MHD issues in Tore Supra steady-state fully non-inductive scenario

Fully non-inductive tokamak plasma discharges are attractive for a fusion reactor, and the understanding of their limitations beyond the linear stability analysis is therefore essential. Specific features of the non-inductive scheme are due to the enhanced impact of non-linear effects: one is due to the bootstrap current, and the other is linked to the intrinsic physics of plasma current generation by the external system used. In the Tore Supra tokamak, a Lower Hybrid wave launcher provides the external source (approx. 85% of the total plasma current), which has non-linear dependences on the plasma current and electron temperature profiles. Such complex interplay is presumably the cause of spontaneous temperature oscillations that are sometimes observed. But it can also cause the plasma to enter a regime with permanent MHD activity (the so-called MHD regime), after the triggering of a double-tearing mode. We report observations and interpretations of MHD modes in such non-inductive discharges, with particular emphasis on their impact on the global equilibrium. We first present numerical simulations of the impact of double-tearing modes, which in experiments trigger the MHD regime. Then we report observation of MHD modes in the Oscillating regimes, at the edge of MHD unstable domain. Finally, we summarize the operational constraints to avoid the MHD regime, from the initial ohmic phase to the non-inductive phase. This study provides a better understanding of MHD limits in non-inductive discharges. (author)