Action diffusion and lifetimes of quasistationary states in the Hamiltonian Mean Field model

Out-of-equilibrium quasistationary states (QSSs) are one of the signatures of a broken ergodicity in long-range interacting systems. For the widely studied Hamiltonian Mean-Field model, the lifetime of some QSSs has been shown to diverge with the number N of degrees of freedom with a puzzling N^1.7 scaling law, contradicting the otherwise widespread N scaling law. It is shown here that this peculiar scaling arises from the locality properties of the dynamics captured through the computation of the diffusion coefficient in terms of the action variable. The use of a mean first passage time approach proves to be successful in explaining the non-trivial scaling at stake here, and sheds some light on another case, where lifetimes diverging as e^N above some critical energy have been reported

Microtearing turbulence: magnetic braiding and disruption limit

International audienceA realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement: the mean residence time of magnetic field lines. Their computation in the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of δB/B above which stochasticity is no longer benign yet leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence δB/B has been measured to grow with the plasma electron density this would also be a density limit

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

Occurrence and impact of the spatial chaos of magnetic field lines

International audienceMagnetic field lines are spatially chaotic at a given time when the space Hamiltonian from which they derive is non integrable. As the minimal number of degrees of freedom required for the possible emergence of chaos is three, situations or models involving only two effective space dimensions cannot develop a spatial chaos of magnetic field lines. The question behind this presentation will be: What is the impact of the spatial chaos of magnetic field lines in hot magnetized plasmas? Two situations will be addressed: i) in tokamak sawteeth, that can be viewed as a laboratory prototype of magnetic reconnection, evidence will be given that magnetic field lines are spatially chaotic during the fast crash (reconnection) phase. ii) the spatial chaos of magnetic field lines will be shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured in toroidal magnetic devices

Axisymmetric steady-state flows in tokamak plasmas under the visco-resistive MHD setting

International audienc

Théorie des Plasmas

International audienceISSN : 1775-038

Transient fields produced by a cylindrical electron beam flowing through a plasma

To appear in Laser and Particle Beams.International audienceThe out-of-equilibrium situation in which an initially sharp-edged cylindrical electron beam, that could e.g. model electrons flowing within a wire, is injected into a plasma is considered. A detailed computation of the subsequently produced magnetic field is presented. The control parameter of the problem is shown to be the ratio of the beam radius to the electron skin depth. Two alternative ways to address analytically the problem are considered: one uses the usual Laplace transform approach, the other one involves Riemann's method in which causality conditions manifest through some integrals of triple products of Bessel functions

Lien entre cisaillement magnétique et résonances dans des dispositifs de fusion par confinement magnétique

International audienc

Benefits of an extended low shear region for the confinement of tokamak plasmas

International audienc