Modelling of advanced tokamak scenarios with LHCD in Alcator C-Mod

A combined model for current profile control and MHD stability analysis has been used to identify stable operating modes near the ideal stability limit (beta(N) similar or equal to 3) in the Alcator C-Mod tokamak. These discharges are characterized by relatively high fractions of bootstrap current (f(BS) = 0.70) and non-monotonic profiles of the safety factor with q(min) > 2. In the absence of a conducting shell, stability was determined by the onset of the low (n = 1) external kink mode. In these studies, current profile control in the plasma periphery (r/a greater than or similar to 0.5) was provided by 2.5-3.0 MW of LHCD power. Internal and edge transport barriers were introduced into the model calculations in the form of density transitions. Excellent wave accessibility and absorption were still found in the presence of an H-mode-like edge density barrier. However, the presence of these barriers resulted in about a 10% decrease in the stability limit, from beta(N) similar or equal to 3 to beta(N) similar or equal to 2.7.