Nonlocal simulation of the transition from ballooning to ion temperature gradient mode turbulence in the tokamak edge

The transition from resistive ballooning to ion temperature gradient (ITG) mode turbulence in the tokamak edge is studied in three dimensions using the electrostatic reduced Braginskii equations with ion temperature dynamics. In contrast to most previous simulations of plasma edge turbulence, which assume the parameters (density, temperature, collisionality, etc.) to be constant in space and time (local approximation), a realistic spatial variation and the nonlinear dependence of the parameters on the fluctuating quantities is taken into account. The influence of nonlocal effects is studied by a scan in the ratio of profile to turbulence scale length. For a large ratio, good agreement between local and nonlocal results on the ITG mode and ballooning turbulence is found. With increasing nonlocality, however, the anomalous transport is more and more suppressed. For the ITG mode, the turbulence quench is due to a decrease of the linear growth rate, while for the ballooning mode a novel, inherently nonlocal saturation mechanism is identified. (C) 2000 American Institute of Physics. [S1070-664X(00)05106-5].