Coupling of the 2 × 24 waveguide grill for lower hybrid waves in ASDEX

Experimental results on the coupling of a 2 × 24 waveguide grill are compared with theory. Graphite tiles attached directly to the grill and intended to protect the grill against particle bombardment have been found to degrade the coupling substantially if they protrude only a few millimeters beyond the actual grill surface. Measurements in this case are compatible with calculations where a vacuum gap is introduced between the grill and the plasma. Low reflection is obtained when the tiles do not protrude. Coupling can be maintained when the RF pulse is started while the plasma is near to the grill and then moved to a larger grill-separatrix distance where otherwise total reflection would occur.