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

Enhanced confinement modes up to a toroidal field of $B_{\mathrm{T}}\text{=8\hspace{0.17em}}$ 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 $\mathrm{(H)}$ and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement $\text{(H=2.0)}$ , high normalized beta ${\mathrm{(\beta }}_{\mathrm{N}}\text{=1.5)}$ , low radiated power fraction ${\mathrm{(P}}_{\mathrm{rad}}^{\mathrm{main}}{\mathrm{/P}}_{\mathrm{loss}}\text{=0.3)}$ , and low effective charge ${\mathrm{(Z}}_{\mathrm{eff}}\text{=1.5)}$ has been obtained in Enhanced D${}_{\alpha }$ H-mode. This type of H-mode has enhanced levels of continuous D${}_{\alpha }$ 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 $\mathrm{(q)}$ 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 $q_0$ .