Impact of MHD equilibrium input variations on the high beta stability boundaries of NSTX

Ideal MHD stability limits of anticipated plasma configurations for NSTX [Ono M. et al. 2000 Nucl. Fusion 40 557] and the dependence on the parameters defining the MHD equilibrium are evaluated. The study provides a quantitative computational evaluation of the stability limit variations induced by changes to the equilibrium of NSTX high β plasmas. The analysis is based on a reference free-boundary equilibrium with β = 41.5%, monotonic safety factor q profile (q_a = 12.1, q₀ = 2.8) and broad pressure profile p (peaking factor F_p ≡ p(0) / 〈p〉 = 1.7). On this reference target local variation of the plasma boundary, and the safety factor q and pressure profiles p are imposed. Localized inflection of the outboard plasma boundary, produced by near field effects from poloidal shaping field coils, weaken the stability due to the destabilization of high n ballooning modes. Variation of the q profile at different radial locations can also degrade stability. Both experimental profiles from existing tokamaks and spherical torus machines and profiles generated from transport modelling of anticipated neutral beam heated plasmas are used. Degraded stability is found at increasing pressure peaking factor due to the destabilization of n = 1 kink/ballooning modes. Direct access to the second region of stability is found in certain configurations and, for the entire set of variations considered, the lower calculated β limit values are still in the range of 20.0% without considering the stabilizing effect of the passive conducting structures.