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

The development of an 8-GHz gyrotron for lower hybrid heating of the FTU tokamak in Frascati, Italy is discussed. In the final system, 8 MW of RF power will be required, and the tokamak will operate for 1 s every 10 min. The gyrotrons will be required to operate into a 4:1 voltage standing wave ratio (VSWR). Because the RF antenna is a rectangular grill, the RF must be delivered to the tokamak in a rectangular, fundamental waveguide. The side-coupled approach has been developed to avoid complicated mode converters and filters. The RF is extracted transversely 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. Three tubes have been tested to date and another is being assembled for test. Test results are described as well as significant design changes that have occurred over the course of the program.<>