Advances in lower hybrid current drive technology on Alcator C-Mod.

Lower hybrid current drive (LHCD) is an attractive option for non-inductive tokamak operation due to its high current drive efficiency and ability to drive current off axis. The parameters of the Alcator C-Mod LHCD system (f₀ = 4.6 GHz, B_Φ ≃ 5.5T, n_e ≃ 10²⁰ m^-3) are similar to the proposed LHCD system on ITER. This paper will describe improvements in LHCD technology on C-Mod designed to increase single-pass absorption at high n_e, extend pulse length (to >3 s), and increase power delivered to the plasma (to ~2 MW). Modelling of lower hybrid (LH) wave propagation indicates that the observed loss of LHCD efficiency at higher n_e can be mitigated by enhancing the single pass power absorption through use of an off mid-plane launcher. The four rows of the launcher are located above the mid-plane (with I_p and B_Φ both clockwise viewing from the top down) in order to exploit the poloidal upshift of n_∥ as rays propagate from the antenna into the plasma. The transmitter protection system (TPS) was redesigned to model the coolant temperature in real time and shut off the klystron beam voltage if the coolant is close to boiling. The TPS upgrade has been installed and operated on C-Mod for pulses up to 4.5 s into dummy loads and 1.0 s into the plasma. A new movable local LH launcher protection limiter was designed to reduce reflection coefficients across a wide range of launcher positions. Finally, a high power waveguide double-stub tuner is under development to provide feedback controlled load matching to reduce power reflected from the antenna under poor coupling conditions. [ABSTRACT FROM AUTHOR]