EXCITATION OF GLOBAL MODES IN TEXTOR AND COMPARISON WITH THEORY

The excitation of fast-wave global eigenmodes is investigated both experimentally for TEXTOR and theoretically using a slab coupling mode. A brief comparison with a cylindrical model shows that in the TEXTOR situation no essential geometrical effect is left out by the slab code. The resonant behaviour associated with eigenmodes is studied for several signals: the resistance and inductance of the antennae, the r.f. magnetic and electric field components picked up by probes at the plasma edge, and the power coupled to a fast-wave antenna used as a receiver. It is shown that all the major features of these signals linked to the existence of eigenmodes can be explained using the theoretical models. Two possibly counterintuitive conclusions of this study are (a) that the absence of resonance peaks in the resistance curves of the pi-phased antennae does not imply that the eigenmodes are not excited at similar levels as for the 0-phasing, and (b) that the variations in local r.f. field magnitude are not closely related to the resonant behaviour of the antenna resistance.