Abstract
Global magnetohydrodynamic (MHD) 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 β 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 β. As a result of these improvements, peak
β values have reached (not simultaneously) βT = 35%, βN = 6.4, βN = 4.5, βN/li = 10, and βP = 1.4. High
βP operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 s with
sustained periods of βN ≈ 6 above the ideal no-wall limit and near the with-wall limit. Details of the β-limit scalings
and β-limiting instabilities in various operating regimes are described.