PAM, multijunction and conventional LHCD grills: Operational experience on FTU

The lower hybrid current drive (LHCD) system for FTU has a modular configuration that gives it a high degree of flexibility in operations. The system normally operates with six conventional grills (CG), each one made of four superimposed rows of 12 waveguides, distributed on two independent launching structures. This modular configuration has given the possibility of testing on FTU a classical multijunction (MJ) and a passive active multijunction (PAM) that is considered as a privileged candidate for the LH launcher for ITER. The tests have confirmed the good coupling properties of the MJ with plasma densities lower than those normally acceptable for a CG, but at expenses of a reduced power directivity and flexibility in N∥. The PAM, with a small reduction on power directivity and flexibility in N∥ with reference to the MJ, has shown an efficient coupling when working close to cut-off density (0.8 × 1018 m−3) flush to the first wall, a relatively quiet region in the harsh plasma environment of ITER. The paper presents an overview of the performances, in term of coupling characteristics, of the three launchers on FTU and gives a comparative analysis of the results obtained in similar plasma conditions, in particular during the same shot.