High power density H-modes in Alcator C-Mod

Optimization studies of energy confinement in H-mode plasmas heated by ICRF waves were performed on the compact high-field tokamak Alcator C-Mod. Reduction of the radiated power from the main plasma by boronization of the molybdenum first wall made a major impact on improving the quality of H-mode, as characterized by the confinement enhancement factor over the ITER89-P L-mode scaling, HITER89-p. Longer ELM-free periods became possible, and HITER89-p = 2.5 was achieved transiently, but in this mode of operation impurity accumulation led to eventual termination of H-mode. More favorable high quality quasi-steady-state H-modes which led to steady-state levels of density (n-bare = 4 x 1020 m-3), stored energy (HITER89-p = 2.0, βN = 1.5), and radiated power (Pmainrad/Ploss ≤ 30%) were also achieved. This mode of operation is characterized by high levels of continuous Dα emission comparable to the L-mode level, with very little or no ELM activity. The H-mode behavior was affected by controlling the neutral pressure by wall conditioning and gas puffing. (author). 18 refs, 7 figs