Load Resilient Transmission Line System for Long-Pulse, High-Power ICRF Operation at KSTAR

We have newly designed, fabricated, and installed a decoupler and a 3 dB hybrid coupler for load-resilient operation of the KSTAR ICRF. The new ICRF TL system for load resilient operation has been successfully performed in 2012 plasma experiments. In L-mode plasmas, the ratio between two powers measured at the isolated port of the hybrid coupler and reflected from two resonant loops becomes approximately 1. Furthermore, a load resilient transmission line (TL) system results in stable ICRF transmitter operation and power transmission in ELMy discharges. The maximum ICRF transmitted power is approximately 500 kW and pulse length is 3 s. With power transmission at higher than 500 kW, breakdowns frequently occurred in the vacuum feedthrough. After the campaign, we discovered that surfaces of alumina, coaxial inner and outer conductor of the vacuum feedthrough were severely damaged by the arcs. We removed the silver coat from 8 feedthroughs and repaired the damaged surfaces. In order to study a higher standoff capability, we tested the repaired feedthroughs in the vacuum chamber with a high power RF test stand.