Identification of Mercier instabilities in Alcator C-Mod tokamak

During current ramp-up discharges, highly localized magnetohydrodynamic (MH D) fluctuations were observed on the electron cyclotron emission diagnostic s of Alcator C-Mod tokamak [I. H. Hutchinson ci al., Phys. Plasmas 1, 1511 (1994)]. The electron temperature profile was hollow, while the density pro file was weakly decreasing. Assuming that the equilibration time was short enough to quickly thermalize ions the pressure profile was also found to be hollow. Using this pressure profile as an additional constraint to the EFI T program, an equilibrium with reversed shear was constructed having a q(0) much greater than1. The localized MHD activity was observed near the inner q =5 rational surface in this reconstructed equilibrium, where the Mercier criterion for ideal MHD stability was violated because of the reversed pres sure gradient (dpldr>0), q>1 and moderate shear. When kinetic effects were added, the ideal Mercier mode was finite ion Larmor radius stabilized. Howe ver, ion Landau damping was found to be strong enough to drive a kinetic Me rcier instability. (C) 2000 American Institute of Physics. [S1070-664X(01)0 0401-3].