Diamagnetic destabilization of magnetic islands in the non-linear regime

The non-linear behaviour of magnetic islands is studied under the assumption that their width delta is small enough so that the microturbulence spectrum near the islands keeps its unperturbed structure, instead of adjusting to the local plasma gradients. The friction forces experienced by the electron and ion assemblies from the turbulent modes then play a leading role in determining the stability and the rotation velocity of the islands. In the presence of a thermal gradient of electrons, a strong diamagnetic effect may destabilize the islands for which the particle diffusion rate D/delta2 is larger than the frequency ω. This effect is similar to the linear destabilization effect due to collisions (νsub(c) > ω). The role of collisions is played in the non-linear regime by the particle diffusion. Even at negative (stable) values of Δ' the studied effect could produce a small persistent MHD activity, detectable by Mirnov probes at low wave numbers. On the other hand, it is found that, under the above assumption, the island rotation should be influenced by the electron diamagnetism. It is only when the turbulent spectrum adjusts to the local plasma gradients within the island structure that the islands should follow the ion mass motion. (author)