Development of a Controller for Liquid Matching Systems

Maximum RF power is transmitted from transmitter to antenna in KSTAR ICRF system when matching condition is sustained between a transmitter and an antenna. ICRF system has liquid matching systems to make a matching condition between the transmitter and the antenna. The liquid matching systems consist of a liquid stub tuner and a liquid phase shifter. The liquid matching systems contain a liquid between the inner and outer conductors of the coaxial transmission line. By changing the liquid level instead of shifting the electric short-end, the phase of the wave can be shifted based on the difference between the RF wavelength in the liquid and gas due to the different relative dielectric constants. In the long pulse test on the transmission system at the high RF voltage, the reflected RF power increased with time. When the RF power was applied in absence of a feedback control at the impedance matching, the reflected RF power, Pref exceeded half the forward power, Pfw within a few minutes and the RF voltage, VRF declined consequently. On the other hand, Pref could be kept at a low fraction of Pfw and VRF was constant in the case of the feedback control. We designed and fabricated liquid level controller with cylinder type stub for feedback control of the matching system