Non-linear temperature oscillations in the plasma centre on Tore Supra and their interplay with MHD

Regular oscillations of the central electron temperature have been observed by means of ECE (Electron Cyclotron Emission) and SXR (Soft X-ray)diagnostics during non-inductively driven discharges on Tore Supra. These oscillations are sustained by lower hybrid currant drive, do not have a helical structure and, therefore, cannot be ascribed as MHD phenomena. The most probable explanation of this oscillating regime (O-regime) is the assumption that the plasma current density (and, thus, the q-profile) and the electron temperature evolve as a non-linearly coupled predator-pray system. The integrated modelling code CRONOS has been used to demonstrate that the coupled heat transport and resistive diffusion equations admit solutions for the electron temperature and the current density which have a cyclic behaviour. Recent experimental results in which the O-regime co-exists with MHD modes will be presented. Because both phenomena are linked to details of the q-profile, some interplay between MHD and oscillations may occur. The localisation of magnetic islands allows to obtain an accurate picture of the q-profile in the plasma core. In some case, MHD-driven reconnection helps in maintaining a weakly inverted q-profile that is found to be, in the CRONOS simulations, a necessary condition to trigger the oscillations. (authors)