Power compensators for phased operation of antenna arrays on JET and DIII-D

A system has been designed to allow operation of the JET A2 antenna arrays with phasing between elements of other than 0 or π at increased power. The system uses a hybrid coupler to transfer power in the direction opposite to that occurring at the antenna elements. This results in nearly equal currents and voltages on the feed lines and antenna structures, with nearly equal input powers at the rf generators. The direction and magnitude of the power transfer is controlled by adjustable tuning elements mounted on two of the four coupler ports. Much insight has been gained on optimization of the circuit design to maximize bandwidth (∼10 MHz for the present JET design with 20 MHz a possibility) and power-handling capability. The concept has been tested initially on the DIII-D tokamak, with a compensator installed in the feed circuit for the four-element FWCD antenna array. In this application it acts to equalize input impedances, and will eventually be used to allow operation with multiple transmitters