Internal global modes, with the lowest wave numbers, can be excited in a magnetically confined plasma column during the process of approaching the fusion ignition regime and possibly prevent reaching this regime. The conditions under which the onset of ideal MHD modes of this kind (m0=1, n0=1) can be suppressed are analysed numerically in terms of plasma pressure and current density profiles that are consistent with those of present day experiments, for strongly shaped plasma configurations with tight aspect ratio. The severity of these conditions (considerably lower values of poloidal beta than previously assumed) may force ignition experiments to adopt, in effect, smaller plasma currents than presently planned, in order to reduce the size of the region affected by the considered mode. This is in contrast with the requirement of very high plasma currents to prevent an excessive enhancement of the plasma thermal energy transport due to microscopic processes
Equilibrium properties at very low aspect ratio in the Pegasus toroidal experiment