Imaging Main-Ion and Impurity Velocities for Understanding Impurity Transport in the Tokamak Boundary

Imaging of ion velocities throughout the scrape off layer (SOL) combined with 2D and 3D numerical fluid modeling is establishing the roles of frictional coupling, ion-thermal forces, and parallel pressure gradients in determining impurity and momentum transport on open magnetic field lines. Velocity measurements of C$_2^+$ impurity ions alongside He$^+$ main-ion species enabled the first quantitative measurements of the entrainment of impurity velocities with the main ion species in the divertor and main-chamber SOL. Changing poloidal location of the parallel-B flow stagnation point in H-mode plasmas has been observed as has velocity slowing in both species of up to 10km/s at the mid-plane during detachment. In these cases the direction of the flow relative to the magnetic field direction implies cross-field drift effects are important for determining parallel transport along field lines. UEDGE simulations of these plasmas identify how the ratio of frictional and grad-T$_\text{i}$ forces balance to determine bulk impurity transport; the degree of entrainment of impurities is expected to vary throughout the SOL, and as a function of power and density. These 2D measurements have been achieved using two coherence imaging spectroscopy systems on DIII-D calibrated with a tunable diode laser to a velocity accuracy better than 1 km/s. In addition, 3D C$_2^+$ flow perturbations were observed in the vicinity of large coherent n=1 islands produced by external RMP coils. A poloidally alternating pattern of acceleration and deceleration, correlated to island positions, was observed with local velocity changes up to 10km/s and a length scale of 30-40cm. Comparison with EMC3-EIRENE simulations indicate that these perturbations result from temperature-driven parallel pressure gradients.