Free-boundary simulations of MHD plasma instabilities in tokamaks

One of the most promising concepts for future fusion reactors is the tokamak. In these devices, a hot ionized plasma is confined with the use of large magnetic fields. As these systems are typically out of thermodynamic equilibrium, they are prone to instabilities that deteriorate the plasma confinement and can also damage the plasma facing components. The subject of this thesis is the study of a particular type of tokamak instabilities with numerical Magnetohydrodynamic (MHD) simulations. The numerical code JOREK-STARWALL is adapted and applied to the simulation of the so-called free-boundary instabilities. The investigation of this type of instabilities requires a special treatment for the plasma boundary conditions for the magnetic field, where the interaction of the plasma with the vacuum and the surrounding conducting structures needs to be taken into account. In this work, the modelling of the electromagnetic plasma-wall-vacuum interaction is reviewed and generalized for time-varying coil currents and for the so-called halo currents, which are electric currents flowing from the plasma to the wall and vice versa. The adapted JOREK-STARWALL code is applied to realistic plasma geometries in order to study the physics of two particular free-boundary instabilities: Edge Localized Modes (ELMs) triggered by vertical position oscillations and Vertical Displacement Events (VDEs). Two major results are obtained. First the triggering of ELMs during vertical position oscillations is for the first time reproduced with self-consistent simulations. These allow for the investigation of the physical mechanism underlying this phenomenon. The simulations reveal that for the international ITER project, a large-scale tokamak, these triggered ELMs are mainly due to an increase in the plasma edge current due to the vertical plasma motion. Secondly for VDEs, several benchmarks are performed with other existing MHD codes showing a good agreement and therefore allowing the performance of ITER simulations to estimate the expected amount of halo currents in ITER. Additionally, preliminary toroidally asymmetric VDE simulations are presented. (author)