Optimization of transport suppression barriers generated by externally driven Alfvèn waves in D-shaped, low aspect ratio tokamaks

In an effort to optimize the internal transport barriers (ITBs) generated by externally launched mode-converted fast waves (FWs) in pre-heated spherical tokamaks (STs), we have carried out a systematic parametric investigation with respect to the rf waves and antenna characteristics; as a study case, a START-like device has been considered. Within the framework of a plasma model including both kinetic effects (collisionless Landau damping on passing electrons) and collisional damping on both trapped and passing electrons and ions, and starting with the solution of the full wave equation for a ST-plasma, we show that optimized ITBs, suitable for the stabilization of plasma turbulence (e.g. overpassing the maximum growth rate of the ITG-instability, γ_ITG) in STs can be generated by the aid of externally launched FW and mode-converted to kinetic Alfvèn waves. This result holds in spite of the limiting trapped-particles associated squeezing factor S present in the non-linear equation for E_r (via the viscosity coefficient μ_θi α |S|^3/2, S = S(dE_r/dr)).