Drift dynamics of magnetic islands

Diamagnetic drifts play an important role in the evolution of magnetic islands when β>ρ*. In particular, they are responsible for the propagation of the magnetic islands through the plasma. This propagation gives rise to a polarization current that is thought to be responsible for the threshold against neoclassical tearing modes. Of course, diamagnetic drifts also give rise to microinstabilities and thus to turbulence that interacts with the magnetic islands. We have investigated the role of the polarization current under both laminar and turbulent conditions. Under laminar conditions three regimes can be distinguished depending on the width W of the island compared to the gyroradius ρs and to the acoustic width ρsLs/Ln where compressibility becomes important (Ls and Ln are the shear and density scale-lengths). Numerical and analytic calculations show that the polarization drift is stabilizing in all three regimes. The amplitude of the polarization current exhibits a peak at the transition between the compressible and incompressible regimes. The electron temperature gradient is also found to have a stabilizing influence for ηe>0. To investigate the interaction of turbulence with a magnetic island we have extended the Hasegawa-Wakatani model to include the effects of an island and of the associated magnetic shear. In the absence of island the fluctuations are most intense in the hydrodynamic channel where the resistive drag causes the electron response to be non-adiabatic. When the island width exceeds the width of the hydrodynamic channel the region of non-adiabatic behavior is broadened, as eddies show a tendency to propagate along the separatrix. The turbulent fluctuations are found to exert a drag on the island, slowing and even weakly reversing the propagation velocity so that after saturation the island is drifting slowly in the ion diamagnetic direction. (author)