Power balance of Lower Hybrid Current Drive in the SOL of High Density Plasmas on Alcator C-Mod Tokamak

Lower Hybrid Current Drive (LHCD) on Alcator C-Mod exhibits low efficiency for densities $ \sim \bar{n}_e > 1\cdot 10^{20}$ m$^{-3}$ for diverted discharges. Emissivity profiles of Hydrogenic Ly$_\alpha$ and H$_\alpha$ show significant changes during the application of LH wave as high density, along with enhanced parametric decay instability (PDI) and the generation of thermoelectric scrape-off-layer (SOL) currents. A corresponding reduction in X-ray emission from fast electrons in the confined plasma suggest damping of the LH waves in the SOL. A wide-viewing, absolutely-calibrated Hydrogen Ly$_\alpha$ camera was installed to characterize fast timescale ($\sim .1$ ms) poloidal dynamics of SOL during the application of LHCD. Analyses and results will be shown characterizing the absolute power deposition LHCD as it relates to various plasma null configurations. Ly$_\alpha$ emission is also correlated to various experimental parameters such as SOL electron density and temperature profiles and Lower Hybrid input power in order to elucidate possible damping mechanisms.