ICRF antenna matching systems with ferrite tuners for the Alcator C-Mod tokamak

Highlights: • We have developed a fast ferrite tuner (FFT) based antenna matching system. • The FFT system works well on the ICRF antennas on the Alcator C-Mod tokamak. • The FFT system maintains <1% RF power reflection for most plasma discharges. • The FFT system can deal with plasma transients. • The FFT system can handle high RF power operation. - Abstract: Real-time fast ferrite tuning (FFT) has been successfully implemented on the ICRF antennas on Alcator C-Mod. The former prototypical FFT system on the E-port 2-strap antenna has been upgraded using new ferrite tuners. A new FFT system with two ferrite tuners and one fixed-length stub has been installed on the transmission line of the D-port 2-strap antenna. These two systems are able to achieve and maintain the reflected power to the transmitters to less than 1% in real time under almost all plasma conditions and help ensure reliable high power operation of the antennas. The loading insensitivity feature vs. plasma conditions of the innovative field-aligned (FA) 4-strap antenna on the J-port allows us to significantly improve the matching by installing a carefully designed stub on each of the two transmission lines. The reduction of the RF voltages in the transmission lines has enabled the J-port FA antenna to deliver 3.7 MW RF power to plasmas out of 4 MW source power. The matching on the J-port antenna can be further improved by adding a single ferrite tuner under real-time control on each transmission line and this scheme will be implemented in the near future.