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Rencontres COFECUB : dynamique complexe des plasmas

par Caroline CHAMPENOIS - publié le

le 22 mars à partir de 14:00 service 361

12 h 00 meeting at 361 before going to lunch together

14 h 00 Ricardo L. Viana
Department of Physics, Federal University of Parana, Curitiba, Parana, BR

Brownian motion and diffusion of density lumps in reaction-diffusion plasma systems

The formation of density lumps in a bounded plasma is studied from the perspective of the nonlinear dynamics, as a time-dependent nonlinear reaction-diffusion problem. The physical variables involved are the injection and recombination rates and the diffusion coefficient, in such a way that the density lumps, or chaotic defects, result from the competition between nonlinearity and diffusion. We investigate some of the dynamical features presented by density lumps such as their difusion properties and Brownian motion with memory. In the latter case we used recently developed techniques in nonlinear time series analysis which employ recurrence-based concepts.

15 h 00 Dominique Escande
Aix-Marseille université et CNRS, Physique des interactions ioniques et moléculaires, Marseille, FR

Relevant heating of the solar corona by quenching Alfvén waves : a result of adiabaticity breakdown

Till now, an efficient direct transfer of energy from Alfvén waves to ions in the solar corona was considered as requiring a broad spectrum of such waves. We show that a narrow spectrum-actually a single low-frequency Alfvén wave-can do it. This is because the corresponding phase space ion dynamics involves a slowly pulsating separatrix, which forces particles to cross it in an almost periodic way. As explained by Hamiltonian neo-adiabatic theory, each separatrix crossing destroys adiabatic invariance, since the period is infinite there, yielding irreversible energy transfer from the wave, and thus particle heating. In a single wave period, two initially close orbits can land in faraway domains in phase space. One of these domains corresponds to energies of the order of the coronal temperature for wave amplitudes in agreement with measurements.

15 h 30Michel Vittot
Centre de physique théorique, CNRS - Aix-Marseille université, Marseille, FR

Reduction of the guiding-center dynamics : Magnetic momentum & bounce-average

We want to reduce the Hamiltonian dynamics in a Tokamak, with strong E & B, and the retro-action of the plasma on the field. This will simplify the dynamics and may permit to build some "Internal Transport Barrier" (ITB).
A good approximation is a classical (relativistic) Hamiltonian field theory E(q) & B(q) with some Vlasov distributions f_+(q ;p) & f_-(q ;p) for the matter (q in R^3 ; p in R^3 ) : this an Eulerian point of view.
So it is crucial to reduce the dimensionality : for instance from 6 to 3 (or 4).
The first "guiding-center" framework reduces the dynamics of particles in a strong, arbitrary, but static magnetic field. Later we will embed this model in the full Maxwell-Vlasov Hamiltonian framework, to build the "gyro-center » corrections to the previous reduction, by adding the retro-action of the plasma on the fields.
We use an extension of the Hamiltonian perturbation theory (and the normal forms) : K.A.M. methods.

16 h

contact : Yves Elskens


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