Reliable high power RF injection through the upgrade of ICRF transmission line system

We have upgraded the ICRF transmission line system installing newly a decoupler and a 3 dB hybrid coupler for load-resilient operation in KSTAR plasma experiments since 2012. Load resilient operation has been successfully performed in 2012 plasma experiments. In L/H-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. However, the maximum injected RF power to the plasma was limited to 500 kW with pulse length of 3 s by breakdowns frequently occurred in the vacuum feedthrough in the 2013 campaign. So we have to focus on the reliable high power RF injection to the plasmas through the upgrade of ICRF transmission line components such as 6-inch vacuum feedthrough and 6-to-9 inch U-link coaxial in order to increase the voltage standoff during the 2014 campaign. With several activities such as modification of outer conductor's shape in the VFTs and low surface roughness of inner conductor of VFT and high voltage RF test on VFTs, we could increase the voltage standoff up to 28 kV. Finally we could successfully inject 1 MW, 3 s pulse into L-mode plasma and 700 kW, 5 s pulse into H-mode plasmas without any arcs during the 2014 plasmas experiments. (author)