The Analysis of Alfvén Wave Antenna Implementation in the ETE Spherical Tokamak

The preliminary design study of the Alfvén wave system for the ETE spherical tokamak is presented. The main objective of the proposed RF system is generation of travelling Alfvén waves, which can be used for the study of the non-inductive current drive and the plasma heating in a wide range of operational regimes, which are specific for tokamaks with small aspect ratio R/a ' 1.2− 1.5. The antenna system consists of four modules, which are separated by 90◦ in toroidal direction. Each module has two sets of RF current carrying poloidal straps that are positioned symmetrically in relation to the equatorial plane of the vacuum chamber at the poloidal coordinates ±45◦. The poloidal extension of each strap is approximately 90◦. The excited mode numbers can be controlled by the phasing of the feeding RF current. In the basic regime of operation, the straps in the same toroidal cross-sections are fed by the RF currents with (0,±π/2) phasing so that they can excite travelling modes M = ±1, N = ±1. In order to decrease the periphery RF power deposition and to improve the selectivity of the Alfvén mode excitation, the inclination of the antenna straps in toroidal direction can be adjusted in the range of angles ±30◦. The dependence of the antenna efficiency on the antenna orientation is studied numerically in the frame of 1-D MHD model.