Digital real-time control for an ICRF fast ferrite tuning system on Alcator C-Mod

A double-stub fast ferrite tuning (FFT) system has been developed and installed on the Alcator C-Mod tokamak for real-time ion cyclotron range of frequencies (ICRF) antenna matching. The FFT uses a digital feedback control system consisting of a Linux server, a compact PCI digitizer and MDSplus interface. The software is written in IDL, and runs with interrupts disabled. During a plasma discharge, the computer records the forward RF power, reflected power, and phase data on both the antenna side and the transmitter side of the tuning system, calculates the required magnetic coil currents in the ferrite tuners to minimize the reflected power to the antenna, and sends out the current demands in each computation cycle. The FFT system has been demonstrated to achieve good matching to antenna loading in low confinement modes (L-mode), high confinement modes (H-mode), and plasmas with edge-localized modes (ELMs) in Alcator C-Mod.