Stable and stationary states with hollow current density profiles have been achieved in Tore Supra with lower hybrid current drive (LHCD) during reduced toroidal magnetic field operation (B t 2 T) and in weak LH absorption regimes. For these plasma conditions, offaxis LH power deposition profiles are obtained in a reproducible manner when the internal LH caustics prevent central absorption of the waves. In the multipass LH wave propagation regime, the validity of the statistical treatment of stochastic wave diffusion is shown both theoretically and experimentally. When a large fraction of the plasma current (above 50%) is non-inductively sustained by the LH waves, the magnetic shear is reversed in the plasma core, i.e. inside a normalized plasma radius of the order of 0.4. The resulting hollow current density profiles have led to an enhancement of the total electron thermal energy content, up to a factor of 1.6 compared with L-mode discharges. The confinement improvement is attributed to a strong reduction of the electron thermal diffusivity in the central reversed shear region, nearly down to its neoclassical level.

As a result of experimental observations of localized heat flux on components magnetically connected to radiating waveguides in Tore Supra and in TdeV, the acceleration of electrons near lower hybrid (LH) antennas has been investigated. A simple analytical model has been developed to compute the dynamics of the particles in the near field approximation. Landau damping of the very high N|| (20 < N|| < 100) component of the launched spectrum on the thermal electrons of the scrape-off layer (SOL) is found to occur. Simulation of a typical LH pulse in Tore Supra indicates that the electrons can be accelerated up to 2-3 keV. Modelling of the interaction of this fast electron population with the edge plasma allows a calculation of the heat flux on plasma facing components that are magnetically connected to the antenna. Model results and the results of experiments in Tore Supra and TdeV are compared. The calculated heat fluxes are found to be fairly consistent when the variation of convective heat flux at the grill aperture is taken into account. From this analysis, it is concluded that, for an LH power density of 25 MW/m2, the resulting heat flux along the field lines (3.5 MW/m2) is manageable for the components connected to the antenna, provided that good coupling can be maintained at a low density in front of the grill.

The TORE SUPRA lower hybrid current drive experiments (8 MWi3.7 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 NETIITER. 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" m-3 I neg I 1.4 X 10l8 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 MWlm'. 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.