Design of 5.0-GHz KSTAR lower-hybrid coupler

The Korean Superconducting Tokamak Advanced Research (KSTAR) tokamak is being constructed to perform long-pulse, high-β, advanced tokamak fusion physics experiments by Korean Basic Science Institute (KBSI). The KSTAR tokamak will use the non-inductive plasma current drive system, lower-hybrid current drive (LHCD) system for lone-pulse operation. A 5.0-GHz microwave coupler is designed for KSTAR LHCD experiments. Four high-power klystrons feed 128 guidelets at front coupler using hundred and twenty 3-dB power dividers including 3-dB power splitters in the coupler. The guidelet has a height of 5.5 cm and the width of 0.55 cm in order for the power flux density not to exceed 4.0 kW/cm2 at the guidelet. The coupler is constructed by laminating a stack of metal plates milled to waveguide patterns. This paper describes detailed design for the 3-dB power splitter, the fixed-phase shifter, and the taper section composing the coupler. Using the High Frequency Structure Simulator (HFSS) program, we achieved a phase difference of ±0.5° and a relative power difference less than 0.2% between two vertical guidelets.