MHD stability of fully non-inductive discharges in Tore Supra

Fully non-inductive discharges with lower hybrid (LH) current drive have been run successfully in Tore Supra, allowing for stationary plasma conditions for more than 6 min. The operational domain has been explored in terms of total plasma current (I_p), magnetic field (B_vac), and parallel refractive index (〈n_∥〉), which characterize the deposition profile of LH waves. MHD activity has been encountered in a number of cases, either at a low saturated level, or at a high level in the so-called MHD regime (Maget P. et al 2004 Nucl Fusion 44 443). This work first addresses the linear MHD stability properties of the operational space (B_vac, I_P, 〈n_∥〉). Although most regions are found to be linearly unstable, the condition for triggering the deleterious MHD regime is more restrictive, and is shown to result from the full reconnection of the double-tearing mode. A simple criterion based on the helical flux ψ*, namely ψ* = 0 at the outer resonant surface, is used for the identification of the region where the MHD regime is triggered.