ICRF driven edge convection and interaction with the H-mode

It is shown that radiofrequency antenna sheaths can bias the edge plasma potential and drive steady state convective cells in the scrape-off layer. The resulting E-vector x B-vector convective flow opposes the natural direction of the sheared flow outside the last confined plasma flux surface. A two-dimensional fluid simulation shows that the interaction of the opposing poloidal flows produces secondary vortices which connect the edge of the confined plasma to the antenna tiles. Estimates for typical tokamak edge parameters suggest that the eddy transit time of particles and energy across these vortices is short enough to cause the broadening of scrape-off layer (SOL) density and temperature profiles observed during high power heating with ICRF antennas in monopole phasing. RF sheath driven convection is also a good candidate for explaining the phasing dependence of the global confinement properties of ICRF H-modes on JET. A comparison of JET H-mode data with the theoretical modelling supports this idea and suggests that ICRF convection may be a useful tool to spread the heat deposition in the divertor and to extend the lifetime of the H-mode. (author). 7 refs, 4 figs