Fast Ferrite ICRF Matching System in Alcator C-Mod

The loading of an ICRF antenna can vary significantly in a plasma discharge due to L-H mode transitions, the occurrence of edge localized modes and changes in plasma shape. The matching systems currently used in Alcator C-Mod have fixed positions during a discharge, which are often inadequate to accommodate all the loading variations. To avoid excessive VSWR to the transmitters, the RF power has to be shut off when mis-match happens. This imposes limitations on the range of plasma parameters that the antennas can couple effectively. In order to make ICRF operation in C-Mod more robust and flexible, a double-stub fast ferrite matching system has recently been developed. The magnetic permeability of the ferrite material can be changed by varying a magnetic field, generated by coils. As a result, the equivalent electrical length of the stubs can be controlled dynamically in a plasma discharge. The coil current control is carried out through a digital feedback system, which computes the outputs from real-time impedance signals. Simulations have shown that this fast ferrite system will allow a good loading match under most plasma conditions in Alcator C-Mod. Bench test results of the system will be presented.