Automatic impedance matching network for ICRH-RF experiments on SST-1

Abstract

Ion Cyclotron Resonance Heating (ICRH) is a promising heating method for a fusion device due to its localized power deposition profile, a direct ion heating at high density, and established technology for high power RF generation and transmission low cost. For the same reason 1.5-Megawatt ICRH system is developed indigenously for steady state super-conducting tokamak (SST-1).Since plasma-loading impedance is generally small as compared to the characteristic impedance of the transmission system, a significant amount of power will be reflected from the antenna back towards the generator giving very high reflections to SWR which can damage the high power RF tube. Hence matching network is used to match the total transmission line system to the antenna impedance so that the RF generator sees a matched load and can operate at high efficiency. In the ICRH system, coarse matching network and the on-line automatic matching network designed for the impedance matching of the system to transfer maximum power to the tokamak plasma during 1000 seconds operation. The plasma impedance varies in time on milliseconds scale and hence on-line impedance matching has to match the impedance on a faster scale to avoid the reflections as well as to transfer maximum power to the plasma for heating.There are two transmission lines connected with Hybrid coupler sourced by RF Generator (RF Power) 45.6 MHz frequency. Automatic matching network is connected with each transmission line, which offers on-line fine matching along with coarse matching system connected with transmission line. The matching system includes stubs, phase shifters and the automatic matching system consists of motorized vacuum variable capacitors connected to 9" transmission line.The method includes the detection of reflected power in transmission line with the help of probes, give a right signal to VME based data acquisition and control system to do calculations and generate the signal to vary the capacitance of motorized vacuum variable capacitors within few milliseconds with the help of the controller of the motor of the variable capacitor.Initially automatic matching system was tested with individual transmission lines and then both the lines were matched simultaneously. In order to deliver power to both the lines from a single RF generator, hybrid coupler was used which also protects RF generator from reflections.Here we present the details of the on-line matching system and its testing results. The significant result is that we could match the variable load impedance with the generator impedance within 40-50 milliseconds with d1600 count of motor controller program.