Engineering Considerations in the Implementation of High Power Fast Ferrite Tuners on Alcator C-Mod

A real-time ion cyclotron range of frequencies (ICRF) antenna matching system has been successfully implemented on Alcator C-Mod. A triple-stub tuning system working at 80 MHz is used, where one stub acts as a pre-matching stub and the other two stubs incorporate fast ferrite tuners (FFT) to realize fast tuning. It uses a computer based digital controller for feedback control (200 uS per iteration) using real-time antenna loading measurements as inputs and the coil currents to the FFT magnets as outputs. The system has obtained and maintained matching for a large range of plasma parameters, including L-mode, H-mode, and plasmas with edge localized modes, and up to 1.8 MW net RF power into H-mode plasma. The RF power loss in the system has been found to be insignificant when the voltage in the system is below 30 kV. Achieving this level of performance involved several engineering challenges. The ferrite tuners available had to be used in their received configuration and their implementation would accommodate the existing characteristics of the tuners. A suitable range of load matching, operational speed, component protection and thermal management were factors that had to be balanced against tuner characteristics, system complexity and cost containment. The FFTs are permanently operational on Alcator C-Mod.