Occam's razor on the mechanism of resistive-wall-mode-induced β limits in diverted tokamaks

External kink modes, believed to be the drive of the β-limiting resistive wall mode, are strongly stabilized by the presence of a separatrix. We thus propose a novel mechanism explaining the appearance of long-wavelength global instabilities in free boundary high-β diverted tokamaks, retrieving the experimental observables within a physical framework dramatically simpler than most of the models employed for the description of such phenomena. It is shown that the magnetohydrodynamic stability is worsened by the synergy of β and plasma resistivity, with wall effects significantly screened in an ideal, i.e., with vanishing resistivity, plasma with separatrix. Stability can be improved by toroidal flows, depending on the proximity to the resistive marginal boundary. The analysis is performed in tokamak toroidal geometry, and includes averaged curvature and essential separatrix effects.