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Experimental observations of LHCD at high density (ne>1020m∼3) on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops sharply in single null discharges well below the density limit previously observed on limited tokamaks (ω/ωLH∼2). Modeling and experimental evidence suggest that the absence of LH driven fast electrons at high density may be due to collisional absorption in the scrape off layer. Experiments show that the expected current drive density dependence is recovered for inner wall limited discharges across the range of densities scanned (0.5×1020 m-3e<1.5×1020 m-3). Increasing Te in the periphery of the plasma (0.8e. Ray tracing/Fokker-Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model. [ABSTRACT FROM AUTHOR]

On Alcator C-Mod, the LH wave coupling was observed to degrade at higher launched n∥≡ck∥/ω, suggesting that the waves have to tunnel through a millimetric evanescent layer between the plasma and the launcher. Subsequent edge density profile measurements by means of an X-mode SOL reflectometer were able to accurately document density depletion in the presence of high power LHRF. Good coupling was recovered for non-perturbative power level experiments (few Watts), confirming the role of high power LHRF waves on the edge plasma profiles. The measured reflection coefficients and density profiles were well reproduced by means of a fullwave Finite Element Method (FEM) simulation in which the density depletion by ponderomotive forces is self-consistently taken into account via an iterative approach. This model has been verified with previous 1-D calculations and has been seamlessly extended to efficiently model arbitrary 2-D or 3-D geometries. Considering a realistic geometry further exacerbates the density depletion in front of the launcher and is a key ingredient to get good agreement with the reflectometer measurements. The characterization of the plasma behavior in the vicinity of the coupler fits into a broader investigation taking place at Alcator C-Mod, which aims at understanding the role of the SOL on the LHCD efficiency. In particular, focus has been given to the loss of current drive which is observed at high density. Several experimental observations, supported by ray tracing/Fokker-Planck modeling, suggesting this is due to parasitic absorption in the SOL. Initial results using the LHEAF fullwave code and a 2D SOL model which includes the effects of flux expansion and parallel heat transport are presented. [ABSTRACT FROM AUTHOR]