Feedback controlled hybrid fast ferrite tuners

A low power ANT-Bosch fast ferrite tuner (FFT) was successfully tested into (1) the lumped circuit equivalent of an antenna strap with dynamic plasma loading, and (2) a plasma loaded antenna strap in DIII-D. When the FFT accessible mismatch range was phase-shifted to encompass the plasma-induced variation in reflection coefficient, the 50 Ω source was matched (to within the desired 1.4:1 voltage standing wave ratio). The time required to achieve this match (i.e., the response time) was typically a few hundred milliseconds, mostly due to a relatively slow network analyser-computer system. The response time for the active components of the FFT was 10 to 20 msec, or much faster than the present state of the art for dynamic stub tuners. Future FFT tests are planned, that will utilize the DIII-D computer (capable of submillisecond feedback control), as well as several upgrades to the active control circuit, to produce a FFT feedback control system with a response time approaching 1 msec