Resonant loop system for the KSTAR ICRF current drive

The phased current distribution at current straps for the KSTAR ICRF antenna causes a power imbalance at each strap owing to the mutual couplings between current straps. In order to mitigate the effect of coupling, a decoupler connecting two phased feeding lines are designed based on both a lumped element antenna model and a distributed transmission line model. Though the decoupler parameter is dependent on the loading resistance, which depends on plasma condition, an analysis shows that the decoupling is effective in the wide range of loading resistance assuming the low variation of mutual inductance between straps. A circuit analysis also shows that the RF characteristics of a complex RF transmission system are matched well for the asymmetric antenna current spectrum aiming for a non-inductive current drive of KSTAR. The calibration result of decoupler after installation is also discussed.

► There has been severe power imbalance problem in compact ICRF current drive antenna for KSTAR.
► Decoupler system is designed based on both a lumped elements model and a distributed antenna model.
► We found simple decoupler is effective for the wide range of loading resistance which depends on plasma condition.
► The calibration result of newly installed decoupler shows good agreement with expectations.