2D and 3D boundary turbulence studies

Pressure driven curvature instabilities at the boundary of a tokamak plasma are studied. First, three-dimensional turbulence simulations are presented including the case where magnetic field lines are stochastic. Three main features are observed: (i) the level of pressure fluctuations decreases in the ergodic layer. (ii) This is essentially due to a suppression of large-scale structures. (iii) The turbulent heat diffusivity does not decrease in the stochastic layer due to an increase of electric field fluctuations. These observations are in agreement with turbulence measurements on Tore Supra. Second, particle flux driven turbulence in the scrape-off layer (SOL) is studied using a two-dimensional code. The transverse transport is found to be ballistic and intermittent. Bursts propagate outwards with a radial velocity of the order of one tenth of the parallel velocity. Test particles undergo outward and inward trajectories. The order of magnitude of the e-folding SOL length is in agreement with experimental data.