Comparison of charge-exchange recombination spectroscopy measurements of the pedestal region of Alcator C-Mod with neoclassical flow predictions

Abstract

The study and prediction of velocities in the pedestal region of Alcator C-Mod are important aspects of understanding plasma confinement and transport. In this study, we examine the simplified neoclassical predictions for impurity flows using equations developed for plasmas with background ions in the Pfirsch-Schliter (PS, high collisionality) and banana (low collisionality) regimes. Measured B5+ flow profiles are derived from the charge-exchange spectroscopy diagnostic on Alcator C-Mod and are compared with calculated profiles for the region just inside the last closed flux surface. For the steep gradient region, reasonable agreement is found between the predictions from the PS regime formalism and the measured poloidal velocities regardless of the collisionality of the plasma. The agreement between the neoclassical predictions using the banana regime formalism and measured velocities is poorer. Additionally, comparisons of measured velocities from the low- and high-field sides of the plasma lead us to infer the strong possiblity of a poloidal asymmetry in the impurity density. This asymmetry can be a factor of 2-3 for the region of the steepest gradients, with the density at the high-field side being larger.