Influence of triangularity and profiles on ideal-MHD beta limits for NET

Ideal-MHD stability calculations are presented to show the effect of triangularity δ on the beta-limit for plasma of NET/ITER type with ellipticity κ=2 and aspect ratio A=3.7. The optimum betas are achieved as a tradeoff between n=1 ideal kink stability and stability against Mercier and ballooning (n=∞) modes. Two different types of beta-optimisations have been made to study the influence of equilibrium profiles. In one set of optimisations, the current profile is prescribed and the pressure profile is optimised. We impose current profiles that are flat in the central region and decrease monotonically to zero at the edge. For such current profiles, maximum beta is obtained for broad pressure profiles, in particular, for low qs and small triangularity. The beta-limit is independent of triangularity up to δ=0.4, but increases slightly for high triangularity, δ=0.6. In the other set of optimisations, we specify a more peaked pressure profile and optimise the current profile. Peaked pressure profiles, when combined with standard current profiles, for which the central q is flat and near unity, tend to be unstable to localised, pressure driven modes. Therefore, careful tailoring of the central current profile is required. After optimisation of the current profile the n=1 and n=∞ modes give a beta-limit for peaked pressure profiles that is almost unchanged from that for broad profiles, if the cross-section is sufficiently triangular, δ≥0.4. By contrast, in an elliptical cross-section, δ=0.0, the maximum beta decreases significantly for the peaked profiles. (author) 3 refs., 3 figs