The Balanced RF Feeding of Resonant Loops for the KSTAR ICRF

An ICRF system for the KSTAR tokamak is planned for the next campaign to deliver nominally 2 MW of RF power to the plasma to heat the ions through minority fundamental or second harmonic ion cyclotron resonance heating. The maximum RF power deliverable to the plasma is limited by the maximum RF voltages on the transmission line or antenna system. The higher RF voltage or current than the limited values may destroy the transmission lines, tuners or antenna by arcing or burning. The convenient method to reduce the RF voltage with the same power is dividing the power to the many branches. It is one of the reasons of installing four current straps within an KSTAR ICRF antenna. To archive this goal, eight power branches connecting to each upper and bottom current straps were designed and among them, four branches of two resonant loops were installed. Balanced power dividing in the ion cyclotron frequency range can be archived by using hybrid divider if the phase difference between the outputs of the divider is +/-90 degrees. For the ion heating schemes, these phase differences should be avoided because the antenna currents having these phase relation could heat the electrons through resonant heating. Therefore, the ICRF circuit for the early experiment of the KSTAR has been prepared by using simple Tee. Using simple Tee cannot guarantee balanced power dividing to the outputs of unequal impedances. This presentation will give the analysis of the unbalanced voltages measured during the last campaign and the technical methods to archive balanced voltages for high power feeding