Resistive n=1 modes in reversed magnetic shear Alcator C-Mod plasmas

Abstract

During current rampup discharges with early auxiliary RF heating, reversed magnetic shear profiles have been obtained on Alcator C-Mod. MHD oscillations are often observed on ECE diagnostics, as well as magnetics. Although no direct q profile measurement was available, a hollow current density profile with q(min) = 2.5 was reconstructed by the EFIT program. The location of the largest electron temperature fluctuations was close to the outer q = 3 rational surface. On the basis of a resistive linear stability code (MARS), the MHD oscillations were identified as n = 1 resistive 'multiple' tearing modes. Since the pressure profile was hollow and q > 1 in the core, a resistive interchange mode was also predicted to be unstable at the inner q = 4 rational surface, despite the low beta(N).