β-Limiting MHD instabilities in improved-performance NSTX spherical torus plasmas

Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) {beta}{sub t} = 35%, {beta}{sub N} = 6.4, = 4.5, {beta}{sub N}/l{sub i} = 10, and {beta}{sub P} = 1.4. High {beta}{sub P} operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of {beta}{sub N} {approx} 6 above the ideal no-wall limit and near the with-wall limit. Details of the {beta} limit scalings and {beta}-limiting instabilities in various operating regimes are described.