The role of magnetic perturbations in runaway electron and sawtooth dynamics

Licentiate thesis, 2011

Nuclear fusion in tokamaks is a possible future energy production mechanism. Some of the still unsolved problems are connected to the effect of magnetic perturbations on the radial plasma transport. Two such problems are addressed in this thesis. The sudden loss of plasma confinement in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious structural damage. To suppress the runaway beam the application of resonant magnetic perturbations (RMP) has been suggested. The numerical analysis of the RMP is based on the relativistic, gyro-averaged drift equations for the runaway electrons in the 3D perturbed equilibria of the TEXTOR and ITER tokamaks. The simulations include a time-dependent electric field, radiation losses and collisions. The simulated runaway current damping rate agrees with experimental observations at TEXTOR. The results indicate that, in a properly chosen perturbation geometry, runaway electrons are rapidly lost from regions where the normalised magnetic perturbation amplitude is larger than 0.1%. This applies in both machines roughly to the outer third - outer half of the confinement value. The second part of the thesis describes the low frequency precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes. Sawtooth crashes are periodic density and temperature redistributions of the plasma core. Besides the well-known internal kink mode, the low frequency sawtooth precursor (LFSP) mode is studied in detail. Power modulation of the LFSP is found to correlate with the power modulation of the kink mode in the quasi-stationary intervals indicating possible non-linear interaction. In all the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash. The spatial structure of the LFSP was estimated and found to equal that of the kink mode. A possible role of this mode in the mechanism of the sawtooth crash is discussed.