Predictive Simulations of Transport in Tokamak

Doctoral thesis, 1999

The understanding of anomalous transport mechanisms in magnetically confined plasmas is one of the most central issues for the achievement of controlled thermonuclear fusion. Even though this problem has been given a very high priority since the very beginning of the magnetic fusion research programs, there are several outstanding issues that remain to be resolved. Because of this the next step in tokamak devices is being designed by using empirical models of energy confinement. Recent advances in theory-based models of anomalous transport, in particular in self-consistent descriptions of the Ion-Temperature-Gradient-driven modes (ITG) and Trapped Electron (TE)-modes have led to a considerable interest in the performance of these models in predictive simulations of currently operating tokamak experiments. More emphasis is now placed on the predictions of the theory based transport models under reactor relevant conditions. In the present work, predictive simulations of current large scale tokamak experiments have been performed using an advanced fluid model describing the toroidal branch of the ITG- and TE-modes. The simulation code self-consistently predicts the time-evolution of temperature and density profiles for electrons, main ions and a single impurity species from local heat and particle sources. A range of L- and H-mode discharges of different experimental characteristics have been simulated. Predicted quantities such as the thermal energy and radial profiles of temperature and density are generally close to or within the experimental error bars. Of particular interest are the results obtained from similarity scaling experiments. These "windtunnel" experiments are set up to maintain close to identical conditions in different sets of discharges while allowing a single parameter to vary. Simulation results are in good agreement with the experimentally obtained scalings also when the intrinsic gyro-Bohm and collisionless model dependencies deviate from experimental results. In particular the Bohm-like dependence of L-mode plasmas can be attributed to variations in the similarity conditions where profile shapes are slightly different between discharges of different gyro-radius thus masking the inherent gyro-Bohm dependence. Electromagnetic corrections to the model description have also been studied and are shown to be important in high performance hot ion H-mode discharges.