Waveguide Coupling during Lower Hybrid Current Drive in a Tokamak

Abstract
By using a lower hybrid current drive, changes in the amplitude and phase of reflected signals at the four-waveguide grill as well as decreases in the edge plasma density measured by double probes could be monitored. The influence of the plasma density and its gradient at the grill mouth on the coupling between waveguides and a plasma was investigated using a simplified linear grill theory based on a step plus ramp density model. It was confirmed from a comparison with the theoretical analysis that changes in waveguide coupling during current drive were mainly caused by a decrease of the density in the scrape-off plasma.