Design and test of an 8 GHz, 500 kW, side-coupled gyrotron

An 8-GHz, 500-kW gyrotron oscillator is being developed for lower hybrid heating of the FTU tokamak in Italy. The final system will be required to deliver 8 MW of RF power, to operate for one second very 10 minutes, and to continue RF operation into a 4:1 voltage standing wave ratio. A tube is being developed that couples the power from the interaction circuit directly into the fundamental, rectangular waveguide. This represents a major departure from previous designs where the power is extracted axially in an overmoded, circular guide. The gyrotron operates at 95 kV and 12 A with the RF output in two waveguides carrying 250 kW each. A standard magnetron injection gun is used with a loading of 3.2 A/cm/sup 2/. Considerable design effort has been required to develop a circuit that will operate at high efficiency without severe mode competition. Techniques were developed to reduce mode conversion and azimuthal distortion of the cavity fields and ensure operation in the desired axial mode. The mechanical characteristics, gun design, circuit, collector, and window design are described. >