The RF conditioning of vacuum feedthrough for high power ICRF heating in KSTAR

• Breakdown voltage of VFT of ICRF in various surface conditions and vacuum pressure was measured.
• Breakdown voltage does not depend on vacuum pressure up to glow discharge pressure.
• Breakdown voltage increases as the number of breakdown with limited energy increases.
• Pressure increase during arc in conditioned VFT is less than 10%.

The high-power coaxial vacuum feedthrough for the ICRF antenna of KSTAR tokamak was designed, fabricated and tested. It utilizes two cylindrical vacuum barriers formed of an alumina ceramic. Two cylinders are coaxially disposed between tapered coaxial conductors to form a vacuum sealed connection between a pressurized coaxial transmission line and an ICRF antenna located within a vacuum chamber of KSTAR. A vacuum feedthrough is one of the most critical parts of an ICRF antenna. It must transmit RF power while keeping the antenna in high vacuum, and support antenna elements against electromagnetic forces and thermal stress. Frequently, high-power pulsed ICRF experiments have been limited due to breakdown at the vacuum feedthrough. In order to study a higher standoff capability, the vacuum feedthrough is tested and conditioned in the vacuum chamber with a high power RF test stand. A fast interlock circuit is also utilized to reduce breakdown energy. The amplitude of RF breakdown voltage of the conditioned vacuum feedthrough is measured to be ∼55 kV in the normal vacuum pressure of 1–5 ×10−5 mbar. The breakdown voltage is not changed severely up to 5 × 10−3 mbar. This paper will show the results of high power RF test and conditioning aiming for higher reliability at high RF power.