Sawteeth and other Transport Phenomena Induced by Ex-drifts in Tokamaks

Doctoral thesis, 1997

Electric fields and the concomitant ExB-drift play a prominent role in many plasma physics phenomena in Tokamaks. Several issues relevant for fusion research and related to the ExB-driven transport, are addressed in the present thesis. At first, the essential role of the ExB-driven plasma transport is illustrated in cases where the plasma is frozen into the magnetic field. An exact analytical solution of the nonlinear magnetohydrodynamical equations is given for the propagation of a perturbation of arbitrary amplitude in a warm magnetized plasma. A condition for whether or not an initially smooth wave profile forms a shock is outlined, and the dependence of the shock formation length on the wave amplitude is given. Secondly, the sawteeth-induced transport of various plasma species including energetic ions in Tokamak plasmas is studied. A consistent picture of the sawteeth-induced transport of impurities is given, and the qualitative agreement with experimental measurements in several Tokamaks is shown to be good. Observations of the sawtooth-induced redistribution of non-thermal confined alpha particles in TFTR DT-experiments are presented. The measured changes in the alpha density profile at sawtooth crashes are consistent with predicted values. Furthermore, a qualitative model is given to describe the redistribution of ICRF heated minority ions at sawtooth crashes. A crucial feature of the model is that it takes into account profile broadening effects due to the finite width of the ICRF heated particle orbits. The predictions of the model are found to be in agreement with experimental JET data. The impact of sawteeth during thermonuclear burn on the thermal confinement, the Helium ash removal rate as well the nonlinear dynamics of the burn are studied analytically and numerically. Depending on parameter regimes, the sawtooth oscillations may or may not be of beneficial character. In particular, the sawteeth may degrade the confinement and quench the burn, but may also enhance the ash removal rate or boost the fusion power. Thirdly, a significant feature of divertor detachment is the strong variation in the total plasma pressure along the magnetic field lines within the divertor volume. It is shown numerically and analytically that the ExB driven flows result in a significant pressure drop in front of the divertor plates, provided that the radial width of the temperature profile is comparable to or less than the poloidal sound ion gyro radius. The question of appropriate boundary condition is addressed, and the generation of parallel return plasma flows during strong drifts is observed. Finally, the formation of transport barriers in Tokamaks has recently attracted considerable attention. Here, simple approximate analytical solutions describing many of the particular features of a characteristic strongly nonlinear diffusion equation modeling the ExB shear suppression are presented.