since myofilament network is an early target of oxidative stress we investigated the molecular changes induced by oxidation on cofilin isoforms and their interplay with the protein phosphorylation state to get insight on whether/how those changes may predispose to early protein aggregation. Using different and complementary approaches we characterized the aggregation properties of cofilin-2 and its phosphomimetic variant (S3D) in response to oxidative stress in silico, in vitro and on isolated cardiomyocytes. We found that the phosphorylated (inactive) form of cofilin-2 is mechanistically linked to the formation of an extended network of fibrillar structures induced by oxidative stress via the formation of a disulfide bond between Cys39 and Cys80. Such phosphorylation-dependent effect is likely controlled by changes in the hydrogen bonding network involving Cys39. We found that the sulfide ion inhibits the formation of such structures. This might represent the mechanism for the protective effect of the therapeutic agent Na2S on ischemic injury
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