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

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 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 P values have reached (not simultaneously) beta(T) = 35%, beta(N) = 6.4, <beta(N)> = 4.5, beta(N)/l(i) = 10, and beta(P) = 1.4. High Pp operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 s with sustained periods of beta(N) approximate to 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.