Beta-limiting MHD instabilities in NSTX spherical torus plasmas

Neutral beam heated plasmas in the National Spherical Torus Experiment (NSTX) have achieved high toroidal beta exceeding 25% and normalized beta values above 4 at plasma current = 1.2 MA, toroidal field = 0.3 Tesla, and aspect ratio A = 1.3. Plasmas with elongation = 1.8, triangularity = 0.5, internal inductance = 0.7, and pressure peaking factor = 2.9 are found to achieve the highest beta. In general, most high-beta disruptions in NSTX are well correlated with the predicted ideal stability limits for core-localized n=1 pressure-driven kink modes. Neoclassical tearing modes can also limit NSTX performance by limiting the poloidal beta to less than 0.5 and are potentially problematic for high bootstrap fraction scenarios. In dedicated experiments to maximize the stabilizing effect of the NSTX passive conducting plates on kink modes, the first resistive wall mode (RWM) was observed on NSTX. The locked-mode detector array used for diagnosing the RWM also discovered a large n=1 vacuum error-field which has since been significantly reduced. The impact of reduced error fields on the MHD stability of NSTX discharges will be assessed in the near term. (author)