Simulation and experimental study of a remote steering system for ECRH/ECCD antenna beams

The present design for the ITER electron cyclotron wave launcher is based on individual circular corrugated waveguides running up to the vacuum vessel. At each waveguide end near to the plasma, a pair of movable mitre bends provides steering of the beam between 0° and 40°. An alternative to this concept could be corrugated square or rectangular waveguides. These waveguides show imaging characteristics, which can be used for remote scanning of the beam, thus avoiding movable parts near to the plasma. To obtain a safe data base for the realisation of this concept, theoretical and experimental studies were carried out. The calculations show that a scanning range of more than ±10° is possible with negligible loss into side lobes. Furthermore, concepts to improve the scanning range can be derived from the calculations. Measurements of amplitude and phase distribution in the output plane of the waveguide and in the far-field show very good agreement with theory for a beam polarisation perpendicular to the scanning direction. For beams polarised parallel to the scanning direction, where the propagation in the waveguide is determined mainly by the grooved walls, a reduced beam quality is measured, which can be attributed to imperfect machining of the grooves. The results show also, that the imaging characteristics are in principle maintained after the introduction of a pair of mitre bends with the bending plane perpendicular to the scanning plane. Finally, the application to ITER is discussed and concepts to improve the scanning range are given.