A Lower Hybrid Current Drive (LHCD) system has been installed on the Alcator C-MOD tokamak at MIT. Twelve klystrons at 4.6 GHz feed a 4x22 waveguide array. This system was designed for maximum flexibility in the launched parallel wave-number spectrum. This flexibility allows tailoring of the lower hybrid deposition under a variety of plasma conditions. Power levels up to 900 kW have been injected into the tokomak. The parallel wave number has been varied over a wide range, n.

On the Alcator C-Mod tokamak, lower hybrid current drive (LHCD) is being used to modify the current profile with the aim of obtaining advanced tokamak (AT) performance in plasmas with parameters similar to those that would be required on ITER. To date, power levels in excess of 1 MW at a frequency of 4.6 GHz have been coupled into a variety of plasmas. Experiments have established that LHCD on C-Mod behaves globally as predicted by theory. Bulk current drive efficiencies, n20IlhR/Plh ~ 0.25, inferred from magnetics and MSE are in line with theory. Quantitative comparisons between local measurements, MSE, ECE and hard x-ray bremsstrahlung, and theory/simulation using the GENRAY, TORIC-LH CQL3D and TSC-LSC codes have been performed. These comparisons have demonstrated the off-axis localization of the current drive, its magnitude and location dependence on the launched n.

Recent research on the high-field, high-density diverted Alcator C-MOD tokamak has focussed on the plasma physics and plasma engineering required for ITER and for attractive fusion reactors. Experimental campaigns over the past two years have focused on understanding the physical mechanisms that affect the plasma performance realized with all-molybdenum walls versus walls with low-Z coatings. RF sheath rectification along flux tubes that intersect the RF antenna is found to be a major cause of localized boron erosion and impurity generation. Initial lower-hybrid current drive (LHCD) experiments (PLH

(cont.) Clear evidence of current drive was seen as the loop voltage decreased by about 50%. Studies were performed by varying the phase, density, and magnetic field as well as changing the direction that the waves were launched. Analysis using CQL3D indicates that a current drive efficiency of 0.15 (1020 m-2A/W) was obtained by driving 167 kA of LH current with 410 kW at a line-average density of 5.5 x 1019 m-3, without an electric field. This efficiency exceeds what was observed on Alcator C, but is within the! range observed on FTU for similar densities. Including the residual electric field increased the LH driven current to 308 kA, corresponding to an increased effective efficiency of 0.28. In addition to experiments, extensive modeling of current profile control through phase variation was studied, including compound spectra, using CQL3D. It was found that over 200 kA of off-axis LH current can be generated in a variety of profile shapes in an H-mode target plasma. Finally, time dependent modeling of an integrated scenario was performed using the transport code TRANSP to explore what performance can be ultimately achieved on C-Mod. Results indicate that fully non-inductive, quasi-steady-state plasmas are possible with bootstrap fractions as high as 75%.