Comparison of neoclassical predictions with measured flows and evaluation of a poloidal impurity density asymmetry

The study and prediction of velocities in the pedestal region of Alcator C-Mod are important for 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–Schlüter (PS) (high collisionality) and banana (low collisionality) regimes. B5+ flow profiles for H-mode plasmas are acquired using the charge-exchange recombination spectroscopy diagnostic on Alcator C-Mod and are compared with calculated profiles for the region just inside the last closed flux surface. Reasonable agreement is found between the predictions from the PS regime formalism and the measured poloidal velocities for the steep gradient region of the H-mode pedestals, regardless of the collisionality of the plasma. The agreement is poorer between the neoclassical predictions and measured velocity profiles using the banana regime formalism. Additionally, comparisons of measured velocities from the low- and high-field sides (LFS and HFS) of the plasma lead us to infer the strong possibility of a poloidal asymmetry in the B5+ density. This asymmetry can be a factor of 2–3 for the region of the steepest gradients, with the density at the HFS being larger. The magnitude of the density asymmetry is found to be correlated with the magnitude of the poloidal velocity at the LFS of the plasma.