Effets cinétiques en reconnexion magnétique

Plasmas are gaseous systems of ions and electrons which interact via electromagnetic fields and display collective properties. Among these, is the notion of the magnetic line "connection". This expresses the fact that, in regimes in which charged particles spiral sufficiently fast along lines of magnetic induction, the latter is linked to the bulk plasma motion and acquire a topological identity which forbids them to break, intersect and reconnect. This topological identity, however, can be locally violated thanks to a number of kinetic effects, such as particle collisions, when the currents in the plasma are sufficiently intense: one speaks of "magnetic reconnection". Magnetic reconnection is an important ingredient of the plasma self-organization and has significance for both space and laboratory plasmas since it is at the basis of natural phenomena like solar flares and polar lights, or of disruptive processes in thermonuclear fusion experiments. A long-standing problem in the study of laboratory and astrophysical plasmas is to understand the mechanisms of acceleration of electrons and ions, as a magnetic field reconnect and release energy. In this work, we studied kinetic effects on reconnection instabilities developing spontaneously in static current sheets (tearing modes) and in combination with a class of kinetic instabilities (Weibel instabilities) that are relevant both to astrophysical plasma jets and to laser-plasma interaction experiments. We performed this study using reduced-fluid and kinetic models and we investigated the competition between tearing-type modes and Weibel-type instabilities by means of both semi-lagrangian full kinetic Vlasov-Maxwell simulations and particles in cell simulations.Les plasmas sont des systèmes gazeux d'ions et d'électrons qui interagissent avec les champs électromagnétiques et affichent des propriétés collectives. Parmi ceux-ci, il y a la notion de "connexion" de lignes magnétiques. Ceci exprime le fait que, dans des régimes dans lesquels les particules chargées s'enroulent suffisamment vite le long des lignes d'induction magnétique, ces dernières sont liées au mouvement du plasma massif et acquièrent une identité topologique qui leur interdit de se rompre, se croiser et se reconnecter. Cette identité topologique peut cependant être localement violée grâce à un certain nombre d'effets cinétiques, comme les collisions entre les particules, lorsque les courants dans le plasma sont suffisamment intenses : on parle de “reconnexion magnétique”. La reconnexion magnétique est un ingrédient important de l'auto-organisation du plasma et a une importance pour les plasmas spatiaux et de laboratoire, car elle est à la base de phénomènes naturels comme les éruptions solaires et les aurores polaires, ou de processus disruptifs dans les expériences de fusion thermonucléaire. Un problème de longue date dans l'étude des plasmas de laboratoire et astrophysiques est de comprendre les mécanismes d'accélération des électrons et des ions, lorsqu’un champ magnétique se reconnecte et libère de l'énergie. Dans ce travail, nous avons étudié les effets cinétiques sur les instabilités de reconnexion se développant spontanément dans les nappes de courant statique (modes de déchirement) et en combinaison avec une classe d'instabilités cinétiques (instabilités de Weibel) qui sont pertinentes à la fois pour les jets de plasma astrophysiques et pour les expériences d'interaction laser-plasma. Nous avons effectué cette étude en utilisant des modèles fluides réduits et cinétiques, et nous avons étudié la concurrence entre les modes de type déchirement et les instabilités de type Weibel au moyen de simulations cinétiques complètes avec codes semi-lagrangiennes Vlasov-Maxwell et de type “Particle-In-Cell“

Collisionless Heating Driven by Vlasov Filamentation in a Counterstreaming Beams Configuration

International audienceWe perform high resolution kinetic simulations of interpenetrating plasma beams. This configuration is unstable to both Weibel-type and two-stream instabilities, which are known to linearly induce a growth of the magnetic and electrostatic energy, respectively, at the expenses of the kinetic energy. “Oblique modes” are further beam-plasma instabilities, which linearly combine the features of the former two. Here we show the possibility of a reversal of the energy flow associated to these beam-plasma instabilities, when secondary propagating oblique modes are excited. This rapid conversion from magnetic to kinetic energy (i.e., kinetic heating), differs from the standard magnetic reconnection scenario and is induced by the reinforcement of the filamentation process of the distribution function in the phase space. This phenomenon—likely of general interest to collisionless dissipation processes in plasmas—can be understood in terms of mode synchronization: the coupling of oblique modes at disparate spatial scales leads to the appearance of synchronized “filamented” modes, which act on the global dynamics of the plasma via kinetic heating, collisionless dissipation, and turbulence

A numerical study of electron-magnetohydrodynamics tearing modes in parameter ranges of experimental interest

International audienceWe perform a numerical study of the linear dynamics of tearing modes in slab incompressible electron- magnetohydrodynamics (EMHD) by considering some parameter ranges which can be of interest for laboratory plasmas (e.g., helicon devices) or for astrophysics (e.g., solar-wind turbulence). To this purpose several non-ideal effects are simultaneously retained (finite electron inertia, resistivity and electron viscosity) and we make distinction between the dissipation coefficients in the direction parallel and perpendicular to the guide field. We thus identify some new recon- nection regimes, characterized by a departure from the customary monotonic power-law scalings of the growth rates with respect to the non-ideal parameters. The results here presented can provide a useful indication for future studies of EMHD regimes relevant to experiments and for extensions of the EMHD tearing mode modelling to more complete regimes including kinetic effects (e.g., "electron-only" reconnection in kinetic regimes)

TRANSPORT ET PERTES DE PARTICULES ALPHA PRODUITES PAR LES REACTIONS DE FUSION EN PRÉSENCE D'INSTABILITÉS MHD Une approche de particules passives utilisant le nouveau simulateur toroïdal accéléré de particules en full-orbit (TAPAS)

International audienceThe confinement of highly energetic alpha particles is of uttermost importance, for they must transfer their energy to the thermal plasma in order to ensure the self-sustainment of fusion reactions. The present paper focuses on this issue by combining gyro-fluid modelling of a recent JET discharge using the FAR3d code and the integration of trajectories of passive fusion-born alpha particles using the recently developed full-orbit Toroidal Accelerated PArticle Simulator (TAPAS). Special emphasis is put on the impact of magnetohydrodynamic activity triggered by energetic particles on the confinement of alpha particles. In the analysed discharge, significant population of trapped Hydrogen from Ion Cyclotron Resonant Heating drives fishbone unstable, which is observed in self-consistent simulations of FAR3d. The obtained electromagnetic perturbation is afterwards introduced in TAPAS to integrate the trajectories of alpha particles in the inner core. It is concluded that there is significant global impact of the fishbone instability on the losses of alpha particles. Preliminary analyses have also been conducted to assess the impact of other instabilities, such as those excited by the passing population resulting in Toroidicityinduced Alfvén Eigenmodes, with frequencies larger than that of the fishbone. The results show that the effect of these modes on the confinement of alpha particles might be negligible