Design of off-midplane launcher (LH3) based on velocity space synergy for Alcator C-Mod

A new LH launcher (LH3) is designed for Alcator C-Mod to increase the net LHCD power to 2 MW. With the existing launcher (LH2), LH3 aims to enhance the single pass power absorption to improve LHCD efficiency at high density. For this purpose, launcher design parameters are surveyed to maximize the synergistic effect between LH2. Ray-tracing is extensively used and the launcher location and the launched N_∥ = c/v_∥ are optimized. At the line averaged density of 1.4x10²⁰ m³, it is predicted that the combination of two launchers can reduce the parasitic edge loss dramatically, and therefore can increase the LH driven current by about 50 - 60% compared to cases in which two launchers are used separately. Based on the parameter survey, an off-midplane launcher (LH3) is designed to be located 15 cm above mid-plane. LH3 employs 8-way power splitters to improve the antenna-plasma coupling. The antenna-plasma coupling simulation is performed using COMSOL and ALOHA codes, predicting 90% of RF power coupling when the launcher front density is high enough (≥ n_cutoff). (author)