Performance of the ITER ICRH system as expected from TOPICA and ANTITER II modelling

The performance on plasma of the antennas of the proposed ITER ICRF system is evaluated by means of the antenna 24 x 24 impedance matrix provided by the TOPICA code and confirmed and interpreted by the semi-analytical code ANTITER II (summarized in an appendix). From this analysis the following system characteristics can be derived: (1) a roughly constant power capability in the entire 40-55MHz frequency band with the same maximum voltage in the eight feeding lines is obtained for all the considered heating and current drive phasings on account of the broadbanding effect of service stubs. (2) The power capability of the array significantly depends on the distance of the antenna to the separatrix, the density profile in the scrape-off layer (SOL) and on the strap current toroidal and poloidal phasings. The dependence on phasing is stronger for wider SOL. (3) To exceed a radiated power capability of 20 MW per antenna array in the upper part of the frequency band, with a separatrix-wall distance of 17 cm and a conservative short decay plasma edge density profile, the system voltage stand-off must be 45 kV and well chosen combinations of toroidal and poloidal phasing are needed. (4) On account of the plasma gyrotropy and of poloidal magnetic field, special care must be taken in choosing the optimal toroidal current drive and poloidal phasings. The ANTITER II analysis shows furthermore that important coaxial and surface mode excitation can only be expected in the monopole toroidal phasing, that strong wave reflection from a steep density profile significantly reduces the coupling even if the separatrix is closer to the antenna and that the part of the edge density profile having a density lower than the cut-off density pertaining to the considered phasing does not significantly contribute to the coupling.