β limiting MHD activity and mode locking in alcator C-mod

Since Alcator C-Mod normally operates at high toroidal field and high collisionality, β limiting instabilities are rarely observed. Under some conditions, however, when operating at low collisionality and high input power (P_ICRF ≤ 5 MW), large amplitude (5 × 10^-5 < [B̃_θ/B_θ]_wall ≤ 5 × 10^-3) low-frequency (f_MHD < 50 kHz) MHD modes appear to limit the achievable β. Modes with m/n = 5/4, 4/3, 3/2, and 2/1 were destabilized when β_p > 0.52 and increased in amplitude with increasing β until a rollover or collapse in β occurred. The largest amplitude modes with m = 2, n = 1 strongly degraded momentum and energy confinement when the modes coupled across the plasma core and locked to the wall, bringing the plasma ion toroidal rotation to zero, within experimental errors, about 50 ms after mode locking. MHD stability was calculated with the linear resistive toroidal MHD code MARS for a discharge with a large m = 2, n = 1 mode. Comparisons with neoclassical tearing mode (NTM) theory and with NTMs found on other tokamaks indicate that these modes may be driven by a combination of resistive, neoclassical, and error field effects.