Coaxial and surface mode excitation by an ICRF antenna in large machines like DEMO and ITER

A study of radio frequency (RF) field excitation in the edge of the plasma of DEMO is performed by means of the semi-analytic coupling code ANTITER II. The modeling uses the designed antenna and a reference low coupling density profile of ITER. The results show the existence of coaxial modes propagating between the wall and the Alfvén resonance region where surface modes are excited leading to large standing wave patterns all around the machine. The excitation of these modes can be strongly reduced for a strap current distribution of the antenna array which fulfills the two conditions ∑i Ii = 0 and ∑i Ii Si = 0(I _i: current of strap i, S _i: toroidal strap position). These conditions are satisfied by the triple strap antenna of AUG that has allowed successful ICRH operation in a W coated machine. They are also achieved for the (0π π0) phasing of the four columns of two poloidal triplets of radiating straps constituting the ITER antenna array. Ways to further optimize the cancellation of these edge modes are also investigated.