Development of Fast Ferrite Tuner for Lower Hybrid current drive

Abstract

We are developing a Fast Ferrite Tuner (FFT) for use in the Lower Hybrid current drive system at 4.6GHz. The FFT consists of two stub tuners mounted on the narrow sides of a WR187 waveguide whose phase lengths are electrically controlled by adjusting the bias magnetic field in ferrite blocks mounted in the stubs. The current in the coils producing the bias magnetic fields will be driven by power supplies which are computer controlled. An algorithm in the computer will change the phase length of the stub tuners to eliminate the reflection from the load. The challenging tasks in the development are obtaining the necessary phase shift with minimum field variation, keeping the losses low (<0.1dB), avoiding resonances in the waveguide during field changes, avoiding breakdown at for high power (>100KW), and developing a power supply that can respond in sub millisecond timescales. This report presents the design concept, the results of measurements on the tasks, and CST simulations of the design.