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(THP2/06) Mercier Instabilities in the Alcator C-Mod Tokamak

R. J. Hastie1), Y. In1), J. J. Ramos1), P. J. Catto1)2), A. Hubbard1), I. H. Hutchinson1), E. S. Marmar1), M. Miklos1), J. A. Snipes1), S. Wolfe1), G. Taylor3), A. Bondeson4)
 
1) Plasma Science and Fusion Center, MIT, Cambridge, MA 02139, U.S.A.
2) Lodestar Research Corporation, Cambridge, MA 02139, U.S.A.
3) Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, U.S.A.
4) Chalmers University of Technology, Goteborg, Sweden

Abstract.  During current ramp discharges, highly localized MHD fluctuations were observed on ECE diagnostics of Alcator C-Mod tokamak. The electron temperature profile was hollow, while the density profile was almost flat. Assuming that the equilibration time was short enough to quickly thermalize ions with Ti/Te $ \approx$ 0.9, the pressure profile was also found to be hollow. Using the pressure profile as an additional constraint to the EFIT program, an equilibrium with reversed shear was constructed, whose q(0) $ \gg$ 1. The localized MHD activity was observed near the inner q=5 rational surface in this reconstructed equilibrium. According to ideal MHD stability theory, it was found to be ideally unstable because of the reversed pressure gradient (dp/dr > 0), q $ \gg$ 1 and moderate shear. When kinetic effects are added, the ideal Mercier mode was finite ion Larmor radius (FLR) stabilized. However, considering that the ions are collisionless ( $ \nu_{ii}^{}$ $ \ll$ $ \omega$), and the thermal ion transit frequency is comparable to the ion diamagnetic drift frequency ( $ \omega_{ti}^{}$ $ \sim$ $ \omega_{*pi}^{}$), ion Landau damping was found to be strong enough to drive kinetic Mercier instability. As a result, the localized fluctuations in C-Mod can be attributed to a FLR modified kinetic Mercier instability.

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IAEA 2001