Waveguide coupling during lower hybrid current drive in a tokamak

During lower hybrid current drive, changes in amplitude and phase of reflected signals at the four waveguide grill, and also decreases in the edge plasma density measured by double probes are monitored. The influence of plasma density and its gradient at the grill mouth on the coupling between waveguides and plasma is investigated by a simplified linear grill theory based on a step plus ramp density model. It is confirmed from the comparison with the theoretical analysis that the change in waveguide coupling during current drive is mainly caused by a reduction of density in the scrape-off plasma. (author)