Ray tracing of lower hybrid and ion cyclotron waves

Abstract

We review the use of ray tracing codes for the investigation of wave propagation and plasma heating in toroidal axisymmetric geometry, with particular emphasis to the lower hybrid and ion cyclotron frequency ranges.

After a summary of the approximations involved, we point out that, at these low frequencies, a full-wave treatment of the launching structure on the one hand, and of singular layers (wave and particle resonances) on the other hand, are an essential part of any ray tracing code. The spectral approach to ray tracing, which makes explicit use of the decomposition of the hf fields in toroidal modes allowed by axisymmetry, is instrumental to cope with electrically short antennas whose radiation pattern is dominated by diffraction, and to allow a plausible evaluation of Landau and cyclotron damping, and of wave behaviour near conversion layers.

Numerical methods and structure of ray tracing briefly discussed, and a few examples are presented, obtained with the RAYLH and RAYIC codes developed by the author. The rapidly growing number of applications of ray tracing in the literature is also briefly summarised; it is the best proof that this approximate method, if its possibilities and limits are properly understood, can give precious insight into the physics of hf heating of tokamak plasmas.