Dynamics of magnetic islands in large Δ′ regimes

Saturation of magnetic islands at large values of the tearing mode stability parameter Δ′ is investigated numerically. In such regimes, the island dynamics exhibit a number of transient features such as coalescence instability, X-point collapse, and plasmoid generation. It is shown that while conditions for these transient instabilities to appear depend on the viscosity and resistivity, the final width of the saturated island is independent of such phenomena, as well as viscosity and resistivity values. It is found that the saturated island width is strongly influenced by global properties of the current profile and related to the equilibrium reconnected flux across the magnetic island.