Influence of conducting side limiters on the excitation of Alfven waves in tokamak plasmas

In recent experiments carried out on the Phaedrus tokamak, the conducting plates that protect both sides of a poloidal antenna set have been replaced by non-conducting boron nitride plates. With this modification, direct heating and current drive by Alfven waves were demonstrated. An analysis proves that elimination of poloidal conducting protectors plays a crucial role in antenna performance improvement. The effect of the conducting side limiters on the wave spectrum and antenna impedance is calculated numerically for a cylindrical plasma column and model antenna current. The results demonstrate that conducting side limiters cause the excitation of additional modes that are deleterious for the Alfven heating and current drive scheme. (author). Letter-to-the-editor. 16 refs, 5 figs