FEEDBACK CONTROL MATCHING SYSTEM FOR TEMPORAL CHANGE OF PLASMA LOADING RESISTANCE IN HIGH POWER/STEADY STATE ICRF HEATING ON THE LARGE HELICAL DEVICE

ICRF heating on the Large Helical Device features high power(up to 9MW) with steady state operation (30min.). We have researched feedback control impedance matching systems with two stub tuners for this device. The preferable method for following changes in plasma loading resistance has been found to be by shifting only one of the stub tuners. In this case the reflected power can be reduced to less than 0.5% for wide range of electrical resistances, 1.0 to 5.6Ω. We also successfully tested frequency feedback control of matching with a long line stub tuner or a twin stub tuner. The twin stub tuner is newly invented to simplify frequency feedback control system. A response time of 1ms has been demonstrated for this system using the twin stubtuner. During tests with a time varying electric resistance, 2 to 6Ω, the frequency always tracked the value that minimized the reflected power to less than 0.2%.