Magnetic Island Dynamics in Relation to Transport Barriers in the TJ-II Heliac

The dynamics of magnetic islands is studied in the context of the role they play in the experiments of the heliac TJ-II that show a correlation between MHD activity and transport barriers [1]. The non-linear island growth is studied in presence of a plasma flow associated with the transport barrier. In previous works it has been found that the presence of a sheared flow reduces the growth rate of the island and is thus stabilizing, when only the modification of the tearing-mode parameter is considered. However, another effect of the plasma sheared flows is to produce a polarization current in the vicinity of the magnetic island separatrix due to the nozzle effect. The influence of this current on the island stability has been computed for axisymmetric systems [2]. Here, we computed it for a non-axisymmetric geometry, as is the case for TJ-II, and is found to be weakly destabilizing. A bifurcation in the torque balance can explain the cycle of the transport barrier formation and island growth with the subsequent barrier destruction. The whole process is modeled using the Astra transport code in which we include a turbulence model based on the resistive ballooning mode. Three impurity species of Carbon are also included in order to obtain the emitted radiation that is measured in the experiments by bolometers. In this way we can have a relatively good correlation with the experimental sequences.