Current profile control and magnetohydrodynamic stability in Tore Supra discharges with edge-plasma control by the ergodic divertor

Although ergodic diverters are primarily designed to control particle and h eat fluxes at the plasma edge, they also happen to affect the MHD stability of tokamak discharges. On Tore Supra, the ergodic divertor has long been k nown to stabilize the m/n = 2/1 tearing mode induced, for instance, by edge radiation and detachment processes, thus allowing safe high-current and hi gh-density operations. More recently, though, in discharges where ergodic d ivertor operations were optimized relative to the control of the edge-plasm a (i.e. with large divertor perturbation), a detrimental increase in the di sruptivity has been observed. The action that the ergodic divertor has on t he MHD activity is interpreted in terms of a redistribution of the current profile. The latter results from a large increase in the edge resistivity, primarily induced by the degradation of the electron energy confinement in the ergodic layer. The possibility that a transport barrier develops in the vicinity of the separatrix strongly affects the considered modelling.