Relativistic ray-tracing of electron Bernstein waves in a spherical tokamak reactor

Electron Bernstein waves (EBW) can propagate between electron cyclotron harmonics in hot, dense plasmas with no high-density cutoff (unlike electron cyclotron waves). ARIES-ST is a spherical tokamak (ST) reactor study. A promising method of heating and driving current in such an ST reactor would be with electron Bernstein waves. These could be mode converted from microwave vacuum modes (X-mode and O-mode) launched from the edge. In this paper, fully relativistic ray-tracing calculations (both damping and propagation) of EBWs in ARIES-ST are presented, using the relativistic dispersion solver R2D2 and the ray-tracing code GENRAY. The ray paths, damping locations, and polarizations are compared with the more commonly used non-relativistic EBW ray-tracing. A range of frequencies are shown for EBWs with small n∥ that could be produced by the X-B mode conversion process, and EBWs with large n∥ ≈ 0.6, produced by the O-X-B mode conversion process. With the density and temperature profile chosen for this paper, the greatest depth into the core that could be reached with mode-converted EBWs is a radial location of approximately ρ = 0.4. Although the radial location of the damping in most cases was not significantly different between the relativistic and non-relativistic cases, there are differences in the poloidal locations, as well as in the polarizations of the wave along the ray path, especially in the damping region.