Dynamics of magnetic islands and confinement transitions in TJ-II

Bolometric observations over a large number of discharges under very different conditions in the TJ-II Heliac, together with magnetic and line-emission diagnostics have indicated that the plasma behavior near rational magnetic surfaces is rather peculiar, which involves the formation and destruction of internal transport barriers associated with magnetohydordynamic (MHD) activity [I]. The interpretation of the results is that around the magnetic islands that appear at resonant low-order rational surfaces there is a sheared flow that gives rise to the formation of a transport barrier, as the anomalous transport is decreased. This is evidenced by low recycling at the edge shown by low Ha emission. Then an MHD event arises, possibly due to magnetic reconnection, which destroys the barrier and releases high-energy particles. This sequence of events is observed as sawtooth-like oscillations in the bolometric emission with a well defined repetition time. In this work a model is presented where this ingredients are incorporated in a transport simulation that includes a turbulence model based on resistive ballooning modes [2]. The sheared flow is described by a neoclassical modeling of the ambipolar electric field given in [3]. A transition model [4] is included in the transport system of equations to verify that the appropriate signals are caused by the mechanisms described above. It is found that the model reproduces in an acceptable way the experimental results, supporting the assumed phenomenology: confinement regulated by rapid particle expulsions related to the break of magnetic islands locally and intermittently. The repetition rates are on the ms time-scale. The breaking of magnetic islands is described in terms of a magnetic reconnection model which is related to the presence of sheared flows just outside the islands. The unstable tearing mode includes the effect of the polarization drift in the outer region.