Modeling refraction of LHCD waves by SOL turbulence with GENRAY/CQL3D

Lower hybrid current drive (LHCD) is a promising actuator for driving off-axis current in a tokamak. RF wave and Scrape Off Layer (SOL) turbulence interaction may affect wave propagation, altering core power absorption. A ray-tracing/Fokker-Planck model GENRAY/CQL3D is used to study how the effects of refraction due to SOL turbulence alone impact core LHCD deposition. A model for the turbulent blob field in real space [1] is coupled to the background plasma in a DIII-D geometry using GENRAY. The wave propagation and absorption are calculated to determine the current drive (CD) profile. Initial results show that edge fluctuations lead to lower CD efficiency and radial broadening of the CD profile. This is due to ray refraction, and evolution of N_|| through the SOL and core plasma. The CD profile is sensitive to the amplitude and wavenumber of SOL density fluctuations. These results could explain why LHCD efficiency is observed to be lower than what is predicted by standard RF wave theory for conditions where SOL fluctuations are high. The effects of refraction will be compared to those of wave scattering in k-space; L and H-mode SOL models will also be compared. [1] Sierchio RSI 2016.