Observations of Centrally Peaked Impurity Profiles Following Pellet Injection in the Alcator-CTokamak

Abstract

Highly peaked carbon and molybdenum profiles occurred after hydrogen pellet injection. 40 ms after injection, carbon was well fitted by ${\left[\frac{T_e}{T_e\mathrm{}\left(0\right)\mathrm{}}\right]}^{1.5Z_{\mathrm{C}}}$ for $r\lesssim 6.5$ cm ($Z_{\mathrm{C}}=6\mathrm{}$). Experimental values of the inward convective velocity and diffusivity were $\sim {10}^3\left(\frac{r}{a}\right)$ cm/s and ∼ 300 ${\mathrm{cm}}^2$/s, respectively, which are of the order of neoclassical predictions. An internal disruption then occurred, which reduced on-axis impurities by frac23; and ended neoclassical-like transport. Carbon is also found to affect sawtooth dynamics dramatically by altering the central resistivity. The implications of these observations for ignited plasmas are discussed.