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Lower Hybrid Current Drive (LHCD) experiments performed at density close to that required for the steady state scenario are reported. On C-Mod, FTU and Tore Supra, a strong decay of the brehmsstrahlung emission is observed when the density is increased, much faster that the prediction of LHCD modelling. On JET, LH power deposition is also found to be sensitive to the plasma density: LH power modulation indicates that the power deposition moves to the very edge of the plasma (r/a ~ 0.9) when the density approaches the requirement of the JET SS scenario. From this experiment but also from the reconstruction of the electron cyclotron emission spectrum, the decrease of the LHCD efficiency with density is also found. From LHCD modelling of different JET pulses performed at different densities and wave parallel refraction indexes, it is concluded that the wave accessibility condition is not the key parameter for explaining the decrease of the efficiency. C-Mod, FTU and Tore Supra experiments indicate that the plasma edge parameters, namely density and temperature but also fluctuations, are affecting the efficiency via loss mechanisms which are likely to be collisional damping (C-Mod), parametric decay instabilities or wave scattering (FTU/ Tore Supra).

Abstract. This paper summarizes highlights of research results from the Alcator C-Mod tokamak covering the period 2006 through 2008. Active flow drive, using mode converted waves in the ion cyclotron range of frequencies (ICRF), has been observed for the first time in a tokamak plasma, using a mix of D and 3He ion species; toroidal and poloidal flows are driven near the location of the mode conversion layer. ICRF induced edge sheaths are implicated in both the erosion of thin boron coatings and the generation of metallic impurities. Lower Hybrid RF has been used for efficient current drive, current profile modification, and toroidal flow drive. In addition, LHRF has been used to modify the H-mode pedestal, increasing temperature, decreasing density, and lowering the pedestal collisionality. Studies of hydrogen isotope retention in solid metallic plasma facing components reveal significantly higher retention than expected from ex-situ laboratory studies; a model to explain the results, based on plasma/neutral induced lattice damage has been developed and tested. During gaspuff mitigation of disruptions, induced MHD causes the magnetic field to become stochastic, resulting in reduction of halo currents, spreading of plasma power loading, and loss of run-away electrons before they cause damage. Detailed pedestal rotation profile measurements have been used to infer ER profiles, and correlation with global H-mode confinement. An improved L-mode regime, obtained at q 95 ≤3 with ion drift away from the active x-point, shows very good confinement without a strong density pedestal, and no evidence of particle or impurity accumulation without the need for ELMs or any other edge density regulation mechanism.