LOWER HYBRID WAVE COUPLING IN TORE SUPRA THROUGH MULTIJUNCTION LAUNCHERS

The TORE SUPRA lower hybrid current drive experiments (8 MW/3.7 GHz) use large phased waveguide arrays, four rows of 32 active waveguides and two passive waveguides for each of the two grills, to couple the waves to the plasma. These launchers are based on the 'multijunction' principle which allows them to be quite compact and is therefore attractive for the design of efficient multi-megawatt antennas in NET/ITER. Extensive coupling measurements have been performed in order to study the radiofrequency (RF) characteristics of the plasma loaded antennas. Measurements of the plasma scattering coefficients of the antennas show good agreement with those obtained from the linear coupling theory (SWAN code). Global reflection coefficients of a few per cent have been measured in a large range of edge plasma densities (0.3 x 10(18) m-3 less-than-or-equal-to n(eg) less-than-or-equal-to 1.4 x 10(18) m-3) or antenna positions (0.02-0.05 m from the plasma edge) and up to a maximum injected RF power density of 45 MW/m2. When the plasma is pushed against the inner wall of the chamber, the reflection coefficient is found to remain low up to distances of the order of 0. 10 m. The coupling measurements allow us to deduce the 'experimental' power spectra radiated by the antennas when all their modules are fed simultaneously with variable phases. Thus, the multijunction launcher is assessed as a viable antenna for high power transmission with good coupling characteristics and spectrum control.