Design of external matching unit with high power vacuum windows for the FAST-ICRH system

FAST (Fusion Advanced Studies Torus) is a new tokamak machine for the researches on Fusion Energy, proposed by the Italian Fusion Association as a satellite Tokamak for accompanying the International Thermonuclear Experimental Torus (ITER) programme and to explore fusion relevant technologies. In FAST peculiar aspect of the physics of burning plasma scenarios will be investigated by accelerating fast ions through additional heating systems. The main heating source is the Ion Cyclotron Resonance Heating (ICRH) System that will couple to the plasma about 30 MW of power in the 30-90 MHz frequency range. The launcher consists in eight current straps arranged in four rows. A load tolerant External Conjugate-T concept with high resilience to the loading variation due to the plasma perturbation is being studied to match the low impedance of the launcher to the feeding coaxial cables (Zo= 30 Ω). The whole Matching System made by passive RF components (hybrids couplers, paralleled stubs and shifters, T-junction and relative shifters) has been carefully evaluated and the results presented in the paper. The RF design of a double conical vacuum window and the evaluation of the power handling capability of the external matching will be presented.