Acceleration of electrons in the near field of LH grills

Following experimental observations of localised heat flux on components magnetically connected to the radiating waveguides, the acceleration of electrons near Lower Hybrid antennas is investigated. A simple analytical model is developed to compute the dynamics of the particles in the near field approximation. It is found that Landau damping of the very high N// (20 < N// < 100) component of the launched spectrum on the thermal electrons of the scrape-off layer occurs. Simulation of a typical LH pulse on plasma indicates that the electrons can be accelerated up to 2-3 keV. The modelling of the interaction of this fast electrons population with the edge plasma allows to calculate the heat flux on the connected components. Experimental and calculated heat fluxes are 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 a LH power density of 25 MW/m2, the heat flux along the field lines (3.5 MW/m2) is easily manageable for the components connected to the antenna. (author)