Lower hybrid wave excitation and propagation in the spherical tokamak Globus-M

The problem of lower hybrid (LH) wave excitation and current drive (CD) in tokamaks with a small aspect ratio (spherical tokamaks) is discussed. It is proposed to solve this problem by exciting the waves slowed down in the poloidal rather than the toroidal direction. As a result, due to the strong poloidal inhomogeneity of the magnetic field in spherical tokamaks, even the waves with comparatively weak slowing down (N approximate to 3-5) excited by a waveguide antenna in the equatorial plane can penetrate into the dense plasma and be absorbed via the Landau mechanism. This approach was applied for modeling the LHCD experiments in the low aspect ratio tokamak Globus-M (R = 0.36 m, a(0) = 0.24 m, B(0) = 0.4T, I(p) = 0.25 MA, vertical elongation k = 1.6, operating frequency 2.45 GHz). The modeling was carried out using four independent codes: (i) the self-consistent antenna coupling code GRILL3D, (ii) the ray-tracing code incorporating a specially developed ray-tracing technique including some corrections necessary in strongly inhomogeneous plasma, (iii) the 2D full-wave code WAVETOP2D and (iv) the Fokker-Plank code combined with the ray-tracing code allowing simulation of the driven current density profile. The results of simulations were cross-checked and appeared to be in a good agreement. It was demonstrated that the proposed scenario provides a possibility for the efficient LHCD in a spherical tokamak.