Fig. 1 : Contours de la composante Bz du champ magnétique normalisée à quatre instants différents pour une simulation réalisée à partir du code VLEM) de l’excitation d’une large spectre pour un mode de filamentation oblique. Après saturation de l’instabilité (en haut à gauche), le transfert de l’énergie est inversé.
Fig. 2 : Flux de particules pour différentes impuretés (simulations numériques réalisées avec GYSELA). Le flux total d’impuretés (ligne continue noire) est divisé en une partie turbulente (ligne pointillée rouge) et une partie néoclassique (ligne pointillée bleue).
This first example concerns the generation of a magnetic field from a Weibel-type instability. A numerical simulation of beam-plasma interaction in the relativistic regime is carried out on Jean Zay using 2,048 cores (2Mh allocation), considering a configuration of two electron beams propagating in opposite directions. Such a configuration, encountered in astrophysics and laser-plasma interaction, is unstable and leads to the generation and amplification of an intense magnetic field.
Using the semi-Lagrangian code VLEM (VLasov ElectroMagnetic solver [1, 2]), we have shown a correlation between entropy violation and energy transfer (kinetic to magnetic energy), induced by microscopic fluctuations in the distribution function. Fig. 1 shows a reversible time evolution of the magnetic field's lactopology. We have thus been able to demonstrate the reversible nature of this energy transfer [3], in line with information theory and a new heating process. This type of problem is also relevant to magnetic reconnection, where the multi-scale character is of prime importance. An MHD AMR code has been developed to deal with the multiscale aspect, as part of a GENCI-DARI project entitled "Nonlinear dynamics of current sheets in magnetic reconnection" [10-11]. This AMR technique could be adapted to solve the Vlasov-Maxwell system.
The second example concerns impurity transport in tokamaks. Impurity transport is complex and results from turbulence (anomalous transport) and collisions (neoclassical transport). There may be synergy between the two channels, as recently demonstrated [4]. The code is currently being developed at IRFM-CEA and is called GYSELA-5D (GYrokinetic Semi-Lagrangian [6]). From a theoretical point of view, and based on the results of nonlinear simulations, the various terms involved in transport (diffusion, pinch velocities, neoclassical and turbulent contributions, etc ) have been highlighted, notably thanks to neoclassical and quasi-linear theories [7 -8] (see fig 2). In addition, a new source of vorticity polarizes the system, triggering ExB velocity shear, a transport barrier and a pedestal (high confinement mode, H-mode). At sufficiently high shear rates, turbulent transport is suppressed and a transport barrier is created. These results have been compared with those given without a transport barrier [8], and the transport barrier is shown to be effective in reducing the amplitude of turbulence [9].
This research corresponds to the doctoral programs of Kyungtak Lim and Guillaume Lo-Cascio, which involve teams from the University of Lorraine and IR- FM-CEA. TERESA simulations [7] have been carried out on Jean Zay-IDRIS for a total of 5 million core-hours. GYSELA simulations [8-9] have been carried out on Joliot-Curie-TGCC for a total of 22 million core-hours. This work is part of a Eurofusion project (TSVV#6).