Trigger of MHD activities by external magnetic perturbations

Full text: In tokamak plasmas, MHD activities are driven not only by the equilibrium profiles but also by an external perturbation, which originates from another MHD activities, error field etc. Penetration of an external perturbation cross the resonant surface is shielded by the plasma flow. Therefore, in order to excite an MHD activity at the resonant surface, there is a critical condition of an external perturbation and the plasma rotation. This critical condition is important to understand the onset of a MHD mode, which appears in the equilibrium stable for a spontaneous MHD one, and also important from the view point of the plasma control. In this study, the nonlinear response of the plasma to external magnetic perturbations will be investigated base on the collisional and collisionless MHD model. As a concrete example of the plasma disruption by an external perturbation, plasma rotation effects on the trigger and evolution of MHD activities in reversed shear profiles will be reported by nonlinear MHD simulations. It is found that, in rotating plasma, magnetic islands formed around inner and outer magnetic resonant surfaces, which are stable for the tearing mode, by an external perturbation, (driven magnetic island) evolve with different growth rate during an initial growth phase. After an initial growth phase, an outer magnetic island grows rapidly prior to the inner one and triggers a rapid growth of an inner magnetic island. At a final phase, enlarged magnetic islands flatten a q-profile in wide radial region including the plasma center. Though this final phase resembles closely a nonlinear destabilization of a spontaneous double tearing mode (DTM), this process can explain time delay of a plasma edge oscillation to trigger an internal MHD events and disruption. (author)