Low frequency MHD activity in NSTX and NSTX-U discharges*

Important performance limiting instability mechanisms in the NSTX spherical tokamak are often linked to low frequency and low toroidal periodicity (n) MHD activity. n=1 modes are quite common in long pulse NSTX plasmas. They have both a core kink component and a tearing component and they have been observed to deleteriously impact performance over a wide range of q95. Such modes can be triggered by edge localized modes (ELMs), energetic particles modes (EPMs) or grow from small amplitudes. Most of their dynamics is typical of the neoclassical tearing modes (NTM), but they also have a clear 1/1 component, which can be ideally unstable, and may account for the observed mode onset in the absence of an observable trigger. The presence of similar n=1 core kink/tearing modes was also observed in NSTX-U. In addition, M3D-C1 simulations suggest that observed low frequency, low-n MHD activity may flatten core temperature profiles via a soft-beta limit. A comparison of the NSTX-U and NSTX low frequency and low n MHD characteristics will be presented as well as predictions made with the resistive DCON code to identify regions of global and tearing stability for NSTX-U.