2D Modeling of the O-X conversion in toroidal plasmas

The conversion of an ordinary (O) mode to an extraordinary (X) mode at electron cyclotron frequencies (Prienhalter and Kopecky 1973 J. Plasma Phys. 10 1) in a spherical tokamak geometry has been examined numerically. A cold plasma dielectric tensor was used; the singularity at the upper hybrid resonance (UHR) was treated by introducing an artificial collision frequency. This allowed us to model the O-X-B mode conversion scheme (B stands for the Bernstein wave) analytically reducing the wave equations to the second order. Our numerical modeling confirmed the predictions of the 2D O-X conversion theory. In particular, it was shown that, first, the pattern of the converted X wave can be correctly described by an analytical formula within its validity limits, and second, there is an asymmetry of the O-X conversion efficiency with respect to the direction of the toroidal magnetic field. The last result is the first numerical confirmation of the effect predicted earlier by the 2D O-X conversion theory. Our modeling did not confirm the discouraging result of the numerical simulation (Vdovin 2006 33rd EPS Conf. on Plasma Physics vol 30I) that the UHR plays an important role in RF power deposition when O-mode is launched at the fundamental harmonic, and no conditions for effective O-X conversion are provided.