Development of the liquid matching components for the KSTAR ICRF system

A liquid stub tuner has been developed which is a method for a liquid matching system for transmitting a MW level of RF power continuously in the KSTAR ICRF heating and current drive system. It is made of a 228.6 mm coaxial transmission line, and it uses the difference between the RF wavelength in a liquid and that in a gas due to their different dielectric constant. The level of the liquid can be changed from 0 to 2.8 m for each liquid stub tuner, which corresponds to a 2.0 m mechanical length variation at 30 MHz. DC high voltage tests of various conditioned silicon oils such as used silicon oil, fresh silicon oil, and vacuum-pumped silicon oil were accomplished. Vacuum-pumped silicon oil showed an improved high voltage characteristic when compared with the used silicon oil and fresh silicon oil. The DC high voltage tests showed that the high voltage characteristic of the silicon oils was worse after a breakdown had occurred. High RF test for the liquid stub tuner was performed with vacuum-pumped silicon oil to ascertain its’ stand-off voltage. A stand-off voltage of 37.5 kV (average) was achieved for the liquid stub tuner with vacuum-pumped silicon oil during a 300 s operation.