Lower Hybrid Current Drive on Alcator C-Mod: System Design, Implementation, Protection, Calibration and Performance

A 4.6 GHz 3 MW lower hybrid current drive (LHCD) system has been designed and implemented on Alcator C-Mod. This RF system will allow C-Mod to access advanced tokamak regimes: high confinement, high beta_n, and high bootstrap fraction and extend them to quasi-steady-state conditions. The LHCD system includes twelve 250 kW klystrons. Power from each klystron is split eight ways using a complex system of waveguides to drive a 96-window coupler array. The amplitude and relative phasing of each klystron is controlled by a computer-based system using I-Q vector modulators and is monitored by I-Q detectors to control the n_par spectrum applied to the plasma. Calibration is accomplished using a network analyzer in conjunction with software programs to generate two-dimensional lookup tables that allow compensation for system non-linearities. Forward and reflected powers are monitored to protect the klystrons, waveguides and coupler array from arcing. During the 2006 experimental campaign, nearly 1 MA of current was driven into Alcator C-Mod plasma using 800 kW of coupled RF power.