The role of magnetic perturbations in runaway electron and sawtooth dynamics

Doktorsavhandling, 2013

As the world's fusion energy program is increasingly focused towards burning plasma experiments, it is important to address the remaining theoretical issues. In this thesis we focus on the effect of magnetic perturbations on the radial plasma transport. 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. In this thesis, 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 results indicate that, in a properly chosen perturbation geometry, runaway electrons are rapidly lost from approximately the outer half of the confinement volume. Simulation studies of runaway evolution with self-consistent electric field in the presence of impurities have been carried out for the JET tokamak with a 1D tool, where we have demonstrated the runaway suppression effect of magnetic perturbation induced radial transport. We also show that runaway electrons can generate high energy positrons, and that their synchrotron radiation may be used for diagnostic purposes. The last part of the thesis describes the low frequency precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes, which 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. Time-frequency analysis indicates non-linear interaction and a similar spatial structure for the two modes. A possible role of this mode in the evolution of the sawtooth crash is discussed in the context of magnetic perturbations.