Performances of different load tolerant external matching units for the FAST-ICRH system

FAST (Fusion Advanced Studies Torus), a new tokamak machine for researches on Fusion Energy, has been proposed to the EU partners by the Italian Fusion Association to support the International Thermonuclear Experimental Torus (ITER) programme and to explore fusion relevant technologies. It is a compact tokamak with high toroidal magnetic field (8.5 T) and plasma current (up to 8 MA). In order to investigate fast particle behaviours in conditions similar to those of ITER and peculiar aspect of the physics of burning plasma, the project foresees a dominant Ion Cyclotron Resonance Heating (ICRH) System, able to couple a RF power of 20 MW in the 60 - 90 MHz frequency range. The launcher consists of eight current straps arranged in four rows. The performances of different load-tolerant external matching units (Conjugate-T in different configurations and 3 dB hybrid coupler options), have been compared considering the resilience to the loading variation, maximum voltage, current and electric field in the transmission line and the cost of the system. All the schemes, made by passive RF components (hybrids couplers, paralleled stubs and shifters, T-junction and relative shifters), have been carefully evaluated and the results are presented in the paper.