Global modelling of magnetic island control in tokamaks

Magneto-Hydro-Dynamic (MHD) instabilities are susceptible to develop within a tokamak plasma. These instabilities manifest themselves as magnetic islands which reduce the plasma confinement. The islands can however be controlled by driving current inside them. In this thesis, we consider the modeling of the magnetic islands and their control using first principle approaches, which rely on a global MHD description of the plasma. We have detailed the inclusion a RF-driven current like source term in an MHD code, which requires special care to be given to the modeling of the current density evolution. The implementation has been benchmarked against the asymptotic models, allowing us to retrieve the influence of parameters such as deposition width or misalignment with respect to the island width and position. Beyond these aspects, we have evidenced new effects, linked to the 3D nature of the current deposition. We have observed a flip instability in which an island, reduced by the ECCD, brutally inverse its phase so that its X-Point faces the current deposition, allowing the mode the grow further. We then moved on to the topic of the best suitable control strategies for the control of the island. We have implemented in XTOR a control system that mimics the experimental ones and adapt the current deposition in function of a preset strategy. Nonlinear MHD simulations have been carried out using different control schemes, allowing us to quantify the gain to expect from each of these methods depending on the characteristics of the current deposition. (author)