RADIALLY GLOBAL GYROKINETIC SIMULATION STUDIES OF TRANSPORT BARRIERS

Improvements in tokamak transport have recently been obtained in a var iety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three-dimensi onal toroidal gyrokinetic simulation is used to study three effects th at are linearly stabilizing and may cause the formation of transport b arriers, namely, sheared toroidal rotation, reversed magnetic shear, a nd peaked density profiles. The effect of toroidal shear flow on ion h eat diffusivity is found to be relatively weak compared to mixing-leng th expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s < 1/2) in combination with a peaked density profile relative to the temperature profile greatly sup presses ion-temperature-gradient-driven turbulence in the central regi on of global nonlinear simulations. Similar features are seen experime ntally in reversed magnetic shear tokamak plasmas. There is some nonlo cal penetration (similar to 20-30 rho(i)) of the turbulence into the s ubcritical core region. (C) 1996 American Institute of Physics.