The interchange mode is the most dangerous MHD instability in zero-current stellarators.In the linear analysis, the eigenfunction for the poloidal magnetic flux corresponding to the maximum growth rate is an odd function in the radial coordinate. Therefore, the flux is close to zero at the resonant surface even with finite resistivity. This means that magnetic islands are hardly generated spontaneously in the linear phase, not like the tearing mode.
On the other hand, in the numerical study with multi-helicity in the toroidal LHD geometry, Ichiguchi et al.[1] showed that the magnetic islands can be generated in the nonlinear saturation phase of the interchange mode evolution. The number of the island on a poloidal cross section is twice of the poloidal mode number of the driving interchange mode. The positions of the islands almost coincides with those of the local vortices induced by the interchange mode. Therefore, it seems that the driven reconnection due to the local vortices results in the island generation.
In order to precisely analyze the generation mechanism of the magnetic island, we study the nonlinear saturation of the interchange mode in the cylindrical geometry and for a single helicity.
In this case, we include both parity of sine and cosine in the Fourier expansion for each perturbation. This allows us to examine the effects of the self-consistent poloidal flow on the island generation. It is obtained that the effect of the flow depends on the viscosity.

References