HEAT-FLUX DRIVEN ION TURBULENCE

This work is an analysis of an ion turbulence in a tokamak in the case where the thermal flux is fixed and the temperature profile is allowe d to fluctuate. The system exhibits some features of self-organized cr itical systems. In particular, avalanches are observed. Also the frequ ency spectrum of the thermal flux exhibits a structure similar to the one of a sand pile automaton, including a 1/f behavior. However, the t ime average temperature profile is found to be supercritical, i.e., th e temperature gradient stays above the critical value. Moreover, the h eat diffusivity is not the same for a turbulence calculated at fixed f lux than at fixed temperature gradient, with the same time averaged te mperature. More precisely the diffusivity at fixed temperature is foun d to be larger in the edge and smaller close to the heat source. (C) 1 998 American Institute of Physics.