Real-time impedance matching system using liquid stub tuners in ICRF heating

For steady-state operation with ion cyclotron range of frequencies (ICRF) heating, real-time impedance matching is necessary in order to inject ICRF power into the plasma against variations in the antenna impedance. Two methods of the real-time impedance matching were employed during a long pulse discharge in the large helical device (LHD). One was the manual-frequency control method and the other was the automatic control of liquid heights in liquid stub tuners using trial-and-error method. Both of these methods entailed disadvantages; a limitation of bandwidth in impedance matching by a frequency control, and in trial-and-error method an abrupt increase of the reflected power fraction and a slow reduction rate of the reflected power fraction. A new real-time impedance matching method solving these problems was developed utilizing liquid stub tuners. The impedance of the resistor attached to the outlet of the stub tuners was measured using a directional coupler attached to the coaxial line near the outlet of the final power amplifier. Optimum liquid heights were predicted and liquid heights were adjusted in real-time using pulse motors. The reduction of the reflected power fraction was demonstrated and was kept low against variations in the impedance of the resistor.