Impact of MHD Equilibrium Input Variations on the High-beta Stability Boundaries of NSTX
Authors: F. Paoletti, S.A. Sabbagh, J. Manickam, J. Menard, R.J. Akers, D. Gates, S.M. Kaye, and L. Lao
Date of PPPL Report: March 2001
Submitted to: Nuclear Fusion
Ideal MHD stability limits of anticipated plasma configurations for the National Spherical Torus Experiment (NSTX) [Ono, M., (et al., Nucl. Fusion 40 (2000) 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-beta plasmas. The analysis is based on a reference free-boundary equilibrium with beta = 41.5%, monotonic safety factor q profile (qa = 12.1; q0 = 2.8), and broad pressure profile p (peaking factor FP always equals p(0)/<p> = 1.7). On this reference target local variation of the plasma boundary, safety factor q, and pressure p profiles 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 location can also degrade stability. Both experimental profiles from existing tokamaks and spherical torus machines and profiles generated from transport modeling 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 beta-limits values are still in the range of 20.0% without considering the stabilizing effect of the passive conducting structures.