Online impedance matching system for ICRH-RF experiments on SST-1 tokamak

Abstract

High-power radio frequency (RF) heating system in the frequency range of ion cyclotron range of frequencies (ICRF) on tokamak needs a mechanism for matching impedance seen by the antenna in presence of plasma to the generator output impedance for maximum transfer of RF power to the plasma with minimum reflections to avoid damage to the generator. The impedance of the antenna is strongly dependent on edge plasma parameters and the impedance sometimes changes as fast as 10−4s, while the RF generators used can deliver full power only into constant load impedance. Hence, the matching system with dynamic response in between generator and antenna is very much essential for high power ICRF experiments when the plasma is of longer duration with variable load impedance. For ICRF system on SST-1 tokamak, two automatic matching systems are employed each for one transmission line, which consists of motorized stub tuners and phase shifters to match the antenna impedance in the time scale of 120ms. As a part of initial testing, online impedance matching system is tested with individual transmission lines and then both the lines are matched simultaneously on a variable dummy load which simulates the plasma load. In order to deliver power to both the lines from a single RF generator, hybrid coupler is used which also protects RF generator from reflections up to certain extent. However for hybrid coupler has to work properly and both the lines should see same load impedance. The automatic matching system is installed on tokamak state super-conducting tokamak (SST-1) and is tested up to 140kW power in the vacuum vessel of the tokamak. Here we present the details of the online matching system and its testing results. The significant result is that we could match the variable load impedance with the generator impedance within 120ms by suitably adjusting the step counts of the motor controller.