Low-to-high confinement transition simulations in divertor geometry

Recent results are presented for turbulence in tokamak boundary plasmas and its relationship to the low-to-high confinement (L-H) transition in a real istic divertor geometry. These results are obtained from a three-dimensiona l (3D) nonlocal electromagnetic turbulence code, which models the boundary plasma using fluid equations for plasma vorticity, density, electron and io n temperatures and parallel momenta. With sources added in the core-edge re gion and sinks in the scrape-off layer (SOL), the code follows the self-con sistent profile evolution together with turbulence. Under DIII-D [Luxon , I nternational Conference on Plasma Physics and Controlled Nuclear Fusion (In ternational Atomic Energy Agency, Vienna, 1986), p. 159] tokamak L-mode con ditions, the dominant source of turbulence is pressure-gradient-driven resi stive X-point modes. These modes are electromagnetic and curvature-driven a t the outside mid-plane region but become electrostatic near X-points due t o magnetic shear and collisionality. Classical resistive ballooning modes a t high toroidal mode number, n, coexist with these modes but are sub-domina nt. Results indicate that, as the power is increased, these modes are stabi lized by increased turbulence-generated velocity shear, resulting in an abr upt suppression of high-n turbulence and the formation of a pedestal in den sity and temperature, as is characteristic of the H-mode transition. The se nsitivity of the boundary turbulence to the direction of the toroidal field B-t is discussed. [S1070-664X(00)96405-X].