Radiofrequency-heated enhanced confinement modes in the Alcator C-Mod tokamak

Enhanced confinement modes up to a toroidal field of [formula omitted]T have been studied with up to 3.5 MW of radiofrequency (rf) heating power in the ion cyclotron range of frequencies (ICRF) at 80 MHz. H-mode is observed when the edge temperature exceeds a threshold value. The high confinement mode (H-mode) with higher confinement enhancement factors [formula omitted] and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement [formula omitted], high normalized beta [formula omitted], low radiated power fraction [formula omitted], and low effective charge [formula omitted] has been obtained in Enhanced D[formula omitted] H-mode. This type of H-mode has enhanced levels of continuous D[formula omitted] emission and very little or no edge localized mode (ELM) activity, and reduced core particle confinement time relative to ELM-free H-mode. The pellet enhanced performance (PEP) mode is obtained by combining core fueling with pellet injection and core heating. A highly peaked pressure profile with a central value of 8 atmospheres was observed. The steep pressure gradient drives off-axis bootstrap current, resulting in a shear reversed safety factor [formula omitted] profile. Suppression of sawteeth appears to be important in maintaining the highly peaked pressure profile. Lithium pellets were found to be more effective than deuterium pellets in raising [formula omitted].